NADPH-diaphorase activity in the rat hypothalamo-neurohypophysial system after salt loading and rehydration

In this work we investigated the histochemical NADPH-diaphorase activity of the rat hypothalamo-magnocellular neurons (with special attention to the nucleus circularis) and of the fibers of hypothalamo-neurohypophysial tract at the level of the median eminence after 5 days of salt loading and salt loading plus rehydration. In all hypothalamo-magnocellular nuclei examined, salt loading increased neuronal NADPH-diaphorase staining, but when the response of the neurons of the nucleus circularis and those of the supraoptic (SON) and paraventricular nuclei (PVN) were compared, nearly all the neurons present in the nucleus circularis were strongly stained whereas in the SON and PVN not all neurons respond to hyperosmotic stimulus, some unresponsive neurons also being observed. In the internal zone of the median eminence some fibers were intensely stained after salt loading but we never observed reactive fibers in the external zone. Rehydration after salt loading reversed the increase in histochemical labelling to the intensity observed in intact animals in all magnocellular neurons. However in the median eminence staining was still stronger after 48 h of rehydration than in untreated rats.