Abstract

Homologous desensitization of muscarinic acetylcholine receptors (mAChR) was studied using primary cultures of corticostriatal neurons from neonatal rats. Prolonged incubation with carbachol attenuated phospholipase C responsiveness to muscarinic agonists and decreased the number of cell surface mAChR, as measured by binding of N-[3H] methylscopolamine to neuronal monolayers. When neurons were exposed to carbachol for 15 min, 40% of the mAChR lost from the membrane domain was recovered in the cytosol; a decrease of the total neuronal receptors was detected following an incubation with the agonist lasting longer than 15 min. Both 8-Br-cyclic AMP and forskolin neither affected N-[3H]methylscopolamine binding to cell monolayers or did they prevent the agonist-mediated mAChR desensitization. 8-Br-cyclic GMP also failed to decrease mAChR number. Pertussis toxin failed to prevent the homologous desensitization of mAChR under conditions that blocked the agonist-mediated inhibition of forskolin-stimulated cyclic AMP formation. The phorbol ester 12-O-tetradecanoyl-phorbol-12, 13-acetate induced a concentration-dependent decrease of N-[3H]methylscopolamine binding to neuronal monolayers. However, the protein kinase C inhibitors sphingosine and the ganglioside monosialosyl-gangliotetraglicosylceramide inhibited the 12-O-tetradecanoyl-phorbol-12,13-acetate-induced but not the agonist-induced desensitization of mAChRs. Furthermore, incubation with muscarinic agonists failed to translocate protein kinase C from cytosol to plasma membranes, as measured by binding of the phorbol ester [3H]-4-beta-phorbol-12,13-dibutyrate to neuronal monolayers. In corticostriatal neurons the agonist-induced desensitization and internalization of mAChR involves neither protein kinase C and protein kinase A activation nor changes in cyclic GMP and cyclic AMP content.