TEMPERATURE-DEPENDENT EFFECTS OF ALPHA-ADRENERGIC AGONISTS AND ANTAGONISTS IN THE COLD

This series of experiments examined whether temperature-dependent effe cts of the alpha-antagonists prazosin and yohimbine compromised their use as blockers of alpha-adrenergic agonist responses in cold-exposed rats. An operant leverpressing task was used to measure the demand for heat in a cold environment. The alpha(1)-antagonist prazosin had mode st effects, but the alpha(2)-antagonist yohimbine was thermolytic in t hat it dose dependently increased operant responding but decreased pos ttest colonic temperature (Tc). These potent effects of the alpha(2)-a ntagonist led to tests of the alpha(2)-agonist clonidine. Clonidine in creased operant responding for heat to an extraordinary degree, result ing in significant increases in posttest Tc. However, clonidine was fo und to be a hypothermic agent when tested in rats at 5 degrees C but d enied the opportunity to increase body temperature by operant lever pr essing, suggesting a central effect on the control of thermal balance. Measurement of changes in metabolic rate at 5 and 23 degrees C showed that yohimbine increased metabolism at 23 degrees C but decreased it in the cold. Prazosin had little effect on metabolism or Tc at either temperature. Prazosin inhibited the decrease in Tc induced by norepine phrine (NE), but had little effect on the lever-pressing response. Yoh imbine had no significant antagonistic effect on NE-induced changes in lever-pressing behavior or posttest Tc, but neither did the thermolyt ic effects of yohimbine exacerbate those of NE. These results show tha t ct-antagonist interactions with agonists can be complicated by tempe rature-dependent effects of each.