[Effect of intraseptal noradrenaline on somatic and visceral pain threshold of rabbits].

With electric stimulation of the splanchnic nerve or the skin of the tip of rabbit's ear to measure visceral or somatic pain threshold, we studied the effects of noradrenaline microinjection into the septal nucleus on visceral pain and somatic pain and the relationship between the intraseptal noradrenaline and the intra-PAG opiate peptidergic system. There was no effect on visceral pain threshold after injections of alpha-agonist clonidine (10 micrograms/2 microliters) or alpha-antagonist phentolamine (10 micrograms/2 microliters). In a group injected with beta-agonist isoprenaline (1 micrograms/2 microliters), visceral pain threshold was raised remarkably, while beta-antagonist propranolol (10 micrograms/2 microliters) injected into bilateral septal nuclei decreased visceral pain threshold. Phentolamine (10 micrograms/2 microliters) or propranolol (10 micrograms/2 microliters) injected into septal nucleus induced an elevation of somatic pain threshold. The results indicate that the beta-receptor in spetal nucleus plays an important role in the modulation of visceral pain. Both alpha-and beta- adrenergic receptors have effects on the modulation of somatic pain. Intra-PAG microinjection of naloxone (1 micrograms/1 microliters) attenuated visceral analgesia produced by injection of isoprenaline (1 micrograms/2 microliters) into septal nucleus. Microinjection of anti-leu-enkephalin antiserum (1:20,000) into PAG also attenuated the analgesia. When microinjection of isoprenaline into septal nucleus produced analgesia, the release of leu-enkephalin immunoreactive-like-substance in PAG was significantly increased. The results suggest that the analgesic effect of intra-septal noradrenaline on visceral pain is somehow related with the endogenous opiate peptidergic system in PAG, and the leu-enkephalin in PAG plays an important role in this process.