The GABAergic innervation of the goldfish pituitary was studied at the light and electron microscope levels by means of radioautography after in vitro incubation in tritiated gamma-aminobutyric acid (GABA) and immunocytochemistry using antibodies against GABA. Following incubation of pituitary fragments in a medium containing tritiated GABA, a selective uptake of the tracer was observed within the digitations of the neurohypophysis. Silver grain clusters were also observed in the adenohypophyseal tissue. At the electron microscope level, this uptake was found to correspond to nerve endings containing small clear and dense-core vesicles. These labeled profiles were located mainly in neurohypophyseal digitations in close apposition with the basement membrane separating the neurohypophysis from the adenohypophysis. However, they were also encountered in direct contact with most adenohypophyseal cell types in the different lobes. These results were confirmed by immunocytochemical data demonstrating the presence of numerous GABA immunoreactive fibers in both anterior and neurointermediate lobes. They were found either in the digitations of the neurohypophysis or in the adenohypophysis in direct contact with the glandular cells with a distribution and an ultrastructural aspect similar to those observed by radioautography. These data demonstrate that the pituitary of teleosts receives a massive GABAergic innervation. Although physiological data providing a functional significance for such an innervation are lacking, the present study suggests that, as already documented in mammals, GABA may be involved in the neuroendocrine regulation of pituitary functions in teleosts.
The GABAergic innervation of the mouse pituitary, including the median eminence, was studied at light-microscopic and ultrastructural levels by use of a pre-embedding immunocytochemical technique with antibodies directed against GABA. In the median eminence, a high density of GABA-immunoreactive fibers was found in the external layer where the GABAergic varicosities were frequently observed surrounding the blood vessels of the primary capillary plexus. In the internal and subependymal layers, only few fibers were immunoreactive. The intense labeling of the external layer was observed in the entire rostro-caudal extent of the median eminence. In the pituitary proper, a dense network of GABA-immunoreactive fibers was revealed throughout the neural and intermediate lobes, entering via the hypophyseal stalk. The anterior and tuberal lobes were devoid of any immunoreactivity. The GABA-immunoreactive terminals were characterized in the median eminence, and in the intermediate and posterior lobes at the electron-microscopic level. They contained small clear vesicles, occasionally associated with dense-core vesicles or neurosecretory granules. In the intermediate lobe they were seen to be in contact with the glandular cells. In the posterior lobe and in the median eminence, GABA-immunoreactive terminals were frequently located in the vicinity of blood vessels. These results further support the concept of a role of GABA in the regulation of hypophyseal functions, via the portal blood for the anterior lobe, directly on the cells in the intermediate lobe, and via axo-axonic mechanisms in the median eminence and posterior lobe.
The localization of oxytocin (OT) binding sites and vasopressin (VP) binding sites of the V1a subtype was investigated by radioautography in kidneys of rabbits, mice and meriones during postnatal development and in the adult, and in the human kidney. Kidney sections were incubated in the presence of selective radioiodinated OT and V1a antagonists, respectively. The localizations were compared with those previously described in the rat. The main finding of the study was the almost constant presence in the cortex of V1a binding sites in the connecting tubule, the cortical collecting duct and in the juxtaglomerular apparatus (on the intra- and extraglomerular mesangium and the afferent arteriole). This distribution suggests an interaction of VP via V1a receptors and the kallikrein-kinin system in the kidney. OT binding sites, in comparison with V1a binding sites, were fewer and less constantly detectable in the kidney of the different species. In the mouse, their presence on the limbs of Henle’s loop in the medulla points to the possibility of their involvement in the medullary concentrating process. In the kidneys of the various species, OT and V1a binding sites occurred always in differential structures. In contrast, in the human kidney cortex, a colocalization of OT and V1a binding sites was almost constantly observed. This raises the question as to the specificity of the neurohypophysial hormone receptors in the human kidney.
This study has evaluated the development of the hypothalamic vasopressin system and nephrons of the kidney in desert rodents, Meriones shawi, which effectively retain water by excretion of highly concentrated urine. The vasopressin system was studied immunocytochemically at the 18th fetal day, at the 2nd, 13th, 27th postnatal days and in adulthood. The kidneys were investigated at the 2nd, 13th postnatal days and in adulthood using microdissection technique. Occasional vasopressin-immunoreactive neurons were observed as early as the 18th fetal day, only in the paraventricular nucleus. From the 2nd postnatal day onwards, vasopressin neurons increased progressively in number, being mainly concentrated in the supraoptic and paraventricular nuclei, as well as in the ventral retrochiasmatic region. Transient neuronal populations were also observed at the 13th postnatal day in the lateral preoptic area and anterior hypothalamic nucleus. Apart from the neurons, the glandular cells of the tuberal lobe showed immunostaining from the 18th fetal day, the first age studied, until the 13th postnatal day. The fibers of differentiating vasopressin neurons grew towards the circumventricular/neurohemal organs, terminating in the organum vasculosum of the lamina terminalis and the lateral ventricles as early as the 18th fetal day, as well as the third ventricle, the posterior lobe and the external zone of the median eminence between the 2nd and 13th postnatal days. The kidney in 2-day-old Meriones comprised nephrons at different stages of development from an S-shaped body to well-differentiated nephrons. At the 13th postnatal day, as in adulthood, the nephrons were well differentiated and characterized by long, thin loops descending to different levels of papilla. Thus, according to our morphological data the hypothalamic vasopressin neurons and nephrons in the kidney of Meriones reach the definitive state by the end of the 2nd postnatal week.
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