DOWN-REGULATION OF MUSCARINIC RECEPTORS AND THE M3 SUBTYPE IN WHITE-FOOTED MICE BY DIETARY EXPOSURE TO PARATHION

The effect of ad libitum dietary exposure (as occurs in the field) to parathion for 14 d was investigated on the muscarinic acetylcholine re ceptor (mAChR) in brains and submaxillary glands of adults of a field species, the white-footed mouse Peromyscus leucopus. Immunoprecipitati on using subtype selective antibodies revealed that the relative ratio s of the m1-m5 mAChR subtypes in Peromyscus brain were similar to thos e in rat brain. There was little variability in acetylcholinesterase ( AChE) activity in control mice brains but large variability in 39 expo sed mice, resulting from differences in food ingestion and parathion m etabolism. Accordingly, data on radioligand binding to mAChRs in each mouse brain were correlated with brain AChE activity in the same mouse , and AChE inhibition served as a biomarker of exposure reflecting in situ paraoxon concentrations. Exposure to parathion for 14 d reduced m aximal binding (B(max)) of [H-3]quinuclidinyl benzilate ([H-3]QNB), [H -3]-N-methylscopolamine ([H-3]NMS), and [H-3]-4-diphenylacetoxy-N-meth ylpiperidine methiodide ([H-3]-4-DAMP) by up to approximately 58% with out affecting receptor affinities for these ligands. Maximal reduction in B(max) of [H-3]QNB and [H-3]-4-DAMP binding occurred in mice with highest AChE inhibition, while equivalent maximal reduction in B(max) of [H-3]NMS occurred in mice with only approximately 10% AChE inhibiti on, without further change at higher parathion doses. This is believed to be due to the hydrophilicity of [H-3]NMS, which limits its accessi bility to internalized desensitized receptors. In submaxillary glands (mAChRs are predominantly m3 subtype), there were significant dose-dep endent reductions in [H-3]QNB binding and m3 mRNA levels in exposed mi ce, revealed by Northern blot analyses. The reduction in m3 receptors is suggested to result mostly from reduced synthesis at the transcript ion level, rather than from translational or posttranslational events. The data suggests that down-regulation of mAChRs occurs after dietary exposure for 14 d to sublethal concentrations of parathion in a field rodent species, and that significant though incomplete recovery in AC hE and mAChRs occurs in 7 d following termination of exposure.