DIFFERENTIAL RECEPTOR OCCUPANCY REQUIREMENTS FOR MUSCARINIC CHOLINERGIC STIMULATION OF INOSITOL LIPID HYDROLYSIS IN BRAIN AND IN NEUROBLASTOMAS

  • 1 July 1987
    • journal article
    • research article
    • Vol. 32  (1) , 81-90
Abstract
The potency with which carbamoylcholine enhances phosphoinositide (PPl) hydrolysis in different brain regions (neostriatum, cerebral cortex, and hippocampus) and in two neuroblastomas (the murine N1E-115 and human SK-N-SH) differs by 10- to 20-fold. To determine whether the presence of a muscarinic receptor (mAChR) reserve might account for these differences, we have examined the effect of propylbenzilylcholine mustard (PrBCM) on mAChR number and on agonist-stimulated PPl hydrolysis. In the cerebral cortex, in hippocampus, and in N1E-115 cells, PrBCM treatment resulted in a loss of the PPl response, as measured by the release of [3H]inositol phosphates, that was equal to or greater than the reduction in receptor number, as determined by the loss of either [3H]quinuclidinylbenzilate- or [3H]N-methylscopolamine-binding sites. From dose response curves for carbamoylcholine, it was determined that alkylation of mAChRs resulted in a reduction in the maximum release of inositol phosphates but had no effect on agonist potency. The KA values for carbamoylcholine obtained following receptor inactivation were similiar to those for the EC50 (120-316 .mu.M). In contrast, in both the neostriatum and SK-N-SH cells, PrBCM treatment resulted in a greater loss of mAChR number than of stimulated inositol phosphate release, and dose response curves for carbamoylcholine were shifted to higher agonist concentrations. The KA values (34-65 .mu.M) were 2- to 9-fold higher than the comparable EC50 values. Moreover, in both tissues the PPl response elicited by partial agonists was more susceptible to receptor alkylation than that elicited by carbamoylcholine. The two groups of tissues also differ in their sensitivity to pirenzepine, which is a markedly weaker antagonist of stimulated PPl hydrolysis in SK-N-SH cells and neostriatum (Ki 160-250 nM), than in the cerebral cortex, hippocampus, and N1E-115 cells (Ki 10-20 nM). These results suggest: 1) that a population of "spare" receptors exists for mAChR-mediated inositol lipid hydrolysis in some neuronal tissues, 2) that both M1 and M2 mAChRs may be coupled to PPl turnover, and 3) that M2 mAChRs appear to be more efficiently coupled to phosphoinositide hydrolysis than their M1 counterparts.