© 1996 Oxford University Press
research-article |
Effects of Chlorpyrifos on High-Affinity Choline Uptake and [3H]Hemicholinium-3 Binding in Rat Brain
Division of Toxicology, College of Pharmacy and Health Sciences, Northeast Louisiana University Monroe, Louisiana 71209-0470
Received May 6, 1996; accepted August 14, 1996
High, subcutaneous doses of the organophosphorus insecticide chlorpyrifos (CPF) in adult male rats can be well-tolerated despite extensive and persistent acetylcholinesterase (AChE) inhibition. We propose that changes in acetylcholine synthesis could modulate the toxicity associated with extensive AChE inhibition following CPF exposure. High-affinity choline uptake (HACU, the rate-limiting step in acetylcholine synthesis) and binding to [3H]-hemicholinium-3 (HC-3, a specific ligand for the choline transporter) were chosen as indicators of acetylcholine synthesis. Female, Sprague-Dawley rats (220–280 g) were treated with either vehicle (peanut oil, 2 ml/kg, sc) or CPF (280 mg/kg, 2 ml/kg, sc), examined daily for clinical signs of toxicity, and sacrificed 1, 2, or 7 days later for neurochemical measurements {AChE inhibition, muscarinic receptor binding using [3H]quinuclidinyl benzilate (QNB) and [3H]cis-methyldioxolane (CD) as ligands, HACU and [3H]HC-3 binding} in frontal cortex. Despite extensive AChE inhibition (90–93%) at all time points, relatively minor degrees of overt toxicity were noted in CPF-treated rats. Binding to the non-selective muscarinic antagonist [3H]QNB was reduced (10–34%), whereas binding to the putative m2-selective agonist [3H]CD was increased (15–23%) at all three time points. HACU was reduced (20%) in crude synaptosomes prepared from CPF-treated rats 1 day following exposure but no significant changes were noted at 2 or 7 days after treatment. CPF-oxon, the active oxidative metabolite of CPF, was a weak inhibitor of HACU in vitro (IC50>200 µM). Binding to [3H]HC-3 was reduced in a dose-related manner 1 day after CPF exposure. Kinetic analyses of [3H]HC-3 binding 1 day after CPF (280 mg/kg) indicated a significant reduction in density {Bmax: control, 187±18 fmol/mg protein; CPF, 104±12 fmol/mg protein) with no apparent change in binding affinity (Kd: control, 25±3 nM; CPF, 19±3 nM). These results suggest that a reduction in HACU/acetylcholine synthesis may contribute, along with compensatory changes in cholinergic receptors, to the diminished toxicity following extensive AChE inhibition by CPF.