Inhibition Of Lordosis Research Articles

α 1-Noradrenergic regulation of hypothalamic progestin receptors and guinea pig lordosis behavior

Experiments were conducted to determine whetherα 1- orα 2-receptors mediate noradrenergic (NA) regulation of guinea pig lordosis behavior and hypothalamic progestin receptors. When infused into a lateral cerebroventricle at a dose that inhibits lordosis and that decreases the concentration of estradiol-inducible hypothalamic progestin receptors, phenoxybenzamine decreased binding of theα 1-ligand [ 3H]WB4101 but not theα 2-ligand [ 3H]clonidine to brain membranes. Thus, under the conditions used, phenoxybenzamine appears to blockα 1-receptors with little or no effect onα 2-receptors. Experiments with the selectiveα 1-antagonist prazosin also indicatedα 1-receptor reguilation of lordosis and hypothalamic progestin receptors. Prazosin inhibited lordosis induced by estradiol benzoate (EB) plus progesterone and by EB + clonidine and decreased the concentration of cytoplamic progrestin receptors in hypothalamus (but not in area or frontal cortex) of EB-primed females. The inhibition of lordosis is apparently not due to some unknown side effect of prazosin because pretreatment with a high dose of clonidine attenuated the inhibition. The possibility that a causal relationship exists between effects ofα 1-NA transmission on hypothalamic progestin receptors and lordosis was discussed. Also, because effects of NA transmission on hypothalamic progestin receptors are dependent on prior treatment with EB, it was suggested that NA transmission might influence estradiol action in addition to progestin action in hypothalamic cells.

Modulation of lordosis behavior of female rats by naloxone, beta-endorphin and its antiserum in the mesencephalic central gray: possible mediation via GnRH.

The role of endogenous opiate peptides in the mesencephalic central gray (MCG) and their possible interactions with gonadotropin-releasing hormone (GnRH) in the regulation of lordosis behavior was assessed in ovariectomized, estrogen-treated and estrogen-progesterone-treated female rats. Lordosis behavior triggered by male mounting was enhanced by microinfusion of naloxone and anti-beta-endorphin-globulin (anti-beta-end-G) but not by anti-met-enkephalin-globulin or anti-dynorphin-globulin into the MCG in both estrogen-treated and estrogen- (low dose) progesterone-treated females. The potentiating effects of naloxone and anti-beta-end-G could be blocked by a preinfusion of either anti-GnRH-globulin or an antagonist analog of GnRH directly into the MCG. However, two potent antagonist analogs of GnRH were not effective in blocking lordosis indicating a dissociation between their neural actions and their known inhibitory effects on luteinizing hormone release. Conversely, beta-endorphin but not met-enkephalin or dynorphin infused into the MCG inhibited lordosis behavior in both estrogen-treated and estrogen-progesterone-treated rats. This beta-endorphin-induced inhibition of lordosis in the estrogen-treated rats could be overcome by GnRH microinfused directly into the MCG which potentiated lordosis to high levels. These observations provide evidence that beta-endorphin may be the sole opiate peptide in the MCG involved in the control of lordosis behavior and also suggests a functional relationship with GnRH systems in the MCG in such a regulatory mechanism.

Inhibition of estrogen-activated sexual behavior by androgens

Experiments to determine the potential of androgen to inhibit estrogen-activated female sexual behavior in rats were conducted. Treatment with either testosterone propionate (0.8 or 1.6 mg/day) or dihydrotestosterone propionate (0.2, 0.4, or 0.8 mg/day) significantly reduced the incidence of lordosis in ovariectomized females receiving estradiol benzoate (1 μg/day). A similar suppression of estrogen-activated lordosis by testosterone was observed in castrated male rats. Flutamide, an androgen-receptor blocker, prevented the inhibition of lordosis by testosterone in females, indicating that the interaction of testosterone or a metabolite with an androgen receptor may be an important feature of this inhibition. Furthermore, the ability of dihydrotestosterone to inhibit lordosis at lower doses than testosterone suggests that the conversion of testosterone to dihydrotestosterone may also be necessary. These experiments demonstrate the potential of testosterone to inhibit the occurrence of female sexual behavior in rats, in contrast to its established facilitative effect on this behavior.

Dihydrotestosterone-induced inhibition of lordosis in estrogen-primed ovariectomized rats following 6-hydroxydopamine or electrolytic septal lesions

The dose-dependent inhibition of lordosis by 5 alpha-dihydrotestosterone propionate in ovariectomized female rats treated concurrently with estradiol benzoate was attenuated neither by intracranial 6-hydroxydopamine injections which reduced dopamine concentrations by approximately 70% in septum or 75% in both septum and n. accumbens septi nor by electrolytic destruction of the lateral or medial septal nuclei or of both septal nuclei. These results suggest that inhibitory effect of dihydrotestosterone on estrogen-induced lordosis does not critically depend on the mesolimbic dopaminergic innervation of the forebrain or on the septal nuclei.

Persistence of dihydrotestosterone inhibition of lordosis in estrogen-primed rats fed a tryptophan-deficient diet

Administering increasing dosages of 5α-dihydrotestosterone propionate (DHTP) (0, 40, or 80 μ/100 g in oil daily) to ovariectomized female rats treated concurrently with estradiol benzoate (EB) (1 μg/100 g in oil daily) caused significant, equivalent reductions in lordotic responsiveness in rats maintained either on a tryptophan-deficient corn diet or on a corn diet to which L-tryptophan was added. Whole brain concentrations of 5-hydroxytryptamine (5-HT) and of 5-hydroxyindoleacetic acid were approximately two times higher in rats fed corn+tryptophan versus corn alone. These findings suggest that the ability of DHTP to inhibit lordosis probably depends upon some mechanism other than a facilitation of 5-HT release from serotonergic neurons.

Cholinergic influences on estrogen-dependent sexual behavior in female rats.

The ability of cholinergic agents to influence hormone-dependent sexual behavior in female rats was examined. In the first experiment, female sexual behavior, indicated by the incidence of lordosis, was significantly increased in estrogen-treated female rats following bilateral infusion of a cholinergic receptor agonist, carbachol (.5 microgram/cannula), into the medial preoptic area of the brain. Infusion of an artificial cerebrospinal fluid vehicle failed to facilitate lordosis. The incidence of lordosis was normally highest 15 min after carbachol infusion, began to wane by 45 min, and had returned to control levels by 90 min. Further, centrally administered carbachol activated lordosis at lower levels of estrogen priming than did systemically administered progesterone. In a second experiment, female rats, brought into sexual receptivity by administration of estrogen and progesterone, received preoptic infusions of an acetylcholine synthesis inhibitor, hemicholinium-3. Significant reductions in the incidence of lordosis were observed following bilateral infusion of hemicholinium-3 (1.25 microgram/cannula). This inhibition of lordosis was prevented when carbachol (.5 microgram/cannula) was infused along with hemicholinium-3. Results confirm the importance of cholinergic influences on sexual behavior in female rats.

Inhibition of lordosis in rats by the antiestrogen CI-628 in the absence of progesterone.

A series of experiments assessed whether the ability of the antiestrogen CI-628 to inhibit estrogen-stimulated lordosis in adult ovariectomized rats depends upon its interference with the synergistic effects of estrogen with progesterone. In Experiment 1, the effect of CI-628 was contrasted in rats brought into estrus by a single subcutaneous injection of estradiol benzoate (EB) combined with an injection of progesterone (P) 42 hr later versus four daily injections of EB (without any P). CI-628 was given at the time of EB injection(s). CI-628 substantially and equally effectively antagonized lordotic responding in both conditions. In the absence of CI-628, rats receiving progesterone had significantly higher lordosis scores than the 4-day EB control animals. In Experiment 2, rats receiving CI-628 on only the first 2 of 4 days of EB injections had significantly decreased lordosis scores unless P was also given on the day of testing. This suggested that the EB from the latter injections was not acting as a progestin "mimic." In Experiment 3, lordotic responding stimulated by EB (without P) was inhibited by CI-628 in rats that were both ovariectomized and adrenalectomized. This suggested that adrenal progestins were not involved in the ability of CI-628 to inhibit lordosis. Taken together, the results suggest that the mechanism of action of CI-628 for the inhibition of lordosis does not depend upon its ability to antagonize an estrogen-induced increase in neural progestin receptors. Implications of this for estrogen-mediated behaviors, for which CI-628 has little or no antagonistic effects, are discussed.

Uncoupling of the sexual receptivity (lordosis) hormone relationships by estradiol benzoate and testosterone propionate injection in neonatal hamsters of both sexes

(1) Estradiol benzoate (EB) at 250 μg when injected into hamsters at 12, 24, 48, 72, 96 or 120 hr after birth was equally effective at all times in suppression of sexual responsiveness measured as total lordosis duration (TLD) per 10 min test interval. (2) Testosterone propionate TP at 500 μg resulted in a significantly greater suppression in TLD when injected at 12, 24 or 48 hr as compared with injections at 72, 96 or 120 hr. (3) All neonatally injected female groups showed a significant reduction in TLD scores when compared with nontreated females with the exception of 96 hr TP treated females. (4) None of the neonatally treated male groups had TLD scores which were significantly different from the nontreated males. (5) All groups of treated females showed lordosis which in the 12- and 24-hr treatment groups did not change significantly as a result of castration or exogenous hormone treatment. (6) The 48-hr TP-treated females were the earliest treatment group in which the TLD score decreased significantly following ovariectomy and increased significantly following treatment with exogenous estrogen and progesterone. (7) Facilitation of lordosis followed by inhibition of lordosis on repeated progesterone injection (biphasic response) was not seen in hamsters of either sex injected with EB between 12 and 120 hr after birth. (8) The noninjected male hamsters and hamsters of both sexes receiving TP treatment at 96 hr or later showed a biphasic response to progesterone. (9) Our observations indicate that defeminization is most readily produced by estrogen treatment which in turn suggests that aromatization of androgen to estrogen is important for defeminization. (10) Since male hamsters respond to estrogen and progesterone, although with lower TLD scores than found for females, the male hamster is only partially defeminized either as a result of low androgen production and/or aromatization enzymes available during the critical stages of development.

Antiestrogens: Effects of tamoxifen, nafoxidine, and CI-628 on sexual behavior, cytoplasmic receptors, and nuclear binding of estrogen

These experiments utilized the estrogen antagonists CI-628, nafoxidine, and tamoxifen as tools to investigate potential molecular mechanisms of estrogen activation of female rat sexual behavior. Adult female rats, ovariectomized 4–7 days previously and matched for body weight, were administered single sc injections of one of the three antiestrogens, and the ability of the antagonists to block estrogen-induced sexual behavior, to deplete and replenish hypothalamic estrogen receptors, and to inhibit the binding of estradiol by hypothalamic nuclei 2 hr, or 1, 2, 4, or 7 days later was assessed. All three compounds produced a dose- and time-dependent inhibition of estrogen-activated lordosis, with tamoxifen being the most potent inhibitor. The three antiestrogens also caused prolonged depletion of hypothalamic estrogen receptors, but there was no correlation between receptor levels and the degree of inhibition of lordosis behavior at any time point following antiestrogen treatment. Rats showed high levels of sexual receptivity when antiestrogens were injected 2, 4, or 7 days before estrogen; however, hypothalamic estrogen receptors were still markedly (up to 70%) reduced at some of these time points. In contrast, there was a large ( r = 0.67), significant correlation between the ability of all three agents to reduce [ 3H]estradiol binding by brain cell nuclei and their ability to reduce the display of estrogen-induced female sexual behavior. Antiestrogen injections which inhibited lordosis always decreased the level of specific estradiol binding by hypothalamic nuclei. These data indicate that delayed receptor replenishment does not adequately explain the antagonism of lordosis behavior by antiestrogens. The results presented here strongly point to the cell nucleus as the critical locus of receptor-mediated interactions which underlie estrogen and antiestrogen regulation of female sexual behavior.

Ovarian hormones regulating sexual and social behaviors in female prairie voles, Microtus ochrogaster

In the induced ovulator, Microtus ochrogaster (the prairie vole), daily injections of estradiol benzoate (EB 0.5 μg or 5.0 μg) facilitated high levels of sexual receptivity in ovariectomized females withing 48 hr of the onset of treatment. A lower level of EB (0.05 μg) with or without progesterone (0.5 mg) for five days did not induce lordosis. Four hr after sexually receptive EB-primed females received progesterone (0.5 mg) they either continued to show lordosis (EB 5.0 μg group) or showed an inhibition of lordosis (EB 0.5 μg group). Within 24 hr lordosis was markedly reduced in both groups. Progesterone-associated inhibitions were no longer apparent 48 hr after progesterone was discontinued. This general pattern of response to ovarian hormones is consistent with data described for other induced ovulators.

Noradrenergic transmission and female sexual behavior of guinea pigs

Treatment with the dopamine beta-hydroxylase (DBH) inhibitor U-14,624 (50, 100, or 150 mg/kg) blocked the induction of lordosis behavior by estradiol benzoate (EB) and progesterone (P) in overiectomized guinea pigs. After treatment with U-14,624 (100 mg/kg), norepinephrine (NE) content of medial basal hypothalamus, preoptic area and cortex was reduced (by 55%) and dopamine (DA) content of medial basal hypothalamus was increased (by 155%) during the period when females treated with EB and P normally display lordosis. Treatment with the NE receptor stimulator clonidine (1.0 mg/kg) restored lordosis behavior in females treated with EB, P, and U-14,624 (100 mg/kg), but the putative DA and serotonin (5-HT) receptor blockers pimozide (1.0 mg/kg) and methysergide (20.0 mg/kg) were ineffective in this respect. Thus, inhibition of lordosis after treatment with U-14,624 appeared to be attributable primarily to a reduction in NE neurotransmission, rather than to an increase in DA or 5-HT activity. Because clonidine induced lordosis in females treated with EB, P, and U-14,624, it seemed unlikely that the facilitatory effects of clonidine on lordosis were mediated by activation of presynaptic alpha-adrenergic receptors (i.e. inhibitory NE autoreceptors) rather than by postsynaptic alpha-receptors. In addition, pretreatment with the postsynaptic alpha-adrenergic antagonist phenoxybenzamine (20.0 mg/kg) blocked the facilitation of lordosis by clonidine (1.0 mg/kg) in females primed with EB alone and with EB plus P. Thus, the facilitatory effects of clonidine on lordosis appear to be mediated by activation of postsynaptic alpha-adrenergic (i.e. NE) receptors. The results of this study provide further evidence that NE neurotransmission facilitates the expression of female sexual behavior in guinea pigs.

Long Term Effects of Estrogen Action Are Crucial for the Display of Lordosis in Female Guinea Pigs: Antagonism by Antiestrogens and Correlations with inVitroCytoplasmic Binding Activity*

The administration of 1.6 μg 17β-estradiol benzoate (EB) 60 h before an injection of 0.5 mg progesterone induced optimal lordosis responses in ovariectomized guinea pigs. To determine whether the presence of estrogen in neural tissue throughout the 60-h period was necessary for the display of lordosis, various antiestrogens (i.e. MER-25, CI-628, enclomiphene, or zuclomiphene) were administered during the late portion of the 60-h period. Dose-dependent inhibition of lordosis resulted when these antiestrogens were injected at 40 and 47 h after EB injection. On the other hand, injection of supplemental EB late in the priming period enhanced the display of lordosis. A correlation between behavioral antiestrogenicity and in vitro competition for cytoplasmic hypothalamic-preoptic area (H-POA) estrogen receptors was observed for all of the antiestrogens tested except MER-25. The rank order of behavioral antiestrogenic activity was: CI-628 > MER-25 = enclomiphene > zuclomiphene. The relative binding affinities of...

The effects of lysergic acid diethylamide on copulatory behaviour in the female rat

The actions of lysergic acid diethylamide (LSD) on a serotonin- and dopamine-dependent functional test, lordotic behaviour in female rats, and other behaviours were investigated. Increasing doses of LSD inhibited the display of lordosis in increasing numbers of females. Also locomotor activity and habituation of a startle reaction were impaired. Pretreatment with a large single dose of p-chlorophenylalanine, which by itself induced lordosis in all subjects tested, caused an enhancement of the LSD effect, prolonging its duration. When p-chlorophenylalanine was replaced by α-methyl- p-tyrosine, all females showed lordosis by this treatment alone, while a combination with LSD had a significantly briefer effect than this dose of LSD alone. Repeated administration of smaller doses of p-chlorophenylalanine did not by itself alter the response level in a group of females, and together with LSD the effect was of a shorter duration than after the single dose of p-chlorophenylalanine. Inhibition of lordosis was not related to impairment of locomotor capacity. These differential effects after pretreatment with the synthesis inhibitors in addition to other data discussed indicate that certain functional effects of LSD could be related to increased serotonin activity.

Effect of subcutaneous vs intraperitoneal administration of an anti-estrogen, CI-628, on estradiol- and estradiol benzoate-stimulated lordosis in the ovariectomized rat

The anti-estrogen CI-628 (3 mg) inhibited estradiol (E, 150 μg, SC in oil)-stimulated lordosis in ovariectomized rats, when the anti-estrogen was given intraperitoneally (IP) at the time of E injection (Hr 0). No lordosis inhibition occurred after similar treatment, if the CI-628 was given subcutaneously. For estradiol benzoate (EB, 2 μg, SC in oil)-stimulated lordosis, CI-628 (Hr 0) had a substantial inhibitory effect whether it was given IP or SC, the greater percentage inhibition occurring after SC CI-628 (Experiment 1). All animals received sequential injections of estrogen and progesterone (500=s,g, SC in oil) and were then tested to 10 mounts by an intact male. These results suggested that, after E injection, neural tissues mediating lordosis have a shorter period of sensitivity to the behavioral effects of CI-628 than after EB injection. Thus, inhibition of E-stimulated lordosis apparently required the more rapid onset of intracellular estrogen retention inhibition resulting from CI-628 given IP rather than SC. This was not the case for EB-stimulated lordosis, where a longer period of sensitivity to CI-628 seemed likely. This hypothesis was supported by the data of Experiment 2, in which CI-628 (3 mg, IP) was first given 3 hr after E or EB treatment. In this paradigm, CI-628 no longer inhibited E-stimulated lordosis. In contrast, the anti-estrogen's effect on EB-stimulated lordosis was at least equal to the inhibition occurring after CI-628 (IP) given at Hr 0.

Lordosis in female rats following medial forebrain bundle lesions

In the first experiment, ovariectomized rats given weekly injections of estrogen and progesterone which were relatively unresponsive during preoperative testing received bilateral medial forebrain bundle (MFB) lesions. Postoperative lordosis quotients were significantly elevated. In the second experiment, MFB lesion and sham lesion rats were tested for responsiveness to decreasing doses of estrogen. Lesions had no significant effect in this experiment. In the final experiment, MFB lesions significantly increased the duration of receptivity following single injections of estrogen and progesterone. These results suggest that, like the preoptic area, under some conditions the MFB may play a role in the suppression of lordosis.

Suppression of lordosis in the hormone-primed female hamster by electrical stimulation of the septal area

The behavior of the female hamster toward the male is altered radically by estrogen and progesterone. Her aggression gives way to sexual receptivity, and the lordosis posture will commonly be maintained in the presence of a sexually active male for greater than 10 min at a time. The septal area is one of the limbic and hypothalamic areas which preferentially accumulate radioactive estradiol in the female hamster. In order to determine whether this region is involved in lordosis control, we examined the effects of electrical stimulation (ES) there on the copulatory behavior of ovariectomized, hormone-primed, sexually receptive females in the presence of sexually active males. Sixty second trains of 100 Hz, 0.2 msec, biphasic, square-wave pulses at either 80 or 90 μA were effective in suppressing the lordosis response in 8 of 13 animals with electrodes in the septal area. In the most effective cases ES would prevent lordosis even during persistent contacts and mount attempts by the male. Some neurons in the septal area may be part of a network acting to inhibit the display of lordosis. One of the ways ovarian hormones could facilitate lordosis is through reduction of the suppressive activity in such a network.

Comparative facilitation and inhibition of lordosis in the guinea pig with progesterone, 5α-pregnane-3,20-dione, or 3α-hydroxy-5α-pregnan-20-one

The major 5α-reduced metabolites of progesterone tentatively identified in neural tissue of the guinea pig were evaluated in this species for their ability to facilitate and inhibit lordosis responses of spayed females after estradiol benzoate (EB) pretreatment. 5α-Dihydroprogesterone was found to be an effective facilitative agent, but at doses of 0.05-0.3 mg administered at time intervals from 12–60 hr after estradiol, it was not as potent as progesterone. The steroids 3α-hydroxy-5α-pregnan-20-one and 5β-pregnane-3,20-dione, evaluated at only one dose level (0.18 mg) and at one time interval after estradiol (36 hr), were found to have moderate facilitative effects, but they were not as effective as 5α-dihydroprogesterone. The inhibitory influences of the metabolites studied were found to be weak relative to progesterone when given at doses of 0.6 mg 1 hr after EB. However, when 5α-dihydroprogesterone was given at a higher dose (3.6 mg) it was then found to be an effective inhibitor of the lordosis response. The results indicate that this metabolite has behavioral influences similar to those of progesterone for both facilitation and inhibition of estrus. It was suggested that the superior potency of injected progesterone may be due to mechanisms of bioavailability, including relative solubility differences of the two steroids when administered subcutaneously.

Inhibition of lordosis in female rats by subcutaneous implants of testosterone, androstenedione or dihydrotestosterone in infancy

Female rats were implanted on the day of birth with Silastic capsules containing nonesterified testosterone, androstenedione, or dihydrotestosterone. The date of vaginal opening was assessed until sacrifice. The animals were ovariectomized, treated with estradiol benzoate and progesterone, and tested for the display of lordosis. The animals were then administered testosterone propionate and the size of the phallus was taken. Testosterone and dihydrotestosterone completely inhibited vaginal opening; androstenedione was partially effective. Testosterone almost completely inhibited lordosis behavior; androstenedione was partially effective and dihydrotestosterone was ineffective. All three androgens facilitated phallic development.

Suppression of lordosis in guinea pigs by ethamoxy-triphetol (MER-25) given at long intervals (34–46 hr) after estradiol benzoate treatment

Ovariectomized guinea pigs were given estradiol benzoate (EB) followed 40 hr later by progesterone (P). Behavioral testing commenced 1 hr after P injection and continued at hourly intervals for 8 hr. This treatment activated lordosis in almost 100% of animals. Administration of the antiestrogen MER-25 (75 mg/kg body wt per injection) between 2 hr before and 6 hr after EB treatment did not cause a significant decline in proportion of animals displaying lordosis, but did cause a decrease in length of time the lordosis position was held (maximum lordosis, sec). In contrast, 13 14 animals given MER-25 at 2 hr before and 2 hr after P and 8 10 animals given MER-25 simultaneously with and 2 hr after P, failed to show lordosis. Administration of supplementary EB at around the time of P injection, partially alleviated these behavior-blocking effects of MER-25. When MER-25 was given 2–6 hr after administration of P there was a significant decrease in duration of heat (hr). These results suggest that in addition to its early “triggering” effects, estrogen has important “maintenance” effects which determine the character of heat in guinea pigs. Continued presence of estrogen in the nervous system may be a requirement for the facilitatory actions of P on sexual behavior in guinea pigs, but such a requirement may not exist in other rodents such as rats.

Hormonal specificity in the suppression of sexual receptivity of the female golden hamster.

SUMMARY Oestrogen—progesterone-induced lordosis behaviour was suppressed in female hamsters which received neonatal treatment with testosterone propionate or diethylstilboestrol. No suppression of lordosis was observed in females which received neonatal treatment with testosterone, androsterone or control substances. In addition, the ano—genital distance and diameter of the vaginal orifice were increased in females treated with testosterone, testosterone propionate, androsterone or diethylstilboestrol. However, testosterone, testosterone propionate and androsterone had a greater effect on ano—genital distance than did diethylstilboestrol. Possible mechanisms for the defeminizing action of the hormones administered were discussed.