Studies relating morphine hypothermia with serotonin reuptake in the rat hypothalamus

Experiments were conducted to determine if the acute hypothermia elicited by morphine in the rat resulted from morphine-induced inhibition of serotonin reuptake into hypothalamic nerve-endings. The acute hypothermia elicited by morphine (30 mg/kg, s.c.) was reversed by pretreatment with naloxone (10 mg/kg, s.c.), a narcotic antagonist, which alone did not alter body temperature. In vitro uptake of serotonin into synaptosomes prepared from rat hypothalamus was inhibited 53% by the addition of morphine (7 X 10(-4) M) and 53.9% by naloxone (1 X 10(-3) M). Simultaneous addition of both drugs in these same concentrations further inhibited synaptosomal serotonin uptake 77.8%. Uptake of serotonin uptake into synaptosomes isolated from rats injected with morphine (30 mg/kg, s.c.) was not altered as compared to controls. These data suggest that the acute hypothermic acition in morphine in the rat is not related to an inhibition of the serotonin reuptake mechanism in hypothalamic nerve-endings.