A small subpopulation of progesterone receptor-containing neurons in the guinea pig arcuate nucleus projects to the median eminence

Regulation of hypothalamic neuropeptide Y Y1 receptor gene expression during the estrous cycle: role of progesterone receptors

Neuropeptide Y (NPY) stimulates the release of GnRH in an estrogen (E2)-dependent manner, which is important in generating preovulatory GnRH surges. We tested the hypothesis that E2 up-regulates NPY's actions by stimulating NPY Y1 receptor (Y1r) gene expression through a mechanism mediated by E2's ability to induce progesterone (P) receptors (PRs). In initial experiments, a specific Y1r antagonist BIBP3226 was used to confirm the involvement of Y1r in the stimulatory effects of NPY on in vivo GnRH release. Hypothalamic Y1r messenger RNA (mRNA) levels were then measured using competitive RT-PCR and were found to be significantly increased at 1000, 1200, and 1400 h on proestrus compared with other times of the day or cycle stage. Ovariectomy eliminated these increases, and E2 treatment restored them. Additional P treatment produced even larger increases in Y1r mRNA levels. To assess the role of PRs in stimulating Y1r expression, proestrous rats were treated with PR antagonist or oil vehicle and killed at 1200 h. Treatment with PR antagonist completely blocked the proestrous rise in Y1r gene expression. In parallel experiments, the same in vivo PR antagonist treatments also blocked NPY stimulation of GnRH release in vitro. Together our findings reveal that 1) Y1r mRNA levels are increased during the late morning and afternoon of proestrus; 2) Y1r mRNA levels are similarly increased by E2, and to an even greater extent by additional P; and 3) PR antagonism blocks both increased Y1r mRNA and induction of GnRH responsiveness to NPY. These observations support the idea that E2 up-regulates GnRH neuronal responses to NPY through stimulation of Y1r gene expression, and that E2's actions are mediated by the induction and subsequent activation of PRs.

A subset of estrogen receptor-containing neurons project to the median eminence in the ewe

The neural pathways responsible for conveying the steroid feedback signals that ultimately affect reproductive neuroendocrine function remain largely undefined. One possibility involves a direct projection from estrogen receptor (ER)-containing neurons to the median eminence (ME), a site of neuroendocrine peptide release. To examine this possibility, 8 ewes received stereotaxic injections of the retrograde neuronal tract-tracing compound cholera toxin-beta subunit (CT beta) into the ME. Neurons sending projections to the ME and containing ER were identified using a dual-label immunoperoxidase method. Double-labeled cells were found in distinct regions: (1) the ER-rich arcuate nucleus (ARC) that contained the greatest number of double-labeled cells, and (2) the organum vasculosum of the lamina terminalis (OVLT) which contained a very consistent, but low, number of double-labeled cells. While a fairly large number of retrogradely-labeled ARC neurons containing ER were identified, the majority of ER-containing ARC neurons were unlabeled and thus send projections elsewhere. Other regions containing high concentrations of ER-positive cells such as the medial preoptic area (MPOA), anterior hypothalamic area, and ventrolateral portion of the ventromedial hypothalamic nucleus, were devoid of double-labeled cells. Similarly, regions rich in neuroendocrine neurons such as the periventricular hypothalamus and paraventricular and supraoptic hypothalamic nuclei contained no double-labeled cells. These results suggest that modulation of neuroendocrine secretory activity may occur directly at the level of the ME by ER-containing neurons located within restricted regions of the hypothalamus and forebrain. However, the relatively low proportion of ER-containing neurons projecting to the ME suggests that the influence of estradiol upon neuroendocrine function also may include target sites other than the ME.

Estrogen modulation of two subpopulations of β-endorphin neurons in ovariectomized guinea pigs distinguished by peripherally injected fluorogold

β-endorphin released by neurons in the arcuate nucleus affects the output of several neuroendocrine systems and estrogen levels modulate the production and secretion of β-endorphin. We used intraperitoneal injection of fluorogold to retrogradely label the cell bodies of neurons that project outside the blood-brain-barrier in conjunction with immunohistochemistry for β-endorphin to dual label the subpopulation of β-endorphin neurons that project to the median eminence or other sites of access to the peripheral circulation. We found that some identified β-endorphin neurons in the arcuate nucleus of ovariectomized guinea pigs sequestered fluorogold. Approximately 7% of β-endorphin-containing cells co-localized with fluorogold. The effect of estrogen on the number of identified β-endorphin cells was examined. A single estradiol benzoate injection to ovariectomized guinea pigs 24 h prior to sacrifice dramatically decreased the total number of β-endorphin cells identified in the rostral, medial and the caudal portions of the arcuate nucleus. Also, a significantly smaller percentage of fluorogold-filled cells was found to contain β-endorphin immunoreactivity in the estrogen-treated group. These data suggest that a subpopulation of β-endorphin neurons has access to the peripheral circulation and may alter the output of neurosecretory terminals at the level of the median eminence. Furthermore, estrogen affects this subpopulation and the general population of β-endorphin neurons in the arcuate nucleus in a similar manner.

Lateral septal projections onto tubero-infundibular neurons in the hypothalamus of the guinea pig

Efferent projections of the lateral septal nucleus (LS) to the preoptic area and the hypothalamus were identified in 20 female guinea pigs after iontophoretic injection of the anterograde axonal tracer Fluoro-Ruby. Tubero-infundibular (TI) neurons of the preoptic area and the hypothalamus were retrogradely labeled after intracardiac injection of Granular Blue or Fluoro-Gold. Magnocellular neurons of the supraoptic and paraventricular nuclei were also labeled. The double labeling procedure allowed an estimation of the extent of the direct relationship between LS efferents and TI neurons. Contacts between lateral septal fibers and TI cell bodies were mainly observed at the light-microscopical level in the preoptic area. A group of labeled fibers coursing along the third ventricle established sparse connections with hypothalamic periventricular TI neurons. A few appositions was observed in the infundibular (arcuate) nucleus, suggestive of a monosynaptic regulation of TI neurons by a septo-arcuate tract. Close association with labeled magnocellular neurons was also noted at the edge of the supraoptic and paraventricular nuclei. The sparse but direct connections between LS and TI neurons may be involved in the neuroendocrine functions of the LS.

Immunocytochemical localization of progesterone receptors in galanin neurons in the guinea pig hypothalamus

A double-label immunocytochemical technique was used to determine whether progesterone receptor-containing neurons in the female guinea-pig hypothalamus also contained galanin. Adult ovariectomized guinea-pigs were primed by estradiol to induce progesterone receptors and injected intracerebroventricularly with colchicine to visualize galanin-immunopositive neurons. A small proportion of progesterone receptor-containing perikarya in the medial preoptic area and the mediobasal hypothalamus were bound to be immunoreactive for galanin. The medial preoptic, periventricular and arcuate nuclei showed the greatest concentration of double-labelled cells. Galanin varicosities appeared in close proximity to neurons with progesterone receptor-containing nuclei. These results provide neuroanatomical evidence that a subset of hypothalamic galanin-immunoreactive neurons is directly regulated by progesterone.

Progesterone Receptor-Containing Neurons in the Guinea-Pig Mediobasal Hypothalamus have Axonal Projections to the Medial Preoptic Area

The location and number of progesterone receptor-containing neurons in the mediobasal hypothalamus that project to the medial preoptic area were determined by combining retrograde fluorescent tract tracing with progesterone receptor immunocytochemistry. Injections of the retrograde tract tracer Fluoro-gold were made in the preoptic area of female guinea-pigs ovariectomized and primed with estradiol. After 5 days survival to allow for retrograde transport, tissue sections were incubated with monoclonal antibodies to the progesterone receptor to detect the presence of progesterone receptor-immunoreactive neurons. Cell bodies were labelled with Fluoro-gold throughout the arcuate nucleus. These neurons were not concentrated in any particular area of the nucleus but were diffusely distributed bilaterally. Retrogradely-labelled neurons were also observed in the ventrolateral and ventromedial nuclei mainly contralateral to the injection site. Progesterone receptor immunofluorescence labelled a subpopulation (7% to 10%) of these retrogradely-labelled cells particularly in the arcuate nucleus, including the median eminence. The double-labelled cells were more numerous in the anterior two-thirds of the arcuate nucleus. Although our estimates of the proportion of hypothalamic progesterone receptor-immunoreactive neurons that sent axons directly to the medial preoptic area were low, (about 0.35%), these neurons may be part of a neural circuit involved in the regulation of reproductive processes.

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

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.