Regulation of muscarinic receptor function in developing oligodendrocytes by agonist exposure

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Abstract
1 Oligodendrocytes, the myelin forming cells in the CNS, express muscarinic acetylcholine receptors (mAChR), primarily M3, coupled to various signal transduction pathways. 2 In the present study we have investigated whether mAChR undergo functional agonist-induced regulation in cultured oligodendrocyte progenitors and differentiated oligodendrocytes. 3 The muscarinic agonist, carbachol (CCh) caused a time-dependent desensitization of phosphoinositide (PI) hydrolysis, and the internalization and down-regulation of receptors. Short-time desensitization (5 min) of PI hydrolysis occurred without receptor internalization and reached 54% by 1 h. The same treatment decreased cell surface receptors labelled with the non-permeable ligand [(3)H]-NMS by 47%, while total receptor density ([(3)H]-scopolamine binding) decreased by 30%. Longer CCh treatment down-regulated receptors by 70% and desensitized the PI response by 80%. 4 Although protein kinase C (PKC) activation desensitized mAChR, CCh-mediated desensitization was independent of PKC. 5 Inhibition of receptor endocytosis by low temperature during the pre-stimulation period or in the presence of hyperosmotic sucrose (0.5 M) blocked desensitization, receptor internalization and down-regulation. 6 Recovery of surface mAChR and their functional activity following down-regulation was slow, returning to control levels by 24 h after agonist removal. In progenitor cells, dose-response curves for CCh-mediated PI hydrolysis and c-fos mRNA expression showed that newly synthesized mAChR were supersensitive after recovery. 7 Overall, the present results provide evidence of functional agonist-mediated mAChR regulation in brain oligodendroglial cells.