Facilitation Of Lordosis Research Articles

Sexual receptivity is reduced in the female mu-opioid receptor knockout mouse

Activation of mu-opioid receptors is critical to steroid regulation of female sexual behavior, lordosis, in rodents. Estrogen treatment activates mu-opioid receptors in the medial preoptic area inhibiting lordosis, but ultimately appears important for progesterone facilitation of lordosis. We investigated the role of mu-opioid receptors in the expression of sexual receptivity in mice lacking mu-opioid receptors. Although estrogen and progesterone facilitated lordosis in mu-opioid receptor knockout mice, they exhibited deficits in lordosis quotient and score compared with wild-type females, indicating reduced sexual receptivity. In contrast, wild-type and mu-opioid receptor knockout female mice did not differ in either active or passive avoidance of the male. These data are most consistent with the hypothesis that mu-opioid receptor activation is necessary for estrogen and progesterone to maximally facilitate lordosis.

Inhibiting Biosynthesis and/or Metabolism of Progestins in the Ventral Tegmental Area Attenuates Lordosis of Rats in Behavioural Oestrus

In the ventral tegmental area (VTA), lordosis of rats is facilitated by 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP). Central 3alpha,5alpha-THP results from metabolism of peripheral progesterone, from the ovaries and/or adrenals, by sequential enzymatic activity of 5alpha-reductase and 3alpha-hydroxysteroid oxidoreductase (3alpha-HSOR). In addition, in glial cells, cholesterol is converted into pregnenolone by the P450 side-chain cleavage enzyme (P450scc), which is then metabolized to progesterone by 3beta-hydroxysteroid dehydrogenase, and subsequently reduced to 3alpha,5alpha-THP. We hypothesize that, in the VTA, formation of 3alpha,5alpha-THP by both metabolism and biosynthesis is necessary for facilitation of lordosis of female rats. In Experiment 1, naturally-receptive rats received bilateral VTA infusions of a P450scc inhibitor, digitoxin (1 microg/side); a 5alpha-reductase inhibitor, finasteride (10 microg/side); digitoxin (1 microg/side)+finasteride (10 microg/side); or vehicle and were tested 3 h later for lordosis. In Experiment 2, the effects of VTA infusions of digitoxin, finasteride, digitoxin+finasteride, or vehicle on lordosis and midbrain and plasma 3alpha,5alpha-THP levels were examined. In Experiment 3, we investigated whether infusions of 3alpha,5alpha-THP to the VTA reinstated lordosis and midbrain 3alpha,5alpha-THP levels following administration of inhibitors. VTA infusions of digitoxin, finasteride, or digitoxin+finasteride, significantly and similarly reduced lordosis and midbrain, but not plasma 3alpha,5alpha-THP levels, compared to vehicle. Following receipt of inhibitor infusions, 3alpha,5alpha-THP to the VTA restored lordosis and midbrain 3alpha,5alpha-THP levels. These data suggest that, in the VTA, both central biosynthesis of progesterone and metabolism of progesterone (from central and/or peripheral sources) to 3alpha,5alpha-THP are important for mediating lordosis of rats.

Effects of midbrain lesions on lordosis and ultrasound production

Previous studies suggest the control of lordosis by competing neural systems. A lordosis-inhibiting system is thought to originate in the lateral septum (LS) or preoptic area (POA) and project to midbrain sites, including the ventral tegmental area (VTA) and ventral half of the periaqueductal gray (vPAG). A lordosis-facilitating system is thought to originate in the ventromedial hypothalamus and project to the dorsal half of the PAG (dPAG). To test the generality of this model, we subjected female hamsters to control operations or lesions of the dPAG, vPAG or VTA. Subjects in hormone-induced estrus were observed for lordosis responses to males and manual stimulation and for rates of production of the “ultrasounds” used by hamsters for courtship. PAG lesions of both types depressed ultrasound rate, lordosis initiation and lordosis maintenance. VTA lesions failed to affect ultrasound rate, lordosis maintenance or lordosis responses to males. However, damage to the dorsomedial tegmentum was correlated with deficits in lordosis initiation. These results suggest the concentration in the PAG of mechanisms controlling vocalizations, including the ultrasounds used by hamsters for courtship. They confirm midbrain control of lordosis initiation and maintenance but fail to support any simple specialization of dorsal and ventral areas for these dimensions. They also fail to support a specialization of the dorsal and ventral halves of the midbrain for lordosis facilitation and inhibition, but do suggest several midbrain areas likely to be involved in the first of these functions.

Effects of midbrain lesions on lordosis and ultrasound production

Previous studies suggest the control of lordosis by competing neural systems. A lordosis-inhibiting system is thought to originate in the lateral septum (LS) or preoptic area (POA) and project to midbrain sites, including the ventral tegmental area (VTA) and ventral half of the periaqueductal gray (vPAG). A lordosis-facilitating system is thought to originate in the ventromedial hypothalamus and project to the dorsal half of the PAG (dPAG). To test the generality of this model, we subjected female hamsters to control operations or lesions of the dPAG, vPAG or VTA. Subjects in hormone-induced estrus were observed for lordosis responses to males and manual stimulation and for rates of production of the “ultrasounds” used by hamsters for courtship. PAG lesions of both types depressed ultrasound rate, lordosis initiation and lordosis maintenance. VTA lesions failed to affect ultrasound rate, lordosis maintenance or lordosis responses to males. However, damage to the dorsomedial tegmentum was correlated with deficits in lordosis initiation. These results suggest the concentration in the PAG of mechanisms controlling vocalizations, including the ultrasounds used by hamsters for courtship. They confirm midbrain control of lordosis initiation and maintenance but fail to support any simple specialization of dorsal and ventral areas for these dimensions. They also fail to support a specialization of the dorsal and ventral halves of the midbrain for lordosis facilitation and inhibition, but do suggest several midbrain areas likely to be involved in the first of these functions.

Regulation of lordosis by cyclic 3',5'-guanosine monophosphate, progesterone, and its 5alpha-reduced metabolites involves mitogen-activated protein kinase.

Progesterone (P) and its ring A-reduced metabolites regulate sexual behavior in ovariectomized, estrogen-primed female rats when they are administered intracerebrally and systemically. The present study tested the hypothesis that the MAPK pathway participates in P facilitation and sequential inhibition of sexual behavior. The role of MAPK in lordosis facilitation by two ring A-reduced metabolites of P, 5alpha-dihydroprogesterone (5alpha-DHP) and 5alpha,3alpha-pregnanolone (5alpha,3alpha-Pgl), was also assessed. In Experiment 1, the MAPK inhibitor PD98059 was infused intracerebroventricularly before progestin administration. Lordosis behavior induced by P, 5alpha-DHP, and 5alpha,3alpha-Pgl was abolished 2 h after progestin administration by PD98059. P and 5alpha,3alpha-Pgl facilitation of proceptive behaviors was also decreased by the MAPK inhibitor. Experiment 2 examined the effects of MAPK inhibition on P sequential inhibition. Estrogen-primed females received intracerebroventricular infusions of PD98059 or vehicle 30 min before systemic administration of P and were tested for lordosis 4 h later. Animals received a second injection of P 24 h later and were retested for lordosis. The MAPK inhibitor blocked both lordosis facilitation and sequential inhibition produced by systemic administration of P. Because cGMP can also facilitate lordosis behavior, and cGMP-dependent protein kinase can activate MAPK, experiment 3 determined whether interference with MAPK would affect cGMP enhancement of lordosis. The icv infusion of PD98059 significantly inhibited lordosis behavior induced by 8-bromo-cGMP, a cell-permeable cGMP analog, at both 2 and 4 h. These data support the hypothesis that the MAPK pathway is involved in lordosis regulation by P and some of its ring A-reduced metabolites as well as by the second messenger, cGMP.

Medial preoptic area δ-opioid receptors inhibit lordosis

Endogenous opioid peptides that activate the δ-opioid receptor (DOR) are thought to facilitate female receptive behavior. This facilitation of lordosis has been demonstrated by intracerebroventricular infusions and injection of DOR-active ligands into the ventromedial hypothalamic nucleus, an area with robust DOR binding. However, DOR binding is distributed throughout the hypothalamus, and the role of DOR in other areas of the hypothalamus has not been examined. In the current study, we demonstrated DOR immunoreactivity in the medial preoptic area (MPO), in particular medial preoptic nucleus (MPN) of the preoptic area. DOR immunoreactive processes were sparsely distributed in the medial and lateral parts of the MPN. Larger DOR immunoreactive fibers were localized in the ventrolateral aspect of the lateral MPN. The MPN is involved in the modulation of female sexual receptivity and the distribution of DOR in this area suggested to us that DOR may regulate lordosis. Ovariectomized rats with unilateral cannulae aimed at the MPN were given 5 μg 17β-estradiol benzoate (EB), once every 4 days and tested for lordosis. [D-Pen 2, D-Pen 5]-enkephalin (DPDPE), a DOR agonist, microinfused into the MPO, 52–54 h after EB-priming, inhibited lordosis when compared with the aCSF (vehicle) control ( P ≤ 0.05). The inhibitory effects of DPDPE were reversed by microinjection of naltrindole, a DOR antagonist ( P ≤ 0.05). Interestingly, the DOR inhibition of lordosis is similar to the μ-opioid receptor inhibition of lordosis in the MPN. These results indicate that DOR in the MPO, particularly in the MPNm, plays an important role in the regulation of lordosis.

GABA A receptor regulation of kyphotic nursing and female sexual behavior in the caudal ventrolateral periaqueductal gray of postpartum rats

Bilateral lesions of the ventrolateral caudal periaqueductal gray inhibit lordosis and kyphosis, the postures of female sexual receptivity and maternal nursing that are characterized respectively by dorsoflexion and ventroflexion of the spinal column. These lesions also inhibit the solicitation behaviors that accompany lordosis, but they do not impair retrieval or licking of pups. We tested the hypothesis that reproductive behaviors affected by these lesions are tonically inhibited by activity of the GABA A receptor via site-specific manipulations of receptor activity. Rats were bilaterally implanted during pregnancy with guide cannulae aimed at the caudal periaqueductal gray and ovariectomized on day 1 postpartum. Microinfusions (0.25 μl/side) of saline or drug took place on days 5 and 7 postpartum into the dorsolateral column and on days 9 and 11 into the ventrolateral column. Five minutes post-infusion dams were reunited with their pups and their maternal behavior was observed for 30 min. Feminine sexual behaviors were evaluated post-weaning after another set of microinfusions in the ventrolateral caudal periaqueductal gray. Potential facilitation of kyphosis and lordosis was tested with the GABA A antagonist bicuculline (15 ng/side) during sub-threshold conditions, i.e., non-suckling pups or sub-threshold ovarian hormone dosages; potential inhibition of these postures was tested with the GABA A agonist muscimol (125 ng/side) during optimal conditions, i.e., suckling pups or supra-threshold ovarian hormone treatments. Dorsolateral drug manipulations were ineffective. In the ventrolateral periaqueductal gray bicuculline significantly increased and muscimol significantly decreased kyphosis, lordosis, and sexual solicitations compared with saline. Retrieval and licking of pups were not altered by GABA A manipulations. These findings suggest that the reproductive postures of female rats, lordosis and kyphosis, as well as sexual solicitations, are tonically inhibited by the neurotransmitter GABA within the ventrolateral caudal periaqueductal gray in the midbrain. In contrast, retrieval and licking of pups appear to be under separate neurochemical or neuroanatomical control, or both. Further, this tonic inhibition is likely relieved by excitatory somatosensory inputs to this site, from mounting and suckling respectively.

GABAergic drugs alter hypothalamic serotonin release and lordosis in estrogen-primed rats

The effects of muscimol, a GABA A agonist, and phaclofen, a GABA B antagonist, on serotonin (5HT) release in the mediobasal hypothalamus and lordosis behavior were studied in freely moving rats using in vivo microdialysis. Two days after implantation of bilateral guide cannulae directed towards the ventromedial nucleus of the hypothalamus (VMH), ovariectomized rats were primed with estradiol (E 2). The rats were implanted with microdialysis probes 24 h later. Following a pretest for lordosis, perfusate 5HT was measured at 20-min intervals until the baseline was stable. The rats were treated with 10, 30 or 100 μM muscimol or 30 and 100 μM phaclofen in artificial CSF delivered via reverse dialysis for 40 min. Control animals were continuously perfused with artificial CSF. Behavior was tested 20, 60 and 180 min after introduction of the drug. Decreased hypothalamic 5HT (40–60% of baseline) and marked facilitation of lordosis were present 20 min after administration of either drug. The effects of 10 and 30 μM muscimol and 30 μM phaclofen on both 5HT and lordosis were reversed after 180 min. Reversal of the behavioral and neurochemical effects were not evident in either the 100 μM muscimol or 100 μM phaclofen groups at the time-points tested. Proceptive responses were observed in phaclofen-treated rats but not in rats treated with muscimol. Levels of hypothalamic 5HT and lordosis quotients in control rats did not significantly differ from initial values. These results suggest that GABAergic effects on lordosis may be mediated through an interaction with 5HT in the mediobasal hypothalamus.

Functional Interactions between Estrogen and Insulin-Like Growth Factor-I in the Regulation of α1B-Adrenoceptors and Female Reproductive Function

The ovarian hormone estradiol (E(2)) and insulin-like growth factor-I (IGF-I) interact in the CNS to regulate neuroendocrine function and synaptic remodeling. Previously, our laboratory showed that 2 d E(2) treatment induces alpha(1B)-adrenoceptor expression and promotes IGF-I enhancement of alpha(1)-adrenoceptor potentiation of cAMP accumulation in the preoptic area (POA) and hypothalamus (HYP). This study examined the hypothesis that E(2)-dependent aspects of female reproductive function, including alpha(1B)-adrenoceptor expression and function in the POA and HYP, are mediated by brain IGF-I receptors (IGF-IRs) in female rats. Ovariohysterectomized rats were implanted with a guide cannula aimed at the third ventricle and treated in vivo with vehicle or E(2) daily for 2 d before experimentation. Intracerebroventricular infusions of JB-1, a selective IGF-IR antagonist, were administered every 12 hr beginning 1 hr before the first E(2) injection. Administration of JB-1 during E(2) priming completely blocks hormone-induced luteinizing hormone release and partially inhibits hormone-dependent reproductive behavior. Reproductive behavior is restored by intracerebroventricular infusion of 8-bromo-cGMP, the second messenger implicated in alpha(1)-adrenergic facilitation of lordosis. In addition, blockade of IGF-IRs during E(2) priming prevents E(2)-induced increases in alpha(1B)-adenoceptor binding density and abolishes acute IGF-I enhancement of NE-stimulated cAMP accumulation in HYP and POA slices. These data document the existence of a novel mechanism by which IGF-I participates in the remodeling of noradrenergic receptor signaling in the HYP and POA after E(2) treatment. These events may help coordinate the timing of ovulation with the expression of sexual receptivity.

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.

A dopamine antagonist blocks vaginocervical stimulation-induced neuronal responses in the rat forebrain

During mating in rats, the male provides vaginocervical stimulation (VCS) to the female via intromissions. VCS, provided manually, mimics many aspects of mating, including facilitation of lordosis, induction of sexual receptivity, abbreviation of the period of sexual receptivity, and induction of twice-daily prolactin surges, which result in pseudopregnancy. VCS also induces the expression of Fos, the protein product of the immediate early gene c-fos, which has been used as a marker for neurons that are responsive to mating stimuli. Because VCS induces the release of dopamine in the forebrain, as well as phosphorylation of DARPP-32, a phosphoprotein associated with activation of the D 1 subtype of dopamine receptor, we tested the hypothesis that VCS induces Fos expression by acting on the D 1 class of dopamine receptors. Injection of SCH 23390, an antagonist of the D 1 class of dopamine receptors, virtually eliminated VCS-induced Fos expression without affecting constitutive levels of Fos-Immunoreactivity (Fos-IR) in all brain areas in which VCS induced Fos expression. In a follow-up experiment, expression of a second immediate early protein, egr-1, was blocked as well, suggesting that these results are not specific to Fos. Therefore, the results are consistent with the idea that VCS induces dopamine release, causing activation of D 1 dopamine receptors, which in turn, results in neuronal response, as seen by both Fos and egr-1 expression.

The Role of Neurosteroids and Nongenomic Effects of Progestins in the Ventral Tegmental Area in Mediating Sexual Receptivity of Rodents

Progesterone (P4) in the ventromedial hypothalamus (VMH) and ventral tegmental (VTA) is important for facilitation of lordosis; however, P4'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 P4'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 GABAA/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 P4's metabolism to 5α-pregnan-3α-ol-20-one (3α,5α-THP), the most effective endogenous positive modulator of GBRs, or the biosynthesis of the neurosteroid 3α,5α-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. 3α,5α-THP is increased in the midbrain of mated > proestrous > diestrous rodents. These data suggest that 3α,5α-THP has a proximate modulatory role on lordosis. In summary, the actions of P4 in the VTA are different from those in the VMH that involve PRs. In the VTA, P4 may facilitate lordosis following metabolism to and/or biosynthesis of 3α,5α-THP, which may have subsequent actions at GBRs and/or MBRs to acutely modulate female sexual behavior in rodents.

Progesterone Blockade of Estrogen Activation of μ-Opioid Receptors Regulates Reproductive Behavior

The mu-opioid receptor (MOR), a G-protein-coupled receptor, is internalized after endogenous agonist binding. Although receptor activation and internalization are separate events, internalization is a good assay for activation because endogenous opioid peptides all induce internalization. Estrogen treatment of ovariectomized rats induces MOR internalization, providing a neurochemical signature of estrogen activation of the medial preoptic nucleus. MOR activation appears to be the mechanism via which estrogen acts in the medial preoptic area to prevent the display of female reproductive behavior during the first 20-24 hr after estrogen treatment. Naltrexone, an alkaloid universal opioid receptor antagonist, prevented MOR internalization, suggesting that estrogen induces the release of endogenous opioid peptides that in turn activate the MOR. Enkephalins and beta-endorphin are nonselective endogenous MOR ligands. The most selective endogenous MOR ligands are the endomorphins. Infusions of selective MOR agonists, H-Tyr-d-Ala-Gly-N-Met-Phe-glycinol-enkephalin (DAMGO) or endomorphin-1, into the medial preoptic nucleus attenuated lordosis, and their effects were blocked with the MOR antagonist H-d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Infusion of endomorphin-1 internalized MOR. To determine whether progestin also acts via the MOR system to facilitate reproductive behavior, ovariectomized rats were primed with 17beta-estradiol and progesterone. Progestin facilitation of lordosis was correlated with a reduction of estrogen-induced MOR internalization. Progestin reversed estrogen-induced MOR internalization, suggesting that progesterone blocked estrogen-induced endogenous opioid release, relieving estrogen inhibition and facilitating lordosis. These results indicate a central role of MOR in the mediation of sex steroid activation of the CNS to regulate female reproductive behavior.

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

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

Intravenous progesterone elicits a more rapid induction of lordosis in rats than does SKF38393

In ovariectomized (ovx), estradiol (E)-primed rats, progesterone (P) facilitates sexual receptivity within 2–30 min following intravenous (i.v.) administration. Intracerebroventricular infusion of P or the dopamine receptor type 1-like (D1) agonist, SKF38393, increases lordosis in ovx, E-primed rats, and intracellular progestin receptor (PR) blockers attenuate P and SKF38393’s facilitation of lordosis. The present experiments examined whether P can have effects via Dl receptors and compared the onset of P’s and SKF38393’s induction of lordosis following i.v. administration. Ovx rats (N = 20) with i.v. jugular catheters were primed daily with 2 µg E subcutaneously and were pretested for sexual receptivity. In Experiment 1, rats (n = 10) were repeatedly tested for receptivity 3–7, 15, 30, 60, and 120 min following i.v. infusion of P (2 µg), SKF38393 (100 ng), and propylene glycol vehicle (0.2 cc). Progesterone increased postinfusion lordosis at all test times, whereas SKF38393’s increases in lordosis were not statistically significant until 15 min following i.v. infusion, relative to lordosis following vehicle or pretest conditions in Experiment 1. In Experiment 2, rats (n = 15; 5 from Experiment 1, and 10 new subjects) received infusions of the antiprogestin, RU38486, the D1 antagonist, SCH23390, or vehicle followed by a second P or vehicle infusion. Although both RU38486 and SCH23390 blocked the facilitatory effects of P on lordosis, their effects varied. RU38486 completely blocked P’s effects, whereas SCH23390 did not. In Experiment 3, rats (n = 15, from Experiment 2) received infusions of RU38486, SCH23390, or vehicle followed by a second SKF38393 or vehicle infusion. RU38486 and SCH23390 both effectively blocked the facilitatory effects of SKF38393 on lordosis. In Experiment 4, rats (n = 15, from Experiment 3) received i.v. infusions of P, which rapidly and significantly increased the number of super-threshold spikes in the ventral tegmental area, but not in the ventral medial hypothalamus or in the parietal cortex. These data suggest that actions at intracellular progestin receptors do not account for all of P’s effects to facilitate receptivity. Interestingly, although PRs may be involved in P and D1 ligand’s activation of female sexual behavior, D1 receptors are not required for P’s effects. However, i.v. P rapidly and significantly alters neuronal activity in sites with the greatest concentration of dopamine neurons (ventral tegmental area > ventral medial hypothalamus > cortex).

Ovarian Hormone Dependence of α1-Adrenoceptor Activation of the Nitric Oxide–cGMP Pathway: Relevance for Hormonal Facilitation of Lordosis Behavior

The ovarian hormones estradiol (E(2)) and progesterone (P) facilitate rat lordosis behavior in part by regulating the expression of and signal transduction by adrenoceptors in the hypothalamus (HYP) and preoptic area (POA). The major adrenoceptor subtype mediating E(2) and P facilitation of lordosis is the alpha(1)-adrenoceptor. In the present studies, we tested the hypotheses that (1) alpha(1)-adrenoceptors in the HYP enhance lordosis responses by activating the nitric oxide (NO)-cGMP signaling pathway, and (2) coupling of alpha(1)-adrenoceptors to this signal transduction pathway is hormone-dependent. Basal levels of cGMP were significantly higher in HYP and POA slices from animals treated with E(2) and P when compared with slices from ovariectomized controls or females treated with only E(2) or P. When slices of HYP and POA from ovariectomized female rats were incubated with norepinephrine or the selective alpha(1)-adrenoceptor agonist phenylephrine, cGMP accumulation was observed only if slices had been derived from females treated with both E(2) and P before experimentation. Moreover, alpha(1)-adrenoceptor stimulation of cGMP synthesis was blocked by an inhibitor of NO synthase, confirming that these receptors act by NO-mediated stimulation of soluble guanylyl cyclase. Behavioral studies demonstrated further that the cell-permeable cGMP analog 8-bromoadenosine-cGMP reverses the inhibitory effects of the alpha(1)-adrenoceptor antagonist prazosin on lordosis behavior in E(2)- and P-treated female rats. Thus, the NO-cGMP pathway mediates the facilitatory effects of alpha(1)-adrenoceptors on lordosis behavior in female rats, and previous exposure of the HYP and POA to both E(2) and P are required to link alpha(1)-adrenoceptors to this pathway.

Facilitatory and Inhibitory Effects of β-Endorphin on Lordosis in Female Rats: Relation to Time of Administration

The purpose of the present study was to investigate the effect of time of β-endorphin (β-EP) administration on lordosis in ovariectomized female rats injected subcutaneously (sc) with estradiol benzoate (EB) and progesterone (Prog). Intracerebroventricular (icv) injections of β-EP and naloxone (NLX), an opioid receptor antagonist, were administered at the various stages of sc steroid hormone priming. Facilitation of lordosis induced by 10 μg β-EP was observed exclusively within the initial 6 h of estrogen action, after which inhibition of lordosis occurred. At 12 h after EB priming, at the time of sc Prog treatment (or 43 h after EB priming), icv injection of 10 μg β-EP significantly inhibited lordosis. Lordosis was significantly facilitated by icv injections of 1 and 10 μg β-EP at the time of sc EB priming, but not by 0.1 μg β-EP. A dose–response relationship was identified for lordosis in experimental animals receiving icv injection of β-EP. Lordosis was inhibited by icv injections of 1 and 10 μg β-EP at 1 h before the test (or 47 h after EB priming). Lordosis was significantly inhibited by icv injection of NLX at all stages. From the present results, it seems that two different mechanisms are involved in endorphinergic modulation of rats' sexual receptivity: (a) the endorphinergic system at the initial stages of estrogen action facilitates the estrogen activation of lordosis; (b) the endorphinergic system at the final stages of steroid action inhibits lordosis. Moreover, there exists a critical time between 6 and 12 h after estrogen priming for endorphinergic mediation to modulate estrogen action.

Indomethacin Inhibits Lordosis Induced by Ring A-Reduced Progestins: Possible Role of 3α-Oxoreduction in Progestin-Facilitated Lordosis

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

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

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

Mating-Related Stimulation Induces Phosphorylation of Dopamine- and Cyclic AMP-Regulated Phosphoprotein-32 in Progestin Receptor-Containing Areas in the Female Rat Brain

Vaginal-cervical stimulation induces a number of physiological and behavioral events, including the facilitation of mating behavior. Although the facilitation of one component of mating behavior, lordosis, by vaginal-cervical stimulation does not require the presence of progesterone, it appears to be mediated by neural progestin receptors. Abundant evidence suggests that dopamine may play a role in the neural circuitry activated by vaginal-cervical stimulation, including the mating-induced release of dopamine in progestin receptor-containing areas of the brain, changes in the activational state of progestin receptors because of dopamine D1 receptor stimulation, facilitation of lordosis by D1 receptor stimulation in estradiol-primed rats via progesterone-independent events, and D1 agonist-induced neuronal responses in progestin receptor-containing areas and cells. We tested the hypothesis that vaginal-cervical stimulation induces phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32; Mr = 32,000), a protein phosphorylated predominantly in response to the stimulation of D1 receptors. At 9 d after ovariectomy, female rats were injected subcutaneously with a behaviorally effective dose of estradiol benzoate. At 48 hr later they received vaginal-cervical or control (perineal) stimulation, and they were perfused 1 hr later. Vaginal-cervical stimulation increased the number of cells expressing pDARPP-32 immunoreactivity by 92% in the medial preoptic nucleus, 134% in the caudal ventromedial hypothalamic nucleus, 123% in the posterodorsal medial amygdala, and 103% in the bed nucleus of the stria terminalis. These results suggest that some of the neuronal effects of vaginal-cervical stimulation, and perhaps other social or environmental stimuli, are mediated by phosphorylation of DARPP-32, perhaps via stimulation of D1 receptors, within progestin receptor-containing areas.