A quantitative study of the effects of some muscarinic antagonists on the guinea‐pig olfactory cortex slice

Abstract

Muscarinic depression of the electrically‐evoked surface‐negative field potential (N‐wave) was measured in guinea‐pig olfactory cortex slices maintained in vitro. The effects of three muscarinic receptor antagonists, pirenzepine, atropine and gallamine on this muscarinic response were analysed in detail. Pirenzepine was a potent competitive antagonist of carbachol (CCh)‐evoked responses. Schild plot analysis yielded a pA2 value of 7.9 (Schild slope constrained to unity). A similar analysis for atropine versus CCh responses gave a pA2 of 8.9. Combination experiments using pirenzepine and atropine produced dose‐ratio shifts close to those expected for two antagonists competing for a similar receptor site. Gallamine was only a weak antagonist of responses to CCh. Oxotremorine behaved as a competitive antagonist at this muscarinic receptor (pA2 = 6.1). It is concluded that the presynaptic muscarinic receptor mediating depression of the N‐wave in the olfactory cortex slice is of the M1‐subtype. 1988 British Pharmacological Society