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 μA) and increased it with higher intensities (20–150 μA). Prazosin (0-5-5.0 μgKg i.v.) dose-dependently attenuated pressor responses concurrently with a reduction in the responsiveness of peripheral alpha-adrenoceptors to phenylephrine (500ng i.V.). Methlothepin (5–10 μgKg i.v.) abolished depressor responses and reduced the pressor effects without altering the response to phenylephrine. Ketanserin (5–10 μgKg i.v.) abolished depressor changes and potentiated pressor responses. High doses (20–200 μgKg) produced a decrease in pressor responses but correspondingly lowered BP and reduced the response to phenylephrlne.