Electronmicroscopic immunocytochemical study on the vasopressin-containing neurons of the thirsting rat

Summary

Whereas in thirsting animals the perikarya of the nucleus supraopticus are nearly empty of neurosecretory granules as evidenced by electron microscopic observation, the perikarya are heavily stained by light microscopic immunohistochemical staining. In an attempt to discover the substrate responsible for the positive immunohistochemical staining in thirsting rats, the neurons of the supraoptic nucleus of normal and long-term thirsting animals were compared by electron microscopic immunocytochemistry (indirect PAP-method). In controls all parts of the vasopressin-synthesizing neuron are filled with elementary granules which render a positive and uniform reaction after immunostaining with the indirect PAP-method. The positively reacting fibers in the external zone of the median eminence contain smaller granules than those of the tractus supraoptico-hypophyseus. Within the nucleus suprachiasmaticus, no positive reaction after immunostaining was found. In long-term thirsting animals PAP-complexes as markers of vasopressin are located over the ergastoplasm and over the few small elementary granules. The processes within the nucleus supraopticus and the ballooned axons in the internal zone of the median eminence exhibit “free”, i.e. non granule-bound, PAP-complexes. Findings in the nucleus suprachiasmaticus and the median eminence of thirsting animals correspond to those in controls. The neurohypophysis is almost completely devoid of PAP-labeled elementary granules.

From these results it can be concluded that during thirst vasopressin synthesis is increased in the ergastoplasm and that the hormone is transported partly in a non granule-bound form. Direct contacts between neurosecretory cells and the basal lamina are found more often in thirst-stressed animals and are typical of neurohemal regions. It is discussed whether these neurohemal regions may develop transitionally under stress.