Identification of neural pathways activated in dehydrated rats by means of Fos-immunohistochemistry and neural tracing.

The distribution of Fos-immunoreactivity (Fos-IR) was studied in the brains of rats deprived of water for 24 or 48 h and compared with that in brains of water-replete rats. Intense Fos-IR was observed in many neurons of the median preoptic nucleus (MnPO), organum vasculosum of the lamina terminalis (OVLT), supraoptic nucleus and hypothalamic paraventricular nucleus. There was less intense and sparse Fos-IR in the subfornical organ. In water-replete rats, Fos-IR was absent or very low in these regions. In other rats, cholera toxin B-gold conjugate was microinjected bilaterally into the supraoptic nucleus to identify retrogradely labelled neurons in the lamina terminalis projecting to the supraoptic nucleus. Approximately 30% of these retrogradely labelled neurons in the OVLT and MnPO also exhibited Fos-IR after 48 h of water deprivation. These data show that neurons in the MnPO, OVLT and, to a lesser extent, the subfornical organ probably play an important role in homeostatic responses to dehydration, such as vasopressin secretion.