Effects of repeated heat exposure on hypothalamic sensitivity to norepinephrine

Bilateral stainless steel guide tubes were stereotaxically implanted above the anterior hypothalamus (AH) of 12 male Sprague-Dawley rats (300-360 g). Beginning 3 days postsurgery, norepinephrine (NE) was injected into the AH each day at increasing depths below the guide tube until colonic temperature fell at least 0.8 degrees C. Subsequently, the effects of treadmill exercise at 21.5 m/min for 3 wk at 22 degrees C (n = 6) or 35 degrees C (n = 6) on the magnitude of the NE-induced hypothermia were investigated. Training duration ranged from 12 to 50 min/day, depending on the time required for colonic temperature of the heated rats to rise to a predetermined level: 40.4 degrees C on day 1, increasing 0.1 degree C/day to 41.5 degrees C on day 12 and thereafter. Before and after training, bilateral injections of NE (10 micrograms) at 22 degrees C at rest and just before exercise (21.5 m/min) caused the following mean +/- SE reductions in colonic temperature (degree C). (Formula: See Text). These reductions in colonic temperature were associated with sharp elevations in tail-skin temperature, but there were no significant differences in the response between the two groups. We conclude that repeated heat exposure increased the sensitivity of the AH to exogenous NE.