Short-term desensitization of muscarinic cholinergic receptors in mouse neuroblastoma cells: Selective loss of agonist low-affinity and pirenzepine high-affinity binding sites

The effects of brief incubation with carbamylcholine on subsequent binding of [³H]N-methylscopolamine were investigated in mouse neuroblastoma cells (clone N1E-115). This treatment demonstrated that the muscarinic receptors in this neuronal clone can be divided into two types; one which is readily susceptible to regulation by receptor agonists, whereas the other is resistant in this regard. In control cells, both pirenzepine and carbamylcholine interacted with high- and low-affinity subsets of muscarinic receptors. Computer-assisted analysis of the competition between pirenzepine and carbamylcholine with [³H]N-methylscopolamine showed that the receptor sites remaining upon desensitization are composed mainly of pirenzepine low-affinity and agonists high-affinity binding sites. Furthermore, there was an excellent correlation between the ability of various muscarinic receptor agonists to induce a decrease in consequent [³H]N-methylscopolamine binding and their efficacy in stimulating cyclic GMP synthesis in these cells. Thus, only the agonists that are known to recognize the receptor's low-affinity conformation in order to elicit increases in cyclic GMP levels were capable of diminishing ligand binding. Taken together, our present results suggest that the receptor population that is sensitive to regulation by agonists includes both the pirenzepine high-affinity and the agonist low-affinity receptor binding states. In addition, the sensitivity of these receptor subsets to rapid regulation by agonists further implicates their involvement in desensitization of muscarinic receptor-mediated cyclic GMP formation.