Somatostatin: a hypothalamic transmitter for thermoregulation in rats.

The changes in both the thermoregulatory responses and brain somatostatin (SS) levels produced by ambient temperature (Ta) changes were assessed in rats after they had been equilibrated to each of the Ta for a period of about 90 min. Cold exposure, in addition to elevating hypothalamic SS-levels, led to increased metabolism and cutaneous vasoconstriction at Ta = 8 degrees C. In contrast, heat exposure, in addition to lowering hypothalamic SS-levels, resulted in decreased metabolism and cutaneous vasodilation at Ta = 30 degrees C. Rats were chronically implanted with a hypothalamic cannula to allow intrahypothalamic injection of SS on the conscious rats. Direct administration of SS (0.1-0.3 micrograms) into the preoptic anterior hypothalamic area caused a dose-related rise in colon temperature at three Ta tested. The SS-induced hyperthermia was produced by increased metabolism at Ta = 8 degrees C, whereas at Ta = 30 degrees C, it was caused by cutaneous vasoconstriction. At Ta = 22 degrees C, the hyperthermia was caused by increased metabolism and cutaneous vasoconstriction. Systemic administration of cysteamine, in addition to lowering hypothalamic SS-levels, produced a dose-related fall in colon temperature at Ta of 8 degrees C and 22 degrees C. The hypothermia induced by cysteamine was produced by decreased metabolism at Ta = 8 degrees C, whereas at Ta = 22 degrees C, it was caused by both decreased metabolism and cutaneous vasodilation. The data indicate that the hypothalamic SS-levels mediate normal body temperature responses in rats.