Autoradiographic Distribution of Muscarinic Acetylcholine Receptor Subtypes in Rat Cochlear Nucleus

The cochlear nucleus (CN) receives cholinergic inputs primarily from centrifugal pathways. There is evidence that the effects of these cholinergic inputs may be mediated mainly by muscarinic acetylcholine receptors. We used 1-[N-methyl-³H]scopolamine (NMS) to study muscarinic receptor binding in the rat CN autoradiographically. To determine which muscarinic receptor subtypes participate in the binding, we included competition assays using the unlabeled subtype-preferential ligands pirenzepine, AF-DX 116, 4-DAMP, HHSiD, and tropicamide to compete with [³H]NMS for binding. Our results suggest that NMS binding density in the CN is about a tenth of that in the facial nucleus. Inside the CN, the highest binding was found in granular regions, followed in order by the dorsal CN (DCN) fusiform soma layer, the DCN molecular layer, the DCN deep layer, the anteroventral CN (AVCN) and posteroventral CN (PVCN). Binding in the interstitial nucleus (auditory nerve root) was similar to background. The results of the competition assays suggest that the M₂ receptor subtype predominates in VCN, M₄ in the DCN fusiform soma layer, and both subtypes in DCN molecular and deep layers. M₄ and M₃ subtypes predominated in the granular region of AVCN, while M₁ and M₂ were more prominent in the granular region of PVCN. The results show similarities to those obtained with pharmacological and immunohistochemical methods, but also some discrepancies. The different distributions of the different muscarinic receptor subtypes suggest that the effects of cholinergic inputs may differ among CN subregions, in agreement with in vivo pharmacological results. Overall, the centrifugal cholinergic influences on information processing in the CN may especially involve M₂ and M₄ receptors.