Sexual Receptivity In Ovariectomized Rats Research Articles

Immediate early gene activity-regulated cytoskeletal-associated protein regulates estradiol-induced lordosis behavior in female rats.

Sensory feedback is an important component of any behavior, with each instance influencing subsequent activity. Female sexual receptivity is mediated both by the steroid hormone milieu and interaction with the male. We tested the influence of repeated mating on the level of sexual receptivity in ovariectomized rats treated with estradiol benzoate (EB) once every fourth day to mimic the normal phasic changes of circulating estradiol. Females were divided into two groups: naïve, which were tested for lordosis behavior once, and experienced rats, which were tested for lordosis after each EB injection. To monitor the effect of mating, the number of neurons expressing the immediate early gene activity-regulated cytoskeleton-associated protein (Arc) were counted in the mediobasal hypothalamus. Females were unreceptive following the first EB treatment, but the mating induced Arc expression. In naïve rats, each subsequent EB injection increased the levels of sexual receptivity. This ramping was not observed in experienced rats, which achieved only a moderate level of sexual receptivity. However, experienced females treated with EB and progesterone were maximally receptive and did not have Arc expression. To test whether the expression of Arc attenuated lordosis, Arc antisense oligodeoxynucleotides (asODN) were microinjected into experienced females' arcuate nuclei. Arc expression was attenuated, and the experienced EB-treated females achieved maximal sexual receptivity. These results demonstrate that Arc expression in the hypothalamus might influence future sexual receptivity and provides evidence of learning in the arcuate nucleus. The loss of Arc results in unrestrained sexual receptivity.

Cyclic Estradiol Treatment Normalizes Body Weight and Restores Physiological Patterns of Spontaneous Feeding and Sexual Receptivity in Ovariectomized Rats

Hypothalamic–pituitary–gonadal axis function strongly influences feeding and body weight in cycling females in many species. To test the sufficiency of cyclic variations in plasma estradiol to reproduce normal patterns of spontaneous feeding, food intake, and body weight, ovariectomized Long–Evans rats were subcutaneously injected every fourth day with 2 μg estradiol benzoate or with the oil vehicle alone. Cyclic estradiol treatment completely normalized the trajectory of body weight gain and total food intake through seven treatment cycles. The hyperphagia of ovariectomized rats was expressed as an increase in spontaneous meal size. Meal frequency decreased, but not enough to compensate for the increase in meal size. Estradiol treatment normalized both parameters. In addition, cyclic estradiol treatment produced a further phasic decrease in meal size (and increase in meal frequency) and a decrease in food intake during the second night after injection. This phasic change is similar to the feeding changes occurring during estrus in intact rats. Sexual receptivity was measured during the eighth estradiol treatment cycle, 4 h after injection of 0.5 mg progesterone. Lordosis scores at the time of the treatment cycle modeling estrus were maximal, and scores at the time modeling diestrus were slightly increased over those of rats that did not receive estradiol. Finally, plasma estradiol levels, measured during the ninth treatment cycle, revealed a near-normal cyclic pattern of plasma estradiol levels. These results provide the first demonstration that the induction of a cyclic, near-physiological pattern of plasma estradiol is sufficient to maintain normal levels of body weight, spontaneous feeding patterns, total food intake, and (together with progesterone) sexual receptivity in ovariectomized rats.

Anabolic-Androgenic Steroid Effects on Sexual Receptivity in Ovariectomized Rats

Anabolic-androgenic steroid (AAS) compounds are synthetic androgens taken by athletes to increase physical strength and endurance. Recent studies in our laboratory have demonstrated that AAS administration disrupts the estrous cycle of Long–Evans rats. The present experiments examined the effects of six commonly abused AAS compounds on sexual receptivity in ovariectomized rats. Adult female Long–Evans rats received estradiol benzoate (EB; 2.0 μg/day sc) for 6 consecutive days followed by 15 days of EB concurrent with daily sc injections of 7.5 mg/kg of one of the following AAS compounds: 17α-methyltestosterone, methandrostenolone, nandrolone decanoate, stanozolol, oxymetholone, testosterone cypionate, or the oil vehicle. On Day 15, all female rats received progesterone (1.0 mg/rat) 4 h before testing. Tests for sexual receptivity were conducted on Days 3, 6, 14, and 15 of AAS treatment. Although the time course of AAS effects on sexual receptivity varied, some overall effects were clear. For example, 17α-methyltestosterone, methandrostenolone, nandrolone decanoate, and stanozolol interfered with the display of sexual receptivity on Day 14, whereas oxymetholone and testosterone cypionate had no effect. Rats in all groups displayed high levels of sexual receptivity after receiving progesterone on Day 15. Our results show that AAS compounds vary in their degree of inhibition of female sexual behavior in ovariectomized rats.

Lesions in the preoptic area suppressed sexual receptivity in ovariectomized rats with estrogen implants in the ventromedial hypothalamus

The objective of the experiment was to determine if electrolytic ablation of a portion of the preoptic area (POA) influenced the activation of female lordosis behavior by implants of estradiol benzoate in the ventromedial hypothalamus (VMH) of ovariectomized (OVX) rats. Two weeks after ovariectomy, rats received either bilateral electrolytic lesions (2 mA for 10 sec in Experiment 1, or 1 mA for 5 sec in Experiment 2) in the POA, or sham lesions (all procedures except passage of current). On the same day (day 0 of the experiment) thirty-gauge stainless steel cannulae containing crystalline estradiol benzoate were stereotaxically placed bilaterally into the VMH of all the rats. Subsequently, females were tested for the lordosis response to stud males on days 2, 4, 6, 8, and 10 in Experiment 1 or on days 7, 14 and 15 in Experiment 2. All rats received 0.5 mg progesterone (SC) only before the last test. A female was considered sexually receptive if she exhibited a lordosis quotient (LQ) greater than or equal to 10 (LQ=No. lordosis responses/10 mounts by male × 100). The frequencies for sexual receptivity in females with POA lesions were significantly lower than those for control females without lesions in both experiments. Additionally the degree of receptivity (lordosis quotient) was significantly lower on each test day for rats with POA lesions than that for rats without POA lesions. The results imply that the maintenance of the integrity of the POA under this experimental condition was important for the expression of the facilitative influence of the VMH on lordotic responsiveness.

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.

Induction of sexual receptivity in ovariectomized rats by pulse administration of oestradiol-17 beta.

Constant-release implants filled with oestradiol-17 beta induced sexual receptivity in ovariectomized rats in response to progesterone treatment if they were implanted 32 h before behavioural testing. A 20 h period of exposure to oestradiol, by implantation 32 h before testing and removal of the implants 20 h later, was sufficient for induction of the behaviour. The exposure time necessary for behavioural responses could be further reduced to two 4 h periods, between 32 and 28 h and between 16 and 12 h, before testing. Serum levels of oestradiol were raised within 1 h of oestradiol implantation and declined rapidly after implant removal. A single injection of oestradiol benzoate was much more potent than a single injection of oestradiol in inducing sexual receptivity in ovariectomized rats, but this difference in potency was reversed if two appropriately timed injections were given. Oestrone- or oestriol-filled implants were relatively ineffective in inducing sexual receptivity. It is suggested that oestradiol has to be present at crucial time points to prepare an ovariectomized rat to respond behaviourally to progesterone treatment and that oestradiol is the principal oestrogen in the stimulation of sexual behaviour in female rats.

Evidence that progesterone does not inhibit the induction of sexual receptivity by oestradiol-17 beta in the rat.

Progesterone-filled constant-release implants facilitated the induction of sexual receptivity in ovariectomized rats given implants of oestradiol-17 beta precisely 32 h before testing, irrespective of the time of implantation. Inhibition by progesterone implants of the behavioural response to an injection of progesterone occurred after the facilitation 32 h after oestradiol implantation. Sexual receptivity could be induced in pseudopregnant rats in the absence of progesterone treatment by injection of 1 microgram oestradiol 32 and 16 h before testing at a time when endogenous serum levels of oestradiol were low and progesterone levels were high. The behavioural response of ovariectomized rats implanted with oestradiol and tested daily was unaffected by implantation of progesterone at the time of oestradiol implantation, although serum levels of progesterone varied with the number of progesterone implants inserted. Inhibition by progesterone implants of the behavioural response to an injection of progesterone 6 h before behavioural testing occurred only if the progesterone implants were present for at least 32 h of a 48 h period. Serum levels of progesterone were raised within 1 h of progesterone implantation and declined within a 6 h period after implant removal. It is concluded that progesterone does not inhibit the behavioral effect of oestradiol and that progesterone does not play an inhibitory role in the regulation of the behavioural oestrous cycle in our strain of rats.

Effect of anti-androgen and 5 alpha-reductase inhibitor on hormone-induced sexual behavior during pregnancy in the rat.

The ability of estradiol benzoate (EB) or EB plus progesterone (P) to induce estrous behavior in the rat is substantially decreased during pregnancy, an effect not solely attributable to high titers of circulating P. The possible contribution of 5o-dihydrotestosterone (DUT) to the inhibition of lordotic responsivenessduring pregnancy was studied. As few as three daily injectionsof DUTpropionate (DHTP) to ovariectomized rats treated concurrently with EB caused a significant reduction in sexual receptivity.Furthermore, daily treatment of ovariectomized rats with an androgen receptor-blocker (Flutamide, 5 mg/day) in combination with EB and DHTP facilitated the display of lordosis. The 5o-reductase inhibitor, 4-androstan-3-one, 17j3 carboxylic acid (1 7�C, 3 mg/day), also resulted in increased sexual receptivity in ovariectomized rats treated concurrently with testosterone propionate (TP). However, pregnant females given 7-9 daily injections of Flutamide or 1 7ilC in these dosages were no more receptive than control animals in tests conducted on postcoital Day 16 following treatment with EB plus P. These data suggest that factors other than circulating DHT are primarily responsible for the suppression of behavioral responsivenss to ovarian hormones which occurs in the rat during pregnancy.

Induction of sexual receptivity by oestradiol benzoate in cyclic female rats: influence of ovarian secretions before injection of oestradiol benzoate.

The ability of cyclic female rats to show sexual receptivity 24 h after an injection of 2 microgram oestradiol benzoate (OB) was lost 24 h after ovariectomy. Exposure of cyclic rats to antioestrogen (nitromophene monocitrate) implants 24 h before ovariectomy and OB treatment prevented the latter from inducing sexual receptivity within 24 h of administration. Treatment of ovariectomized rats with constant release implants filled with an oil solution of 15 microgram oestradiol/ml had no behavioural effect in itself, but prepared the rats to show lordosis 24 h after administration of OB. Progesterone treatment (4 mg) induced sexual behaviour in cyclic rats on days other than that of the oestrous cycle when the rats are normally receptive. Evidence is presented that a lower level of oestradiol stimulation than that present duing pro-oestrus was needed for the induction of sexual receptivity in ovariectomized rats. It is suggested that the low basal level of oestradiol which was present throughout the oestrous cycle was necessary for the induction of sexual receptivity and that an increase in oestradial stimulation served to increase the behavioural sensitivity to progesterone.

Effects of subcutaneous implants of progesterone on the induction and duration of sexual receptivity in ovariectomized rats.

Ovariectomized rats, pre-implanted with elastomer capsules containing oestradiol, became sexually receptive after exposure to progesterone (implanted in elastomer capsules) for 4-6 h. Implantation of progesterone capsules facilitated receptivity in oestradiol-implanted rats independently of both previous exposure to progesterone implants and the presence of progesterone at the time of implantation. The duration of sexual receptivity in ovariectomized rats implanted with oestradiol and progesterone capsules was dependent upon the length of both the oestradiol and progesterone capsules, but the decline in sexual behaviour of receptive rats was independent of the continued presence of either oestradiol or progesterone. Repeated implantation of progesterone capsules at 6 hourly intervals prevented the decline of sexual receptivity.

Effects of the prostaglandin synthesis inhibitor, indomethacin on estrogen- and estrogen plus progesterone-induced sexual receptivity in ovariectomized rats

Implants of crystalline PGE2 in the basal preoptic-anterior hypothalamic areas stimulates high levels of sexual receptivity in ovariectomized, estrogen-primed rats. Indomethacin, which blocks the synthesis of PGE2, failed to inhibit either estrogen- or estrogen plus progesterone-induced receptivity. Neither intracerebral nor subcutaneous administration of indomethacin diminished the display of steroid induced reproductive behavior without also causing a depression in open-field activity, and in some cases, causing gastrointestinal problems and even death. These results suggest that prostaglandin synthesis is not a required step in the mechanism by which estrogen and progesterone exert their behavioral effects. The possibility that PGE2 and LH-RH synthesis and/or release might contribute to a collateral mechanism for the induction of sexual receptivity was discussed.

Effects of prostaglandin E 2 and indomethacin on sexual behavior in the female rat

Prostaglandin E 2 (PGE 2) facilitated sexual behavior in estrogen-primed ovariectomized or ovariectomized-adrenalectomized rats. Administration of indomethacin, an inhibitor of prostaglandin synthesis, attenuated the effectiveness of estrogen and progesterone in inducing sexual receptivity in ovariectomized rats. Concurrent administration of PGE 2 with indomethacin restored sexual behavior only when administered early in the estrogen-priming period but not if administered along with the progesterone. Our studies support the likelihood of a role of prostaglandins in the control of sexual behavior in the female rat.

Effect of clomiphene citrate and its isomers on sexual behavior in ovariectomized rats.

The effects of clomiphene citrate and its two isomers were evaluated as to their ability to induce sexual receptivity in ovariectomized rats. Ten–µg, 100–µg, 1–mg and 10–mg doses of either cis—clomiphene—, trans—clomiphene—, or racemic clomiphene—citrate were given to individual groups of 8 rats for 10 days. On the eleventh day each group received 500 µg sc of progesterone in oil. Ten mount behavior tests were performed on the afternoon of treatment days 5, 10 and 11. One mg of trans—clomiphene citrate caused a significant increase in the lordosis response on day 5. Cis—clomiphene citrate was ineffective in facilitating the lordosis response. Racemic clomiphene citrate, alone, caused a significant increase in the lordosis response after 10 mg per day for 10 days and treatment with progesterone increased the lordosis response in animals treated with either 1 or 10 mg per day. Based on the response to trans—clomiphene, clomiphene citrate did not cause as significant an increase in the lordosis response as would be expected for a racemic mixture. Similarly, the trans isomer was 10 times more effective than the cis isomer in causing vaginal cornification and 20 times more effective than clomiphene citrate. Because of these unexpected responses with the racemic form of clomiphene it is suggested that the cis isomer may interfere with the trans isomer. It is concluded that the trans isomer of clomiphene is particularly effective in facilitating sexual behavior. (Endocrinology92: 1079, 1973)

Effect of anti-estrogen on steroid induced sexual receptivity in ovariectomized rats

Ovariectomized female rats were injected with estradiol benzoate (EB) and 48 hr later with progesterone, in order to induce sexual receptivity as measured by the lordosis quotient (LQ) over 30 mounts. The antiestrogen CN-55, 945-27 (CN) markedly reduced the LQ when administered by gavage at the time of EB injection. There was no effect of CN when given at the time of progesterone injection, nor when given 24 hr after EB. These data demonstrate that CN counteracts the behavioral effects of EB, and suggest that exposure of the brain to EB for less than 24 hr is sufficient to trigger processes which culminate in the lordosis response of the female rat to the male.