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

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Peripheral desensitization was induced in 36 2–4 day old Wistar male rats by sc administration of capsaicin in increasing doses (350; 700; 1,400; 2,100; and 2,800 μg) on 5 consecutive days. When adult, 9 treated and 5 control Ss were chronically implanted with a lateral icv guide cannula. This tube was used for icv capsaicin injection. 11 treated Ss and 6 controls were implanted with a hypothalamic thermode. Hypothalamic heating was activated with the aid of this water-perfused thermode. 28 Ss, 12 control and 16 treated, were injected ip and sc with capsaicin. Results show that in control Ss, 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 Ss, 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 Ss thermoregulatory behavior for fresh air was identical to that of controls at all ambient temperatures before and during hypothalamic heating. Results suggest that the central heat receptors responsible for autonomic and behavioral thermoregulation in the treated Ss functioned normally when icv capsaicin was injected or when the hypothalamus was 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). (30 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)

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