Progesterone versus estrogen facilitation of female sexual behavior by intracranial administration to female rats

The CNS sites of action for progesterone facilitation of female sexual behavior are disputed. Among the areas most often cited are the ventromedial hypothalamus and the ventral midbrain. There is also a controversy about whether estradiol may substitute for progesterone for the facilitation of receptive behavior when given systemically or intracranially. We tested VMH and ventral midbrain applications of estradiol versus progesterone for the facilitation of female sexual behavior in estrogen-primed, ovariectomized female rats. Subjects were implanted with bilateral guide tubes aimed at ventral hypothalamic or midbrain sites. Estrogen-primed rats received either 28-gauge insert cannulae filled at the lumen with pure progesterone, estradiol, or cholesterol, or empty tubes, and were tested for receptivity with intact, experienced stud males just before, and 1 and 4 hr after, intracranial hormone administration. Significant estrous responsiveness was seen only in the 4-hr test after progesterone was implanted in the VMN in the first intracranial cannula test. We conclude, in contrast to some previous reports, that administration of progesterone to the VMN is more effective in the facilitation of female sexual behavior than when it is implanted in the ventral midbrain, and that administration of estradiol to either site is ineffective.     

Facilitation of sexual receptivity in hamsters by simultaneous progesterone implants into the VMH and ventral mesencephalon

Intracranial implantation experiments have shown that the ventromedial hypothalamus (VMH) is the most sensitive site for the facilitation of female sexual behavior by progesterone in estrogen-primed rats. However, similar implantation techniques have been much less successful in hamsters. Several lines of evidence indicate that both hypothalamic and midbrain structures are important for hamster lordosis. Therefore we compared the effect of progesterone (P) implants administered simultaneously to VMH and ventral midbrain on opposite sides of the brain to the effects of bilateral implants to each of these sites separately. Ovariectomized female hamsters were stereotaxically implanted with 24-gauge thin-wall guide tubes according to one of five patterns. Bilaterally symmetrical cannulae were aimed at VMH or ventral mesencephalon (vMES) or asymmetrical implants were aimed at one of the following pairs of sites, on opposite sides of the brain: VMH-vMES, VMH-preoptic area (VMH-POA), or anterior hypothalamus-anterior mesencephalon (AH-aMES). After recovery from surgery, females were primed with 10 micrograms estradiol benzoate and given pellets of P or cholesterol through a 30-gauge injector in the targeted sites. Latency, frequency, and duration of lordosis were recorded in 10-min tests with sexually active male hamsters. Sexual receptivity was significantly facilitated by simultaneous contralateral P implants into the VMH-vMES. P implants in any other combination of sites did not significantly facilitate lordosis compared to cholesterol control implants, nor did bilateral administration of this dose of P in either VMH or vMES have a reliable effect. The results support the hypothesis that P action is required in both VMH and vMES to reliably stimulate receptivity in hamsters.     

Role of postnatal androgens in sexual differentiation of the lordosis-inhibiting effect of central injections of cholecystokinin

The neuropeptide cholecystokinin (CCK) inhibits lordosis behavior when infused into the ventromedial nucleus of the hypothalamus (VMN) of female rats and has no effect when infused into the VMN of male rats. To test whether this sex difference develops under the control of perinatal steroids, male rats were castrated or given sham surgeries within 3 h of birth and female rats were injected with either 0 or 100 micrograms testosterone propionate on postnatal day 5. As adults, these rats were castrated as necessary, implanted with unilateral cannulae directed at the VMN, and tested for their ability to display female sexual behavior and to respond to CCK. Neonatal castration of males prevented defeminization of this response. When treated with 5 micrograms estradiol benzoate (EB), neonatally castrated males showed both lordosis behavior and a profound inhibition of that behavior after infusions of CCK. Neonatally castrated males did not display lordosis behavior when treated with 2 micrograms EB. Control males showed no lordosis behavior and, therefore, no response to CCK. Both doses of EB induced lordosis behavior in neonatally androgenized females. Significantly, these neonatally androgenized females were less responsive to CCK's inhibition of lordosis and were also anovulatory. These results imply that androgens alter the development of CCK responsive circuits as well as defeminize cyclic gonadotropin release. Levels of 125I-sCCK-8 binding in the VMN were correlated closely with an individual's ability to respond to sCCK-8. In summary, the inhibition of female sexual behavior caused by exogenously administered CCK in normal adult female rats appears to be controlled at least partially by levels of CCK receptors in the VMN and to differentiate under the control of perinatally present testosterone.     

The ventromedial nucleus of the hypothalamus and the hormonal arousal of sexual behaviors in the female rat

Bilateral lesions of the ventromedial nucleus of the hypothalamus interfered with the estrogenic induction of sexual receptivity in the female rat, but seemingly did not affect the ability of female rats to show lordosis following combined stimulation with estrogen and progesterone. In addition, ventromedial hypothalamic lesions did not affect the ability of females to show male-like sexual activity in response to exogenous androgenic stimulation.     

The role of neurosteroids and nongenomic effects of progestins in the ventral tegmental area in mediating sexual receptivity of rodents

Progesterone (P(4)) in the ventromedial hypothalamus (VMH) and ventral tegmental (VTA) is important for facilitation of lordosis; however, P(4)'s actions in these brain areas are different. Using lordosis in rodents as in vivo experimental models, we have examined the effects progestins exert in the midbrain and hypothalamus. Localization and blocker studies indicate that P(4)'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 alter lordosis, which suggests that progestins may interact with GBRs to facilitate receptivity by enhancing the function of GABAergic neurons. Interfering with P(4)'s metabolism to 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha,5 alpha-THP), the most effective endogenous positive modulator of GBRs, or the biosynthesis of the neurosteroid 3 alpha,5 alpha-THP in the VTA attenuates female sexual behavior in rodents. Stimulation of mitochondrial benzodiazepine receptors (MBRs), which enhance neurosteroid production, by infusions of a MBR agonist to the VTA enhances lordosis. 3 alpha,5 alpha-THP is increased in the midbrain of mated > proestrous > diestrous rodents. These data suggest that 3 alpha,5 alpha-THP has a proximate modulatory role on lordosis. In summary, the actions of P(4) in the VTA are different from those in the VMH that involve PRs. In the VTA, P(4) may facilitate lordosis following metabolism to and/or biosynthesis of 3 alpha,5 alpha-THP, which may have subsequent actions at GBRs and/or MBRs to acutely modulate female sexual behavior in rodents.     

Selective brain stem transections affecting reproductive behavior of female rats: The role of hypothalamic output to the midbrain

In estrogen-treated, ovariectomized rats, selective transections were used to interrupt, together or separately, the medial and lateral pathways by which efferent fibers from the ventromedial nucleus (VMN) of the hypothalamus reach the lower brain stem. Dorsal hemisections placed to interrupt both projections reduced or eliminated lordotic behavior. Transections placed to intercept all of the medially descending fibers, but spare the lateral pathway, did not reduce lordosis in mating or manual stimulation tests. The lateral pathway was interrupted at two different locations. Parasagittal transections at the level of the VMN showed that the lateral pathway, as a whole, was not required for lordosis when the medial pathway was left intact. Also, no particular subset of fibers assuming a lateral trajectory from the VMN to the brain stem was required for the display of lordosis. However, the fibers running through the lateral brain stem do play some role in the expression of the reflex, and more caudal bilateral lateral transections did reduce lordotic behavior. The absence of lordosis in mating tests was not a result of a systematic increase in rejecting behaviors. The observation of intermittent lordotic responses, or improved lordotic behavior following additional treatment with estrogen and/or progesterone revealed that these laterally transected animals were still able to produce the motor pattern of lordosis. The deficits seen were attributed to the interruption of fibers mediating the control of lordosis by the hypothalamus. This role of the ventromedial nucleus can be described as a tonic, estrogen-dependent facilitation of supraspinal mechanisms which control lordosis and are located more caudally in the brain stem. The laterally descending VMN efferents may play a quantitatively more important role in the control of lordosis than the medially descending fibers.     

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH–ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH–ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol + progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol + progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol + progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.     

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.     

Specificity and neural sites of action of anisomycin in the reduction or facilitation of female sexual behavior in rats

These experiments were designed to investigate the role of neuronal protein synthesis in the hormonal activation of female sexual behavior using intracranial implants of the protein synthesis inhibitor, anisomycin. In the first experiment, female rats receiving bilateral cannulae implants in the medial preoptic area (POA), septal region (SEPT), ventromedial hypothalamus (VMH), or midbrain central gray (CG) were injected with 2.5 micrograms estradiol benzoate (EB), followed 48 hr later by 500 micrograms progesterone (P). Females receiving anisomycin in the VMH at the time of EB injection had lower levels of lordosis and darting compared to tests without anisomycin. Sexual behavior was unaffected in females receiving anisomycin implants in the POA, SEPT, or CG. In a second experiment, we replicated the finding that anisomycin could attenuate lordotic responsivity when placed in the VMH of female rats injected with 2.5 micrograms EB and 500 micrograms P. In addition, we found that POA implants of anisomycin could facilitate lordosis in females given a low dose of EB (1.25 microgram) plus 500 micrograms P. In a third experiment, we assessed the effects of anisomycin application to the VMH or POA of female rats receiving estradiol (E; diluted 1:250 with cholesterol) implants in the VMH and systemic P. Treatment of the VMH with anisomycin prior to E in the VMH suppressed lordotic responding, whereas anisomycin application to the POA prior to E in the VMH had no effect on lordosis. The results of these experiments suggest that reducing protein synthesis in the region of the VMH disrupts the action of estrogen on the VMH, and that the facilitative action of anisomycin in the POA of female rats requires more estrogen treatment than threshold stimulation of the VMH alone.     

Identification of neural circuits involved in female genital responses in the rat: a dual virus and anterograde tracing study

The spinal and peripheral innervation of the clitoris and vagina are fairly well understood. However, little is known regarding supraspinal control of these pelvic structures. The multisynaptic tracer pseudorabies virus (PRV) was used to map the brain neurons that innervate the clitoris and vagina. To delineate forebrain input on PRV-labeled cells, the anterograde tracer biotinylated dextran amine was injected in the medial preoptic area (MPO), ventromedial nucleus of the hypothalamus (VMN), or the midbrain periaqueductal gray (PAG) 10 days before viral injections. These brain regions have been intimately linked to various aspects of female reproductive behavior. After viral injections (4 days) in the vagina and clitoris, PRV-labeled cells were observed in the paraventricular nucleus (PVN), Barrington's nucleus, the A5 region, and the nucleus paragigantocellularis (nPGi). At 5 days postviral administration, additional PRV-labeled cells were observed within the preoptic region, VMN, PAG, and lateral hypothalamus. Anterograde labeling from the MPO terminated among PRV-positive cells primarily within the dorsal PVN of the hypothalamus, ventrolateral VMN (VMNvl), caudal PAG, and nPGi. Anterograde labeling from the VMN terminated among PRV-positive cells in the MPO and lateral/ventrolateral PAG. Anterograde labeling from the PAG terminated among PRV-positive cells in the PVN, ventral hypothalamus, and nPGi. Transynaptically labeled cells in the lateral hypothalamus, Barrington's nucleus, and ventromedial medulla received innervation from all three sources. These studies, together, identify several central nervous system (CNS) sites participating in the neural control of female sexual responses. They also provide the first data demonstrating a link between the MPO, VMNvl, and PAG and CNS regions innervating the clitoris and vagina, providing support that these areas play a major role in female genital responses.     

The reversible inhibition of steroid-induced sexual behavior by intracranial cycloheximide

Cycloheximide(Cyclo), an inhibitor of protein synthesis by a direct action on protein synthesis at the ribosomal level, was used to reversibly inhibit estrogen-induced sexual receptivity. Cyclo (100 μg per rat) was infused into the preoptic area(POA) of ovariectomized rats at varying times before, simultaneously with, and after 3 μg of subcutaneous estradiol benzoate (EB). All animals received 0.5 mg progesterone (P) 36 hr after EB, and were tested for sexual receptivity 4–6 hr after P. The females were placed with stud males and a lordosis quotient was computed for each female (lordosis quotient = number of lordosis responses/20 mounts by the male × 100). Females receiving Cyclo 6 hr before, simultaneously with, or 12 hr after EB showed significantly lower levels of sexual receptivity when compared to females receiving Cyclo 36 hr before and 18 and 24 hr after EB. When those animals that showed low levels of sexual behavior after Cyclo infusion were reprimed with EB and P 7 days later and presented with a male they showed high levels of sexual receptivity. Thus, the effect of Cyclo was reversible. Only Cyclo infusions into the POA (bilateral) and third ventricle were effective in suppressing sexual behavior. Caudate nucleus, lateral ventricle, and unilateral POA infusions were without effect.The data presented are in agreement with earlier work that utilized actinomycin D to inhibit steroid-induced sexual behavior. Cyclo was found to be less toxic than actinomycin D. All of the available evidence is consistent with the hypothesis that estrogen stimulates RNA and/or protein synthesis in its facilitation of sexual behavior in the female rat.     

Oxytocin immunoreactivity in the hypothalamus of female hamsters

In Syrian hamsters (Mesocricetus auratus), oxytocin (OXT) activity within the medial preoptic-anterior hypothalamus (MPOA-AH) and the ventromedial hypothalamus (VMH) plays an important role in the expression of sexual receptivity. Immunocytochemical analysis with OXT-specific antibodies was used to identify the distribution of OXT-containing cell bodies and fibers in female hamster brain and to determine the possible sources of OXT important for sexual receptivity. Oxytocin-immunoreactive cell bodies and fibers were found in several regions of the preoptic area, including the medial preoptic area, the medial preoptic nucleus, and the bed nucleus of the stria terminalis. Large numbers of cell bodies and fibers were localized within the paraventricular and supraoptic nuclei, and in anterior hypothalamus. OXT-immunoreactive fibers were observed in the VMH and the ventral tegmental area. The anatomical data from the present study support the hypothesis that OXT activity in the MPOA-AH and the VMH plays an important role in the regulation of sexual receptivity in hamsters.     

Inhibition of estrogen facilitation of sexual behavior by the intracerebral infusion of actinomycin-D

It has been shown previously that intracerebral actinomycin-D (Act-D) pellets inhibit estrogen facilitated female sexual behavior, but it was not possible to test the reversibility of this effect. In the present study an attempt was made to distinguish between the possible temporary interruption by Act-D of the biochemical action of estrogen which facilitates sexual receptivity and permanent toxic effects of the drug. Act-D in saline was infused into the third ventricle or the preoptic area (POA) to determine whether a reversible suppression of sexual behavior as measured by the lordosis quotient (LQ) could be produced. Ovariectomized rats were implanted with midline guide tubes entering the third ventricle (eight rats) or with bilateral tubes extending to the corpus callosum above the POA (67 rats). Each animal served as its own control since pretest and Act-D and recovery tests were performed 10–14 days apart in most subjects. For each behavioral test implanted subjects were primed with 3μg estradiol benzoate (EB) and 0.5 mg progesterone (P) 48 hr later. Behavioral tests, each involving 50 mounts, were performed 4–6 hr after P. Following the pretest the animals were retested under experimental conditions. Inner cannulae were inserted into the POA through the guide tubes and 0.11 μg Act-D infused 24 or 12 hr before, simultaneously with, or 6, 12, 18, or 26 hr after EB. A recovery test was performed 10–14 days later with no intracerebral infusion. The control procedure (infusion of of saline either simultaneously with or 12 hr after EB) did not alter the LQ. Act-D infusion produced a reversible suppression of lordosis which was dependent upon the time of administration of Act-D. Intraventricular infusion of Act-D 6 hr after EB reversibly inhibited lordosis behavior and no lesions were produced. Act-D infused into the POA simultaneously with EB or 6 hr later reversibly suppressed the LQ. In the 6 hr group, for example, the LQ fell from 78.3 to 35.7, but 10–14 days later reached 74.3. Although brain lesions of varying extent were produced by Act-D, the marked but reversible suppression of lordosis behavior is consistent with the view that Act-D inhibits estrogen facilitation of lordosis behavior by means of a biochemical rather than cytotoxic action.     

Release of orphanin FQ/nociceptin in the medial preoptic nucleus and ventromedial nucleus of the hypothalamus facilitates lordosis

Opioid regulation of reproduction has been widely studied. However, the role of opioid receptor-like 1 receptor (NOP; also referred to as ORL-1 and OP4) and its endogenous ligand orphanin FQ/nociceptin (OFQ/N) have received less attention despite their extensive distribution throughout nuclei of the limbic-hypothalamic system, a circuit that regulates reproductive behavior in the female rat. Significantly, the expression of both receptor and ligand is regulated in a number of these nuclei by estradiol and progesterone. Activation of NOP in the ventromedial nucleus of the hypothalamus (VMH) of estradiol-primed nonreceptive female rats facilitates lordosis. NOPs are also expressed in the medial preoptic nucleus (MPN), however, their roles in reproductive behavior have not been studied. The present experiments examined the role of NOP in the regulation of lordosis in the MPN and tested whether endogenous OFQ/N in the MPN and VMH mediates reproductive behavior. Activation of NOP by microinfusion of OFQ/N in the MPN facilitated lordosis in estradiol-primed sexually nonreceptive female rats. Passive immunoneutralization of OFQ/N in either the MPN or the VMH reduced lordosis in estradiol-primed females, but had no effect on lordosis in estradiol+progesterone-primed sexually receptive rats. These studies suggest that OFQ/N has a central role in estradiol-only induced sexual receptivity, and that progesterone appears to involve additional circuits that mediate estradiol+progesterone sexual receptivity.     

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH–ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH–ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol + progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol + progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol + progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.     

Hypothalamic Grafts Induce the Recovery of Lordosis in Female Hamsters with Lesions of the Ventromedial Hypothalamus

Previous studies have documented the ability of neural grafts to stimulate the recovery of lordosis from neurochemical deficits. However, it was unclear if grafts also could reverse deficits in lordosis caused by lesions at critical points in the neural circuit controlling this response. To address this question, female hamsters were subjected to unilateral lesions of the ventromedial hypothalamus (VMN), a structure well known for its mediation of hormonal effects on lordosis. The effects of these lesions were described by noting the ability of manual stimulation of one flank to reinstate a deteriorating lordosis response. Consistent with past results, unilateral VMN lesions decreased responsiveness to stimulation of just the contralateral flank. Females showing such lateralized decrements then received control treatments or implants into the lesioned area of basal hypothalamic tissue from a neonatal male or female hamster. Approximately 1 month later, tests of lordosis reinstatement by ipsi- or contralateral manual stimuli were repeated. Whereas lateralized decrements in responsiveness persisted in control subjects, implants of tissue from male or female neonates led to reliable improvements in lordosis, reversing the lesion-induced decrease in contralateral responsiveness. The mechanism responsible for this change is unclear, but could involve an elevation in a lordosis-facilitating neuromodulator. Alternatively, it could depend on the reinforcement or replacement of neural circuits for lordosis, possibly including those that connect the two VMNs with each other or with the periaqueductal gray of the midbrain.     

Tamoxifen and ICI 182,780 activate hypothalamic G protein-coupled estrogen receptor 1 to rapidly facilitate lordosis in female rats

In the female rat, sexual receptivity (lordosis) can be facilitated by sequential activation of estrogen receptor (ER) α and G protein-coupled estrogen receptor 1 (GPER) by estradiol. In the estradiol benzoate (EB) primed ovariectomized (OVX) rat, EB initially binds to ERα in the plasma membrane that complexes with and transactivates metabotropic glutamate receptor 1a to activate β-endorphin neurons in the arcuate nucleus of the hypothalamus (ARH) that project to the medial preoptic nucleus (MPN). This activates MPN μ-opioid receptors (MOP), inhibiting lordosis. Infusion of non-esterified 17β-estradiol into the ARH rapidly reduces MPN MOP activation and facilitates lordosis via GPER. Tamoxifen (TAM) and ICI 182,780 (ICI) are selective estrogen receptor modulators that activate GPER. Therefore, we tested the hypothesis that TAM and ICI rapidly facilitate lordosis via activation of GPER in the ARH. Our first experiment demonstrated that injection of TAM intraperitoneal, or ICI into the lateral ventricle, deactivated MPN MOP and facilitated lordosis in EB-primed rats. We then tested whether TAM and ICI were acting rapidly through a GPER dependent pathway in the ARH. In EB-primed rats, ARH infusion of either TAM or ICI facilitated lordosis and reduced MPN MOP activation within 30 min compared to controls. These effects were blocked by pretreatment with the GPER antagonist, G15. Our findings demonstrate that TAM and ICI deactivate MPN MOP and facilitate lordosis in a GPER dependent manner. Thus, TAM and ICI may activate GPER in the CNS to produce estrogenic actions in neural circuits that modulate physiology and behavior.     

Modulation of the arcuate nucleus–medial preoptic nucleus lordosis regulating circuit: A role for GABAB receptors

Estradiol rapidly activates a microcircuit in the arcuate nucleus of the hypothalamus (ARH) that is needed for maximal female sexual receptivity. Membrane estrogen receptor-α complexes with and signals through the metabotropic glutamate receptor-1a stimulating NPY release within the ARH activating proopiomelanocortin (POMC) neurons. These POMC neurons project to the medial preoptic nucleus (MPN) and release β-endorphin. Estradiol treatment induces activation/internalization of MPN μ-opioid receptors (MOR) to inhibit lordosis. Estradiol membrane action modulates ARH gamma-aminobutyric acid receptor-B (GABA_B) activity. We tested the hypothesis that ARH GABA_B receptors mediate estradiol-induced MOR activation and facilitation of sexual receptivity. Double-label immunohistochemistry revealed expression of GABA_B receptors in NPY, ERα and POMC expressing ARH neurons. Approximately 70% of POMC neurons expressed GABA_B receptors. Because estradiol initially activates an inhibitory circuit and maintains activation of this circuit, the effects of blocking GABA_B receptors were evaluated before estradiol benzoate (EB) treatment and after at the time of lordosis testing. Bilateral infusions of the GABA_B receptor antagonist, CGP52432, into the ARH prior to EB treatment of ovariectomized rats prevented estradiol-induced activation/internalization of MPN MOR, and the rats remained unreceptive. However, in EB-treated rats, bilateral CGP52432 infusions 30 min before behavior testing attenuated MOR internalization and facilitated lordosis. These results indicated that GABA_B receptors were located within the lordosis-regulating ARH microcircuit and are necessary for activation and maintenance of the estradiol inhibition of lordosis behavior. Although GABA_B receptors positively influence estradiol signaling, they negatively regulate lordosis behavior since GABA_B activity maintains the estradiol-induced inhibition.     

PAC1 receptors mediate pituitary adenylate cyclase-activating polypeptide- and progesterone-facilitated receptivity in female rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts as a feed-forward, paracrine/autocrine factor in the hypothalamic ventromedial nucleus (VMN) for receptivity and sensitizes pituitary hormone release for ovulation. The present study examined receptor(s) and signaling pathway by which PACAP enhances rodent lordosis. PACAP binds to PACAP (PAC1)- and vasoactive intestinal peptide-preferring receptors (VPAC1, VPAC2). Ovariectomized rodents primed with estradiol (EB) were given PACAP or vasoactive intestinal peptide directly onto VMN cells. Only PACAP facilitated receptivity. Pretreatment with VPAC1 and VPAC2 inhibitors blocked both PACAP- and progesterone (P)-induced receptivity. Antisense (AS) oligonucleotides to PAC1 (not VPAC1 or VPAC2) inhibited the behavioral effect of PACAP and P. By real-time RT-PCR, EB, P and EB+P enhanced VMN mRNA expression of PAC1. Within the total PAC1 population, EB and EB+P induced expression of short form PAC1 and PAC1hop2 splice variants. Finally, blocking cAMP/protein kinase A signaling cascade by antagonists to cAMP activity and protein kinase A or by antisense to dopamine- and cAMP-regulated phosphoprotein of 32 kDa blocked the PACAP effect on behavior. Collectively, these findings provide evidence that progesterone receptor-dependent receptivity is, in part, dependent on PAC1 receptors for intracellular VMN signaling and delineate a novel, steroid-dependent mechanism for a feed-forward reinforcement of steroid receptor-dependent reproductive receptivity.