Recently in the pig hypothalamus a vasopressin- and oxytocin-containing nucleus was identified which, like the supraoptic nucleus, becomes sexually dimorphic after puberty. Following the increase in circulating steroids at puberty, the vasopressin- and oxytocin-containing nucleus becomes twice as large in both males and females. In adulthood, the vasopressin- and oxytocin-containing nucleus of females is approximately twice as large as that in males. Because these alterations are possibly due to an influence of gonadal steroids, i.e. estrogens, the vasopressin- and oxytocin-containing the presence of estrogen receptors. In addition to the area of the vasopressin- and oxytocin-containing nucleus, the present study documented the distribution of estrogen receptors in the septal area and other parts of the hypothalamus of intact post-pubertal male and female pigs, by utilizing immunocytochemical methodology. Intense nuclear estrogen receptor staining was found in a number of areas, i.e. the medial preoptic area, the oxytocin-containing dorsomedial extension of the supraoptic nucleus, a possible homologue of the sexually dimorphic nucleus of the preoptic area, the median preoptic nucleus, the medial and lateral part of the bed nucleus of the stria terminalis, the ventromedial hypothalamus and the arcuate nucleus. In the ventral part of the lateral septum, the septohypothalamic nucleus, the nucleus subfornicalis and the stigmoid nucleus estrogen receptor immunoreactivity was less intense. Dorsolaterally of the vasopressin- and oxytocin-containing nucleus, estrogen receptor positive cells were observed, but the vasopressin- and oxytocin-containing nucleus itself lacked such receptors. In the magnocellular supraoptic nucleus and paraventricular nucleus no nuclear estrogen receptor staining was found. However, a weak cytoplasmic staining was present in all cells. There was a clear sex difference in the estrogen receptor-immunoreactive cell number in a possible homologue of the sexually dimorphic nucleus of the preoptic area. Compared to male pigs, in female pigs the number of cells showing estrogen receptor immunoreactivity in this area, which is known to be sexually dimorphic in various species, was twice as high. In other areas, such as the medial part of the bed nucleus of the stria terminalis, the medial preoptic area, the arcuate and ventromedial hypothalamic nucleus, a similar sex difference was found. In addition estrogen receptor immunoreactivity was generally more intense in females. No sex differences were noted in the overall distribution of estrogen receptor cells in the areas studied.The lack of nuclear estrogen receptors in the vasopressin- and oxytocin-containing nucleus, paraventricular hypothalamic nucleus and the main part of the supraoptic nucleus does not support the hypothesis that gonadal steroids “directly” control these structures. Similar to the control of gonadotropin release seen in other mammals, the present data suggest that gonadal steroids “indirectly” influence the pig vasopressin- and oxytocin-containing nucleus and supraoptic nucleus.
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