Effects of mu-opioid receptor stimulation in the hypothalamic paraventricular nucleus on basal and stress-induced catecholamine secretion and cardiovascular responses.

Previous work from this laboratory has demonstrated that opioid peptides, acting at mu-receptors in the brain, stimulate central sympathetic outflow thereby increasing plasma catecholamine concentrations in unstressed rats. Brain sites involved in opioid-mediated catecholamine secretion have not been characterized fully. Additionally, brain opioid effects on sympathoadrenal catecholamine secretion during stress have not been defined. Because the paraventricular hypothalamic nucleus (PVN) plays a central role in autonomic and cardiovascular regulation, we administered the mu-selective enkephalin analog, D-Ala2-NMe-Phe4-Gly(ol)5enkephalin (DAGO), directly into PVN in conscious, unstressed rats and determined the changes in plasma catecholamine concentrations, blood pressure and heart rate. Then, during the peak response, rats were subjected to restraint stress and the same parameters were again measured. Under basal conditions, picomolar doses of DAGO injected into PVN increased plasma concentrations of catecholamines, especially epinephrine, and raised blood pressure. These effects were dose-related (0.01-0.3 nmol) and antagonized by naloxone given either systemically or directly into PVN. Tachycardia was also observed except at the highest dose of DAGO (0.3 nmol). Thus, mu-receptor stimulation in PVN increases central sympathetic outflow in nonstressful situations, producing increased adreno-medullary catecholamine secretion, blood pressure and heart rate. During restraint stress, PVN microinjections of DAGO blunted stress-induced tachycardia, apparently by a vagal mechanism as this effect was blocked by atropine methyl nitrate. PVN DAGO had no significant effect on the plasma catecholamine responses to restraint stress. In contrast, naloxone injected into PVN augmented stress-induced epinephrine secretion. Thus, PVN mu-receptors may regulate heart rate during stress, and an endogenous opioid released during restraint stress may modulate adrenomedullary responses to stress.