Lordotic Behavior Research Articles

Implantation of dihydrotestosterone propionate into the lateral septum inhibits sexual receptivity in estrogen-primed, ovariectomized rats.

Two experiments were conducted to identify sites in the female rat brain at which dihydrotestosterone acts to inhibit estradiol-induced feminine sexual behavior. 27 gauge cannulae containing either crystalline dihydrotestosterone propionate (DHTP) or cholesterol were implanted unilaterally into the lateral septum, preoptic area-anterior hypothalamic area, ventromedial nucleus of the hypothalamus or caudate putamen. In experiment 1, rats were given stereotaxic implants of steroids prior to being injected daily with estradiol benzoate (EB; 0.5 microgram/100 g body weight) and tested for sexual receptivity. In experiment 2, animals were injected daily with EB (0.9 microgram/100 body weight) and tested for sexual receptivity prior to and after stereotaxic implantation of steroids. In both experiments significant reductions in lordotic behavior were obtained only with lateral septal implants of DHTP.

Cycloheximide activation of the lordotic response in low-dose, estrogen-treated ovariectomized rats: evidence for modulating inhibitory influences of the limbic system on sexual behavior

This study tested the effect of intracranially injected cycloheximide (CHX), an inhibitor of protein synthesis, on facilitation of sexual receptivity in ovariectomized rats. The rats received 0.5 μg estradiol benzoate (EB), s.c., once daily on days 8 through 12 after ovariectomy (OVX). Either CHX (in 0.5 μl saline) or 0.5 μl saline was injected into the lateral septum (LS), cortical nucleus (ACO) or medial nucleus of the amygdala or medial preoptic area on day 11 after OVX. The dose of EB was insufficient to facilitate lordotic behavior on day 10 or day 12 after OVX unless CHX was injected into the LS or ACO. Injection of saline did not influence lordosis.

Role of aromatization in sexual differentiation: Effects of prenatal ATD treatment and neonatal castration

Pregnant female rats were administered either the aromatization inhibitor ATD (1,4,6-androstatriene-3,17-dione) or propylene glycol from Days 10 to 21 of gestation. On the day of birth one-half of the offspring from each group were gonadectomized. The remaining offspring were gonadectomized 35 days after birth. When adult the animals were given eight weekly mating tests following treatment with 2 or 8 μg of estradiol benzoate (EB) and 25 or 200 μg of progesterone (P). The probability of lordotic behavior as well as the frequency of ear-wiggle and hop and dart responses was measured. Prenatal ATD treatment resulted in a slight increase in lordotic behavior in the males. Lordotic potential was greatly facilitated by castration at birth. ATD treatment also increased the frequency of proceptive behaviors in males and combined ATD treatment and neonatal castration produced a dramatic increase in these behaviors. Prenatal ATD treatment and neonatal ovariectomy had only modest effects on the display of receptive and proceptive behaviors in females. Two weeks after the last test for female mating behaviors, the animals received daily injections of 200 μg of testosterone propionate. Four weekly tests for male-typical responses were given starting 1 week after the first injection. Prenatal ATD treatment did not markedly affect masculine behavior in the males. Castration at birth eliminated the ejaculatory response and reduced the frequency of mounting and intromission behavior. Prenatal ATD treatment and ovariectomy at birth had no appreciable effects on the display of male-typical behaviors in the females. Testosterone-stimulated masculine behavior of the female was similar to that of the male castrated at birth.

The effect of LHRH antagonist analogs and an antibody to LHRH on mating behavior in female rats

The action of two antagonist analogs and an antibody to luteinizing hormone-releasing hormone (LHRH) on sexual receptivity was studied in avariectomized, estrogen-progesterone primed female rats. Small amounts of each LHRH substance or saline was infused through a cannula positioned in either the third ventricle or arcuate-ventromedial (ARC-VMH) area of the hypothalamus. Infusions were carried out at the time of progesterone priming, which was 42 hrs post-estrogen treatment, and sexual receptivity, as denoted by the lordosis-to-mount ratio, was measured six hrs later. One antagonist analog, [D-pGlu1, D-Phe2, D-Trp3,6]-LHRH[1], had little or no effect on sexual receptivity when tested in either site. Similarly, an antibody to LHRH, tested only in the ARC-VMH, had no observable effect on lordotic behavior. However, the second and the most potent antagonist analog, [Ac-dehydro-Pro1, pCl-D-Phe2, D-Trp3,6]-LHRH[2], produced a marked and significant decrement in lordotic behavior when infused into either the third ventricle or ARC-VMH. These results suggest that this potent and long-acting, competitive antagonist analog of LHRH prevented endogenous LHRH from exerting its normal role in the induction of sexual receptivity and provide evidence to support the contention that the role of LHRH in mediating receptivity in the female rat is physiologically relevant.

Intralimbic Progesterone and Methysergide Facilitate Lordotic Behavior in Estrogen-Primed Female Rats

Intracranial administration of progesterone or a serotonergic receptor blocker into either the dorsal hippocampus or corticomedial amygdala, but not the septum, activate lordotic behavior in estrogen-primed female rats. The same intra-amygdaloid sites mediated the effects of both treatments, whereas in the hippocampus the regio superior was responsive to serotonergic receptor blockade and the dentate gyrus was responsive to progesterone. This is the first demonstration that the amygdala and hippocampus participate both in the facilitation of lordosis by progesterone and in a serotonergic system which inhibits female sexual behavior.

Bisexual behavior in male rats treated neonatally with antibodies to luteinizing hormone-releasing hormone.

For investigating the possibility that the neuropeptide luteinizing hormone-releasing hormone (LHRH) is involved in the process of sexual differentiation, male rat pups were injected on Days 1 and 3 of life with specific antibodies to LHRH (AB-LHRH); control rats were treated with normal rabbit serum (NRS). At maturity, males treated with AB-LHRH were as fertile as controls, and their amount and intromission latencies and the postejaculatory interval were extended only slightly. However, they showed high levels of lordotic behavior, including ear wiggling, when castrated and primed with estrogen or with estrogen plus progesterone. Testosterone propionate, administered neonatally together with AB-LHRH, did not reverse these effects. There was no evidence that neonatal treatment with antiserum to LHRH affected testosterone levels, as measured by radioimmunoassay. Males treated with AB-LHRH and castrated as adults did not respond to estrogen priming by releasing a surge of luteinizing hormone, a result indicating that they did not possess the female type of gonadotropin regulation. These findings indicate that neutralization of endogenous LHRH during neonatal life selectively blocks defeminization of behavior without affecting the process of masculinization.

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.

Inhibition of lordotic behavior in female rats following intracerebral infusion of anticholinergic agents

Cholinergic antagonists were bilaterally infused into forebrain areas of ovariectomized female rats brought into sexual receptivity by administration of estrogen and progesterone. Infusion of hemicholinium-3 (HC-3, 5 or 7.5 μg/cannula), an acetylcholine synthesis inhibitor believed to interfere with choline uptake, decreased the incidence of lordotic behavior displayed by females. This inhibition was prevented by infusion of choline chloride (120 μg/cannula) along with HC-3. Atropine sulfate, an acetylcholine receptor blocker, also reduced lordotic behavior in females following intracerebral infusion (20 μg/cannula). These results lend support to the suggestion that lordotic behavior is facilitated by central cholinergic activity.

Sexual behavior in male rats following intracerebral estrogen application.

The effects on sexual behavior and morphology of estradiol benzoate (EB) applied directly to the medial preoptic nucleus were assessed in three groups of male rats with varying sexual behavior potentials. High levels of lordotic behavior were observed in all EB-treated groups. Cholesterol was uniformly ineffective. Central EB failed to facilitate male copulatory behavior in any group. However, there was evidence that cannula implantation may have impaired the potential for male behavior. Significant reductions in testis and epididymis weights were evident in some but not all EB-treated males. It was concluded that EB can activate lordotic behavior consistently in male rats if applied in sufficient amounts to the medial preoptic region.

Differential effects of testosterone metabolites in the neonatal period on open-field behavior and lordosis in the rat

Two experiments were done to compare the effects of neonatal exposure to testosterone and its major metabolites, dihydrotestosterone (DHT) and estradiol (E 2), on the development of sex differences in open-field behavior in the rat. In Experiment 1 female rats administered either testosterone propionate (TP), DHT, or estradiol benzoate (EB) were found as adults to have low activity scores, more typical of adult males, when compared to the high scores of oil-treated females. In Experiment 2 the adult open-field behavior of female rats treated neonatally with testosterone or the metabolites was compared to that of male rats treated from Day 1 to 10 of life with the aromatizing enzyme inhibitor, androst-1,4,6-triene-3,17-dione (ATD). These same animals were later tested for lordotic behavior after gonadectomy and priming with EB and progesterone. All male animals and female animals exposed neonatally to testosterone or to either of the metabolites had suppressed open-field activity scores compared to oil-treated females. However, the lordotic behavior of females exposed to DHT and of males exposed to ATD was not defeminized and was comparable to that of oil-treated females. These observations were discussed in terms of a role for the androgenic actions of testosterone in establishing sex differences in nonreproductive behavior in the rat.

Effects of prenatal stress on avoidance acquisition, open-field performance and lordotic behavior in male rats

Prenatal stress enhanced lordotic behavior potentials in male rats but did not feminize patterns of active avoidance acquisition or open-field performance. These results suggest that prenatal stress selectively feminizes some but not all behavior patterns shown to differentiate under the influence of perinatal gonadal hormones. In the rat, the critical period for the differentiation of active avoidance behavior appears to span prenatal and early neonatal ontogeny.

Midbrain central gray: LHRH infusion enhances lordotic behavior in estrogen-primed ovariectomized rats

Ovariectomized rats were implanted with 23 gauge stainless steel cannulae in the ventrolateral midbrain central gray. Twelve sexually active rats were estrone-primed and infused with saline and 50 ng luteinizing hormone-releasing hormone (LHRH) in counterbalanced order. Infusion of luteinizing hormone-releasing hormone significantly enhanced the lordotic response to coital stimulation compared to saline infusion. These results support the role of hypothalamo-mesencephalic LHRH-containing pathways in modulating lordotic behavior in estrogen-primed Ovariectomized rats.

Role of hypothalamic dopaminergic receptors in the control of lordosis behavior in the female rat

The role of hypothalamic dopaminergic receptors in lordotic behavior was studied by infusing dopaminergic agents into either the medial preoptic area (MPOA), arcuate-ventromedial area (ARC-VM) or lateral hypothalamic area (LHA). Dopaminergic receptor stimulants, dopamine (DA) and apomorphine (APO), enhanced sexual receptivity when infused into the MPOA or ARC-VM in ovariectomized (OVX) rats primed with low doses of estrone. Under these conditions of low preinfusion receptivity (mean preinfusion L M =0.159 ), haloperidol (HALO), a dopaminergic receptor blocker, had no statistically significant effect upon lordotic behavior. Infusions of DA, APO, or HALO into the LHA also had no effect upon lordotic behavior in this model. A second experiment, in which OVX rats were primed with higher doses of estrone to maintain high preinfusion receptivity (mean preinfusion L M =0.869 ), was used to evaluate the effects of dopaminergic receptor blockade upon elevated sexual behavior. Using the second protocol, dopaminergic blockers, HALO and α flupenthixol, were observed to significantly depress lordotic behavior when infused into the MPOA and ARC-VM. In this model no alterations in sexual behavior were observed following MPOA or ARC-VM infusions of APO or the inactive stereoisomer of α flupenthixol, β fluqenthixol. Thus, the hypothalamic activation of dopaminergic receptors was shown to be stimulatory upon lordotic behavior. A third experiment was designed to evaluate the effects of dopaminergic receptor blockade upon luteinizing hormone-releasing hormone (LRH) enhanced lordotic behavior. In this protocol comparisons were made among the lordotic responses to MPOA and ARC-VM infusions of LRH, LRH with HALO and vehicle. Infusions of LRH into the MPOA and ARC-VM significantly enhanced lordotic behavior, whereas the addition of HALO to the LRH infusates abolished this response. It was proposed that hypothalamic dopaminergic receptor activation may contribute to the stimulatory effects of LRH upon lordotic behavior.

Rat strain differences in copulatory behavior after para-chlorophenylalanine and hormone treatment.

The potentiation of masculine copulatory behavior in castrated male rats following systemic para-chlorophenylalanine (PCPA) treatment was found to be dependent upon the strain of rat and the recency of castration. Wistar (W) and Sprague-Dawley (SD) males displayed a decline in behavioral responsiveness to PCPA treatment following castration, the W males retaining their behavioral responsiveness longer than SD males. Castrated W males were also behaviorally more responsive to androgen replacement than were SD males. Ovariectomized W and SD females receiving estrogen replacement displayed a strain difference in hormonal sensitivity. In tests for feminine copulatory behavior, ovariectomized W females were behaviorally more sensitive to estrogen treatment than SD females. Ovariectomized and estrogen-primed females of both strains displayed potentiated lordotic behavior following chronic PCPA treatment. Concomitant treatment with dexamethasone to reduce adrenal steroid output abolished the potentiation of lordotic behavior found with PCPA treatment.

Hypothalamic injection of cycloheximide in the 4-day cyclic rat. Acute suppression of lordotic behavior and ovulation.

The results of serveral studies imply that estrogen can act upon the central nervous system via a protein synthetic step. Our objective was to determine if the intrahypothalamic (preoptic area, POA) injection of cycloheximide (CHX), an inhibitor of protein synthesis, at 17.00 h on diestrus II of the 4-day cycle altered lordotic behavior and (or) ovulation in the intact rat (sexual receptivity to males normally begins on the evening of proestrus as herein defined; ovulation occurs on estrus of the cycle). CHX-treated females were tested for lordotic behavior at 23.00 h on proestrus, then killed at 17.00 h on the following day. None of the CHX-POA rats were receptive to the males and 90% of these rats did not ovulate. Thus, CHX significantly suppressed sex behavior and ovulation in the cyclic rat.

Evidence that a factor besides progesterone, prolactin, or plasma-estradiol-binding protein inhibits estrogen-induced sexual receptivity in pregnant rats.

Daily administration of estradiol benzoate stimulated significantly less lordotic behavior in rats during the second half of pregnancy than in ovariectomized females that received subcutaneous progesterone implants, pituitary grafts that raised plasma prolactin, or both treatments combined. Following an initial facilitation of receptivity, females with progesterone implants showed only moderate reductions in lordosis quotients over 3 test days. The capacity of plasma from pregnant rats to bind estradiol was found to increase significantly during the second half of pregnancy. However, daily administration to pregnant rats of a synthetic estrogen, R 2858, which is not bound by plasma protein, was no more effective than estradiol benzoate in stimulating receptive behavior. Administration of estradiol benzoate also stimulated significantly lower levels of sexual behavior in pregnant females than in females in which pseudopregnancy had been prolonged by previous hysterectomy or induction of uterine decidualization. These findings suggest that some endocrine factor other than progesterone, prolactin, or estradiol-binding protein is primarily responsible for the potent suppression of behavioral responsiveness to estrogen which occurs in pregnant rats. It is suggested that 5 alpha-reduced androgens may cause these behavioral effects.

Hormonally mediated lordosis in female rats: Actions of flutamide and an aromatization inhibitor

Agents which interfere with androgen metabolism or receptor binding diminished androgen-induced, but not estrogen-induced lordotic behavior in female rats. The non-steroidal antiandrogen, flutamide, reduced the amount of lordosis displayed by females receiving 250 μg testosterone propionate (TP) plus progesterone (P). Flutamide (10 mg) did not affect the lordotic behavior of animals receiving estradiol benzoate (0.5 μg or 0.25 μg EB) plus P. Similarly, the aromatization inhibitor 1,4,6-androstatriene-3,17-dione (ATD; 10 mg) blocked TP plus P induced lordosis but failed to alter EB (10 μg) plus P induced lordosis. These results suggest that androgen-induced lordosis occurs after the testosterone molecule becomes bound to a receptor and is enzymatically converted to estrogenic metabolites.

Prednisolone modifies estrogen-induced sexual differentiation

It has been hypothesized that during the perinatal period androgen alters sexual development only after it is aromatized to estrogen intracellularly. The dose of estrogen needed to cause sexual differentiation, however, is larger than might be expected on the basis of the rate of estrogen production. This could be because estrogens are normally sequestered by plasma fetoneonatal estrogen-binding protein. To test this possibility neonatal female rats were administered a low dose of estrogen alone or in combination with the glucocorticoid prednisolone, a steroid that reduces blood levels of the estrogen-binding protein. The estrogen significantly inhibited adult lordotic behavior only when combined with the prednisolone, a treatment which had no effect by itself. The findings support the “aromatization hypothesis” and the suggestion that the fetoneonatal estrogen-binding protein protects the developing rat from differentiating effects of estrogen.

Dimethystilbestrol is not antiestrogenic for rat sexual behavior or gonadotropin secretion.

Antiestrogens have been used recently to elucidate the critical steps in the mechanism of action of estrogen (E). The present study was designed to assess the actions of the presumably potent antiestrogen dimethylstilbestrol (DMS) on E-induced sexual behavior. gonadotropin secretion regulation and uterine growth. DMS failed to block estradiol bencoate (EB)-induced lordosis behavior in rats and, when given concurrently with a low dose of EB, acted synergistically to facilitate receptivity. When given in combination with progesterone (P). DMS induced lordotic behavior in a dose-dependent manner. When given in combination with a low dose of estradiol (E2), DMS also facilitated the inhibitory effect of E2 on compensatory ovarian hypertrophy. When tested for its ability to block E2-stimulated uterine growth. DMS neither stimulated nor inhibited the action of E2. When given alone. DMS again induced sexual receptivity, while having no significant effect on uterine growth in the same animals. The antiestrogenic effects of DMS, as revealed by studies of uterine growth and vaginal cornification in mice, were not evident in the rat where DMS acted as an estrogen with respect to CNS functions, while having few or no estrogenic or antiestrogenic effects on peripheral target tissues.

Role of hypothalamic alpha and beta adrenergic receptors in the control of lordotic behavior in the ovariectomized-estrogen primed rat

The role of hypothalamic α- and β-adrenergic receptors in the control of lordotic behavior was studied by infusing α and β receptor stimulants and blockers into either the medial preoptic area (MPOA), arcuate-ventromedial area (ARC-VM) or lateral hypothalamic area (LHA) in ovariectomized (OVX), estrone primed rats. In the first experiment, OVX rats were primed with low doses of estrone (100–250 μg) in order to maintain low preinfusion receptivity (mean preinfusion L/M=0.169). The infusion of α receptor blockers, phentolamine and phenoxybenzamine; β receptor stimulants, isoroterenol; dopamine, norepinephrine; or epinephrine into either the MPOA or ARC-VM produced increases in lordotic behavior in OVX rats primed with low doses of estrone. Using the same protocol, MPOA or ARC-VM infusions of methoxamine, an α receptor stimulant, or propranolol, a β receptor blocker, produced decreases in the lordotic behavior. Infusions of any of these agents into the LHA had no effect upon sexual receptivity. In order to corroborate the pharmacological effects observed with low preinfusion receptivity, a second experiment was conducted in which high preinfusion receptivity (mean preinfusion L/M=0.898) was maintained by priming the OVX rats with higher doses of estrone. Using this second protocol, methoxamine and propranolol infusions into the MPOA or ARC-VM depressed lordotic response. However, further enhancement by α receptor blockade or β receptor stimulation from the initially near maximal lordotic response could not be obtained. A third experiment was designed to evaluate the effects of adrenergic receptor activity on the stimulatory effects of luteinizing hormone-releasing hormone (LRH) upon lordotic behavior. This protocol allowed comparisons among lordotic responses to MPOA and ARC-VM infusions of vehicle, LRH, LRH with methoxamine and LRH with propranolol. Infusions of LRH into the MPOA or ARC-VM significantly enhanced lordotic behavior, whereas the addition of either methoxamine or propranolol to LRH infusates abolished this response. Alternative mechanisms were proposed for the possible roles of α- and β-adrenergic receptors in the hypothalamic control of estrogen-induced and LRH-facilitated lordotic behavior.