Molecular Pharmacology, Vol 14, 246-255, Copyright © 1978 by the American Society for Pharmacology and Experimental Therapeutics

Muscarinic Cholinergic Receptors in Rat Corpus Striatum and Regulation of Guanosine Cyclic 3',5'-Monophosphate

¹ Medical Research Council Neurochemical Pharmacology Unit, Department of Pharmacology, Cambridge University Medical School, Cambridge CB2 2QD, England

Binding of cholinergic agonists and antagonists to muscarinic receptors in slices of rat striatum was assessed by their ability to displace the specific ligand [³H]quinuclidinyl benzilate (QNB), and these results were compared with the effects of these drugs in eliciting changes in cyclic 3',5'-GMP in the same preparation in vitro. The agonists oxotremorine, arecoline, and carbachol caused transient increases in cyclic GMP, reaching a maximum after 2 min of incubation. The increase in cyclic GMP amounted to a 6-fold change over basal levels in the absence of the phosphodiesterase inhibitor isobutylmethylxanthine, or a doubling over basal levels in its presence. There was also a significant increase in tissue cyclic AMP, which, like the cyclic GMP response, was antagonized by muscarinic blocking drugs. The EC₅₀ values for agonists in increasing cyclic GMP were similar to the estimated K_i values for these compounds in inhibiting QNB binding; thus the cyclic GMP response appears to be linearly related to muscarinic receptor occupancy. Muscarinic antagonists and some neuroleptic drugs antagonized the cyclic GMP response elicited by oxotremorine, with relative potencies as follows: scopolamine > QNB > atropine > clozapine > thioridazine > chlorpromazine > haloperidol. Similar rank orders and relative potencies were seen for these drugs as inhibitors of QNB binding. Pilocarpine increased cyclic GMP at low concentrations, but this effect was no longer seen at higher concentrations. The cyclic GMP response to oxotremorine required the presence of calcium ions in the external medium, and was maximal at calcium concentrations above 1 mM. Sodium azide caused an increase in tissue cyclic GMP which was additive to that elicited by oxotremorine. It is suggested that occupation of muscarinic receptors may activate a soluble intracellular guanylate cyclase indirectly, with calcium possibly acting as the intracellular mediator. The ability of some neuroleptic drugs to inhibit the cyclic GMP response to muscarinic agonists in brain slices adds further weight to the view that such compounds may possess important antimuscarinic actions in mammalian central nervous system.

Note:
ACKNOWLEDGMENTS We thank Drs. Ken Minneman and Piers Emson for valuable advice and assistance with methodology. In addition, we are grateful to Dr. Michael Young for critical discussions and supplies of cholinergic drugs, and to Susan Gardiner for excellent technical assistance. Chlorpromazine and thioridazine were kindly supplied by May and Baker, Ltd; clozapine, by Wander (Sandoz) Laboratories; and haloperidol, by Janssen Pharmaceutica.