EFFECTS OF SINOAORTIC DENERVATION ON FOS EXPRESSION IN THE BRAIN EVOKED BY HYPERTENSION AND HYPOTENSION IN CONSCIOUS RABBITS

We have previously shown [Li and Dampney (1994) Neuroscience 61, 613-6 34] that periods of sustained hypertension and hypotension each induce s a distinctive and reproducible pattern of neuronal expression of Fos (a marker of neuronal activation) in specific regions of the brainste m and forebrain of conscious rabbits. The aim of this study was to det ermine the contribution of afferent inputs from arterial baroreceptors to the activation of neurons in these various brain regions that is c aused by a sustained change in arterial pressure. Experiments were car ried out on rabbits in which the carotid sinus and aortic depressor ne rves were cut in a preliminary operation. Following a recovery period of seven to 10 days, a moderate hypertension or hypotension (increase or decrease in arterial pressure of 20-30 mmHg) was induced in conscio us barodenervated rabbits for 60 min by the continuous infusion of phe nylephrine or sodium nitroprusside, respectively. In control experimen ts, barodenervated rabbits were subjected to the identical procedures except that they were infused with the vehicle solution alone. Compare d with the effects seen in barointact rabbits, [Li and Dampney (1994) Neuroscience 61, 613-634] the number of neurons that expressed Fos in response to hypertension was reduced by approximately 90% in the nucle us of the solitary tract and in the caudal and intermediate parts of t he ventrolateral medulla. In supramedullary regions, baroreceptor dene rvation resulted in a reduction of approximately 60% in hypertension-i nduced Fos expression in the central nucleus of the amygdala and in th e bed nucleus of the stria terminalis, but no significant reduction in the parabrachial complex in the pons. Following hypotension, the numb er of neurons that expressed Fos in barodenervated rabbits, compared w ith barointact rabbits, [Li and Dampney (1994) Neuroscience 61, 613-63 4] was reduced by approximately 90%, in the nucleus of the solitary tr act, area postrema, and caudal, intermediate and rostral parts of the ventrolateral medulla. Baroreceptor denervation also resulted in a sim ilar large reduction in hypotension-induced Fos expression in many sup ramedullary regions (locus coeruleus, midbrain periaqueductal grey, hy pothalamic paraventricular nucleus, and in the central nucleus of the amygdala and the bed nucleus of the stria terminalis in the basal fore brain). In the supraoptic nucleus, hypotension-induced Fos expression in barodenervated rabbits was reduced by 75% compared to barointact an imals, but was still significantly greater than in control animals. Th ere was also a high level of Fos expression, much greater than in cont rol animals, in the circumventricular organs surrounding the third ven tricle (subfornical organ and organum vasculosum lamina terminalis). T he results indicate that in conscious rabbits the activation of neuron s that occurs in several discrete regions at all levels of the brain f ollowing a sustained change in arterial pressure is largely dependent upon inputs from arterial baroreceptors, with the exception of neurons in the circumventricular organs surrounding the third ventricle that are activated by sustained hypotension. The latter group of neurons ar e known to project to vasopressin-secreting neurons in the supraoptic nucleus, and may therefore via this pathway trigger the hypotension-in duced release of vasopressin that occurs in tile absence of barorecept or inputs. (C) 1997 IBRO.