OPIOID RECEPTOR MODULATION OF NEURAL TRANSMISSION IN THE RABBIT COELIAC GANGLION AND GANGLIONIC OPIOID RECEPTOR ACTIVATION BY BUNITROLOL

SUMMARY 1 We examined the preganglionic splanchnic nerve activity and postganglionic renal nerve activity before and after a local injection of naloxone (20 ug/kg) into the coeliac ganglion of anaesthetized rabbits. This was done during graded hypertension, induced by the administration of phenylephrine (0.5-10 ug/kg, i.v.) and with selective intraganglionic injection of methionine-enkephalin (ME) and bunitrolol, which is a beta-blocker. 2 During hypertension both pre-and postganglionic discharge decreased, but only postganglionic discharge was inhibited by naloxone treatment into the ganglion. 3 Local injection of ME (0.1-10 ug/kg) into the coeliac ganglion decreased postganglionic activity by 9.0 ± 1.0 to 41.2 ± 4.7% from control, and this decrease was inhibited by naloxone. 4 Administration of bunitrolol (1-300 ug/kg) decreased postganglionic discharge by 3.9 ± 1.4 to 39.7 + 2.4% of the control and this decrease was also inhibited by naloxone. 5 These results suggest that opioid receptors in the coeliac ganglion play an inhibitory role in neural ganglionic transmission and that this inhibitory action reduces postganglionic sympathetic discharge.