Pain-induced inhibition of gastric motility is mediated by adrenergic and vagal non-adrenergic reflexes in the rat

Painful stimuli have been shown to inhibit gastric motility in animal experiments and delay gastric emptying in humans. The aim of the present study was to investigate in detail mechanisms involved in pain-induced gastric inhibition. Pain stimulation by exerting pressure on a testicle induced a prompt gastric relaxation which lasted throughout the period of stimulation. Pain-induced gastric relaxation was significantly reduced by the selective α-1 blocker, prazosin, and by the non-selective β-blocker, propranolol. Similarly pain-induced inhibition of gastric tone was significantly reduced by bilateral cervical vagotomy. In contrast, gastric relaxation following pain stimulation was significantly potentiated by the selective adrenergic α-2 blocker, yohimbine. Combined administration of prazosin and propranol followed by bilateral cervical vagotomy abolished gastric relaxation in response to pain stimulation. In conclusion, gastric relaxation in response to painful stimulation was found to be reflex-mediated via sympathetic neurons acting on α-1 and β receptors and possibly also via vagal non-adrenergic fibres. Pain-induced inhibition of gastric tone was significantly increased by yohimbine. It is suggested that yohimbine by blocking presynaptic inhibitory receptors on adrenergic neurons facilitates the release of noradrenaline in response to pain stimulation.