AGONIST-INDUCED MUSCARINIC CHOLINERGIC RECEPTOR INTERNALIZATION, RECYCLING AND DEGRADATION IN CULTURED NEURONAL CELLS - CELLULAR MECHANISMSAND ROLE IN DESENSITIZATION

Short-term incubation of intact neuronal cells with muscarinic choline rgic agonists resulted in a rapid decrease of the specific binding of [H-3]methylscopolamine to cell surface receptors indicative of recepto r internalization. The agonists induced the internalization of both th e muscarinic receptor subtypes coupled to adenylyl cyclase and those c oupled to phosphoinositide turnover. Receptor internalization, which w as inhibited at 0-4 degrees and by depletion of intracellular K+ is th ought to occur through coated pits formation and was rapidly reversibl e. Receptor recycling did not imply protein synthesis. Down-regulation of muscarinic receptors occurred slowly in the presence of agonists, needed intact cytoskeleton (demonstrated by the inhibitory effect of c olchicine) and involved lysosomal activity. Both receptor internalizat ion and down-regulation were prevented by muscarinic receptor antagoni sts. Receptor internalization and down-regulation are agonist-induced cellular mechanisms that with receptor phosphorylation and uncoupling, may induce desensitization. These processes may contribute to complex intracellular regulatory processes and may be involved in some of the long-term effects of neurotransmitters (mainly neuropeptides and grow th hormones) or drugs.