Histaminergic responses by hypothalamic neurons that regulate lordosis and their modulation by estradiol

How do fluctuations in the level of generalized arousal of the brain affect the performance of specific motivated behaviors, such as sexual behaviors that depend on sexual arousal? A great deal of previous work has provided us with two important starting points in answering this question: (i) that histamine (HA) serves generalized CNS arousal and (ii) that heightened electrical activity of neurons in the ventromedial nucleus of the hypothalamus (VMN) is necessary and sufficient for facilitating the primary female sex behavior in laboratory animals, lordosis behavior. Here we used patch clamp recording technology to analyze HA effects on VMN neuronal activity. The results show that HA acting through H1 receptors (H1R) depolarizes these neurons. Further, acute administration of estradiol, an estrogen necessary for lordosis behavior to occur, heightens this effect. Hyperpolarization, which tends to decrease excitability and enhance inhibition, was not affected by acute estradiol or mediated by H1R but was mediated by other HA receptor subtypes, H2 and H3. Sampling of mRNA from individual VMN neurons showed colocalization of expression of H1 receptor mRNA with estrogen receptor (ER)-α mRNA but also revealed ER colocalization with the other HA receptor subtypes and colocalization of different subtypes with each other. The latter finding provides the molecular basis for complex “push-pull” regulation of VMN neuronal excitability by HA. Thus, in the simplest causal route, HA, acting on VMN neurons through H1R provides a mechanism by which elevated states of generalized CNS arousal can foster a specific estrogen-dependent, aroused behavior, sexual behavior.     

Sex and estrogenic effects on coexpression of mRNAs in single ventromedial hypothalamic neurons

Regulated gene expression in single neurons can be linked to biophysical events and behavior in the case of estrogen-regulated gene expression in neurons in the ventrolateral portion of the ventromedial nucleus (VMN) of the hypothalamus. These cells are essential for lordosis behavior. What genes are coexpressed in neurons that have high levels of mRNAs for estrogen receptors (ERs)? We have been able to isolate and measure certain mRNAs from individual VMN neurons collected from rat hypothalamus. Large numbers of neurons express mRNA for ERalpha, but these neurons are not identical with the population of VMN neurons expressing the likely gene duplication product, ERbeta. An extremely high proportion of neurons expressing either ER also coexpress mRNA for the oxytocin receptor (OTR). This fact matches the known participation of oxytocin binding and signaling in sexual and affiliative behaviors. In view of data that ER and OTR can signal through PKCs, we looked at coexpression of selected PKCs in the same individual neurons. The most discriminating analysis was for triple coexpression of ERs, OTR, and each selected PKC isoform. These patterns of triple coexpression were significantly different for male vs. female VMN neurons. Further, individual neurons expressing ERalpha could distribute their signaling across the various PKC isoforms differently in different cells, whereas the reverse was not true. These findings and this methodology establish the basis for systematic linkage of the brain's hormone-sensitive signaling pathways to biophysical and behavioral mechanisms in a well studied mammalian system.     

Sex differences in serotonergic but not gamma-aminobutyric acidergic (GABA) projections to the rat ventromedial nucleus of the hypothalamus

Hormonal conditions that elicit lordosis in female rats are ineffective in males, suggesting that this behavior is actively suppressed in males. Previous studies theorize that serotonergic and gamma-aminobutyric acidergic (GABA) inputs to the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMNvl) may contribute to lordosis inhibition in males. Using triple-label immunofluorescent techniques, the present studies explored potential sex differences in the density of these projections within three hypothalamic sites: the VMNvl, the arcuate nucleus (ARC), and the dorsomedial nucleus of the hypothalamus. Antibodies directed against HuC/D, estrogen receptor (ER)-alpha and either serotonin (5-HT) or the gamma-aminobutyric acid synthetic enzyme glutamic acid decarboxylase-65 were used to compare the densities of glutamic acid decarboxylase (GAD)-65- and 5-HT-containing fibers in each brain area, the percentage of VMNvl HuC/D immunoreactive (ir) neurons that contained ERalpha, and the percentage of HuC/D and ERalpha double-labeled cells receiving apparent contacts from 5-HT fibers between adult, gonadectomized male and female rats. The densities of VMNvl and ARC 5-HT immunolabeled fibers were significantly higher in the males, and the percentage of VMNvl HuC/D-ir neurons containing ERalpha was significantly higher in the females. The percentage of HuC/D-ir cells contacted by 5-HT fibers was significantly higher in the males, compared with the females, but there was no sex difference in the proportion of those cells receiving contacts that were ERalpha-ir. Neonatal administration of estradiol but not genistein masculinized 5-HT content in the adult female VMNvl, but the percentage of HuC/D-ir cells colabeled with ERalpha was not significantly affected by treatment. A similar, but not statistically significant, pattern was observed in the ARC. These findings suggest that the development of serotonergic inputs to the male VMNvl is orchestrated by neonatal estradiol exposure. The hormone-dependent organization of these 5-HT projection patterns may be an important developmental mechanism accounting for sex-specific behaviors in adulthood.     

GABAergic control of receptivity in the female rat

GABAergic neurotransmission has been implicated in the control of the steroid-dependent behavior, lordosis. GABA has dual effects on lordosis: it facilitates lordosis through actions in the medial hypothalamus (mHYP) and it inhibits lordosis through actions in the preoptic area (POA). In the present study, gonadally intact and ovariectomized female rats were behaviorally tested with a sexually active male. Brains were removed from sexually receptive female either 1 or 24 h after behavioral testing. There was a significant difference in endogenous GABA concentration in HYP and POA between receptive, postreceptive and ovariectomized nonreceptive females. Specifically, GABA levels in postreceptive females were higher in the HYP (20%) and lower in the POA (21%) in comparison to receptive females (p less than 0.05). There was also a significant change in binding parameters of 3H-muscimol in the HYP and POA of receptive females as compared to 24 h postreceptive and ovariectomized rats. Attempts to modulate 3H-GABA release from hypothalamic tissue slices by estrogen or progesterone in ovariectomized rats yielded no effect on this parameter.     

Sex differences in estrogenic regulation of neuronal activity in neonatal cultures of ventromedial nucleus of the hypothalamus

Estrogenic effects have been implicated in sexual differentiation of brain and behavior, in part by affecting neuronal activity in the ventromedial nucleus of the hypothalamus (VMN). We report here a remarkable sex difference in estrogenic regulation of neuronal activity in male vs. female neural networks. Spontaneous synaptic currents originating from a population of neurons were recorded in primary VMN cultures using the whole-cell patch-clamp technique. Treatment with 17beta-estradiol (E2, 10 nM) for 24 h induced opposite effects in the two sexes: the frequency of spontaneous synaptic events decreased significantly in neurons derived from males but increased in those from females. Interestingly, the 24-hour E2 effect was partially reversed by an acute application (5 min) of a second dose of E2 (10 nM), suggesting an interaction between extended (24-hr) and acute (5-min) effects of E2 in VMN neurons. To understand the underlying mechanism of this sexually dimorphic action of E2, we analyzed the E2 effect on GABAergic neurotransmission by recording miniature inhibitory postsynaptic currents. After 24-hour E2 treatment, both the amplitude and frequency of miniature inhibitory postsynaptic currents increased in neurons derived from males but decreased in those from females. These results suggest that E2-induced changes in GABAergic inhibition could at least partially explain E2 effects on neuronal activity. We conclude that E2 may have sexually dimorphic effects on the synaptic output of VMN neurons by modulating GABAergic neurotransmission.     

Transmitter content and afferent connections of estrogen-sensitive progestin receptor-containing neurons in the primate hypothalamus.

Progestin receptor-containing cells in the hypothalamus of the adult female green monkey (Cercopithecus aethiops) were examined by double-label immunocytochemical methods to determine their anatomical location, neurotransmitter content and afferent connections. Animals were ovariectomized and administered either estradiol valerate or the oil injection vehicle, and were sacrificed after 10 days of treatment. Using a monoclonal antibody raised against rabbit uterine progestin receptor (PR), the distribution of PR-immunoreactive cells in the mediobasal hypothalamus and the effect of estrogen treatment on this distribution was determined. PR-immunoreactive cells were found throughout the ventromedial nucleus (VMN), in the area between the VMN and fornix, and in the medial portion of the infundibular nucleus. Estrogen treatment dramatically increased both the number of labeled cells and the intensity of immunoreaction product in these regions. In double-immunostained sections, boutons immunoreactive for antigens indicative of serotonin, pro-opiomelanocortin derived peptides, GABA, catecholamine, neuropeptide Y, substance P, cholecystokinin, and somatostatin were demonstrated to establish synaptic contact with the soma of PR-immunoreactive hypothalamic neurons. In colchicine-pretreated animals, all PR-containing neurons in the mediobasal hypothalamus were found to contain immunoreactivity for glutamic acid decarboxylase, the enzyme required for synthesis of GABA. No evidence of colocalization with other antigens, including LHRH, was observed. Because LHRH neurons are known to receive a rich GABAergic innervation PR-containing GABAergic cells may represent steroid-sensitive sites of integration for inputs from other neural systems involved in the control of gonadotropin secretion.     

Dilute estradiol implants and progestin receptor induction in the ventromedial nucleus of the hypothalamus: correlation with receptive behavior in female rats

The ventromedial nucleus of the hypothalamus (VMN) is thought to constitute an essential neural substrate for hormonal induction of female sexual behavior. In previous work, implants of dilute (0.4%) estradiol (E2) have been found to prime progesterone-facilitated female sexual behavior in rats only when they are within or very close to the VMN. In other studies, induction of cytosolic progestin receptors (PRc) in the hypothalamus has been correlated with systemic E2 priming of receptive behavior. We combined dilute E2 implants with a Palkovits punch microassay for PRc in the an effort to examine the relationship between estrous behavior and estrogen-induced PRc in the VMN of individual female rats. Ovariectomized rats were given bilateral guide cannulae aimed at the VMN and were primed for 3 days with 1) blank implants plus oil sc, 2) blank implants plus 0.5 micrograms E2 benzoate sc, 3) implants of 0.4% E2 in cholesterol, or 4) implants of 2.0% E2. On day 4 all rats received progesterone (P) sc and were tested for receptive behavior. The following week all subjects received the same estrogen-priming treatment but were killed on day 4 without P treatment. The VMN was punch dissected and assayed for PRc. High levels of lordosis were seen in the E2 benzoate-treated and 2.0% E2-implanted animals, while little behavior was seen in the oil-treated animals. Among 0.4% E2-implanted animals, higher levels of lordosis were observed in subjects with implant placements in the rostral than in the caudal VMN; however, lordosis quotients were not correlated with PRc induction measured in the entire VMN of individual animals. In general, our findings suggest that local estrogenic stimulation of target cells in the VMN is capable of both priming the estrous response and induction of P receptors. Whether these two effects are causally related awaits future study.     

Localized actions of progesterone in hypothalamus involve oxytocin.

Two ovarian hormones, estradiol and progesterone, which facilitate mating behavior in the female rat by acting on the ventromedial nuclei (VMN) of the hypothalamus, induce changes in oxytocin receptor binding in this brain region. Estradiol induced a 4-fold increase in the oxytocin receptor binding of the VMN and surrounding area and increased the number and immunostaining of oxytocin fibers in an area lateral to the ventral VMN. Progesterone, in estrogen-primed rats, caused the induced oxytocin receptors to spread over the area containing the oxytocin fibers. Infusion of oxytocin into the ventromedial hypothalamus increased the display of lordosis behavior only in females primed with both estradiol benzoate and progesterone. Thus, the sequential actions of two ovarian hormones bring a neuropeptide and its receptors into register and enable the neuropeptide to exert behavioral effects.     

Estradiol modulation of phenylephrine-induced excitatory responses in ventromedial hypothalamic neurons of female rats

Estrogens act within the ventromedial nucleus of the hypothalamus (VMN) to facilitate lordosis behavior. Estradiol treatment in vivo induces alpha(1b)-adrenoreceptor mRNA and increases the density of alpha(1B)-adrenoreceptor binding in the hypothalamus. Activation of hypothalamic alpha(1)-adrenoceptors also facilitates estrogen-dependent lordosis. To investigate the cellular mechanisms of adrenergic effects on VMN neurons, whole-cell patch-clamp recordings were carried out on hypothalamic slices from control and estradiol-treated female rats. In control slices, bath application of the alpha(1)-agonist phenylephrine (PHE; 10 microM) depolarized 10 of 25 neurons (40%), hyperpolarized three neurons (12%), and had no effect on 12 neurons (48%). The depolarization was associated with decreased membrane conductance, and this current had a reversal potential close to the K(+) equilibrium potential. The alpha(1b)-receptor antagonist chloroethylclonidine (10 microM) blocked the depolarization produced by PHE in all cells. From estradiol-treated rats, significantly more neurons in slices depolarized (71%) and fewer neurons showed no response (17%) to PHE. PHE-induced depolarizations were significantly attenuated with 4-aminopyridine (5 mM) but unaffected by tetraethylammonium chloride (20 mM) or blockers of Na(+) and Ca(2+) channels. These data indicate that alpha(1)-adrenoceptors depolarize VMN neurons by reducing membrane conductance for K(+). Estradiol amplifies alpha(1b)-adrenergic signaling by increasing the proportion of VMN neurons that respond to stimulation of alpha(1b)-adrenergic receptors, which is expected in turn to promote lordosis.     

Electrophysiological actions of oxytocin on hypothalamic neurons in vitro: neuropharmacological characterization and effects of ovarian steroids.

Oxytocin (OT) neurotransmission in the brain has a facilitatory effect on sexual receptivity in rats. This effect of OT is dependent on priming by ovarian steroids, estrogen and progesterone. These steroids modulate OT binding in specific brain nuclei, including the ventrolateral portion of the ventromedial hypothalamic nucleus (vlVMN). In the present study, single-unit activity was recorded from the vlVMN in hypothalamic slices to characterize the electrophysiological actions of OT. To examine the effects of ovarian steroids on OT actions, we used brain slices prepared from ovariectomized rats either treated with estrogen or not, and some slices were treated with progesterone in vitro. OT had little modulatory action on neuronal responses to other agents, but affected the activity of large numbers of vlVMN units. Of those neurons affected, 94% responded with excitation. This predominant stimulatory action of OT is consistent with its lordosis-facilitating effect, because increases in the activity of VMN neurons are generally associated with the facilitation of lordosis. Pharmacological analyses with selective OT agonists and antagonists as well as structurally related peptides showed that the excitatory action of OT is mediated by OT receptors. Estradiol modulated several aspects of OT transmission. First, it increased neuronal responsiveness to OT, especially at the lowest concentration used (0.2 nM). In addition, it caused neuronal responses to OT to correlate significantly with responses to acetylcholine and norepinephrine, which also can act on the ventromedial hypothalamus to facilitate lordosis. Finally, estradiol enhanced the excitability of laterally projecting neurons, which have been implicated in lordosis. In estrogen-pretreated slices, addition of progesterone in vitro caused little further effect on responses of individual neurons to exogenous OT. Altogether, the present electrophysiological findings are consistent with the hypothesis that estrogen potentiates OT action by increasing functional OT receptors preferentially in lordosis-relevant neurons, thereby enabling OT to efficiently facilitate female reproductive behavior.     

Lactation alters gamma-aminobutyric acid neuronal activity in the hypothalamus and cerebral cortex in the rat

Gamma-aminobutyric acid (GABA) neurons terminating in the hypothalamus have been implicated in the neuroendocrine regulation of reproductive hormones, particularly luteinizing hormone (LH) and prolactin. The aim of this study was to examine whether GABAergic neuronal activity in the hypothalamus was modified during lactation, and whether any observed changes correlated with changes in secretion of these hormones. Animals were divided into three experimental groups: diestrous controls, lactating with pups present (with pups), and lactating with pups removed for 4 h (without pups). Animals were decapitated either without treatment, or 60 min after inhibition of GABA degradation by aminooxyacetic acid (AOAA) (100 mg/kg, i.p.). The rate of GABA accumulation in the tissue after AOAA is a measure of GABA turnover. GABA turnover was estimated in 13 microdissected brain regions, and serum prolactin and LH measured by radioimmunoassay. Suckling was associated with significantly increased prolactin and significantly decreased LH compared with diestrous rats. In lactating rats with pups, GABA turnover was significantly increased in the cingulate cortex compared with diestrous rats. GABA turnover was significantly increased in the ventrolateral preoptic nucleus of lactating rats with pups compared with diestrous rats or lactating rats without pups. There was significantly lower GABA turnover in the anterior hypothalamic area, ventromedial and dorsomedial hypothalamic nuclei in lactating rats without pups compared with diestrous rats. There were no significant changes in other brain regions examined. The results demonstrate that activity of GABAergic neurons in specific parts of the hypothalamus and cerebral cortex is altered during lactation.     

Decrease of serotonin and metabolite in the forebrain and facilitation of lordosis by dorsal raphe nucleus lesions in male rats

In castrated male rats, a radiofrequency lesion was made in the dorsal raphe nucleus (DRL) and lordosis behavior was observed following treatment with estrogen. After the behavioral test, brains were removed and the contents of 5-HT and 5-HIAA in the forebrain were measured by high pressure liquid chromatography (HPLC). In the results, only 2 of 16 control males without brain surgery showed lordosis, and the mean lordosis quotient (LQ) was extremely low when compared to that in control females. In contrast, all male rats with DRL displayed lordosis and the mean LQ was higher than that of control males without brain surgery but lower than that in control females (P < 0.001). In the DRL males, 5-HT and 5-HIAA contents in the septum (SPT), the preoptic area (POA), the ventromedial hypothalamus (VMH) and the striatum (STM) were lower than those in control male and female groups (P < 0.001). These results suggest that the dorsal raphe nucleus prevents male rats from showing lordosis by serotonergic influence in the forebrain. In addition, HPLC results showed that levels of the 5-HT in the SPT, the POA and the VMH in the female group were higher than those in the control male group (P < 0.05). In female rats, the POA (P < 0.01) and the VMH (P < 0.05) contained larger 5-HT than those in the SPT and the STM, but there were no difference of 5-HT contents in the male rat.     

Decreased hypothalamic and medullary GABA turnover in spontaneously hypertensive rats.

OBJECTIVE: The aim was to assess whether Gamma-aminobutyric acid (GABA) neurone activities in the central nervous system, especially in the hypothalamus and medulla oblangata, are altered in hypertension. METHODS: Central GABA content and turnover rate were measured in spontaneously hypertensive rats (SHR) and their normotensive Wistar Kyoto controls (WKY). GABA content was determined with high performance liquid chromatography, and in vivo GABA turnover rates were estimated by GABA accumulation after injection of amino-oxyacetic acid, a selective inhibitor of GABA degrading system. Two groups of nine week old male rats (32 SHR and 32 WKY) were used. RESULTS: GABA concentrations in cerebrospinal fluid were lower in SHR than in WKY. Since hypothalamus and medulla oblongata are the possible active sites of this system, basal GABA contents and in vivo GABA turnover rates were measured in hypothalamus and medulla oblongata. Basal GABA content in the medulla oblongata and hypothalamus was almost equal in SHR and WKY. On the other hand, GABA turnover rates were significantly lower in SHR than in WKY in both the hypothalamus and the medulla. CONCLUSIONS: Since it is known that GABA is an inhibitory neurotransmitter in the central nervous system and that it controls autonomic and cardiovascular activities, the findings suggest that the decreased hypothalamic and medullary GABAergic activities may permit sympathetic hyperactivity to contribute to the increase in blood pressure in SHR.     

Effects of age on GABA turnover rates in specific hypothalamic areas in female rats.

During aging in female rats the estrous cycle ceases and the animals develop phases of constant estrous (CE) or constant diestrous (CD) prior to the irreversible transition into anestrous. In young rats, gamma-aminobutyric acid (GABA) is of pivotal importance for the release of GnRH. In the medial-preoptic area (MPO) where the majority of the GnRH perikarya are located in the rat, GABA release decreases at the time of the preovulatory LH surge. The suprachiasmatic nucleus (SCN) contains numerous GABA neurons. Neurochemical signals from this hypothalamic nucleus provide temporal information to GnRH neurons and thereby influence the preovulatory LH surge and the length of estrous cycles. To investigate aging-related changes of the activity of hypothalamic GABAergic neurons, we determined GABA turnover rates in various hypothalamic nuclei of CE and CD rats and compared them to those determined in young estrous (E) or diestrous rats (D1). In old female rats, GABA activity in the MPO was significantly decreased compared to E and D1 rats. A selective increase of GABA turnover rates was observed in the SCN of CE animals. No age-related changes were observed in the other examined brain areas. These data provide the first evidence for alterations in GABAergic activity in specific hypothalamic areas that depend on age and reproductive status. These may cause changes in ability to induce preovulatory LH surges and to maintain regular estrous cyclicity.     

Regulation of pseudosexual behavior in the parthenogenetic whiptail lizard, Cnemidophorus uniparens

Neuroendocrine mechanisms underlying complementary behaviors like male-typical mounting and female-typical receptivity are most often studied independently in males and females, respectively. Cnemidophorus uniparens is a unisexual lizard species consisting only of females that alternately express male- and female-like pseudosexual behavior across the ovarian cycle. Intact, postovulatory (PostOv), and ovariectomized (OVX), androgen-implanted animals [OVX plus testosterone (T)] exhibit male-like mounting, but not receptivity, whereas intact, preovulatory (PreOv), and OVX lizards injected with estradiol [OVX plus estrogen (E)] express receptivity, but not mounting. We tested whether the serotonergic system in the preoptic area (POA) and ventromedial nucleus of the hypothalamus (VMN) gates the reciprocal inhibition characterizing this alternating expression of mounting and receptivity. Serotonergic signaling at the POA appears to be key to gating male-like behavior. Postovulatory and OVX plus T animals have lower intracellular serotonin (5-HT) levels, and greater abundance of inhibitory 5-HT1A receptor mRNA in the POA compared with both PreOv and OVX plus E lizards. Moreover, injecting 5-HT into the POA of OVX plus T animals suppresses mounting, whereas injection into VMN of OVX plus E lizards suppresses receptivity. Although 5-HT levels in the VMN do not differ across the ovarian cycle or between hormonally manipulated animals, PreOv and OVX plus E lizards have a lower abundance of 5-HT2A mRNA in the VMN. Stimulating 5-HT1A receptors using systemic drug administration inhibits mounting, whereas activating 5-HT2A receptors facilitates receptivity. This study illuminates how male- and female-typical sexual behaviors share common neural circuits, and that 5-HT regulates these naturally complementary, and mutually exclusive, behaviors.     

Hormonal induction of lordosis and ear wiggling in rat pups: gender and age differences

To assess how early can estrogens induce female mating behaviors, rat pups 8–29 days old (D8–D29, respectively) were injected twice daily with estradiol benzoate (E) or oil (O) followed by progesterone (P) or oil, and then observed for the estrogen-dependent ear wiggling (EW) and lordosis in response to natural stimulation from male rats. In female pups treated with E + E + P, the incidence of EW appeared as early as D13 and increased gradually to reach maximum at D18, when all pups tested showed EW. EW also occurred in E + E + O females, but never in O + O + P females or in any E + E + P male. Lordosis in E + E + P, as well as E + E + O, female pups occurred later, starting at D15. O + O + P females or E + E + P males never display lordosis. To explore the possibilities that the age and gender differences are due to distribution and/or function of estrogen receptor-α (ERα) or progesterone receptor (PR), separate pups were used for immunocytochemical (ICC) staining of these receptors in the hypothalamic ventromedial nucleus (VMN). There was no age difference in female pups in the density of ERα or the induction of PR between D11/D12, when no sexual behavior was observed, and D19/D20, when almost all pups tested performed the behaviors. There were gender differences: male pups had less ERα than females at D19/D20, though not at D11/D12, and did not respond to E in the induction of PR in the VMN. These results show that ERs and their signaling systems in the VMN of rat pups are functional at least after D11 but only in females, and that the gender differences appeared to be due to differences in the molecular biology of ERα.     

Restraining action of GABA on estradiol-induced LH surge in the rat: GABA activity in brain nuclei and effects of GABA mimetics in the medial preoptic nucleus.

The relationship between GABA dynamics and LH release was studied on day 2 after subcutaneous estrogen implant in short-term ovariectomized rats. GABA accumulation, used as an index of GABA turnover, was determined in the medial preoptic nucleus (MPN), medial (MS) and lateral (LS) septal nuclei, median eminence-mediobasal hypothalamus (MBH) and locus ceruleus (LC). Measurements of GABA were performed at two different times of day (11.00 and 15.00 h), 3 h after intraperitoneal administration of gamma-vinyl-GABA (GVG), an irreversible inhibitor of GABA transaminase. Either morning or afternoon ovariectomized rats (OVX) showed a significant increase in GABA accumulation after GVG treatment in all the areas studied. Estrogen-treated OVX rats showed in the morning a lower GABA accumulation in the MPN, MBH and LC, and GABA levels remained unchanged in the LS and MS. In the afternoon, the MPN and LS showed a lower rate of GABA accumulation whereas in the MBH and LC the GABA increase was not observed. In contrast the MS showed a rate of GABA accumulation similar as in the OVX rats. Local administration in the MPN of 20 micrograms GVG, or GABA-A receptor stimulation by muscimol (50 ng), prior to the increase in plasma LH levels, prevented the occurrence of the estradiol-induced LH surge. The effect of muscimol was reversed by bicuculline (30 ng), a GABA-A receptor antagonist. Bicuculline in low doses lacked effect by itself. In conclusion, these results strongly suggest that a decreased GABAergic activity in MPN, MBH and LC precedes the estradiol-evoked LH surges in ovariectomized rats. Moreover, that in septal nuclei, a low GABAergic activity takes place well before the occurrence of plasma LH increase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of 5alpha-reductase enzyme or GABA(A) receptors in the VMH and the VTA attenuates progesterone-induced sexual behavior in rats and hamsters.

Progesterone (P) to the ventromedial hypothalamus (VMH) and the ventral tegmental area (VTA) of ovariectomized (OVX), estradiol benzoate (EB)-primed rats and hamsters produces female sexual behavior similar to that seen in proestrous, receptive rodents. Because P's 5alpha-reduced metabolites can have facilitative effects on female sexual receptivity through actions at GABA(A)/benzodiazepine receptor complexes (GBRs), the role of 5alpha-reductase and GBRs in the VMH and the VTA was investigated. In Experiment 1, 5alpha-reductase immunoreactivity (5alpha-red-IR) and GBR immunoreactivity (GBR-IR) in the VMH and the VTA of OVX, EB (10 microg) and P (500 microg)-primed rats and hamsters was examined. More 5alpha-red-IR and GBR-IR was seen in the VMH and the VTA of receptive (EB and P-primed) compared to non-receptive (sesame oil vehicle) rodents. In Experiment 2, OVX, EB and P-primed rats and hamsters received implants of finasteride, a 5alpha-reductase inhibitor, or no implants to the VMH and the VTA and were tested for sexual receptivity with a male. Ovariectomized EB and P-primed rats and hamsters receiving finasteride to the VMH and the VTA had decreased lordosis compared to rodents receiving control implants to the VMH and the VTA. In Experiment 3, OVX, EB and P-primed rats and hamsters received infusions of picrotoxin, a GBR antagonist, or vehicle infusion to the VMH and the VTA and were tested for sexual receptivity with a male. Ovariectomized EB and P-primed rats and hamsters receiving picrotoxin to the VMH and the VTA had decreased lordosis compared to rodents receiving vehicle infusions to the VMH and the VTA. These data suggest that 5alpha-reductase and GBRs are present in the VMH and VTA, and that inhibiting 5alpha-reductase activity or blocking GBRs in the VMH and the VTA attenuates EB+P-primed sexual receptivity of OVX rats and hamsters.     

Estradiol modulation of oxytocin binding in the ventromedial hypothalamic nucleus of male and female rats

It has been suggested that estradiol and oxytocin (OT) may interact as neuroendocrine components in the regulation of sexual behavior. In the present study the effect of estradiol benzoate (EB) treatment (50 micrograms/kg body weight/2 days) on [3H]-OT binding was evaluated in adult and 21-day-old gonadectomized male and female rat brains. Coronal sections through the ventro-medial nucleus of the hypothalamus (VMN) were analyzed in three different section planes. EB priming induced an increase in [3H]-OT binding in the VMN of both male and female rats. Greater binding site density and significant EB effects were found in the most caudal plane where the ventrolateral portion of the VMN is well defined at both ages. OT binding in the central amygdaloid nucleus was not affected by this treatment but higher binding levels were found in the most caudal sections irrespective of hormonal status or sex. No sex differences were detected in OT binding in the VMN of basal or EB-treated animals. These results suggest that a dose of EB which activates female sexual behavior in female but not in male rats is able to induce similar levels of OT binding in the VMN of animals of both sexes.     

Environmental-like exposure to low levels of estrogen affects sexual behavior and physiology of female rats

Xenoestrogens are endocrine-disrupting chemicals that mimic the action of endogenous estrogen hormones. Effects of xenoestrogen on aquatic wildlife are well documented, whereas the experimental evidence for impairment of reproductive behavior and physiology in mammals after exposure to xenoestrogens has been debated. The strongest arguments against such studies have been that the route, time course, and intensity of exposure did not simulate environmental exposure and that the chemicals tested have additional nonestrogenic toxic effects, hindering generalization of actual xenoestrogenic effects. Here we show that environmental-like exposure to the pure estrogen 17alpha-ethinylestradiol during development alters reproductive behavior and physiology in adult female Sprague-Dawley rats. We simulated environmental exposure by giving low doses (0.4 and 0.004 microg/kg.d) of 17alpha-ethinylestradiol orally to pregnant females from conception to weaning of the pups, which continued to receive the treatment until puberty. We studied the sexual behavior, estrous cycle, and estradiol plasma levels of intact female rats when they reached 3 months of age. Exposure to the higher dose strongly affected female sexual behavior and physiology, with suppression of lordosis and the estrous cycle and enhanced aggression toward males. The lower dose disrupted appetitive components of sexual behavior that influence the rate of copulation. Estradiol plasma levels were not affected by the treatment. Our study revealed that exposure to low oral doses of a pure estrogen during development alters female sexual behavior and physiology. These results suggest potential risks of reproductive failure from xenoestrogen exposure in realistic ecological conditions.