EFFECT OF LESIONS OF FOREBRAIN CIRCUMVENTRICULAR ORGANS ON C-FOS EXPRESSION IN THE CENTRAL-NERVOUS-SYSTEM TO PLASMA HYPERNATREMIA

Experiments were carried out on conscious adult male Wistar rats to in vestigate the effect of selective ablation of the subfornical organ (S FO), and/or the anteroventral third ventricular (AV3V) region on the i nduction of Fos in central structures in response to plasma hypernatre mia. Fos induction, detected immunohistochemically, was used as a mark er for neuronal activation. Intravenous infusions of hypertonic saline resulted in dense Fos-like immunoreactivity in several forebrain (par aventricular nucleus of the hypothalamus (PVH), supraoptic nucleus (SO N), median preoptic nucleus (MnPO), medial preoptic nucleus, organum v asculosum of the laminae terminalis and SFO) and brainstem (nucleus of the solitary tract, ventrolateral medulla, and parabrachial nucleus) structures. Intravenous infusions of the hypertonic saline solution in to animals with lesions of either the SFO, the AV3V or both resulted i n a decreased number of Fos-like immunoreactive neurons in the MnPO, P VH and SON. In addition, the number of Fos-labeled neurons in the SON after lesions of both the SFO and the AV3V was significantly greater t han that observed in isotonic saline infused controls. Finally, lesion s of the forebrain circumventricular structures did not alter the Fos labeling in brainstem structures as a result of the infusion of the hy pertonic solution. These data suggest that changes in plasma osmolalit y and/or concentration of sodium alter the activity of SON and brainst em neurons in the absence of afferent inputs from the SFO and AV3V.