PREJUNCTIONAL MUSCARINIC RECEPTORS REGULATING NEUROTRANSMITTER RELEASE IN AIRWAYS

Prejunctional pA(2) values of five muscarinic antagonists were determi ned in the guinea-pig trachea under stimulation conditions in which th e antagonists alone did not enhance acetylcholine release. The antagon ists were partly selective at M(1) (pirenzepine), M(2) (AQ-RA 741, him bacine) and M(3) receptors (hexahydrosiladifenidol, dicyclomine). The profile of the antagonist affinities was different from that obtained at cardiac M(2) receptors but resembled the profile reported in the li terature for the cloned m4 receptor. This suggests that autoinhibition of acetylcholine release in the trachea is mediated via M(4) receptor s.