Pharmacological evidence that circumventricular organs may participate in the pressor effect of central serotonin.

The present study was to investigate the role of '5-HT2-like' receptors in the circumventricular organs in the pressor response elicited by the central injection of serotonin (5-HT) to conscious rats. The increase in blood pressure induced by intracerebroventricular (i.c.v.) 5-HT (2.5 micrograms) was attenuated by intravenous pretreatment will, the peripheral 5-HT2 antagonist xylamidine (100 or 300 micrograms/kg). A combination of xylamidine with prazosin or a V1-vasopressin antagonist did not produce a further attenuation in the pressor response. Our data suggest that when administered i.c.v., 5-HT increases blood pressure by acting on 5-HT2-like receptors located in circumventricular organs as well as areas of the brain protected by the blood-brain barrier.