Several aspects of the regulation of phospholipase C-coupled receptors have been studied in cerebellar granule cells. In response to stimulation with carbachol, both m3- and m2-muscarinic receptor mRNA are down-regulated but with a distinct time-course. Exposures of cells to subtype selective and nonselective antagonists induce differential up-regulation of m2- and m3-receptor mRNA. M3-receptor mRNA and c-fos mRNA are also up-regulated by endothelin, a novel neuropeptide. Disruption of microtubules by colchicine results in a loss of muscarinic receptor number, m3-receptor mRNA, and carbachol-induced phosphoinositide turnover, while m2-receptor mRNA is markedly up-regulated. Prestimulation of cells with 5-HT or DOI causes a time-dependent decrease of 5-HT2 receptor-mediated phospoinositude turnover; however, 3H-ketanserin binding to intact cell is not down-regulated. Prestimulation of cells with GABA for 7 days inhibits phosphoinositide hydrolysis stimulated by glutamate and KC1. GABA dependence and withdrawal supersensitivity are expressed in these GABA-pretreated neurons due to persistent stimulation of GABA A receptors. In addition, diazepam was observed to inhibit phosphoinositide responses to glutamate, carbachol and KC1 by a mechanism independent of its well known effect on GABA A receptor transmission.
