Muscarinic acetylcholine receptor-mediated phosphoinositide turnover in cultured cerebellar granule cells: desensitization by receptor agonists.

Cultured granule cells prepared from 8-day postnatal rats were used for the study of carbachol-induced phosphoinositide turnover. The addition of carbachol induced a 20- to 30-fold increase in [3H]inositol monophosphate (IP1) accumulation within 30 min in the presence of 20 mM LiCl in granule cells prelabeled with [3H]myoinositol. Carbachol also stimulated the formation of [3H]inositol bisphosphate and [3H]inositol trisphosphate assayed either in the presence or in the absence of lithium; the increase in [3H]inositol triphosphate and [3H]inositol biphosphate synthesis appeared to be faster in the time course but much smaller in amount when compared with the formation of [3H]IP1. The EC50 of carbachol was approximately 7 microM, and the saturation concentration was about 100 microM. This carbachol-induced response was completely blocked by two muscarinic acetylcholine receptor (mAChR) antagonists, atropine and pirenzepine, with a Ki of 0.5 and 120 nM, respectively. Saturation studies of the binding of [3H]N-methylscopolamine and [3H]quinuclidinyl benzilate to mAChRs in intact granule cells revealed the presence of a single class of binding sites with a Kd of 140 and 126 pM, respectively, and a Bmax of approximately 70 fmol/10(6) cells for both ligands. Within 1 hr after pre-exposure of cells to carbachol, the subsequent carbachol-induced [3H]IP1 accumulation was reduced by about 50%, whereas mAChR binding, assessed by using either [3H]N-methylscopolamine or [3H]quinuclidinyl benzilate, was unchanged. A second slower phase of desensitization was associated with a loss of mAChR binding sites; at 18 hr, the decrease of responsiveness was about 80%, whereas the loss of mAChR sites was about 60%.(ABSTRACT TRUNCATED AT 250 WORDS)