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

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 I:C) into the brain at Ta = 22 degrees C. Intracerebroventricular administration of Poly I:C, in addition to elevating hypothalamic TRH levels, produced a fever with a latency of onset of about 30 min. The fever induced by Poly I:C was brought about by increased metabolism and cutaneous vasoconstriction in rats. The results suggest that either cold stress or Poly I:C injection elevates TRH levels in rat hypothalamus and thus induces thermogenic reactions.