Seasonal variation in opioid modulation of feeding in the 13-lined ground squirrel.

Opioids are well recognized to modulate ingestive behaviors in a variety of species. To study the potential role of opioids in the alteration of ingestive behaviors that occur prior to hibernation, we have administered opiate agonists and the antagonist, naloxone, to the 13-lined ground squirrel (Spermophilus tridecemlineatus) during periods when they were hyperphagic and hypophagic. Naloxone consistently reduced feeding during both phases of the feeding cycle. Hypophagic animals, however, were 10 times more sensitive to the effects of naloxone. The effect of the opiate agonists (morphine and butorphanol, 1 and 10 mg/kg) on feeding also varied between seasons. The low dose of morphine produced a slight, but significant increase in feeding at one hour in hyperphagic animals, while the high doses tended to decrease feeding. When administered to hypophagic animals, feeding was decreased by both doses of each agonist. Immunoreactive (IR)-dynorphin levels in the cortex, hypothalamus and striatum were higher during the hypophagic phase compared with the hyperphagic phase. These data indicate that the 13-lined ground squirrel possesses an opiate sensitive feeding system which is affected by season.