Dissociation between peripheral and central heat loss mechanisms induced by neonatal capsaicin

Fifty-eight 4-day-old male rats were studied. Peripheral desensitization was induced in 36 rats by sc administration of capsaicin in increasing doses (350, 700, 1,400, 2,100, and 2,800 micrograms) on 5 consecutive days. When adult, 9 treated and 5 control rats were chronically implanted with a lateral intracerebroventricular (icv) guide cannula. This tube was used for icv capsaicin injection. Eleven treated rats and 6 controls were implanted with a hypothalamic thermode. Hypothalamic heating was activated with the aid of this water-perfused thermode. Twenty-eight rats, 12 control and 16 treated, were injected ip and sc with capsaicin. Results showed that in control rats, icv, ip, and sc capsaicin injections produced a fall in body (core) temperature and an increase in cutaneous temperature. These changes are attributed to cutaneous vasodilation. In capsaicin-treated rats, sc and ip injections had no effect on body temperature. However, neonatal treatment did not inhibit the effect of capsaicin on body temperature. Furthermore, in these rats thermoregulatory behavior for fresh air was identical to that of controls at all ambient temperatures before and during hypothalamic heating. These results suggest that the central heat receptors responsible for autonomic and behavioral thermoregulation in the treated rats function normally when icv capsaicin is injected or when the hypothalamus is heated. In contrast, neonatal injection of capsaicin produced an inhibition of the heat loss mechanisms induced by ip or sc capsaicin, i.e., by stimulation of the peripheral nervous system.