Neural regulation of muscarinic receptors in chick expansor secundariorum muscle

We have recently observed the loss of muscarinic receptors from a smooth muscle of the chicken wing, the expansor secundariorum, during post-hatch development¹. The nerve supplying the muscle has been shown to consist mainly of sympathetic fibres that, when stimulated, cause muscle contraction by the release of noradrenaline and subsequent activation of α-adrenoreceptors. Although no acetylcholine release has been detected in isolated nerve-muscle preparations², the presence of a cholinergic input has not been clearly excluded. Kuromi and Hagihara² observed that atropine was ineffective in blocking the nerve stimulation-induced contraction of the muscle, whereas phentolamine partly and guanethedine totally abolished the contraction. We have now examined the effect of denervation of the muscle 1 day after hatch on the binding of ³H-quinuclidinyl benzilate (³H-QNB), a radioligand for muscarinic receptors ^3,4. Our results indicate that the normal loss of muscarinic receptors from the expansor secundariorum is prevented by denervation. This observation is important as it demonstrates the regulation of a receptor population by a nerve supply that apparently does not release the corresponding transmitter.