SEROTONIN IS NOT SYNTHESIZED, BUT SPECIFICALLY TRANSPORTED, IN THE NEURONS OF THE HYPOTHALAMIC DORSOMEDIAL NUCLEUS

A small group of neurons in the hypothalamic dorsomedial nucleus (DMN) have been reported to contain serotonin after pharmacological treatme nts enhancing brain serotonin levels. This study aimed at elucidating whether these neurons are able to synthesize serotonin de novo, and wh ether they possess a specific serotonin transport mechanism. Serotonin content in these neurons was raised by administration of L-tryptophan and pargyline. Double immunostaining for serotonin and tryptophan hyd roxylase (TpOH), the serotonin synthesizing enzyme, revealed that none of the serotonin-containing neuronal somata expressed TpOH. Intracere broventricular colchicine treatment did not result in TpOH-IR in these neurons. Fluoxetine, a specific serotonin transport inhibitor, preven ted the accumulation of serotonin in these neurons. The present result s thus indicate that the serotonin-containing DMN neurons are not able to synthesize serotonin. Instead, they take up exogenous serotonin vi a a specific serotonin transport mechanism. As serotonin and DMN are a ssociated with various physiological functions, such as regulation of food intake and modulation of fear and anxiety, the mechanisms reveale d in the present study may participate in these clinically important b rain functions.