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

Microinjections (50 nl) of carbachol (2.5-500 microM) 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 and methoctramine (1 mM, each) into the PMN, in separate experiments, significantly attenuated the carbachol-induced inhibition of PN activity. These results suggest that inhibition of PN activity induced by microinjections of carbachol into the PMN is mediated via M(1) and M(2) receptors. Since pirenzepine and methoctramine microinjections into the PMN did not alter the control PN activity, it was concluded that in anesthetized rats cholinergic inputs to the PMN, if any, are not tonically active. It is possible that muscarinic receptors in the PMN come into play only under specific conditions such as activation of a reflex mechanism which alters PN activity. These hypotheses remain to be tested.