Estrogen receptive neurons in the preoptic area of the rat are postsynaptic targets of a sexually dimorphic enkephalinergic fiber plexus

Estrogen receptors in the central nervous system and their implication for dopamine-dependent cognition in females

This article is part of a Special Issue "Estradiol and cognition". Over the past 30 years, research has demonstrated that estrogens not only are important for female reproduction, but also play a role in a diverse array of cognitive functions. Originally, estrogens were thought to have only one receptor, localized exclusively to the cytoplasm and nucleus of cells. However, it is now known that there are at least three estrogen receptors (ERs): ERα, ERβ and G-protein coupled ER1 (GPER1). In addition to being localized to nuclei, ERα and ERβ are localized to the cell membrane, and GPER1 is also observed at the cell membrane. The mechanism through which ERs are associated with the membrane remains unclear, but palmitoylation of receptors and associations between ERs and caveolin are implicated in membrane association. ERα and ERβ are mostly observed in the nucleus using light microscopy unless they are particularly abundant. However, electron microscopy has revealed that ERs are also found at the membrane in complimentary distributions in multiple brain regions, many of which are innervated by dopamine inputs and were previously thought to contain few ERs. In particular, membrane-associated ERs are observed in the prefrontal cortex, dorsal striatum, nucleus accumbens, and hippocampus, all of which are involved in learning and memory. These findings provide a mechanism for the rapid effects of estrogens in these regions. The effects of estrogens on dopamine-dependent cognition likely result from binding at both nuclear and membrane-associated ERs, so elucidating the localization of membrane-associated ERs helps provide a more complete understanding of the cognitive effects of these hormones.

Reproductive aging in Japanese quail, Coturnix japonica is associated with changes in central opioid receptors

Quantitative in vitro autoradiography was used to measure specific mu and delta opioid receptor densities in regions of the Japanese quail, Coturnix japonica, brain that regulates reproductive endocrine and behavioral responses to determine the possible involvement of the opioid system in reproductive decline seen during aging. Densities were measured in selected brain regions of young sexually active (YAM), young photoregressed (YPM), old reproductively senescent (OIM) male, young active (YF), and old senescent female (OF) Japanese quail. Medial and lateral septum (SM, SL), medial preoptic area (POM), and n. intercollicularis (ICo) were of particular interest for reproductive responses. Similar to previous observations, mu and delta opioid receptors showed differential distributions in the areas measured. Some age-related changes were observed, with lower SM mu receptor densities in aged males (OIM) than females or young males (YAM). Densities of mu receptors in the POM and in other areas examined did not vary with sex or age. Similarly, OIM males had lower densities of delta receptors in the SM than young males (YAM and YPM); POM delta receptor densities were also low in OIM males compared to the YPM males, and YAM males were intermediate. Interestingly, photoregressed males (YPM) had higher SL delta receptor densities than any other group. Thus there were age-related differences detected in mu receptor densities among groups in the SM of OIM relative to other groups; and the mu and delta receptor densities did not differ in females with brain region. Additionally for delta receptors specifically, YF and OF did not differ from OIM for any brain region and similarly had lower densities of delta receptors compared to YAM males. These data provide support for regional differences in opioid receptor distribution and for age- and sex-related differences in delta opioid receptor densities. The direction of change presents an interesting dichotomy in that, compared to young active males, delta opioid receptor densities increased with loss of reproductive function in the YPM, whereas receptor densities decreased in the OIM. Plasma androgen levels were relatively low in both these groups compared to the young active males. This observation suggests that there is an age-related loss in the ability of this receptor system to respond to circulating and centrally produced steroid hormones in the POM and in some septal regions, compared to young animals that are responding to environmental cues. Furthermore, these data support an active role of the opioid peptide system in the inhibition of the reproductive axis in photoregression.

Immunohistochemistry and neural connectivity of the Ov shell in the songbird and their evolutionary implications

The neuropeptide immunohistochemistry and neural connectivity of areas surrounding the thalamic auditory nucleus (the nucleus ovoidalis [Ov]), as well as the areas to which it is connected, were investigated in a songbird, the Bengalese finch. The results showed that met-enkephalin was present in the Ov shell and most of the areas connected to it, but not in the Ov core. Anterograde and retrograde tracing studies showed that the Ov shell was more widely connected than the Ov core. The Ov shell was mainly connected to: 1). areas flanking the primary telencephalic auditory field (i.e., fields L2b, L1, and L3) and areas surrounding the robust nucleus of the archistriatum (RA); 2). several hypothalamic areas such as the nucleus ventromedialis hypothalami (VMN) and the nucleus anterior medialis hypothalami (AM). Some of these areas connected to the Ov shell are thought to be involved in auditory mediated neurosecretory activities. These results, which are similar to those reported previously in non-songbirds, suggest that the Ov shell and other surrounding areas of auditory and song-control nuclei are conserved in birds. These findings are discussed in terms of the evolution of the core-and-surround organization of auditory and song-control nuclei.

Sexual dimorphism in the GABAergic control of gonadotropin release in intact rats

GABA is a potent regulator of gonadotropin release both in male and female rats. We reported 24 h profiles of GABA release in the medial preoptic area (MPO) where gonadotropin-releasing hormone (GnRH) surge generator resides in female rats. In this article, we review the sex difference in 24 h profiles of GABA release. GABA release is high and episodic in male rats without any time dependency, but female rats showed a surge-like secretion of GABA in the early morning of the proestrous day. GABA release rapidly decreased until the afternoon of the day of proestrus followed by the preovulatory luteinizing hormone (LH) surge. The peak time of GABA episodes changes with estrous cycle in female rats. Fitting with the double cosinor method demonstrated that the acrophase of the GABA release in proestrous female rats occurs in the early morning, whereas the acrophases in diestrous females, estrous females and males occur at various time of day. Proestrous female rats showed significant difference in the peak time and acrophase of the GABA release compared with other estrous stages of female and male rats. These results demonstrated further sexual dimorphism of GABA release in the MPO, suggesting that coupling between the GABA release and the circadian clock may be a determining factor in the sex difference of the hypothalamo-pituitary-gonadal (HPG) axis in rats.

Estrogen receptor-alpha immunoreactive neurons in the ventrolateral periaqueductal gray receive monosynaptic input from the lumbosacral cord in the rhesus monkey

Estrogen affects female sexual behavior, analgesia, and micturition in mammals. One of the possible sites at which estrogen might exert its effect on these functions is the periaqueductal gray (PAG). The PAG is involved in each of these functions, it receives sensory input relevant to these functions from the lumbosacral cord, and contains estrogen receptor-alpha immunoreactive (ER-alpha IR) neurons. The present light (LM) and electron microscopic (EM) study seeks to determine whether there are monosynaptic projections from the lumbosacral cord to ER-alpha IR neurons in the PAG of the female rhesus monkey. Tracer was injected into the lumbosacral cord to visualize the lumbosacral-PAG projection, and the distribution of ER-alpha IR neurons in the PAG was studied immunohistochemically. The medial part of the ventrolateral caudal PAG received the densest projection from the lumbosacral cord. Another prominent projection was found in the lateral PAG at the intercollicular level. Although ER-alpha IR neurons were widely distributed throughout the PAG, approximately 40% of ER-alpha IR PAG neurons were located as a distinct cluster in the medial portion of the ventrolateral, caudal PAG. Double labeling experiments showed that the location of this cluster precisely overlapped with the densest lumbosacral-PAG projection. EM revealed that axons from the lumbosacral cord made asymmetrical synaptic contacts with unlabeled dendrites and ER-alpha IR neuronal somata in the ventrolateral PAG. It is concluded that there exists a specific, monosynaptic pathway from lumbosacral neurons to ER-alpha expressing PAG neurons in the rhesus monkey. This pathway might be involved in the mechanisms of analgesia, blood pressure, mating behavior, and micturition.

Alterations in the estrogen sensitivity of hypothalamic proenkephalin mRNA expression with age and prenatal exposure to alcohol

Studies suggest that exposure to alcohol in utero causes reproductive and neuroendocrine deficits in adult female rats. The ventromedial nucleus of the hypothalamus (VMN) is an estrogen-sensitive brain region which is regarded as a primary locus for modulating female reproduction. Proenkephalin (PE) mRNA expression in the VMN is dramatically increased by estrogen and this elevation is thought to be involved in modulating female reproductive behavior and neuroendocrine function. To examine whether prenatal alcohol exposure has long-term effects on the ability of estrogen to influence hypothalamic PE mRNA levels, female rats at 2-3, 6-7 or 15-18 months of age, derived from alcohol- or control-fed dams, were studied. 7 days following ovariectomy, animals received either estrogen or sham treatment for 2 days prior to sacrifice. PE mRNA levels in the VMN and striatum were determined by in situ hybridization histochemistry. Film autoradiogram density, numbers of PE mRNA-expressing cells and exposed silver grains/cell were analyzed. Estrogen treatment increased hybridization density, the number of PE mRNA-expressing cells and PE mRNA (grains) level/cell in the VMN of normal adult female rats. In old rats, estrogen increased the number of PE mRNA-expressing cells without up-regulating PE mRNA grain density/cell. In fetal alcohol-exposed (FAE) female rats, the number of cells that expressed PE mRNA did not increase following estrogen treatment at any age. Elevation of grain density/cell following estrogen was observed in FAE animals but only at 7-8 months of age. Overall, these data indicate that the estrogen responsiveness of PE mRNA expression in the VMN declines with age and, furthermore, prenatal exposure to alcohol blunts estrogen's effects on PE mRNA expression in the adult VMN. These finding may help to explain the mechanisms underlying the loss of reproductive function observed in FAE females.

Co-expression of steroid hormone receptors in opioid peptide-containing neurons correlates with patterns of gene expression during the estrous cycle

The anteroventral periventricular nucleus (AVPV) of the preoptic region represents an essential component of neural pathways regulating gonadotropin secretion, and contains sexually dimorphic populations of neurons that express dynorphin or enkephalin. In the present study we used in situ hybridization to measure prodynorphin (PDYN) and proenkephalin (PENK) mRNA in the AVPV of intact animals killed on each day of the cycle. Levels of PDYN mRNA were lowest in animals killed on the afternoon of proestrus and then increased by over 60% by the morning of the following day. Expression of PENK mRNA was generally stable during the cycle, but a small yet significant reduction was detected on proestrus relative to levels of PENK mRNA in animals killed on the day of diestrus. In addition, we used double in situ hybridization to demonstrate that the majority of PDYN mRNA-containing neurons express both estrogen (50%) and progesterone receptor (85%) mRNAs. Only one quarter of the PENK-containing neurons also co-express estrogen receptor mRNA, and fewer than 10% of the PENK mRNA neurons express PR mRNA. Thus, the differential expression of PDYN and PENK during the cycle generally correlates with distinct differences in the degree of colocalization of ER and PR mRNA in PDYN and PENK mRNA-containing neurons in the AVPV.

The hypothalamic ventromedial nucleus sends a met-enkephalin projection to the preoptic area's periventricular zone in the female rat

The female but not the male rat possesses a dense network of methionine enkephalin (m-Enk) fibers in the periventricular zone of the preoptic area (pePOA). The potential source of these fibers was determined by injection of the tracer fluorogold, FAu, into the preoptic area of adult female rats. Twenty-four hours before they were killed, the rats were administered colchicine (intraventricularly) to enable immunocytochemical visualization of m-Enk cells. Upon examination of the brains with fluorescence microscopy, double-labeled cells showing fluorogold and immunofluorescence for m-Enk were consistently observed in the preoptic area, the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMHvl) and nearby medial tuberal area (MTA), the arcuate nucleus, periventricular area of the hypothalamus, perifornical area, and dorsomedial nucleus of the hypothalamus. A series of lesion and knife cut experiments using glass, Halasz, and wire knives determined that the pePOA m-Enk fibers arose from the hypothalamus, near or within the VMH. Ibotenic acid lesions further determined that the source of the m-Enk projection was the VMHvl with a possible additional contribution from the MTA.

Estrogen-receptive neurons in the anteroventral periventricular nucleus are synaptic targets of the suprachiasmatic nucleus and peri-suprachiasmatic region

The anteroventral periventricular nucleus (AVPv) in the rat preoptic area is a key site underlying control of the steroid dependent preovulatory gonadotropin surge. Estrogen and progesterone receptor-containing neurons in the preoptic/hypothalamic continuum, particularly those in the AVPv, are believed to transduce steroidal signals and, in turn convey this information to the LHRH system, which lacks steroid receptors. In addition to the influence of the gonadal steroids, the precise timing of the preovulatory gonadotropin surge is believed to be regulated by the hypothalamic suprachiasmatic nucleus (SCN). The SCN and peri-SCN neurons send efferent projections rostrally to the anterior preoptic area suggesting that circadian signals are communicated synaptically to steroid-responsive neurons in the AVPv. To test this hypothesis, ultrastructural double label immunocytochemistry was conducted to determine whether SCN efferents contact estrogen receptor-immunoreactive neurons in the AVPv. Brain sections with SCN injections of phaseolus vulgaris leucoagglutinin (PHA-L) were immunostained for estrogen receptors and PHA-L. Light and electron microscopic data show that the anterior preoptic area received robust PHA-L-immunoreactive efferents from SCN neurons and immediately adjacent subparaventricular zone. In particular, the AVPv contained abundant labeled fibers and terminal boutons. Ultrastructurally, SCN- and subparaventricular zone-derived terminals synaptically contacted the perikaryon of many estrogen receptor-immunoreactive neurons in the AVPv. The perikarya of unlabeled neurons were also contacted, but the majority of the labeled contacts were observed upon neuronal processes. These results demonstrate that estrogen responsive AVPv neurons are regulated by SCN efferents. Furthermore, the present data provide strong support to the idea of collective control of pituitary gonadotropin release by steroid sensitive and circadian signal neural pathways.

Changes in preproenkephalin messenger RNA level in the rat ventromedial hypothalamus during the estrous cycle

To gain a better understanding of the relationship between the female rat reproductive system and preproenkephalin (PPE) expressing neurons under physiological conditions, we examined changes in PPE mRNA levels in the mediobasal hypothalamus during the rat estrous cycle by means of northern blotting and in situ hybridization histochemistry (ISHH). In the Northern blot studies, we found that PPE mRNA levels in the mediobasal hypothalamus were significantly increased by noon of proestrus compared to those in the morning and stayed high until diestrus day 1, and returned toward low levels on diestrous day 2. In contrast, measured as controls, glyceraldehyde-3-phosphate-dehydrogenase mRNA levels were significantly higher on proestrus regardless of time of day compared to diestrus day 2, and levels of calcineurin mRNA on proestrous and estrous were significantly lower than diestrous day 1 and day 2. ISHH studies revealed that these changes in PPE mRNA levels were specific in the ventromedial hypothalamic nucleus pars ventrolateralis (VMHVL), since we could not see any significant changes in signal in other parts including ventromedial hypothalamic nucleus pars dorsomedialis and arcuate hypothalamic nucleus. In the VMHVL, PPE mRNA levels in the afternoon of proestrous were significantly higher than those in the afternoon of diestrous day 2 whereas no significant change in PPE mRNA was observed in the caudate-putamen. The present study provides additional information relevant to possible implications of PPE gene expression in female reproductive systems, since changes in PPE mRNA levels may be associated with estrogen as well as progesterone or other hormonal concentrations during the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen receptor-immunoreactive neurons contain calcitonin gene-related peptide, methionine-enkephalin or tyrosine hydroxylase in the female rat preoptic area

We have shown in our previous studies that estrogen treatment selectively influences calcitonin gene-related peptide (CGRP)-, methionine-enkephalin (Met-Enk)- and tyrosine hydroxylase (TH)-immunoreactive (IR) intensities in the neurons of the periventricular preoptic nucleus (PPN) and the medial preoptic area (MPA) of the female rat. In the present study, we examined whether estrogen receptor (ER)-IR neurons in the PPN and MPA contain CGRP, Met-Enk, or TH using a double-labeling immunohistochemical method and investigated changes in the number of double-labeling cells upon treatment with estrogen. Brain sections of ovariectomized rats and ovariectomized and estrogen-treated rat were stained using the avidin-biotin-peroxidase complex method followed by the peroxidase-anti-peroxidase method. The sections were first incubated with an anti-ER antibody in conjunction with nickel diaminobenzidine which produces a dark blue reaction product in the nucleus. Subsequently, CGRP, Met-Enk or TH antisera were applied to these sections and the resulting brown diaminobenzidine reaction product in the cytoplasm was examined. Neurons that were double-labeled for ER and CGRP, Met-Enk or TH were investigated in the PPN and MPA. The number of doubly labeled ER/CGRP- and ER/TH-IR neurons was large, whereas the number of ER/Met-Enk-IR neurons was small. These results suggest that ER in the PPN and MPA may be more closely related to the mechanism of changes in CGRP- and TH-IR intensities upon estrogen treatment than that in Met-Enk-IR intensity.

Sex differences in the behavioural response to persistent pain in rats

The behavioural response to formalin-induced persistent pain was examined in male and female rats both unfamiliar and familiar with the test apparatus. Rats were subcutaneously injected with 50 microliters of formalin (10%) in the hindpaw and placed in the test cage (60 min). Licking and Flexing duration and Paw-Jerk frequency were recorded. Licking and Flexing lasted longer in females than males, while Paw-Jerk occurred in both sexes with comparable frequencies. Flexing and Paw-Jerk were lower in animals unfamiliar with the test apparatus. Therefore, behavioural responses to pain appeared to be affected by sex and familiarization with the experimental setting in different and independent ways.

Somatostatin-immunoreactive neurones in the hypothalamic ventromedial nucleus possess oestrogen receptors in the male and female rat

Neurones containing oestrogen receptors (ERs) in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) are believed to play an important role in mediating oestrogen's regulatory influence on reproductive behaviour. As somatostatin (SOM)-immunoreactive neurones are found exclusively within the ventrolateral division of VMN, this study has used double-labelling immunocytochemistry techniques to evaluate whether ER-immunoreactive cells in the VMNvl synthesize SOM in both the male and female rat. Rats gonadectomized 1 week earlier were perfused and brain sections through the mediobasal hypothalamus processed for ER and SOM immunoreactivity using the H222 monoclonal rat ER antibody and a polyclonal rabbit SOM antiserum. Within the VMN, cells immunoreactive for SOM were found predominantly in the rostral portion of the VMNvl while ER-immunoreactive cells were distributed throughout the VMNvl. Sequential double-labelling studies revealed that many ER-containing cells in the rostral VMNvl were also immunoreactive for SOM. A semi-quantitative analysis of double-labelled cells in the rostral VMNvl of male and female rats, respectively, estimated that 52 +/- 2% and 55 +/- 8% of SOM-IR cells possess ERs while 35 +/- 1% and 28 +/- 3% of ER-IR cells synthesise SOM in the rostral VMNvl. No sex differences were detected at this level. These results show that approximately half of the SOM-synthesizing neurones in the rostral VMN possess ERs and indicate that SOM should now be considered alongside enkephalin and Substance P as a putative mediator of oestrogen's regulatory influence on reproduction through the VMN of the rat.

Estrogen affects calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive neuron in the female rat preoptic area

The effects of estrogen on the neurons of the medial preoptic area of the ovariectomized rat were immunohistochemically investigated using antisera to calcitonin gene-related peptide (CGRP) and methionine-enkephalin (Met-Enk). To visualize CGRP- and Met-Enk-immunoreactive (IR) cell somata, colchicine was injected into the cerebroventricle. CGRP- and Met-Enk-IR neurons were distributed in the medial preoptic nucleus (MPN) but few in the periventricular preoptic nucleus (PPN) in the ovariectomized rat. After estrogen treatment, CGRP immunoreactivity was markedly increased in the PPN and MPN, whereas Met-Enk immunoreactivity was increased in the MPN. These results, along with our previous data, suggest that estrogen accelerates CGRP- and Met-Enk expression in a different manner in the PPN and MPN neurons.

Time-course analysis of changes in calcitonin gene-related peptide-and methionine-enkephalin-immunoreactivity in the female rat preoptic area after estrogen treatment

The time-course effects of one month of estrogen upon calcitonin gene-related peptide - and methionine-enkephalin-immunoreactivity in the periventricular preoptic nucleus and medial preoptic nucleus were semi-quantitatively investigated with a computer-based image analysis system. Female Wistar rats were ovariectomized and implanted subcutaneously with a 10-mm-long silastic capsule containing estradiol-17 beta, or with a blank capsule, as a control. Estradiol-17 beta-treated rats were killed at days 1, 4, 7, 10, 14 and 28 after the implantation of estradiol-17 beta. To investigate the details of changes in calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive fibers in the periventricular preoptic nucleus and medial preoptic nucleus, a grid, made up of 8 x 16 squares (one square corresponding to 50 x 50 microns in the true section), was set on the wall of the third ventricle, and immunoreactivity within each square was measured with an image analyser. In the control rats, calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive fibers were distributed in the periventricular preoptic nucleus and medial preoptic nucleus. In the estradiol-17 beta-treated rats, calcitonin gene-related peptide-immunoreactive fibers increased prominently at day 1, day 7 and day 10 in the periventricular preoptic nucleus, whereas methionine-enkephalin-immunoreactive fibers increased at day 1, day 14 and day 28 in the periventricular preoptic nucleus and medial preoptic nucleus. These findings suggest that the mechanism underlying the increases in these calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive fibers after estrogen treatment might be different.