The comparative potency of various steroids to complete the priming process for lordosis in guinea pigs

Ovariectomized adult guinea pigs were treated with a regimen of estradiol benzoate (0.2 μg/animal estradiol benzoate at hr 0 and 19) that was shown to be minimally effective for the induction of lordosis. They were then treated with 10, 20, or 80 mg of enclomiphene, 5, 20, 40, or 100 μg of estradiol, or testosterone, cortisol, estrone, estriol, diethylstilbestrol, catechol estradiol, or catechol estrone (all at a dose equivalent to 5 μg of estradiol) at hr 28. At hr 39 all females were given 0.5 mg progesterone, and subsequently tested for lordosis behavior. Of the various agents injected at hr 28 only estradiol (at all doses given), estrone, estriol, and diethylstilbestrol were effective in supporting display of lordosis behavior. The results indicate that the antiestrogen enclomiphene, the catechol estrogens, and at least some C₁₉ and C₂₁ steroids are weaker than E₂ or ineffective in facilitating lordosis behavior when given late in the priming period. Because previous work had shown that enclomiphene has partial estrogenic effects on lordosis behavior when administered early in the priming period (i.e., at hr 0, 19), it is suggested the early and late phases of the priming process induced by E₂ entail qualitatively different neural processes.     

Facilitation of Sexual Receptivity by Ventromedial Hypothalamic Implants of the Antiprogestin RU 486

RU 486 is known primarily as an antagonist to progestins and glucocorticoids. However, RU 486 has also been shown to have agonistic progestational properties in biochemical and behavioral studies. In the current study, RU 486 was implanted directly into tim ventromedial hypothalamus (VMH) to test for facilitative action on the receptive behavior of female ovariectomized Long-Evans rats primed with 5 μg of estradiol benzoate. Cannulae containing RU 486, progesterone (P), or empty cannulae were implanted 48 hr after estrogen priming. The lordosis quotient and the lordosis score were assessed 4 hr after the cannulae were lowered by a standardized test consisting of 10 mounts by a stimulus male. P and RU 486 significantly facilitated receptivity compared to blank implants in terms of lordosis quotient and lordosis score, with no significant difference between the hormone treatments. While only a single dose of each treatment was given in the current study, RU 486 facilitated lordosis when implanted to the VMH as well as progesterone in contrast to our previous results where the steroids were administered systemically.     

Independence of progesterone-induced facilitation and inhibition of lordosis behavior in ovariectomized guinea pigs

In ovariectomized Hartley guinea pigs, 15 μg progesterone was shown to facilitate lordosis in 48% of animals when administered 36 hr after a 3.3 μg injection of estradiol benzoate. This dose of progesterone also inhibited lordosis behavior in 65% of animals administered an additional 0.6 mg progesterone at 60 hr. Significant inhibition of lordosis response to the 0.6 mg progesterone existed among animals in which lordosis was not facilitated by the initial 15 μg dose of progesterone. These results show that progesterone-induced inhibition can occur without prior facilitation of lordosis as tested by the manual stimulation technique.     

Development of steroid-induced lordosis in female guinea pigs: effects of different estradiol and progesterone treatments, clonidine, and early weaning

The ability of developing ovariectomized (OVX) guinea pigs to display lordosis following a variety of steroid treatments which are behaviorally effective in adults was examined. Females OVX at 11 days of age did not display lordosis at 20 days of age, following treatment with several dose combinations of estradiol benzoate (EB, 10-50 micrograms) and progesterone (0.5-5 mg). By 30 days of age, 25% of the animals responded to EB plus progesterone, and by 40 days of age, adult-typical responses were observed. The developmental profile of responsiveness to steroids was not altered by varying the age at OVX, or by allowing pups to remain with a lactating mother. OVX females given estradiol (E2) implants did not exhibit progesterone-facilitated lordosis earlier than those treated with EB: however, the former group did show an unusually high incidence of progesterone-independent lordosis at 40 days of age. Twenty-day-old females also did not respond behaviorally to discrete pulses of E2 followed by progesterone, a treatment which was very effective in adults. Finally, lordosis was not facilitated in EB-primed, 20-day-old females by the alpha-noradrenergic agonist, clonidine, a treatment which was effective in adult females. These data illustrate a variety of conditions under which juvenile female guinea pigs do not exhibit steroid-induced lordosis. Since 20-day-old, EB-treated females also did not exhibit clonidine-facilitated lordosis, incomplete development of the central steroid-responsive and/or noradrenergic systems may contribute to the inability to display steroid-induced estrous behavior at this age.     

Effect of adrenalectomy and 5-hydroxytryptophan on phasic release of luteinizing hormone

The effect of 5-hydroxytryptophan (5-HTP) on serum progesterone and the possible role of adrenal progesterone in mediating stimulation by 5-HTP of phasic release of luteinizing. hormone (LH) were investigated in estradiol benzoate (EB)-treated ovariectomized rats. LH surges were induced in long-term (at least two weeks) ovariectomized rats by two injections of EB (20 micrograms/rat, s.c.) with an interval of 72 hrs. Administration of 5-HTP (50 mg/kg, i.p.) at 1000 hr in EB-treated ovariectomized rats resulted in a four-fold increase in serum progesterone within 30 mins, and significantly stimulated the LH surge at 1600 hr. This facilitative effect of 5-HTP on serum LH, but not progesterone, was further potentiated in rats pretreated with P-chlorophenylalanine (PCPA) 72 hrs earlier. Adrenalectomy shortly before 5-HTP administration attenuated the LH surge in saline treated controls, and completely blocked the facilitative effect of 5-HTP on the afternoon surge of LH in rats pretreated with PCPA 72 hrs earlier. On the other hand, chronic adrenalectomy (for 6 days) followed by hydrocortisone (0.2 mg/rat/day) replacement not only had no effect on the LH surge in saline treated controls, but also failed to prevent 5-HTP from facilitating the LH surge in PCPA pretreated rats. On the first day of bleeding, the basal LH value at 1000 hr in sham operated controls was significantly suppressed by PCPA pretreatment 48 hrs earlier. The second dose of 5-HTP administered on the next day failed to potentiate LH surges in either sham operated or adrenalectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-term (39–51 hr) exposure to estradiol in silastic capsules and lordosis behavior of guinea pigs

Three dosage levels of estradiol (E₂) in Silastic capsules were administered to ovariectomized guinea pigs for periods of 39 or 51 hr. At 39 hr., a systemic injection of progesterone was given, and hourly testing began. Dose-dependency of various aspects of lordosis behavior was established at both time intervals, and the serum E₂ levels produced by the medium and high dosage levels bracketed those found at the proestrus peak in intact guinea pigs. Although there were no significant behavioral differences between the 39- and 51-hr groups at the individual dosage levels, when the data from all three dosage groups were collapsed, exposure for 51 rather than 39 hr was seen to produce increases in maximum lordosis and heat-duration measures. The results suggest that subtle facilitative effects of estrogen occur very late in the estrogen-conditioning process (even after progesterone is administered).     

The effects of progestins,mineralocorticoids, glucocorticoids,and steroid solubility on the induction of sexual receptivity in rats

In the first experiment, progesterone and its 5α-reduced metabolite, 5α-dihydroprogesterone, dissolved in two different vehicles were compared for their effectiveness in facilitating lordosis behavior in ovariectomized estrogen-primed rats. When dissolved in oil vehicle, 5α-dihydroprogesterone was less effective than progesterone. However, when dissolved in Tween 80 solution, the two progestins were equally effective. In the second experiment, adrenal corticoids dissolved in Tween 80 solution were tested for their relative ability to facilitate sexual receptivity. Progesterone, desoxycorticosterone, and desoxycorticosterone acetate were equally effective in facilitating sexual receptivity. Aldosterone, corticosterone, and corticosterone acetate were no more effective than the vehicle in facilitating sexual receptivity.     

Facilitation of receptive behavior in estrogen-primed female rats by the anti-progestin, RU 486

The progestin receptor antagonist RU 38486 (henceforth referred to as RU 486) was tested for facilitative effects on female receptive behavior in ovariectomized Long-Evans rats primed with 2 micrograms estradiol benzoate (EB). RU 486 (0, 0.5, 1.6, or 5.0 mg) was administered 48 hr after estrogen priming. The lordosis quotient (LQ) and lordosis score (LS) were assessed 4 hr after RU 486 administration in a standardized test consisting of a 10-mount test by a stimulus male. A significant dose effect was found by both LQ and LS, with those subjects receiving 5 mg of RU 486 being significantly more receptive than vehicle control animals. Thus RU 486 acted as a weak progestin agonist under testing conditions typical for assessment of progestin facilitation of female sexual behavior in rats. Low levels of proceptive behavior (hops and darts) were seen in a minority of the tests, and did not vary systematically as a function of the dose of RU 486 administered. We also examined the effects of RU 486 given before progesterone (P) on receptivity in a blocking paradigm and confirmed previous reports that the antagonist significantly attenuates facilitation of sexual behavior when given in combination with P. A progestin receptor assay of the cytosols of the hypothalamus-preoptic area in estrogen-primed female rats treated with 5 mg RU 486 revealed a significantly greater depletion of available cytosolic P receptors than when rats were treated with a similarly facilitating dose of P (100 micrograms). The results suggest a possible dual mode of action for RU 486--a weak, receptor-mediated agonistic effect on sexual behavior when given alone to estrogen-primed rats, and a competitive blocking effect on receptivity when administered with P.     

GABAergic drugs and lordosis behavior in the female rat

Agents modifying GABAergic neurotransmission were administered to ovariectomized rats treated with different doses of estradiol benzoate (EB) + progesterone (P) or with EB alone. Hormone treatments were designed to induce an intermediate level of receptivity in order to be able to observe both stimulatory and inhibitory effects on lordosis behavior. Both the GABAA receptor agonist THIP and the GABAB receptor agonist baclofen inhibited lordosis behavior at doses from 20 and 5 mg/kg, respectively. The GABA transaminase inhibitor gamma-acetylen GABA (GAG) and the GABA agonist 3-aminopropanesulfonic acid had no effects, even when high doses were administered. The GABAA receptor antagonist bicuculline had no effect by itself nor did it block the effects of THIP. It is therefore suggested that the GABAA receptor is of slight importance in the control of lordosis behavior. No evidence could be found supporting the hypothesis that an interaction between P and GABA is important for hormone-induced receptivity. It does not appear likely that motor disturbances are responsible for the inhibitory effects of baclofen and THIP. The exact mechanism by which these drugs inhibit lordosis behavior is not clear at present.     

Homotypical sexual behavior in gonadectomized female and male rats treated with 5α-19-hydroxytestosterone: Comparison with related androgens

Ovariectomized female rats were treated in turn over several weeks with estradiol benzoate (EB), testosterone (T), 19-hydroxytestosterone (19HT), dihydrotestosterone (DHT) and 5α-19-hydroxytestosterone (5α19HT). EB was given as a single dose, the androgens were given over 3 days, and progesterone (P) was given 48 hr after the last injection. Each week, rats were tested for lordosis behavior 4–6 hr after P. High levels of receptivity were seen after EB + P, 19HT + P and T + P. Rats treated with DHT + P or 5α19HT + P were unreceptive. Four groups of castrated male rats were treated with T, 19HT, DHT and 5α19HT for 4 weeks starting from castration. In weekly sexual behavior tests, only T and 19HT maintained normal copulatory performance throughout the experiment. 19HT and 5α19HT had negligible effects on peripheral androgen target organs. The failure of 5α19HT to stimulate sexual behavior in rats of either sex supports the view that this steroid does not undergo central aromatization.     

Estrogen and the induction of lordosis in female and male prairie voles (Microtus ochrogaster)

Estrogen elicited lordosis in ovariectomized female prairie voles (Microtus ochrogaster). Treatment with estradiol benzoate (EB) was particularly effective if administered as multiple injections. Very high dose levels were not, in general, any more effective than lower doses. Individual animals typically showed lordosis within 24 to 48 hr following the onset of EB treatment and prolonged treatments did not increase the percentage of females responding to EB. Castrated male prairie voles did not respond with lordosis to repeated daily injections of 10 micrograms EB given for a period of 15 consecutive days.     

Estrogen and progesterone interactions influencing sexual and social behavior in the brown lemming, Lemmus trimucronatus.

The effects of estrogen and progesterone on the social and sexual behavior of brown lemmings, Lemmus trimucronatus, were investigated. The behavior of hormone-treated and untreated ovariectomized females and sexually vigorous males was observed in six consecutive daily 5-min dyadic encounters. Sexual receptivity, as measured by lordosis, and other social behaviors including nasonasal contact, boxing postures, allogrooming, perineal investigation, and male mounting increased following 48 hr of exposure to daily injections of 0.5 μg estradiol benzoate (EB). Lordosis in EB-primed females was not facilitated or inhibited by short-term (4 hr) exposure to 0.5 mg progesterone (P). Long-term (greater than 24 hr) exposure to P apparently inhibited lordosis and other social behaviors in EB-treated females, although males continued to attempt to mount these females. In EB-treated females a dramatic increase in threat-leaps, directed by the female toward the male, was observed within 4 hr of P injection. Threat-leaps declined when P was withdrawn. Threat-leaps were also observed in ovariectomized females after prolonged exposure to P only (0.5 mg/day). Vaginal perforation and cornification were first apparent 48 hr after EB injection. P-alone treated ovariectomized females also showed vaginal perforation but cornified cells were infrequent and these animals did not show lordosis.     

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

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

Anti-estrogenic suppression of the lordosis response in female rats

The anti-estrogen, CI 628, was used to suppress the lordosis response induced by sequential injections of estrogen and progesterone in ovariectomized (OVX) female rats. Appropriate doses of CI 628 completely abolished sexual receptivity in females administered estradiol benzoate (EB) in sesame oil. This behavioral effect could be attenuated by providing increased quantities of EB or decreased quantities of CI 628. Anti-estrogenic effects on lordosis induced by free estradiol in saline (E) were assessed after first establishing behaviorally equivalent doses of EB and E. This was accomplished by determining thresholds for E-induced lordosis. OVX females were approximately seven times less sensitive to E than to EB. CI 628 had no significant effects on E-induced lordosis, in contrast to the complete abolition of lordosis in females treated with behaviorally equivalent EB doses. A possible mechanism to explain this differential responsiveness of EB- and E-treated females is discussed.     

Molecular mechanisms of the antihormonal and antiimplantation effects of norethisterone and its a-ring reduced metabolites

Norethisterone (NET) has been used as a contragestational postcoital agent. It is biotrans-formed to 5α dihydro-NET (5α-NET) and 3β,5α tetrahydro-NET (3β,5α-NET) in target tissues. The participation of these metabolites in NET effects is unknown. We have examined the antiimplantation and antiprogestational effects of NET and its metabolites, in adult mated female rabbits, by assessing the number of implantation sites and the expression products of the uteroglobin (UTG) gene in the uterus, and by comparing them with those of RU-486 and estradiol. Steroids were daily administered s.c. at several doses for 7 consecutive days, starting 24 hr after coitus. To assure that fertilization occurred in all animals, the presence of early pregnancy factor was determined. The results demonstrated that high doses (5 mg/kg) of NET reduced both implantation and the expression of the UTG gene. On the other hand, lower doses (1.5 mg/kg) of 5α-NET produced an antiimplantation effect and suppressed UTG synthesis and its mRNA. These effects were similar to those of RU-486. At lower doses (1 mg/kg), both estradiol and the estrogenic metabolite 3β,5α-NET were also effective in inhibiting implantation and UTG gene expression. The overall results suggest that NET metabolites exert antiimplantation and antiprogestational effects through their interaction with progesterone and estrogen receptors, and provide an explanation for the molecular mechanisms involved in the postcoital contraceptive action of NET. © 1995 Wiley-Liss, Inc.     

A role for Src kinase in progestin facilitation of estrous behavior in estradiol-primed female rats

This study tested the hypothesis that the Src/Raf/MAPK signaling pathway is involved in the facilitation of the lordosis and proceptive behaviors induced by progesterone (P) and its ring A-reduced metabolites in ovariectomized, estradiol-primed rats. Intraventricular (icv) infusion of PP2 (7.5, 15 and 30 µg), a Src kinase inhibitor, significantly depressed P-dependent estrous behavior (lordosis and proceptivity) in estradiol-primed rats. Icv infusion of 30 µg of PP2 also significantly attenuated estrous behavior induced by the ring A-reduced P metabolites 5α-dihydroprogesterone (5α-DHP) and 5α-pregnan-3α-ol-20-one (allopregnanolone). PP2 did not inhibit estrous behavior induced by administration of high doses of estradiol alone to ovariectomized rats. We also assessed if the ventromedial hypothalamus (VMH) is one of the neural sites at which progestins activate Src signaling to facilitate estrous behavior. Bilateral administration of 15 µg of PP2 into the VMH inhibited the stimulation of both lordosis and proceptive behaviors elicited by subcutaneous P administration to estradiol-primed rats. These results suggest that progestins act through Src/Raf/MAPK signaling to initiate estrous behaviors in estrogen-primed rats. This event is one component of the cellular pathways leading to the display of estrous behaviors induced by P and its ring A-reduced metabolites in female rats.     

Effects of the antiestrogens,MER-25 and CI 628, on rat and hamster lordosis

Antiestrogens were used to test the hypothesis that estrogen exerts a “maintenance,” as well as a “priming,” effect on rat and hamster sexual receptivity as it apparently does for guinea pigs. MER-25 (75 or 150 mg/kg) significantly reduced rat LQ when given ?2 hr or 8 hr after EB injection. MER-25 given at 34 hr (2 hr prior to P) failed to diminish rat LQ. With hamsters, MER-25 in large doses (750 mg/kg) given either at ?2 hr or 34 hr reduced lordosis duration to 40% of controls, but this effect was confounded by severe illness among the MER-25 injected animals. Lower doses failed to block behavior, but still produced some toxicity. CI 628 (50 mg/kg) greatly reduced hamster lordosis duration and increased lordosis latency when given 0 hr, but not 34 hr, after EB. The results are consistent with similar previous work on rats and do not support the concept of estrogen “maintenance” in either rats or hamsters.     

Hysterectomy-induced facilitation of lordosis behavior in the rat

The present study investigated the effect of hysterectomy on hormone-induced lordosis behavior. Lordosis quotients (LQ) were measured in hysterectomized-ovariectomized (HO) and ovariectomized-sham hysterectomized (OSH) rats after several treatments including either estradiol benzoate (EB) alone or EB plus progesterone (P) 44 hr later. Testing consisted of placing the females with sexually active males 48 hr after EB. In Experiment 1, HO animals treated with 5 μg/kg EB and 0.5 mg P had significantly higher LQs than OSH animals; groups treated with 10 μg/kg plus P were not different. Experiment 2 showed that a single injection of 50 μg/kg EB resulted in equally high levels of receptivity in both groups. The LQs of HO animals injected with 3 μg/kg for 4 days did not differ from those of OSH animals; however, the administration of 0.5 mg P 24 hr after the fourth EB injection resulted in significantly higher LQs in the HO group (Experiment 3). In Experiment 4, HO rats injected with 5 μg/kg EB and 0.1 mg P 44 hr later displayed higher levels of lordosis behavior than OSH animals. It was concluded that hysterectomy facilitated the lordosis behavior of ovariectomized rats injected with both EB and P and that the mechanism for this potentiation remains to be determined.     

Progesterone facilitation of lordosis in male and female Sprague-Dawley rats following priming with estradiol pulses

Adult male Sprague-Dawley rats rarely exhibit progesterone-facilitated lordosis following steroid treatments which are effective in females. In contrast, progesterone-facilitated lordosis has been observed following priming with estradiol pulses in another strain. The aim of this study was to compare progesterone-facilitated feminine sexual behavior in adult male and female Sprague-Dawley rats following priming with estradiol benzoate (EB) or estradiol pulses. Female sexual behavior was measured in adult, gonadectomized males and females treated as follows: Two pulses of estradiol followed by progesterone or oil the next day; EB (two doses) for 3 days, and progesterone or oil the next day. These protocols were repeated at 4- or 6-day intervals, respectively. Progesterone-facilitated lordosis was observed consistently in both sexes treated with estradiol pulses. By the fifth test, lordosis quotients did not differ between the sexes, but the lordosis ratings in progesterone-treated males remained lower than those observed in females. Proceptivity (hop-darting) was facilitated by progesterone in females, but was never observed in males. Lordosis was induced in both sexes by 15 micrograms EB, but was not reliably facilitated by progesterone. Treatment with the lower dose of EB (1.5 micrograms) induced high levels of receptivity in females (occasionally facilitated by progesterone), but not in males regardless of subsequent treatment (i.e, progesterone or oil). These data suggest that progesterone-facilitated lordosis can be induced in male Sprague-Dawley rats, if a regimen of estradiol pulses is used. Thus, the brain of the adult male is not inflexibly differentiated with regard to progesterone facilitation of feminine receptive behavior.     

Mu-, delta-, and kappa-opioid receptor agonists selectively modulate sexual behaviors in the female rat: differential dependence on progesterone.

Previous studies suggested that opioid receptor agonists infused into the lateral ventricles can inhibit (through mu receptors) or facilitate (through delta receptors) the lordosis behavior of ovariectomized (OVX) rats treated with estrogen and a low dose of progesterone. The present study investigated the behavioral and hormonal specificity of those effects using more selective opioid receptor agonists. Sexually experienced OVX rats were implanted stereotaxically with guide cannulae aimed at the right lateral ventricle. One group of rats was treated with estradiol benzoate (EB, 10 micrograms) 48 hr and progesterone (P, 250 micrograms) 4 hr before testing, whereas the other group was treated with EB alone. Rats were infused with different doses of the selective mu-receptor agonist DAMGO, the selective delta-receptor agonist DPDPE, or the selective kappa-receptor agonist U50-488. The females were placed with a sexually vigorous male in a bilevel chamber (Mendelson and Gorzalka, 1987) for three tests of sexual behavior, beginning 15, 30, and 60 min after each infusion. DAMGO reduced lordosis quotients and magnitudes significantly in rats treated with EB and P, but not in rats treated with EB alone. In contrast, DPDPE and U50-488H increased lordosis quotients and magnitudes significantly in both steroid-treatment groups. Surprisingly, measures of proceptivity, rejection responses, and level changes were not affected significantly by mu or kappa agonists, although proceptivity and rejection responses were affected by DPDPE treatment. These results suggest that the effects of lateral ventricular infusions of opioid receptor agonists on the sexual behavior of female rats are relatively specific to lordosis behavior. Moreover, the facilitation of lordosis behavior by delta- or kappa-receptor agonists is independent of progesterone treatment, whereas the inhibitory effect of mu-receptor agonists on lordosis behavior may require the presence of progesterone.