LIGHT-MICROSCOPIC DISTRIBUTION AND PARASAGITTAL ORGANIZATION OF MUSCARINIC RECEPTORS IN RABBIT CEREBELLAR CORTEX

Recent studies on the effects of intrafloccular injections of muscarin ic agonists and antagonists on compensatory eye movements in rabbit, i ndicate that muscarinic receptors may play a modulatory role in the ra bbit cerebellar circuitry. It was previously demonstrated by Neustadt et al. (1988), that muscarinic receptors in rabbit cerebellar cortex a re distributed into alternating longitudinal zones of very high and ve ry low receptor density, In the present study, the zonal and cellular distribution of muscarinic receptors in the rabbit cerebellar cortex i s investigated in detail using in vitro ligand autoradiography with th e non-selective high-affinity antagonist [H-3]quinuclidinyl benzilate (QNB), and the M2-specific antagonist [H-3]AF-DX384, and immunocytoche mistry with a monoclonal antibody specific for the cloned m2 muscarini c receptor protein. [H-3]QNB and [H-3]AF-DX384 binding sites and m2-im munoreactivity had similar overall distributions: dense labeling occur red in the dendritic arbors of a subset of Purkinje cells that are org anized into parasagittal bands. A high level of muscarinic receptor la beling was also observed in a thin substratum of the molecular layer i mmediately above the Purkinje cell layer of the vestibule-cerebellar l obules, i.e. the nodulus, the ventral uvula and the flocculus, Labelin g in this stratum was associated with densely packed fibres, which wer e putatively identified as parallel fibres, Also Golgi cells, which we re localized in part in the molecular layer, and a subset of messy fib re rosettes, primarily concentrated in lobule VI, were immunoreactive for the m2 receptor. The parasagittal bands of labeled Purkinje cell d endrites were most prominent in the anterior lobe (lobules I-V), in cr us 1 and 2, in the flocculus, the ventral paraflocculus and the rostra l folium of the nodulus. In other lobules, only infrequent Purkinje ce lls contained muscarinic receptors. The parasagittal organisation of m uscarinic receptors differed from that of zebrin I, a Purkinje cell-sp ecific protein which is often used as a marker of parasagittal parcela tion of the cerebellar cortex. In the anterior lobe, however, there wa s a partial correspondence between muscarinic receptor and zebrin I ba nds. In the flocculus the distribution of muscarinic-receptor-positive Purkinje cells was related to the distinct white matter compartments as revealed with acetylcholinesterase (AChE) histochemistry. Muscarini c receptor-containing Purkinje cells were located primarily in the flo ccular zone 1, which is implicated in the control of eye movements abo ut a horizontal axis. In order to relate the distribution of muscarini c receptor labeling to that of cholinergic nerve terminals, [H-3]QNB b inding sites and sodium-dependent [H-3]hemicholinium-3 binding were co mpared. Sodium-dependent [H-3]hemicholinium-3 binding sites mainly occ urred in the granule cell layer of the vestibulo-cerebellum, which cor responds well with the distribution of the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT). However, sodium-dependent [H -3]hemicholinium binding complemented, rather than co-localized with, muscarinic receptors which were primarily distributed in the molecular layer of the lobules of the vestibulo-cerebellar lobules, Their funct ional significance is puzzling, since their distribution does not corr espond to that of markers of cholinergic innervation.