Presence of estrogen receptor immunoreactivity in the oxytocin-containing magnocellular neurons projecting to the neurohypophysis in the guinea-pig

Abstract

The immunoperoxidase technique was used on adjacent sections of guinea-pig brain to compare precisely the distribution of estrogen receptor-immunoreactive cells and progesterone receptor-immunoreactive cells in the supraoptic nucleus and the paraventricular nucleus. Only estrogen receptor-immunoreactive neurons were found in the supraoptic nucleus. A large number of estrogen receptor-positive cells were observed in the periventricular magnocellular groups throughout the rostrocaudal extent of the paraventricular nucleus, whereas only a few progesterone receptor-immunoreactive cells were scattered in the anterior portion of this region. We used a combination of axonal tracing with double immunocytochemical detection to determine whether estradiol acts directly on the oxytocin-immunoreactive neurons which project to the neurohypophysis. Oxytocin-immunoreactive cells were found in the supraoptic nucleus, ventrally to the optic pathways, in subchiasmatic and retrochiasmatic areas, and in the anterior hypothalamic area. These cells were also retrogradely labeled by Granular Blue when this tracer was injected intravenously. In the paraventricular nucleus, the Granular Blue/oxytocin-positive cells were observed in the periventricular magnocellular groups whereas Granular Blue labeled neurons were found in both Parvocellular and Magnocellular components. We Found that almost all the oxytocin-immunoreactive cells revealed estrogen receptor immunoreactivity. In conclusion, the comparative study of distribution of estrogen receptors and progesterone receptors in the guinea-pig supraoptic and paraventricular nuclei indicates that, in the supraoptic nucleus, only estrogen receptors are present and that, in the paraventricular nucleus, they are far more numerous than progesterone receptors. The present findings demonstrate that the magnocellular cells which contain estrogen receptors are oxytocinergic. In addition, these cells are retrogradely labeled pointing to a neurohypophysial projection. It is likely that estradiol controls the hypothalamo-neurohypophysial oxytocin system by direct action on the magnocellular neurons.