Effects of enkephalins and morphine on spontaneous electrical activity and on junction potentials elicited by parasympathetic nerve stimulation in cat and rabbit colon.

1 The effects of Leu-enkephalin, Met-enkephalin and morphine on excitatory junction potentials (e.j.ps) and inhibitory junction potentials (i.j.ps) elicited by stimulation of efferent parasympathetic nerves were studied in cats and rabbits, anaesthetized and in vitro.2 Enkephalins (0.008 mg/kg in vivo and 10(-6)M in vitro) enhanced e.j.p. amplitude in rabbit proximal colon and decreased it in rabbit distal colon and in cat colon. Enkephalins decreased i.j.p. amplitude in all the three models.3 Morphine (0.2 mg/kg in vivo and 10(-6)M in vitro) had the same effects as enkephalins on e.j.ps. In contrast, morphine decreased i.j.p. amplitude in rabbit proximal and distal colon and increased it in cat colon.4 Enkephalins and morphine induced (especially in the cat) spike activity which was potentiated by atropine (0.1 mg/kg in vivo or 10(-6)M in vitro).5 All the effects of enkephalins and morphine were antagonized by naloxone (0.2 mg/kg in vivo or 10(-6)M in vitro).6 These results suggest that the facilitatory effects of enkephalins and morphine on e.j.ps of rabbit proximal colon are due to the absence of opiate receptors on the excitatory nerve pathway and to a removal of inhibition by blockade of the non-adrenergic, non-cholinergic inhibitory pathway. Enkephalinergic intramural neurones may modulate the activation of either excitatory or inhibitory pathways in intramural reflexes.