Dissociation of Fos-like immunoreactivity in lamina terminalis and magnocellular hypothalamic nuclei induced by hypernatremia.

Rats were given either slow (1 h) or rapid (10 min) intravenous infusions of either 6 or 12 mmol NaCl/kg body weight. Fos-like immunoreactivity (FLI) induced by the infusions was measured in several brain regions. The higher dose of NaCl induced FLI in structures of the lamina terminalis, including organum vasculosum (OVLT) and subfornical organ (SFO), as well as in the magnocellular supraoptic (SON) and paraventricular hypothalamic (PVN) nuclei. The lower dose of hypertonic NaCl induced FLI in only the SON and PVN. Faster delivery of the solute load tended to amplify the FLI in SFO and OVLT. These data confirm and extend previous reports of osmotically-induced FLI in rat brain and demonstrate that the discrepancies between these studies result from different dosage regimens of NaCl. The data are discussed as they relate to the lamina terminalis as a primary osmosensitive region in brain.