The influence of the paraventricular nucleus on baroreceptor dependent caudal ventrolateral medullary neurones of the rat.

The question of whether neurones in the paraventricular nucleus (PVN) of the hypothalamus have an influence on barosensitive vasomotor neurones of the caudal ventrolateral medulla (CVL) was addressed in this study using anaesthetised rats. Extracellular microelectrode recordings were made from 27 vasomotor neurones in the CVL, identified by their cardiac cycle-related probability of discharge, by the increase in activity in response to an increase in arterial blood pressure produced by i.v. phenylephrine and by the decrease in activity in response to a decrease in blood pressure produced by i.v. nitroprusside. Activation of neurones at different sites in the PVN with a microinjection of d,l-homocysteic acid (DLH) activated 12 CVL neurones and inhibited 6 CVL neurones. In four CVL units single-shock electrical stimulation at a PVN pressor site, first identified with DLH, elicited a synaptic action potential with a latency of 29+/-0.3 ms. It is concluded that PVN neurones can influence the CVL vasomotor neurones directly. This could be one means by which PVN-depressor and sympatho-inhibitory effects are produced.