Effects of alpha-agonists on circulatory responses to somatic afferent nerve stimulation.

The effect of intracisternal (i.c.m.) injection of clonidine, noradrenaline, and piperoxane on the pressor response to electrical stimulation of a peripheral somatic afferent nerve was investigated using anaesthetized cats. It was found that noradrenaline caused a depression of the magnitude of the pressor response and this effect was antagonized by subsequent injection of piperoxane i.c.m. In contrast, clonidine had no significant effect on the magnitude of the somatic pressor reflex but caused a dose-dependent prolongation of the reflex after cessation of nerve stimulation. This prolongation was antagonized by piperoxane which also caused an increase in the magnitude of the reflex. Piperoxane alone had no significant effect on the magnitude or duration of the reflex. Neither magnitude nor duration of the somatic pressor reflex was influenced significantly by a reduction in resting blood pressure. It is suggested that clonidine and noradrenaline act at different sites within the central nervous system to produce qualitatively different changes in the efferent sympathetic discharge pattern modulating the somatic pressor reflex.