Effect of acute and chronic morphine treatment on serotonin uptake into rat hypothalamic synaptosomes

The effect of morphine on uptake of serotonin into hypothalamic synaptosomes was determined following acute and chronic morphine treatment in the rat. The uptake of serotonin was noncompetitively inhibited by in vitro morphine (IC50 = 7 X 10(-4) M) but not by an acute morphine treatment (30 mg/kg s.c.) which produces hypothermia. Uptake of serotonin into hypothalamic synaptosomes derived from rats rendered tolerant to the hypothermic effect of morphine or rats withdrawn from morphine was not altered either in the absence or presence of in vitro morphine as compared to control rats. These data indicate that alterations in thermoregulatory responses of the rat to either acute or chronic morphine treatment do not result from morphine-induced modification of serotonin uptake into hypothalamic neurons.