IMMUNOHISTOCHEMISTRY OF MUSCARINIC ACETYLCHOLINE-RECEPTORS IN RAT COCHLEAR NUCLEUS

Neurons of the cochlear nucleus (CN) receive extrinsic and intrinsic c holinergic inputs, the effects of which appear to be mediated primaril y by muscarinic acetylcholine receptors (mAChRs). To investigate the d istribution of mAChRs and the correlation of pre- and post-synaptic ch olinergic markers in CN, we used a monoclonal anti-mAChR antibody (M35 ) and a monoclonal antibody against choline acetyltransferase (ChAT) t o perform immunohistochemistry on rat brain sections, and we also carr ied out histochemistry for acetylcholinesterase (AChE) activity. The d ensity distributions of ChAT immunohistochemistry and AChE activity hi stochemistry agreed well with previous quantitative measurements of th e activity distributions of these enzymes. A generally close correlati on between densities of M35 and ChAT immunoreactivities was found acro ss subregions of rat CN. The localizations of M35 and ChAT immunoreact ivities suggest that cholinergic synapses are mostly axosomatic in ven tral CN and possibly mostly axodendritic in dorsal CN. Prominent diffe rences in density and appearance between M35 immunoreactivity and AChE activity were found in the regions of the rat CN where granule cells predominate.