Pharmacological characterization of amine receptors on embryonic chick sensory neurones

Abstract

The effects of noradrenaline, dopamine and 5‐hydroxytryptamine were investigated on the duration of the action potential of embryonic chick sensory neurones in vitro. All three amines, like γ‐aminobutyric acid, decreased the duration of the action potential evoked by current injection. 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. 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. Saturating doses of noradrenaline and dopamine or 5‐hydroxytryptamine were not additive. 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. 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. 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. 1984 British Pharmacological Society