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 b rains 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 n eurons of the median preoptic nucleus (MnPO), organum vasculosum of th e lamina terminalis (OVLT), supraoptic nucleus and hypothalamic parave ntricular nucleus. There was less intense and sparse Fos-IR in the sub fornical organ. In water-replete rats, Fos-IR was absent or very low i n these regions. In other rats, cholera toxin B-gold conjugate was mic roinjected bilaterally into the supraoptic nucleus to identify retrogr adely labelled neurons in the lamina terminalis projecting to the supr aoptic nucleus. similar to 30% of these retrogradely labelled neurons in the OVLT and MnPO also exhibited Fos-IR after 48 h of water depriva tion. These data show that neurons in the MnPO, OVLT and, to a lesser extent, the subfornical organ probably play an important role in homeo static responses to dehydration, such as vasopressin secretion.