Acute baroreceptor unloading evokes Fos expression in anesthetized rat brain.

Functional organization of brain pathways subserving the baroreceptor reflex was investigated by mapping immunoreactivity of Fos protein, a neuronal activity marker, in response to acute baroreceptor unloading in anesthetized rats. Compared with normal control and sham operation, sinoaortic denervation (SAD) evoked a distinctive pattern of Fos expression in the nucleus tractus solitarii (NTS), the primary termination of baroreceptor afferents. The sinoaortic denervation also induced a prominent and reproducible Fos expression in specific regions of the brainstem and forebrain, which receive direct or indirect inputs from the nucleus tractus solitarii. These brain regions included the rostral ventrolateral medulla (RVLM), lateral parabrachial nucleus (LPBN), hypothalamic paraventricular nucleus (PVN), supraoptic nucleus (SON), and central amygdaloid nucleus (CeA). These findings help us to identify brain regions that are specifically responsive to decreased arterial baroreceptor activity, without the accompanying confounding variables of behavioral arousal or stress.