Effects of selective sinoaortic denervations on phenylephrine-induced activational responses in the nucleus of the solitary tract

Intravenous administration of phenylephrine provokes a pattern of cellular activation in the nucleus of the solitary tract that resembles the central distributions of primary baroreceptor afferents supplied by the carotid sin us and aortic depressor nerves. Transganglionic transport and denervation m ethods were used in an experimental setting to test the dependence of pheny lephrine induced Fos immunoreactivity on the integrity of buffer nerve affe rents, and to identify the subregions of the nucleus of the solitary tract supplied by each. Cholera toxin B-horseradish peroxidase injections into ei ther or both nerves revealed terminal labeling concentrated in, but not res tricted to, the dorsal commissural part of the nucleus of the solitary trac t at the level of the apex of calamus scriptorius, and extending into the d orsal subnucleus at the level of the area postrema. Preferential ramificati ons of carotid sinus and aortic depressor nerve afferents at the levels of the commissural part of the nucleus and the area postrema, respectively, we re reflected in the extent to which labeled fibers comingled with neurons e xhibiting phenylephrine-induced Fos in dual labeling experiments. Complete sinoaortic denervation reduced by 90% the number of neurons exhibiting drug -induced Fos expression. Selective carotid and aortic sinus denervations ef fected partial reductions manifest preferentially in the caudal and rostral foci of the distribution, respectively. Reduced activational responses at the level of the area postrema of aortic sinus-denervated rats were accompa nied by a reduction in cellular nicotinamide adenine dinucleotide phosphate -diaphorase activity in this region. Animals killed 30 days after complete sinoaortic denervation displayed no evidence of recovery of phenylephrine-i nduced Fos, while the strength and distribution of the response in rats tha t received selective carotid sinus denervation were indistinguishable from those seen in controls. These findings (i) support the dependence of phenylephrine induced Fos expr ession on the integrity of carotid sinus and aortic depressor nerve afferen ts, (ii) provide anatomical and functional evidence that the two buffer ner ves distribute differentially within the nucleus of the solitary tract, and (iii) implicate central reorganization as a likely basis for functional re covery of baroreflex mechanisms following partial sinoaortic denervation. ( C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.