Opioids and hibernation. I. Effects of naloxone on bear HIT'S depression of guinea pig ileum contractility and on induction of summer hibernation in the ground squirrel.
Summer hibernation in ground squirrels (Citellus tridecemlineatus) can be induced by intravenous injection of hibernation-induction trigger (HIT) from winter bear plasma or its albumin fraction. In this study, we show that bear HIT depresses electrically-induced contraction of the guinea pig ileum myenteric plexus-longitudinal muscle preparation, and that naloxone, at 100, 1,000, or even 4,000 nM, fails to reverse that effect. In a simultaneous study, four sets of ground squirrels were implanted with osmotic minipumps which delivered solutions at a controlled and continuous rate. Two of the groups had pumps delivering naloxone at 1 mg/kg body weight per hour. The other two groups had saline-filled pumps (controls). One set of squirrels from each of the saline- and naloxone-filled pump groups were then injected intravenously with winter bear plasma. The remaining two groups of squirrels were injected with winter bear albumin fraction. Hibernation frequency was determined by measurements of core temperature (from surgically-implanted radio capsules), respiratory rate, and bouts of activity. Squirrels with saline-filled pumps hibernated four times more frequently than the naloxone groups. To confirm these findings, three squirrels from each naloxone group were reinjected with bear HIT after removal of the pumps. These six squirrels then hibernated over four times their previous frequency. Results suggest that bear HIT is not itself an opioid (since naloxone did not reverse bear HIT's depression of electrically-induced contraction of guinea pig ileum). The fact that bear HIT's effect of inducing summer hibernation in ground squirrels is effectively blocked in vivo by naloxone leads to the speculation that HIT may be either a precursor of endogenous opioids or a potent releaser of them, which, in turn, induce hibernation.
Bruce
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Department of Biology, Wheaton College, IL 60187.
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D000818:Animals; D002335:Carnivora / Q000097:blood*; D004558:Electric Stimulation; D006168:Guinea Pigs; D006605:Hibernation / Q000187:drug effects*; D007082:Ileum / Q000502:physiology*; D007260:Infusion Pumps; D009119:Muscle Contraction / Q000187:drug effects*; D009197:Myenteric Plexus / Q000502:physiology; D009270:Naloxone / Q000494:pharmacology*; D010455:Peptides; D011506:Proteins / Q000494:pharmacology*; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology; D012589:Sciuridae / Q000502:physiology*; D012621:Seasons; D001503:Ursidae / Q000097:blood*
D010455:Peptides; D011506:Proteins; D011957:Receptors, Opioid; C036932:hibernation induction trigger; D009270:Naloxone