Hypothermic Responses to Intrahypothalamic Capsaicin in the New-Born Rat
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Capsaicin has been known to produce a hypothermia in the adult animal, probably by its action on the warm-detectors responsible for thermoregulation. The effect of capsaicin on the body temperature was studied in the new-born rats aged 6 hours to 5 days old. Under ether anesthesia, a stainless-steel guide cannula (0.5mm, o.d.) was subacutely implanted into the brain, aimed more or less at the preoptic area and anterior hypothalamus (PO/AH) . The animals were held in a prone position on a sponge rubber mat in a'neutral' enviroment where the animal could maintain the rectal temperature (T<SUB>re</SUB>) at 36-37°C. The subcutaneous injection of capsaicin (5-10μg) produced a sudden fall in T<SUB>re</SUB> (1-2°C) . With repeated injections of increasing doses, the hypothermia diminished progressively and finally the animal became completely unresponsive to a large amount (1mg) of the drug (desensitization) . Direct injection of capsaicin (5μg) into PO/AH also produced a prompt fall in T<SUB>re</SUB> even in 1 dayold rats and the repeated injection resulted in the diminished responses. The injection outside PO/AH or the injection of solvent alone had no effect. Since the PO/AH warm-sensitive neurons in the new-born rat specifically respond to capsaicin, it is suggested that the capsaicin-induced hypothermia is brought about, at least in part, by its action on the PO/AH warm-sensitive neurons. These results indicate that the neonatal PO/AH warm-sensitive neurons attain some degree of maturity to respond to capsaicin, to lower the body temperature and to be desensitized by capsaicin.
- 日本生気象学会の論文
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