Evidence for an interrelationship between ventral noradrenergic bundle and CNS endorphins in the control of nociception in the rat.

The present paper examines the role of the ventral noradrenergic bundle (VB) in relation to endorphins in the control of nociception in the rat. Selective, bilateral destruction of the VB produced a substantial fall in hypothalamic levels of noradrenaline. On day 4 post-surgery, VB-lesioned rats displayed a pronounced elevation in basal nociceptive threshold. This proved to be reversible by the specific opioid antagonist, naloxone, evidential of its mediation by endorphins. It was, however, unaffected by dexamethasone, a suppressor of corticotrophic secretion of beta-endorphin, indicative that this pituitary pool of beta-endorphin was not responsible. On day 12, at which time the elevation in nociceptive threshold had disappeared, neither the time course nor the intensity of the antinociception elicited by acute stress or various doses of morphine was attenuated in VB-lesioned as compared to sham rats. These data are evidential that the VB may influence nociceptive thresholds via an interaction with a CNS endorphinergic network. They demonstrate, further, that the VB does not mediate a significant component of the antinociception generated by either morphine or stress.