Neuronal expression of Fos protein in the rat brain after baroreceptor stimulation

The purpose of this study was to identify the CNS neurons that express Fos protein after repeated activation of the baroreceptor reflex. This was done in Wistar rats anesthetized with urethane and alpha-chloralose with careful physiological controls. The intact control rat showed few Fos-immunoreactive (ir) neurons, whereas the anesthetized control rat showed many Fos-ir neurons in the CNS from the medulla oblongata to the forebrain. After repeated stimulation of baroreceptors by pressor responses to phenylephrine (dose), we counted the amounts of Fos-ir neurons (response). The correlation coefficient of the dose-response relationship was high, and significant only in the medial part of the nucleus tractus solitarii (NTS) in the medulla and periaqueductal gray (PAG) in the midbrain, whereas it was comparatively high but insignificant in the commissure and lateral parts of the NTS, caudal and rostral ventrolateral medulla, periambiguus nucleus, dorsal and ventral medullary reticular nuclei, lateral parabrachial nucleus, paraventricular nucleus thalamus, and dorsomedial nucleus hypothalamus. No significant correlation was found in the humoral control nuclei in the preoptico-hypothalamic structure. Fos expression was never detected in the sensory neurons in the ganglia petrosum and nodosum, and in the sympathetic preganglionic neurons in the intermediolateral nucleus of the thoracic spinal cord. This study shows that Fos expression in the CNS neurons is induced not only by baroreceptor stimulation but also by anesthesia and/or sham-operation, and that Fos expression in the NTSm and PAG neurons faithfully responds to baroreceptor stimulation.