Relationship between desensitization and sequestration of muscarinic cholinergic receptors in two neuronal cell lines

Agonist-induced sequestration and desensitization of muscarinic receptors was studied in two cell lines that each express differentially-coupled receptors. The NG108-15 glioma x neuroblastoma hybrid cells have muscarinic receptors coupled only to the inhibition of adenylate cyclase, whereas SK-N-SH human neuroblastoma cells have muscarinic receptors coupled only to the turnover of phosphoinositide. In both NG108-15 and in SK-N-SH cells, muscarinic agonists caused a rapid, temperature-dependent, loss of binding sites for [3H] N-methylscopolamine. In contrast, the density of binding sites for [3H] quinuclidinyl benzilate remained almost unchanged after treatment with carbachol. Since carbachol also caused a redistribution of muscarinic receptors from the plasma membrane to a lighter membrane fraction, the loss of binding sites for [3H] N-methyl-scopolamine was interpreted as being due to sequestration of receptors. In addition to agonist-induced sequestration, muscarinic agonists also caused desensitization of the receptor-mediated response in NG108-15 cells. In SK-N-SH cells, however, no such desensitization of the receptor-mediated response was found, despite the agonist-induced sequestration of receptors. These findings demonstrate that agonist-induced sequestration of receptors does not always lead to desensitization of receptors.