Estrogen receptor expression in brainstem noradrenergic neurons of the sheep

Magnetic Resonance Imaging, a New Tool for Neuroendocrine Research in Sheep

Magnetic resonance imaging (MRI) brain analysis is used in rodents and for clinical investigation in humans, and it becomes also possible now for large animal models studies. Specific facilities are available with clinical scanners and benefit to neuroendocrine investigations in sheep. Sheep has a large gyrencephalic brain and its organization is very similar to primates and human, and among physiological regulations, oestrous cycle of the ewes is similar to women. Therefore, this animal is a good model for preclinical researches using MRI, as illustrated with steroids impact on the brain. New data were obtained concerning the effect of sexual steroids on neuronal networks involved in the control of reproduction and in the influence of sexual steroids on cognition. In addition to the importance of such data for understanding the role of these hormones on brain functions, they give new insights to consider the sheep as a powerful model for preclinical studies in the field of neuroendocrinology. These points are discussed in this short review.

Sex differences in the expression of estrogen receptor alpha within noradrenergic neurons in the sheep brain stem

In female sheep, high levels of estrogen exert a positive feedback action on gonadotropin releasing hormone (GnRH) secretion to stimulate a surge in luteinizing hormone (LH) secretion. Part of this action appears to be via brain stem noradrenergic neurons. By contrast, estrogen action in male sheep has a negative feedback action to inhibit GnRH and LH secretion. To investigate whether part of this sex difference is due to differences in estrogen action in the brain stem, we tested the hypothesis that the distribution of estrogen receptor α (ERα) within noradrenergic neurons in the brain stem differs between rams and ewes. To determine the distribution of ERα, we used double-label fluorescence immunohistochemistry for dopamine β-Hydroxylase, as a marker for noradrenergic and adrenergic cells, and ERα. In the ventrolateral medulla (A1 region), most ERα-immunoreactive (-ir) cells were located in the caudal part of the nucleus. Overall, there were more ERα-ir cells in rams than ewes, but the proportion of double-labeled cells was did not differ between sexes. Much greater numbers of ERα-ir cells were found in the nucleus of the solitary tract (A2 region), but <10% were double labeled and there were no sex differences. The majority of ERα-labeled cells in this nucleus was located in the more rostral areas. ERα-labeled cells were found in several rostral brain stem regions but none of these were double labeled and so were not quantified. Because there was no sex difference in the number of ERα-ir cells in the brain stem that were noradrenergic, the sex difference in the action of estrogen on gonadotropin secretion in sheep is unlikely to involve actions on brain stem noradrenergic cells.

Importance of sex to pain and its amelioration; relevance of spinal estrogens and its membrane receptors

Estrogens have a multitude of effects on opioid systems and are thought to play a key role in sexually dimorphic nociception and opioid antinociception. Heretofore, classical genomic actions of estrogens are largely thought to be responsible for the effects of these steroids on nociception and opioid antinociception. The recent discovery that estrogens can also activate estrogen receptors that are located in the plasma membrane, the effects of which are manifest in seconds to minutes instead of hours to days has revolutionized our thinking concerning the ways in which estrogens are likely to modulate pain responsiveness and the dynamic nature of that modulation. This review summarizes parameters of opioid functionality and nociception that are subject to modulation by estrogens, underscoring the added dimensions of such modulation that accrues from rapid membrane estrogen receptor signaling. Implications of this mode of signaling regarding putative sources of estrogens and its degradation are also discussed.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Changes in oestrogen receptor alpha expression in the nucleus of the solitary tract of the rat over the oestrous cycle and following ovariectomy

Oestrogen is capable of modulating autonomic outflow and baroreflex function via actions on groups of neurones in the brainstem. We investigated the presence of oestrogen receptor (ER) alpha in a part of the nucleus of the solitary tract (NTS) associated with central cardiovascular control, aiming to determine whether ERalpha mRNA and protein expression is correlated with levels of circulating oestrogen during the oestrous cycle. Polymerase chain reaction (PCR) detected ERalpha mRNA in the NTS at each stage of the oestrous cycle, from ovariectomised, sham-operated and male rats. Real-time PCR showed variations in ERalpha mRNA expression during the oestrous cycle, with the highest levels seen in oestrus, and lowest levels in metoestrus (P < 0.05 versus oestrus) and proestrus (P < 0.05 versus oestrus). Expression in males was lower than in dioestrus and oestrus females (P < 0.05). After ovariectomy, ERalpha mRNA levels were decreased compared to sham-operated animals (P < 0.01). Confocal fluorescence immunohistochemistry with stereological analysis showed that numbers of ERalpha immunoreactive cell nuclei per mm(3) of tissue in the caudal NTS were significantly greater in proestrus than in other groups of rats (P < 0.05). There were also differences among the groups in the extent of colocalisation of ERalpha in neurones immunoreactive for tyrosine hydroxylase and nitric oxide synthase. These results imply a complex pattern of region-specific oestrogen signalling in the NTS and suggest that ERalpha expression in this important autonomic nucleus may be related to circulating oestrogen levels. This may have consequences for the regulation of autonomic tone and baroreflex sensitivity when oestrogen levels decline, for example following menopause.

Oestrogen receptors in the central nervous system and evidence for their role in the control of cardiovascular function

Oestrogen is considered beneficial to cardiovascular health through protective effects not only on the heart and vasculature, but also on the autonomic nervous system via actions on oestrogen receptors. A plethora of evidence supports a role for the hormone within the central nervous system in modulating the pathways regulating cardiovascular function. A complex interaction of several brainstem, spinal and forebrain nuclei is required to receive, integrate and co-ordinate inputs that contribute appropriate autonomic reflex responses to changes in blood pressure and other cardiovascular parameters. Central effects of oestrogen and oestrogen receptors have already been demonstrated in many of these areas. In addition to the classical nuclear oestrogen receptors (ERalpha and ERbeta) a recently discovered G-protein coupled receptor, GPR30, has been shown to be a novel mediator of oestrogenic action. Many anatomical and molecular studies have described a considerable overlap in the regional expression of these receptors; however, the receptors do exhibit specific characteristics and subtype specific expression is found in many autonomic brain areas, for example ERbeta appears to predominate in the hypothalamic paraventricular nucleus, whilst ERalpha is important in the nucleus of the solitary tract. This review provides an overview of the available information on the localisation of oestrogen receptor subtypes and their multitude of possible modulatory actions in different groups of neurochemically and functionally defined neurones in autonomic-related areas of the brain.

Estrogen receptor-alpha immunoreactive neurons in the brainstem and spinal cord of the female rhesus monkey: species-specific characteristics

The distribution pattern of estrogen receptors in the rodent CNS has been reported extensively, but mapping of estrogen receptors in primates is incomplete. In this study we describe the distribution of estrogen receptor alpha immunoreactive (ER-alpha IR) neurons in the brainstem and spinal cord of the rhesus monkey. In the midbrain, ER-alpha IR neurons were located in the periaqueductal gray, especially the caudal ventrolateral part, the adjacent tegmentum, peripeduncular nucleus, and pretectal nucleus. A few ER-alpha IR neurons were found in the lateral parabrachial nucleus, lateral pontine tegmentum, and pontine gray medial to the locus coeruleus. At caudal medullary levels, ER-alpha IR neurons were present in the commissural nucleus of the solitary complex and the caudal spinal trigeminal nucleus. The remaining regions of the brainstem were devoid of ER-alpha IR neurons. Spinal ER-alpha IR neurons were found in laminae I-V, and area X, and were most numerous in lower lumbar and sacral segments. The lateral collateral pathway and dorsal commissural nuclei of the sacral cord and the thoracic intermediolateral cell column also contained ER-alpha IR neurons. Estrogen treatment did not result in any differences in the distribution pattern of ER-alpha IR neurons. The results indicate that ER-alpha IR neurons in the primate brainstem and spinal cord are concentrated mainly in regions involved in sensory and autonomic processing. Compared with rodent species, the regional distribution of ER-alpha IR neurons is less widespread, and ER-alpha IR neurons in regions such as the spinal dorsal horn and caudal spinal trigeminal nucleus appear to be less abundant. These distinctions suggest a modest role of ER-alpha in estrogen-mediated actions on primate brainstem and spinal systems. These differences may contribute to variations in behavioral effects of estrogen between primate and rodent species.

The nucleus para-retroambiguus: a new group of estrogen receptive cells in the caudal ventrolateral medulla of the female golden hamster

Receptive female hamsters display very rigid lordotic postures. Estradiol facilitates this behavior via activation of estrogen receptors. In the hamster brainstem estrogen receptor-alpha-immunoreactive neurons (ER-alpha-IR) are present in various brainstem regions including nucleus retroambiguus (NRA) in the caudal ventrolateral medulla (CVLM) and nucleus of the solitary tract. ER-alpha-IR neurons in the CVLM project to the thoracic and upper lumbar cord. However, A1 neurons in this region do not project to the spinal cord, in contrast to overlapping C1 neurons. The question now arises: are ER-alpha-IR cells in the CVLM part of the A1/C1 group, or do they belong to the NRA or do they compose a separate cluster. A study in ovariectomized female hamsters using a combination of double immunostaining and retrograde tracing techniques and measurement of soma diameters was carried out. The results showed that A1/C1 neurons in the CVLM are almost never ER-alpha-positive; neurons inside or bordering the NRA can be divided in two different types: large multipolar and small; the large NRA-neurons, projecting caudally, are neither tyrosine hydroxylase- (TH) nor ER-alpha-IR; the small neurons, bordering the NRA and projecting caudally, are ER-alpha-IR but not TH-IR. From the available evidence and the present findings it can be concluded that the group of small ER-alpha-IR neurons in the CVLM has to be considered as a distinct entity, probably involved in the autonomic physiological changes concurring with successive phases of the estrous cycle. Because the location is closely related to the NRA itself the nucleus is called nucleus para-retroambiguus, abbreviated (NPRA).

Sexual behavior in ewes and other domestic ruminants

Similarities as well as differences across species in the control of sexual behavior are helping to fully understand the subtle relations between physiology and eco-ethological constraints and how the brain integrates such information. We will illustrate this with sexual behavior in domestic ruminants and especially ewes. Females of these species like humans, but unlike rodents, have a long luteal phase. A prolonged exposure to progesterone (Pg) before the preovulatory estradiol rise is necessary for estrous behavior to be displayed. Estradiol action and receptor localization is very similar to that observed in other species. But not too surprisingly, the role of Pg is rather different with a priming effect not observed in rodents. However, as in rodents, Pg also has an inhibitory effect, is necessary for the display of proceptivity and is responsible for the timing of the different periovulatory events. These steroids act on the central nervous system in similar areas across mammalian species to regulate estrous behavior. Steroid fluctuations during the estrous cycle cause changes in catecholaminergic activity in the hypothalamus. Interestingly, these neurotransmitters seem to have very similar effects in ewes and rats as illustrated by the norepinephrine rise after male-female interactions observed in both species. Similar comparisons can be made regarding the action of some neuropeptides, including oxytocin and GnRH, and more integrative processes like sexual differentiation and modulation of reproduction by social interactions. Data on sheep, goats and cows will be compared with those of rodents.

Sex-specific densities of estrogen receptors alpha and beta in the subnuclei of the nucleus tractus solitarius, hypoglossal nucleus and dorsal vagal motor nucleus weanling rats

In rats ventilatory responses to N-methyl-d-aspartate (NMDA) receptor modulation are sexually dimorphic and may be altered by manipulating brain levels of estrogen receptors. Here we used image analysis and immunohistochemistry in weanling male and female rats to quantitate areas and densities of ER alpha and ER beta-positive neurons within medullary regions associated with cardiopulmonary regulation including the hypoglossal nucleus, subnuclei of the nucleus of the solitary tract (NTS), and the dorsal motor nucleus of the vagus. Weanling rats were selected because ventilation, metabolic rate, and body and brain weights are comparable at this age and there are no large fluctuations in plasma hormone levels. Females, relative to males, had smaller areas in the A2 region and parts of the NTS. Counts and densities for ER alpha were greater in females than males in almost all regions studied. In contrast sex differences in ER beta were found in fewer nuclei, but in those higher counts and densities were noted in females. In general, ER beta-positive neurons in the brainstem regions examined were less prevalent than ER alpha neurons. Thus, in weanling rats sex affected ER alpha and ER beta neuronal densities in brainstem regions associated with cardiopulmonary regulation that may be responsible for sex differences in control of breathing.

Estrogen synthesis in the spinal dorsal horn: a new central mechanism for the hormonal regulation of pain

The data summarized here suggest the existence of a new central pathway for the hormonal regulation of pain. These data mainly collected in quail, a useful model in neuroendocrinology, demonstrate that numerous neurons in the superficial laminae of the spinal cord express aromatase (estrogen-synthase). Chronic and systemic blockade of this enzyme in quail alters nociception within days, indicating that the slow genomic effects of sex steroids on nociception classically observed in mammals also occur in birds and require aromatization of androgens into estrogens. However, by contrast with these slow effects, acute intrathecal inhibition of aromatase in restricted spinal cord segments reveals that estrogens can also control nociception much faster, within 1 min, presumably through the activation of a nongenomic pathway and in a manner that depends on an immediate response to fast activation/deactivation of local aromatase activity. This emergent central and rapid paracrine mechanism might permit instantaneous and segment-specific changes in pain sensitivity; it draws new interesting perspectives for the study of the estrogenic control of pain, thus far limited to the classical view of slow genomic changes in pain, depending on peripheral estrogens. The expression of aromatase in the spinal cord in other species and in other central nociception-related areas is also briefly discussed.

Estrogen and hypertension

Menopause is accompanied by a dramatic rise in the prevalence of hypertension in women, suggesting a protective role of endogenous estradiol on blood pressure (BP). Both animal experimental and human clinical investigations suggest that estrogen engages several mechanisms that protect against hypertension, such as activation of the vasodilator pathway mediated by nitric oxide and prostacyclin and inhibition of the vasoconstrictor pathway mediated by the sympathetic nervous system and angiotensin. However, emerging evidence from recent clinical trials indicates a small increase, rather than decrease, in systolic BP with oral estrogen administration in postmenopausal women, without any detectable effect on diastolic BP. Mechanisms underlying this selective rise in systolic BP in postmenopausal women and oral contraceptive-induced hypertension in premenopausal women remain unknown, but the rise may be related to supraphysiologic concentration of estrogen in the liver. To date, transdermal delivery of estrogen, which avoids the first-pass hepatic metabolism of estradiol, appears to have a small BP-lowering effect in postmenopausal women and may be a safer alternative in hypertensive women.

The aryl hydrocarbon receptor pathway and sexual differentiation of neuroendocrine functions

Historically, much of the research on health effects of environmental pollutants focused on ascertaining whether compounds were carcinogenic. More recent findings show that environmental contaminants also exert insidious effects by disrupting hormone action. Of particular concern are findings that developmental exposure to dioxins, chemicals that act through the aryl hydrocarbon receptor pathway, permanently alters sexually differentiated neural functions in animal models. In this review, we focus on mechanisms through which dioxins disrupt neuroendocrine development as exemplified by effects on a brain region critical for ovulation in rodents. We also provide evidence that dysregulation of GABAergic neural development may be a general mechanism underlying a broad spectrum of effects seen after perinatal dioxin exposure.

Estrogen receptor-alpha and -beta immunoreactive neurons in the brainstem and spinal cord of male and female mice: relationships to monoaminergic, cholinergic, and spinal projection systems

For many populations of estrogen-sensitive neurons it remains unknown how they are associated with central nervous system circuitries that mediate estrogen-induced modulation of behavioral components. With the use of double-labeling immunohistochemistry and tracing techniques, the relationships of estrogen receptor (ER)-alpha- and ER-beta-immunoreactive (IR) neurons in the mouse brainstem and spinal cord to monoaminergic, cholinergic, and spinal projection systems are explored. Similar distributions of ER-IR neurons were present in females and males, with differences in labeling intensity of ER-alpha immunoreactivity among males and estrogen-, and oil-treated females. Barrington's nucleus, the ventrolateral medulla, and the nucleus of the solitary tract contained spinal-projecting ER-alpha-IR neurons, whereas ER-alpha-IR neurons in the periaqueductal gray, parabrachial nucleus, and catecholaminergic A1 cell group received spinal input. Numerous tyrosine hydroxylase (TH)-IR ER-alpha-IR neurons were present in the ventral periaqueductal gray, nucleus of the solitary tract, A1 cell group, and lumbosacral cord. The dorsal raphe nucleus contained ER-alpha-IR and ER-beta-IR neurons that colocalized with serotonin (5HT), and the reticulotegmental nucleus contained 5HT-IR ER-alpha-IR neurons. Fibers IR for vesicular acetylcholine transporter (VAChT), TH, and 5HT were located among ER-alpha-IR neurons in the dorsal horn and spinal autonomic regions. Robust staining for TH and VAChT, but not 5HT, was present among ER-alpha-IR neurons in the lumbosacral lateral collateral pathway. Possible modulatory actions of estrogen on each of these ER-IR populations are discussed in the context of their specific function, including micturition, sexual behavior, ejaculation, cardiovascular and respiratory control, tactile and nociceptive sensory processing, anti-nociception, endocrine regulation, and feeding.

Evidence that projections from the bed nucleus of the stria terminalis and from the lateral and medial regions of the preoptic area provide input to gonadotropin releasing hormone (GNRH) neurons in the female sheep brain

The arcuate nucleus/ventromedial hypothalamic nucleus (ARC/VMH) region is thought to relay estrogen feedback signals to gonadotropin-releasing hormone (GnRH) cells in the sheep brain. This region sends major projections to the lateral preoptic area (lPOA), ventral bed nucleus of the stria terminals (vBnST) and the ventro-caudal division of the median preoptic nucleus (vcMePON) with little direct input to GnRH cell bodies, suggesting interneuronal relay to GnRH neurons. The brain stem also provides input to the POA. The present study aimed to identify possible relay circuits in the POA and BnST to GnRH neurons. Biotinylated dextran amine (BDA) was injected into lPOA (n=6), vBnST (n=2), vcMePON (n=3) and periventricular nucleus (PeriV; n=1) of ewes for anterograde tracing. GnRH immunoreactive (IR) perikarya appearing to receive input from BDA-containing varicosities were identified by fluorescence microscopy, with further analysis by confocal microscopy. When BDA was injected into rostral and caudal regions of lPOA (n=3), no tracer-filled varicose fibers were found in contact with GnRH-IR perikarya. Injections into the center of the lPOA (n=3) indicated direct projections to GnRH-IR cells. Injections into the vBnST, vcMePON and PeriV indicated that cells of these regions also provide input to GnRH cells. BDA-containing varicosities found in the MPOA were immunoreactive for NPY or were GABAergic or glutamatergic when the tracer was injected into vBnST and lPOA, but not when injections were placed in the vcMePON. With injection into the PeriV, tracer-filled varicosities in the MPOA were not immunoreactive for somatostatin or enkephalin. Injection of FluoroGold into ventral POA retrogradely labeled cells in the above mentioned areas, but few were also immunoreactive for estrogen receptor-alpha. Thus, cells of the vBnST, lPOA, vcMePON and PeriV project to GnRH neurons. These cells may provide an interneuronal route to GnRH neurons from the ARC/VMH, the brain stem and other regions of the brain.

Immunocytochemical localization of aromatase in sensory and integrating nuclei of the hindbrain in Japanese quail (Coturnix japonica)

The distribution of the estrogen synthesizing enzyme (aromatase) in the hindbrain (rhombencephalon and mesencephalon) of male adult quail was investigated by immunocytochemistry. Aromatase-immunoreactive neuronal structures (perikarya and fibers bearing punctate structures) were observed in sensory (trigeminal, solitary tract, vestibular, optic tectum) and integrating (parabrachial, periaqueductal, cerulean, raphe) nuclei. Besides the expression of aromatase in these well-delineated nuclei, dense to scattered networks of immunoreactive fibers were found dispersed throughout the hindbrain and, in particular, in its rostral and dorsal parts. To a lesser extent, they were also present throughout the premotor nuclei of the reticular formation and in various fiber tracts. In contrast, no immunoreactive signal was found in motor nuclei, and in most of the statoacoustic (cerebellum, cochlear, olive, pontine, part of vestibular) nuclei. The expression of aromatase in perikarya and fibers in areas of the adult hindbrain where estrogen receptors have been identified previously suggests a role for estrogens locally produced in the regulation of sensory and integrating functions, contrary to the widespread assumption that these functions are regulated exclusively by steroids produced in the gonads.

Luteinizing hormone and luteinizing hormone-releasing hormone secretion is under locus coeruleus control in female rats

It has been suggested that norepinephrine (NE) from the locus coeruleus (LC) plays an important role in triggering the preovulatory surge of gonadotropins. This work intended to study the role of LC in luteinizing hormone (LH) secretion during the estrous cycle and in ovariectomized rats treated with estradiol and progesterone (OVXE(2)P) and to correlate it with LH releasing hormone (LHRH) content in the medial preoptic area (MPOA) and median eminence (ME). Female rats on each day of the estrous cycle and OVXE(2)P were submitted to jugular cannulation and LC electrolytic lesion or sham-operation, at 09:00 h. Blood samples were collected hourly from 11:00 to 18:00 h, when animals were decapitated and their brains removed to analyze LC lesion and punch out the MPOA and ME. Plasma LH levels and LHRH content of MPOA and ME were determined by radioimmunoassay. During metestrus, diestrus and estrus, LC lesion did not modify either LH plasma concentrations or LHRH content, but completely abolished the preovulatory LH surge during proestrus and the surge of OVXE(2)P. These blockades were accompanied by an increased content of LHRH in the MPOA and ME. The results suggest that: (1). LC does not participate in the control of basal LH secretion but its activation is essential to trigger spontaneous or induced LH surges, and (2). the increased content of LHRH in the MPOA and ME may be due to a decreased NE input to these areas. Thus, LC activation may be required for depolarization of LHRH neurons and consequent LH surges.

Regulation of norepinephrine in the medial preoptic area of Siberian hamsters by gonadal steroids

Photoperiod has profound effects upon the neuroendocrine axis underlying reproductive physiology in seasonally breeding mammals. For long-day (LD) breeders, such as the Siberian hamster, exposure to a short-day (SD) photoperiod results in declines in circulating levels of gonadal steroids, luteinizing hormone (LH), and prolactin (PRL). The current study sought to investigate the effects of photoperiod and steroid levels on norepinephrine (NE), one of the major neurochemical regulators of gonadotropin-releasing hormone (GnRH) function. Since NE release within the medial preoptic area (mPOA) has been shown to stimulate the activity of GnRH cells, it was hypothesized that exposure to a short photoperiod would decrease NE levels. Furthermore, since gonadal steroids show negative feedback on GnRH function, it was hypothesized that gonadectomy would result in increased levels of NE. Adult male and female Siberian hamsters were gonadectomized and implanted with silastic capsules containing either cholesterol (C) or a mixture of estradiol (E) or testosterone (T). Microdialysis sampling within the mPOA was conducted after 8 weeks of exposure to either an LD or an SD photoperiod. Blood samples were analyzed for LH and PRL, while dialysis samples were analyzed for NE and its major metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG). The results revealed significant suppression of LH and PRL by exposure to the SD photoperiod in both males and females. For LH, the steroid implants suppressed circulating hormone levels under both photoperiods, whereas for PRL, steroid treatment facilitated circulating levels. In contrast, there were no significant effects of photoperiod on NE or MHPG release for either males or females, but there was a significant decrease in extracellular levels of these neurochemicals in steroid-treated animals. These data suggest that photoperiodic modulation of GnRH neuronal function by NE is achieved largely through the indirect effects of photoperiod on circulating gonadal steroids.

Effects of estradiol on glucoprivic transactivation of catecholaminergic neurons in the female rat caudal brainstem

Hyperphagic and hypothalamic neuroendocrine responses to acute glucose deprivation are modified by the ovarian steroid estradiol (E). Observations of genomic activation of catecholaminergic (CA) neurons in the hindbrain lateral reticular nucleus, nucleus of the solitary tract, and area postrema (AP) by glucopenia support their potential function in pathways mediating regulatory effects of this metabolic challenge within the brain. Expression of E receptors by these cells suggests that their activity may be sensitive to steroid modulation during glucopenia. The present studies investigated the role of E on transcriptional activation of caudal brainstem CA neurons by the glucose antimetabolite, 2-deoxy-D-glucose (2DG). Ovariectomized rats were implanted with s.c. Silastic capsules containing E (30 or 250 microg/ml) or sesame oil, and injected i.p. 7 days later with 400 mg 2DG/kg or saline. Tyrosine hydroxylase (TH)-immunoreactive (-ir) neurons in the C(1)/A1, C2, C3, A2, A5, and A6 cell groups and AP were colabeled for Fos following antimetabolite administration, whereas vehicle injection resulted in negligible nuclear staining of these cells. With the exception of A2, A6, and AP cells, mean numbers of Fos- and TH-/Fos-ir-positive neurons in these brain sites did not differ between E- and sesame oil-implanted groups. Numbers of TH-positive A2 and A6 neurons that expressed Fos in response to 2DG were significantly greater in rats implanted with the high E dose vs. either the low steroid dose or sesame oil. These results show that the magnitude of cellular Fos labeling within discrete hindbrain CA neuron populations varies in accordance with circulating E levels. These findings suggest that E may exert potential modulatory effects on glucoprivic activation of the Fos stimulus/transcription cascade and consequent compensatory genomic responses within specific areas of the female rat caudal brainstem.

Relationship between norepinephrine release in the hypothalamic paraventricular nucleus and circulating prolactin levels in the Siberian hamster: role of photoperiod and the pineal gland

The impact of norepinephrine (NE) and its metabolite, 3-methoxy4-hydroxyphenylglycol (MHPG), on circulating prolactin (PRL) was evaluated in the paraventricular region of the hypothalamus as a function of photoperiod and integrity of the pineal gland. In Experiment 1, whole tissue content of NE and MHPG was assessed in male and female hamsters that had been pinealectomized or sham-pinealectomized and exposed to long or short photoperiods for 5 weeks. The results revealed a marginal effect of photoperiod in males, but no overall effects of surgery. Because analysis of whole tissue content can be complicated by concurrent changes in synthesis and storage rates, Experiment 2 was conducted using microdialysis to assess extracellular levels of NE and MHPG in female hamsters. Pinealectomy completely prevented the short-day-induced suppression of luteinizing hormone, but it only partially prevented the effects of short days on PRL. Furthermore, both NE and MHPG levels were significantly elevated in short-day-exposed pinealectomized and sham-operated animals. These results suggest that NE release within the paraventricular nucleus inhibits the circulating PRL levels and is one mechanism by which direct photic information can influence the neuroendocrine axis independently of the pineal melatonin signal.

Gonadal steroid receptors in the regulation of GnRH secretion in farm animals

The sites of action and mechanisms by which gonadal steroids regulate gonadotrophin-releasing hormone (GnRH) in domestic animals remain largely unknown. This review summarises information gained from sheep regarding the distribution of the gonadal steroid receptors in the brain, the neurochemical identity and the projections of these steroid receptor-containing neurones. The cells in the hypothalamus that contain each of the gonadal steroid receptors (oestrogen receptor alpha (ERalpha), oestrogen receptor beta (ERbeta), progesterone receptor (PR) and androgen receptor (AR)) show a remarkably similar distribution, although the PR and AR-containing cells are less widespread than oestrogen receptors (ERs). There is considerable overlap in the distribution of ERalpha- and ERbeta-containing cells but also some unique sites for each subtype. This suggests differential regulation of the actions of oestrogen. There appears to be little sexual dimorphism in the distribution of the gonadal steroid receptors in the hypothalamus, with the notable exception of the ventromedial nucleus where females appear to have greater numbers of both ERalpha- and ERbeta-containing cells. Neuronal tracing studies have identified projections of some of the ERalpha-containing cells to sites that may allow interaction with the GnRH system. The receptor mapping, neuronal tracing and microimplantation studies suggest that the ventromedial nucleus is likely to be a key hypothalamic nucleus in the steroid regulation of GnRH secretion in sheep.

Neuronal projections to the lateral retrochiasmatic area of sheep with special reference to catecholaminergic afferents: immunohistochemical and retrograde tract-tracing studies

The retrochiasmatic area contains the A15 catecholaminergic group and numerous monoaminergic afferents whose discrete cell origins are unknown in sheep. Using tract-tracing methods with a specific retrograde fluorescent tracer, fluorogold, we examined the cells of origin of afferents to the retrochiasmatic area in sheep. The retrogradely labeled cells were seen by observation of the tracer by direct fluorescence or by immunohistochemistry with specific antibodies raised in rabbits or horses. Among the retrogradely labeled neurons, double immunohistochemistry for tyrosine hydroxylase, dopamine-beta-hydroxylase, and serotonin were used to characterize catecholamine and serotonin FG labeled neurons. The retrochiasmatic area, which included the A15 dopaminergic group and the accessory supraoptic nucleus (SON), received major inputs from the lateral septum (LS), the bed nucleus of the stria terminalis (BNST), the thalamic paraventricular nucleus, hypothalamic paraventricular and supraoptic nuclei, the perimamillary area, the amygdala, the ventral part of the hippocampus and the parabrachial nucleus (PBN). Further, numerous scattered retrogradely labeled neurons were observed in the preoptic area, the ventromedial part of the hypothalamus. the periventricular area, the periaqueductal central gray (CG), the ventrolateral medulla and the dorsal vagal complex. Most of the noradrenergic afferents came from the ventro-lateral medulla (Al group), and only a few from the locus coeruleus complex (A6/A7 groups). A few dopaminergic neurons retrogradely labeled with flurogold were observed in the periventricular area of the hypothalamus. Rare serotoninergic fluorogold labeled neurons belonged to the dorsal raphe nucleus. Most of these afferents came from both sides of the brain, except for hypothalamic supraoptic and paraventricular nuclei. In the light of these anatomical data, we compared our results with data obtained from rats, and we discussed the putative role of these afferents in sheep in the regulation of several specific functions in which the retrochiasmatic area may be involved, such as reproduction.

Colocalization of arginine-vasotocin and chicken luteinizing hormone-releasing hormone-I (cLHRH-I) in the preoptic-hypothalamic region of the chicken

To characterize a possible relationship between chicken luteinizing hormone-releasing hormone-I (cLHRH-I) and arginine-vasotocin (AVT) we performed immunocytochemical double-stainings throughout the preoptic-hypothalamic region of the chicken brain. This study clearly reveals a partial colocalization between both neuropeptides. Single-labeled neurons, containing either cLHRH-I or AVT are found intermingled with double stained cells, immunoreactive (ir) for both peptides. A significant number of double-labeled perikarya is found in the preoptic area, more specifically in the ventral and external portion of the supraoptic nucleus (SOv and SOe) and in the medial preoptic nucleus (MPOv). At the level of the anterior hypothalamus, double-labeled cells are predominantly observed near the third ventricle in the nucleus paraventricularis magnocellularis (PVN) and the nucleus periventricularis hypothalami (PHN). Next to this colocalization, a number of cLHRH-I-ir cell bodies are found in close apposition to AVT-ir fiber profiles in the very same areas. Taken together, these data are the first to provide morphological evidence indicating that the AVT system might be involved in the regulation of cLHRH-I release and thus of reproductive functions in birds.

Oestrogen receptors in the brainstem of the female sheep: relationship to noradrenergic cells and cells projecting to the medial preoptic area

Oestrogen regulates the secretion of gonadotropin releasing hormone (GnRH) and this could be mediated by noradrenergic systems originating in the brainstem. Whilst it is known that noradrenergic cells possess oestrogen receptors (ER), it is not known whether ER-immunoreactive (-ir) cells in the brainstem project to the regions of the hypothalamus in which GnRH neurons are found. We have used dual-label immunocytochemistry to determine the extent to which ER-alpha is found in noradrenergic cells in the brainstem of the ovariectomized (OVX) ewe. Noradrenergic/adrenergic cells were identified by immunostaining for dopamine beta-hydroxylase (DBH). Cells that stained for both DBH and ER were found in both the A1 and A2 cell groups, with the highest levels found in the most caudal regions. In the A1 group, at the most caudal extent, 73% of ER-ir cells were DBH-positive and 19% of DBH-ir cells were ER-positive. The degree of co-localization decreased in a linear manner towards the rostral brainstem. In the caudal half of A2, 9-14% of ER-ir cells were DBH-positive and 20-25% of DBH cells were ER-positive. Less than 2% of DBH-ir cells in the A5 group were dual-labelled and none of the cells in the A6 and A7 groups were ER-positive. The retrograde tracer FluoroGold was injected into the preoptic area of nine OVX ewes and labelled cells were examined in the brainstem to determine the extent of co-localization of ER. Only injections in the rostroventral part of the medial preoptic area near to the organum vasculosum of the lamina terminalis resulted in the labelling of cells in the brainstem. One ewe with very strong labelling of the brainstem was selected for detailed mapping. In the ventrolateral medulla, half the ER-ir cells in the most caudal regions were retrogradely labelled. Almost all the ER-ir cells in the mid-region of the ventrolateral medulla were retrogradely labelled but no co-localization of retrograde tracer and ER was observed rostral to obex. There were many ER-ir cells and retrogradely-labelled cells in the nucleus of the solitary tract but only a few double-labelled cells. Similarly, numerous ER-ir cells and retrogradely labelled cells were observed around the lateral edges of the caudal fourth ventricle and across to the lateral parabrachial nucleus but there were few double-labelled cells. These results suggest differential regulation of noradrenergic cells by oestrogen, with a direct action of the hormone confined to the cells in the most caudal region of the A1 and A2 cell groups. The cells of the caudal ventrolateral medulla which contain ER-ir cells that project to the preoptic area may be important in the mediation by noradrenaline of the actions of oestrogen on GnRH secretion in the ewe.

Localization of estrogen-receptive neurons projecting to the GnRH neuron-containing rostral preoptic area of the ewe

Estrogen exerts important feedback effects upon the biosynthetic and secretory behavior of gonadotropin-releasing hormone (GnRH) neurons to control reproductive functioning. The mechanism of estrogen action upon these neurons is unclear and seems likely to involve the transsynaptic regulation of GnRH neurons. The objective of the present study was to identify the estrogen-receptive neural populations which project to the general vicinity of the GnRH perikarya in the rostral preoptic area and diagonal band of Broca (rPOA/DBB) of the ewe. Intact breeding-season ewes received an injection of the retrograde tracer fluorogold (FG) into the rPOA/DBB, and their hypothalami and brainstems examined for the presence of FG and estrogen receptor alpha (ERalpha) immunocytochemistry. Retrogradely labeled neurons were identified principally within the lateral septum (LS), lamina terminalis, bed nucleus of the stria terminalis, POA, arcuate nucleus (ARN), ventromedial nucleus (VMN) and median eminence. Smaller numbers of FG-immonoreactive cells were found in the caudal brainstem where they resided mostly in the ventrolateral medulla (VLM). Dual-labeled cells exhibiting both FG and ERalpha staining were prominent in the POA, LS and at all rostrocaudal levels of the VMN and ARN. Small numbers of dual-labeled cells were found in the VLM. These observations indicate that a number of distinct ERalpha-expressing neural populations project to the rPOA/DBB where the majority of the GnRH perikarya are found in the ewe. Although it is not possible to determine the direct connectivity of these projections with GnRH neurons, the findings provide an initial neuroanatomical framework through which the transsynaptic actions of estrogen on ovine GnRH neurons may be tested.

Suppression of tonic luteinizing hormone secretion and norepinephrine release near the GnRH neurons by estradiol in ovariectomized rats

One of the major neurotransmitters that controls pulsatile luteinizing hormone (LH) secretion is norepinephrine (NE). NE pulses detected in the median eminence of ovariectomized rhesus monkeys are highly correlated with both GnRH and LH pulses. In contrast, previous reports suggest that this is not the case in rats, thus it remains to be determined whether NE stimulates LH release on a pulse-by-pulse basis in that species. Further, a variety of indirect evidence supports the hypothesis that in rats, estradiol exerts its negative feedback action on LH secretion in part by inhibiting noradrenergic neurotransmission that is stimulatory to LH release, but there is no direct evidence to support this hypothesis. Therefore the following study was designed to test the hypothesis that estradiol suppresses NE release in the vicinity of the GnRH neurons after ovariectomy. In addition, we examined whether episodes of NE release are correlated with LH pulses in ovariectomized rats. Blood samples and microdialysates of the diagonal band of Broca/medial preoptic area (DBB/MPOA) were collected every 5 min from 09:00 to 14:00 h from untreated or estradiol-treated (4-5 days), long-term ovariectomized (1-4 months) rats for determination of plasma LH by RIA and NE release by HPLC. The results indicate that in both untreated and estradiol-treated ovariectomized rats, LH pulses are not correlated with episodes of NE. Thus, NE may play a permissive role in the control of pulsatile LH secretion in rats. Further, estradiol treatment leads to a suppression of both plasma LH levels and NE release in the DBB/MPOA, supporting the hypothesis that a decrease in NE neurotransmission that is stimulatory to LH release mediates the negative feedback action of estradiol on tonic LH secretion.

Levels of dopamine beta hydroxylase immunoreactivity in the preoptic hypothalamus of the ovariectomised ewe following injection of oestrogen: evidence for increased noradrenaline release around the time of the oestrogen-induced surge in luteinizing hormone

We have measured dopamine beta hydroxylase (DBH) immunoreactivity in the preoptic hypothalamus as an index of release of noradrenaline in the ovariectomised (OVX) ewe at the time of an oestrogen-induced surge in luteinizing hormone (LH) release. OVX ewes (n=5) were given an injection of 50 microg oestradiol benzoate (or oil), which causes a surge in the secretion of LH. Blood samples were taken and sheep were killed 16 h later. Other groups (n=3) were given oil or oestrogen and killed 6 h later. Brains were collected for immunohistochemistry and image analysis. The number of DBH-stained cells and the integrated optical density of the cells was measured throughout the A1 field of the brainstem. The DBH staining was measured in the terminal beds of the hypothalamus. There was no difference between the controls and the EB-treated OVX ewes in the number of DBH positive cells or the optical density of DBH-staining cells in the A1 field. Within the preoptic area, there was reduced (P<0.02) DBH staining in the 16 h EB-treated ewes. There was no change in the DBH staining in the paraventricular nucleus or the arcuate nucleus of the hypothalamus. These data suggest that there is release of noradrenaline in the preoptic area at the time of the E-induced GnRH/LH surge in the OVX ewe.