Repeated estradiol benzoate treatment protects against the lordosis-inhibitory effects of restraint and prevents effects of the antiprogestin, RU486.

The following experiment was designed to test two specific questions: (1) Does the antiprogestin, RU486, reduce emergence of lordosis behavior and/or proceptivity in rats given repeated treatment with 10µg estradiol benzoate (EB) and/or a single high dose (40µg) of EB? (2) Does RU486 accentuate the effects of a 5min restraint experience on sexual behaviors in rats given repeated treatment with estradiol benzoate (EB) and/or a high dose of EB? RU486 was used to determine if a high dose and/or repeated treatment with EB enhanced proceptivity and reduced the response to mild stress through an intracellular progesterone receptor-mediated process. Ovariectomized Fischer rats were injected with a single dose of 10 or 40µg estradiol benzoate (EB) or received 4consecutiveweeks of treatment with 10µg EB. Forty-eight hours after the last treatment with EB, rats were injected with 5mg/kg of the antiprogestin, RU486, or the RU486 vehicle. That afternoon, rats were monitored for sexual behaviors. Sexually-receptive rats were then restrained for 5min and again tested for sexual behaviors. A separate set of rats received 4consecutiveweeks of 10µg EB treatment before treatment with a higher (5mg/rat) dose of RU486. Lordosis to mount ratios, lordosis quality, proceptivity, and resistance were monitored. RU486 had no effect on the emergence of sexual behaviors but did accentuate the lordosis-inhibitory effect of restraint in rats given a single treatment with EB. Rats treated for 4consecutiveweeks with EB showed no effect of restraint and were unaffected by RU486. These findings lead to the suggestion that repeated EB initiates select behavioral effects that are not mimicked by acute EB treatment and that the intracellular progesterone receptor may not be involved.

Allopregnanolone's attenuation of the lordosis-inhibiting effects of restraint is blocked by the antiprogestin, CDB-4124.

A brief restraint experience reduces lordosis behavior in ovariectomized females that have been hormonally primed with estradiol benzoate. The addition of progesterone to the priming prevents the lordosis inhibition. Based on prior studies with an inhibitor of progesterone metabolism, we have implicated the intracellular progesterone receptor, rather than progesterone metabolites, as responsible for this protection. However, the progesterone metabolite, allopregnanolone (3α-hydroxy-5α-pregnan-20-one), also prevents lordosis inhibition after restraint. In a prior study, we reported that the progestin receptor antagonist, RU486 (11ß-(4-dimethylamino)phenyl-17ß-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one), attenuated the effect of allopregnanolone. Because RU486 can also block the glucocorticoid receptor, in the current studies, we evaluated the effect of the progestin receptor antagonist, CDB-4124 (17α-acetoxy-21-methoxy-11ß-[4-N,N-dimethyaminopheny]-19-norpregna-4,9-dione-3,20-dione), which is relatively devoid of antiglucocorticoid activity. Ovariectomized, Fischer rats were injected with 10 µg estradiol benzoate. Two days later, rats received either 60 mg/kg CDB-4124 or 20% DMSO/propylene glycol vehicle 1 h before injection with 4 mg/kg allopregnanolone. After a pretest to confirm sexual receptivity, rats were restrained for 5min and immediately tested for sexual behavior. Lordosis behavior was reduced by the restraint and attenuated by allopregnanolone. Pretreatment with CDB-4124 reduced allopregnanolone's effect. These findings support prior suggestions that allopreganolone reduces the response to restraint by mechanisms that require activation of the intracellular progesterone receptor.

Prior hormonal treatment, but not sexual experience, reduces the negative effects of restraint on female sexual behavior.

These experiments were designed to determine if prior sexual experience reduced the negative effect of mild stress on female sexual behavior. In the first experiment, ovariectomized rats were hormonally primed with estradiol benzoate and progesterone for 3 consecutive weeks during which they received six mating experiences in a male's home cage or received no sexual experience. The next week, females were primed with 10 µg estradiol benzoate two days before a 5 min restraint. Both groups were resistant to the negative effects of the stressor. In the second experiment, females received 0, 1, 2, or 3 weeks of 10 µg estradiol benzoate and were restrained on the fourth week after priming with 10 µg estradiol benzoate. Rats without prior hormonal priming showed a decline in lordosis behavior after restraint but prior priming with estradiol benzoate reduced this effect. In the third experiment, rats received 3 weeks of hormonal priming with estradiol benzoate and progesterone with or without sexual experience. An additional group received no sexual experience or hormonal priming. Females were then given a 3-week hormone vacation before testing in the restraint paradigm. All groups showed a decline in lordosis behavior after restraint. The fourth experiment was identical to the third except that sexual experience in the male's cage and in a pacing apparatus were compared. There was no effect of either type of sexual experience on the response to restraint. Possible mechanisms responsible for effects of prior hormonal priming are presented and the absence of an effect of sexual experience is discussed in comparison to findings in male rats.

Comparison of female Fischer and Sprague-Dawley rats in the response to ketanserin.

The effects of the 5-HT2A/2C receptor antagonist, ketanserin, on lordosis behavior were examined in hormonally primed, ovariectomized Fischer and Sprague-Dawley females. Rats were primed with 0.067µg/g body weight estradiol benzoate and 3.33µg/g body weight progesterone. After a pretest for sexual behavior, rats were injected with 0.416 to 10mg/kg ketanserin. In both strains, lordosis behavior, lordosis quality, and proceptivity were significantly reduced by ketanserin. There was modest evidence of a strain difference with Sprague-Dawley females slightly more sensitive to ketanserin. In a second experiment, the effects of 10mg/kg fluoxetine, 1mg/kg ketanserin, and their combination were examined to determine if the two drugs would have additive effects on sexual behavior. There was no evidence that the drugs were additive in their effect and the strains did not differ in their response to the combined treatment. These findings are discussed in relation to prior evidence for strain differences in the sexual behavioral response to fluoxetine and to a receptor agonist acting preferentially at 5-HT1A receptors.

RU486 blocks effects of allopregnanolone on the response to restraint stress.

These experiments were designed to provide information about the potential involvement of progesterone receptors in the ability of allopregnanolone (3α-hydroxy-5α-pregnan-20-one) to reduce the lordosis-inhibiting effects of restraint stress. Ovariectomized Fischer rats were hormonally primed with 10 µg estradiol benzoate and 4 mg/kg allopregnanolone or vehicle. One hour before allopregnanolone, rats were injected with the progesterone receptor antagonist, RU486 (11ß-(4-dimethylamino)phenyl-17ß-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one), or vehicle. Four hours after allopregnanolone or vehicle, sexual behavior was examined before and after a 5-min restraint stress. Lordosis behavior of rats primed only with estradiol benzoate declined after the 5 min of restraint while allopregnanolone prevented this decline. RU486 attenuated the ability of allopregnanolone to prevent the restraint-induced decline in lordosis behavior. These findings are consistent with earlier suggestions that progesterone receptors are involved in allopregnanolone's ability to reduce the effects of restraint stress.

Sprague-Dawley and Fischer female rats differ in acute effects of fluoxetine on sexual behavior.

INTRODUCTION: The selective serotonin reuptake inhibitor (SSRI), fluoxetine, leads to sexual dysfunction in a substantial proportion of women. In studies with the Fischer inbred rat, the 5-HT(1A) receptor has been implicated in this sexual dysfunction. Whether this association with 5-HT(1A) receptors holds for other rat strains is not known. AIM: The effects of acute fluoxetine on sexual behavior in two strains of rats that differ in their response to a 5-HT(1A) receptor agonist were examined. Whether the strain difference is comparable in naturally cycling and hormonally primed, ovariectomized rats was determined. METHODS: Proestrous rats and ovariectomized rats, hormonally primed with estradiol benzoate and progesterone, were treated with varying doses of fluoxetine. Sexual behavior was examined before and after treatment with the SSRI. MAIN OUTCOME MEASURES: Lordosis to mount ratios, lordosis quality, and proceptive behaviors were quantified. Sprague-Dawley and Fischer females were compared on each of these measures. The IC(50) for inhibition of lordosis behavior was determined. RESULTS: In both the intact and the hormonally primed, ovariectomized model, Sprague-Dawley females were less sensitive to the effects of fluoxetine on sexual behavior. In both groups, fluoxetine showed dose dependency in behavioral inhibition, but a higher dose was required for Sprague-Dawley than for Fischer females. Naturally cycling, proestrous rats required a higher dose of fluoxetine than hormonally primed ovariectomized rats to produce significant inhibition of sexual behavior. Thus, the strain difference in the response to fluoxetine does not parallel strain differences in the response to a 5-HT(1A) receptor agonist. CONCLUSIONS: Acute treatment with fluoxetine inhibits lordosis behavior in both Fischer and Sprague-Dawley females and the strain difference cannot be explained by reported strain differences in the response to a 5-HT(1A) receptor agonist. Fluoxetine's inhibition of female rat sexual behavior may involve effects of the SSRI in addition to activation of the 5-HT(1A) receptor.

An antiprogestin, CDB4124, blocks progesterone's attenuation of the negative effects of a mild stress on sexual behavior.

These experiments were designed to test the hypothesis that a progesterone receptor antagonist would block progesterone's ability to reduce the negative effects of a 5 min restraint on female rat sexual behavior. Ovariectomized Fischer rats were injected with 10 µg estradiol benzoate. Two days later, rats were injected subcutaneously (sc) with the progesterone receptor antagonist, CDB4124 (17α-acetoxy-21-methoxy-11ß-[4-N,N-dimethyaminopheny]-19-norpregna-4,9-dione-3,20-dione) (60 mg/kg), or vehicle (20% DMSO+propylene glycol). One hour later, rats were injected sc with 500 µg progesterone or vehicle (sesame seed oil). Rats were assigned to one of three different treatment conditions: (1) (ECV) estradiol benzoate, CDB4124, sesame seed oil vehicle, (2) (ECP) estradiol benzoate, CDB4124, progesterone, and (3) (EVP) estradiol benzoate, DMSO/propylene glycol vehicle, progesterone. That afternoon sexual behavior was examined before and after a 5 min restraint experience. Before restraint, lordosis behavior was comparable across treatment conditions but only progesterone-treated rats exhibited proceptive behavior. CDB4124 did not block progesterone's induction of proceptivity. However, after restraint, CDB4124 attenuated the positive effects of progesterone on all sexual behaviors examined. The restraint experience inhibited sexual behavior in rats treated with estradiol benzoate and CDB4124 and in rats treated with estradiol benzoate, CDB4124, and progesterone but not in rats given estradiol benzoate and progesterone without CDB4124. These findings are consistent with the hypothesis that progesterone receptors mediate progesterone's ability to reduce the negative sexual behavioral effects of a mild stressor.

Mechanisms responsible for progesterone's protection against lordosis-inhibiting effects of restraint I. Role of progesterone receptors.

Progestins and antiprogestins are widely used therapeutic agents in humans. In many cases, these are indicated for the treatment of reproductive activities. However, progesterone has widespread physiological effects including a reduction of the response to stress. We have reported that 5 min of restraint reduced lordosis behavior of ovariectomized rats hormonally primed with estradiol benzoate. When ovariectomized rats received both estradiol benzoate and progesterone priming, restraint had minimal effects on lordosis. Progesterone influences behavior through classical intracellular progesterone receptor-mediated nuclear events as well as extranuclear events. How these multiple events contribute to the response to stress is unclear. The current project was designed to initiate examination of the mechanisms responsible for progesterone's ability to protect against the effects of the restraint. In the first experiment, ovariectomized rats, primed with 10 µg estradiol benzoate, received 500 µg progesterone 4 h, 1 h, or 30 min before restraint. When progesterone was injected 4h before restraint, progesterone eliminated the effects of restraint. In contrast, progesterone 30 min before restraint offered no protection. Effects of progesterone 1h before restraint were equivocal allowing the suggestion that less than 4h of progesterone priming might be sufficient. In the second experiment, the synthetic progestin, medroxyprogesterone, was shown to mimic effects of progesterone in preventing effects of restraint. Finally, the progesterone receptor antagonist, RU486, attenuated progesterone's protection against restraint. These findings offer evidence that ligand-activated progesterone receptor mechanisms contribute to the maintenance of lordosis behavior in the presence of mild stress.

Mechanisms responsible for progesterone's protection against lordosis-inhibiting effects of restraint II. Role of progesterone metabolites.

When ovariectomized Fischer female rats are hormonally primed with 10 µg estradiol benzoate, a 5 min restraint experience rapidly inhibits lordosis behavior. Addition of progesterone to the hormonal priming prevents this restraint-induced inhibition. In prior work, we reported evidence that progesterone receptors (PR) may contribute to this protective effect of progesterone. In the current manuscript, we provide evidence that progesterone metabolites may also contribute to progesterone's ability to reduce the effects of restraint. Ovariectomized female rats were hormonally primed with 10 µg estradiol benzoate followed 2 days later with 4.0 mg/kg of the progesterone metabolite, allopregnanolone. Allopregnanolone, administered either 4 h or 2 h before the restraint experience, was as effective as progesterone in reducing the lordosis-inhibitory effects of restraint. In the second experiment, progesterone metabolism was blocked with 50 mg/kg of the 5α-reductase inhibitor, finasteride. Surprisingly, finasteride did not prevent progesterone from reducing the effects of restraint. In a third experiment, we tested the possibility that allopregnanolone acted through metabolism to dihydroprogesterone. Rats were treated with allopregnanolone or with allopregnanolone plus the 3α-hydroxysteroid dehydrogenase inhibitor, indomethacin. Indomethacin did not prevent allopregnanolone from reducing the effects of restraint. Mechanisms are discussed whereby cross-talk between PR-mediated and metabolite-mediated events may converge in producing progesterone's attenuation of the effect of restraint.

Tropisetron increases the inhibitory effect of mild restraint on lordosis behavior of hormonally primed, ovariectomized rats.

Ovariectomized rats, hormonally primed with 10 µg estradiol benzoate and 500 µg progesterone are resistant to the lordosis-inhibiting effects of a 5 min restraint experience. However, modulation of the serotonergic (5-HT) system alters this resistance to stress. In the following experiment, ovariectomized Fischer inbred rats were hormonally primed with 10 µg estradiol benzoate and 500 µg progesterone. The effect of 5 min restraint on sexual behavior was examined after bilateral hypothalamic infusion or intraperitoneal (ip) treatment with the 5-HT(3) receptor antagonist, 3-tropanylindole-3-carboxylate hydrochloride (tropisetron). Infusion with 50 or 100 ng tropisetron inhibited lordosis behavior. When rats were infused with 10 or 25 ng tropisetron, rats showed normal lordosis behavior. However, when infusion with 10 or 25 ng tropisetron was combined with 5 min restraint, lordosis behavior was inhibited. These findings are consistent with prior work that has implicated hypothalamic serotonin in control of lordosis behavior and in the effect of mild restraint on the behavior. In contrast to the effects of the intracranial infusion, intraperitoneal injection with 1.0 or 2.0 mg/kg tropisetron did not amplify the effects of restraint.

Progesterone reduces the inhibitory effect of a serotonin 1B receptor agonist on lordosis behavior.

Ovariectomized Fischer inbred rats were hormonally primed with 10µg estradiol benzoate and sesame seed oil (EO rats) or with estradiol benzoate and 500µg progesterone (EP rats). Four to six hours after progesterone or oil, rats were pretested for sexual behavior and then infused bilaterally into the ventromedial nucleus of the hypothalamus with 0, 50, 100 or 200ng of the 5-HT(1B) receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrol[3,2-bi]pyridin-5-one-dihydrochloride (CP 93129). Sexual receptivity was monitored by the lordosis to mount (L/M) ratio. EO rats showed a transient decline in lordosis behavior following infusion with the saline vehicle and this was amplified by CP 93129. There were no effects of any infusion in EP rats. These findings are discussed in terms of the possible stress effect of the intracranial infusion in EO rats and their implications for a role of 5-HT(1B) receptors in the response to a mild stress.

Role of 5-HT(1A) receptors in fluoxetine-induced lordosis inhibition.

The selective serotonin reuptake inhibitor (SSRI), fluoxetine (Prozac(R)), is an effective antidepressant that is also prescribed for other disorders (e.g. anorexia, bulimia, and premenstrual dysphoria) that are prevalent in females. However, fluoxetine also produces sexual side effects that may lead patients to discontinue treatment. The current studies were designed to evaluate several predictions arising from the hypothesis that serotonin 1A (5-HT(1A)) receptors contribute to fluoxetine-induced sexual dysfunction. In rodent models, 5-HT(1A) receptors are potent negative modulators of female rat sexual behavior. Three distinct experiments were designed to evaluate the contribution of 5-HT(1A) receptors to the effects of fluoxetine. In the first experiment, the ability of the 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635), to prevent fluoxetine-induced lordosis inhibition was examined. In the second experiment, the effects of prior treatment with fluoxetine on the lordosis inhibitory effect of the 5-HT(1A) receptor agonist, (+/-)-8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), were studied. In the third experiment, the ability of progesterone to reduce the acute response to fluoxetine was evaluated. WAY100635 attenuated the effect of fluoxetine; prior treatment with fluoxetine decreased 8-OH-DPAT's potency in reducing lordosis behavior; and progesterone shifted fluoxetine's dose-response curve to the right. These findings are consistent with the hypothesis that 5-HT(1A) receptors contribute to fluoxetine-induced sexual side effects.

Progesterone reduces the effect of the serotonin 1B/1D receptor antagonist, GR 127935, on lordosis behavior.

Ovariectomized rats were hormonally primed with 10 microg estradiol benzoate or with estradiol benzoate plus 500 microg progesterone. Rats received a bilateral infusion with 200 ng of the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1-1'-biphenyl-4-carboxamide hydrochloride (GR 127935), into the ventromedial nucleus of the hypothalamus (VMN), followed by a 5 min restraint or home cage experience. In estrogen-primed females that had experienced minimal handling between ovariectomy and use in the experiment, infusion with the water vehicle transiently inhibited lordosis behavior, and the 5-HT(1B/1D) receptor antagonist amplified this inhibition. There were no effects in rats hormonally primed with estrogen and progesterone. Handling for two days before the experiment reduced the effects of the infusions in estrogen-primed rats. However, when a 5 min restraint experience followed infusion with GR 127935, there was a significant decline in lordosis behavior that persisted for 10 to 15 min after the experience. Regardless of the prior experience or type of infusion, the addition of progesterone to the hormonal priming completely prevented the lordosis inhibition. These findings are consistent with prior evidence that progesterone protects against the inhibitory effects of a 5 min restraint experience on lordosis behavior. Moreover, these are the first experiments to demonstrate an inhibitory effect of a selective 5-HT(1B/1D) receptor antagonist in the VMN on lordosis behavior of estrogen primed, but not estrogen and progesterone primed, ovariectomized rats.

Daily male exposure attenuates estrous cycle disruption by fluoxetine.

Fluoxetine (Prozac) produces sexual dysfunction in a substantial number of patients. In the few animal studies designed to address this sexual dysfunction in females, data have been inconsistent. Some investigators report that the drug disrupts sexual behavior without affecting the estrous cycle while we have reported robust effects of fluoxetine on the estrous cycle. The current studies were designed to initiate examination of procedural differences that may account for these contradictory outcomes. In the first experiment, intact, regularly cycling female rats were injected daily for 10 days with 10 mg/kg fluoxetine (intraperitoneally) or vehicle. Male-exposed, fluoxetine- or vehicle-treated rats were housed in a room with males and placed for 5 min/day into a male's cage. Other fluoxetine-treated females were housed in a room separate from males. In the second experiment, this protocol was repeated for 20 days and an additional group of females were exposed to male bedding for 5 min/day. Without male exposure, fluoxetine rapidly disrupted vaginal estrus and sexual receptivity so that approximately 50% of the rats failed to show vaginal estrus during the first 5 days; and the majority of the rats had a blocked cycle by 10 days of treatment. With male exposure, these reproductive effects were attenuated. The majority of rats cycled normally during the first 5 days of treatment and more than half cycled throughout the experiment. Loss of behavioral receptivity occurred even when normal estrous cyclicity was present. Although exposure to the male's bedding may have delayed the onset of estrous cycle disruption, five min daily exposure to a male's bedding did not prevent the disruptive effects of fluoxetine. These findings are consistent with evidence that fluoxetine's effect on female sexual dysfunction may result, in part, from the drugs' disruption of the hypothalamic-pituitary-gonadal axis. However, the data also evidence dissociation between the effects of fluoxetine on vaginal and behavioral estrus. These findings may also explain why different laboratories have reported the presence or absence of estrous cycle disturbances following daily treatment with fluoxetine.

Serotonin receptor involvement in effects of restraint on female rat lordosis behavior.

Ovariectomized Fischer (CDF-344) rats, with bilateral cannulae in the mediobasal hypothalamus (MBH) near the ventromedial nucleus of the hypothalamus (VMN), were used to test the hypothesis that serotonin receptors in the VMN contribute to the lordosis-inhibiting effects of mild restraint. Rats were hormonally primed with 10 microg estradiol benzoate (EB) followed 48 h later with sesame seed oil. Four to six hours later (during the dark portion of the light-dark cycle), rats were pretested for sexual behavior. Thereafter, they were infused with saline, 2 microg of the serotonin (5-HT) 2 receptor agonist, (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCl (DOI), or 1 microg of the 5-HT(1A) receptor antagonist, N-{2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY100635). After a 5 min restraint, rats were tested for sexual receptivity. Rats infused with saline showed a significant decline in lordosis behavior after restraint. Infusion with either DOI or WAY100635 attenuated these effects of restraint. These findings extend earlier observations that the lordosis-disruptive effects of mild restraint include activation of 5-HT(1A) receptors in the VMN and are the first to implicate VMN 5-HT(2) receptors in protection against mild restraint.

Restraint accentuates the effects of 5-HT2 receptor antagonists and a 5-HT1A receptor agonist on lordosis behavior.

The effect of restraint on lordosis behavior was examined in proestrous and ovariectomized, hormone-primed rats. Restraint durations from 5 to 60 min had no effect on lordosis behavior of proestrous rats. There was also no effect of 5 min restraint on lordosis behavior of ovariectomized rats hormonally primed with 10 microg estradiol benzoate and 500 microg progesterone. However, after intraperitoneal treatment with 1.0 mg/kg ketanserin tartrate (ketanserin), 5 min of restraint significantly reduced lordosis behavior of both groups of rats. The 5-min restraint combined with 0.50 or 0.75 mg/kg ketanserin reduced lordosis to mount (L/M) ratios of ovariectomized rats, while L/M ratios of proestrous rats were inhibited only by the 1.0 mg/kg dose. Increasing the restraint duration (10 or 15 min) reduced the dose of ketanserin necessary to reduce the L/M ratios of proestrous rats. Treatment with the selective serotonin (5-HT)(2C) receptor antagonist, SB206553 (2.5 or 5.0 mg/kg), in combination with 5 min of restraint, also reduced L/M ratios of hormonally primed, ovariectomized rats. The neural sites responsible for ketanserin's additivity with restraint are unknown, but infusion of the drug into the ventromedial nucleus of the hypothalamus (VMN) did not mimic the systemic treatment. However, 5 min of restraint did enhance the effects of VMN infusion with the 5-HT(1A) receptor agonist, 8-OH-DPAT. In contrast, 8-OH-DPAT's systemic potency was not enhanced by restraint. These findings support the hypothesis that a mild stressor increases the lordosis-inhibiting effects of 5-HT(1A) receptor agonists and that 5-HT(2) receptors may protect against such disruption of lordosis behavior.

Progesterone attenuates the effect of the 5-HT1A receptor agonist, 8-OH-DPAT, and of mild restraint on lordosis behavior.

Ovariectomized, hormone-primed rats were used to test the hypothesis that progesterone treatment attenuated the effects of the 5-HT(1A) receptor agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on female rat lordosis behavior. Based upon prior evidence that prepriming with estradiol benzoate (EB) reduced the ability of 8-OH-DPAT to inhibit lordosis behavior, rats were preprimed with 10 microg EB 7 days before a second priming with 10 microg EB followed 48 h later with 500 microg progesterone or vehicle. Independent of the presence of progesterone, prepriming with EB attenuated the lordosis-inhibiting effects of systemic treatment with 8-OH-DPAT. However, progesterone also reduced the effects of 8-OH-DPAT and this effect was also seen in females primed only once with EB. In contrast, progesterone was relatively ineffective in attenuating the effects of bilateral infusion with 8-OH-DPAT into the ventromedial nucleus of the hypothalamus (VMN). The failure of progesterone to substantially reduce the effects of VMN infusion with 8-OH-DPAT contrasts with prior studies in which estrogen's protective action against the drug did include the VMN. Thus, while both estrogen and progesterone reduce the lordosis-inhibiting effect of 8-OH-DPAT, the mechanisms responsible for the effects of the two gonadal hormones may be different. Priming with progesterone also prevented the effects of 5 min of restraint. When rats were hormonally primed with EB and oil, rats showed a transient, but significant, decline in lordosis behavior 5 and 10 min after restraint. Rats primed with EB and progesterone were unaffected by the restraint. These results are discussed in terms of their implications for the role of progesterone in altering the 5-HT(1A) receptor modulation of lordosis behavior.