3 alpha-OH-DHP and 5 alpha-THDOC implants to the ventral tegmental area facilitate sexual receptivity in hamsters after progesterone priming to the ventral medial hypothalamus

Establishing connectivity through microdissections of midbrain stimulation-related neural circuits

Comprehensive understanding of the neural circuits involving the ventral tegmental area is essential for elucidating the anatomofunctional mechanisms governing human behaviour, in addition to the therapeutic and adverse effects of deep brain stimulation for neuropsychiatric diseases. Although the ventral tegmental area has been targeted successfully with deep brain stimulation for different neuropsychiatric diseases, the axonal connectivity of the region is not fully understood. Here, using fibre microdissections in human cadaveric hemispheres, population-based high-definition fibre tractography and previously reported deep brain stimulation hotspots, we find that the ventral tegmental area participates in an intricate network involving the serotonergic pontine nuclei, basal ganglia, limbic system, basal forebrain and prefrontal cortex, which is implicated in the treatment of obsessive-compulsive disorder, major depressive disorder, Alzheimer's disease, cluster headaches and aggressive behaviours.

Effects of the neuroactive steroid allopregnanolone on intracranial self-stimulation in C57BL/6J mice

RATIONALE

The neuroactive steroid (3α,5α)-3-hydroxy-pregnan-20-one (3α,5α-THP, allopregnanolone) has effects on reward-related behaviors in mice and rats that suggest that it may activate brain reward circuits. Intracranial self-stimulation (ICSS) is an operant behavioral technique that detects changes in the sensitivity of brain reward circuitry following drug administration.

OBJECTIVE

To examine the effects of the neuroactive steroid allopregnanolone on ICSS and to compare these effects to those of cocaine.

METHODS

Male C57BL/6J mice implanted with stimulating electrodes implanted into the medial forebrain bundle responded for reinforcement by electrical stimulation (brain stimulation reward (BSR)). Mice received cocaine (n = 11, 3.0-30.0 mg/kg, intraperitoneal (i.p.)) or the neuroactive steroid allopregnanolone (n = 11, 3.0-17.0 mg/kg, i.p.). BSR thresholds (θ 0) and maximum (MAX) operant response rates after drug treatments were compared to those after vehicle injections.

RESULTS

Cocaine and allopregnanolone dose dependently lowered BSR thresholds relative to vehicle injections. Cocaine was maximally effective (80 % reduction) in the second 15 min following the 30 mg/kg dose, while allopregnanolone was maximally effective (30 % reduction) 15-45 min after the 17 mg/kg dose. Neither drug had significant effects on MAX response rates.

CONCLUSIONS

The effects of allopregnanolone on BSR thresholds are consistent with the previously reported effects of benzodiazepines and alcohol, suggesting that positive modulation of GABAA receptors can facilitate reward-related behaviors in C57BL/6J mice.

Membrane progestin receptors in the midbrain ventral tegmental area are required for progesterone-facilitated lordosis of rats

Progesterone (P₄) and its metabolites, rapidly facilitate lordosis of rats partly through actions in the ventral tegmental area (VTA). The study of membrane progestin receptors (mPRs), of the Progestin and AdipoQ Receptor (PAQR) superfamily, has been limited to expression and regulation, instead of function. We hypothesized that if mPRs are required for progestin-facilitated lordosis in the VTA, then mPRs will be expressed in this region and knockdown will attenuate lordosis. First, expression of mPR was examined by reverse-transcriptase polymerase chain reaction (RT-PCR) in brain and peripheral tissues of proestrous Long-Evans rats. Expression of mPRα (paqr7) was observed in peripheral tissues and brain areas, including hypothalamus and midbrain. Expression of mPRβ (paqr8) was observed in brain tissues and was abundant in the midbrain and hypothalamus. Second, ovariectomized rats were estrogen (E₂; 0.09 mg/kg, SC), and P₄ (4 mg/kg, SC) or vehicle-primed, and infused with antisense oligodeoxynucleotides (AS-ODNs) targeted against mPRα and/or mPRβ intracerebroventricularly or to the VTA. Rats were assessed for motor (open field), anxiety (elevated plus maze), social (social interaction), and sexual (lordosis) behavior. P₄-facilitated lordosis was significantly reduced with administration of AS-ODNs for mPRα, mPRβ, or co-administration of mPRα and mPRβ to the lateral ventricle, compared to vehicle. P₄-facilitated lordosis was reduced, compared to vehicle, by administration of mPRβ AS-ODNs, or co-administration of mPRα and mPRβ AS-ODNs, but not mPRα AS-ODNs alone, to the VTA. No differences were observed for motor, anxiety, or social behaviors. Thus, mPRs in the VTA are targets of progestin-facilitated lordosis of rats.

6-hydroxydopamine lesions enhance progesterone-facilitated lordosis of rats and hamsters, independent of effects on motor behavior

The Ventral Tegmental Area (VTA) is an important brain area for progesterone (P(4))'s effects to facilitate female sexual behavior of rodents. We investigated the importance of dopaminergic neurons in the VTA, and two dopaminergic projection sites, the Nucleus Accumbens (NAc), and Caudate Nucleus of the Striatum (CN), in modulating P(4)-facilitated sex and motor behavior. Ovariectomized (ovx) rats and hamsters, administered estradiol benzoate (10 microg) and P(4) (0, 50, 100, 200, or 500 microg), were tested for motor behavior in a chamber that automatically records horizontal beam breaks, and for sexual behavior in response to a sexually-experienced male. Animals were tested once a week until each P(4) dosage was received; animals then had bilateral 6-hydroxydopamine (6-OHDA) or sham lesions to the VTA, NAc, or CN and were re-tested at each P(4) dosage on subsequent weeks. Fixed brains were stained with cresyl violet and processed for dopamine transporter (DAT) immunoreactivity. The number of cresyl violet stained cells was significantly lower in all 6-OHDA infusion sites compared to non-6-OHDA infusion sites of rats and hamsters. Also, in rats, the number of DAT-immunoreactive neurons was lower in all 6-OHDA infusion sites compared to non-6-OHDA infusion sites. In rats, 6-OHDA but not sham, lesions to the VTA, NAc, or CN produced P(4)-dependent increases in lordosis quotients and resulted in modest increases in motor behavior. In hamsters, 6-OHDA, but not sham, lesions to the VTA, NAc, or CN produced P(4)-dependent increases in total lordosis durations and produced modest decreases in motor behavior. This suggests that the dopaminergic output neurons of midbrain VTA may play an important role in modulation of P(4)-facilitated sexual lordosis among rodents.

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

The progesterone (P(4)) metabolite and neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) acts in the midbrain ventral tegmental area (VTA) to modulate lordosis of female rats. 3alpha,5alpha-THP also mediates exploratory, affective, and social behaviors; whether actions of 3alpha,5alpha-THP in the VTA mediate these behaviors is of interest. To elucidate the role of the VTA in mediating exploratory, affective, and social behaviors, the present study examined effects of inhibiting 3alpha,5alpha-THP formation in the VTA. Rats received intra-VTA infusions of either PK11195 (400ng/mul, which inhibits de novo 3alpha,5alpha-THP production), indomethacin (10mug/mul, which blocks metabolism of P(4) to 3alpha,5alpha-THP), PK11195 and indomethacin together, or beta-cyclodextrin vehicle and tested on a battery of anxiety (open field and elevated plus maze), social (partner preference and social interaction), and sexual (paced mating) tasks. Compared to rats infused with vehicle to the VTA, rats infused with inhibitor(s) demonstrated significant reductions in central entries in the open field, time on open arms of an elevated plus maze, time spent interacting with a conspecific, initiation and intensity of lordosis, sexual solicitations, and midbrain 3alpha,5alpha-THP levels. These findings suggest that actions of 3alpha,5alpha-THP in the VTA are important for mediating aspects of exploration, anxiety, and social behavior related to mating.

The neurophysiology of sexual arousal

Our understanding of the process and initiation of sexual arousal is being enhanced by both animal and human studies, inclusive of basic science principles and research on clinical outcomes. Sexual arousal is dependent on neural (sensory and cognitive) factors, hormonal factors, genetic factors and, in the human case, the complex influences of culture and context. Sexual arousal activates the cognitive and physiologic processes that can eventually lead to sexual behavior. Sexual arousal comprises a particular subset of central nervous system arousal functions which depend on primitive, fundamental arousal mechanisms that cause generalized brain activity, but are manifest in a sociosexual context. The neurophysiology of sexual arousal is seen as a bidirectional system universal to all vertebrates. The following review includes known neural and genomic mechanisms of a hormone-dependent circuit for simple sex behavior. New information about hormone effects on causal steps related to sex hormones' nuclear receptor isoforms expressed by hypothalamic neurons continues to enrich our understanding of this neurophysiology.

Differential effect of kinase A and C blockers on lordosis facilitation by progesterone and its metabolites in ovariectomized estrogen-primed rats

Dose response curves for lordosis behavior was obtained for progesterone (P) and its two ring A-reduced metabolites: 5alpha-pregnanedione (alpha-DHP) and 5alpha,3alpha-pregnanolone (5alpha,3alpha-Pgl) by infusing these progestins in the right lateral ventricle (rlv) of ovariectomized (ovx) estradiol-treated rats (2 microg estradiol benzoate; EB), 40 h before intracerebro-ventricular (icv) injection. Effective doses 50 (ED50) revealed that ring A-reduced progestins were more potent than P itself to induce lordosis behavior. Two dose levels, one producing the maximal effect and the other one producing a submaximal response (ED50-ED60), were selected for testing the capacity of RpAMPS, a kinase A blocker, and H7, a kinase C blocker, to modify the response to the three progestins. rlv injection of RpAMPS significantly depressed the lordosis response to the two dose levels of P and alpha-DHP but failed to significantly inhibit that of 5alpha,3alpha-Pgl. The administration of H7 prevented the effect of both 5alpha-reduced progestins without affecting the response to P. The results suggest that P and its ring A-reduced metabolites stimulate lordosis behavior through different cellular mechanisms: P acting mainly through the cAMP-kinase system; alpha-DHP through both kinase A and kinase C signaling pathways and 5alpha,3alpha-Pgl through the kinase C system.

Escalated aggression as a reward: corticosterone and GABA(A) receptor positive modulators in mice

RATIONALE

Individuals seek out the opportunity to fight, but the mechanisms behind this positively reinforcing effect of aggression have yet to be understood.

OBJECTIVES

The aims of this study were to (1) describe behavioral and corticosterone elevations that occur in aggressive mice conditioned to respond for the opportunity to fight another mouse, (2) determine if corticosterone elevations are necessary for operant responding and escalated aggression, and (3) determine if corticosterone elevations alter the aggression-heightening effects of gamma-aminobutyric acid (GABA)(A) receptor positive modulators.

METHODS AND RESULTS

Aggressive male CFW mice were conditioned to respond under the control of a fixed-interval 10-min (FI10) schedule that reinforced their operant behavior by the presentation of an intruder mouse into their home cage. After the FI10, aggressive behavior was ca. 75% higher than the species-typical levels of fighting and plasma corticosterone was more than twice as high after briefly fighting and/or responding on the FI10 schedule. Inhibition of corticosterone synthesis by metyrapone (30-100 mg/kg) reduced both conditioned responding as well as the aggressive behavior after the FI. Although the benzodiazepine midazolam (0.3-3 mg/kg) heightened species-typical aggressive behavior, it did not increase the high level of aggression engendered by the FI schedule. However, midazolam (0.3 mg/kg) and the neurosteroid allopregnanolone (17 mg/kg) both heightened aggression when given after corticosterone synthesis inhibition by metyrapone (56 mg/kg).

CONCLUSIONS

These data suggest that corticosterone elevations are required for responding that is motivated by aggressive behavior and for escalated aggression that follows this responding. Corticosterone elevations also appear to inhibit the aggression heightening effect of GABA(A) receptor positive modulators.

The nitric oxide pathway participates in estrous behavior induced by progesterone and some of its ring A-reduced metabolites

Intracerebral and intravenous administration of progesterone (P) and its ring A-reduced metabolites induces intense sexual behavior (lordosis and proceptivity) in estrogen-primed rats. The present study tested the hypothesis that the nitric oxide-cGMP-protein kinase G pathway is involved in the facilitation of sexual behavior induced by the intracerebroventricular (i.c.v.) administration of P (130 ng) and its ring A-reduced metabolites 5alpha-dihydroprogesterone (5alpha-DHP; 13 ng) and 5alpha,3alpha-pregnanolone (5alpha,3alpha-Pgl; 13 ng). In Experiment 1, we tested the relevance of the nitric oxide/cGMP pathway by infusing a nitric oxide synthase inhibitor or a nitric oxide-dependent, soluble guanylyl cyclase inhibitor icv before progestin administration. The lordosis induced by P, 5alpha-DHP and 5alpha,3alpha-Pgl was significantly reduced at 2 h after progestin infusion by the previous injection of either a nitric oxide synthase inhibitor or by a soluble guanylyl cyclase inhibitor. Lordosis behavior returned to control values by 4 h. In Experiment 2, i.c.v. infusion of the protein kinase G inhibitor KT5823 significantly inhibited the lordosis behavior induced by all three progestins at 2 h. These data support the hypothesis that the nitric oxide/cGMP/protein kinase G pathway is involved in the lordosis induced by P and some of its ring A-reduced metabolites.

Neurosteroids, GABAA receptors, and escalated aggressive behavior

Aggressive behavior can serve important adaptive functions in social species. However, if it exceeds the species-typical pattern, it may become maladaptive. Very high or escalated levels of aggressive behavior can be induced in laboratory rodents by pharmacological (alcohol-heightened aggression), environmental (social instigation), or behavioral (frustration-induced aggression) means. These various forms of escalated aggressive behavior may be useful in further elucidating the neurochemical control over aggression and violence. One neurochemical system most consistently linked with escalated aggression is the GABAergic system, in conjunction with other amines and peptides. Although direct stimulation of GABA receptors generally suppresses aggression, a number of studies have found that positive allosteric modulators of GABAA receptors can cause increases in aggressive behavior. For example, alcohol, benzodiazepines, and many neurosteroids are all positive modulators of the GABAA receptor and all can cause increased levels of aggressive behavior. These effects are dose-dependent and higher doses of these compounds generally shift from heightening aggressive behavior to being sedative and anti-aggressive. In addition, these modulators interact with each other and can have additive effects on the GABAA receptor and on behavior, including aggression. The GABAA receptor is a heteropentameric protein that can be constituted from various subunits. It has been shown that subunit composition can affect sensitivity of the receptor to some modulators and that subunit composition differentially affects the sedative vs anxiolytic actions of benzodiazepines. Initial studies targeting alpha subunits of the GABAA receptor point to their significant role in the aggression-heightening effects of alcohol, benzodiazepines, and neurosteroids.

Non-ligand activation of estrous behavior in rodents: cross-talk at the progesterone receptor

Estrous behavior in rodents is triggered by the binding of progesterone (P) to its intracellular receptor (PR). Non-steroidal agents (i.e., gonadotropin-releasing hormone, noradrenaline, dopamine and others), acting at the membrane, can facilitate estrous behavior in estrogen-primed rats. This action is mediated through the generation of second messengers (cyclic AMP, cyclic GMP, calcium) which, in turn, phosphorylate through diverse kinase systems (protein kinases A, G or C) either the PR or associated effector proteins linking the PR to the trans-activation machinery. P or its metabolites also activate cyclic AMP-signaling pathways by acting directly on the membrane or by modulating neurotransmitter release. Molecular processes resulting from second messenger signaling pathways and those from the progesterone-RP interaction synergize to elicit a full behavioral response.

Mitochondrial benzodiazepine receptors in the ventral tegmental area modulate sexual behaviour of cycling or hormone-primed hamsters

Hamsters are highly dependent upon the central actions of progesterone (P4) for facilitation of sexual behaviour. In the ventral tegmental area (VTA), P4 has actions through its neurosteroid metabolite 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP). The effects of enhancing or inhibiting neurosteroidogenesis (and thereby 3alpha,5alpha-THP concentrations), through manipulations of mitochondrial benzodiazepine receptors, in the VTA on socio-sexual behaviour of female hamsters were examined. Intact, naturally receptive hamsters and ovariectomized (OVX), hormone-primed hamsters were unilaterally infused via chronic guide cannula to the VTA with the mitochondrial benzodiazepine receptor antagonist 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboximide (PK-11195) or the mitochondrial benzodiazepine receptor agonist, N,N-dihexyl-2-(4-fluorophenyl)indole-30-acetamide (FGIN 1-27) and tested for sexual responsiveness and lordosis. PK-11195 (5.6, 11.2 or 22.4 nm) to the VTA attenuated sexual responsiveness of naturally receptive or oestradiol benzoate (EB) + P4-primed hamsters compared to vehicle. In addition, FGIN 1-27 (11.4 nm) infusions to the VTA increased sexual responsiveness and lordosis of cycling or OVX, EB + P4-primed hamsters, compared to vehicle infusions. In OVX, EB + P4-primed hamsters, decrements in sexual responsiveness produced by VTA infusions of PK-11195 (5.6 nm) were attenuated by VTA infusions of 3alpha,5alpha-THP. VTA infusions of PK-11195 (5.6 nm) or FGIN 1-27 (11.4 nm), respectively, decreased and increased midbrain levels of 3alpha,5alpha-THP compared to each other. Together, these findings indicate that manipulating actions of mitochondrial benzodiazepine receptors in the VTA can augment and inhibit neurosteroidogenesis and sexual responsiveness of hormone-primed and naturally receptive hamsters.

Lordosis of rats is modified by neurosteroidogenic effects of membrane benzodiazepine receptors in the ventral tegmental area

Progestins modulate lordosis through actions in the ventral tegmental area (VTA). Whether neurosteroidogenesis of 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), involving mitochondrial benzodiazepine receptors (MBR), is important for lordosis was investigated. Ovariectomized (Ovx), hormone-primed rats (experiments 1, 3, 5, 6) and rats in behavioral estrus (experiments 2 and 4) were unilaterally infused via chronic guide cannula to the VTA with a MBR agonist, N,N-dihexyl-2-(4-fluorophenyl) indole-30-acetamide (FGIN 1-27) or antagonist 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboximide (PK-11195). Experiment 1: Estradiol benzoate (EB)-primed (25 microg) rats administered 0 or 25 microg progesterone (P4) SC showed increased lordosis when infused with 5.0 microg FGIN 1-27 to the VTA; those administered 100 or 200 microg P4 SC exhibited greater lordosis when infused with 2.5 or 5.0 microg FGIN, relative to saline-infused rats. Experiment 2: Rats, near the termination of behavioral estrus, infused with 2.5 or 5.0 microg of FGIN 1-27 to the VTA, showed increased lordosis compared to that seen following vehicle administration. Experiment 3: EB-primed rats administered 200 or 500 microg P4 SC showed decreased lordosis when infused with 100, 200, or 400 ng PK-11195, relative to saline-infused rats. Experiment 4: Rats infused at the peak of behavioral estrus with 100, 200, or 400 ng PK-11195 to the VTA exhibited reduced lordosis compared to that seen following vehicle administration. Experiment 5: 3alpha,5alpha-THP (100 ng) infusions to the VTA reinstated lordosis of hormone-primed rats infused with PK-11195 (100 ng) to the VTA. Experiment 6: FGIN 1-27 (5.0 microg) and PK-11195 (100 ng) infusions aimed at the VTA respectively increased and decreased midbrain levels of 3alpha,5alpha-THP compared to vehicle. Notably, the specific effects observed with infusions to the VTA were not seen with infusions to the control site, the substantia nigra. These data suggest that neurosteroidogenesis involving MBRs in the VTA mediates lordosis of hormone-primed or behavioral estrous rats.

Ventral tegmental area infusions of inhibitors of the biosynthesis and metabolism of 3alpha,5alpha-THP attenuate lordosis of hormone-primed and behavioural oestrous rats and hamsters

The importance of progesterone biosynthesis and metabolism to 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), which exerts its effects via GABAA/benzodiazepine receptor complexes (GBRs) rather than intracellular progestin receptors (PRs), was investigated for its effects on sexual receptivity. Epostane, a 3beta-hydroxysteroid dehydrogenase inhibitor, blocks progesterone and 3alpha,5alpha-THP biosynthesis. Finasteride, a 5alpha-reductase inhibitor, blocks the metabolism of progesterone to dihydroprogesterone (DHP), which is subsequently metabolized to 3alpha,5alpha-THP. Indomethacin, a 3alpha-hydroxysteroid oxidoreductase inhibitor, blocks DHP's metabolism to 3alpha,5alpha-THP, and its oxidation to DHP. Epostane, finasteride, indomethacin or vehicle were infused intracranially in the ventral tegmental area (VTA) of hormone-primed or naturally receptive rats and hamsters and sexual behaviour was recorded. Epostane, finasteride and indomethacin to the VTA significantly reduced lordosis, compared to vehicle infusions, in hormone-primed and behavioural oestrous rats and hamsters. Radioimmunoassay revealed that concentrations of midbrain 3alpha,5alpha-THP were reduced following epostane, finasteride or indomethacin infusions that significantly decreased lordosis. Immunocytochemistry for 3alpha,5alpha-THP revealed the number of immunoreactive cells were significantly reduced in the VTA following epostane, finasteride or indomethacin infusion to the VTA, but not other midbrain sites. These data suggest that biosynthesis of progestins, and the metabolism of progesterone to 3alpha,5alpha-THP in the VTA, are important for progestin-facilitated sexual receptivity of rats and hamsters.

The role of neurosteroids and non-genomic effects of progestins and androgens in mediating sexual receptivity of rodents

Progestins and androgens modulate sexual receptivity in rodents, in part through mechanisms independent of traditional intracellular steroid receptors. Progesterone (PROG) in the ventromedial hypothalamus (VMH) and ventral tegmental (VTA) facilitates lordosis but has different actions in these brain areas. Primarily using lordosis in rodents as an in vivo experimental model, we have examined the effects that progestins exert in the midbrain and hypothalamus. Localization and blocker studies indicate that PROG's actions in the VMH require intracellular progestin receptors (PRs) but in the VTA they do not. Progestins that have rapid, membrane effects, and/or are devoid of affinity for PRs, facilitate lordosis when applied to the VTA. Manipulation of GABA and/or GABA(A)/benzodiazepine receptor complexes (GBRs) in the VTA alters lordosis, which suggests that progestins may interact with GBRs to facilitate receptivity by enhancing the function of GABAergic neurons. Interfering with PROG's metabolism to, or the biosynthesis of, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-TH PROG or allopregnanolone), the most effective endogenous GBR agonist, in the VTA attenuates female sexual behavior in rodents. Stimulation of mitochondrial benzodiazepine receptors (MBRs), which enhances neurosteroid production, by infusions of an MBR agonist to the VTA enhances lordosis. 3alpha,5alpha-TH PROG is increased in the midbrain of mated>proestrous>diestrous rodents. These data suggest that in the VTA, PROG may facilitate lordosis following metabolism to and/or biosynthesis of 3alpha,5alpha-TH PROG, which may have subsequent actions at GBRs and/or MBRs to acutely modulate female sexual behavior in rodents. The 3alpha-hydroxysteroid oxidoreduced metabolite of dihydrotestosterone (DHT), 5alpha-androstane-3alpha,17beta-diol (3alpha-androstanediol), is important for termination of sexual receptivity in rodents and has these effects in the absence of functional intracellular androgens receptors. As well, altering GBR function in the hypothalamus can influence 3alpha-androstanediol's inhibition of sexual receptivity. Through actions in the hypothalamus that are independent of intracellular androgen receptors but involving GBRs, 3alpha-androstanediol inhibits lordosis. These findings suggest that the PROG metabolite and pregnane neurosteroid, 3alpha,5alpha-TH PROG, and the testosterone metabolite and androstane neurosteroid, 3alpha-androstanediol, can have proximate influences on lordosis that is via nonclassical actions at intracellular steroid receptors.

A membrane-associated progesterone-binding protein, 25-Dx, is regulated by progesterone in brain regions involved in female reproductive behaviors

The ventromedial hypothalamus (VMH) plays a central role in the regulation of the female reproductive behavior lordosis, a behavior dependent upon the sequential activation of receptors for the ovarian steroid hormones estradiol (E) and progesterone (P). These receptors function as transcription factors to alter the expression of target genes. To discover behaviorally relevant genes targeted by E and P in the VMH, we used the differential display PCR to identify messenger RNAs that are differentially expressed in the hypothalamus of ovariectomized (ovx) rats treated with E alone compared with ovariectomized rats treated with E and P. We show here that one interesting mRNA within the hypothalamus that is repressed by P after E priming encodes the protein 25-Dx, the rat homolog of the human membrane-associated P-binding protein Hpr6.6. Neurons in the brain containing the highest levels of 25-Dx are located in several nuclei of the basal forebrain, including the VMH. 25-Dx expression is also higher in the hypothalamus of female P receptor "knockout" mice than in their wild-type littermates. These findings suggest a mechanism in which the activation of nuclear P receptor represses expression of a membrane P receptor, 25-Dx, during lordosis facilitation.

Gender differences in brain and behavior: hormonal and neural bases

This article briefly discusses the difficulties in determining the brain-behavior relationship and reviews the literature on some potential mechanisms underlying gender differences in behavioral responses. Mechanisms that are discussed include genetic effects, organizational effects of gonadal hormones, genomic actions of steroids, nongenomic effects of steroids, and environmental influences. The review is an introduction to the articles presented in this special volume on gender differences in brain and behavior.

Progestins' rapid facilitation of lordosis when applied to the ventral tegmentum corresponds to efficacy at enhancing GABA(A)receptor activity

Progestins may have actions in the midbrain though gamma-aminobutyric acid (GABA)A/benzodiazepine receptor complexes (GBRs) that are relevant for sexual receptivity. The efficacy and time course of various progestins to enhance lordosis when applied to the ventral tegmental area (VTA), following progesterone to the ventral medial hypothalamus (VMH) was investigated. Ovariectomized, oestrogen-primed rats and hamsters with contralateral VMH/VTA cannulae were tested for lordosis before and after implants of P to the VMH and progestins to the VTA. The progestins were P, 5alpha-pregnan-3,20-dione (DHP), 5alpha-pregnan-3alpha-ol-20-one(3alpha,5alpha-TH P), 5alpha-pregnan-3alpha,21-diol-20-one (THDOC), 5beta-pregnan-3alpha-ol-20-one(3alpha,5beta-THP) , 17alpha-ol-6-methyl-4,6-pregnadiene-3,20-dione-17-acetate (megestrol acetate, MA), and 6-chloro-17-ol-4,6-pregnadiene-3, 20-dione-17-acetate (chlormadinone acetate, CA). Progestins' effects on GABA-mediated chloride influx and SR 95531 binding in cortical and midbrain tissue, respectively, were examined in rats and hamsters. 3alpha,5alpha-THP and THDOC implants to the VTA were the most effective at immediately facilitating lordosis of rats and hamsters. Two hours later all other progestins, except MA and CA, increased lordosis in rats; only P, 3alpha,5alpha-THP, and THDOC were effective in hamsters. The progestins' effectiveness at facilitating lordosis were similar to their effects on GABA-stimulated chloride influx and SR 95531 receptor binding (3alpha,5alpha-THP and THDOC>P>DHP>3alpha, 5beta-THP>MA and CA). These findings suggest that progesterone lordosis enhancing effects in the rodent VTA may be via GBRs.

GABA(A), D1, and D5, but not progestin receptor, antagonist and anti-sense oligonucleotide infusions to the ventral tegmental area of cycling rats and hamsters attenuate lordosis

In hamsters, progesterone (P) in the hypothalamus and ventral tegmental area (VTA) is necessary for receptivity; in rats, hypothalamic P induces receptivity and midbrain P further enhances it. How P exerts its effects in the VTA on lordosis is of interest because few estrogen-induced P receptors (PRs) have been identified there. Sexual receptivity of rats and hamsters is enhanced when P's actions in the VTA are restricted to the membrane and when the gamma-aminobutyric acid (GABA)A agonist, muscimol, is infused into the VTA, but attenuated with infusions of the GABA(A) antagonist, bicuculline. The dopamine (DA) agonist. SKF38393, rapidly enhances receptivity when infused intravenously; this effect can be blocked by both DA receptor (DR) and PR antagonists. This study investigated the importance of PRs, glutamic acid decarboxylase (GAD), the enzyme responsible for GABA production, GABA(A) receptors (GBRs), and DRs in the VTA of cycling rats and hamsters for the expression of lordosis. Proestrous and diestrous animals implanted with bilateral VTA cannulae were pre-tested for receptivity, infused with either an antagonist (RU38486 (20 microg), bicuculline (100 ng), SCH23390 (100 ng)), anti-sense oligonucleotide (against PR (250 ng), GAD (500 ng), D1 (500 ng), D5 (250 ng)), or control infusions to each cannulae and re-tested. Vehicle and scrambled oligonucleotides were infused as controls and elicited similar effects. Antagonists of GBRs and DRs significantly reduced lordosis on post-tests compared to the PR antagonist and control conditions in rats and hamsters. Lordosis was significantly reduced, compared to controls, only by anti-sense oligonucleotides for GAD and D1- and D5-DR subtypes. These data suggest that in the VTA GABAergic and dopaminergic neurons may be more important in the mediation of sexual receptivity than neurons containing intracellular PRs.

Indomethacin inhibits lordosis induced by ring A-reduced progestins: possible role of 3alpha-oxoreduction in progestin-facilitated lordosis

Progestins with a delta-4-3-keto configuration bind to the progestin receptor (PR) and facilitate estrous behavior in estrogen-primed rats. Some ring A-reduced progestins [5alpha-dihydroprogesterone (alphaDHP), allopregnanolone, and epipregnanolone] are more potent estrus-inducing agents than progesterone when iv injected despite their lower affinity for the PR. Yet the estrus-inducing action of such progestins is reduced by the antiprogestin RU486, suggesting that binding to the PR is required for this effect. Because allo- and epi-pregnanolone are oxidized to alpha- and betaDHP, respectively, by 3alpha-hydroxysteroid oxo-reductase (3alphaHSOR), part of their estrus-inducing action may occur through the binding of such DHPs to the PR. Conversely, because 3alphaHSOR reduces alpha- and betaDHP to allo- or epi-pregnanolone, both of which exert membrane effects, the estrus-inducing effect of DHPs may involve actions independent of the PR. To test these possibilities we assessed the effect of indomethacin, a blocker of 3alphaHSOR, on the estrus-inducing action of such progestins. Because indomethacin also inhibits cyclooxygenases, we selected a dose and treatment schedule that does not interfere with prostaglandin-mediated brain processes (e.g., LHRH release). Indomethacin did not significantly modify the effect of progesterone or megestrol acetate on lordosis. Yet, it significantly reduced the action of all ring A-reduced progestins. Results suggest that: (a) oxidation is essential for lordosis facilitation by 3alpha-pregnanolones and (b) reduction of 3-keto progestins generates 3alpha-hydroxy metabolites which synergize with processes triggered by occupation of the PR by 3-keto progestins. The possible participation in this response of other events influenced by indomethacin (e.g., prostaglandin or melatonin synthesis) is discussed.

Progesterone has rapid and membrane effects in the facilitation of female mouse sexual behavior

Ovariectomized (ovx) mice require both estradiol (E2) and progesterone (P) administration to reinstate feminine sexual behavior (lordosis). The importance of P's actions at E2-induced intracellular progestin receptors (PRs) to facilitate lordosis was investigated in PR knockout (PRKO) mice, PRKO's wild type littermates (C57X129), and wild type C57BL/6J (C57) mice. Subjects were ovx, E2-primed (0.5 microg) and tested following intravenous (i. v.) and intercereberal P. Intravenous P (200 microg) significantly increased lordosis of all mice within 10 min of P, but vehicle infusion did not (Experiment 1). Intravenous P significantly increased the amount and duration and reduced the latency of lordosis, over that seen with vehicle infusion, in PRKO and wild type mice. Whole brain concentrations of P and its 5alpha-reduced metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), which has low affinity for intracellular PRs, were also increased following P compared to vehicle infusion. Progesterone, but not vehicle infusions, significantly increased the number of PR-immunoreactive (PR-IR) cells in the ventromedial hypothalamus (VMH) of C57 and C57X129 mice and increased number of 3alpha, 5alpha-THP-immunoreactive (3alpha,5alpha-THP-IR) cells in the ventral tegmental area (VTA) of all mice. In Experiment 2, P conjugated to bovine serum albumin (P:BSA) increased lordosis when applied bilaterally to both the VMH and VTA of E2-primed mice more than BSA implants. Progesterone implants increased the number of PR-IR cells in the VMH of C57 and C57X129 mice and the number of 3alpha,5alpha-THP-IR cells in the VTA of all mice. The rapid facilitation of lordosis with i.v. P infusion and increases in lordosis when P's effects are relegated to the membrane in the VMH and VTA of PRKO and wild type mice suggest that P may facilitate lordosis through actions at substrates other than intracellular PRs. The present findings suggest a role of 3alpha,5alpha-THP.

The neurosteroids, progesterone and 3alpha,5alpha-THP, enhance sexual motivation, receptivity, and proceptivity in female rats

The effects of progesterone (P) and the neurosteroid and P metabolite, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) on ovariectomized (ovx), estradiol-3-benzoate (EB)-primed rats on sexual motivation, receptivity, and proceptivity were examined. Changes in central P and 3alpha,5alpha-THP were measured following administration of EB, EB+P, EB+3alpha,5alpha-THP, or EB+inhibitor of 5alpha-reductase or P metabolism (epostane and finasteride)+P (Expt. 1). Partner preference was measured as the duration of time females in these different hormonal treatments spent in proximity to a male vs. female conspecific (Expt. 2). Receptivity (lordosis quotients and ratings) and proceptivity (darting, hopping, ear wiggling, and pacing), for different hormone treatments were assessed (Expt. 3 and Expt. 4, respectively). Conditioned place preference following hormone treatments and paced mating enabled assessment of sexual motivation (Expt. 5). Central P and 3alpha,5alpha-THP were measured in various combinations of hormone/mating conditions (Expt. 6). Studies revealed that 3alpha,5alpha-THP has a significant role in these reproductive measures. Brain concentrations of 3alpha, 5alpha-THP were significantly higher in animals receiving EB+P or EB+3alpha,5alpha-THP compared to animals receiving EB alone, or EB+P in conjunction with an inhibitor of P metabolism. EB+P and EB+3alpha, 5alpha-THP significantly increased time spent in proximity to the male, receptivity and proceptivity. When administered to ovx, EB-primed rats, the progestin metabolite, 3alpha,5alpha-THP, had effects on these behaviors similar to P. Epostane, an inhibitor of P and 3alpha,5alpha-THP biosynthesis, and finasteride, an inhibitor of P metabolism to 3alpha,5alpha-THP, administered to EB+P animals reduced male partner preference, proceptive, and receptive behaviors to levels seen in EB+vehicle animals. Notably, whole brain 3alpha, 5alpha-THP levels were significantly increased and whole brain P levels were significantly reduced in paced mated rats compared to standard mated, and receptive non-mated animals. These studies suggest that P and 3alpha,5alpha-THP may have some common effects on reproductive behavior, e.g., sexual motivation, receptivity, and proceptivity.

Gonadal steroids and neuronal function

Gonadal steroid hormones may affect, simultaneously, a wide variety of neuronal targets, influencing the way the brain reacts to many external and internal stimuli. Some of the effects of these hormones are permanent, whereas others are short lasting and transitory. The ways gonadal steroids affect brain function are very versatile and encompass intracellular, as well as, membrane receptors. In some cases, these compounds can interact with several neurotransmitter systems and/or transcription factors modulating gene expression. Knowledge about the mechanisms implicated in steroid hormone action will facilitate the understanding of brain sexual dimorphism and how we react to the environment, to drugs, and to certain disease states.

Ring A reductions of progestins are not essential for estrous behavior facilitation in estrogen-primed rats

In Experiment 1 six dose levels (range 0.66-2000 microg) of progesterone (P) and two synthetic progestins with a double bond at C6: megestrol acetate (MA) and chlormadinone acetate (CA), which cannot be reduced at C5, were injected to estrogen-primed (2 microg estradiol benzoate 42 h earlier) ovariectomized (ovx) rats. The three progestins elicited significant lordosis and proceptive behaviors. Potency analysis showed that MA was the most potent progestin for stimulating estrous behavior, followed by P and CA. These results suggest that ring A reduction of progestins to 5alpha/5beta metabolites is not essential for the facilitation of estrous behavior in ovx estrogen-primed rats. Progestins with the 3-ketone group and a double bond at C4 can also be reduced at C3 to yield 3alpha-hydroxysteroid metabolites potentially capable of stimulating estrous behavior. In Experiment 2, the relevance of the formation of 3alpha-hydroxysteroid metabolites for estrous behavior facilitation was tested by concurrently injecting indomethacin (1.5 mg), a blocker of 3alpha-hydroxysteroid oxidoreductase, with 400 microg of P, MA, or CA to ovx estrogen-primed rats. Indomethacin failed to block the stimulatory effect of these progestins on estrous behavior. These results suggest that 3-ketosteroid reduction is also not essential for estrous behavior facilitation by progestins.

Emerging mechanisms of the behavioral effects of steroids

Within two models of steroid-modulated behavior, sodium appetite and sexual receptivity, novel mechanisms of steroid action have emerged. These include interactions between different types of steroid receptors, plasticity of synapses, activation of unliganded steroid receptors, and rapid effects or steroids. These mechanisms highlight the diversity of steroid action in the central nervous system.

Behavioral effects of 3 alpha-androstanediol. II: Hypothalamic and preoptic area actions via a GABAergic mechanism

We investigated whether 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-androstanediol; 3 alpha-Diol), a neurosteroid whose effects are primarily inhibitory to sexual behavior, may act through interactions with gamma-aminobutyric acid (GABA) receptor complexes (GBRs) in the medial basal hypothalamus (MBH) and the preoptic area (POA). In Experiment (Exp.) 1, ovariectomized (ovx) rats were implanted with bilateral guide cannulae aimed above the MBH and were later treated with 17 beta-estradiol (E2, 2 injections of 1 microgram/0.2 ml in 10% ethanol) and either 3 alpha-Diol (3.0 mg/kg, s.c.) or vehicle. Progesterone (0.5 mg, s.c.) was given 24 h after the first E2 injection and a pre-test for lordosis responsiveness was carried out 4 h later. The GABAA agonist, muscimol (50 ng), then was infused into the MBH and rats were tested 10, 30 and 60 min later. Muscimol infusion facilitated lordosis behavior in vehicle-treated controls, but 3 alpha-Diol-treated animals failed to show this facilitation. To ascertain whether 3 alpha-Diol would also prevent muscimol's action in the POA, a site in which muscimol inhibits, rather than facilitates, sexual receptivity, ovx animals in Exp.2 were implanted with bilateral guide cannulae aimed above the POA and were treated with E2, 3 alpha-Diol, and P and infused and tested as in Exp. 1. Muscimol and 3 alpha-Diol each significantly inhibited receptivity; when they were combined, the inhibition was more pronounced. In Exp. 3, POA infusions of the GABAA antagonist, bicuculline, counteracted muscimol's and 3 alpha-Diol's inhibition of sexual behavior. In Exp. 4, in vitro treatment of POA and MBH membrane fractions with 3 alpha-Diol (30 microM) enhanced maximal [3H]muscimol binding without altering the affinity of the binding sites for the agonist. These data suggest that 3 alpha-Diol inhibits E2 and progestin-induced lordosis behavior via actions at the GBR in both the MBH and POA.

Behavioral effects of 3 alpha-androstanediol. I: Modulation of sexual receptivity and promotion of GABA-stimulated chloride flux

Pregnane neurosteroids may initiate sexual receptivity not only via actions at intracellular receptors, but by affecting gamma-aminobutyric acid (GABA) receptor complexes (GBRs). To investigate whether GBR-mediated actions of an androgenic neurosteroid 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-androstanediol; 3 alpha-Diol) may influence the expression of sexual behavior, ovariectomized (ovx) rats received daily injections of 3 alpha-Diol (0.6, 3.0, 6.0 and 7.5 mg/kg) or vehicle (10% (v/v) ethanol in propylene glycol) at 10.00 h, and s.c. injections of estradiol-17 beta (E2: 1 microgram/0.2 ml in 10% ethanol) at 13.00 h and 19.00 h. Progesterone (P: 0.5, 1.0, 2.0 and 4.0 mg/kg) or sesame-oil vehicle was given at 12.30 h on the day following two days of 3 alpha-Diol and E2 treatment. In Expt. 1, levels of sexual receptivity were measured at 18.00-19.00 h, 56-57 h after the first injection of 3 alpha-Diol and 4 h after P or vehicle injection. 3 alpha-Androstanediol (6.0 mg/kg) attenuated sexual behavior (lordosis quotient, lordosis rating) and facilitated aggressive/rejection behaviors following 0.0, 1.0, 2.0 and 4.0 mg/kg P. The highest dosage of 3 alpha-Diol (7.5 mg/kg) facilitated sexual behavior and inhibited aggression behaviors following 0.0, 1.0, 2.0 and 4.0 mg/kg P. In Expt. 2, GABA-stimulated chloride flux was greater in cortical synaptoneurosomes of animals that received hormone treatments associated with inhibited receptivity (E2 + P + 3 alpha-Diol 3.0 mg/kg) than following treatments that facilitated receptivity (E2 + P and E2 + P + 3 alpha-Diol 7.5 mg/kg) or unreceptive ovx animals. In Expt. 3, circulating concentrations of 3 alpha-Diol resulting from the 0.0, 3.0 and 7.5 mg/kg s.c. doses administered to E2- and P-primed animals was measured by radioimmunoassay. Circulating levels of 3 alpha-Diol at the completion of behavioral testing were comparable to those previously ascertained across the estrous cycle. These data indicate that 3 alpha-Diol influences the expression of E2 and P-induced receptivity, and suggest that 3 alpha-Diol, like other neurosteroids, may exert its effects on sexual behavior by actions at GBRs.

Testosterone and progesterone metabolism in the central nervous system: cellular localization and mechanism of control of the enzymes involved

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons and in the glia. 1. The activities of 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone; DHT) and of 3 alpha-hydroxy steroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol; 3 alpha-diol) were first evaluated in primary cultures of neurons, oligodendrocytes, and type-1 and type-2 astrocytes, obtained from the fetal or neonatal rat brain. The formation of DHT and 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or labeled DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, also type-2 astrocytes and oligodendrocytes possess considerable 5 alpha-reductase activity. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes, and oligodendrocytes. Moreover, the results indicate that, in type 1 astrocytes, both 5 alpha-reductase and 3 alpha-HSD are stimulated by coculture with neurons and by the addition of neuron-conditioned medium, suggesting that secretory products released by neurons might intervene in the control of glial cell function. 2. Subsequently it was shown that, similarly to what happens when testosterone is used as the substrate, 5 alpha-reductase, which metabolizes progesterone into 5 alpha-pregnane-3,20-dione, (DHP), shows a significantly higher activity in neurons than in glial cells; however, also type-1 and type-2 astrocytes as well as oligodendrocytes possess some ability to 5 alpha-reduce progesterone. On the contrary, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one (THP), appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lowe than that present in type-1 astrocytes. The existence of isoenzymatic forms of the enzymes involved in androgen and progesterone metabolism is discussed.

Gonadal and adrenal steroids regulate neurochemical and structural plasticity of the hippocampus via cellular mechanisms involving NMDA receptors

1. The hippocampus is an important brain structure for working and spatial memory in animals and humans, and it is also a vulnerable as well as plastic brain structure as far as sensitivity to epilepsy, ischemia, head trauma, stress, and aging. 2. The hippocampus is also a target brain area for the actions of hormones of the steroid/thyroid hormone family, which traditionally have been thought to work by regulating gene expression. "Genomic" actions of steroid hormones involve intracellular receptors, whereas "nongenomic" effects of steroids involve putative cell surface receptors. Although this distinction is valid, it does not go far enough in addressing the variety of mechanisms that steroid hormones use to produce their effects on cells. This is because cell surface receptors may signal changes in gene expression, while genomic actions sometimes affect neuronal excitability, often doing so quite rapidly. 3. Moreover, steroid hormones and neurotransmitters may operate together to produce effects, and sometimes these effects involve collaborations between groups of neurons. For example, a number of steroid actions in the hippocampus involve the coparticipation of excitatory amino acids. These interactions are evident for the regulation of synaptogenesis by estradiol in the CA1 pyramidal neurons of hippocampus and for the induction of dendritic atrophy of CA3 neurons by repeated stress as well as by glucocorticoid injections. In addition, neurogenesis in the adult and developing dentate gyrus is "contained" by adrenal steroids as well as by excitatory amino acids. In each of these three examples, NMDA receptors are involved. 4. These results not only point to a high degree of interdependency between certain neurotransmitters and the actions of steroid hormones, but also emphasize the degree to which structural plasticity is an important aspect of steroid hormone action in the adult as well as developing nervous system.

Analgesic effects of the neurosteroid 3 alpha-androstanediol

The efficacy of 5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-Androstanediol; 3 alpha-Diol) and 4-pregnen-3,20-dione (progesterone; P) in promoting analgesia was investigated. Ovariectomized rats received daily injections of 3 alpha-Diol (0.6, 3.0, 6.0 and 7.5 mg/kg) or vehicle and twice daily injections of estradiol-17 beta (E2: 1 microgram) for 2 days. Progesterone (0.5, 1.0, 2.0 and 4.0 mg/kg) or its vehicle was given on the third day and nociceptive testing using the radiant heat tailflick method was carried out 4 h later. In Expt. 1, P and 3 alpha-Diol both produced analgesia and had biphasic dose-response effects when administered singly. 3 alpha-Diol (3.0 mg/kg) elevated tailflick latencies in E2-primed animals above those following vehicle, 6.0 or 7.5 mg/kg 3 alpha-Diol; 6.0 and 7.5 mg/kg produced elevations that were greater than vehicle but less than 3.0 mg/kg. Progesterone (0.5 and 1.0 mg/kg) also elevated tailflick latencies above vehicle controls, while 2.0 and 4.0 mg/kg produced intermediate effects. In Expt. 2, 3 alpha-Diol (3 alpha-Diol:BSA) and P (P:BSA) conjugated to bovine serum albumin (BSA) were applied to the medial basal hypothalamus (MBH) and preoptic area (POA) to ascertain whether the steroids' analgesic actions were mediated by membrane actions in these sites. Free P and P:BSA both increased tailflick latencies when applied to the MBH, while 3 alpha-Diol and 3 alpha-Diol:BSA elevated latencies when applied to the POA, suggesting the steroids' effects occur in part at the neuronal membrane. In Expt. 3, free P or P:BSA applied to the MBH did not increase tailflick latencies if systemic P was given concurrently. Similarly, free 3 alpha-Diol and 3 alpha-Diol:BSA implants into the POA failed to increase tailflick latencies if s.c. 3 alpha-Diol was co-administered. These data indicate that P and 3 alpha-Diol at moderate doses have analgesic effects in part via membrane actions within the MBH and POA, respectively.

Ring A reduced progestins potently stimulate estrous behavior in rats: paradoxical effect through the progesterone receptor

The effect of ring A reductions at C5 and C3 on the capacity of the progesterone (P) molecule to stimulate estrous behavior was studied in ovariectomized estrogen primed rats (5 micrograms estradiol benzoate, EB, 40 h before progestin administration). Dose-response curves (dose range: 0.75-200 micrograms) for the lordosis quotient (LQ), lordosis score (LS), and proceptivity were constructed for P and all its ring A reduced metabolites: 5 alpha-pregnanedione (alpha DHP), 5 beta-pregnanedione (beta DHP), 3 alpha,5 alpha-pregnanolone (3 alpha,5 alpha-Pgl), 3 alpha,5 beta-pregnanolone (3 alpha,5 beta-Pgl), 3 beta,5 alpha-pregnanolone (3 beta,5 alpha-Pgl), and 3 beta,5 beta-pregnanolone (3 beta,5 beta-Pgl). Progestins were dissolved in propylene glycol and IV injected through an indwelling jugular catheter. Tests for lordosis and proceptivity were made at 5, 30, and 120 min after progestin injection. Weak, though significant lordosis behavior was observed at 5 min following the injection of some of the progestins, particularly the pregnanolones. Maximal responses were obtained at 120 min postinjection for all progestins. Dose response curves of the LQ, LS, and proceptivity were dualistic for alpha DHP and both 3 alpha pregnanolones, smaller responses being observed with high doses. Relative potency analysis revealed that alpha DHP, 3 alpha,5 beta-Pgl, 3 beta,5 alpha-Pgl, and 3 alpha,5 alpha-Pgl were considerably more potent for eliciting lordosis than P (14, 13.7, 9, and 4-fold, respectively). The same order of relative potencies was found for both LS and proceptivity. 3 beta,5 beta-Pgl and beta DHP were only slightly more potent than P (2 and 1.5-fold, respectively). In a second study, the antiprogestin RU486 (5 mg, SC), injected 60 min before one of four selected progestins (alpha DHP, 3 alpha,5 alpha-Pgl, 3 alpha,5 beta-Pgl, and 3 beta,5 beta-Pgl), significantly inhibited their action on estrous behavior (lordosis and proceptivity) when tested at 60 and 120 min postinjection. On the other hand, RU486 failed to inhibit early lordotic responses obtained at 5 and 30 min following 3 alpha,5 alpha-Pgl and 3 alpha,5 beta-Pgl. Similarly RU486 was ineffective in inhibiting lordosis in ovariectomized rats treated only with estradiol (3 micrograms of EB/day for 7 days). Data suggest that: (i) ring A reduction of the P molecule plays an important role in the normal facilitation of estrous behavior in the rat; and (ii) ring A reduced progestins provoke this effect by acting, at least partially, through the progesterone receptor.

Benzodiazepine binding varies with stage of estrous cycle in unwashed membranes from mouse brain

The influence of the stage of the estrous cycle on binding of [3H]diazepam was examined in membranes from brains of female mice. In order to conserve endogenous factors such as progesterone, other steroids, or GABA, the assay was performed without the extensive washing procedures typically employed in measurements of benzodiazepine binding. Significant variations in the apparent maximal numbers of binding sites (Bmax) were noted during the estrous cycle in both hypothalamus and cortex. The Bmax measured in membranes from proestrus female mice was significantly higher than in membranes from mice at other stages in the estrous cycle. Variations in apparent equilibrium binding dissociation constants (Kd) were not statistically significant by stage of the estrous cycle. The demonstrated variations in binding suggest the existence of a factor which varies with the estrous cycle in female mice and modulates the activity of the GABAA receptor complex.

Male preference for the odors of estrous female mice is reduced by the neurosteroid pregnenolone sulfate

The effects of the neurosteroid, pregnenolone sulfate (PS), on the responses of male mice to the odors of estrous female mice were examined in an odor preference test. Control untreated mice displayed a significant preference for the odors of an estrous female, spending more time in a Y-maze in the vicinity of the odors of an estrous than a non-estrous female. Administration of PS decreased male preference for the odors of estrous females, causing a significant dose-related (0.01-10 mg/kg) decrease in the amount of time spent in the proximity of the odors of the estrous female, while having significantly less of an effect on the responses to the non-estrous female odors. Neither pregnenolone nor sodium sulfate had any significant effects on the olfactory responses. The effects of PS were significantly reduced by peripheral administrations of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, but were not significantly affected by either the GABAA antagonists, bicuculline and picrotoxin, or the benzodiazepine antagonist, Ro 15-1788. These results suggest that pregnenolone sulfate has inhibitory effects on olfactory mediated male sexual interest, preference, or 'motivation' that, in part, involve interactions with NMDA receptor mediated mechanisms.

Anti-anxiety effects of progesterone and some of its reduced metabolites: an evaluation using the burying behavior test

In this study, pregnenolone sulfate, progesterone and some of its main reduced metabolites were tested for anxiolytic properties in rats using the burying behavior paradigm. All steroids were subcutaneously injected. Progesterone (0.5, 1.0, 2.0 and 4.0 mg/rat) and its 3 alpha-5 alpha-reduced metabolite (0.125, 0.250, 0.5 and 1.0 mg/rat) produced a clear dose-dependent anxiolytic response, without affecting the spontaneous ambulatory behavior. 5 alpha-Pregnanedione decreased burying behavior only at the highest dose (4.0 mg/rat). Pregnenolone sulfate (1.0, 2.0 and 4.0 mg/rat), 5 beta-pregnanedione and 3 beta- or 5 beta-pregnenolones were devoid of effects. The 3 beta-5 beta-reduced metabolite of progesterone (2.0 and 4.0 mg/rat) decreased motor activity, without altering the burying behavior. These results demonstrate that the 3 alpha-5 alpha metabolite of progesterone shows the highest anxiolytic potency in the burying behavior test, when compared with all steroids evaluated. The data are discussed in terms of the close structure-activity relationship requirements of steroids to stimulate the GABA/benzodiazepine receptor-chloride ionophore complex.

How do sex and stress hormones affect nerve cells?

Gonadal and adrenal hormones regulate both structure and neurochemical function in neurons that express receptors for them. Gonadal hormone effects mediate sexual differentiation of the brain and reproductive tract, and their actions during early development program groups of cells in the nervous system to respond in male- or female-typical ways to hormones in adulthood. Induction of synapse formation is one consequences of brain sexual differentiation, but hormonally directed synaptic plasticity is by no means confined to early development and in fact occurs cyclically during reproductive cycles in a number of brain regions of the female rat, including the hippocampus. The hippocampus responds to adrenal steroids as well and undergoes changes in dendritic branching as a result of repeated stress. Implications of hormonally directed changes in brain structure and neurochemistry are discussed with respect to human pathophysiology.

Male preference for the odors of estrous female mice is enhanced by the neurosteroid 3 alpha-hydroxy-4-pregnen-20-one (3 alpha HP)

The effects of the centrally produced allylic neurosteroid, 3 alpha-hydroxy-4-pregnen-20-one (3 alpha HP), on the responses of male mice to the odors of estrous female mice were examined in an odor preference test. Control untreated mice displayed a significant preference for the odors of an estrous female, spending more time in a Y-maze in the vicinity of the odors of an estrous than a non-estrous female. Intracerebroventricular (i.c.v.) administrations of 3 alpha HP enhanced male preference for the odors of estrous females, causing a significant dose-related (0.01-1.0 microgram) increase in the amount of time spent in the proximity of the odors of the estrous female, while having no significant effect on the responses to the non-estrous female odors. These effects of 3 alpha HP were stereospecific, with the stereoisomer, 3 beta-hydroxy-4-pregnen-20-one (3 beta HP), having no significant effects on odor preferences. The analgesic, morphine, also had no significant effects on the responses to female odors suggesting that the enhanced preference for estrous female odors were unlikely to be directly due to any analgesic effects of 3 alpha HP. The effects of 3 alpha HP were significantly reduced by peripheral administrations of the GABAA antagonists, bicuculline and picrotoxin, but were unaffected by either the benzodiazepine antagonist, Ro 15-1788, or the opiate antagonist, naloxone. These results suggest that the neurosteroid 3 alpha HP has facilitatory effects on olfactory mediated male sexual interest or motivation that involve interactions with the GABAA receptor.

Progesterone 5-alpha-reduction in neuronal and in different types of glial cell cultures: type 1 and 2 astrocytes and oligodendrocytes

Progesterone, like testosterone, can be converted in the brain into 5-alpha-reduced metabolites (5-alpha-pregnan-3,20-dione, DHP; 5-alpha-pregnan-3-alpha-ol-20-one, THP). Recently we have shown that testosterone is 5-alpha-reduced to DHT mainly in neurons, while glial cells possess this enzymatic activity only in limited amounts. On the other hand, a glial cell type (type 1 astrocytes) is almost exclusively responsible for the further metabolism of DHT into 3-alpha-diol. The aim of the present studies was that of evaluating the formation of the 5-alpha-reduced metabolites of progesterone in cultures of neurons, type 1 and 2 astrocytes and oligodendrocytes. The data here presented indicate that, similarly to what happens when testosterone is used as the substrate, the 5-alpha-reductase which metabolizes progesterone shows a significantly higher activity in neurons than in glial cells; however, also type-1 and type-2 astrocytes as well as oligodendrocytes possess some ability to 5-alpha-reduce progesterone. On the contrary, the 3-alpha-hydroxysteroid dehydrogenase (3-alpha-HSD), the enzyme which converts DHP into THP, appears to be mainly present in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained utilizing androgens as precursors, oligodendrocytes show a considerable 3-alpha-HSD activity, even if this is statistically lower than that present in type-1 astrocytes. The existence of isoforms of the enzymes involved in androgen and progesterone metabolism may explain these data.

Progesterone and the neural mechanisms of hamster sexual behavior

Stimulation of both the ventral medial hypothalamus (VMH) and the ventral tegmental area (VTA) by progesterone is necessary to facilitate sexual behavior in female hamsters. Recently obtained evidence indicates that progesterone exerts its behaviorally relevant actions in the VTA by acting on cell membranes. When progesterone conjugated to bovine serum albumin, which cannot permeate the cell membrane, is applied to the VTA concurrent with free progesterone to the VMH, estrogen-primed hamsters become sexually receptive. Since the reverse treatment is ineffective, this suggests that progesterone's nongenomic effects in the VTA may require concurrent genomic activation by progesterone in the VMH. The nongenomic action of progesterone on sexual receptivity may involve the GABAA receptor complex, as progestins are known to modulate this receptor complex. VTA infusions of GABAA agonists enhance, and antagonists inhibit, progesterone's effectiveness on receptivity. Finally, the behavioral effectiveness of progesterone metabolites in the VTA, concurrent with progesterone in the VMH, is consistent with their relative biochemical efficacy at the GABAA complex. These data suggest that progesterone may exert its behavioral effects in the VTA through GABAA. However, it is not yet clear whether progesterone normally acts directly on GABAA in the VTA. Progesterone may also act at some other membrane binding site and GABAA may represent an indirect mechanism for progesterone.

5 alpha-reduced progesterone metabolites are essential in hamster VTA for sexual receptivity

Progesterone's (P) actions in the ventral medial hypothalamus (VMH) and the ventral tegmental area (VTA) are essential for sexual receptivity in hamsters. This study investigates the notion that P works in the VTA in the absence of intracellular P receptors (PRs) by 5 alpha-reduction to progestins, which would subsequently bind to membrane gamma-Aminobutyric Acid receptor complexes (GBRs). To test the importance of P metabolism, a 5 alpha-reductase inhibitor, 17 beta-N, N-diethylcarbamoyl-4-methyl-4aza-5 alpha-androstan-3-one (4MA) was administered SC (0, 3, 9, 15, 24, or 30 mg/kg) to ovx estradiol benzoate(EB)-primed hamsters, followed by one of six doses of SC P (0, 25, 50, 100, 200, or 500 micrograms) and sexual receptivity testing. 200 micrograms P-treated animals administered higher (24 and 30 mg/kg) doses of 4MA had significantly decreased total lordosis durations (TLDs) compared to 0 mg/kg 4MA controls (exp 1). In exp 2, 4MA was aimed bilaterally at the VTA prior to SC P. After 200 micrograms P, animals had significantly lower TLDs than after 500 micrograms P, 3 hours following bilateral VTA implantation of 4MA, but not cholesterol. In exp 3, cycling female hamsters were infused with 1.0 microgram 4MA or vehicle unilaterally into the VTA on diestrus, proestrus, and estrus. 4MA, but not vehicle, infusions into the VTA interrupted receptivity in estrus and proestrus animals, but had no effect on diestrus animals. 4MA's reduction of receptivity when given systemically and intracranially strongly supports the hypothesis that 5 alpha-reduced P metabolites, possibly interacting with GBRs in the VTA, are essential for sexual receptivity in hamsters.

Androgen and progesterone metabolism in the central and peripheral nervous system

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons, in the glia, and in neuroblastoma cells. The activities of the 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone, DHT), and of the 3 alpha-hydroxysteroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol, 3 alpha-diol) have been first evaluated in primary cultures of neurons, oligodendrocytes and type-1 and -2 astrocytes, obtained from the fetal or neonatal rat brain. All the cultures were used on the fifth day. The formation of DHT of 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, type-2 astrocytes and oligodendrocytes also possess considerable 5 alpha-reductase activity, while type-1 astrocytes show a much lower enzymatic concentration. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes and oligodendrocytes. The compartmentalization of two strictly correlated enzymes (5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase) in separate central nervous system (CNS) cell populations suggests the simultaneous participation of neurons and glial cells in the 5 alpha-reductive metabolism of testosterone. Subsequently it has been shown that, similarly to what happens when testosterone is used as the substrate, the 5 alpha-reductase which metabolizes progesterone into 5 alpha-pregnane-3,20-dione (DHP) shows a significantly higher activity in neurons than in glial cells; however, type-1 and -2 astrocytes as well as oligodendrocytes also possess some ability to 5 alpha-reduce progesterone. On the other hand, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one, appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lower than that present in type-1 astrocytes. The existence of isoforms of the enzyme involved in androgen and progesterone metabolism is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)