Hypothalamic modulation of baroreceptor afferent unit activity.

Unit responses to sinus nerve stimulation were recorded in the medulla. A conditioning stimulus to the posterior hypothalamus produced inhibition of 65% of unit responses to sinus nerve stimulation as early as 7 ms and extending as long as 790 ms after conditioning; 50% recovered after 300 ms. Unit responses to hypothalamic stimulation alone were also recorded in the medulla, some in the same loci as other unit responses to sinus nerve stimulation. They could be activated by contralateral as well as ipsilateral hypothalmic stimulation and showed recurrent bursts of firing over a 1,000-ms poststimulus interval. Evoked potentials and unit responses were recorded in the posterior hypothalamus, some occurring within 10--20 ms poststimulation of the sinus nerve, indicating that baroreceptor information is ascending in a time sufficiently short to involve the hypothalamus in reflex regulation of blood pressure as well as more generalized homeostatic responses which include the cardiovascular system.