Acquisition of sexual receptivity: roles of chromatin acetylation, estrogen receptor-alpha, and ovarian hormones

Stress during pubertal development affects female sociosexual behavior in mice

Puberty is a crucial phase for the development of female sexual behavior. Growing evidence suggests that stress during this period may interfere with the development of sexual behavior. However, the neural circuits involved in this alteration remain elusive. Here, we demonstrated in mice that pubertal stress permanently disrupted sexual performance without affecting sexual preference. This was associated with a reduced expression and activation of neuronal nitric oxide synthase (nNOS) in the ventrolateral part of the ventromedial hypothalamus (VMHvl). Fiber photometry revealed that VMHvl nNOS neurons are strongly responsive to male olfactory cues with this activation being substantially reduced in pubertally stressed females. Finally, treatment with a NO donor partially restored sexual performance in pubertally stressed females. This study provides insights into the involvement of VMHvl nNOS in the processing of olfactory cues important for the expression of female sexual behavior. In addition, exposure to stress during puberty disrupts the integration of male olfactory cues leading to reduced sexual behavior.

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.

Mistimed restricted feeding disrupts circadian rhythms of male mating behavior and female preovulatory LH surges in mice

In rodents, eating at atypical circadian times, such as during the biological rest phase when feeding is normally minimal, reduces fertility. Prior findings suggest this fertility impairment is due, at least in part, to reduced mating success. However, the physiological and behavioral mechanisms underlying this reproductive suppression are not known. In the present study, we tested the hypothesis that mistimed feeding-induced infertility is due to a disruption in the normal circadian timing of mating behavior and/or the generation of pre-ovulatory luteinizing hormone (LH) surges (estrogen positive feedback). In the first experiment, male+female mouse pairs, acclimated to be food restricted to either the light (mistimed feeding) or dark (control feeding) phase, were scored for mounting frequency and ejaculations over 96 h. Male mounting behavior and ejaculations were distributed much more widely across the day in light-fed mice than in dark-fed controls and fewer light-fed males ejaculated. In the second experiment, the timing of the LH surge, a well characterized circadian event driven by estradiol (E2) and the SCN, was analyzed from serial blood samples taken from ovariectomized and E2-primed female mice that were light-, dark-, or ad-lib-fed. LH concentrations peaked 2 h after lights-off in both dark-fed and ad-lib control females, as expected, but not in light-fed females. Instead, the normally clustered LH surges were distributed widely with high inter-mouse variability in the light-fed group. These data indicate that mistimed feeding disrupts the temporal control of the neural processes underlying both ovulation and mating behavior, contributing to infertility.

Sensory Detection by the Vomeronasal Organ Modulates Experience-Dependent Social Behaviors in Female Mice

In mice, social behaviors are largely controlled by the olfactory system. Pheromone detection induces naïve virgin females to retrieve isolated pups to the nest and to be sexually receptive to males, but social experience increases the performance of both types of innate behaviors. Whether animals are intrinsically sensitive to the smell of conspecifics, or the detection of olfactory cues modulates experience for the display of social responses is currently unclear. Here, we employed mice with an olfactory-specific deletion of the G protein Gαi2, which partially eliminates sensory function in the vomeronasal organ (VNO), to show that social behavior in female mice results from interactions between intrinsic mechanisms in the vomeronasal system and experience-dependent plasticity. In pup- and sexually-naïve females, Gαi2 deletion elicited a reduction in pup retrieval behavior, but not in sexual receptivity. By contrast, experienced animals showed normal maternal behavior, but the experience-dependent increase in sexual receptivity was incomplete. Further, lower receptivity was accompanied by reduced neuronal activity in the anterior accessory olfactory bulb and the rostral periventricular area of the third ventricle. Therefore, neural mechanisms utilize intrinsic sensitivity in the mouse vomeronasal system and enable plasticity to display consistent social behavior.

Sex chromosome complement influences vulnerability to cocaine in mice

Women acquire cocaine habits faster and are more motivated to obtain drug than men. In general, female rodents acquire intravenous cocaine self-administration (SA) faster and show greater locomotor responses to cocaine than males. Sex differences are attributed to differences in circulating estradiol. We used the four core genotype (FCG) mouse to ask whether sex chromosome complement influences vulnerability to cocaine's reinforcing and/or locomotor-activating effects. The FCG cross produces ovary-bearing mice with XX or XY genotypes (XXF, XYF) and testes-bearing mice with XX or XY genotypes (XXM, XYM). A greater percentage of gonadal females acquired cocaine SA via infusions into jugular catheters as compared with XYM mice, but XXM mice were not significantly different than any other group. Discrimination of the active versus inactive nose poke holes and cocaine intake were in general greater in gonadal females than in gonadal males. Progressive ratio tests for motivation revealed an interaction between sex chromosomes and gonads: XYM mice were more motivated to self-administer cocaine taking more infusions than mice in any other group. Locomotor responses to cocaine exposure revealed effects of sex chromosomes. After acute exposure, activity was greater in XX than in XY mice and the reverse was true for behavioral sensitization. Mice with XY genotypes displayed more activity than XX mice when given cocaine after a 10-day drug-free period. Our data demonstrate that sex chromosome complement alone and/or interacting with gonadal status can modify cocaine's reinforcing and locomotor-activating effects. These data should inform current studies of sex differences in drug use.

Sexual experience with a known male modulates c-Fos expression in response to mating and male pheromone exposure in female mice

Sexually naïve female mice are not sexually receptive in their first mating opportunity. Four to five sexual encounters are needed to display high sexual receptivity as assessed by the lordosis reflex. The neuronal changes induced by sexual experience are not well understood. In this study, we evaluated if repeated sexual stimulation with the same male was associated with an increase in the neuronal activity evaluated by c-Fos expression in brain structures associated with the control of sexual behavior such as the accessory olfactory bulb (AOB), ventromedial hypothalamus (VMH), and the medial preoptic area (MPOA). Ovariectomized female mice were randomly distributed into three groups: sexually naïve (SN), with no prior sexual stimulation; sexually inexperienced (SI), with one prior mating session; and sexually experienced (SE), with six prior mating sessions. Females were primed with estradiol benzoate and progesterone once a week for 7 weeks. Neuronal activation in response to mating or soiled bedding was evaluated in the 7^th week. Each group was subdivided into three subgroups: clean (exposure to clean bedding), male bedding (exposure to sawdust soiled with secretions from a male), or mating. Each female mated with her assigned male; in the exposure subgroup, soiled bedding was obtained from the male with whom she mated. Neuronal activity data showed that SE females had a higher c-Fos response in the VMH when they mated in comparison to females exposed to clean bedding. SI females that mated had a decrease c-Fos expression in the glomerular cell layer of the AOB, compared to females exposed to male bedding. The mitral cell layer showed a higher c-Fos response in SI females that mated in comparison to those exposed to male bedding. Comparisons between groups presented with the same stimulus indicate that SI females exposed to male bedding showed a decrease in c-Fos response in the mitral cell layer in comparison to SE and SN females. Correlation analysis demonstrated that the lordosis quotient from the last mating test correlated positively with the number of c-Fos-positive cells in the mitral cell layer in SE and SI groups. A similar correlation was found in the MPOA in SI females. Prior mating in female mice is required to increase sexual receptivity. Changes in the neuronal activity in the AOB and VMH may be involved in the neuronal plasticity induced by repeated sexual stimulation.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Effect of Ovarian Hormones and Mating Experience on the Preference of Female Mice to Investigate Male Urinary Pheromones

In female mice, the expression of receptive lordosis behavior requires estradiol and progesterone actions in the nervous system; however, the contribution of these hormones to females' motivation to seek out male pheromones is less clear. In an initial experiment, sexually naïve ovary-intact female mice preferred to investigate (make nasal contact with) testes-intact male as opposed to estrous female urine, provided they were in vaginal estrus. In a second experiment, groups of sexually naïve and mating-experienced, ovariectomized females were tested for urinary pheromone preference first without and then with ovarian hormone replacement. Without hormone replacement, sexually naïve ovariectomized females showed no preference for male over female urinary pheromones whereas mating-experienced females preferred to investigate male pheromones. Ovariectomized females in both groups preferred male over female urine after sequential s.c. injections with estradiol benzoate followed 2 days later with progesterone and after prolonged (7 days) exposure to estradiol alone. Our results indicate that in sexually naïve female mice estradiol, perhaps aided by progesterone, is required to motivate a preference to seek out male pheromones whereas after mating experience females' preference to investigate male pheromones persists even in the absence of ovarian hormone action.

Tph2-/- female mice restore socio-sexual recognition through upregulating ERα and OTR genes in the amygdala

The central 5-hydroxytryptamine system impairs sociosexual behaviors and olfaction preferences in sexually naive mice. However, it remains unknown whether reproductive experiences impart an effect on the sexual olfactory preferences of female mice lacking central serotonin. Here, we aimed at examining such effects and the underlying mechanisms using Tph2 knockout female mice. Sexually naive Tph2^−/− female mice failed to recognize olfactory cues regarding sex, genetic relatedness, and social hierarchy despite exhibiting normal olfactory discrimination. However, reproduction-experienced Tph2^−/− female mice recovered sexual olfactory preferences, as did sexually naive Tph2^+/+ females. Meanwhile, both the estrogen receptor α and oxytocin receptor in the amygdala of reproduction-experienced Tph2^−/− females presented upregulated expression at the mRNA level and an upward tendency at the protein level vs. sexually naive Tph2^−/− females. Intracerebroventricular administration of a combination of estrogen receptor α and oxytocin receptor agonists, but not either agent alone, could restore the sexual olfactory preferences of sexually naive Tph2^−/− female mice to some degree. We speculate that estrogen receptor α and oxytocin receptor activation in the amygdala after reproductive experiences restores sexual olfactory recognition in Tph2^−/− female mice.

Epigenetic regulation of motivated behaviors by histone deacetylase inhibitors

Growing evidence has begun to elucidate the contribution of epigenetic mechanisms in the modulation and maintenance of gene expression and behavior. Histone acetylation is one such epigenetic mechanism, which has been shown to profoundly alter gene expression and behaviors. In this review, we begin with an overview of the major epigenetic mechanisms including histones acetylation. We next focus on recent evidence about the influence of environmental stimuli on various motivated behaviors through histone acetylation and highlight how histone deacetylase inhibitors can correct some of the pathologies linked to motivated behaviors including substance abuse, feeding and social attachments. Particularly, we emphasize that the effects of histone deacetylase inhibitors on motivated behaviors are time and context-dependent.

DREADD-induced silencing of the medial amygdala reduces the preference for male pheromones and the expression of lordosis in estrous female mice

Sexually naïve estrous female mice seek out male urinary pheromones; however, they initially display little receptive (lordosis) behavior in response to male mounts. Vomeronasal-accessory olfactory bulb inputs to the medial amygdala (Me) regulate courtship in female rodents. We used a reversible inhibitory chemogenetic technique (Designer Receptors Exclusively Activated by Designer Drugs; DREADDs) to assess the contribution of Me signaling to females' preference for male pheromones and improvement in receptivity normally seen with repeated testing. Sexually naïve females received bilateral Me injections of an adeno-associated virus carrying an inhibitory DREADD. Females were later ovariectomized, treated with ovarian hormones, and given behavioral tests following intraperitoneal injections of saline or clozapine-N-oxide (CNO; which hyperpolarizes infected Me neurons). CNO attenuated females' preference to investigate male vs. female urinary odors. Repeated CNO treatment also slowed the increase in lordosis otherwise seen in females given saline. However, when saline was given to females previously treated with CNO, their lordosis quotients were as high as other females repeatedly given saline. No disruptive behavioral effects of CNO were seen in estrous females lacking DREADD infections of the Me. Finally, CNO attenuated the ability of male pheromones to stimulate Fos expression in the Me of DREADD-infected mice but not in non-infected females. Our results affirm the importance of Me signaling in females' chemosensory preferences and in the acute expression of lordosis. However, they provide no indication that Me signaling is required for the increase in receptivity normally seen after repeated hormone priming and testing with a male.

A comparison of the effects of male pheromone priming and optogenetic inhibition of accessory olfactory bulb forebrain inputs on the sexual behavior of estrous female mice

Previous research has shown that repeated testing with a stimulus male is required for ovariectomized, hormone-primed female mice to become sexually receptive (show maximal lordosis quotients; LQs) and that drug-induced, epigenetic enhancement of estradiol receptor function accelerated the improvement in LQs otherwise shown by estrous females with repeated testing. We asked whether pre-exposure to male pheromones ('pheromone priming') would also accelerate the improvement in LQs with repeated tests and whether optogenetic inhibition of accessory olfactory bulb (AOB) projection neurons could inhibit lordosis in sexually experienced estrous female mice. In Experiment 1, repeated priming with soiled male bedding failed to accelerate the progressive improvement in LQs shown by estrous female mice across 5 tests, although the duration of each lordosis response and females' investigation of male body parts during the first test was augmented by such priming. In Experiment 2, acute optogenetic inhibition of AOB inputs to the forebrain during freely moving behavioral tests significantly reduced LQs, suggesting that continued AOB signaling to the forebrain during mating is required for maximal lordotic responsiveness even in sexually experienced females. Our results also suggest that pheromonal stimulation, by itself, cannot substitute for the full complement of sensory stimulation received by estrous females from mounting males that normally leads to the progressive improvement in their LQs with repeated testing.

Bridging progestogens in pregnancy and pregnancy prevention

Steroid hormones have been in use for more than a half a century as contraceptive agents, and only now are researchers elucidating the biochemical mechanisms of action and non-target effects. Progesterone and synthetic progestins, critical for women's health in the US and internationally, appear to have important effects on immune functioning and other diverse systems. Apart from the contraceptive world is a separate field that is devoted to understanding progesterone in other contexts. Based on research following a development timeline parallel to hormonal contraception, progesterone and 17-hydroxyprogesterone caproate are now administered to prevent preterm birth in high-risk pregnant women. Preterm birth researchers are similarly working to determine the precise biochemical actions and immunological effects of progesterone. Progesterone research in both areas could benefit from increased collaboration and bringing these two bodies of literature together. Progesterone, through actions on various hormone receptors, has lifelong importance in different organ systems and researchers have much to learn about this molecule from the combination of existing literatures, and from future studies that build on this combined knowledge base.

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.

Epigenetic regulation of Progesterone Receptor isoforms: from classical models to the sexual brain

Progesterone Receptor is a member of the nuclear receptor superfamily, which regulates several functions in both reproductive and non-reproductive tissues. Progesterone Receptor gene encodes for two main isoforms, A and B, and contains two specific promoters with their respective transcription start sites. The mRNA expression of both isoforms is mainly regulated by estrogens and specifically via the Estrogen Receptor Alpha, in a context specific manner. Furthermore, it has been reported in extensive physiological and pathological models that Progesterone Receptor isoforms regulation is related to the epigenetic state of their respective promoters. Epigenetic regulation of Progesterone Receptor isoforms in the brain is a recent and scarcely explored field in neurosciences. This review focuses on the epigenetic mechanisms involved in Progesterone Receptor regulation, emphasizing the implications for the sexual brain. Future directions for research about this important field are also discussed.

Reversible DNA methylation regulates seasonal photoperiodic time measurement

In seasonally breeding vertebrates, changes in day length induce categorically distinct behavioral and reproductive phenotypes via thyroid hormone-dependent mechanisms. Winter photoperiods inhibit reproductive neuroendocrine function but cannot sustain this inhibition beyond 6 mo, ensuring vernal reproductive recrudescence. This genomic plasticity suggests a role for epigenetics in the establishment of seasonal reproductive phenotypes. Here, we report that DNA methylation of the proximal promoter for the type III deiodinase (dio3) gene in the hamster hypothalamus is reversible and critical for photoperiodic time measurement. Short photoperiods and winter-like melatonin inhibited hypothalamic DNA methyltransferase expression and reduced dio3 promoter DNA methylation, which up-regulated dio3 expression and induced gonadal regression. Hypermethylation attenuated reproductive responses to short photoperiods. Vernal refractoriness to short photoperiods reestablished summer-like methylation of the dio3 promoter, dio3 expression, and reproductive competence, revealing a dynamic and reversible mechanism of DNA methylation in the mammalian brain that plays a central role in physiological orientation in time.

Permanent and plastic epigenesis in neuroendocrine systems

The emerging area of neuroepigenetics has been linked to numerous mental health illnesses. Importantly, a large portion of what we know about early gene×environment interactions comes from examining epigenetic modifications of neuroendocrine systems. This review will highlight how neuroepigenetic mechanisms during brain development program lasting differences in neuroendocrine systems and how other neuroepigenetic processes remain plastic, even within the adult brain. As epigenetic mechanisms can either be stable or plastic, elucidating the mechanisms involved in reversing these processes could aid in understanding how to reverse pathological epigenetic programming.

Histone deacetylase inhibitors facilitate partner preference formation in female prairie voles

In the socially monogamous prairie vole (Microtus ochrogaster), mating induces enduring pair-bonds initiated by partner preference formation and regulated by a variety of neurotransmitters including oxytocin, vasopressin, and dopamine. Here we examined potential epigenetic mechanisms mediating pair-bond regulation. We show that the histone deacetylase inhibitors sodium butyrate and TrichoStatin A (TSA) facilitate partner preference formation in female prairie voles in the absence of mating. This was associated with a specific up-regulation of oxytocin (OTR) and vasopressin V1a receptors (V1aR) in the nucleus accumbens, through an increase in histone acetylation at their respective promoter. Furthermore, TSA-facilitated partner preference was prevented by OTR or V1aR blockade in the nucleus accumbens. Importantly, mating-induced partner preference triggered the same epigenetic regulation of OTR and V1aR gene promoters as TSA. These observations thus indicate that TSA and mating facilitate partner preference through epigenetic events, providing the first direct evidence for an epigenetic regulation of pair-bonding.

Monoaminergic and neuropeptidergic neurons have distinct expression profiles of histone deacetylases

Monoaminergic and neuropeptidergic neurons regulate a wide variety of behaviors, such as feeding, sleep/wakefulness behavior, stress response, addiction, and social behavior. These neurons form neural circuits to integrate different modalities of behavioral and environmental factors, such as stress, maternal care, and feeding conditions. One possible mechanism for integrating environmental factors through the monoaminergic and neuropeptidergic neurons is through the epigenetic regulation of gene expression via altered acetylation of histones. Histone deacetylases (HDACs) play an important role in altering behavior in response to environmental factors. Despite increasing attention and the versatile roles of HDACs in a variety of brain functions and disorders, no reports have detailed the localization of the HDACs in the monoaminergic and neuropeptidergic neurons. Here, we examined the expression profile of the HDAC protein family from HDAC1 to HDAC11 in corticotropin-releasing hormone, oxytocin, vasopressin, agouti-related peptide (AgRP), pro-opiomelanocortin (POMC), orexin, histamine, dopamine, serotonin, and noradrenaline neurons. Immunoreactivities for HDAC1,-2,-3,-5,-6,-7,-9, and -11 were very similar among the monoaminergic and neuropeptidergic neurons, while the HDAC4, -8, and -10 immunoreactivities were clearly different among neuronal groups. HDAC10 expression was found in AgRP neurons, POMC neurons, dopamine neurons and noradrenaline neurons but not in other neuronal groups. HDAC8 immunoreactivity was detected in the cytoplasm of almost all histamine neurons with a pericellular pattern but not in other neuropeptidergic and monoaminergic neurons. Thus, the differential expression of HDACs in monoaminergic and neuropeptidergic neurons may be crucial for the maintenance of biological characteristics and may be altered in response to environmental factors.

Estrogen dependent activation function of ERβ is essential for the sexual behavior of mouse females

We previously generated and characterized a genuine estrogen receptor (ER) β-null mouse line (named ERβ(ST)(L-/L-)) and showed that ERβ(ST)(L-/L-) mice were sterile, due to an ovulation impairment in females and to an unknown reason in males, as their reproductive organs and spermatozoid motility appeared normal. We report here an assessment of the sexual behavior of ERβ(ST)(L-/L-) null mice. We found that ERβ(ST)(L-/L-) males display mildly impaired sexual behavior and that ERβ(ST)(L-/L-) females are significantly less receptive and less attractive than wild-type (WT) females. Decreased attractivity is also exhibited by ERβAF2(0) but not by ERβAF1(0) mutant females (females devoid of either AF2 or AF1 activation function of ERβ). Interestingly, by using an odor preference test, we have determined that the low attractiveness of ERβ(ST)(L-/L-) and ERβAF2(0) females is related to a deficiency of a volatile chemosignal.

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.

Sexual differentiation of the rodent brain: dogma and beyond

Steroid hormones of gonadal origin act on the neonatal brain to produce sex differences that underlie adult reproductive physiology and behavior. Neuronal sex differences occur on a variety of levels, including differences in regional volume and/or cell number, morphology, physiology, molecular signaling, and gene expression. In the rodent, many of these sex differences are determined by steroid hormones, particularly estradiol, and are established by diverse downstream effects. One brain region that is potently organized by estradiol is the preoptic area (POA), a region critically involved in many behaviors that show sex differences, including copulatory and maternal behaviors. This review focuses on the POA as a case study exemplifying the depth and breadth of our knowledge as well as the gaps in understanding the mechanisms through which gonadal hormones produce lasting neural and behavioral sex differences. In the POA, multiple cell types, including neurons, astrocytes, and microglia are masculinized by estradiol. Multiple downstream molecular mediators are involved, including prostaglandins, various glutamate receptors, protein kinase A, and several immune signaling molecules. Moreover, emerging evidence indicates epigenetic mechanisms maintain sex differences in the POA that are organized perinatally and thereby produce permanent behavioral changes. We also review emerging strategies to better elucidate the mechanisms through which genetics and epigenetics contribute to brain and behavioral sex differences.

Epigenetic methodologies for behavioral scientists

Hormones are essential regulators of many behaviors. Steroids bind either to nuclear or membrane receptors while peptides primarily act via membrane receptors. After a ligand binds, the conformational change in the receptor initiates changes in cell signaling cascades (membrane receptors) or direct alternations in DNA transcription (steroid receptors). Changes in gene transcription that result are responsible for protein production and ultimately behavioral modifications. A significant part of how hormones affect DNA transcription is via epigenetic modifications of DNA and/or the chromatin in which it is entwined. These alterations lead to transcriptional changes that ultimately define the phenotype and function of a given cell. Importantly we now know that environmental stimuli influence epigenetic marks, which in the context of neuroendocrinology can lead to behavioral changes. Importantly tracking epigenetic states and profiling the epigenome within cells require the use of epigenetic methodologies and subsequent data analysis. Here we describe the techniques of particular importance in the mapping of DNA methylation, histone modifications and occupancy of chromatin bound effector proteins that regulate gene expression. For researchers wanting to move into these levels of analysis we discuss the application of modern sequencing technologies applied in assays such as chromatin immunoprecipitation and the bioinformatics analysis involved in the rich datasets generated.