1. In rats the injection of capsaicin into the pre‐optic area of the anterior hypothalamus produces a prompt fall in body temperature and abolishes shivering. With repeated injections of capsaicin the hypothermic effect gradually diminishes and finally vanishes (local desensitization). 2. Rats desensitized by hypothalamic injections exhibit a behaviour similar to rats pre‐treated parenterally with capsaicin: put in a heat box at 37–39° C they lose their ability to regulate against overheating of their bodies and respond with an enhanced hyperthermia to strong sensory stimuli such as repeated pinching of the tail. 3. Parenteral desensitization strongly inhibits the effect of capsaicin given into the hypothalamus. On the other hand in intrahypothalamically desensitized rats the hypothermic response to subcutaneously given capsaicin is also reduced. 4. The hypothermic response to local heating of the anterior hypothalamus by diathermy (from 1 to 4° C above the initial temperature) was markedly reduced or even abolished in rats pre‐treated parenterally with large doses of capsaicin. 5. It is concluded that the hypothalamic warmth detectors are stimulated and subsequently desensitized by capsaicin. Thus, in the thermoregulatory disturbances caused by capsaicin the impairment of the hypothalamic warmth detectors plays an important role. 6. Capsaicin is proposed as a tool in studying the function of the hypothalamic warmth detectors.
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