Pharmacological characterization of amine receptors on embryonic chick sensory neurones

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Abstract
1 The effects of noradrenaline, dopamine and 5-hydroxytryptamine were investigated on the duration of the action potential of embryonic chick sensory neurones in vitro. 2 All three amines, like γ-aminobutyric acid, decreased the duration of the action potential evoked by current injection. 3 The onset of the noradrenaline-induced decrease in action potential duration was fast (< 1 s) and the recovery phase was dependent upon the dose of noradrenaline applied. Rapid washout of the noradrenaline revealed a minimum 30 s recovery time which was independent of the initial noradrenaline concentration. 4 Dopamine and 5-hydroxytryptamine could mimic the effects of noradrenaline on action potential duration. The ED50 for all three amines was approximately 1 μm. At a saturating concentration of 10 μm, noradrenaline was more potent than dopamine and 5-hydroxytryptamine. 5 Saturating doses of noradrenaline and dopamine or 5-hydroxytryptamine were not additive. 6 Responses to all three amines were affected similarly by antagonists: they were antagonized by yohimbine, phentolamine, haloperidol and mianserin but not by propranolol, prazosin, domperidone, spiperone or methysergide. Clonidine and xylazine (α2-adrenoceptor agonists) were also without effect. 7 In contrast to the amines, saturating concentrations of γ-aminobutyric acid were additive with those of noradrenaline. Responses to GABA were not antagonized by the amine receptor antagonists. 8 The evidence described here suggests that the amines and γ-aminobutyric acid decrease sensory neurone action potential duration via pharmacologically-distinct membrane receptors. In addition, it is likely that the amines are acting via a single class of receptor whose pharmacology is different from classical adrenoceptors, dopamine receptors and 5-hydroxytryptamine receptors.