Fine structure and plasticity of barosensitive neurons in the nucleus of solitary tract

Intravenous phenylephrine (PE) activates neurons in the nucleus of the soli tary tract (NTS) whose distribution conforms to those of central projection s of the carotid sinus and aortic depressor nerves. This was exploited to p ermit fine structural characterization of cells presumed to compose the fir st station in the processing of arterial baroreceptor input, and their resp onses to stimulation. Rats were perfused at varying intervals after PE inje ction, and sections through the baroreceptor afferent zone of the NTS prepa red for preembedding immunolocalization of Fos-immunoreactivity. Labeled ne urons composed a continuous strip extending from the dorsal part of the com missural NTS (NTScom) to the dorsal subnucleus at the level of the area pos trema (NTSap). PE-sensitive neurons in these regions were medium-sized, rou nd to ovoid in shape, with scant cytoplasm and an unremarkable complement o f organelles. Distinctive features included extensively invaginated nuclei and well-developed Golgi apparati; Fos-ir cells in the NTSap were distingui shed from those in NTScom by virtue of better-developed rough endoplasmic r eticulum and Golgi, and less convoluted nuclei. Proximal synaptic input to PE-sensitive neurons was sparse and was provided by terminals containing pr edominantly small, clear synaptic vesicles that formed mainly symmetric jun ctions with somata and primary dendrites. Prolonged stimulation was accompa nied by accentuation of nuclear invaginations, marked accumulation of heter ochromatin at their apices, and evidence of enhanced Golgi activity (vesicu lar budding). These may represent adaptations to facilitate changes in gene expression, to maintain neurotransmitter availability, or both, in the fac e of a persistent hypertensive challenge. J. Comp. Neurol. 422:338-351, 200 0. (C) 2000 Wiley-Liss, Inc.