EVIDENCE AGAINST THE UNITARY HYPOTHESIS OF AGONIST AND ANTAGONIST ACTION AT PRESYNAPTIC ADRENOCEPTORS

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Abstract
1 The concept that presynaptic receptors regulate noradrenergic transmitter release via a system of inhibitory receptors mediating negative feedback relies on a supposed association between increases in stimulation-induced efflux of [3H]-noradrenaline by antagonists and blockade by them of the inhibitory effects of exogenous noradrenaline. 2 It was shown in guinea-pig ureter, that yohimbine (3 × 10−7 m), a presumed selective presynaptic antagonist, increased transmitter efflux substantially at 1 Hz and 5 Hz with 100 pulses, purportedly representing antagonism of the inhibitory effect of locally released noradrenaline but did not reduce the inhibitory effect of exogenous noradrenaline (1.8 × 10−6 m or 1.8 × 10−7 m) except in one case. 3 Additionally, the inhibitory effect of oxymetazoline (1.0 × 10−7m or 1.0 × 10−8m) on stimulation-induced efflux was in no way antagonized by yohimbine (3 × 10−7 m). 4 It is concluded that the increased efflux of [3H]-noradrenaline produced by antagonists and the decreased efflux produced by exogenous agonists may represent actions at different loci and that the hypothesis of presynaptic feedback regulatory sites is still not substantiated.

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