Circannual variations in bear plasma albumin and its opioid-like effects on guinea pig ileum.

Previous studies suggest that hibernation is controlled by an opioid system. In this study we examined the effect of plasma albumin fractions drawn from black bears at timed intervals while in hibernation or during the awake state in fall and winter, on induced contractility of the guinea pig ileum. Four hundred nM morphine produced typical suppression of contractility and 400 or 1000 nM naloxone (an opiate antagonist) restored it. Twenty mg of lyophilized albumin fraction from the winter hibernating bear caused similar suppression, the effect being greater than that of either summer bear or winter-active bear plasma albumin. Naloxone reversed the suppression in all cases. Strong suppression of contractility was also demonstrated with 2.5 nM [D-Pen2.5]-enkephalin (DPDPE), a delta agonist, but only minor suppression with 2.5 nM dynorphin A, a kappa agonist. Results support the opioid nature of the albumin-bound hibernation-induction trigger substance, that it binds to the delta opiate receptor, and that delta agonist opioid production may increase during the hibernation season.