Changes in cold- and heat-defence following electrolytic lesions of raphe nuclei in the guinea-pig

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Abstract
In conscious guinea-pigs the effects of electrolytic lesions of the nucleus raphe magnus (NRM) and of adjacent areas in the lower brainstem on cold- and heat-defence were studied. Changes in core, temperature, heat production and heat loss effectors as well as their threshold temperatures were compared in the same animals before and after lesioning. As an additional index of heat loss, ear skin temperature and a derived parameter — vasomotor index — were also measured. Three days after NRM lesion the fall in core temperature evoked by an exposure to 14–15°C was smaller than before lesion, furthermore the body temperature threshold for shivering increased. Cold-induced heat production was also higher following NRM lesions. Lesions outside the NRM or sham-operation did not influence colddefence. After NRM lesion heat-defence was also improved; the rise in core temperature elicited, by an exposure to 36–37°C was smaller and the body temperature threshold for car skin vasoconstriction during recovery from hyperthermia decreased. No change in respiratory evaporative heat loss could be observed after NRM lesion. Lesions outside the NRM or sham-operation did not influence, heat-defence. An attempt has been made to explain the observed improvements in cold- and heat-defence by discussing relevant data on mechanisms in central temperature control.