Microinjections of carbachol into the phrenic motor nucleus inhibit phrenic nerve activity in the rat

Microinjections (50 nl) of carbachol (2.5-500 mu M) into the phrenic motor nucleus (PMN) of anesthetized rats caused a dose-dependent decrease in the phrenic nerve (PN) burst-amplitude. Prior microinjections of pirenzepine an d methoctramine (1 mM, each) into the PMN, in separate experiments, signifi cantly attenuated the carbachol-induced inhibition of PN activity. These re sults suggest that inhibition of PN activity induced by microinjections of carbachol into the PMN is mediated via M-1 and M-2, receptors. Since pirenz epine and methoctramine microinjections into the PMN did not alter the cont rol PN activity, it was concluded that in anesthetized rats cholinergic inp uts to the PMN, if any, are not tonically active. It is possible that musca rinic receptors in the PMN come into play only under specific conditions su ch as activation of a reflex mechanism which alters PN activity. These hypo theses remain to be tested. (C) 1999 Elsevier Science B.V. All rights reser ved.