Stimulation of the paraventricular nucleus modulates firing of neurons in the nucleus of the solitary tract

The present study assessed whether the baroreflex inhibition elicited by electrical stimulation of the hypothalamic paraventricular nucleus (PVN) involves altered activity in the nucleus of the solitary tract (NTS). Unit recordings were made from 107 neurons in the NTS in anesthetized rabbits, Intravenous phenylephrine was used to induce a presser response and to activate baroreflexes. Of the neurons that responded to presser responses, two-thirds were excited and one-third was inhibited. Stimulation of the PVN inhibited 70% of the phenylephrine-responsive NTS neurons, with or without concurrent baroreceptor stimulation. When PVN stimulation was delivered concurrently with phenylephrine injection, more NTS neuronal inhibition and less excitation occurred than with phenylephrine alone. Usually PVN stimulation inhibited NTS neurons that were excited by presser responses; less commonly, PVN stimulation excited NTS neurons that were inhibited by presser responses. The findings are consistent with the view that PVN activation during the defense reaction inhibits baroreflexes by altering firing of NTS neurons.