Gonadal Steroid Hormone Receptors and Social Behaviors

Specialized androgen synthesis in skeletal muscles that actuate elaborate social displays

Androgens mediate the expression of many reproductive behaviors, including the elaborate displays used to navigate courtship and territorial interactions. In some vertebrates, males can produce androgen-dependent sexual behavior even when levels of testosterone (T) is low in the bloodstream. One idea is that select tissues make their own androgens from scratch to support behavioral performance. We first study this phenomenon in the skeletal muscles that actuate elaborate sociosexual displays in downy woodpeckers and two songbirds. We show that the woodpecker display muscle maintains elevated T when the testes are regressed in the non-breeding season. Both the display muscles of woodpeckers, as well as the display muscles in the avian vocal organ (syrinx or SYR) of songbirds, express all transporters and enzymes necessary to convert cholesterol into bioactive androgens locally. In a final analysis, we broaden our study by looking for these same transporters and enzymes in mammalian muscles that operate at different speeds. Using RNA-seq data, we find that the capacity for de novo synthesis is only present in "superfast" extraocular muscle. Together, our results suggest that skeletal muscle specialized to generate extraordinary twitch-times and/or extremely rapid contractile speeds may depend on androgenic hormones produced locally within the muscle itself. Our study therefore uncovers an important new dimension of androgenic regulation of behavior.

The reduction in sexual behavior of adult female rats exposed to immune stress in the neonatal period is associated with reduced hypothalamic progesterone receptor expression

PURPOSE

We examined the mechanism by which neonatal immune stress reduces the sexual behavior of female rats in adulthood.

METHODS

Neonatal female rats were randomly divided into 3 groups: control (n = 11), postnatal day 10 lipopolysaccharide (PND10LPS) (n = 23), and PND25LPS (n = 11) groups, which received intraperitoneal injections of LPS (100 μg/kg) or saline on PND10 and 25. Daily inspections of the vaginal opening (VO) were performed from PND27 to PND37. Thereafter, the frequency of estrus was assessed for 15 days. Female rats (at 11-12 weeks of age) were placed in a cage with male rats, and their sexual behavior was monitored for 30 min. The hypothalamic mRNA expression levels of factors related to sexual behavior were examined via real-time PCR.

RESULTS

VO occurred later and the frequency of estrus was lower in the PND10LPS group compared to the control group. The number of lordosis behaviors and the total number of mounts performed by male partners were lower in the PND10LPS and PND25LPS groups than in the control group. Acceptability: The lordosis quotient and lordosis rating were lower in the PND10LPS group than in the control group. Proceptive behavior: the number of ear wiggling events was lower in the PND10LPS group than in the other groups, and the number of hops/darts was lower in the PND10LPS group than in the control group. The hypothalamic mRNA expression level of progesterone receptors (PR)A + B was lower in the PND10LPS group than in the control group, and the hypothalamic PRB mRNA expression level was lower in the PND10LPS and PND25LPS groups than in the control group.

CONCLUSION

Neonatal immune stress impeded sexual behavior and hypothalamic PR mRNA expression in female rats. Decreased progesterone activity in the hypothalamus might explain the reduction in sexual behavior seen in these rats.

New concepts in the study of the sexual differentiation and activation of reproductive behavior, a personal view

Since the beginning of this century, research methods in neuroendocrinology enjoyed extensive refinements and innovation. These advances allowed collection of huge amounts of new data and the development of new ideas but have not led to this point, with a few exceptions, to the development of new conceptual advances. Conceptual advances that took place largely resulted from the ingenious insights of several investigators. I summarize here some of these new ideas as they relate to the sexual differentiation and activation by sex steroids of reproductive behaviors and I discuss how our research contributed to the general picture. This selective review clearly demonstrates the importance of conceptual changes that have taken place in this field since beginning of the 21st century. The recent technological advances suggest that our understanding of hormones, brain and behavior relationships will continue to improve in a very fundamental manner over the coming years.

Estrogen receptor α and β are involved in the activation of lordosis behavior in estradiol-primed rats

The present study was designed to assess the participation of estrogen receptors alpha (ERα) and beta (ERβ) in the short-term facilitation of lordosis behavior in ovariectomized (ovx), estradiol (E₂) primed rats. In experiment 1, dose response curves for PPT and DPN (ERα and ERβ agonists, respectively) facilitation of lordosis behavior (lordosis quotient and lordosis score) were established by infusing these agonists into the right lateral ventricle (icv) in female rats injected 40h previously with 5μg of E₂ benzoate. PPT doses of 0.08 and 0.4ng produced high lordosis quotients starting at 30min and continuing at 120 and 240min post-injection. DPN induced high levels of lordosis behavior at all times tested. However, the intensity of lordosis induced by both agonists was weak. In experiment 2, we tested the involvement of each ER in facilitation of lordosis by icv infusion of MPP (ERα-selective antagonist) or PHTPP (ERβ-selective antagonist) prior to infusion of 2ng of free E₂. Icv infusion of either MPP or PHTPP 30min before free E2 significantly depressed E₂ facilitation of lordosis. The results suggest that both forms of ER are involved in the short-latency facilitation of lordosis behavior in E₂-primed rats.

Maternal programming of sexual attractivity in female Long Evans rats

In mammals, maternal care influences the developing offspring across multiple domains. In Long Evans rats, for example, the quality of maternal care received as a pup influences later cognitive function, neuroendocrine responses to stress and behavioral measures of emotionality. Data from humans, non-human primates, and rodents also suggest that early life events may similarly perturb measures of sexual reproduction, with possible consequences for reproductive fitness. The current study examined whether or not male conspecifics differentially prefer females, as adult mating partners, that were reared under varying maternal conditions (assessed via the quantity of licking and grooming received; LG). Additionally, the impact of maternal care on adult female sexual motivation and behavior were quantified to determine if these behavioral characteristics are associated with any preference observed. In a mate preference task, male rats chose, almost exclusively, to mount, copulate and ejaculate with female rats reared under Low LG conditions. Under non-paced mating conditions, female Low LG rats display significantly more paracopulatory and copulatory behaviors compared to High LG rats. Due to its critical role in female paracopulatory behavior, progesterone receptor immunoreactivity (PR-ir) in the ventromedial nucleus of the hypothalamus (VMH) was also assessed in both groups of female rats. Estradiol induced PR-ir in the VMH was significantly higher in Low LG relative to High LG rats. Together, these data suggests that early life parental care may developmentally program aspects of behavior and physiology that subsequently influence sexual attractivity and behavior in adult females.

Activation of progestin receptors in female reproductive behavior: Interactions with neurotransmitters

The steroid hormone, progesterone (P), modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and reproductive behavior in female mammals. A wide body of evidence indicates that these neural effects of P are predominantly mediated via their intracellular progestin receptors (PRs) functioning as "ligand-dependent" transcription factors in the steroid-sensitive neurons regulating genes and genomic networks. In addition to P, intracellular PRs can be activated by neurotransmitters, growth factors and cyclic nucleotides in a ligand-independent manner via crosstalk and convergence of pathways. Furthermore, recent studies indicate that rapid signaling events associated with membrane PRs and/or extra-nuclear, cytoplasmic PRs converge with classical PR activated pathways in neuroendocrine regulation of female reproductive behavior. The molecular mechanisms, by which multiple signaling pathways converge on PRs to modulate PR-dependent female reproductive behavior, are discussed in this review.

Progesterone receptor in the forebrain of female gray short-tailed opossums: effects of exposure to male stimuli

Progesterone receptor immunoreactivity (PRir) in brain areas involved in reproductive behavior in eutherian species was examined for the first time in a female marsupial, the gray short-tailed opossum (Monodelphis domestica, hereinafter, opossum). PRir in nuclei of neurons, measured as area covered by stained nuclei, was seen in the arcuate nucleus (Arc); anteroventral periventricular nucleus (AVPv); bed nucleus of the stria terminalis (BST); medial preoptic area (MPOA), and ventromedial hypothalamus (VMH), but not in control areas adjacent to the hypothalamus or cortex. Female opossums are induced into cytological, urogenital sinus (UGS), estrus by male pheromones and into behavioral estrus, i.e., receptivity, by pairing with a male, and both estradiol (E) and progesterone (P) are involved in induction of receptivity in intact and ovariectomized females. PRir in the AVPv, MPOA, and VMH was very low in females that had never been exposed to males or their scent marks, i.e., naïve anestrous (NVA) females, and either previous or current exposure to males or their scent marks was associated with elevated PRir. PRir was significantly higher in the AVPv and MPOA of anestrous females with previous but no current exposure to males and their scent marks, i.e., experienced anestrous (EXPA) females, than in NVA females, but PRir was significantly lower in the MPOA and VMH of EXPA females than in females that were behaviorally receptive and had recently copulated, i.e., behavioral receptive estrous (BRE) females. PRir was higher in the VMH of both UGS estrous (UGSE) and BRE females compared to that in EXPA animals, but PRir did not differ between UGSE and BRE females in any of the 3 brain areas examined, including the MPOA These results provide evidence that pheromonal induction of estrus and sexual receptivity in opossums is associated with elevation of PRir in the VMH and MPOA and that prior exposure to males or their pheromones, even in the absence of current male stimuli, is associated with persistent elevation of PRir in the AVPv and MPOA.

The activation of birdsong by testosterone: multiple sites of action and role of ascending catecholamine projections

Birdsong is a species-typical stereotypic vocalization produced in the context of reproduction and aggression. Among temperate-zone songbirds, it is produced primarily by males, and its frequency and quality are enhanced by the presence of the gonadal steroid hormone testosterone in the plasma. In the brain, the effects of testosterone on song behavior involve both estrogenic and androgenic metabolites of testosterone that are locally produced and act via their cognate receptors. Androgen, and in some cases estrogen, receptors are present in many specialized forebrain song control nuclei. Testosterone can regulate catecholamine steady-state levels and turnover in these song control regions. Tracing studies combined with immunocytochemistry for tyrosine hydroxylase (a marker of catecholamine synthesis) reveal several catecholamine cell groups that project to forebrain song control nuclei. These brain areas also express the mRNA for either androgen receptors or estrogen receptor alpha, and androgens enhance the expression of tyrosine hydroxylase. Dopaminergic cell groups that project to song nuclei express the protein product of the immediate early gene fos in association with the production of territorial song. Thus, testosterone may be acting on song behavior via these ascending catecholamine cell groups. Chemical lesioning studies suggest that noradrenergic projections to the song system are involved in the latency to produce song and the ability to discriminate conspecific from heterospecific song. The song control circuit may thus be modulated in significant ways via the androgen regulation of forebrain catecholamine systems.

Progestin receptors: neuronal integrators of hormonal and environmental stimulation

Although it originally was believed that neuronal steroid hormone receptors require binding to cognate ligand for activation, more recent evidence suggests that the receptors can be activated indirectly by other compounds, such as neurotransmitters and growth factors, acting through their own membrane receptors and specific intracellular signaling pathways. For example, as is the case with facilitation of sexual behavior by progesterone, facilitation of sexual behavior by D(1)/D(5) dopamine receptor agonists is blocked by disruption of progestin receptors. Therefore, some dopamine agonists facilitate sexual behavior at least in part by a progestin receptor-dependent mechanism, as does progesterone. This "ligand-independent activation" of neuronal progestin receptors is not limited to dopamine agonists; a variety of other compounds, as well as mating stimulation, facilitate sexual receptivity by a progestin receptor-dependent process. Steroid hormone receptors also can be regulated by afferent input in another way. Various neurotransmitters upregulate or downregulate steroid hormone receptors in some neurons. This, in turn, presumably confers greater or decreased sensitivity to the particular factors that can activate the particular steroid receptor in those particular neurons. Therefore, steroid hormones are but one class of factors that can regulate and activate steroid hormone receptors. Some additional factors that activate steroid hormone receptors have been identified, as have some factors that can regulate concentrations of receptors. Relatively little is known at this time about the range of neurotransmitters, humoral factors, and intracellular signaling pathways that are involved.

Response of ERalpha-IR and ERbeta-IR cells in the forebrain of female rats to mating stimuli

Sexual behavior in female rats depends on the action of estradiol on estrogen receptors (ERs) found in particular brain regions. While hormonal regulation of female sexual behavior requires ERalpha, the possible functions of ERbeta remain to be clarified. Mating stimulation has several behavioral and physiological consequences and induces Fos expression in many brain areas involved in the regulation of reproductive behavior and physiology. In addition, some cells in which mating induces Fos expression coexpress ERalpha. To determine whether cells in which Fos is induced by a particular mating stimulus coexpress ERalpha, ERbeta, or both, we used a triple-label immunofluorescent technique to visualize ERalpha-, ERbeta-, and mating-induced Fos-immunoreactivity (Fos-ir) in neurons in which mating stimulation reliably increases Fos expression. Ovariectomized, hormone-primed rats were either unmated, received 15 mounts, or received 15 intromissions. In the rostral medial preoptic area, Fos-ir was induced by mounts alone primarily in cells coexpressing ERalpha-ir, while Fos-ir was induced by intromissions mainly in cells coexpressing both ERalpha-ir and ERbeta-ir (ERalpha/ERbeta-ir). In the dorsal part of the posterodorsal medial amygdala, Fos-ir was induced by intromissions in cells coexpressing ERalpha-ir and ERalpha/ERbeta-ir. However, in the ventral part of the posterodorsal medial amygdala, Fos-ir was induced by intromissions primarily in cells coexpressing only ERbeta-ir. These data suggest that qualitatively different sexual stimuli may be integrated through distinct ER-containing circuits in the rostral medial preoptic area and posterodorsal medial amygdala. The diversity in coexpression of type of ER in cells in different brain areas after various mating stimuli suggests a role for both ERalpha and ERbeta in the integration of hormonal information and information related to mating stimuli.

Photoperiodic and temporal influences on chemosensory induction of brain fos expression in female prairie voles

Prairie voles (Microtus ochrogaster) typically stop breeding during winter. Male prairie voles respond to winter day lengths with gonadal regression, whereas female voles are relatively unresponsive to photoperiod. Unlike commonly studied laboratory rodents, female prairie voles do not exhibit spontaneous oestrous cycles. Instead, females are induced into oestrus by chemosensory cues from conspecific male urine. The present study investigated the interaction among day length, chemosensory cues and the initial brain responses during oestrus induction in female voles. A single drop of male conspecific urine, saline or skimmed milk was applied to the nares of female prairie voles housed for 9 weeks in either long (LD 16 : 8 h) or short (LD 8 : 16 h) days. Animals were killed 0.5, 1, 2 or 24 h after chemosensory treatment and their brains were processed for Fos immunocytochemistry. Body mass and ovarian fat pad mass were higher, but uterine and ovarian mass were lower, in short-day compared to long-day females. Regardless of photoperiod, Fos- immunoreactivity increased in the granule layer of the accessory olfactory bulb (AOB), the supraoptic nucleus and bed nucleus of the stria terminalis (BNST) (anterior medial) in females treated with male urine compared to the two control groups. Fos staining intensified in the AOB, medial and posterocortical medial amygdala and BNST (posterior ventral), 1 h and 2 h after urine treatment. In the medial preoptic area, anterior and lateral hypothalamus, and ventromedial nucleus of the hypothalamus, Fos-immunoreactivity was elevated in females 2 h after receiving urine. Overall, long-day females displayed higher Fos expression in response to urine than females maintained in short days. These results identify a putative neural circuitry of oestrus induction in this species, and provide an approximate time line of activation in the brain circuit responsible for oestrus induction in prairie voles.

Mating stimulation required for mating-induced estrous abbreviation in female rats: effects of repeated testing

Mating stimulation, particularly vaginal-cervical stimulation, causes estrous abbreviation in female rats. In most previous studies, female rats were repeatedly tested for sexual behavior until estrous termination occurred. Thus, it was not clear whether sensory stimulation (e.g., flank stimulation, olfactory cues) received during the repeated testing procedure contributed to estrous abbreviation. In Experiment 1, we determined the effect of premating to two or four ejaculations on the rate of estrous termination when a repeated testing procedure was used. We compared ovariectomized, hormone-primed, female rats receiving (1) four ejaculations, (2) two ejaculations, or (3) no premating. Females premated to either two or four ejaculations showed significantly lower levels of sexual receptivity 12 h later than did nonpremated females. These results confirm that premating induces estrous abbreviation when a repeated testing procedure is used. In Experiment 2, we determined whether the repeated testing procedure was necessary for estrous abbreviation. Ovariectomized, hormone-primed female rats were premated to two ejaculations or not premated. The rats were then tested for sexual behavior repeatedly or only once. Females that were premated and repeatedly tested for sexual behavior showed a statistically significant decrease in sexual receptivity compared to females that were not premated; however, the level of sexual receptivity in premated females did not differ from that in non-premated females when they were tested only once. The results suggest that heat duration is the result of a complex interplay between those factors that promote the expression of sexual receptivity and those that inhibit it.

Neuroendocrinology of song behavior and avian brain plasticity: multiple sites of action of sex steroid hormones

Seasonal changes in the brain of songbirds are one of the most dramatic examples of naturally occurring neuroplasticity that have been described in any vertebrate species. In males of temperate-zone songbird species, the volumes of several telencephalic nuclei that control song behavior are significantly larger in the spring than in the fall. These increases in volume are correlated with high rates of singing and high concentrations of testosterone in the plasma. Several song nuclei express either androgen receptors or estrogen receptors, therefore it is possible that testosterone acting via estrogenic or androgenic metabolites regulates song behavior by seasonally modulating the morphology of these song control nuclei. However, the causal links among these variables have not been established. Dissociations among high concentrations of testosterone, enlarged song nuclei, and high rates of singing behavior have been observed. Singing behavior itself can promote cellular changes associated with increases in the volume of the song control nuclei. Also, testosterone may stimulate song behavior by acting in brain regions outside of the song control system such as in the preoptic area or in catecholamine cell groups in the brainstem. Thus testosterone effects on neuroplasticity in the song system may be indirect in that behavioral activity stimulated by testosterone acting in sites that promote male sexual behavior could in turn promote morphological changes in the song system.

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.

Ligand-independent activation of progestin receptors in sexual receptivity

Ovarian steroid hormones, estradiol and progesterone, regulate cellular functions in the central nervous system, resulting in the alterations in physiology and reproductive behavior. One means by which steroid hormones exert their neural effects on reproductive behavior is via their intracellular receptors functioning as ligand-dependent transcription factors. Studies from our laboratory in the past few years have shown that in addition to their cognate ligands, neurotransmitters like dopamine can activate intracellular steroid receptors in a ligand-independent manner. Using biochemical and molecular approaches we have demonstrated that the effects of neurotransmitter dopamine, on reproductive behavior in female rats and mice, occur by means of cross talk between membrane receptors for dopamine and intracellular progestin receptors (PRs). In this article, our studies on the integration of intracellular signaling pathways leading to the activation of PRs and its impact on modulation of reproductive behavior are summarized.

Progesterone receptors as neuroendocrine integrators

Intracellular progesterone receptors (PRs) are ligand-inducible transcription factors that mediate the majority of the effects of progesterone (P) on neuroendocrine functions. During the past decade, evidence has accumulated which suggest that PRs can also be activated independently of P, by signals propagated through membrane-bound receptors to the interior of cells. The activation of PRs by this type of "cross-talk" mechanism has been implicated in the physiological regulation of several important neuroendocrine processes, including estrous behavior and periovulatory hormone secretions. We review evidence that both ligand-dependent and ligand-independent activation of PRs occurs in central neurons and in anterior pituitary cells and that the convergence and summation of these signals at the PR serves to integrate neural and endocrine signals which direct several critically important neuroendocrine processes. An integrative function for PRs is reviewed in several physiological contexts, including the display of lordosis behavior in female rodents, the neurosecretion of gonadotropin-releasing hormone surges, secretion of preovulatory gonadotropin surges, and release of periovulatory follicle stimulating hormone surges. The weight of evidence indicates that cross talk at the intracellular PR is an essential component of the integrative mechanisms that direct each of these neuroendocrine events. The recurrence of PR's integrative actions in several different physiological contexts suggests that other intracellular steroid receptors similarly function as integrators of neural and endocrine signals in other neuroendocrine processes.

Progesterone receptor and dopamine receptors are required in 9-tetrahydrocannabinol modulation of sexual receptivity in female rats

Ovarian steroids, estrogen and progesterone, influence the sensitivity of certain neural processes to cannabinoid treatment by modulation of brain dopaminergic activity. We examined the effects of the active ingredient of cannabis, Delta(9)-tetrahydrocannabinol (THC), on sexual behavior in female rats and its influence on steroid hormone receptors and neurotransmitters in the facilitation of sexual receptivity. Our results revealed that the facilitatory effect of THC was inhibited by antagonists to both progesterone and dopamine D(1) receptors. To test further the idea that progesterone receptors (PR) and/or dopamine receptors (D(1)R) in the hypothalamus are required for THC-facilitated sexual behavior in rodents, antisense and sense oligonucleotides to PR and D(1)R were administered intracerebroventricularly (ICV) into the third cerebral ventricle of ovariectomized, estradiol benzoate-primed rats. Progesterone- and THC-facilitated sexual behavior was inhibited in animals treated with antisense oligonucleotides to PR or to D(1)R. Antagonists to cannabinoid receptor-1 subtype (CB(1)), but not to cannabinoid receptor-2 subtype (CB(2)) inhibited progesterone- and dopamine-facilitated sexual receptivity in female rats. Our studies indicate that THC acts on the CB(1) cannabinoid receptor to initiate a signal transduction response that requires both membrane dopamine and intracellular progesterone receptors for effective induction of sexual behavior.

Estrogen supplement prevents the calcium hypersensitivity of cardiac myofilaments in ovariectomized rats

Our previous biochemical and mechanical studies have demonstrated an increase in Ca2+ sensitivity of cardiac myofilaments in ovariectomized rats. To test whether the body weight gain associated with ovariectomy contributed some effects to the changes in myofibrillar functions, the relations of pCa (-log Ca2+ molar concentration) to actomyosin adenosine triphosphatase (ATPase) activity of isolated myofibrillar preparations from 10-week pair-fed ovariectomized rats were compared with those from sham-operated controls. Despite similar body weights, the maximum myofibrillar ATPase activity was significantly lower in pair-fed ovariectomized rats as compared to that of sham-operated controls. In addition, the pCa-actomyosin ATPase relationship of pair-fed ovariectomized hearts still demonstrated a significant leftward shift in pCa50 (-log half-maximally Ca2+ activation) from that of sham-operated controls. To find out which hormone was responsible for the observed increase in myofibrillar Ca2+ sensitivity, different sex hormone supplemental regimens were administered to ovariectomized rats. Subcutaneous injection of estrogen (5 microg/rat) or estrogen plus progesterone (1 mg/rat) three times a week could effectively prevent the changes in body weight, heart weight, and uterine weight of the ovariectomized animals. Moreover, supplements of either estrogen or progesterone could prevent a decrease in maximum ATPase activity. In contrast, only the estrogen replacement could abolish the Ca2+ hypersensitivity of the myofilaments in these ovariectomized rats. These results suggest differential cardio-regulatory effects of ovarian sex hormones on the Ca2+ activation of the myofilaments.

Estrogen binding and estrogen receptor characterization (ERalpha and ERbeta) in the cholinergic neurons of the rat basal forebrain

Estrogen is thought to enhance cognitive functions by modulating the production of acetylcholine in basal forebrain neurons; a system that projects to the cerebral cortex and hippocampus and plays a central role in learning and memory. To elucidate the mechanism of estrogen action in the cholinergic system, we utilized a combined in vivo autoradiography/immunocytochemistry technique to evaluate the distribution of estrogen binding sites in cholinergic neurons of the rat basal forebrain. The results of these studies revealed that a portion of the cholinergic neurons in the medial septum (41%), vertical (32%) and horizontal (29%) limbs of the diagonal band and in the substantia innominata/nucleus basalis (4%) contained estrogen receptors. Through the use of a double-label in situ hybridization/immunocytochemistry technique we have shown that estrogen receptor-alpha is the predominant estrogen receptor in the cholinergic neurons, with only a few cells containing estrogen receptor-beta. The results of these studies provide evidence that biologically active estrogen receptors are present in the basal forebrain cholinergic neurons of the adult rat brain, with estrogen receptor-alpha being the predominant receptor subtype. The demonstration that cholinergic neurons contain estrogen receptors is consistent with the possibility that estrogen directly modulates the activity of cholinergic neurons in rats and may provide insight as to how estrogen improves cognitive functions in women.

Progesterone, but not progesterone-independent activation of progestin receptors by a mating stimulus, rapidly decreases progestin receptor immunoreactivity in female rat brain

Recent studies suggest that progestin receptors may be activated in vivo by neurotransmitters in the absence of ligand. More specifically, vaginal-cervical stimulation (VCS) can influence sexual behavior by activating progestin receptors in the absence of progesterone. Another way to test if progestin receptors are influenced by particular stimuli is to examine progestin receptor immunostaining. We report that progestin receptor immunoreactivity is decreased in the forebrain of estradiol-primed ovariectomized (OVX) rats within 1 h after a subcutaneous injection of progesterone, a time by which rapid down-regulation of progestin receptors does not seem to have occurred. In estradiol-primed OVX rats, VCS also decreased progestin receptor immunoreactivity within 1 h in the medial preoptic area, but not in any other area examined. To determine if the decrease in immunoreactivity by VCS was due to adrenal secretions or by ligand-independent activation of progestin receptors, we repeated the experiment in estradiol-primed OVX/adrenalectomized rats. Prior removal of the adrenal glands blocked the rapid decrease in progestin receptor immunoreactivity, even though data from other experiments suggest that progestin receptors are activated by VCS at this time. These studies suggest the possibility that progestin receptors may be affected differentially by progesterone-dependent or by progesterone-independent pathways. This raises the possibility that activation of progestin receptors by these two distinct pathways may lead to different neuronal consequences.

Genomic and membrane actions of progesterone: implications for reproductive physiology and behavior

Progesterone, produced by the ovaries and adrenal glands, regulates reproductive behavior and the surge of luteinizing hormone which precedes ovulation by acting on neurons located in different parts of the hypothalamus. The study of the activation of these reproductive functions in female rats has allowed to explore the different mechanisms of progesterone action in the brain. It has allowed to demonstrate that new actions of the hormone, which have been observed in particular in vitro systems, are also operational in vivo, and may thus be biologically relevant. This mainly concerns the direct actions of progesterone on receptors of neurotransmitters such as oxytocin and GABA. Activation of the progesterone receptor in the absence of ligand by phosphorylation may also play a role.

Ovarian steroid effects on serotonin 1A, 2A and 2C receptor mRNA in macaque hypothalamus

This study mapped the location of serotonin (5HT) 1A, 2A and 2C receptor mRNA expression in the female macaque hypothalamus and determined whether the expression was regulated by estrogen plus or minus progesterone treatment using in situ hybridization (ISH) and densitometric analysis of autoradiographic films. The experimental groups of pigtail macaques (Macaca nemestrina) were spayed controls (n=4), estrogen treated (28 days, n=4) and estrogen+progesterone-treated animals (14 days estrogen+14 days estrogen and progesterone, n=4). Monkey specific 5HT1A (432 bp), 2A (411 bp) and 2C (294 bp) receptor probes were generated with PCR. Moderate 5HT1A receptor hybridization signal was detected in the preoptic area and the ventromedial nuclei. Less intense 5HT1A receptor signal was detected in a contiguous area from the dorsomedial nuclei through the posterior hypothalamus and in the supramammillary area. There was no change in 5HT1A receptor hybridization signal in any area with ovarian steroid treatment. Dense 5HT2A receptor hybridization signal was morphologically confined to the paraventricular, supraoptic, and mammillary nuclei and the external capsule of the thalamus. Light 5HT2A mRNA signal was inconsistently observed in the ventromedial nuclei. There was no change in the 5HT2A receptor hybridization signal in any area with ovarian steroid treatment. The 5HT2C receptor mRNA was widely distributed in the macaque hypothalamus. The preoptic area and anterior hypothalamus were largely positive for 5HT2C mRNA with a more concentrated signal in a narrow periventricular area. Dense 5HT2C receptor signal was detected lateral to the ventromedial nuclei (capsule), in the tuberomammillary nuclei, arcuate nucleus, dorsomedial nuclei, infundibular area and choroid plexus. Moderate 5HT2C receptor signal was detected in the ventromedial nuclei, lateral hypothalamus and dorsal to posterior hypothalamus. There was a significant decrease in total 5HT2C mRNA hybridization signal with ovarian steroid treatment in the ventromedial nuclei, dorsal and posterior hypothalamus. In summary, macaque 5HT1A, 2A and 2C receptor mRNAs are located in distinct hypothalamic loci which play a role in a number of autonomic functions and behavior. Ovarian steroids decreased the expression of 5HT2C receptor mRNA in the ventromedial nuclei, dorsal and posterior hypothalamus. The expression of 5HT1A and 5HT2A receptor mRNA was not altered by treatment with ovarian steroids.

Effects of cold exposure on estrous behavior and neural estrogen receptor in Syrian hamsters

Two experiments examined the effects of 72-h exposure to reduced environmental temperature (5 degrees C) on steroid-induced estrous behavior and neural estrogen-receptor immunoreactivity (ERIR) in ovariectomized Syrian hamsters. Cold exposure significantly inhibited sexual receptivity induced by sequential injections of estradiol benzoate (2.5 microg) and progesterone (500 microg), but only if the animals were not permitted to overeat (limited to 110% of ad lib intake at 22 degrees C). The suppression of sexual receptivity was accompanied by decreases in the number of detectable ERIR cells in the ventromedial hypothalamus (VMH) and by increases in the number of ERIR cells in the medial preoptic area (mPOA). The cold-induced decreases in estrous behavior and in VMH ERIR cells were prevented by lesions of the area postrema (AP), but AP lesions did not prevent the increases in mPOA ERIR cells. Thus, cold exposure mimics the effects of treatment with metabolic inhibitors, experimental diabetes, food deprivation, and insulin-induced fattening on these endpoints. These findings are consistent with the hypothesis that dwelling in the cold affects reproduction indirectly via its actions on metabolic fuel availability, rather than by acting directly on neuroendocrine processes.

Intraventricular injection of neuropeptide Y antisera curbs weight gain and feeding, and increases the display of sexual behaviors in obese Zucker female rats

Obese Zucker rats are hyperphagic, overweight, and infertile. It has been postulated that neuropeptide Y (NPY) overproduction may contribute to obesity and infertility in these animals. To test this hypothesis, ovariectomized, adult obese Zucker rats were implanted with cannulae in the third ventricle and subsequently injected with NPY antisera or normal rabbit sera (NRS) 6, 4 and 2 h before experimental observation. Steroid-treated females injected with NPY antisera were significantly more receptive and were more likely to show proceptive behaviors than after treatment with NRS (e.g., lordosis quotient: NPY antisera, 65.5+/-6.9%; NRS, 30.9+/-11.6%, P < 0.02; 91% displaying proceptivity after NPY antisera injection vs. 36% after NRS, P < 0.03). Injection of NPY antisera also curbed food intake and weight gain (24 h food intake: NPY antisera, 10.5+/-2.1 g; NRS, 20.5+/-1.7 g, P < 0.01; 24 h weight gain: NPY antisera, -5.4+/-2.2 g; NRS, 5.8+/-0.7 g, P < 0.01). Locomotor activity was similar after NRS and NPY antisera treatment (P > 0.5) suggesting that general malaise was not responsible for the effects of NPY antisera on food intake or body weight. These data suggest that endogenous neuropeptide Y contributes to excessive feeding and weight gain, and suppressed reproductive behaviors in obese Zucker female rats.

Effect of photoperiod on neural estrogen and progestin receptor immunoreactivity in female Syrian hamsters

This study explored the possibility that reduced behavioral responsiveness to estradiol and progesterone in female Syrian hamsters exposed to a short photoperiod is associated with a reduction in the concentration of neural steroid receptors. The effects of long and short photoperiod (LP; SP) exposure on steroid receptor immunoreactivity were examined in the ventromedial hypothalamus (VMH), medial tuberal region (mTu), medial preoptic area (mPOA), medial nucleus of the amygdala (mAMYG), and the arcuate nucleus (ARC) of ovariectomized hamsters. In Experiment 1, exposure to SP for ten weeks attenuated the lordosis response following sequential treatment with estradiol and progesterone. In a separate group of animals not given hormones, SP decreased the staining intensity of estrogen receptor immunoreactive (ERIR) cells in the mPOA while increasing the number of detectable ERIR cells in part of the mAMYG. In Experiment 2, SP diminished the lordosis response as it did in Experiment 1. One week later, the same females were administered estradiol systemically to induce progestin receptors (PR). Animals housed in SP showed significantly reduced progestin receptor immunoreactivity (PRIR) in the VMH, mTu, mPOA, mAMYG, and ARC. Experiment 3 examined whether the results of Experiment 2 might have been influenced by photoperiodic effects on peripheral metabolism of estradiol. Among hamsters housed in LP or SP, PRs were induced by estradiol implanted unilaterally in the medial basal hypothalamus, thus bypassing possible photoperiodic effects on peripheral estradiol availability. This treatment resulted in significantly fewer cells with detectable PRIR in the VMH and mPOA of SP females, suggesting that the photoperiodic influences on PR induction observed in Experiment 2 do not depend on alterations in the peripheral availability of estradiol.

Lordosis-enhancing medial preoptic area lesions do not alter hypothalamic estrogen receptor- or progestin receptor-immunoreactivity in prepubertal female guinea pigs

Female guinea pigs rarely display adult-typical lordosis responses to ovarian steroid hormones until 40-50 days of age. Behavioral hyporesponsiveness in prepubertal females may be due, in part, to deficiencies in hypothalamic estrogen receptors and/or estradiol-induced progestin receptors. This study was designed to test the hypothesis that bilateral medial preoptic area (MPOA) lesions, which enhance the display of progesterone-facilitated lordosis in juvenile females, increase levels of hypothalamic estrogen receptors and/or estradiol-induced progestin receptors. Hartley guinea pigs were ovariectomized at 11-12 days of age and at 14-15 days of age received bilateral electrolytic or sham lesions aimed at the MPOA. At approximately 3 weeks of age, lesioned and sham-lesioned animals were either tested for the display of progesterone-facilitated lordosis or perfused, and their hypothalamic tissue processed for estrogen receptor- or estradiol-induced progestin receptor-immunostaining. Although a significantly higher percentage of MPOA-lesioned than sham-lesioned guinea pigs displayed progesterone-facilitated lordosis (85.7% vs. 5. 8%, respectively, p<0.05), there were no significant lesion-related differences in the number or staining intensity of cells containing estrogen receptor- or estradiol-induced progestin receptor-immunoreactivity in the ventrolateral hypothalamus or arcuate nucleus. These data do not support the hypothesis that the enhanced display of progesterone-facilitated lordosis in prepubertal guinea pigs following MPOA lesions is due to increased hypothalamic concentrations of estrogen receptors or estradiol-induced progestin receptors.

Insulin-induced repartitioning of metabolic fuels inhibits hamster estrous behavior: role of area postrema

Excessive diversion of metabolic fuels away from oxidation and into adipose tissue storage depots, such as underfeeding or extraordinary levels of energy expenditure, can induce nutritional infertility. Treatment with pharmacological doses of insulin reduces metabolic fuel availability and suppresses both ovulatory cyclicity and pulsatile luteinizing hormone release in females of several mammalian species, but little attention has been paid to the effects of insulin treatments on reproductive behaviors. Ovariectomized Syrian hamsters were injected with long-acting insulin every 12 h for 72 h and were prevented from overeating by limiting their intake to approximately 110% of pretreatment levels. When given estradiol and progesterone, insulin-treated hamsters exhibited significantly reduced levels of sexual receptivity compared with saline-treated controls. This insulin-induced inhibition of estrous behavior was prevented by lesions of the area postrema. Insulin treatments also caused changes in the number of detectable estrogen receptor immunoreactive cells in the hypothalamus and preoptic area. Therefore, insulin-induced repartitioning of metabolic fuels induces changes in estrous behavior and neural estrogen receptors that are indistinguishable from those caused by food deprivation, cold exposure, or treatment with metabolic inhibitors.

Sexual behavior of Flinders Line female rats bred for differential cholinergic sensitivities

Flinders Lines are two strains of rats selectively bred for their divergent physiological responses to cholinergic drug challenges. Flinders Sensitive Line (FSL) rats are highly sensitive to cholinergic stimulation of various autonomic and behavioral responses compared to Flinders Resistant Line (FRL) rats. Because cholinergic innervation contributes to the regulation of female sexual behaviors in rats, a study was conducted to compare the sexual responses of FSL females to those of FRL females, as well as to those of Long-Evans (LE) females, a conventional rat strain. Ovariectomized FSL rats exhibited significantly higher incidences of lordosis and proceptive behaviors than ovariectomized FRL and LE rats over a range of estrogen doses (2, 3, 4, 5, or 20 microgram(s)/kg estradiol benzoate at 48 h before testing) administered in combination with progesterone (1 or 2 mg/kg at 4-6 h before testing). In addition, the muscarinic antagonist scopolamine inhibited lordosis behavior strongly in FRL females over a range of doses (0.25, 0.5, 1, 2, or 4 mg/kg), but failed to inhibit lordosis in FSL females. Results indicate that FSL females are highly sensitive to the behavioral effects of gonadal steroids and highly insensitive to the effects of a muscarinic antagonist. The enhanced sexual behavioral responses of FSL females could be a consequence of their well-established cholinergic hypersensitivity or a consequence of other undocumented characteristics of FSL females such as hypersensitivity to ovarian hormones. FSL females could provide a valuable model for the study of estrogen action at behavioral, cellular, and molecular levels.

Differential expression of progestin receptor isoforms in the hypothalamus, pituitary, and endometrium of rhesus macaques

The progestin receptor exists in at least two isoforms: a long form (PR-B) and a short form (PR-A), which can be separated and detected with Western blot analysis. It has been suggested from in vitro transfection experiments that differential expression of the two isoforms may provide one mechanism for tissue specific actions of progesterone (P). However, more information from in vivo experimentation is needed. It has been reported that P down-regulates the expression of PR in the endometrium and pituitary of E primed macaques. However, PR protein and PR messenger RNA expression in the hypothalamus is maintained with P treatment of E-primed macaques. Thus, there is tissue-specific regulation of PR by its cognate ligand in the nonhuman primate. To gain insight into the tissue-specific regulation of PR by P, we questioned whether differential expression of the isoforms of PR exists in the endometrium, pituitary, and hypothalamus of rhesus monkeys. The expression of PR-A and PR-B was examined after E (28-30 days) and E + P (14 days E + 14 days E + P) treatment in the primate endometrium, pituitary, and hypothalamus. After E or E + P treatment, the levels of PR-A were 5 times higher than PR-B in the endometrium. PR-A was 1.6-fold higher than PR-B in the pituitary. In the hypothalamus, the ratio of A to B ranged from less than 1 (B exceeds A) to unity (A and B equimolar). There was no difference in the ratio of A to B between E-treated and E + P-treated groups in any tissue examined. These observations (a) provide further support of the hypothesis that differential expression of the isoforms of PR may subserve the tissue specific actions of P and (b) also suggest that P does not differentially affect the expression of the isoforms of its cognate receptor in the endometrium, pituitary, or hypothalamus.

Estrogen receptor function as revealed by knockout studies: neuroendocrine and behavioral aspects

Estrogens are an important class of steroid hormones, involved in the development of brain, skeletal, and soft tissues. These hormones influence adult behaviors, endocrine state, and a host of other physiological functions. Given the recent cloning of a second estrogen receptor (ER) cDNA (the ER beta), work on alternate spliced forms of ER alpha, and the potential for membrane estrogen receptors, an animal with a null background for ER alpha function is invaluable for distinguishing biological responses of estrogens working via the ER alpha protein and those working via another ER protein. Data generated to date, and reviewed here, indicate that there are profound ramifications of the ER alpha disruption on behavior and neuroendocrine function. First, data on plasma levels of estradiol (E2), testosterone (T), and luteinizing hormone (LH) in wild-type (WT) versus ER alpha- mice confirm that ER alpha is essential in females for normal regulation of the hypothalamic-pituitary gonadal axis. Second, ovariectomized female ER alpha- mice do not display sexual receptivity when treated with a hormonal regime of estrogen and progesterone that induces receptivity in WT littermates. Finally, male sexual behaviors are disrupted in ER alpha- animals. Given decades of data on these topics our findings may seem self-evident. However, these data represent the most direct test currently possible of the specific role of the ER alpha protein on behavior and neuroendocrinology. The ER alpha- mouse can be used to ascertain the specific functions of ER alpha, to suggest functions for the other estrogen receptors, and to study indirect effects of ER alpha on behavior via actions on other receptors, neurotransmitters, and neuropeptides.

Progesterone treatment increases Fos-immunoreactivity within some progestin receptor-containing neurons in localized regions of female rat forebrain

In female rats, the sequential release of estradiol and progesterone from the ovaries is required for the expression of sexual behavior during the estrous cycle. Many of the neuronal effects of estradiol and progesterone involve estrogen and progestin receptors. Treatment with a behaviorally-effective dose of estradiol increases Fos expression, suggestive of neuronal response, and subsequent treatment with a behaviorally-effective dose of progesterone further increases Fos expression within a few hours in female rat brain. In order to determine if neurons that respond to progesterone with increase Fos expression also contain progestin receptors, we used a double-label immunofluorescent technique to label both progestin receptors and Fos protein following progesterone or vehicle treatment of estradiol-primed female rats. As shown previously, progesterone treatment increased Fos expression in progestin receptor-containing regions, such as the ventromedial nucleus of the hypothalamus and the medial preoptic area. In addition, progesterone treatment induced a statistically-significant increase in Fos-immunoreactivity within progestin receptor-containing cells in the medial preoptic area and the ventromedial nucleus of the hypothalamus, but not in the arcuate nucleus. Therefore, many but not all of the neurons that respond to progesterone with increased Fos expression also contain progestin receptor-immunoreactivity. The progesterone-induced Fos expression within progestin receptor-containing neurons may or may not be associated with the effects of progesterone on sexual or other reproductive behaviors, as it remains to be tested. However, the Fos expression provides a useful marker to aid in identification of neurons that respond to a behaviorally-relevant dose of progesterone.

Co-expression of steroid hormone receptors in opioid peptide-containing neurons correlates with patterns of gene expression during the estrous cycle

The anteroventral periventricular nucleus (AVPV) of the preoptic region represents an essential component of neural pathways regulating gonadotropin secretion, and contains sexually dimorphic populations of neurons that express dynorphin or enkephalin. In the present study we used in situ hybridization to measure prodynorphin (PDYN) and proenkephalin (PENK) mRNA in the AVPV of intact animals killed on each day of the cycle. Levels of PDYN mRNA were lowest in animals killed on the afternoon of proestrus and then increased by over 60% by the morning of the following day. Expression of PENK mRNA was generally stable during the cycle, but a small yet significant reduction was detected on proestrus relative to levels of PENK mRNA in animals killed on the day of diestrus. In addition, we used double in situ hybridization to demonstrate that the majority of PDYN mRNA-containing neurons express both estrogen (50%) and progesterone receptor (85%) mRNAs. Only one quarter of the PENK-containing neurons also co-express estrogen receptor mRNA, and fewer than 10% of the PENK mRNA neurons express PR mRNA. Thus, the differential expression of PDYN and PENK during the cycle generally correlates with distinct differences in the degree of colocalization of ER and PR mRNA in PDYN and PENK mRNA-containing neurons in the AVPV.

Food deprivation and the facilitatory effects of estrogen in female hamsters: the LH surge and locomotor activity

Two experiments investigated short-term food deprivation effects on neuroendocrine processes influenced by estrogen. These studies were prompted by prior work indicating that food deprivation increased the number of immunocytochemically identified cells containing estradiol receptors in the medial preoptic area of ovariectomized female hamsters. Presumably, this is one way that changes in metabolic fuel availability might alter the responsiveness of one or more systems to estradiol. The purpose of this study was to investigate two effects of estradiol that might be affected by food deprivation; these were 1) the positive feedback effects of estradiol on the luteinizing hormone (LH) surge, and 2) the facilitating effects of estradiol on locomotor activity. In Experiment 1, ovariectomized hamsters were administered estradiol, before or after 48 h of food deprivation. Two days after hormone treatment, blood was obtained by cardiac puncture, once in the morning (1100 h) and twice during the afternoon (1600-1800 h). These times were chosen to best characterize the magnitude of the LH surge. Food deprivation enhanced the amplitude of the LH surge in response to estradiol when this treatment preceded, but not when it followed, the administration of estradiol. However, there was variability in the dose of estradiol at which this effect of food deprivation occurred. In Experiment 2, the locomotor (running wheel) activity of two groups of gonadally intact female hamsters was quantified; one group was tested during the early (days 1 + 2; low estradiol) part of the estrous cycle, and the other group was tested during the late (days 3 + 4; high estradiol part of the estrous cycle. In both groups, testing was performed first under ad lib feeding conditions and again during 48 h of food deprivation. On average, the days 3 + 4 group was more active than the days 1 + 2 group, reflecting their differing levels of endogenous estradiol. Food deprivation significantly increased locomotor activity, independently of the stage of the estrous cycle during which it was imposed. These results are discussed in terms of the influence that altered estradiol receptor expression in the medial preoptic area might play in generating the effects we observed following short-term food deprivation.

Importance of target innervation in recovery from axotomy-induced loss of androgen receptor in rat perineal motoneurons

In adult male rats, axotomy of the spinal nucleus of the bulbocavernosus (SNB) motoneurons transiently down-regulates androgen receptor (AR) immunoreactivity. The present study investigates the importance of target reinnervation in the recovery of AR expression in axotomized SNB motoneurons after short (up to 5 days) and long (1 to 6 weeks) periods of recovery. In the long-term recovery experiment, animals were divided into two groups. In one, the two stumps of the cut pudendal nerve, which carries the axons of the SNB motoneurons, were sutured together immediately after axotomy. In the second group, the proximal stump was ligated immediately after axotomy to prevent target reinnervation. Axotomy of the SNB motoneurons caused a significant down-regulation in AR immunoreactivity within 3 days. At 6 weeks, AR immunoreactivity was still depressed in ligated animals but had recovered to control levels in resutured animals. The recovery in the resutured group was coincident with the first signs of reinnervation of the target perineal muscles, although reinnervation seemed to lag behind AR immunoreactivity. SNB soma size was significantly reduced 2 weeks after axotomy and returned to control levels after 6 weeks of recovery only in the resutured animals. These findings suggest that the target perineal muscles play a role in the regulation of AR expression and androgen sensitivity in the SNB motoneurons, perhaps mediated by muscle-derived trophic factors.

Estrogen receptor immunoreactivity in ferret brain is regulated by estradiol in a region-specific manner

The effects of estrogen on estrogen receptor (ER) immunoreactivity in the male ferret brain were examined. Estrogen treatment reduced the mean number of ER-immunopositive (ER+) cells/unit area in periventricular preoptic area but increased the mean number of ER+ cells/unit area in the medial division of the ventromedial hypothalamic nucleus, while having no effect on the number of ER+ cells/unit area in the lateral VMH and arcuate nucleus. Thus, estrogen regulates brain ER immunoreactivity in male ferrets and the direction and magnitude of this regulation are brain region-specific.

Convergent pathways for steroid hormone- and neurotransmitter-induced rat sexual behavior

Estrogen and progesterone modulate gene expression in rodents by activation of intracellular receptors in the hypothalamus, which regulate neuronal networks that control female sexual behavior. However, the neurotransmitter dopamine has been shown to activate certain steroid receptors in a ligand-independent manner. A dopamine receptor stimulant and a D1 receptor agonist, but not a D2 receptor agonist, mimicked the effects of progesterone in facilitating sexual behavior in female rats. The facilitory effect of the neurotransmitter was blocked by progesterone receptor antagonists, a D1 receptor antagonist, or antisense oligonucleotides to the progesterone receptor. The results suggest that in rodents neurotransmitters may regulate in vivo gene expression and behavior by means of cross-talk with steroid receptors in the brain.

Estrogen-receptor immunoreactivity in hamster brain: preoptic area, hypothalamus and amygdala

The distribution of estrogen-receptor containing cells in the preoptic area, hypothalamus and amygdala of female Syrian hamster brain was studied by immunocytochemical methods. Dense populations of estrogen-receptor immunoreactive (ER-IR) cells were found in the medial preoptic area, the bed nucleus of the stria terminalis, amygdala, ventral and lateral parts of the hypothalamus, and the arcuate nucleus. Injection of estradiol caused a decrease in estrogen-receptor immunoreactivity (ERIR) containing cells within one hour, a decrease that may reflect a change in the ability of the occupied estrogen receptor to bind the particular antibody (H222) used rather than down-regulation of the estrogen receptor. Our findings on the distribution of estrogen-receptor containing cells in these areas using an immunocytochemical technique are consistent with and extend the findings of others using autoradiographic and in vitro binding techniques to study estrogen receptor distribution in hamster brain.

Genetic differences in progestin sensitivity in CD-1 and SW female mice

The lordotic behavior of ovariectomized CD1 and SW female mice was tested in response to estradiol benzoate(EB)+progesterone (P) or EB + 5 alpha-dihydroprogesterone (DHP) to determine (i) the role of progestin metabolism in the regulation of sexually receptive behavior, and (ii) strain differences in sensitivity to these progestins. A greater proportion of SW females resisted mounting attempts than CD-1 females, regardless of the progestin administered. However, the differences in mounting were not correlated with the expression of lordotic behavior. A greater proportion of SW females exhibited sexually receptive behavior when given EB+P in comparison to the CD-1 females. There were no genotypic differences in lordotic behavior in response to EB+DHP. The potential mechanisms mediating genotype-based differences in responsiveness are discussed, and the relationship of this report to previously described strain differences in progestin sensitivity are considered.

Immunocytochemical characterization of afferents to estrogen receptor-containing neurons in the medial preoptic area of the rat

Double-label immunocytochemistry has been employed to elucidate the chemical nature of the afferent neuronal projections to the estrogen receptor-containing neurons located in the medial preoptic area of the rat brain. To ensure a clear separation of the immunolabelled afferent profiles from the estrogen receptors, the former were visualized first and the diaminobenzidine reaction product was silver-gold intensified. Using a monoclonal antibody raised against purified human estrogen receptors, we observed an intense nuclear immunoreactivity in Vibratome, semithin and ultrathin sections. Neuropeptide-Y, serotonin-, phenylethanolamine N-methyltransferase- and adrenocorticotrophin-immunoreactive axons and varicosities were observed in close apposition to the estrogen receptor-positive cells. At the ultrastructural level, neuropeptide-Y-immunoreactive boutons were seen in synaptic contact with cells showing estrogen receptor immunoreactivity in their nucleus. These results indicate that neurons located in the medial preoptic area, one of the principal sites for the control of female reproductive function, may be influenced by both estrogen and neurotransmitters/neuropeptides via, respectively, nuclear receptors and synaptic contacts.

Photoperiod modulates the effects of steroids on sociosexual behaviors of hamsters

Female hamsters (Mesocricetus auratus) exposed to a short photoperiod for 8 weeks were ovariectomized, as were hamsters that were maintained in a long photoperiod. We then tested the effect of photoperiod on the ability of progesterone (P) to reduce aggression, as well as on the effects of estrogen (E), alone and with P, on the display of lordosis. Before hormone treatments, photoperiod did not affect the display of aggressive behaviors. When treated with steroids, short-day females were more aggressive than long-day housed animals, especially when tested using male intruders. Also, exposure to short days reduced the activational effects of E and E+P upon lordosis behavior. These findings confirm that photoperiod can modulate the behavioral effects of E and E+P in female hamsters. Furthermore, the results indicate that behavioral effects of P which do not depend upon E priming are also influenced by photoperiod.

Ontogeny of cells containing estrogen receptor-like immunoreactivity in the Brazilian opossum brain

In this study, we have used the Brazilian short-tailed opossum (Monodelphis domestica) as a model to study the ontogeny of estrogen receptors in the mammalian brain. Monodelphis is a small, pouchless marsupial which breeds well under laboratory conditions and whose young are born in an immature sexually undifferentiated state. The Abbott H222 monoclonal rat estrogen receptor antibody (gift of Abbott Laboratories) was utilized in an indirect immunohistochemical procedure to detect estrogen receptors in developing opossum brains. Estrogen receptors were first expressed in the dorsomedial and ventromedial hypothalamus of the opossum 10 days after birth (10PN). Most regions that contained estrogen receptor-like immunoreactivity (ER LI) in the adult opossum contained ER LI at 15 PN. These areas include the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, periventricular preoptic area and hypothalamus, amygdala, dorsomedial and ventromedial hypothalamic nuclei, arcuate nucleus, ventral premammillary nucleus, and the midbrain central grey. The number of cells that contain ER LI increased through 60PN in all regions that will contain ER LI in the adult opossum. These results indicate that estrogen receptors are present in early development of the Monodelphis brain and may mark the beginning of a critical period for sexual differentiation of the opossum brain.

Estrogen receptor binding in regions of the rat hypothalamus and preoptic area after inhibition of dopamine-beta-hydroxylase

Previous studies have shown that administration of diethyldithiocarbamate (DDC), a dopamine-beta-hydroxylase inhibitor, results in a decreased concentration of estrogen receptors measured in the rodent hypothalamus and preoptic area. To determine if this modulation of receptor content is region-specific, in vitro estrogen binding assays were performed on cytosol and cell nuclear extracts of microdissected brain regions from female rats treated with DDC. For cytosol binding comparisons, ovariectomized (OVX) rats were treated with 550 mg DDC/kg b. wt. or the saline vehicle 12 h before sacrifice. For cell nuclear binding comparisons, OVX rats received a maximal dose of estradiol 12 h after DDC or saline treatment and 1 h before sacrifice. No region-specific decreases in estrogen binding were observed in either cytosol or nuclear extracts. To further examine possible regional specificity, quantitative autoradiographic analysis of the in vivo hypothalamic uptake of an iodinated analog of estradiol, 11 beta-methoxy-16 alpha-[125I]iodoestradiol (MIE2), in OVX rats treated with DDC was conducted. Animals received a saturating dose of [125I]MIE2 12 h after DDC or saline treatment and 1 h before sacrifice. DDC treatment resulted in higher background levels of radioactivity and a trend toward higher uptake levels in all brain regions, but with no evidence of marked regional specific effects in any area of the brain. In tissue uptake studies, DDC treatment resulted in higher levels of radioactivity recovered from serum and neural tissues of [125I]MIE2-injected rats, suggesting that DDC slows the clearance of MIE2.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of testosterone on a selected neuronal population within the preoptic sexually dimorphic nucleus of the Japanese quail

The effects of testosterone on the volume and cytoarchitecture of the sexually dimorphic nucleus of the preoptic area (POM) were investigated in male and female Japanese quail. It was confirmed that castration decreases the POM volume in males and that, in gonadectomized birds of both sexes, testosterone increases this volume to values similar to those observed in intact sexually mature males. This suggests that the sex difference in POM volume results from a differential activation by T so that this brain morphological characteristic is not truly differentiated in the organizational sense. This conclusion was extended here by demonstrating that males exposed to a photoperiod simulating long days and that are known to have high plasma levels of testosterone have a larger POM than short-day males that have inactive testes. Detailed morphometric studies of POM neurons revealed a structural heterogeneity within the nucleus. A population of large neurons (cross-sectional area larger than 70-80 microns2) was well represented in the dorsolateral but was almost absent in the medial part of POM. This lateral population of neurons was sensitive to variations of testosterone levels in males but not in females. The cross-sectional area, diameter, and perimeter of the dorsolateral neurons were significantly increased in males exposed to high testosterone levels (intact birds exposed to long days or castrated birds treated with the steroid). These changes were not observed in the medial part of the nucleus. Interestingly, the size of the dorsolateral neurons was not affected by testosterone treatments in females. These results suggest that the swelling of neurons in the lateral POM of males might be responsible for the increase in total volume of the nucleus, which is observed in physiological situations associated with a high testosteronemia. In addition, the sensitivity to testosterone of the dorsolateral neurons in the POM appears to be sexually differentiated. This differential response to testosterone might represent a truly dimorphic feature in the organizational sense and additional studies manipulating the early steroid environment should be performed to test this possibility.

Effects of the nonsteroidal inhibitor R76713 on testosterone-induced sexual behavior in the Japanese quail (Coturnix coturnix japonica)

A new triazole derivative, R76713 (6-[4-chlorophenyl)(1H-1,2,4-triazol-1-yl)methyl]-1-methyl-1H- benzotriazole), was recently shown to inhibit aromatase selectively without affecting other steroid-metabolizing enzymes and without interacting with estrogen, progestin, or androgen receptors. This compound was tested for its capacity to intefere with the induction of copulatory behavior by testosterone (T) in castrated Japanese quail (Coturnix coturnix japonica). In a first experiment, R76713 inhibited (range 0.01 to 1 mg/kg) the activation of sexual behavior by T silastic implants and hypothalamic aromatase activity in castrated male quail in a dose-dependent manner. The 5 alpha- and 5 beta- reductases of T were not systematically affected. Stereotaxic implantation of R76713 in the medial preoptic area similarly blocked the behavior activated by systemic treatment with T, demonstrating that central aromatization of androgen is implicated in the activation of behavior. These inhibiting effects of R76713 on behavior were observed when implants were placed in the medial part of the nucleus preopticus medialis, confirming the implication of this brain area in the control of male copulatory behavior. Finally, the behavioral inhibition produced by R76713 could be reversed by simultaneous treatment with a dose of estradiol, which was not behaviorally effective by itself. This suggests that the behavioral deficit induced by the inhibitor was specifically due to the suppression of estrogen production. This also shows that the activation of copulatory behavior probably results from the interaction of androgens and estrogens at the brain level, as the two treatments separately providing these hormonal stimuli (T with the aromatase inhibitor on one hand and a low dose of estradiol on the other hand) had almost no behavioral effects but they synergized to activate copulation when given concurrently. These data confirm the critical role of preoptic aromatase in the activation of reproductive behavior and demonstrate that R76713 is a useful tool for the in vivo study of estrogen-dependent processes.

Down-regulation of progestin receptors in guinea pig brain: new findings using an immunocytochemical technique

Progesterone injection in estradiol-primed, ovariectomized guinea pigs results in down-regulation of hypothalamic progestin receptors determined by in vitro binding assays. In order to determine if progesterone also decreases immunostaining of progestin receptors and if progestin receptors are down-regulated preferentially in particular neuroanatomical areas, ovariectomized guinea pigs were injected with doses of estradiol benzoate (10 micrograms at 42 h before progesterone injection) and progesterone (500 micrograms at 4, 12, or 24 h before perfusion) that reliably induce the expression of lordosis and subsequent behavioral refractoriness to progesterone. Progestin receptor-immunoreactive cells were counted in sections from discrete parts of the preoptic area and hypothalamus. As expected, estradiol dramatically increased cell nuclear, and, to a lesser extent, cytoplasmic, immunostaining in defined regions of the preoptic area and hypothalamus. By 12 h after progesterone injection, the number of progestin receptor-immunoreactive cells had decreased in some areas, but not others. The rostral and caudal aspects of the ventrolateral hypothalamus were particularly responsive showing a substantial decrease in progestin receptor-immunoreactivity by 12 h after injection. No decreases in the progestin receptor-immunoreactive cell number were observed in any of the preoptic regions examined, although obvious decreases in immunostaining intensity were seen. The results of these immunocytochemical experiments extend earlier findings from in vitro progestin binding experiments and demonstrate that as with progestin binding, progestin receptor-immunoreactivity decreases when progesterone is injected in a behavioral desensitization procedure. Furthermore, they point to the ventrolateral hypothalamus as one site in which the down-regulation of progestin receptors may be particularly responsive to progesterone.

Many progestin receptor-containing neurons in the guinea pig ventrolateral hypothalamus contain substance P: immunocytochemical evidence

Behavioral and neuroanatomical evidence in rats suggests that substance P may mediate hormonally induced sexual receptivity. To determine if progestin receptor-containing neurons in the guinea pig hypothalamus also contained substance P, we used a double antibody, immunocytochemical technique. Ovariectomized, adult guinea pigs were estradiol-primed for one week. Two days prior to perfusion, colchicine was infused into the lateral ventricle. Substance P-immunoreactive neurons and progestin receptor-immunoreactive neurons were found throughout the hypothalamus and caudal preoptic area. However, substance P and progestin receptor immunoreactivity occurred in the same neurons almost exclusively in the ventrolateral area of the hypothalamus, an important site in the hormonal regulation of female sexual behavior in guinea pigs. Within this area, approximately 35% of the progestin receptor-immunoreactive neurons also contained substance P immunoreactivity. These neuroanatomical data are consistent with the hypothesis that substance P may mediate some of the neural actions of progesterone.

Prazosin treatment does not affect progestin receptor induction in microdissected regions of the rat hypothalamus

Sex differences in estrogen-induced progestin receptors have been described in specific regions of the rat brain. To determine if alpha 1-neurotransmission plays a role in the expression of the sex differences in progestin receptor induction, the effects of the alpha 1-antagonist, prazosin, on progestin binding in microdissected regions of the rat brain was determined. Adrenalectomized/gonadectomized male or female rats were administered various doses of estradiol benzoate (EB) in combination with prazosin. With all treatment paradigms, and in both sexes, no significant effect of prazosin treatment on progestin receptor levels was observed. These results are consistent with the idea that sex differences in the estrogen-induction of progestin receptors in the rat hypothalamus are not due to sex differences in the alpha 1-adrenergic regulation of progestin receptor synthesis.

Long-term ovariectomy and hormone-induced sexual behavior, progestin receptors, and hypothalamic morphology in female rats

Long-term ovariectomy reduces the ability of estradiol and progesterone treatment to induce sexual receptivity in female rats. Previous researchers suggested that this effect may be due to a decreased induction of neural progestin receptors by estradiol in the long-term ovariectomized rats. The present study was designed to replicate and extend this finding, and to search for neuroanatomical correlates by measuring the volume of the ventromedial nucleus (VMN) of the hypothalamus, a putative site of action of estradiol and progesterone for the induction of female sexual behavior. Long-term ovariectomy (5 to 6 weeks) as compared to short-term ovariectomy (1 week) reduced the ability of estradiol-17 beta and progesterone treatment to induce sexually receptive and proceptive behaviors. Consistent with previous reports, our data show that the reduced levels of cytosol progestin receptors after long-term ovariectomy and estradiol treatment are related to a reduced ability of estradiol to induce the receptors. Long-term ovariectomy did not affect the concentration of cytosol progestin receptors in the preoptic area, suggesting a neuroanatomical specificity to this effect. Contrary to our predictions, long-term ovariectomy did not affect the volume of the VMN. In fact, estradiol treatment, while blocking the effect of long-term ovariectomy on sexual behavior, decreased the volume of the VMN. Therefore, the measurement of the volume of the VMN is not a good predictor of the responsiveness to steroid hormone induction of sexual behavior.