Thermal effects of injecting norepinephrine into hypothalamus of the rat during rest and exercise

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Norepinephrine (NE) was injected bilaterally through implanted guide cannulas into the anterior hypothalamus (AH) of conscious male Sprague-Dawley rats (300-360 g) at rest and just before treadmill exercise (21.5 m/min for 30 min). Colonic (Tc), tail-skin (Ts), and ambient (Ta) temperatures were monitored each minute by use of a telethermometer. At rest (Ta = 23 degrees C), intrahypothalamic injections of 0.5-40.0 micrograms of NE, in a total of 1.0 microliter delivered within 30 s, produced a dose-dependent hypothermia with a 3-5 degrees C rise in Ts. NE (10 micrograms) injected 2 min before exercise (Ta = 23 degrees C) caused an immediate rise in Ts and a mean decline in Tc of 0.9 +/- 0.1 degree C (SE) below resting levels during exercise. This decline in Tc was statistically the same as that observed after the injections of 10 micrograms of NE at rest. Under control exercise conditions, Tc rose approximately 1 degree C and was associated with an initial decline (0.5 degrees C) followed by a rise (4.0 degrees C) in Ts. When the alpha-adrenergic receptor blocker phentolamine was injected 5-15 min before exercise, it caused a significant (P < 0.01) elevation in Tc during the ensuing exercise compared with controls. These results indicate that 1) over a dose range 0.5-40.0 micrograms, NE microinjected into the AH of the rt produces only hypothermia; 2) alpha-adrenergic receptors in the AH participate in the mediation of heat dissipation when the thermoregulatory system is challenged by the endogenous heat stress of exercise; and 3) exercise per se provides a nonthermal input to the temperature regulatory system that enhances heat dissipation.