ToxSci Advance Access originally published online on April 15, 2003
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Toxicological Sciences 73, 403-415 (2003)
Copyright © 2003 by the Society of Toxicology
NEUROTOXICOLOGY |
Disassociation of Carbon Disulfide-Induced Depression of Flash-Evoked Potential Peak N166 Amplitude and Norepinephrine Levels
* Neurotoxicology Division, MD B105-05, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Exposure to organic solvents frequently causes functional impairment of the central nervous system (CNS). One method to examine the effects of solvent exposure on visual function is flash-evoked potentials (FEPs). Greater knowledge of the role of various neurotransmitters in generating FEP peaks would be beneficial for understanding the basis of neurotoxicant-induced changes. FEP peak N166 is influenced by the psychological construct of arousal, which in turn is believed to be influenced by the function of neurons containing norepinephrine (NE). Because of its known effects on both NE and FEPs, we utilized carbon disulfide (CS2) as a means to examine the possible role of NE in modulating the amplitude of FEP peaks N36 and N166. Our hypothesis was that CS2-induced alterations in cortical NE levels would be correlated with changes in FEP peak N36 and N166 amplitudes. Adult male Long-Evans rats were implanted with electrodes over their visual cortex and allowed to recover. To develop peak N166, FEPs were recorded for two days prior to dosing. On the third day, FEPs were recorded prior to dosing, and one group of animals was sacrificed to serve as pretreatment controls. The remaining animals were dosed ip with 0 (corn oil vehicle; 2 ml/kg), 100, 200, or 400 mg/kg CS2. The treated animals were retested at 1, 4, 8, or 24 h after dosing, immediately sacrificed, and samples of the cortex, cerebellum, striatum, and brain stem were frozen for high performance liquid chromatography (HPLC) analysis of monoamine levels. Treatment with CS2 decreased peak N166 amplitude at 1 h, and peak N36 amplitude was depressed at 4 h, relative to the subject’s pretreatment values. Peak latencies were increased, and colonic temperature was decreased by treatment with CS2. Exposure to CS2 depressed NE levels in the cortex, brain stem, and cerebellum 4 h after treatment. Conversely, at 4 h, levels of dopamine (DA) and its metabolite 3,4-dihydroxyphenylacetic acid were increased in the brain stem and cerebellum, and levels of the DA metabolite homovanillic acid were increased in the brain stem. Levels of serotonin were unaffected by CS2 treatment. There was a slight increase in striatal levels of the serotonin metabolite 5-hydroxyindole acetic acid at all times after treatment with CS2. There was no apparent association between the decreases in NE levels and the reductions in amplitudes for peaks N36 and N166. The neurochemical mechanism for CS2-induced reductions in FEP peak amplitudes remains to be determined.
Key Words: carbon disulfide (CS2); flash-evoked potential (FEP); norepinephrine (NE).
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Exposure to organic solvents frequently results in functional impairment of the central nervous system (CNS). At high enough levels, acute exposure may result in CNS depression, sedation, or even anesthesia. Lower exposure levels for longer periods of time may change the ability to perform cognitive tasks (Dick, 1995
; Wood, 1981), or to perceive and interpret external stimuli such as auditory signals (Clerici and Fechter, 1991; Crofton et al., 1994; Hirata et al., 1992; Lataye et al., 1999; Simonsen and Lund, 1995), or touch (Chu et al., 1996; Husman and Karli, 1980; Lilis, 1974). The effects of solvent exposure on visual function may be tested using flash-evoked potentials (FEPs) (Dyer et al., 1984, 1988; Herr and Boyes, 1997; Herr et al., 1992; Mattsson et al., 1990; Rebert et al., 1989a, 1990). Flash-evoked potentials are the electrophysiological responses produced by presentation of light flash stimuli, and they are often recorded from electrodes placed over the visual cortex (Herr and Boyes, 1997; Herr et al., 1991, 1992, 1994). Although FEPs provide information regarding the anatomical locus of visual function changes (Herr and Boyes, 1995; Mattsson and Albee, 1988; Mattsson et al., 1992), little is known about the role of different neurotransmitters in generating and/or modulating FEPs. Experiments using selected toxicants to increase our knowledge of the role of various neurotransmitters in generating FEP peaks would be beneficial for interpreting the mechanistic basis of neurotoxicant-induced changes.
Two FEP peaks that may be differentially altered by solvents are a negative peak occurring at approximately 30–36 ms (N36), and a negative peak at about 166 ms latency (N166) following flash stimulation. Dichloromethane reduces the amplitude of FEP peak N36 (0.25 h after treatment), 1,3-dichloropropane reduces the amplitudes of both N36 (0.5 and 1 h) and N166 (at 0.5, 1, and 4 h after treatment), and 1,2-dichlorobenzene only reduces the amplitude of N166 (at 0.5, 1, 2, and 4 h after treatment) (Herr and Boyes, 1997). Treatment with toluene (Dyer et al., 1988; Rebert et al., 1989b,c) or p-xylene (Dyer et al., 1988) has also been shown to decrease the latter portions of FEPs. Interestingly, the decreases in peak N166 amplitude produced by these solvents were observed in the absence of decrements in peak N36 amplitude. Little is known regarding the possible neurochemical basis for these changes in FEPs. Carbon disulfide (CS2) is another solvent which is known to alter the amplitude of both peak N36 and N166, but with a distinctly different time-course after exposure (Herr et al., 1992). Additionally, treatment with CS2 is known to decrease levels of brain norepinephrine (NE), supposedly mediated via inhibition of dopamine ß-hydroxylase (Bus, 1985; McKenna and DiStefano, 1977), suggesting one possible neurochemical mechanism for the alterations observed in FEPs.
The function of the noradrenergic system has been associated with modulating a subject’s level of arousal and cortical responsiveness to external stimuli (Aston-Jones et al., 1984, 1999; Foote et al., 1983; Grant et al., 1988; McCormick, 1989). The NE-containing neurons of the locus coeruleus (LC) discharge slowly during slow-wave sleep, not at all during REM sleep, and rapidly during times of increased vigilance such as during orientation toward an unexpected or preferred stimulus (Foote et al., 1983). Neuroanatomical tracing studies have shown fibers from the LC infiltrating the visual cortex (Aston-Jones et al., 1984; Foote et al., 1983; Parnavelas and Papadopoulos, 1989). Both excitatory and inhibitory responses of cortical cells to NE have been reported (Devilbiss and Waterhouse, 2000; Waterhouse et al., 1990, 2000). One interpretation of such data is that NE may enhance the relative responsiveness to external stimuli by producing an increased signal-to-noise ratio (Aston-Jones et al., 1984; Foote et al., 1983; Hasselmo et al., 1997; Waterhouse et al., 1990), and may also be involved in synaptic plasticity of the visual cortex (Kirkwood et al., 1999). The increased signal-to-noise ratio is achieved via inhibition of spontaneous discharges and a resulting net increase above background of the visually evoked response (Aston-Jones and Bloom, 1981; Aston-Jones et al., 1999; Waterhouse et al., 1990). Changes in the cortical processing of the "salience" of the flash stimulus could be involved in the increases in FEP peak N166 amplitude (and/or the photic after-discharge (PhAD)) observed with repeated testing (Dyer, 1989; Herr et al., 1991, 1994).
Arousal levels have been implicated in modulating the amplitude of the late portions of FEPs. While stimulus intensity is very important in influencing the amplitude of peak N36 (Herr et al., 1991, 1994), additional factors such as the subject’s state of arousal are believed to modulate the amplitude of peak N166 and the PhAD. For these portions of the FEP, maximum amplitudes are obtained when the subject is believed to be in a state of relaxed awareness, neither inattentive nor anxious (Bigler, 1977; Bigler and Fleming, 1976a; Bigler et al., 1976; Joseph et al., 1981; Shearer and Creel, 1978). Behavioral and/or parametric test manipulations have been used to alter arousal levels and modulate the amplitude of the late FEP peaks (Bigler et al., 1976; Dyer, 1989; Schaefer et al., 1974; Standage and Fleming, 1978). Thus, the late portions of FEPs appear to be modified by the subject’s arousal level, which in turn may be influenced by the function of the NE system.
In this study, we have utilized CS2 as a means to examine the relationship between altered brain levels of NE and the amplitude of FEP peaks N36 and N166. This study replicated our previously reported differential effects of CS2 on peak N36 and N166 amplitudes (Herr et al., 1992). Additionally, we examined the dose- and time-course of CS2-induced depression of NE levels (as well as other catecholamine and indolamine neurotransmitters) in four brain regions. The purpose of the experiment was to increase our knowledge of the role of cortical NE in modulating FEP peaks, especially peak N166, which is believed to be influenced by psychological constructs such as arousal level, which may be modulated by NE.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Animals and surgery.
Naive, 90–120-day-old male Long-Evans rats (total n = 200) purchased from Charles River Laboratories (Raleigh, NC) were used in this experiment. Animals were housed in a facility approved by the Association for Assessment and Accreditation of Laboratory Animal Care International, and the National Health and Environmental Effects Research Laboratory, Animal Care and Use Committee of the U.S. EPA, which requires compliance with the National Institutes of Health guidelines, approved all investigations. Animals were housed singly and allowed to acclimate to the colony for at least 5 days (12 h light/dark cycle, lights on 06:00 h; 22 ± 2°C; 40 ± 20% relative humidity) prior to surgery. Food (#5001, Purina Lab Chow, St. Louis, MO) and tap water were provided ad libitum. Surgical electrode implantation procedures have been described previously (Herr and Boyes, 1997
; Herr et al., 1992, 1994). Briefly, the rats were anesthetized and implanted with presoldered stainless-steel epidural screw electrodes (Dyer et al., 1987). A ground electrode was located 2 mm anterior to bregma and 2 mm left of midline, a reference electrode was placed 2 mm anterior to bregma and 2 mm right of midline, and a visual cortex electrode was implanted 4 mm left of midline and 1 mm anterior of lambda (Herr and Boyes, 1993). An antibiotic (10% Povidone Iodine Creme; E. Fougera & Co., Melville, NY) was applied at the conclusion of surgery, and the rats were allowed approximately a week to recover prior to testing.
Flash-evoked potential testing and CS2 treatment.
Animals were randomly assigned to treatment groups, which were counterbalanced across testing chambers. Flash-evoked potential testing took place in sound and light attenuating Faraday cages (Hamm et al., 2000). Unanesthetized rats were restrained in cones made of plastic film (decapicones; Braintree Scientific, Inc., Braintree, MA), their head and pinnae exposed, placed on a platform inside the test chambers, and allowed to acclimate for at least 1 min before stimulation. The rat’s eyes were located approximately 37 cm below a photic stimulator (Model PS22, Grass Instrument Division, Astro-Med, West Warwick, RI) that was encased in a foam-lined and electrically shielded box. Acoustic white noise (80 dB SPL; calibrated at the animal’s ear level) was presented using a 10-cm cone speaker located about 57 cm above the subject’s ear level (Hamm et al., 2000). The white noise was present during testing to mask auditory potentials produced by the strobe discharge (Herr et al., 1996; Shaw, 1992). An overhead DC-powered light bulb produced an ambient illumination of about 20.3 lux. A 10 µs flash stimulus was delivered at 203.1 lux-s (strobe setting 16) for 75 trials (collection time: 50 ms prestimulus baseline + 500 ms data). The EEG signals were amplified (10,000x; filter bandpass: 0.1–1000 Hz; Model 12A5 Neurodata Acquisition System, Grass Instrument Division, Astro-Med, Inc.), digitized at 4000 Hz, and averaged using custom written software (Hamm et al., 2000). Prior to any testing, the amplitude and latency response factors of the amplifiers and computer were calibrated using sine waves of 178 µV RMS at 3, 100, and 300 kHz. Flash intensity and ambient light were calibrated using a photometer (Model 450 with Model 550–3 Pulse Integration Module, Gamma Scientific, Inc., San Diego, CA). Masking noise, auditory calibration was performed at the rat’s ear level in the test chamber using a 1.27 cm microphone (Model 4166, Brüel & Kjær) and a measuring amplifier (Model 2636, Brüel & Kjær).
Animals had FEPs collected for two consecutive days prior to dosing with CS2, in order to develop FEP peak N166 (Dyer, 1989; Herr et al., 1991, 1994). Colonic temperature was monitored during testing using a rectal probe (Model RET-1; Thermalert Thermometer Model TH-8, Physitemp Instruments, Clifton, NJ) inserted approximately 8 cm and was recorded at the conclusion of FEP testing. On the third day, FEPs were recorded as a pretreatment baseline. One group of animals (10 rats/dose) was immediately sacrificed following pretreatment baseline FEP testing, to serve as non-injected controls (NIC) for the biogenic amine assays. The remaining animals were injected with 0 (corn oil vehicle, Mazola), 100, 200, or 400 mg/kg CS2 (ip, 2 ml/kg dosing volume). All treatment groups used 10 animals/dose/time point. The doses of CS2 were based on previous work, which showed a different time-course for CS2-induced depression of FEP peak N36 and N166 amplitudes (Herr et al., 1992). The animals were then retested 1, 4, 8, or 24 h posttreatment. Separate groups of animals were used at each time point.
Tissue collection, biogenic amine assay, and protein assay.
Immediately following posttreatment FEP testing, on the third day, the animals were sacrificed by decapitation, and their brains removed and dissected following a modification of Glowinski and Iversen (1966). One hemisphere of cortical tissue (posterior to the optic chiasm and with the hippocampus removed), cerebellum, brain stem, and striatum were collected, weighed, immediately frozen on dry ice, and stored at -80°C until analysis.
The brain regions used for high performance liquid chromatography (HPLC) analysis were chosen based on their NE and DA innervation, the generator sites for FEPs, and predicted differences in neurochemical alterations produced by CS2. The visual cortex contains the neurons responsible for the generation of FEPs (Branca
k et al., 1990; Kenan-Vaknin and Teyler, 1994; Kraut et al., 1985) and has NE and 5-HT terminal innervation (Baumgarten and Lachenmayer, 1985; Lindvall and Björklund, 1983). The brain stem contains the cell bodies of NE and 5-HT neurons (Baumgarten and Lachenmayer, 1985; Lindvall and Björklund, 1983). The cerebellum receives both NE and 5-HT innervation (Baumgarten and Lachenmayer, 1985; Bishop and Ho, 1985; Lindvall and Björklund, 1983), and the striatum has a rich DA innervation and also contains 5-HT terminals (Li et al., 2001; Lindvall and Björklund, 1983; Vertes, 1991). The striatum was chosen as a negative control area (e.g., changes in NE synthesis should produce none-to-minimal changes in striatal neurotransmitter levels).
Frozen brain samples were sonicated on ice at 15 watts for 5–15 s in 1 ml of 0.1 N perchloric acid (PCA) containing 12.83 µM ethylenediaminetetraacetic acid (EDTA) and the internal standard NT-methyl-5-hydroxytryptamine (5-MHT). The samples were then centrifuged at 4°C for 10 min (12,500 x g) and the supernatants removed. The striatum was diluted 1:4 (v:v) and the brain stem was diluted 2:1 (v:v) with 0.1 N PCA. For all samples, 50 µl of the final supernatant (maintained at 4°C) were injected into the HPLC systems using a refrigerated autosampler. The final injected concentration of 5-MHT was 0.25 ng/µl (or 0.5 ng/µl for striatal samples using System 1). Samples were maintained on ice under light-shielded conditions at all times. The sample size for all groups for the HPLC analysis was 10, with the exceptions of the NIC striatum for the 100 mg/kg group (n = 9) and the cortex for the 0 mg/kg group at the 24-h time (n = 9).
HPLC analysis of brain catecholamines, NE and dopamine (DA), and its metabolites 3,4-dihyroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the indolamine serotonin (5-HT) and its metabolite 5-hydroxyindole acetic acid (5-HIAA) were performed using two HPLC systems. The first system (System 1) consisted of a Waters 510 pump, a WISP 712 autosampler with a cooling unit (Waters Corp., Milford, MA), an Eppendorf CH-30 column heater and TC-50 controller (Eppendorf, Madison, WI), a pulse dampener (Model LP-21, Scientific Systems, Inc., State College, PA), and an ESA Coulochem electrochemical detector (Model 5100A, ESA, Inc., Chelmsford, MA). The second HPLC system (System 2) consisted of an Alliance 2690 Separations Module with column heater module (Waters), and an ESA Coulochem II electrochemical detector (Model 5200A', ESA, Inc.). All chromatographic separations were performed using a Guard-Pak precolumn holder and 10µ µBondpak C18 insert (Waters), and a 5µ Microsorb C18 column (250 x 4.6 mm; Varian Associates, Walnut Creek, CA). The columns were thermostated to 35°C (System 1) or 30°C (System 2). Both systems had a Guard Cell (Model 5020, ESA, Inc.) set at +450 mV, and an Analytical Cell (Model 5011, ESA Inc.) with detector 1 set at +50 mV, and the monitored detector 2 set at +420 mV. The gain setting for detector 2 was 500 nA/V for all tissues using System 2, and 10 x 15 (666.7 nA/V) for striatum, 100 x 3 (333.3 nA/V) for brain stem and cerebellum, and 100 x 6 (166.7 nA/V) for cortex using System 1. Standards for NE, DOPAC, DA, 5-HIAA, HVA, 5-HT, and 5-MHT were purchased from Sigma Chemical Co. (St. Louis, MO). The mobile phase contained 80 mM H3PO4 (Fischer Scientific, Fair Lawn, NJ), 2.59 mM heptanesulfonic acid (Sigma), and 12.83 µM EDTA (Sigma), at apparent pH of 2.7. The absolute concentration of MeOH varied from 9–11%, based on the age of the columns. The mobile phase was filtered (0.22 µ GVWP filters, Milipore, Bedford, MA), sparged with helium, and held under a helium blanket in a reservoir with UV protective coating (Kontes Glass Co., Vineland, NJ). Isocratic elutions were performed with a mobile phase flow rate of 1.2 ml/min for 60 min. Data was collected and analyzed using either Maxima or Millennium32 chromatography software (Waters), using area ratios to determine sample concentrations.
The tissue pellets remaining from the PCA extraction were analyzed for total protein content by the Pierce BCA assay (Pierce, Rockford, IL) against a bovine serum albumin (RIA Grade, Sigma) standard in a Thermomax microplate reader with SOFTmax® PRO software (Molecular Devices, Sunnyvale, CA). Previously frozen pellets were thawed and sonicated on ice in 1 ml of 0.1 N NaOH (Sigma), digested overnight, resonicated, and diluted to a final concentration of 0.01 N NaOH.
Statistical analysis.
Peak amplitudes and latencies were measured from each animal’s average waveform. Peak amplitudes (in µV) were measured from baseline (defined as the average voltage over the prestimulus period). Peak latencies (in ms) were calculated from stimulus onset. Peaks were identified by their polarity and latency according to the average waveform from each treatment group. Based on our previous results (Herr et al., 1992), statistical power was focused on changes in the amplitude of FEP peaks N36 and N166. Other peak amplitudes and latencies were examined in an exploratory manner using Bonferroni-corrected 
levels (critical = 0.05/[number of peak amplitudes and latencies]) (Abt, 1981; Muller et al., 1983). Only effects related to CS2 treatment are discussed in the manuscript. Significant effects were followed by step-down analysis, with further Bonferroni corrections to maintain the familywise 
0.05. Within-subject effects had a Greenhouse-Geisser correction factor (
) applied to their degrees of freedom (Geisser and Greenhouse, 1958; Greenhouse and Greisser, 1959; Keselman and Rogan, 1980). Due to the potentially decreased statistical power produced by these adjustments (Muller et al., 1983), if an overall effect was significant but subsequent step-down ANOVAs failed to reach the corrected significance level (but had a p < 0.05), the actual probability values are reported. In all analysis, following significant ANOVAs, post hoc group mean comparisons were performed using a Tukey-Kramer Multiple Comparison Test ( = 0.05) (Kramer, 1956). These comparisons established which doses of CS2 produced significant changes in FEP peak amplitudes or neurotransmitter levels. All analyses were performed using SAS (PROC GLM) (SAS Institute, Inc., 1989, 1997). Group averaged waveforms were calculated from individual animal data, and are reported for illustrative purposes only.
The initial analysis examined all the groups over the two pretreatment days and the pretreatment baseline test on the third day, to assure the similarity of each group’s development of peak N166 amplitude prior to treatment with CS2 (Dyer, 1989; Herr et al., 1991, 1994). A repeated-measures analysis of variance (ANOVA) was performed for the amplitude of peaks N36 and N166. In this analysis, the animals were coded with their eventual treatment code (representing dose of CS2 and time of testing), even though they were all untreated at this time. Thus, dose of CS2 and time of posttreatment testing were "dummy" between-subject variables, and test day was a repeated within-subject variable.
The data from the post-CS2 treatment times were analyzed in a manner to facilitate comparisons with our previous work (Herr et al., 1992). The previous study employed a repeated-measures design with time as a within-subject factor (e.g., each animal was repeatedly tested at different times following treatment with CS2). Because in this study, the animals were sacrificed at each time point, a within-subjects design was impossible. Therefore, a difference score was calculated for each animal as follows: the third day pretreatment peak value minus the peak value at the time of testing. This difference score was then analyzed using an ANOVA with dose of CS2 and time of testing (sacrifice) as between-subject variables. The NIC groups, which were sacrificed immediately after the pretreatment FEP test on day 3, were dropped from the analysis (no posttreatment data was available). This analysis examined CS2-induced changes in FEP peak amplitudes at different times of posttreatment testing, relative to the animal’s pretreatment amplitudes as well as non-dose-related changes in FEP peak amplitudes between the test times. Data are reported as mean ± standard error (SE).
The neurochemical data was analyzed using a repeated-measures ANOVA, the dose of CS2, and time of testing as between-subject factors, and the brain region as a within-subject factor. This analysis examined the dose-related changes, differences between times of sacrifice, and regional differences in neurotransmitter levels (as well as the interactions between the three factors). An overall critical = 0.0083 was used in this analysis (0.05/6 compounds). Step-down analysis for each brain region and time of testing were conducted using further Bonferroni-corrected levels as indicated by the overall ANOVA. This analysis examined changes in neurotransmitter levels produced by treatment with CS2 for each of the brain regions at each of the various times of testing (sacrifice). Data are reported as ng neurotransmitter (or metabolite)/mg protein ± standard error.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
FEPs and Colonic Temperature
Pretreatment FEPs.
Repeated FEP testing for three days prior to dosing with CS2 resulted in an increase in peak N166 amplitude over days (F[2,360] = 182.68, p
0.0001, = 0.8901). There were no significant differences in the development of peak N166 amplitude between the animals assigned to the different treatment groups or different times of sacrifice. The amplitude of peak N36 differed over the three test days (F[2,360] = 9.97, p 0.0001, = 0.9735), but there were no differences between the various test groups over the three pretreatment test sessions (all p values > 0.05). Additionally, for the day 3 pretreatment FEP test session, there were no significant (p < 0.025; 0.05/2 peaks) differences between any of the test groups for either peak N36 or N166 amplitudes (Fig. 1).
|
Posttreatment FEPs.
Treatment with CS2 decreased the amplitudes of FEP peaks N36 and N166 at different time points (Figs. 1
and 2). Significant differences between the times of testing (all F values[3,144] 
4.78, all p values 0.0033) and doses of CS2 (all F values[3,144] 4.27, all p values 0.0064) were observed for both peaks. Further analysis indicated that the change in peak N166 amplitude was altered by CS2 only at the 1-h time point (F[3,36] = 9.33, p 0.0001). At this 1 h test time, 400 mg/kg CS2 decreased peak N166 amplitude relative to the pretreatment amplitude when compared to the 0, 100, or 200 mg/kg doses. In contrast, the change in peak N36 amplitude was affected by CS2 only at the 4-h time point (F[3,36] = 3.28, p = 0.0317). The 400-mg/kg dose of CS2 decreased peak N36 amplitude (relative to the pretreatment value) when compared to the vehicle controls. CS2-related changes in peak N36 and N166 amplitudes at other time points were not statistically significant (all p values > 0.05).
|
Posttreatment FEPs: Other peak amplitudes.
Treatment with CS2 did not significantly change the posttreatment amplitudes from their pretreatment values for other FEP peaks (critical
= 0.0031; 0.5/8 peaks/2 for amplitudes and latencies). The dose-related change in amplitude for peak P102 failed to reach corrected significance levels (dose effect: F[3,144] = 3.44, p = 0.0185), but suggested that 400 mg/kg CS2 increased peak P102 amplitude compared to the 0 and 200 mg/kg groups (graphs of data means not shown). This is likely to be due to the general increase in the positivity of this region of the FEP waveforms following treatment with 400 mg/kg CS2 (Fig. 1).
Posttreatment FEPs: Peak latencies.
Peak latencies of FEPs were affected by treatment with CS2 (Fig. 1; graphs of mean latencies are not shown). Significant (critical = 0.0031; 0.5/8 peaks/2 for amplitudes and latencies) dose-related increases in latency relative to the pretreatment values were indicated for peaks N36 and N166 (dose effect: all F values[3,144] 12.55, all p values 0.0001). Treatment with 400 mg/kg CS2 increased these peak latencies relative to the 0, 100, and 200 mg/kg doses. Additionally, treatment with 200 mg/kg CS2 increased peak N36 latency compared to the 0 and 100 mg/kg doses. The time-related increases in latency for peaks N36 (time by dose interaction: F[9,144] = 2.45, p = 0.0127) and N166 (time effect: F[3,144] = 2.87, p = 0.0388) failed to reach corrected significance levels. However, there was evidence of dose-related increases in peak N36 latency at 1, 4, and 8 h (all F values[3,36] 13.05, all p values 0.0001). At each of these times, 400 mg/kg CS2 produced an increase in peak N36 latency relative to pretreatment values, compared to the 0, 100, and 200 mg/kg CS2 doses. Additionally, at 4 h, 200 mg/kg CS2 increased peak N36 latency relative to controls. The latency of peak N166 was increased at 8 h by 400 mg/kg CS2 compared to the 0, 100, and 200 mg/kg doses.
Latencies of other FEP peaks were also affected by treatment with CS2. Significant dose-related increases in latency relative to pretreatment values were observed for peaks P28, P59, N65, P72, N84, and P102 (dose effect: all F values[3,144] 8.63, all p values 0.0001). Treatment with 400 mg/kg CS2 increased the latencies of all these peaks relative to the 0, 100, and 200 mg/kg doses. Additionally, the 200 mg/kg dose of CS2 increased the latency of peak P28 compared to the 0 and 100 mg/kg doses, and increased the latency of peak P102 relative to controls.
Colonic temperature.
Treatment with CS2 decreased colonic temperature in a dose- and time-dependent manner (Fig. 3; time by dose interaction: F[12,180] = 5.89, p 0.0001). Significant dose-related decreases in colonic temperature were observed 1, 4, 8, and 24 h after treatment (dose effect: all F values[3,36] 4.69, all p values 0.0073). No differences in the NIC animals were observed. Treatment with 400 mg/kg CS2 decreased colonic temperature compared to the 0, 100, and 200 mg/kg groups at 1, 4, and 8 h, and compared to the 0 and 100 mg/kg groups at 24 h. Also, the 200 mg/kg dose decreased colonic temperature compared to the 0 and 100 mg/kg groups at 4 h. Thus, the time of maximum depression of colonic temperature was observed 4 h after treatment with CS2.
|
Neurotransmitters
Norepinephrine.
Treatment with CS2 altered NE levels (Fig. 4
) in both a time-related (dose by time interaction: F[12,178] = 4.61, p 0.001) and regional manner (dose by region interaction: F[9,534] = 3.64, p = 0.0020, = 0.6357). Cortical NE levels were decreased by CS2 in a dose- and time-related manner (dose by time interaction: F[12,178] = 5.37, p 0.0001). Changes in cortical NE at 1 h (p = 0.0167) failed to reach corrected significance levels (critical = 0.0004; 0.0083/4 regions/5 groups). Decreases in cortical NE were observed at 4 h (F[3,36] = 18.47, p 0.0001). The 400 mg/kg CS2 depressed NE levels compared to the 0, 100, and 200 mg/kg groups. Also at 4 h, 200 mg/kg CS2 decreased cortical NE compared to the control group. The changes in NE levels at 8 h (p = 0.0011) failed to reach corrected significance levels.
|
Treatment with CS2 decreased brain stem NE levels in both a dose- (F[3,178] = 7.58, p
0.0001) and time-related (F[4,178] = 4.59, p = 0.0015) fashion. Brain stem NE levels were decreased by CS2 treatment only at the 4 h time point (F[3,36] = 13.10, p 0.0001). At this time, 400 mg/kg CS2 decreased brain stem NE compared to the 0, 100, and 200 mg/kg groups, and 200 mg/kg CS2 decreased NE compared to the vehicle controls.
Cerebellar NE levels were also decreased in a dose- (F[3,178] = 5.71, p = 0.0009) and time-related (F[4,178] = 6.34, p 0.0001) manner. Significant changes in NE levels resulting from CS2 treatment were observed only at 4 h (F[3,36] = 25.27, p 0.0001). Treatment with 400 mg/kg CS2 decreased cerebellar NE levels compared to the 0, 100, and 200 mg/kg groups.
Levels of NE in the striatum varied over the different times of sacrifice (F[4,178] = 4.49, p = 0.0018). Average levels of NE at 8 h were lower than in the NIC group or at 1 h, and average NE levels at 4 h were less than in the NIC group. However, the decreased levels of striatal NE produced by 400 mg/kg CS2 at 4 (p = 0.0063) and 8 h (p = 0.0069) failed to reach corrected significance levels.
Dopamine and metabolites.
Changes in DA, DOPAC, and HVA produced by treatment with CS2 (Figs. 5, 6, and 7) had a differential regional distribution. All three analytes had different levels in the various brain regions (region effect: all F values[3,534] 2,393.87, all p values 0.0001, ’s 0.3536), reflecting the much greater levels in the striatum than in the other brain areas. There were no significant changes (critical = 0.0021; 0.05/6 analytes/4 regions) in the levels of DA or DOPAC observed in the cortex or striatum following CS2 treatment (all F values[12,178] 0.67, all p values 0.7821). Similarly, striatal levels of HVA were not affected by treatment with CS2 (F[12,178] = 1.00, p = 0.4509). Cortical levels of HVA (Fig. 7) were greater in animals treated with 400 mg/kg CS2 than those treated with 0 or 100 mg/kg (dose effect: F[3,178] = 6.01, p = 0.0006). However, there were no significant treatment-related effects at any of the individual time points (all p values 0.05). In contrast to the relative lack of effects in the cortex and striatum, there were significant CS2-induced increases in DA, DOPAC, and HVA in both the brain stem and cerebellum.
|
|
|
In the brain stem, the increases in DA, DOPAC, and HVA produced by CS2 varied over the different sampling times (all F values[12, 178]
3.85, all p values 0.0001). Treatment with 400 mg/kg CS2 increased brain stem DA levels compared to the 0, 100, or 200 mg/kg doses at 4 h (Fig. 5). The increased DA levels at 8 h (p = 0.0010) failed to reach corrected significance levels (critical = 0.0005; 0.0021/4 times). In a similar manner, 400 mg/kg CS2 increased brain stem DOPAC levels at 4 h, compared to the 0, 100, and 200 mg/kg doses (Fig. 6). The increase in DOPAC levels at 1 h (p = 0.0020) and 8 h (p = 0.0134) failed to reach corrected significance levels. Also, brain stem levels of HVA (Fig. 7) were increased by 400 mg/kg CS2 at 4 h compared to the 0, 100, and 200 mg/kg doses. The increases in brain stem HVA observed at 1 h (= 0.0006) and 8 h (0.0011) failed to reach corrected significance levels.
In the cerebellum, the increases in DA and DOPAC resulting from treatment with CS2 (Figs. 5 and 6) varied over the different sampling times (all F values[12,178] 3.73, all p values 0.0001). Treatment with 400 mg/kg CS2 increased cerebellar DA levels (Fig. 5) at 4 h compared to the 0, 100, and 200 mg/kg doses (F[3,36] = 14.72, p 0.0001). Levels of DOPAC (Fig. 6) in the cerebellum were increased by 400 mg/kg CS2 at 4 h (F[3,36] = 20.76, p 0.0001) compared to the 0, 100, and 200 mg/kg treatments. The increased levels of cerebellar DOPAC at 1 (p = 0.0018) and 8 h (p = 0.0090) failed to reach corrected significance levels. Cerebellar levels of HVA were increased by CS2 (Fig. 7; dose effect: F[3, 178] = 5.93, p = 0.0007), but the changes over time failed to reach corrected significance levels (p = 0.0083). Treatment with 400 mg/kg CS2 increased levels of cerebellar HVA compared to the 0, 100, and 200 mg/kg doses. However, the only time point which possibly showed an effect of CS2 treatment on cerebellar HVA levels was at 4 h, which failed to meet corrected significance levels (p = 0.0011).
Serotonin and metabolites.
Treatment with CS2 had less effects on serotonin and 5-HIAA levels (Figs. 8 and 9) than those observed for NE, DA, DOPAC, and HVA. Regional differences in 5-HT levels were observed (region effect: F[3,534] = 933.11, p 0.0001), with the highest levels found in the brain stem, followed by the striatum, cortex, and cerebellum. However, treatment with CS2 did not alter 5-HT levels (Fig. 8) in any brain region at any time point (all F values [12,178] 0.44, all p values 0.9460).
|
|
Levels of 5-HIAA in different brain regions (Fig. 9
) were altered by treatment with CS2 (region by dose interaction: F[9,534] = 4.02, p = 0.0005). No significant changes in 5-HIAA levels were produced by treatment with CS2 at any time point in the cortex, brain stem, or cerebellum (all F values[12,178] 0.36, all p values 0.9751). However, a significant dose-related increase in 5-HIAA levels was detected in the striatum (dose effect: F[3,178] = 8.95, p 0.0001), but this effect did not vary over the different time points (dose-by-time interaction: p = 0.9487). Treatment with 400 mg/kg CS2 increased average striatal 5-HIAA levels when compared to the 0, 100, and 200 mg/kg doses.
Neurochemical correlations with FEP Peaks N36 and N166.
In the cortex (site of generation of FEP peaks N36 and N166), the only neurotransmitter with CS2-induced dose- and time-related changes was NE. Therefore, as a further analysis on an individual animal basis, a linear regression analysis was performed between cortical levels of NE and the amplitudes of peaks N36 and N166 at the times of CS2-induced depressions (4 and 1 h, respectively; Fig. 10). The slope of the regression lines did not differ from zero, indicating that there was no relationship between the amplitude of peak N36 and cortical NE levels at 1 (p = 0.6618) or 4 h (p = 0.0654), or between peak N166 amplitude and cortical NE levels at 1 (p = 0.5979) or 4 h (p = 0.9129) after treatment with CS2.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Of primary importance for this study was the replication of the different time-courses of decreases in the amplitudes of FEP peaks N36 and N166, where 400 mg/kg CS2 decreased peak N166 amplitude at 1 h and peak N36 amplitude at 4 h (Herr et al., 1992
). This difference in the time-course of effects on peaks N36 and N166 may have implications regarding the neurogenerators of these peaks. Peak N36 is believed to reflect the cortical response to the flash-evoked thalamocortical afferent volley (Bigler, 1977; Brancak et al., 1990; Givre et al., 1994; Kraut et al., 1985; Mitzdorf, 1986, 1987). In contrast, peak N166 may arise from a reverberating thalamocortical circuit activated by the neural inputs responsible for the early FEP components (Bigler, 1977; Brancak et al., 1990). Subcortical relay loops involving the superior colliculus may also be involved in the generation of peak N166 and the PhAD (Sumitomo and Klingberg, 1972a,b). As such, these two portions of the FEP may be considered to reflect the direct cortical response to the light flash (N36) and latter cortical integration (N166) of the visual signal produced by the stimulus. However, the presence of a well developed peak N166 at a time when peak N36 is depressed indicates that, although the neural circuitry involved in producing these peaks may involve serial connections, a nonlinear relationship is indicated. Future studies to examine the interplay between these two portions of FEPs may be warranted. Because both peak N36 and N166 are generated at the cortical level, we were able to examine the potential role of altered cortical NE levels in producing the decreases in FEP amplitudes in this study.
As reported in other studies using rats (Magos and Jarvis, 1970; Magos et al., 1974; McKenna and DiStefano, 1977), treatment with CS2 decreased brain levels of NE and altered levels of DA. The mechanism responsible for this effect is believed to be: (1) reaction of CS2 with endogenous amino acids to form dithiocarbamate metabolites, (2) chelation of copper ions by the dithiocarbamate metabolites, and (3) subsequent inhibition of dopamine ß-hydroxylase (DßH; the enzyme responsible for the synthesis of NE from DA), which requires Cu2+ as a necessary cofactor (Bus, 1985; McKenna and DiStefano, 1977). The cortex, brain stem, and cerebellum were selected due to their NE terminal innervation or NE cell bodies, and were the brain regions showing the largest changes in monoamine neurotransmitter levels (and metabolite levels). Significantly decreased levels of NE in the cortex, brain stem, and cerebellum were observed 4 h after treatment with CS2 (Fig. 4). The decreased NE levels were accompanied by increased levels of DA and DOPAC in the brain stem and cerebellum (Figs. 5 and 6) and HVA in the brain stem (Fig. 7). These increased levels of DA and its metabolites in the brain stem and cerebellum at 4 h may result from CS2-induced inhibition of NE synthesis, rather than any direct effect on DA neurotransmission. In contrast, the decreases in striatal NE levels failed to reach corrected significance levels (Fig. 4). At 4 h, there was approximately a 44% decrease in striatal NE compared to about a 68, 58, or 65% decrease in cortical, brain stem, or cerebellar NE, respectively. Thus, the decreased levels of NE produced by treatment with 400 mg/kg CS2 were slightly less in the striatum (which has less NE innervation) than in the other brain regions. In further support of an effect on NE synthesis by CS2, the striatum (which has a large number of DA projections) did not have any changes in the levels of DA or its metabolites. As predicted, 5-HT levels were not altered in any brain region (Fig. 8). Thus, the data are consistent with a CS2-induced inhibition of DßH, decreased NE synthesis, an accumulation of DA, and metabolism of the excess DA to DOPAC and HVA in NE neurons.
The data presented in this report do not support the hypothesis that CS2-induced decreases in cortical NE are related to the reduced amplitude of FEP peak N166 observed after treatment with CS2. Maximal depression of FEP peak N166 was observed 1 h after treatment (Figs. 1 and 2). In contrast, CS2-induced decreases in NE levels were not observed until 4 h after treatment (Fig. 4). At this time, there was approximately a 68% decrease in cortical NE levels compared to controls. This difference in the time-courses of decreases in peak N166 amplitude, followed by later decreases in cortical NE (at a time when peak N166 amplitude was returning to control levels), suggest that changes in NE-induced cortical responsiveness to the flash stimuli were not related to the observed decreases in peak N166 amplitude. This conclusion is also supported by the lack of a correlation between an individual animal’s peak N166 amplitude and its cortical NE level at 1 or 4 h (Fig. 10). Similarly, there was no relationship between an individual animal’s peak N36 amplitude and its cortical NE levels at 1 or 4 h. Because we used tissue homogenates from large portions of the cortex, the levels of NE at the synapse were not quantified and we cannot rule out changes in the ability of NE to modulate FEP peak amplitudes at the synaptic level. However, the data do indicate that the global reductions in NE levels produced by CS2 treatment do not appear to produce the observed changes in the amplitudes of FEP peaks N36 and N166.
Changes in DA function have been postulated to alter FEPs. Previous investigators have shown that treatment with -methyl-para-tryosine (AMT; to deplete NE and DA) increased the latencies of FEP peaks P1 (P28), N1 (N36), and P2 (P59). Importantly, while decreasing the amplitude of peaks N1P2 (N36P59) and P2N2 (P59N84), this treatment failed to alter the amplitude of peak P3N3 (P102N166) (Dyer et al., 1981). The actions of AMT were attributed to DA depletion in the retina, as increased FEP peak latencies were not observed after electrical stimulation of the optic tract. These same researchers reported that treatment with FLA-63 (bis(4-methyl-1-homopiperazinylthiocarbonly)-disulfide; a DBH inhibitor) failed to alter any FEP peak amplitudes or latencies, suggesting the lack of a role for NE in the effects produced by AMT (Dyer et al., 1981). In agreement, other researchers have also found that treatment with AMT increases the latencies, and decreases the amplitudes of FEP peaks, and attributed the effects to decreased levels of retinal DA (Onofrj and Bodis-Wollner, 1982). It should be noted that Onofrj and Bodis-Wollner (1982) did not quantify NE or DA depletions, and Dyer and coworkers (1981) only measured DA levels in retinal tissue (>50% depletion), to verify the depletion levels of NE and/or DA.
In contrast to reports of DA depletion increasing FEP peak latencies and decreasing the amplitudes of early peaks, treatment with CS2 increased brain levels of DA and its metabolites at 4 h, when peak N36 amplitude was decreased. However, the changes in DA, DOPAC, and HVA were not observed in the cortex where peak N36 is generated (Bigler, 1977; Brancak et al., 1990; Givre et al., 1994; Kraut et al., 1985; Mitzdorf, 1986, 1987) (Figs. 5, 6, and 7). Additionally, there were no changes in DA or its metabolites observed in the striatum (Figs. 5, 6, and 7), where alterations would presumably reflect direct effects on DA neurons, rather than secondary effects on NE neurons. Therefore, it is unlikely that the decreased amplitude of peak N36 was mediated by changes in DA neurotransmission.
The amplitude of FEP peak N166 is reported to be modified by pharmacologic agents reported to interact with non-monoamine-based neurotransmitter systems. Treatment of rats with physostigmine (a cholinesterase inhibitor) has been shown to decrease the amplitude of peak P3-N3 (P102N166) and/or the PhAD (of which peak N166 is the first portion) (Bigler and Fleming, 1976b; Bigler et al., 1978). Similarly, the amplitude of peak P3-N3 was reduced by the anticonvulsant trimethadione (Shearer et al., 1976). Conversely, treatment with pentylenetetrazol (a pro-convulsant) has been shown to increase the amplitude of the PhAD (Bigler, 1977; Rhodes and Fleming, 1970). While not conclusively proven, studies such as these suggest a role for the cholinergic and possibly the gamma-aminobutyric acid systems in the expression of peak N166 and/or the PhAD. However, the influence of treatment with CS2 on the function of these neurotransmitter systems remains to be examined.
Treatment with CS2 decreased the animal’s colonic temperature, similar to the results reported in our previous study (Herr et al., 1992). Although significant hypothermia was observed at all time points, the effect was maximal 4 h after treatment with approximately a 2.4°C decrease in temperature relative to controls (Fig. 3). Decreased colonic temperature has been shown to increase FEP peak latencies (Dyer and Boyes, 1983; Fitzgibbon et al., 1984; Hetzler et al., 1988). In this study, treatment with CS2 increased average FEP peak latencies relative to their pretreatment values. Similar to the decrease in colonic temperature, the increases in peak latencies were greatest over the 1–8 h period, but were still detectable at 24 h. The similar time-courses for the decreases in colonic temperature and the increases in peak latencies, coupled with the known effects of decreased body temperature on FEP peak latencies, suggests that the increases in FEP peak latencies may be secondary to CS2-induced hypothermia. However, the decreases in amplitudes of peaks N36 and N166 do not have the same time-course as the CS2-induced reductions in colonic temperature. Decreases in peak N166 amplitude were observed only 1 h, and peak N36 amplitude was decreased only at 4 h after dosing with CS2, in contrast to the reduced colonic temperatures observed at 1, 4, 8, and 24 h after treatment. These difference in time-courses of effects argue that the changes in FEP peak amplitudes were not secondary to depressed colonic temperature. Additionally, it has been shown that hypothermia has minimal effects on FEP peak P1N1 (P28N36) amplitude as long as colonic temperatures remain above 30°C (as were observed in this study, Fig. 3) (Hetzler et al., 1988). Other investigators have shown that a 6°C decrease in colonic temperature (to 31°C) can result in an increase in peak P1N1 amplitude (Dyer and Boyes, 1983). The maximum hypothermia was observed at 4 h after treatment with CS2 (Fig. 3), at a time when peak N36 amplitude was decreased and the reduction in peak N166 amplitude observed at 1 h was dissipating. Therefore, it would appear that mechanisms other than a reduction in colonic temperature are involved in the decreased amplitudes of FEP peaks N36 and N166 produced by treatment with CS2.
Another possibility is that CS2-induced changes in colonic temperature could influence levels of brain neurotransmitters (and/or metabolites). Most enzymes have an optimum temperature for maximal activity. As such, absolute levels of neurotransmitters would depend on the balance of synthesis and degradation related to the activities of multiple enzymes. In this study, brain levels of NE decreased, levels of DA, DOPAC, HVA, and 5-HIAA increased, while levels of 5-HT were unchanged following treatment with CS2. One might predict similar changes in multiple neurotransmitter systems in response to a generalized decrease in temperature. Additionally, the changes in neurotransmitter levels were statistically significant only at 4 h after treatment with CS2, while significantly decreased colonic temperature was observed 1, 4, 8, and 24 h after dosing with CS2. Coupled with the known inhibition of DBH by dithiocarbamate metabolites of CS2, these observations argue that the observed changes in neurotransmitter levels were the result of toxicological actions of CS2, rather than an indirect result of decreases in colonic temperature.
In summary, we have replicated the differential time-course of CS2-induced depressions of FEP peak N36 and N166 amplitudes. The presence of a large and robust peak N166 at a time when peak N36 is severely depressed argues against the hypothesis that the neurogenerators for these two peaks are arranged in a simple serial relationship. We have also verified that this treatment regime resulted in alterations in brain NE and DA levels. This is the first study to examine the dose- and time-related changes in brain NE levels with respect to visual electrophysiology after CS2 treatment. However, the different time-courses for depression of FEP peak N166 amplitude and decreases in brain NE levels argues that the observed changes in brain monoamine levels were not responsible for the changes in visual system function. Thus, the neurochemical basis for the different time-courses of CS2-induced depression of peaks N36 and N166 remains to be elucidated.
|
ACKNOWLEDGMENTS |
|---|
The authors wish to thank Dr. William K. Boyes and Dr. Gaylia J. Harry for helpful discussions and critique of the manuscript. We also wish to thank Mr. C. Hamm for the design and implementation of the software used for electrophysiological recordings.
|
NOTES |
|---|
The information in this document has been funded wholly (or in part) by the U.S. Environmental Protection Agency. It has been subjected to review by the National Health and Environmental Effects Research Laboratory and approved for publication. Approval does not signify that the contents reflect the views of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
Portions of this manuscript were presented as a poster at the 27th annual meeting of the Society for Neuroscience (1997, Soc. Neuroscience Abstr. 23(1), 1028).
1 To whom correspondence should be addressed at 109 T.W. Alexander Drive, MD B105-05, NHEERL/NTD/NPTB, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. Fax: (919) 541-4849. E-mail: herr.david{at}epamail.epa.gov. 
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Abt, K. (1981). Problems of repeated significance testing. Controlled Clin. Trials 1, 377–381.[CrossRef][Web of Science][Medline]
Aston-Jones, G. and Bloom, F. E. (1981). Norepinephrine-containing locus coeruleus neurons in behaving rats exhibit pronounced responses to non-noxious environmental stimuli. J. Neurosci. 1, 887–900.[Abstract]
Aston-Jones, G., Foote, S. L., and Bloom, F. E. (1984). Anatomy and Physiology of Locus Coeruleus Neurons: Functional Implications. In Norepinephrine (M. G. Ziegler and C. R. Lake, Eds.), Vol. 2, pp. 92–116. Williams & Wilkins, Baltimore, MD.
Aston-Jones, G., Rajkowski, J., and Cohen, J. (1999). Role of locus coeruleus in attention and behavioral flexibility. Biol. Psychiat. 46, 1309–1320.[CrossRef][Web of Science][Medline]
Baumgarten, H. G., and Lachenmayer, L. (1985). Anatomical features and physiological properties of central serotonin neurons. Pharmacopsychiatry 18, 180–187.[Web of Science][Medline]
Bigler, E. D. (1977). Neurophysiology, neuropharmacology, and behavioral relationships of visual system evoked after-discharges: A review. Biobehav. Rev. 1, 95–112.
Bigler, E. D., and Fleming, D. E. (1976a). Effects of shock-induced arousal on the elicitation and waveform elaboration of photically evoked afterdischarges. Physiol. Psychol. 4, 86–90.
Bigler, E. D., and Fleming, D. E. (1976b). Pharmacological suppression of photically evoked after-discharges in rats: Incremental dose, hippocampal EEG and behavioral activity correlates. Psychopharmacologia 46, 73–82.[CrossRef][Web of Science][Medline]
Bigler, E. D., Fleming, D. E., and Shearer, D. E. (1976). Stabilization of photically evoked after-discharge activity: Control procedures and effects of classical trace conditioning. Behav. Biol. 16, 425–437.[CrossRef][Web of Science][Medline]
Bigler, E. D., Shearer, D. E., Dustman, R. E., and Fleming, D. E. (1978). Differential effects of convulsants on visually evoked responses in the albino rat. Pharmacol. Biochem. Behav. 8, 727–733.[CrossRef][Web of Science][Medline]
Bishop, G. A., and Ho, R. H. (1985). The distribution and origin of serotonin immunoreactivity in the rat cerebellum. Brain Res. 331, 195–207.[CrossRef][Web of Science][Medline]
Brancak, K. J., Schober, W., and Klingberg, F. (1990). Different laminar distribution of flash-evoked potentials in cortical areas 17 and 18b of freely moving rats. J. Hirnforsch. 31, 525–533.[Web of Science][Medline]
Bus, J. S. (1985). The relationship of carbon disulfide metabolism to development of toxicity. Neurotoxicology 6, 73–80.[Web of Science][Medline]
Chu, C.-C., Huang, C.-C., Chu, N.-S., and Wu, T.-N. (1996). Carbon disulfide-induced polyneuropathy: Sural nerve pathology, electrophysiology, and clinical correlation. Acta Neurol. Scand. 94, 258–263.[Web of Science][Medline]
Clerici, W. J., and Fechter, L. D. (1991). Effects of chronic carbon disulfide inhalation on sensory and motor function in the rat. Neurotoxicol. Teratol. 13, 249–255.[CrossRef][Web of Science][Medline]
Crofton, K. M., Lassiter, T. L., and Rebert, C. S. (1994). Solvent-induced ototoxicity in rats: An atypical selective mid-frequency hearing deficit. Hear. Res. 80 25–30.[CrossRef][Web of Science][Medline]
Devilbiss, D. M., and Waterhouse, B. D. (2000). Norepinephrine exhibits two distinct profiles of action on sensory cortical neuron responses to excitatory synaptic stimuli. Synapse 37, 273–282.[CrossRef][Web of Science][Medline]
Dick, R. B. (1995). Neurobehavioral assessment of occupationally relevant solvents and chemicals in humans. In Handbook of Neurotoxicology (L. W. Chang and R. S. Dyer, Eds.), Vol. 36, pp. 217–322. Marcel Dekker, New York.
Dyer, R. S. (1989). Peak N160 of rat flash-evoked potential: Does it reflect habituation or sensitization? Physiol. Behav. 45, 355–362.[CrossRef][Medline]
Dyer, R. S., Bercegeay, M. S., and Mayo, L. M. (1988). Acute exposures to p-xylene and toluene alter visual information processing. Neurotoxicol. Teratol. 10, 147–153.[CrossRef][Web of Science][Medline]
Dyer, R. S., and Boyes, W. K. (1983). Hypothermia and chloropent anesthesia differentially affect the flash-evoked potentials of hooded rats. Brain Res. Bull. 10, 825–831.[CrossRef][Web of Science][Medline]
Dyer, R. S., Howell, W. E., and MacPhail, R. C. (1981). Dopamine depletion slows retinal transmission. Exp. Neurol. 71, 326–340.[CrossRef][Web of Science][Medline]
Dyer, R. S., Jensen, K. F., and Boyes, W. K. (1987). Focal lesions of visual cortex: Effects on visual-evoked potentials in rats. Exp. Neurol. 95, 100–115.[CrossRef][Web of Science][Medline]
Dyer, R. S., Muller, K. E., Janssen, R., Barton, C. N., Boyes, W. K., and Benignus, V. A. (1984). Neurophysiological effects of 30-day chronic exposure to toluene in rats. Neurobehav. Toxicol. Teratol. 6, 363–368.[Web of Science][Medline]
Fitzgibbon, T., Hayward, J. S., and Walker, D. (1984). EEG and visual evoked potentials of conscious man during moderate hypothermia. Electroencephalogr. Clin. Neurophysiol. 58, 48–54.[CrossRef][Web of Science][Medline]
Foote, S. L., Bloom, F. E., and Aston-Jones, G. (1983). Nucleus locus ceruleus: New evidence of anatomical and physiological specificity. Physiolog. Rev. 63, 844–914.
Geisser, S., and Greenhouse, S. W. (1958). An extension of Box’s results on the use of the F distribution in multivariate analysis. Annal. Mathemat. Stat. 29, 885–891.
Givre, S. J., Schroeder, C. E., and Arezzo, J. C. (1994). Contribution of extrastriate area V4 to the surface-recorded flash VEP in the awake macaque. Vision Res. 34, 415–438.[CrossRef][Web of Science][Medline]
Glowinski, J., and Iversen, L. L. (1966). Regional studies of catecholamines in the rat brain: I. The disposition of [3H]norepinephrine, [3H]dopamine, and [3H]dopa in various regions of the brain. J. Neurochem. 13, 655–669.[Web of Science][Medline]
Grant, S. J., Aston-Jones, G., and Redmond, D. E., Jr. (1988). Responses of primate locus coeruleus neurons to simple and complex sensory stimuli. Brain Res. Bull. 21, 401–410.[CrossRef][Web of Science][Medline]
Greenhouse, S. W., and Geisser, S. (1959). On methods in the analysis of profile data. Psychometrika 24, 95–112.[CrossRef][Web of Science]
Hamm, C. W., Ali, J. S., and Herr, D. W. (2000). A system for simultaneous multiple subject, multiple stimulus modality, and multiple channel collection and analysis of sensory-evoked potentials. J. Neurosci. Meth. 102, 95–108.[CrossRef][Web of Science][Medline]
Hasselmo, M. E., Linster, C., Patil, M., Ma, D., and Cekic, M. (1997). Noradrenergic suppression of synaptic transmission may influence cortical signal-to-noise ratio. J. Neurophysiol. 77, 3326–3339.
Herr, D. W. and Boyes, W. K. (1995). Electrophysiological analysis of complex brain systems: Sensory-evoked potentials and their generators. In Neurotoxicology. Approaches and Methods (L. W. Chang and W. Slikker, Jr., Eds.), pp. 205–221. Academic Press, New York.
Herr, D. W., and Boyes, W. K. (1997). A comparison of the acute neuroactive effects of dichloromethane, 1,3-dichloropropane, and 1,2-dichlorobenzene on rat flash-evoked potentials (FEPs). Fundam. Appl. Toxicol. 35, 31–48.[CrossRef][Web of Science][Medline]
Herr, D. W., Boyes, W. K., and Dyer, R. S. (1991). Rat flash-evoked potential peak N160 amplitude: Modulation by relative flash intensity. Physiol. Behav. 49, 355–365.[CrossRef][Medline]
Herr, D. W., Boyes, W. K., and Dyer, R. S. (1992). Alterations in rat flash and pattern reversal evoked potentials after acute or repeated administration of carbon disulfide (CS2). Fundam. Appl. Toxicol. 18, 328–342.[CrossRef][Web of Science][Medline]
Herr, D. W., King, D., Griffin, V. T., Watkinson, W. P., Boyes, W. K., Ali, J. S., and Dyer, R. S. (1994). Within-session changes in peak N160 amplitude of flash evoked potentials in rats. Physiol. Behav. 55, 83–99.[CrossRef][Medline]
Herr, D. W., Vo, K. T., King, D., and Boyes, W. K. (1996). Possible confounding effects of strobe "clicks" on flash evoked potentials in rats. Physiol. Behav. 59(2), 325–340.[CrossRef][Medline]
Hetzler, B. E., Boyes, W. K., Creason, J. P., and Dyer, R. S. (1988). Temperature-dependent changes in visual evoked potentials of rats. Electroencephalogr. Clin. Neurophysiol. 70, 137–154.[CrossRef][Web of Science][Medline]
Hirata, M., Ogawa, Y., Okayama, A., and Goto, S. (1992). Changes in auditory brainstem response in rats chronically exposed to carbon disulfide. Arch. Toxicol. 66, 334–338.[CrossRef][Web of Science][Medline]
Husman, K., and Karli, P. (1980). Clinical neurological findings among car painters exposed to a mixture of organic solvents. Scand. J. Work Environ. Health 6, 33–39.[Web of Science][Medline]
Joseph, R., Forrest, N. M., Fiducia, D., Como, P., and Siegel, J. (1981). Electrophysiological and behavioral correlates of arousal. Physiol. Psychol. 9, 90–95.
Kenan-Vaknin, G., and Teyler, T. J. (1994). Laminar pattern of synaptic activity in rat primary visual cortex: Comparison of in vivo and in vitro studies employing the current source density analysis. Brain Res. 635, 37–48.[CrossRef][Web of Science][Medline]
Keselman, H. J., and Rogan, J. C. (1980). Repeated measures F tests and psychophysiological research: Controlling the number of false positives. Psychophysiology 17, 499–503.[Web of Science][Medline]
Kirkwood, A., Rozas, C., Kirkwood, J., Perez, F., and Bear, M. (1999). Modulation of long-term synaptic depression in visual cortex by acetylcholine and norepinephrine. J. Neurosci. 19, 1599–1609.
Kramer, C. Y. (1956). Extension of multiple range tests to group means with unequal numbers of replications. Biometrics 12, 307–310.[CrossRef][Web of Science]
Kraut, M. A., Arezzo, J. C., and Vaughan, H. G., Jr. (1985). Intracortical generators of the flash VEP in monkeys. Electroencephalogr. Clin. Neurophysiol. 62, 300–312.[CrossRef][Web of Science][Medline]
Lataye, R., Campo, P., and Loquet, G. (1999). Toluene ototoxicity in rats: Assessment of the frequency of hearing deficit by electrocochleography. Neurotoxicol. Teratol. 21, 267–276.[CrossRef][Web of Science][Medline]
Li, Y.-Q., Kaneko, T., and Mizuno, N. (2001). Collateral projections of nucleus raphe dorsalis neurones to the caudate-putamen and region around the nucleus raphe magnus and nucleus reticularis gigantocellularis pars in the rat. Neurosci. Lett. 299(1–2), 33–36.[CrossRef][Web of Science][Medline]
Lilis, R. (1974). Behavioral effects of occupational carbon disulfide exposure. In Behavioral Toxicology (C. Xintaras, B. L. Johnson, and I. deGroot, Eds.), pp. 51–60. NIOSH, Cincinnati, OH.
Lindvall, O., and Björklund, A. (1983). Dopamine- and norepinephrine-containing neuron systems: Their anatomy in the rat brain. In Chemical Neuroanatomy (P. C. Emson, Ed.), pp. 229–255. Raven Press, New York.
Magos, L., Green, A., and Jarvis, J. A. E. (1974). Half-life of CS2 in rats in relation to its effect on brain catecholamines. Int. Arch. Arbeitsmed. 32, 296.
Magos, L., and Jarvis, J. A. E. (1970). The effects of carbon disulphide exposure on brain catecholamines in rats. Br. J. Pharmacol. 39, 26–33.[Web of Science][Medline]
Mattsson, J. L., and Albee, R. R. (1988). Sensory evoked potentials in neurotoxicology. Neurotoxicol. Teratol. 10, 435–443.[CrossRef][Web of Science][Medline]
Mattsson, J. L., Boyes, W. K., and Ross, J. F. (1992). Incorporating evoked potentials into neurotoxicity test schemes. In Neurotoxicology (H. Tilson and C. Mitchell, Eds.), pp. 125–145. Raven Press, New York.
Mattsson, J. L., Gorzinski, S. J., Albee, R. R., and Zimmer, M. A. (1990). Evoked potential changes from 13 weeks of simulated toluene abuse in rats. Pharmacol. Biochem. Behav. 36, 683–689.[CrossRef][Web of Science][Medline]
McCormick, D. A. (1989). Cholinergic and noradrenergic modulation of thalamocortical processing. Trends Neurosci. 12, 215–221.[CrossRef][Web of Science][Medline]
McKenna, M. J., and DiStefano, V. (1977). Carbon disulfide: II. A proposed mechanism for the action of carbon disulfide on dopamine ß-hydroxylase. J. Pharmacol. Exp. Ther. 202, 253–266.
Mitzdorf, U. (1986). The physiological causes of VEP: Current source density analysis of electrically and visually evoked potentials. In Evoked Potentials (R. Q. Cracco and I. Bodis-Wollner, Eds.), pp. 141–154. Alan R. Liss, New York.
Mitzdorf, U. (1987). Properties of the evoked potential generators: Current source-density analysis of visually evoked potentials in the cat cortex. Int. J. Neurosci. 33(1–2), 33–59.[Web of Science][Medline]
Muller, K. E., Otto, D. A., and Benignus, V. A. (1983). Design and analysis issues and strategies in psychophysiological research. Psychophysiology 20, 212–218.[Web of Science][Medline]
Onofrj, M., and Bodis-Wollner, I. (1982). Dopaminergic deficiency causes delayed visual evoked potentials in rats. Ann. Neurol. 11, 484–490.[CrossRef][Web of Science][Medline]
Parnavelas, J. G., and Papadopoulos, G. C. (1989). The monoaminergic innervation of the cerebral cortex is not diffuse and nonspecific. Trends Neurosci. 12, 315–319.[CrossRef][Web of Science][Medline]
Rebert, C. S., Matteucci, M. J., and Pryor, G. T. (1989a). Acute effects of inhaled dichloromethane on the EEG and sensory-evoked potentials of Fischer-344 rats. Pharmacol. Biochem. Behav. 34, 619–629.[CrossRef][Web of Science][Medline]
Rebert, C. S., Matteucci, M. J., and Pryor, G. T. (1989b). Acute electrophysiologic effects of inhaled toluene on adult male Long-Evans rats. Pharmacol. Biochem. Behav. 33(1), 157–165.[CrossRef][Web of Science][Medline]
Rebert, C. S., Matteucci, M. J., and Pryor, G. T. (1989c). Multimodal effects of acute exposure to toluene evidenced by sensory-evoked potentials from Fischer-344 rats. Pharmacol. Biochem. Behav. 32, 757–768.[CrossRef][Web of Science][Medline]
Rebert, C. S., Matteucci, M. J., and Pryor, G. T. (1990). Acute interactive pharmacologic effects of inhaled toluene and dichloromethane on rat brain electrophysiology. Pharmacol. Biochem. Behav. 36, 351–365.[CrossRef][Web of Science][Medline]
Rhodes, L. E., and Fleming, D. E. (1970). Sensory restriction in the albino rat: Photically evoked after-discharge correlates. Electroencephalogr. Clin. Neurophysiol. 29, 488–495.[CrossRef][Web of Science][Medline]
SAS Institute, Inc. (1989). SAS/STAT User’s Guide, Vol. 2, V. 6, 4th ed., pp. 1–846. SAS Institute, Inc., Cary, NC.
SAS Institute, Inc. (1997). SAS/STAT Software: Changes and Enhancements through Release 6.12, pp. 1–1167. SAS Institute, Inc., Cary, NC.
Schaefer, C. F., Jr., Kreinick, C. J., and Schwartzbaum, J. S. (1974). Behavioral reactivity, appetitive behavior, and visual-evoked potentials to photic stimuli following amygdaloid lesions in rats. J. Comp. Physiol. Psychol. 86, 793–811.[CrossRef][Web of Science][Medline]
Shaw, N. A. (1992). Auditory potentials elicited by the Grass photic stimulator in the rat. Physiol. Behav. 52, 401–403.[CrossRef][Medline]
Shearer, D. E., and Creel, D. (1978). The photically evoked afterdischarge: Current concepts and potential applications. Physiol. Psychol. 6, 369–376.
Shearer, D. E., Fleming, D. E., and Bigler, E. D. (1976). The photically evoked afterdischarge: A model for the study of drugs useful in the treatment of petit mal epilepsy. Epilepsia 17, 429–435.[Web of Science][Medline]
Simonsen, L., and Lund, S. P. (1995). Four weeks inhalation exposure to n-heptane causes loss of auditory sensitivity in rats. Pharmacol. Toxicol. 76, 41–46.[Web of Science][Medline]
Standage, G. P., and Fleming, D. E. (1978). Photic after-discharge bursting during habituation of a leg flexion response. Physiol. Behav. 20, 411–415.[CrossRef][Medline]
Sumitomo, I., and Klingberg, F. (1972a). Influence of visual cortex lesions upon photically evoked after-discharges of the superior colliculus in freely moving rats. Acta Biol. Med. Germ. 29, 239–246.[Web of Science][Medline]
Sumitomo, I., and Klingberg, F. (1972b). The role of lateral geniculate body in the generation of photically and electrically evoked after discharges in freely moving rats. Acta Biol. Med. Germ. 29, 43–54.[Web of Science][Medline]
Vertes, R. P. (1991). A PHA-L analysis of ascending projections of the dorsal raphe nucleus in the rat. J. Comparat. Neurol. 313, 643–668.[CrossRef][Web of Science][Medline]
Waterhouse, B. D., Azizi, S. A., Burne, R. A., and Woodward, D. J. (1990). Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis. Brain Res. 514(2), 276–292.[CrossRef][Web of Science][Medline]
Waterhouse, B. D., Mouradian, R., Sessker, F. M., and Lin, R. C. S. (2000). Differential modulatory effects of norepinephrine on synaptically driven responses of layer V barrel field cortical neurons. Brain Res. 868, 39–47.[CrossRef][Web of Science][Medline]
Wood, R. W. (1981). Neurobehavioral toxicity of carbon disulfide. Neurobehav. Toxicol. Teratol. 3, 397–405.[Web of Science][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||