Estrogen-receptor occurrence in the female mouse brain: effects of maternal experience, ovariectomy, estrogen and anosmia

Plasticity in auditory cortex during parenthood

Most animals display robust parental behaviors that support the survival and well-being of their offspring. The manifestation of parental behaviors is accompanied by physiological and hormonal changes, which affect both the body and the brain for better care giving. Rodents exhibit a behavior called pup retrieval - a stereotyped sequence of perception and action - used to identify and retrieve their newborn pups back to the nest. Pup retrieval consists of a significant auditory component, which depends on plasticity in the auditory cortex (ACx). We review the evidence of neural changes taking place in the ACx of rodents during the transition to parenthood. We discuss how the plastic changes both in and out of the ACx support the encoding of pup vocalizations. Key players in the mechanism of this plasticity are hormones and experience, both of which have a clear dynamic signature during the transition to parenthood. Mothers, co caring females, and fathers have been used as models to understand parental plasticity at disparate levels of organization. Yet, common principles of cortical plasticity and the biological mechanisms underlying its involvement in parental behavior are just beginning to be unpacked.

Effects of maternal experience on pup-induced activation of maternal neural circuits in virgin mice

Maternal experience can promote a long-lasting increase in maternal motivation. This maintenance of caregiving behaviors, rather than avoidant or agnostic responses towards young, is advantageous for the survival of subsequent offspring. We have previously reported that maternal motivation is associated with differential immediate early gene expression in central motivation circuits and aversion circuits. Here we ask how these circuits come to differentially respond to infant cues. We used Targeted Recombination in Active Populations (TRAP) to identify cells that respond to pups in maternally hesitant TRAP2;Ai14 virgin female mice. Following an initial 60 min exposure to foster pups, virgin TRAP2;Ai14 mice were injected with 4-hydroxytamoxifen to induce recombination in c-Fos expressing cells and subsequent permanent expression of a red fluorescent reporter. We then examined whether the same cells that encode pup cues are reactivated during maternal memory retrieval two weeks later using c-Fos immunohistochemistry. Whereas initial pup exposure induced c-Fos activation exclusively in the medial preoptic area (MPOA), following repeated experience, c-Fos expression was significantly higher than baseline in multiple regions of maternal and central aversion circuits (e.g., ventral bed nucleus of the stria terminalis, nucleus accumbens, basolateral amygdala, prefrontal cortex, medial amygdala, and ventromedial nucleus of the hypothalamus). Further, cells in many of these sites were significantly reactivated during maternal memory retrieval. These data suggest that cells across both maternal motivation and central aversion circuits are stably responsive to pups and thus may form the cellular representation of maternal memory.

Estradiol Regulation of the Prelimbic Cortex and the Reinstatement of Cocaine Seeking in Female Rats

Relapse susceptibility in women with substance use disorders (SUDs) has been linked to the estrogen, 17β-estradiol (E2). Our previous findings in female rats suggest that the influence of E2 on cocaine seeking can be localized to the prelimbic prefrontal cortex (PrL-PFC). Here, we investigated the receptor mechanisms through which E2 regulates the reinstatement of extinguished cocaine seeking. Sexually mature female rats underwent intravenous cocaine self-administration (0.5 mg/inf; 14 × 2 h daily) and extinction, and then were ovariectomized before reinstatement testing. E2 (10 µg/kg, i.p.) alone did not reinstate cocaine seeking, but it potentiated reinstatement when combined with an otherwise subthreshold priming dose of cocaine. A similar effect was observed following intra-PrL-PFC microinfusions of E2 and by systemic or intra-PrL-PFC administration of the estrogen receptor (ER)β agonist, DPN, but not agonists at ERα or the G-protein-coupled ER1 (GPER1). By contrast, E2-potentiated reinstatement was prevented by intra-PrL-PFC microinfusions of the ERβ antagonist, MPP, or the GPER1 antagonist, G15, but not an ERα antagonist. Whole-cell recordings in PrL-PFC layer (L)5/6 pyramidal neurons revealed that E2 decreases the frequency, but not amplitude, of GABAA-dependent miniature IPSCs (mIPSC). As was the case with E2-potentiated reinstatement, E2 reductions in mIPSC frequency were prevented by ERβ and GPER1, but not ERα, antagonists and mimicked by ERβ, but not GPER1, agonists. Altogether, the findings suggest that E2 activates ERβ and GPER1 in the PrL-PFC to attenuate the GABA-mediated constraint of key outputs that mediate cocaine seeking.

Significant increases of the amygdala between immediate and late postpartum: Pronounced effects within the superficial subregion

Research exploring the underlying neuroanatomical correlates of early motherhood seems to suggest that the period after giving birth is marked by tissue increases in the mother's brain. While some studies point to the amygdala as one of the areas undergoing postpartum changes, existing analyses did not discriminate between the different subregions of this functionally heterogeneous structure. Thus, to further extend this understudied field of research and to better understand the potential role of the amygdala when transitioning to motherhood, we applied an advanced region-of-interest technique that enabled us to analyze the amygdala as a whole as well as its different subareas, specifically the left and right centromedian (CM), laterobasal (LB), and superficial (SF) regions. Comparing the brains of 14 healthy women between immediate postpartum (within 1-2 days of childbirth) and late postpartum (at 4-6 weeks after childbirth), we revealed increases of the amygdala. However, effects manifested differentially across subareas, with particularly strong effects for the SF region, moderate effects for the CM region, and no effects for the LB region. These findings might reflect region-specific adaptations of the mother's brain tuning into the distinct and ever-changing needs of a newborn, either as a cause for it or as a consequence thereof.

Sex differences in daily timekeeping and circadian clock circuits

The circadian system regulates behavior and physiology in many ways important for health. Circadian rhythms are expressed by nearly every cell in the body, and this large system is coordinated by a central clock in the suprachiasmatic nucleus (SCN). Sex differences in daily rhythms are evident in humans and understanding how circadian function is modulated by biological sex is an important goal. This review highlights work examining effects of sex and gonadal hormones on daily rhythms, with a focus on behavior and SCN circuitry in animal models commonly used in pre-clinical studies. Many questions remain in this area of the field, which would benefit from further work investigating this topic.

Estrogens and the circadian system

Circadian rhythms are ~24 h cycles of behavior and physiology that are generated by a network of molecular clocks located in nearly every tissue in the body. In mammals, the circadian system is organized hierarchically such that the suprachiasmatic nucleus (SCN) is the main circadian clock that receives light information from the eye and entrains to the light-dark cycle. The SCN then coordinates the timing of tissue clocks so internal rhythms are aligned with environmental cycles. Estrogens interact with the circadian system to regulate biological processes. At the molecular level, estrogens and circadian genes interact to regulate gene expression and cell biology. Estrogens also regulate circadian behavior across the estrous cycle. The timing of ovulation during the estrous cycle requires coincident estrogen and SCN signals. Studies using circadian gene reporter mice have also elucidated estrogen regulation of peripheral tissue clocks and metabolic rhythms. This review synthesizes current understanding of the interplay between estrogens and the circadian system, with a focus on female rodents, in regulating molecular, physiological, and behavioral processes.

A Scientometric Approach to Review the Role of the Medial Preoptic Area (MPOA) in Parental Behavior

Research investigating the neural substrates underpinning parental behaviour has recently gained momentum. Particularly, the hypothalamic medial preoptic area (MPOA) has been identified as a crucial region for parenting. The current study conducted a scientometric analysis of publications from 1 January 1972 to 19 January 2021 using CiteSpace software to determine trends in the scientific literature exploring the relationship between MPOA and parental behaviour. In total, 677 scientific papers were analysed, producing a network of 1509 nodes and 5498 links. Four major clusters were identified: “C-Fos Expression”, “Lactating Rat”, “Medial Preoptic Area Interaction” and “Parental Behavior”. Their content suggests an initial trend in which the properties of the MPOA in response to parental behavior were studied, followed by a growing attention towards the presence of a brain network, including the reward circuits, regulating such behavior. Furthermore, while attention was initially directed uniquely to maternal behavior, it has recently been extended to the understanding of paternal behaviors as well. Finally, although the majority of the studies were conducted on rodents, recent publications broaden the implications of previous documents to human parental behavior, giving insight into the mechanisms underlying postpartum depression. Potential directions in future works were also discussed.

Activation of the medial preoptic area (MPOA) ameliorates loss of maternal behavior in a Shank2 mouse model for autism

Impairments in social relationships and awareness are features observed in autism spectrum disorders (ASDs). However, the underlying mechanisms remain poorly understood. Shank2 is a high‐confidence ASD candidate gene and localizes primarily to postsynaptic densities (PSDs) of excitatory synapses in the central nervous system (CNS). We show here that loss of Shank2 in mice leads to a lack of social attachment and bonding behavior towards pubs independent of hormonal, cognitive, or sensitive deficits. Shank2^−/− mice display functional changes in nuclei of the social attachment circuit that were most prominent in the medial preoptic area (MPOA) of the hypothalamus. Selective enhancement of MPOA activity by DREADD technology re‐established social bonding behavior in Shank2^−/− mice, providing evidence that the identified circuit might be crucial for explaining how social deficits in ASD can arise.

A neural mechanism of nuclear receptor expression and regionalization

Spatially restricted expression of genes by global circulating inducers (hormones, secreted proteins, growth factors, neuromodulators, etc.) was a prerequisite for the evolution of animals. Far from a random occurrence, it is a systematically occurring, certain event, implying that specific information is invested for it to happen. In this minireview, we show for the first time that the expression and regionalization takes place at the level of receptors via a neural mechanism and make an attempt to reconstruct the causal chain from neural signaling to expression of nuclear receptors.

Neural Regulation of Paternal Behavior in Mammals: Sensory, Neuroendocrine, and Experiential Influences on the Paternal Brain

Across the animal kingdom, parents in many species devote extraordinary effort toward caring for offspring, often risking their lives and exhausting limited resources. Understanding how the brain orchestrates parental care, biasing effort over the many competing demands, is an important topic in social neuroscience. In mammals, maternal care is necessary for offspring survival and is largely mediated by changes in hormones and neuropeptides that fluctuate massively during pregnancy, parturition, and lactation (e.g., progesterone, estradiol, oxytocin, and prolactin). In the relatively small number of mammalian species in which parental care by fathers enhances offspring survival and development, males also undergo endocrine changes concurrent with birth of their offspring, but on a smaller scale than females. Thus, fathers additionally rely on sensory signals from their mates, environment, and/or offspring to orchestrate paternal behavior. Males can engage in a variety of infant-directed behaviors that range from infanticide to avoidance to care; in many species, males can display all three behaviors in their lifetime. The neural plasticity that underlies such stark changes in behavior is not well understood. In this chapter we summarize current data on the neural circuitry that has been proposed to underlie paternal care in mammals, as well as sensory, neuroendocrine, and experiential influences on paternal behavior and on the underlying circuitry. We highlight some of the gaps in our current knowledge of this system and propose future directions that will enable the development of a more comprehensive understanding of the proximate control of parenting by fathers.

Impact of Proestrus on Gene Expression in the Medial Preoptic Area of Mice

The antero-ventral periventricular zone (AVPV) and medial preoptic area (MPOA) have been recognized as gonadal hormone receptive regions of the rodent brain that-via wiring to gonadotropin-releasing hormone (GnRH) neurons-contribute to orchestration of the preovulatory GnRH surge. We hypothesized that neural genes regulating the induction of GnRH surge show altered expression in proestrus. Therefore, we compared the expression of 48 genes obtained from intact proestrous and metestrous mice, respectively, by quantitative real-time PCR (qPCR) method. Differential expression of 24 genes reached significance (p < 0.05). Genes upregulated in proestrus encoded neuropeptides (kisspeptin (KP), galanin (GAL), neurotensin (NT), cholecystokinin (CCK)), hormone receptors (growth hormone secretagogue receptor, μ-opioid receptor), gonadal steroid receptors (estrogen receptor alpha (ERα), progesterone receptor (PR), androgen receptor (AR)), solute carrier family proteins (vesicular glutamate transporter 2, vesicular monoamine transporter 2), proteins of transmitter synthesis (tyrosine hydroxylase (TH)) and transmitter receptor subunit (AMPA4), and other proteins (uncoupling protein 2, nuclear receptor related 1 protein). Proestrus evoked a marked downregulation of genes coding for adenosine A2a receptor, vesicular gamma-aminobutyric acid (GABA) transporter, 4-aminobutyrate aminotransferase, tachykinin precursor 1, NT receptor 3, arginine vasopressin receptor 1A, cannabinoid receptor 1, ephrin receptor A3 and aldehyde dehydrogenase 1 family, member L1. Immunocytochemistry was used to visualize the proteins encoded by Kiss1, Gal, Cck and Th genes in neuronal subsets of the AVPV/MPOA of the proestrous mice. The results indicate that gene expression of the AVPV/MPOA is significantly modified at late proestrus including genes that code for neuropeptides, gonadal steroid hormone receptors and synaptic vesicle transporters. These events support cellular and neuronal network requirements of the positive estradiol feedback action and contribute to preparation of the GnRH neuron system for the pre-ovulatory surge release.

Knowledge About Sounds-Context-Specific Meaning Differently Activates Cortical Hemispheres, Auditory Cortical Fields, and Layers in House Mice

Activation of the auditory cortex (AC) by a given sound pattern is plastic, depending, in largely unknown ways, on the physiological state and the behavioral context of the receiving animal and on the receiver's experience with the sounds. Such plasticity can be inferred when house mouse mothers respond maternally to pup ultrasounds right after parturition and naïve females have to learn to respond. Here we use c-FOS immunocytochemistry to quantify highly activated neurons in the AC fields and layers of seven groups of mothers and naïve females who have different knowledge about and are differently motivated to respond to acoustic models of pup ultrasounds of different behavioral significance. Profiles of FOS-positive cells in the AC primary fields (AI, AAF), the ultrasonic field (UF), the secondary field (AII), and the dorsoposterior field (DP) suggest that activation reflects in AI, AAF, and UF the integration of sound properties with animal state-dependent factors, in the higher-order field AII the news value of a given sound in the behavioral context, and in the higher-order field DP the level of maternal motivation and, by left-hemisphere activation advantage, the recognition of the meaning of sounds in the given context. Anesthesia reduced activation in all fields, especially in cortical layers 2/3. Thus, plasticity in the AC is field-specific preparing different output of AC fields in the process of perception, recognition and responding to communication sounds. Further, the activation profiles of the auditory cortical fields suggest the differentiation between brains hormonally primed to know (mothers) and brains which acquired knowledge via implicit learning (naïve females). In this way, auditory cortical activation discriminates between instinctive (mothers) and learned (naïve females) cognition.

Rat dams exposed repeatedly to a daily brief separation from the pups exhibit increased maternal behavior, decreased anxiety and altered levels of receptors for estrogens (ERα, ERβ), oxytocin and serotonin (5-HT1A) in their brain

In the present study we investigated the neurobiological mechanisms underlying expression of maternal behavior. Increased maternal behavior was experimentally induced by a brief 15-min separation between the mother and the pups during postnatal days 1 to 22. On postnatal days (PND) 12 and 22, we determined in experimental and control dams levels of anxiety in the elevated plus maze (EPM) as well as the levels of receptors for estrogens (ERα, ERβ), oxytocin (OTR) and serotonin (5-HT1AR) in areas of the limbic system (prefrontal cortex-PFC, hippocampus, lateral septum-SL, medial preoptic area-MPOA, shell of nucleus accumbens-nAc-Sh, central-CeA and basolateral-BLA amygdala), involved in the regulation of maternal behavior. Experimental dams, which showed increased maternal behavior towards their offspring, displayed reduced anxiety in the EPM on both PND12 and PND22. These behavioral differences could be attributed to neurochemical alterations in their brain: On both PND12 and PND22, experimental mothers had higher levels of ERα and OTRs in the PFC, hippocampus, CeA, SL, MPOA and nAc-Sh. The experimental manipulation-induced increase in ERβ levels was less widespread, being localized in PFC, the hippocampal CA2 area, MPOA and nAc-Sh. In addition, 5-HT1ARs were reduced in the PFC, hippocampus, CeA, MPOA and nAc-Sh of the experimental mothers. Our results show that the experience of the daily repeated brief separation from the pups results in increased brain ERs and OTRs, as well as decreased 5-HT1ARs in the dam's brain; these neurochemical changes could underlie the observed increase in maternal behavior and the reduction of anxiety.

Histone deacetylase inhibition induces long-lasting changes in maternal behavior and gene expression in female mice

In many species, including mice, maternal responsiveness is experience-dependent and permanent, lasting for long periods (months to years). We have shown that after brief exposures to pups, virgin female mice continue to respond maternally toward pups for at least one month. Administration of a histone deacetylase inhibitor (HDACi) reduces the amount of maternal experience required to affect maternal behavior and gene expression. In this set of studies, we examined the epigenetic mechanisms that underlie these motivated behaviors. We assessed whether the effects of HDACi persisted 1 month after the initial experience (in the absence of continued pup experience or HDACi treatment) and whether the maintenance of maternal memory was associated with stable changes in gene expression. Using chromatin immunoprecipitation, we examined whether Esr2 and Oxt gene expression might be mediated by recruitment of the histone acetyltransferase cAMP response element binding protein (CBP) to their promoter regions after maternal memory consolidation. We report that HDACi treatment induced long-lasting changes in maternal responsiveness. Maternal learning was associated with increased recruitment of CBP to the Esr2 and Oxt gene promoters during the consolidation of maternal memory as well as a persistent increase in estrogen receptor-β (Esr2) mRNA and decreased expression of the de novo DNA methyltransferase Dnmt3a within the medial preoptic area. The consolidation of the maternal experience may involve the CBP recruitment and stable changes in gene expression, which maintain increased maternal responsiveness for long periods of time.

Neonatal overexpression of estrogen receptor-α alters midbrain dopamine neuron development and reverses the effects of low maternal care in female offspring

Maternal behavior is dependent on estrogen receptor-alpha (ERα; Esr1) and oxytocin receptor (OTR) signaling in the medial preoptic area (MPOA) of the hypothalamus, as well as dopamine signaling from the ventral tegmental area (VTA) to forebrain regions. Previous studies in rats indicate that low levels of maternal care, particularly licking/grooming (LG), lead to reduced levels of MPOA ERα and VTA dopamine neurons in female offspring and predict lower levels of postpartum maternal behavior by these offspring. The aim of this study was to determine the functional impact on maternal behavior of neonatal manipulation of ERα in females that had experienced low versus high levels of postnatal maternal LG. Adenovirus expressing ESR1 was targeted to the MPOA in female pups from low and high LG litters on postnatal day 2-3. Overexpression of ESR1 in low LG offspring elevated the level of ERα-immunoreactive cells in the MPOA and of tyrosine hydroxylase cells in the VTA to that observed in high LG females. Amongst juvenile female low LG offspring, ESR1 overexpression also decreased the latency to engage in maternal behavior toward donor pups. These results show that virally mediated expression of ESR1 in the neonatal rat hypothalamus results in lasting changes in ESR1 expression through the juvenile period, and can "rescue" hormone receptor levels and behavior of offspring reared by low LG dams, potentially mediated by downstream alterations within reward circuitry. Thus, the transmission of maternal behavior from one generation to the next can be augmented by neonatal ERα in the MPOA.

Adult plasticity in the subcortical auditory pathway of the maternal mouse

Subcortical auditory nuclei were traditionally viewed as non-plastic in adulthood so that acoustic information could be stably conveyed to higher auditory areas. Studies in a variety of species, including humans, now suggest that prolonged acoustic training can drive long-lasting brainstem plasticity. The neurobiological mechanisms for such changes are not well understood in natural behavioral contexts due to a relative dearth of in vivo animal models in which to study this. Here, we demonstrate in a mouse model that a natural life experience with increased demands on the auditory system - motherhood - is associated with improved temporal processing in the subcortical auditory pathway. We measured the auditory brainstem response to test whether mothers and pup-naïve virgin mice differed in temporal responses to both broadband and tone stimuli, including ultrasonic frequencies found in mouse pup vocalizations. Mothers had shorter latencies for early ABR peaks, indicating plasticity in the auditory nerve and the cochlear nucleus. Shorter interpeak latency between waves IV and V also suggest plasticity in the inferior colliculus. Hormone manipulations revealed that these cannot be explained solely by estrogen levels experienced during pregnancy and parturition in mothers. In contrast, we found that pup-care experience, independent of pregnancy and parturition, contributes to shortening auditory brainstem response latencies. These results suggest that acoustic experience in the maternal context imparts plasticity on early auditory processing that lasts beyond pup weaning. In addition to establishing an animal model for exploring adult auditory brainstem plasticity in a neuroethological context, our results have broader implications for models of perceptual, behavioral and neural changes that arise during maternity, where subcortical sensorineural plasticity has not previously been considered.

The maternal brain: an organ with peripartal plasticity

The time of pregnancy, birth, and lactation, is characterized by numerous specific alterations in several systems of the maternal body. Peripartum-associated changes in physiology and behavior, as well as their underlying molecular mechanisms, have been the focus of research since decades, but are still far from being entirely understood. Also, there is growing evidence that pregnancy and lactation are associated with a variety of alterations in neural plasticity, including adult neurogenesis, functional and structural synaptic plasticity, and dendritic remodeling in different brain regions. All of the mentioned changes are not only believed to be a prerequisite for the proper fetal and neonatal development, but moreover to be crucial for the physiological and mental health of the mother. The underlying mechanisms apparently need to be under tight control, since in cases of dysregulation, a certain percentage of women develop disorders like preeclampsia or postpartum mood and anxiety disorders during the course of pregnancy and lactation. This review describes common peripartum adaptations in physiology and behavior. Moreover, it concentrates on different forms of peripartum-associated plasticity including changes in neurogenesis and their possible underlying molecular mechanisms. Finally, consequences of malfunction in those systems are discussed.

Sex differences in behavioral circadian rhythms in laboratory rodents

There is a strong bias in basic research on circadian rhythms toward the use of only male animals in studies. Furthermore, of the studies that use female subjects, many use only females and do not compare results between males and females. This review focuses on behavioral aspects of circadian rhythms that differ between the sexes. Differences exist in the timing of daily onset of activity, responses to both photic and non-photic stimuli, and in changes across the lifespan. These differences may reflect biologically important traits that are ecologically relevant and impact on a variety of responses to behavioral and physiological challenges. Overall, more work needs to be done to investigate differences between males and females as well as differences that are the result of hormonal changes across the lifespan.

Experience-facilitated improvements in pup retrieval; evidence for an epigenetic effect

The quality and quantity of maternal care received during infancy are highly predictive of successful infant development. It has been well established, primarily in rats, that the combination of hormonal and infant stimuli at birth modifies neural circuits that regulate maternal responsiveness. During subsequent interactions, infant stimuli are more likely to elicit rapid maternal responsiveness. Some species, such as humans, can display maternal care in the absence of the endocrine events of pregnancy and birth. Similarly, virgin C57BL/6J female mice, display maternal care toward infants, and experience with infants elicits long-lasting increases in maternal care. We hypothesized that these experience-induced changes in behavior may be mediated by chromatin modifications, which in turn change expression of genes that promote maternal care. One site of action is the medial preoptic area (MPOA). To test our hypothesis we treated virgin female mice with sodium butyrate, a histone deacetylase inhibitor. This treatment potentiated maternal responsiveness as well as the expression of several genes: estrogen receptor β (Esr2), oxytocin (Oxt), and cyclicAMP response element binding protein (CREB) binding protein (Crebbp; a histone acetyltransferase) in the MPOA. These data suggest that experience induces high levels of maternal care via epigenetic modifications.

Limbic brain activation for maternal acoustic perception and responding is different in mothers and virgin female mice

Mothers are primed to become maternal through hormonal changes during pregnancy and delivery of young, virgin females need experience with young for performing maternally. The activation of brain areas controlling maternal behavior can be studied by stimulus-induced expression of the immediate-early gene Fos and immunocytochemical labeling of the FOS protein in activated cells. With this technique we identified areas of the mouse limbic system stimulated by acoustically adequate or inadequate models of pup ultrasounds that, if perceived as adequate, direct the search for lost pups (phonotaxis). Behavioral observations and neural activation data suggest that adequate (50 kHz long tones) and inadequate ultrasound models (50 kHz short or 20 kHz long tones) are differently processed in limbic areas of mothers and virgin females with 1 or 5 days of pup-caring experience depending on the news value and the recognition of the stimuli: High numbers of FOS-positive cells in the medial preoptic area, lateral septum, and bed nucleus of the stria terminalis (mothers and virgins) relate to the salience (news value) of the perceived sounds; contextual stress may be reflected by high activation in parts of the amygdala and the ventromedial hypothalamus (virgins); high activation in the piriform cortex suggests associative learning of adequate sounds and in the entorhinal cortex remembering associations of adequate sounds with pups (virgins). Thus brain areas were differently activated in animals with maternal emotions, however different responses to pup cues depending on how they got primed to behave maternally and on how they evaluated the stimulation context.

Gonadotropin-releasing hormone receptor system: modulatory role in aging and neurodegeneration

Receptors for hormones of the hypothalamic-pituitary-gonadal axis are expressed throughout the brain. Age-related decline in gonadal reproductive hormones cause imbalances of this axis and many hormones in this axis have been functionally linked to neurodegenerative pathophysiology. Gonadotropin-releasing hormone (GnRH) plays a vital role in both central and peripheral reproductive regulation. GnRH has historically been known as a pituitary hormone; however, in the past few years, interest has been raised in GnRH actions at non-pituitary peripheral targets. GnRH ligands and receptors are found throughout the brain where they may act to control multiple higher functions such as learning and memory function and feeding behavior. The actions of GnRH in mammals are mediated by the activation of a unique rhodopsin-like G protein-coupled receptor that does not possess a cytoplasmic carboxyl terminal sequence. Activation of this receptor appears to mediate a wide variety of signaling mechanisms that show diversity in different tissues. Epidemiological support for a role of GnRH in central functions is evidenced by a reduction in neurodegenerative disease after GnRH agonist therapy. It has previously been considered that these effects were not via direct GnRH action in the brain, however recent data has pointed to a direct central action of these ligands outside the pituitary. We have therefore summarized the evidence supporting a central direct role of GnRH ligands and receptors in controlling central nervous physiology and pathophysiology.

Ventromedial and medial preoptic hypothalamic ibotenic acid lesions potentiate systemic morphine analgesia in female, but not male rats

Sex differences in systemic morphine analgesia occur with male rodents displaying significantly greater analgesic magnitudes and potencies than females. Neonatal androgenization, and to a lesser degree, adult ovariectomy enhance systemic morphine analgesia in female rats, implicating both organizational and activational effects of gonadal hormones. The neuroanatomical circuits sensitive to sex-related hormones by which females display a smaller opiate analgesic effect is not clear, but the ventromedial (VMH) and medial preoptic (MPOA) hypothalamic nuclei are critical in the monitoring of estradiol and other sex hormone levels. To assess the contribution of these nuclei to sex and adult gonadectomy differences in systemic morphine analgesia, intact male, intact female and adult ovariectomized (OVEX) female rats received bilateral saline (SAL) or ibotenic acid (IBO) microinjections into either the VMH or MPOA. Following surgeries, baseline tail-flick latencies over 120 minutes (min) were assessed over 4 days in all nine groups with intact females tested in the estrus phase of their cycle. All animals then received an ascending series of morphine (1.0, 2.5, 5.0, 7.5, 10.0mg/kg) injections 30min prior to the tail-flick test time course with 8-12 day inter-injection intervals between doses. Baseline latencies failed to differ between SAL-treated intact males and females, but were significantly higher in SAL-treated OVEX females. Both VMH IBO and MPOA IBO lesions increased baseline latencies in intact male and female rats, but not in OVEX females. SAL-treated intact males (ED(50)=4.0mg/kg) and SAL-treated OVEX females (ED(50)=3.5mg/kg) displayed significantly greater potencies of systemic morphine analgesia than SAL-treated intact females (ED(50)=6.3mg/kg), confirming previous gender and gonadectomy differences. Neither VMH IBO (ED(50)=3.7 mg/kg) nor MPOA IBO (ED(50)=4.1mg/kg) males differed from SAL-treated males in the potency of systemic morphine analgesia. In contrast, VMH IBO (ED(50)=4.1mg/kg) and MPOA IBO (ED(50)=3.5mg/kg) intact females displayed significantly greater potencies in systemic morphine analgesia than SAL-treated intact females. However, VMH IBO OVEX (ED(50)=3.5mg/kg) and MPOA IBO OVEX (ED(50)=3.9 mg/kg) failed to differ from SAL-treated OVEX females in the potency of systemic morphine analgesia. The magnitudes of systemic morphine analgesia as measured by Maximum Percentage Effect values displayed similar patterns, but lesser degrees, of effects. These data suggest that VMH and MPOA nuclei act to tonically inhibit endogenous pain-inhibitory circuits in the intact female, but not intact male brain, and that removal of circulating gonadal hormones by OVEX and/or excitotoxic destruction of these estrogen receptor accumulating nuclei disinhibit the female analgesic response to systemic morphine.

Repeated estradiol administration alters different aspects of neurogenesis and cell death in the hippocampus of female, but not male, rats

Estradiol has been shown to have neuroprotective effects, and acute estradiol treatment enhances hippocampal neurogenesis in the female brain. However, little is known about the effects of repeated administration of estradiol on the female brain, or about the effects of estradiol on the male brain. Gonadectomized male and female adult rats were injected with 5-bromo-2-deoxyuridine (BrdU) (200 mg/kg), and then 24 h later were given subcutaneous injections of either estradiol benzoate (33 mug/kg) or vehicle daily for 15 days. On day 16, animals were perfused and the brains processed to examine cells expressing Ki-67 (cell proliferation), BrdU (cell survival), doublecortin (young neuron production), pyknotic morphology (cell death), activated caspase-3 (apoptosis), and Fluoro-Jade B (degenerating neurons) in the dentate gyrus. In female rats, repeated administration of estradiol decreased the survival of new neurons (independent of any effects on initial cell proliferation), slightly increased cell proliferation, and decreased overall cell death in the dentate gyrus. In male rats, repeated administration of estradiol had no significant effect on neurogenesis or cell death. We therefore demonstrate a clear sex difference in the response to estradiol of hippocampal neurogenesis and apoptosis in adult rats, with adult females being more responsive to the effects of estradiol than males.

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.

Estradiol receptor-alpha expression in hypothalamic and limbic regions of ewes is influenced by physiological state and maternal experience

The influence of estrus, pregnancy, parturition, and maternal experience on the expression of estrogen receptor-alpha (ERalpha) was investigated in hypothalamic and limbic regions of the sheep brain, using immunocytochemistry. Four days before parturition, previous maternal experience was associated with a higher density of ERalpha-labeled neurons in the paraventricular and supraoptic nuclei, the medial preoptic area, and the medial amygdala, but not in the mediobasal hypothalamus. Furthermore, an interaction was found between physiological state and experience in the peripartum period as the effect of experience existing 4 days prepartum was not found at parturition, when densities were lowest both in primiparous and in multiparous ewes. An additional effect of physiological state was also observed between parturition and estrus, densities being significantly lower at parturition than at estrus in the SON, PVN, and MPOA, but not in the medial amygdala. These results indicate that in sheep ERalpha expression is influenced by previous physiological and/or maternal experience at specific times of the reproductive cycle. They are also congruent with the higher ability of multiparous than nulliparous ewes to show maternal behavior several days prepartum.

Infant Rodent Ultrasounds ? A Gate to the Understanding of Sound Communication

Components of the communication system between infant and adult rodents based on ultrasonic vocalizations (USVs) of infants are analyzed. USVs are most often emitted from a pup lost outside the nest in response to changes of: (i) body temperature, (ii) contact with adults/littermates, (iii) handling, and (iv) smell. These changes modulate the state of arousal and the emotional/motivational states and, as a result, USVs are produced. Acoustic properties of USVs seem to reflect the degrees of changes in arousal and emotion/motivation. Adult rodents are aroused by perceiving the USVs, locate the sender and show a phonotaxic approach to the sender. Acoustic properties of USVs in the frequency and time domains are described based on which adult rodents discriminate the USVs from other ultrasounds and take the USVs or adequate models of them as preferred goals of their phonotaxic approach. The preferred approach to adequate USVs is modulated by emotions/motivations, the sex of the receiver, hormonal states, experience with pups and neurotransmitter systems of the brain. The phonotaxis can be understood as the appetitive component of a pup-caring instinct. The consummatory act of the instinct is the retrieval of the lost pup. This retrieval is independent of USV presence, but it closes the communication loop activated by the emission of USVs. Communication with USVs can be used as a tool to investigate genetic and brain mechanisms of behavioral control.

Minireview: Neuronal steroid hormone receptors: they're not just for hormones anymore

The ovarian steroid hormones have numerous effects on the brain, many of which are mediated, at least in part, by interaction with intracellular steroid hormone receptors acting as regulators of transcription. These intracellular steroid hormone receptors have often been considered to be activated solely by cognate hormone. However, during the past decade, numerous studies have shown that the receptors can be activated by neurotransmitters and intracellular signaling systems, through a process that does not require hormone. Although most of these have been in vitro experiments, others have been in vivo. Evidence from a wide variety of tissues and cells suggests that steroid hormone receptors are transcription factors that can be activated by a wide variety of factors, only one of which is cognate hormone. Furthermore, ligand-independent activation of neural steroid hormone receptors, rather than being a pharmacological or in vitro curiosity, seems to be a process that occurs in the normal physiology of animals. Thinking of steroid hormone receptors only as ligand-activated proteins may constrain our thinking about the many factors that may activate the receptors and cause receptor-dependent changes in neural gene expression and neuroendocrine function.

Expression of 112-kDa estrogen receptor in mouse brain cortex and its autoregulation with age

The mouse brain cortex expresses ER whose size is larger than uterus and level changes with age and hormonal treatment. Western blot analysis shows the presence of 66 kDa ER in uterus but 112 and 116 kDa ER in the brain. Whereas 116 kDa ER is phosphorylated and predominantly present in nuclei, 112 kDa ER is unphosphorylated and present in both cytosol as well as nuclei. Similar to uterus, the level of ER decreases drastically in the brain cortex of old mice. Hormonal regulation of ER shows age-specific pattern. Ovariectomy reduces the level of ER in adult but increases it significantly in old. While testosterone does not alter the level of ER in either age, estradiol decreases it considerably in old but shows no detectable difference in adult. Our results provide evidence for the expression of a novel form of ER in mouse brain cortex and its regulation by sex steroid hormones in age-specific manner.