The intracisternal administration of 5,7-dihydroxytryptamine (5,7-DHT) to rats resulted in a potentiated response to 5-hydroxytryptophan (5-HTP) when the animals were tested 30 days later. The 5-HTP-induced changes include elevation of serum prolactin decrease in operant responding, and the magnitude of the "serotonin behavioral syndrome" observed after 5-HTP administration. The serotonin concentration in brains of 5,7-DHT-treated animals reached maximum earlier and remained elevated longer than that of controls following administration of 5-HTP. Brain norepinephrine and dopamine concentration were not affected by 5-HTP in either group of animals. The increase in serum prolactin concentration elicited by administration of the serotonergic agonists quipazine or 5-methyxy-N,N-dimethyltryptamine and by the serotonin uptake inhibitor fenfluramine also was potentiated by pretreating rats with 5,7-DHT. These data suggest that both serotonergic receptor supersensitivity and the absence of presynaptic uptake sites contribute to the enhanced responses to 5-HTP occurring in rats previously treated with 5,7-DHT. The findings further demonstrate that both behavioral and hormonal measures can be used to assess the sensitivity of serotonergic receptors and indicate that 5,7-DHT may be useful in evaluating the role of serotonergic neurons in neuroendocrine function.
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