Serotonin outflow in the hypothalamus of conscious rats: origin and possible involvement in cardiovascular control

The push-pull technique was used to investigate the effects of neuroactive compounds and experimentally induced blood pressure changes on the release of endogenous serotonin in the posterior hypothalamic area of the rat. Hypothalamic superfusion with artificial cerebrospinal fluid which contained 80 mM K+ or 1 microM veratridine enhanced the rate of serotonin release. Superfusion with tetrodotoxin (5 microM) led to a pronounced decrease in the serotonin release rate. Increases in blood pressure elicited by intravenous infusions of noradrenaline (3-4 micro g/kg/min) or phenylephrine (10 microg/kg/min) enhanced the release of serotonin in the hypothalamus. Similarly, the serotonin release rate was enhanced by hypervolaemia. Decreases in blood pressure elicited by intravenous administration of nitroprusside (30-40 microg/kg/min) or chlorisondamine (3 mg/kg) reduced the release of serotonin. Likewise, the serotonin release rate was decreased by hypovolaemia. With one exception (hypothalamic superfusion with tetrodotoxin) neither neuroactive drugs, nor experimentally elicited blood pressure changes modified the release rate of the metabolite 5-hydroxyindoleacetic acid (5-HIAA). These findings show that changes in blood pressure lead to counteractive alterations in the release of serotonin. Thus, serotoninergic neurons of the posterior hypothalamus seem to be involved in the homeostasis of blood pressure by exerting a hypotensive function. At least in the hypothalamus, the concentration of 5-HIAA in the superfusate does not seem to be a reliable marker for the activity of serotoninergic neurons.