Pharmacological modifications of sympathetic responses elicited by hypothalamic stimulation in the rat

1 . The rise in blood pressure and the eyelid contractions elicited by electrical stimulation of the posterior hypothalamus in anaesthetized rats were studied for the assessment of drug effects on the sympathetic system. They were compared with the noradrenaline pressor effect and eyelid responses to cervical sympathetic nerve stimulation. 2 . Phentolamine caused a similar reduction of the pressor responses induced by hypothalamic stimulation and by noradrenaline. It also reduced the eyelid contractions elicited by central and by peripheral electrical stimulation. 3 . Guanethidine caused an immediate inhibition of the pressor response to hypothalamic stimulation, while it potentiated noradrenaline effects. Eyelid contractions elicited both by central and by peripheral electrical stimulation were inhibited. 4 . Chlorpromazine inhibited the pressor responses both to hypothalamic stimulation and to noradrenaline, but it caused a much greater reduction in the centrally evoked eyelid responses than in those due to peripheral stimulation. 5 . Diazepam caused a reduction of sympathetic responses to central stimulation but not to peripherally elicited responses. 6 . In unanaesthetized rats the rise in blood pressure induced by hypothalamic stimulation was accompanied by increased locomotor activity culminating in a flight reaction. In contrast to the pressor effect, which was reduced by all four of the above‐mentioned drugs, the flight reaction was not affected by phentolamine and guanethidine and only delayed by chlorpromazine and diazepam at dose levels which impaired the spontaneous locomotor activity.