Heat acclimation: role of norepinephrine in the anterior hypothalamus

Abstract

The hypothesis that anterior hypothalamic (AH) sensitivity to norepinephrine (NE) is altered by chronic exercise in the heat was tested in male Sprague-Dawley rats. Treadmill exercise 6 days/wk for 3 wk at 21 m/min was performed at 23 degrees C (control; C) or at 35 degrees C (heat acclimated; HA), progressing from 20 to 50 min/day in 2 wk. Time for core temperature (Tco) to rise from 39.5 to 40.5 degrees C during a heat-tolerance test after conditioning increased (P less than 0.05) in the HA group. To test for a change in AH sensitivity, the change in Tco to 2-, 5-, 10-, 20-, and 40-micrograms doses of NE injected bilaterally into the AH was determined after conditioning. Dose-response regression lines showed that exercise in the heat increased the slope and shifted the Tco-NE dose relation to the left. In a separate series of experiments on 6 sedentary(s), 10 C, and 10 HA animals, the amounts of NE, dopamine, and 3,4-dihydroxyphenylglycol (DOPEG) were determined by high-pressure liquid chromatography in the AH, median preoptic area (PO), cortex, and cerebellum after 9 wk of conditioning. Results showed that in the PO there was a significant increase in NE and DOPEG in the HA vs. C group and a trend of increasing NE from the S to C to HA groups. The data indicate that exercise in the heat increases NE-induced peripheral heat-dissipating capacity and increases catecholamine storage in the PO.