Neurally mediated cardiovascular responses to stimulation of cell bodies in the hypothalamus of the rat.

To study the cardiovascular responses to selective activation of neuronal cell bodies in the hypothalamus, DL-homocysteate (5-50 nl of a 0.15-M solution, pH 7.4) was injected into 417 histologically verified sites in the hypothalamus of 46 urethan-anesthetized, paralyzed and artificially ventilated rats. Injections resulted in depressor responses (-5 to -32 mm Hg) in 271 sites, in pressor responses (5-47 mm Hg) in 77 sites and 69 sites were not responsive. Depressor effects had a shorter latency (85% started within 5 s) than pressor effects (42% started within 5 s). Control injections of 0.15 M NaCl into 126 of the responsive sites were ineffective. Arterial pressure (AP) responses showed a positive correlation (r = 0.61, P less than 0.001) with changes in heart rate (HR). Analysis of the anatomical distribution of responsive sites showed that in all hypothalamic subdivisions depressor responses predominated except in the paraventricular nucleus, where mainly pressor effects and tachycardia were elicited. These results demonstrate that excitation of cell bodies in most hypothalamic regions elicits neurally mediated changes in AP and HR and that the traditional functional division of the hypothalamus into a rostral depressor and a caudal pressor area is probably based on the combined excitation of fibers of passage and cell bodies.