Cold stress or a pyrogenic substance elevates thyrotropin-releasing hormone levels in the rat hypothalamus and induces thermogenic reactions.

ABSTRACT

Both thyrotropin-releasing hormone (TRH) levels in the hypothalamus and thermoregulatory responses were assessed in rats after they had been equilibrated to each of three ambient temperatures (Ta 8, 22 and 30 degrees C) tested. Cold exposure, in addition to elevating TRH levels in the hypothalamus, led to increased metabolism and cutaneous vasoconstriction in rats at Ta = 8 degrees C. In contrast, heat exposure resulted in decreased metabolism and cutaneous vasodilatation in rats accompanied by no change in hypothalamic TRH levels at Ta = 30 degrees C. In addition, rats were chronically implanted with a cerebroventricular cannula to allow administration of the pyrogenic substance polyriboinosinic acid-polyribocytidylic acis (Poly IC) into the brain at Ta = 22 degrees C. Intracerebroventricular administration of Poly IC, in addition to elevating hypothalamic TRH levels, produced a fever with a latency of onset of about 30 min. The fever induced by Poly IC was brought about by increased metabolism and cutaneous vasoconstriction in rats. The results suggest that either cold stress or Poly IC injection elevates TRH levels in rat hypothalamus and thus induces thermogenic reactions.