[3H]N‐Methylscopolamine Binding Studies Reveal M2 and M3 Muscarinic Receptor Subtypes on Cerebellar Granule Cells in Primary Culture

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Abstract
Saturation experiments with the muscarinic antagonist [3H]N‐methylscopolamine ([3H]NMS) indicated that cerebellar granule cells in primary culture possess a high density of muscarinic acetylcholine receptors (mAChRs): Bmax= 1.85 ± 0.01 pmol/mg of protein at 10 days in culture; KD= 0.128 ± 0.01 nM The selective M1 antagonist pirenzepine displaced [3H]NMS binding with a low affinity (Ki= 273 ± 13 nM), whereas the M2/M3 muscarinic antagonist 4‐diphenylacetoxy‐N‐methylpiperidine methiodide competed with [3H]NMS with Ki values in the nanomolar range, a result suggesting that some of the mAChRs on cerebellar granule cells belong to the M3 subtype. Methoctramine, which discriminates between M2 and M3 subtypes with high and low affinity, respectively, displayed a high and low affinity for [3H]NMS binding sites (Ki(H)= 31 ± 5 nM; Ki(L)= 2,620 ± 320 nM). These results provide the first demonstration that both M2 and M3 mAChR subtypes may be present on cultured cerebellar cells. In addition, complete death of neurons induced by N‐methyl‐D‐aspartate (100 μM for 1 h) reduced by 85% the specific binding of [3H]NMS, a result indicating that most mAChRs were associated with neuronal components. Finally, the evolution of the density of mAChRs, labeled by [3H]NMS, correlated with the neuronal maturation during the in vitro development of these cells.