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1 Changes in the response to acetylcholine of expansor secundariorum muscles from chicks have been analyzed by pharmacological techniques and by [3-3H)-quinuclidinyl benzilate ([3-3H]-QNB) binding to quantify the muscarinic receptor population. 2 The expansor secundariorum muscle responded to acetylcholine up to the age of 30 days; the response declined thereafter. This developmental decrease in response to acetylcholine was prevented by surgical denervation. 3 In chicks aged less than 25 days, denervation did not affect the sensitivity of the expansor muscle to acetylcholine. In order chicks (above 40 days) denervation gradually restored the sensitivity of the expansor muscle to acetylcholine. Responses of the expansor muscle were always abolished by atropine (1 microM) indicating they were mediated by muscarinic receptors. 4 Binding studies with [3-3H]-QNB showed that changes in response of expansor muscle to acetylcholine were primarily due to changes in the muscarinic receptor population. 5 It is suggested that the noradrenergic innervation of the expansor muscle influences the number of muscarinic receptors expressed in the tissue.

The effects of chemical sympathectomy with 6-hydroxydopamine on the response of the expansor secundariorum muscle to noradrenergic nerve stimulation, noradrenaline and acetylcholine have been investigated. Expansor muscles from 60 day old chicks were sensitive to noradrenergic nerve stimulation and exogenous noradrenaline but virtually unresponsive to acetylcholine. Chemical sympathectomy with 6-hydroxydopamine caused loss of function of noradrenergic nerves of the expansor muscle, induced supersensitivity to exogenous noradrenaline and gradually increased the response of the expansor muscle to acetylcholine. As the patency of noradrenergic nerves reappeared there was a decline in the extent of supersensitivity to noradrenaline and the response to acetylcholine gradually declined. The time courses of these changes differed, indicating that the mechanisms responsible for changes in response to noradrenaline and acetycholine are different.