Effects of cold acclimation and central opioid processes on thermoregulation in rats

Abstract

Two experiments, using centrally administered [ d-Ala 2-MePhe 4-Gly(ol) 5]enkephalin (DAMGO), a selective μ-opioid agonist, assessed the thermoregulatory consequences of cold acclimation. Experiment 1 assessed whether cold acclimation influenced DAMGO hyperthermia at room temperature. Sialoadenectomized rats were implanted with ICV cannulae and IP Mini-Mitters. After 3 weeks of exposure to 5 °C (cold acclimation) or 22 °C (non-cold acclimation) rats were pretreated with IP naltrexone HC1 (2 mg/kg b.wt.) or vehicle (0.15 M saline) and later administered a 5-μI ICV injection of 0.15 M saline, 0.1, or 1.0 μg DAMGO. Cold acclimation exerted little effect on core temperature but potentiated DAMGO hyperthermia in a dose-dependent, naltrexone-reversible, activity-independent manner. Experiment 2 assessed the effect these same manipulations exerted on operant escape from a convective source of mild heat (37 °C). Duration of heat escape increased with cold acclimation in a naltrexone-resistant manner, yet was not influenced by DAMGO in either non-cold-acclimated or cold-acclimated rats. These findings suggest that two central adaptations occur with cold acclimation: A non-μ-opioid process that increases heat sensitivity and a μ-opioid process that potentiates hyperthermia but fails to alter heat escape due to μ-opioid-mediated analgesia.