Involvement of 5-hydroxytryptamine in the blood pressure changes evoked by electrical stimulation of the median raphe nucleus.

Electrical stimulation of the median raphe nucleus (MRN) in urethane-anaesthetised rats decreased systemic blood pressure at low intensities of stimulation (5-25 microA) and increased it with higher intensities (20-150 microA). Prazosin (0-5-5.0 micrograms/Kg i.v.) dose-dependently attenuated pressor responses concurrently with a reduction in the responsiveness of peripheral alpha-adrenoceptors to phenylephrine (500ng i.v.). Methiothepin (5-10 micrograms/Kg i.v.) abolished depressor responses and reduced the pressor effects without altering the response to phenylephrine. Ketanserin (5-10 micrograms/Kg i.v.) abolished depressor changes and potentiated pressor responses. High doses (20-200 micrograms/Kg) produced a decrease in pressor responses but correspondingly lowered BP and reduced the response to phenylephrine. The results suggest the presence of 5-HT-containing links between the MRN and the peripheral cardiovascular effector systems.