Exploratory, anti-anxiety, social, and sexual behaviors of rats in behavioral estrus is attenuated with inhibition of 3alpha,5alpha-THP formation in the midbrain ventral tegmental area

Allopregnanolone: An overview on its synthesis and effects

Allopregnanolone, a 3α,5α-progesterone metabolite, acts as a potent allosteric modulator of the γ-aminobutyric acid type A receptor. In the present review, the synthesis of this neuroactive steroid occurring in the nervous system is discussed with respect to physiological and pathological conditions. In addition, its physiological and neuroprotective effects are also reported. Interestingly, the levels of this neuroactive steroid, as well as its effects, are sex-dimorphic, suggesting a possible gender medicine based on this neuroactive steroid for neurological disorders. However, allopregnanolone presents low bioavailability and extensive hepatic metabolism, limiting its use as a drug. Therefore, synthetic analogues or a different therapeutic strategy able to increase allopregnanolone levels have been proposed to overcome any pharmacokinetic issues.

Allopregnanolone in mood disorders: Mechanism and therapeutic development

The neuroactive steroid allopregnanolone (ALLO) is an endogenous positive allosteric modulator of GABA type A receptor (GABA_AR), and the down-regulation of its biosynthesis have been attributed to the development of mood disorders, such as depression, anxiety and post-traumatic stress disorder (PTSD). ALLO mediated depression/anxiety involves GABAergic mechanisms and appears to be related to brain-derived neurotrophic factor (BDNF), dopamine receptor, glutamate neurotransmission, and Ca²⁺ channel. In the clinical, brexanolone, as a newly developed intravenous ALLO preparation, has been approved for the treatment of postpartum depression (PPD). In addition, traditional antidepressants such as selective serotonin reuptake inhibitor (SSRI) could reverse ALLO decline. Recently, the translocation protein (TSPO, 18 kDa), which involves in the speed-limiting step of ALLO synthesis, and ALLO derivatization have been identified as new directions for antidepressant therapy. This review provides an overview of ALLO researches in animal model and patients, discusses its role in the development and treatment of depression/anxiety, and directs its therapeutic potential in future.

Neonatal exposure to chlordecone alters female social behaviors and central estrogen alpha receptor expression in socially monogamous mandarin voles

Chlordecone (CD) is one of the common persistent organic pollutants in nature and has a profound impact on the environment and on public health. Accumulating evidence has demonstrated that neonatal exposure of CD influences adult physiology and behavior due to its estrogenic properties. Using socially monogamous mandarin voles as an experimental animal model, the present study aimed to evaluate the impact of neonatal exposure to CD on female social behaviors and central estrogen receptor alpha (ERα) expression in adulthood. After receiving a single subcutaneous injection with sesame seed oil (female control group), 17 beta-estradiol (E₂ group), or CD group on postnatal Day 1, the social behaviors of adult animals and ERα expression in specific brain regions were assessed. The data indicated that CD or E₂-treated female animals displayed increased affiliative behaviors and decreased aggressive behaviors with regard to the unfamiliar females in the social interaction test. In addition, CD or E₂-treated female voles exhibited significant preferences to females over males in the sexual preference test. Moreover, CD-treated female animals exhibited higher levels of ERα expression in the bed nucleus of the stria terminalis, the central amygdala, the medial amygdala and the medial preoptic area compared with those of the control voles. The results suggested that neonatal exposure to CD may masculinize female social behaviors, possibly via CD-induced changes in the ERα expression of relevant brain regions.

Mating Enhances Expression of Hormonal and Trophic Factors in the Midbrain of Female Rats

Among female rats, mating enhances neurosteroid formation in the midbrain ventral tegmental area (VTA; independent of peripheral steroid-secreting glands, ovaries, and adrenals). The sources/targets for these actions are not well understood. In Experiment 1, proestrous rats engaged in a mating paradigm, or did not, and the midbrains had been assessed via the Affymetrix rat genome microarrays. In Experiment 2, the influence of gonadal and adrenal glands on the expression of these genes was assessed in rats that were proestrous, ovariectomized (OVX), or OVX and adrenalectomized (ADX). The microarrays revealed 53 target genes that were significantly up-regulated (>2.0-fold change) in response to mating. Mating significantly enhanced the midbrain mRNA expression of genes involved in hormonal and trophic actions: Gh1, S100g, and Klk1b3 in proestrous, but not OVX and/or ADX, rats; Fshb in all but OVX/ADX rats; and lutenizing hormone β and thyroid-stimulating hormone (TSH) β in all rats. Thus, mating enhances midbrain gene expression independent and dependent of peripheral glands.

Physiopathological role of the enzymatic complex 5α-reductase and 3α/β-hydroxysteroid oxidoreductase in the generation of progesterone and testosterone neuroactive metabolites

The enzymatic complex 5α-reductase (5α-R) and 3α/3β-hydroxysteroid oxidoreductase (HSOR) is expressed in the nervous system, where it transforms progesterone (PROG) and testosterone (T) into neuroactive metabolites. These metabolites regulate myelination, brain maturation, neurotransmission, reproductive behavior and the stress response. The expression of 5α-R and 3α-HSOR and the levels of PROG and T reduced metabolites show regional and sex differences in the nervous system and are affected by changing physiological conditions as well as by neurodegenerative and psychiatric disorders. A decrease in their nervous tissue levels may negatively impact the course and outcome of some pathological events. However, in other pathological conditions their increased levels may have a negative impact. Thus, the use of synthetic analogues of these steroids or 5α-R modulation have been proposed as therapeutic approaches for several nervous system pathologies. However, further research is needed to fully understand the consequences of these manipulations, in particular with 5α-R inhibitors.

Central Actions of 3α,5α-THP Involving NMDA and GABAA Receptors Regulate Affective and Sexual Behavior of Female Rats

The neurosteroid, 5α-pregnan-3α-ol-20-one (known as “allopregnanolone” or 3α,5α-THP), is produced in the midbrain ventral tegmental area (VTA), independent of peripheral sources of progestogens, where it has potential actions at N-methyl-D-aspartate (NMDA) and GABA_A receptors to facilitate rodent sexual behavior. Progestogens can also have anti-anxiety effects, but whether these involve actions of centrally-derived 3α,5α-THP or these receptors to support reproductively-relevant behavior is not well understood. We investigated the extent to which 3α,5α-THP’s actions via NMDA and/or GABA_A receptors in the midbrain VTA influence reproductive behaviors. Estradiol-primed, ovariectomized/adrenalectomized (OVX/ADX) rats received midbrain VTA infusions of vehicle, an NMDA receptor blocker (MK-801; 200 ng), or a GABA_A receptor blocker (bicuculline; 100 ng) followed by a second infusion of vehicle or 3α,5α-THP (100 ng). Reproductively-relevant behaviors were assessed: sexual (paced mating), anxiety-like (elevated plus maze), and social (partner preference, social interaction) behavior. Compared to vehicle, intra-VTA infusions of MK-801 exerted anxiolytic-like effects on elevated plus maze behavior and enhanced lordosis. Unlike prior observations in gonadally-intact rats, intra-VTA bicuculline had no effect on the behavior of OVX/ADX rats (likely due to a floor effect). Subsequent infusions of 3α,5α-THP reversed effects on lordosis and infusions of bicuculline inhibited 3α,5α-THP-facilitated lordosis. Thus, NMDA and GABA_A receptors may act as mediators for reproductive behavioral effects of 3α,5α-THP in the midbrain VTA.

Interaction of hormonal and social environments in understanding body image concerns in adolescent girls

During adolescence, peer approval becomes increasingly important and may be perceived as contingent upon appearance in girls. Concurrently, girls experience hormonal changes, including an increase in progesterone. Progesterone has been implicated in affiliative behavior but inconsistently associated with body image concerns. The current study sought to examine whether progesterone may moderate the association between perceived social pressures to conform to the thin ideal and body image concerns. Secondary analyses were conducted in cross-sectional data from 813 girls in early puberty and beyond (ages 8-16) who completed assessments of the peer environment, body image concerns, and progesterone. Models for mediation and moderation were examined with BMI, age, and menarcheal status as covariates. Belief that popularity was linked to appearance and the experience of weight-related teasing were both positively associated with greater body image concerns, but neither was associated with progesterone once adjusting for covariates. Progesterone significantly interacted with perceived social pressures in predicting body image concerns. At higher progesterone levels, appearance-popularity beliefs and weight-related teasing were more strongly related to body image concerns than they were at lower progesterone levels. Findings support a moderating role for progesterone in the link between social pressures and body image concerns in girls. This study adds to a growing literature examining how girls' hormonal environments may modulate responses to their social environments. Longitudinal and experimental work is needed to understand temporal relations and mechanisms behind these associations.

Prenatal stress programs neuroendocrine stress responses and affective behaviors in second generation rats in a sex-dependent manner

An adverse environment in early life is often associated with dysregulation of the hypothalamo-pituitary-adrenal (HPA) axis and higher rates of mood disorders in adulthood. In rats, exposure to social stress during pregnancy results in hyperactive HPA axis responses to stress in the adult offspring and heightened anxiety behavior in the males, but not the females. Here we tested whether, without further intervention, the effects of prenatal stress (PNS) in the first filial generation (F1) are transmitted to the F2 generation via the maternal line. F1 control and PNS female rats were mated with control males and housed under non-stress conditions throughout pregnancy. HPA axis responses to acute stress, anxiety- and depressive-like behavior were assessed in the adult F2 offspring. ACTH and corticosterone responses to an acute stressor were markedly enhanced in F2 PNS females compared with controls. This was associated with greater corticotropin releasing hormone (Crh) mRNA expression in the paraventricular nucleus and reduced hippocampal glucocorticoid (Gr) and mineralocorticoid receptor (Mr) mRNA expression. Conversely, in the F2 PNS males, HPA axis responses to acute stress were attenuated and hippocampal Gr mRNA expression was greater compared with controls. F2 PNS males exhibited heightened anxiety-like behavior (light-dark box and elevated plus maze) compared with F2 control males. Anxiety-like behavior did not differ between F2 control and PNS females during metestrus/diestrus, however at proestrus/estrus, F2 control females displayed a reduction in anxiety-like behavior, but this effect was not observed in the F2 PNS females. Heightened anxiety in the F2 PNS males was associated with greater Crh mRNA expression in the central nucleus of the amygdala compared with controls. Moreover, Crh receptor-1 (Crhr1) mRNA expression was significantly increased, whereas Crhr2 mRNA was significantly decreased in discrete regions of the amygdala in F2 PNS males compared with controls, with no differences in the F2 females. No differences in depressive-like behavior (sucrose preference or forced swim test) were observed in either sex. In conclusion, the effects of maternal stress during pregnancy on HPA axis regulation and anxiety-like behavior can be transmitted to future generations in a sex-dependent manner. These data have implications for human neuropsychiatric disorders with developmental origins.

Role of pregnane xenobiotic receptor in the midbrain ventral tegmental area for estradiol- and 3α,5α-THP-facilitated lordosis of female rats

RATIONALE

Progesterone and its metabolite, 5α-pregnan-3α-ol-20-one (3α,5α-THP), have actions in the ventral tegmental area (VTA) that are required for lordosis, a characteristic mating posture of female rodents. 17β-estradiol (estradiol) co-varies with progestogens over natural cycles, enhances production of 3α,5α-THP, and is required for successful reproductive behavior.

OBJECTIVES

A question of interest is the role of pregnane xenobiotic receptor (PXR), a nuclear receptor that regulates enzymes needed for the production of 3α,5α-THP, for estradiol-mediated lordosis. The hypothesis tested was that if PXR is involved in estradiol-mediated biosynthesis of 3α,5α-THP and reproductive behavior, knocking down expression of PXR in the VTA of estradiol-primed, but not vehicle-primed, rats should decrease lordosis and midbrain 3α,5α-THP; effects may be attenuated by 3α,5α-THP administered to the VTA.

METHODS

Ovariectomized rats were administered subcutaneous injections of oil vehicle or estradiol. Rats were then administered PXR antisense oligonucleotides (PXR AS-ODNs; which are expected to locally knock down expression of PXR), or control (saline), infusions to the VTA. Rats were administered 3α,5α-THP or vehicle via infusions to the VTA. Reproductive behavior (paced mating task) of rats was determined in addition to exploratory (open field), affective (elevated plus maze), and pro-social (social interaction task) behavior.

RESULTS

Reproductive behavior (i.e., increased lordosis) was enhanced with estradiol-priming and infusions of 3α,5α-THP to the VTA. Infusions of PXR AS-ODNs to the VTA attenuated responses in estradiol-, but not vehicle-, primed rats, compared to control infusions.

CONCLUSIONS

PXR may be involved in a neuroregulatory response involving biosynthesis of 3α,5α-THP in the midbrain VTA of estradiol-primed rats.

The pregnane xenobiotic receptor, a prominent liver factor, has actions in the midbrain for neurosteroid synthesis and behavioral/neural plasticity of female rats

A novel factor of interest for growth/plasticity in the brain is pregnane xenobiotic receptor (PXR). PXR is a liver factor known for its role in xenobiotic clearance and cholesterol metabolism. It is expressed in the brain, suggesting a potential role for plasticity, particularly involving cholesterol-based steroids and neurosteroids. Mating induces synthesis of neurosteroids in the midbrain Ventral Tegmental Area (VTA) of female rodents, as well as other “plastic” regions of the brain, including the hippocampus, that may be involved in the consolidation of the mating experience. Reducing PXR in the VTA attenuates mating-induced biosynthesis of the neurosteroid, 5α-pregnan-3α-ol-20-one (3α,5α-THP). The 18 kDA translocator protein (TSPO) is one rate-limiting factor for 3α,5α-THP neurosteroidogenesis. The hypothesis tested was that PXR is an upstream factor of TSPO for neurosteroidogenesis of 3α,5α-THP in the VTA for lordosis, independent of peripheral glands. First, proestrous rats were administered a TSPO blocker (PK11195) and/or 3α,5α-THP following infusions of PXR antisense oligonucleotides (AS-ODNs) or vehicle to the VTA. Inhibiting TSPO with PK11195 reduced 3α,5α-THP levels in the midbrain and lordosis, an effect that could be reversed with 3α,5α-THP administration, but not AS-ODN+3α,5α-THP. Second, proestrous, ovariectomized (OVX), or ovariectomized/adrenalectomized (OVX/ADX) rats were infused with a TSPO enhancer (FGIN 1-27) subsequent to AS-ODNs or vehicle to the VTA. PXR AS-ODNs blocked actions of FGIN 1–27 for lordosis and 3α,5α-THP levels among proestrous > OVX > OVX/ADX rats. Thus, PXR may be upstream of TSPO, involved in neurosteroidogenesis of 3α,5α-THP in the brain for plasticity. This novel finding of a liver factor involved in behavioral/neural plasticity substantiates future studies investigating factors known for their prominent actions in the peripheral organs, such as the liver, for modulating brain function and its augmentation.

Allopregnanolone's attenuation of the lordosis-inhibiting effects of restraint is blocked by the antiprogestin, CDB-4124

A brief restraint experience reduces lordosis behavior in ovariectomized females that have been hormonally primed with estradiol benzoate. The addition of progesterone to the priming prevents the lordosis inhibition. Based on prior studies with an inhibitor of progesterone metabolism, we have implicated the intracellular progesterone receptor, rather than progesterone metabolites, as responsible for this protection. However, the progesterone metabolite, allopregnanolone (3α-hydroxy-5α-pregnan-20-one), also prevents lordosis inhibition after restraint. In a prior study, we reported that the progestin receptor antagonist, RU486 (11β-(4-dimethylamino)phenyl-17β-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one), attenuated the effect of allopregnanolone. Because RU486 can also block the glucocorticoid receptor, in the current studies, we evaluated the effect of the progestin receptor antagonist, CDB-4124 (17α-acetoxy-21-methoxy-11β-[4-N,N-dimethyaminopheny]-19-norpregna-4,9-dione-3,20-dione), which is relatively devoid of antiglucocorticoid activity. Ovariectomized, Fischer rats were injected with 10 μg estradiol benzoate. Two days later, rats received either 60 mg/kg CDB-4124 or 20% DMSO/propylene glycol vehicle 1 h before injection with 4 mg/kg allopregnanolone. After a pretest to confirm sexual receptivity, rats were restrained for 5min and immediately tested for sexual behavior. Lordosis behavior was reduced by the restraint and attenuated by allopregnanolone. Pretreatment with CDB-4124 reduced allopregnanolone's effect. These findings support prior suggestions that allopreganolone reduces the response to restraint by mechanisms that require activation of the intracellular progesterone receptor.

Female mice with deletion of Type One 5α-reductase have reduced reproductive responding during proestrus and after hormone-priming

The capacity to form progesterone (P₄)'s 5α-reduced metabolite, 5α-pregnan-3α-ol-20-one (3α,5α-THP; a.k.a. allopregnanolone), in the brain may be related to facilitation of lordosis among estrogen-primed (E₂) mice. We investigated this idea further by comparing effects of endogenous and exogenous progestogens in mice that are deficient in the Type One 5α-reductase enzyme (5α-reductase knockout mice; 5α-RKO), and their wildtype counterparts for sexual behavior. Comparisons were made following administration of progestogens that are expected to increase 3α,5α-THP or not. Sexual receptivity of 5α-RKO mice and their wildtype counterparts was examined when mice were naturally-cycling (Experiment 1); ovariectomized (OVX), E₂-primed (10 μg, subcutaneous; SC) and administered P₄ (0, 125, 250, or 500 μg SC; Experiment 2); and OVX, E₂-primed and administered P₄, medroxyprogesterone acetate (MPA, 4 mg/kg, SC, which does not convert to 3α,5α-THP) or 3α,5α-THP (4 mg/kg, SC; Experiment 3). The percentage of mounts that elicited lordosis (lordosis quotient) or aggression/rejection behavior (aggression quotient), as well as the quality of lordosis (lordosis rating), was scored. Wildtype, but not 5α-RKO, mice in behavioral estrus demonstrated significantly greater lordosis quotients and lordosis ratings, but similar aggression quotients, compared to their diestrous counterparts. Among OVX and E₂-primed mice, P₄ facilitated lordosis of wildtype, but not 5α-RKO, mice. MPA neither facilitated lordosis of wildtype, nor 5α-RKO mice. 3α,5α-THP administered to wildtype or 5α-RKO mice increased lordosis quotients and lordosis ratings and decreased aggression quotients. 3α,5α-THP levels in the midbrain, one brain region important for sexual behavior, were increased during behavioral estrus, with P4 administered to WT, but not 5α-RKO mice, and 3α,5α-THP administered to WT and 5α-RKO mice. MPA did not increase 3α,5α-THP. Thus, deletion of Type One 5α-reductase among female mice may attenuate reproductive responding during the estrous cycle and after hormone-priming.

Pregnane xenobiotic receptors and membrane progestin receptors: role in neurosteroid-mediated motivated behaviours

Progestogens have actions in the midbrain ventral tegmental area (VTA) to mediate motivated behaviours, such as those involved in reproductive processes, among female rodents. In the VTA, the formation and actions of one progestogen, 5α-pregnan-3α-ol-20-one (3α,5α-THP), are necessary and sufficient to facilitate sexual responding (measured by lordosis) of female rodents. Although 3α,5α-THP can be produced after metabolism of ovarian progesterone, 3α,5α-THP is also a neurosteroid produced de novo in brain regions, such as the VTA. There can be dynamic changes in 3α,5α-THP production associated with behavioural experience, such as mating. Questions of interest are the sources and targets of 3α,5α-THP. Regarding sources, the pregnane xenobiotic receptor (PXR) may be a novel factor involved in 3α,5α-THP metabolism in the VTA (as well as a direct target of 3α,5α-THP). We have identified PXR in the midbrain of female rats, and manipulating PXR in this region reduces 3α,5α-THP synthesis and alters lordosis, as well as affective and social behaviours. Regarding targets, recent studies have focused on the role of membrane progestin receptors (mPRs). We have analysed the expression of two of the common forms of these receptors (mPRα/paqr7 and mPRβ/paqr8) in female rats. The expression of mPRα was observed in peripheral tissues and brain areas, including the hypothalamus and midbrain. The expression of mPRβ was only observed in brain tissues and was abundant in the midbrain and hypothalamus. To our knowledge, studies of these receptors in mammalian models have been limited to expression and regulation, instead of function. One question that was addressed was the functional effects of progestogens via mPRα and mPRβ in the midbrain of hormone-primed rats for lordosis. Studies to date suggest that mPRβ may be an important target of progestogens in the VTA for lordosis. Taken together, the result of these studies demonstrate that PXR is involved in the production of 3α,5α-THP in the midbrain VTA. Moreover, mPRs may be a target for the actions of progestogens in the VTA for lordosis.

Revisiting the roles of progesterone and allopregnanolone in the nervous system: resurgence of the progesterone receptors

Progesterone is commonly considered as a female reproductive hormone and is well-known for its role in pregnancy. It is less well appreciated that progesterone and its metabolite allopregnanolone are also male hormones, as they are produced in both sexes by the adrenal glands. In addition, they are synthesized within the nervous system. Progesterone and allopregnanolone are associated with adaptation to stress, and increased production of progesterone within the brain may be part of the response of neural cells to injury. Progesterone receptors (PR) are widely distributed throughout the brain, but their study has been mainly limited to the hypothalamus and reproductive functions, and the extra-hypothalamic receptors have been neglected. This lack of information about brain functions of PR is unexpected, as the protective and trophic effects of progesterone are much investigated, and as the therapeutic potential of progesterone as a neuroprotective and promyelinating agent is currently being assessed in clinical trials. The little attention devoted to the brain functions of PR may relate to the widely accepted assumption that non-reproductive actions of progesterone may be mainly mediated by allopregnanolone, which does not bind to PR, but acts as a potent positive modulator of γ-aminobutyric acid type A (GABA(A) receptors. The aim of this review is to critically discuss effects of progesterone on the nervous system via PR, and of allopregnanolone via its modulation of GABA(A) receptors, with main focus on the brain.

Motivated behaviors and levels of 3α,5α-THP in the midbrain are attenuated by knocking down expression of pregnane xenobiotic receptor in the midbrain ventral tegmental area of proestrous rats

INTRODUCTION

Progesterone (P4 ) and its product, 5α-pregnan-3α-ol-20-one (3α,5α-THP), act in the midbrain ventral tegmental area (VTA) to alter motivated behaviors, such as mating, and motor and anxiety behavior. Of interest is whether 3α,5α-THP formation requires the pregnane xenobiotic receptor (PXR), which is expressed in the midbrain of rats.

AIM

The role of PXR in the midbrain for 3α,5α-THP formation, which precedes modulation of motivated behaviors, was investigated.

METHODS

Rats had estrous cycle phase determined and were assessed when they were in diestrus or proestrus. Diestrous and proestrous rats were infused with control or antisense oligodeoxyribonucleotides (AS-ODNs) targeted against PXR to the VTA.

MAIN OUTCOME MEASURES

In pilot studies, PXR gene and protein expression in the midbrain were determined with quantitative reverse transcriptase polymerase chain reaction and Western blotting, respectively. Diestrous and proestrous rats infused with control or AS-ODNs to the VTA were tested for anxiety (open field and plus maze), social (social interaction), and sexual (paced mating) behavior. Expression of PXR in the midbrain was verified with Western blotting. Plasma estradiol, P4 , dihydroprogesterone (DHP), and 3α,5α-THP levels, and brain P4 , DHP, and 3α,5α-THP levels were measured. We predicted that proestrous rats infused with PXR AS-ODNs would have decreased anti-anxiety, social, and sexual behavior, lower midbrain expression of PXR, and lower midbrain levels of 3α,5α-THP compared with controls.

RESULTS

Results supported the hypothesis that formation of 3α,5α-THP requires PXR and may be important for motivated behaviors. PXR AS-ODN, compared with control, infusions to the VTA reduced PXR expression and 3α,5α-THP levels in the midbrain and attenuated sexual receptivity of proestrous rats.

CONCLUSIONS

Knockdown of PXR in the midbrain reduces 3α,5α-THP levels and sexual receptivity of proestrous rats. Thus, PXR in the midbrain may be required for the observed increase in 3α-5α-THP during proestrus, which has subsequent effects on motivated, reproductive behaviors.

An antiprogestin, CDB4124, blocks progesterone's attenuation of the negative effects of a mild stress on sexual behavior

These experiments were designed to test the hypothesis that a progesterone receptor antagonist would block progesterone's ability to reduce the negative effects of a 5 min restraint on female rat sexual behavior. Ovariectomized Fischer rats were injected with 10 μg estradiol benzoate. Two days later, rats were injected subcutaneously (sc) with the progesterone receptor antagonist, CDB4124 (17α-acetoxy-21-methoxy-11β-[4-N,N-dimethyaminopheny]-19-norpregna-4,9-dione-3,20-dione) (60 mg/kg), or vehicle (20% DMSO+propylene glycol). One hour later, rats were injected sc with 500 μg progesterone or vehicle (sesame seed oil). Rats were assigned to one of three different treatment conditions: (1) (ECV) estradiol benzoate, CDB4124, sesame seed oil vehicle, (2) (ECP) estradiol benzoate, CDB4124, progesterone, and (3) (EVP) estradiol benzoate, DMSO/propylene glycol vehicle, progesterone. That afternoon sexual behavior was examined before and after a 5 min restraint experience. Before restraint, lordosis behavior was comparable across treatment conditions but only progesterone-treated rats exhibited proceptive behavior. CDB4124 did not block progesterone's induction of proceptivity. However, after restraint, CDB4124 attenuated the positive effects of progesterone on all sexual behaviors examined. The restraint experience inhibited sexual behavior in rats treated with estradiol benzoate and CDB4124 and in rats treated with estradiol benzoate, CDB4124, and progesterone but not in rats given estradiol benzoate and progesterone without CDB4124. These findings are consistent with the hypothesis that progesterone receptors mediate progesterone's ability to reduce the negative sexual behavioral effects of a mild stressor.

RU486 blocks effects of allopregnanolone on the response to restraint stress

These experiments were designed to provide information about the potential involvement of progesterone receptors in the ability of allopregnanolone (3α-hydroxy-5α-pregnan-20-one) to reduce the lordosis-inhibiting effects of restraint stress. Ovariectomized Fischer rats were hormonally primed with 10 μg estradiol benzoate and 4 mg/kg allopregnanolone or vehicle. One hour before allopregnanolone, rats were injected with the progesterone receptor antagonist, RU486 (11β-(4-dimethylamino)phenyl-17β-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one), or vehicle. Four hours after allopregnanolone or vehicle, sexual behavior was examined before and after a 5-min restraint stress. Lordosis behavior of rats primed only with estradiol benzoate declined after the 5 min of restraint while allopregnanolone prevented this decline. RU486 attenuated the ability of allopregnanolone to prevent the restraint-induced decline in lordosis behavior. These findings are consistent with earlier suggestions that progesterone receptors are involved in allopregnanolone's ability to reduce the effects of restraint stress.

Effects and Mechanisms of 3α,5α,-THP on Emotion, Motivation, and Reward Functions Involving Pregnane Xenobiotic Receptor

Progestogens [progesterone (P(4)) and its products] play fundamental roles in the development and/or function of the central nervous system during pregnancy. We, and others, have investigated the role of pregnane neurosteroids for a plethora of functional effects beyond their pro-gestational processes. Emerging findings regarding the effects, mechanisms, and sources of neurosteroids have challenged traditional dogma about steroid action. How the P(4) metabolite and neurosteroid, 3α-hydroxy-5α-pregnan-20-one (3α,5α-THP), influences cellular functions and behavioral processes involved in emotion/affect, motivation, and reward, is the focus of the present review. To further understand these processes, we have utilized an animal model assessing the effects, mechanisms, and sources of 3α,5α-THP. In the ventral tegmental area (VTA), 3α,5α-THP has actions to facilitate affective, and motivated, social behaviors through non-traditional targets, such as GABA, glutamate, and dopamine receptors. 3α,5α-THP levels in the midbrain VTA both facilitate, and/or are enhanced by, affective and social behavior. The pregnane xenobiotic receptor (PXR) mediates the production of, and/or metabolism to, various neurobiological factors. PXR is localized to the midbrain VTA of rats. The role of PXR to influence 3α,5α-THP production from central biosynthesis, and/or metabolism of peripheral P(4), in the VTA, as well as its role to facilitate, or be increased by, affective/social behaviors is under investigation. Investigating novel behavioral functions of 3α,5α-THP extends our knowledge of the neurobiology of progestogens, relevant for affective/social behaviors, and their connections to systems that regulate affect and motivated processes, such as those important for stress regulation and neuropsychiatric disorders (anxiety, depression, schizophrenia, drug dependence). Thus, further understanding of 3α,5α-THP's role and mechanisms to enhance affective and motivated processes is essential.

Mechanisms responsible for progesterone's protection against lordosis-inhibiting effects of restraint I. Role of progesterone receptors

Progestins and antiprogestins are widely used therapeutic agents in humans. In many cases, these are indicated for the treatment of reproductive activities. However, progesterone has widespread physiological effects including a reduction of the response to stress. We have reported that 5 min of restraint reduced lordosis behavior of ovariectomized rats hormonally primed with estradiol benzoate. When ovariectomized rats received both estradiol benzoate and progesterone priming, restraint had minimal effects on lordosis. Progesterone influences behavior through classical intracellular progesterone receptor-mediated nuclear events as well as extranuclear events. How these multiple events contribute to the response to stress is unclear. The current project was designed to initiate examination of the mechanisms responsible for progesterone's ability to protect against the effects of the restraint. In the first experiment, ovariectomized rats, primed with 10 μg estradiol benzoate, received 500 μg progesterone 4 h, 1 h, or 30 min before restraint. When progesterone was injected 4h before restraint, progesterone eliminated the effects of restraint. In contrast, progesterone 30 min before restraint offered no protection. Effects of progesterone 1h before restraint were equivocal allowing the suggestion that less than 4h of progesterone priming might be sufficient. In the second experiment, the synthetic progestin, medroxyprogesterone, was shown to mimic effects of progesterone in preventing effects of restraint. Finally, the progesterone receptor antagonist, RU486, attenuated progesterone's protection against restraint. These findings offer evidence that ligand-activated progesterone receptor mechanisms contribute to the maintenance of lordosis behavior in the presence of mild stress.

Mechanisms responsible for progesterone's protection against lordosis-inhibiting effects of restraint II. Role of progesterone metabolites

When ovariectomized Fischer female rats are hormonally primed with 10 μg estradiol benzoate, a 5 min restraint experience rapidly inhibits lordosis behavior. Addition of progesterone to the hormonal priming prevents this restraint-induced inhibition. In prior work, we reported evidence that progesterone receptors (PR) may contribute to this protective effect of progesterone. In the current manuscript, we provide evidence that progesterone metabolites may also contribute to progesterone's ability to reduce the effects of restraint. Ovariectomized female rats were hormonally primed with 10 μg estradiol benzoate followed 2 days later with 4.0 mg/kg of the progesterone metabolite, allopregnanolone. Allopregnanolone, administered either 4 h or 2 h before the restraint experience, was as effective as progesterone in reducing the lordosis-inhibitory effects of restraint. In the second experiment, progesterone metabolism was blocked with 50 mg/kg of the 5α-reductase inhibitor, finasteride. Surprisingly, finasteride did not prevent progesterone from reducing the effects of restraint. In a third experiment, we tested the possibility that allopregnanolone acted through metabolism to dihydroprogesterone. Rats were treated with allopregnanolone or with allopregnanolone plus the 3α-hydroxysteroid dehydrogenase inhibitor, indomethacin. Indomethacin did not prevent allopregnanolone from reducing the effects of restraint. Mechanisms are discussed whereby cross-talk between PR-mediated and metabolite-mediated events may converge in producing progesterone's attenuation of the effect of restraint.

I. Levels of 5α-reduced progesterone metabolite in the midbrain account for variability in reproductive behavior of middle-aged female rats

At middle-age, the reproductive capacity of female rats begins to decline. Whether there are consequences for social and reproductive behaviors related to changes in estradiol (E(2)), progesterone (P(4)) and its 5α-reduced metabolites, dihydroprogesterone (DHP) and 5α-pregnan-3α-ol-20-one (3α,5α-THP), is of interest. In Experiment 1, 1-year-old female breeder rats that had "maintained their reproductive status" (having 4-5 days estrous cycles, > 60% successful pregnancies after mating, > 10 pups/litter) or their age-matched counterparts with "declining reproductive status" were assessed in social interaction, standard mating, and paced mating when in proestrus. Rats that maintained reproductive status tended to have higher levels of proceptivity, and significantly reduced aggression, towards males, compared to rats with declining reproductive status. Basal midbrain E(2) and DHP levels accounted for a significant proportion of variance in lordosis. In Experiment 2, 1-year-old, age-matched, female breeders that had maintained reproductive status or were in reproductive decline were compared to three-month old, nulliparous females that had regular (4-5 days) or irregular estrous cycles. Age did not influence paced mating but younger rats had greater diencephalon E(2) than did middle-aged rats. After mating, rats with declining/irregular reproductive status had higher P(4) and DHP levels in midbrain than did rats with maintaining/regular reproductive status, albeit differences in midbrain 3α,5α-THP were not seen. Middle-aged rats that maintained reproductive function had greater 3α,5α-THP formation in diencephalon compared to other groups. Thus, age-related changes in central progestogen formation in midbrain or diencephalon may contribute to some variability in expression of reproductive behaviors.

Low doses of cocaine decrease, and high doses increase, anxiety-like behavior and brain progestogen levels among intact rats

There are sex and hormonal differences in response to cocaine that have been demonstrated in people and animal models. Cocaine can alter secretion of progestogens, such as progesterone (P), and its neuroactive metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP). However, little research has been done on the neuroendocrine effects in the initiation phase of cocaine use. We hypothesize that some sex/hormonal differences in initiation phase responses to cocaine may be related to formation of progestogens. To investigate the role of progestogens in sex differences in response to acute cocaine, male and female rats in the high (proestrous) or low (diestrous) progestogen phase of the estrous cycle were administered cocaine (0, 5, 10, or 20mg/kg, IP). We examined cocaine's acute neuroendocrine effects on P and 3alpha,5alpha-THP levels, as well as its effects on acute psychomotor stimulation, anxiety, and sexual behaviors. Among rats that had P and/or 3alpha,5alpha-THP levels increased in response to cocaine, enhanced acute psychomotor stimulation was observed. Results suggest that cocaine produces U-shaped curves for progestogens, and anxiety-like behaviors. Male rats were less susceptible to these effects of cocaine than were proestrous or diestrous female rats. However, cocaine's disruption of sexual behaviors was similar among males and proestrous females. These data suggest a complex interaction between hormonal milieu and the neuroendocrine and behavioral effects of cocaine.

Infusions of bicuculline to the ventral tegmental area attenuates sexual, exploratory, and anti-anxiety behavior of proestrous rats

Actions of 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), in the midbrain ventral tegmental area (VTA) modulate sexual receptivity of female rats. Actions of 3alpha,5alpha-THP at GABAergic substrates in the VTA are known to modulate consummatory aspects of sexual behavior among rodents, such as lordosis. However, the extent to which GABA(A) receptors in the VTA are important for appetitive (exploratory, anti-anxiety, social) aspects of sexual receptivity is not well-understood. Proestrous rats were bilaterally-infused with saline or bicuculline (100 ng), a GABA(A) receptor antagonist, to the VTA or missed control sites. Rats were assessed for exploratory/anti-anxiety (open field/elevated plus maze), social (social interaction), and sexual (paced-mating) behavior. Compared to saline or missed site controls, intra-VTA bicuculline significantly reduced the number of central entries in an open field, time spent on the open arms of an elevated plus maze, frequency and intensity of lordosis, anti-aggression towards a male, pacing of sexual contacts, and 3alpha,5alpha-THP concentrations in midbrain and hippocampus. Bicuculline-infused rats also displayed less affiliation with a novel conspecific, fewer sexual solicitations, and had lower 3alpha,5alpha-THP concentrations in diencephalon and cortex, albeit these were not significant differences. Thus, actions at GABA(A) receptors in the midbrain VTA are essential for appetitive and consummatory aspects of sexual receptivity among rats.

Sex differences in psychopathology: of gonads, adrenals and mental illness

Stress-related disorders such as anxiety and depression are disproportionately prevalent in women. Women are more likely to experience depression and anxiety disorders during periods of marked hormonal fluctuations, suggesting that gonadal hormones are involved in stress pathology. Depression and anxiety are both associated with aberrant secretion of glucocorticoids, which also show marked fluctuations across the reproductive cycle and in response to gonadal steroids. Thus, interactions between gonadal and stress hormones may play a major role in predisposing females to stress-related disease. The purpose of this brief review is to highlight preclinical data regarding the role of estrogens in depression and anxiety-like behaviors. While it is evident the exogenous estrogens modulate affective behavior in rodents, there is some disagreement in the literature, perhaps related to experimental designs that vary with respect to administration parameters and stress. Beneficial effects of estrogens on mood are most likely due to estrogen receptor (ER)beta signaling. The antidepressant and anxiolytic effects of ERbeta are consistent with its role in attenuating glucocorticoid responses to stress, suggesting that estrogens, acting at ERbeta, may improve mood by suppressing glucocorticoid hyperactivity. However, additional studies demonstrate that ERbeta signaling in the hippocampus is sufficient to induce antidepressant and anxiolytic behaviors. Thus, ERbeta may improve mood via primary actions on hypothalamic (i.e., paraventricular nucleus) and/or extra-hypothalamic sites. Overall, the preclinical research suggests that selective ER modulators targeting ERbeta may be an attractive alternative or adjunct treatment to currently prescribed antidepressants or anxiolytics.

Estrogen is necessary for 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) infusion to the ventral tegmental area to facilitate social and sexual, but neither exploratory nor affective behavior of ovariectomized rats

The progesterone metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP, allopregnanolone), acts in the ventral tegmental area (VTA) to facilitate exploratory, anti-anxiety, and socio-sexual behavior among ovariectomized (OVX), estrogen (E(2))-primed rats and gonadally-intact rats with high (proestrus) or low (diestrus) endogenous E(2) levels. The extent to which E(2) is required for these effects of 3alpha,5alpha-THP is not known. OVX rats were primed with systemic 17beta-estradiol (10 microg) or oil vehicle and were infused 44 h later with 3alpha,5alpha-THP (100 ng) or beta-cyclodextrin vehicle to the VTA, substantia nigra (SN), or central grey (CG). Rats were assessed in a battery of exploratory (open field), anxiety (elevated plus maze), social (partner preference, social interaction), and sexual (paced mating) tasks. E(2)-priming was necessary for 3alpha,5alpha-THP infusions to facilitate social interaction and mating and midbrain 3alpha,5alpha-THP levels were higher among E(2)-compared to vehicle-primed rats. Irrespective of E(2)-priming, rats infused with 3alpha,5alpha-THP to the VTA, but not SN or CG, demonstrated increased exploration in an open field, anti-anxiety behavior on an elevated plus maze, and preference for a male. Thus, actions of 3alpha,5alpha-THP in the VTA to enhance social and sexual behaviors were reliant on E(2) but increases in exploratory and anti-anxiety behavior were not.