mu-receptor mediates elevated glucose and corticosterone after third ventricle injection of opioid peptides

Four experiments were done to determine which receptor type(s) mediates the effects of third ventricular microinjections of four opioid peptide agonists on blood levels of glucose, free fatty acids, and corticosterone. Tests were performed in unanesthetized adult male albino rats having chronic intraventricular cannulas; blood samples were taken from the tail tip at 0, 15, 30, 60, 90, and 120 min postmicroinjection. In experiment 1, the agonists DAGO (Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol), beta-endorphin, DSLET (d-Ser2-Leu-enkephalin-Thr), and dynorphin A-(1-17) (0, 0.3, 1, 3, and 10 nmol/rat) produced three distinct patterns of changes in serum glucose, free fatty acid, and corticosterone values. Experiment 2 showed that the effects of DAGO and beta-endorphin were inhibited by prior injection with the opiate-receptor blocker naloxone (1 mg/kg sc) and that the effects of dynorphin were not diminished. Experiment 3 determined that dynorphin effects were also not diminished by naloxone given intraventricularly. Experiment 4 found that blockade of the mu-receptor by intraventricular pretreatment with the specific antagonist beta-funaltrexamine (20 micrograms/rat, 24 h before) completely abolished the effects of DAGO and beta-endorphin on glucose and corticosterone. The mu-receptor is critical to the mediation of the hyperglycemia and hypercorticosteronemia induced by the central administration of opiate agonists. These results imply that mu-opioid binding sites previously identified in central autonomic regions may be involved in the regulation of circulating glucose and corticosterone.

Bombesin microinfusion into the rat hypothalamic paraventricular nucleus increases blood glucose, free fatty acids and corticosterone

Bombesin is a particularly potent hyperglycemic agent when administered intraventricularly or intracisternally in the rat. Because bombesin-like immunoreactivity is found in several forebrain regions implicated in glucoregulation, the ability of direct hypothalamic microinfusions of this peptide to affect serum metabolic fuel levels was tested. Three experiments, using anesthetized, acutely infused rats, or unanesthetized rats with chronic intracranial implants, showed that microinfusion of bombesin into the hypothalamic paraventricular nucleus caused significant, dose-related increases in serum glucose; infusions into the lateral hypothalamus or the caudate nucleus were ineffective. Infusions into the ventromedial nucleus significantly elevated glucose only in acutely anesthetized rats. In unanesthetized rats with chronic intracranial cannulae, bombesin infusions into all 3 hypothalamic sites, but not the caudate-putamen, significantly elevated blood free fatty acids, while only infusions into the paraventricular nucleus caused significant dose-related increases in blood corticosterone. The results demonstrate that the paraventricular nucleus is a sensitive site for bombesin-induced elevation of blood glucose, free fatty acids, and corticosterone. They also imply that the bombesin binding sites and immunoreactive terminals previously identified in these regions may be involved in the central regulation of circulating metabolic fuel levels and the pituitary-adrenal axis, and that the effects of acute surgery may augment the hyperglycemic response to intrahypothalamic bombesin administration.

Intrahypothalamic microinfusion of corticotropin-releasing factor elevates blood glucose and free fatty acids in rats

Three experiments examined whether intrahypothalamic microinfusions of corticotropin releasing factor (CRF) can affect circulating levels of the metabolic fuels, glucose and free fatty acids. Infusions of CRF into the paraventricular nucleus dose-dependently increased serum glucose levels; greater increases were seen in acute than in chronic preparations. The greater effectiveness could not be accounted for by anesthetization per se. CRF infusion into the ventromedial nucleus did not affect serum glucose. Infusions into both sites, however, significantly increased serum free fatty acids. Neither glucose nor free fatty acids were altered by infusions into the lateral hypothalamus or the caudate-putamen. These data suggest that the previously identified CRF binding sites and CRF neuronal terminals in the paraventricular and ventromedial nuclei may be involved in the central regulation of metabolic fuel release. Additionally, it appears that the importance of CRF in the paraventricular nucleus in regulating serum glucose may be greater under some conditions than others.