Effect of environmental temperature on the turnover of noradrenaline in hypothalamus and other areas of rat brain

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Abstract
1. The hypothesis that noradrenaline (NA) may be a transmitter in the temperature regulating centre in the hypothalamus is based on the changes in rectal temperature induced by injection of large doses of NA into the brain. As an alternative approach, the effect of environmental temperature on the rate of turnover of endogenous NA in the hypothalamus has been studied.2. Small amounts of tritium labelled noradrenaline [3H]NA were injected into the c.s.f. of rats in order to label radioactively the endogenous NA in the brain. The rats were then exposed to environmental temperatures of 9, 17, 24 and 32° C. The rates of disappearance of [3H]NA from discrete areas of brain were taken as indices of the rates of turnover of endogenous NA in those areas.3. The rate of disappearance of [3H]NA from the hypothalamus was three times as fast at 9 and 32° C as at 17 or 24° C. There were no such significant differences from the pre‐optic area or ‘rest of brain’ (whole brain minus hypothalamus, pre‐optic area, cerebellum and medulla).4. The endogenous concentrations of NA were not altered by the experimental procedures in any of the areas of brain studied.5. The rats maintained normal rectal temperatures at environmental temperatures of 9, 17 and 24° C but became 2·8° C hyperthermic at 32° C.6. It is concluded that mild conditions of both heat and cold resulted in an increased turnover of NA in specific nerve terminals in the hypothalamus. Since the rats were thermoregulating normally, the nerve terminals involved are regarded as forming a part of the central temperature regulating centre.