Seasonal difference in thermoregulatory responses to opiates in a mammalian hibernator

Abstract

Accumulated evidence suggests that increased endogenous opioid activities may facilitate the onset of hibernation. The present study investigated the change in thermoregulatory responses following ICV infusion of morphine or [D-Ala 2]-Met enkephalinamide (EK) in unanesthetized, unrestrained Columbian ground squirrels ( Spermophilus columbianus) during its annual hibernation cycle. In the nonhibernating phase, low doses of either morphine (<160 μg) or EK (<400 μg) elicited a dose-related hyperthermia and an increase in heat production, whereas a higher dose of opiates caused hypotherma and a decrease in metabolic rate. Naloxone (5 mg/kg, SC) pretreatment reduced or reversed both the hyper- and hypothermic responses to opiates. Lower ambient temperature (5°C) enhanced the hypothermic response and attenuated the hyperthermic response. In the hibernating phase, euthermic ground squirrels exhibited reduced responsiveness to exogenous opiates: the hyperthermic response to low dose of morphine (10 μg) was significantly reduced and hyperthermia, rather than hypothermia was observed at the highest dose of morphine (160 μg). The reduced responsiveness to opiates observed during the hibernating phase seems to suggest a reduction in opiate receptor efficacy which is in agreement with the contention that an increase in endogenous opioid activities may be incumbent with the commencement of hibernation.