Organizing action of prenatally administered testosterone propionate on the Tissues mediating mating behavior in the female guinea pig

Our “Organizing Action” paper published in 1959 put forward the concept that prenatal exposure to testosterone masculinized the behavior of genetic female guinea pigs. Specifically, we proposed that testosterone or some metabolite acted on the central nervous tissues in which patterns of sexual behavior are organized. We later went on to demonstrate similar effects in rhesus monkeys by showing that play behavior by female monkeys prenatally treated with testosterone was masculinized as well. These findings support the organizing actions of androgens as a general process of sexual differentiation.     

Non-verbal behavior as courtship signals: the role of control and choice in selecting partners

In this work, we provide evidence based on direct observation of behavior in encounters of opposite-sexed strangers, that women initiate and “control” the outcome. In the first minute of these videotaped 10-min interactions, neither female “solicitation” behavior nor “negative” behavior is strongly related to professed interest in the man, while female “affirmative” behavior at this stage modulates male verbal output in later stages (4–10 min). Although the rate of female courtship-like behavior is significantly higher in the first minute, it is only in the fourth to tenth minute that the rate of female courtship-like behavior is correlated with professed female interest. We hypothesize that this serves as a strategic dynamic reflecting sexual asymmetry in parental investment and the potential cost of male deception to women. Ambiguous protean behavioral strategies veil individuals' intentions and make their future actions unpredictable. These behavioral strategies may result in men's overestimation of female sexual interest.     

Preference for an estrous female over a non-estrous female evinced by female rats requires dihydrotestosterone plus estradiol

The effects were studied of long-term treatment with testosterone metabolites (dihydrotestosterone, DHT, and estradiol, E2, in sc Silastic implants) on preference behavior of ovariectomized female rats for an estrous female over a non-estrous female. For measuring this behavior a residential plus-maze was used which harbored two ovariectomized “stimulus” females on the top of peripheral boxes, one of which was made estrus by injection of estradiol benzoate and progesterone. When both steroids (DHT plus E2) were circulating simultaneously they evoked preference for an estrous female, while neither steroid by itself sufficed. In earlier work with adult male rats castrated on the day of birth, E2 was effective in the absence of DHT. This sex difference, therefore, seems to have arisen before birth. Further, administration of DHT alone caused a profound lack of interest in both “stimulus” females, which cannot be fully explained by the reduced locomotor activity which has been found to be induced by DHT in earlier Studies.     

Reversal of progesterone-induced sequential inhibition by progesterone metabolites

Previous reports have shown that intrabrain administration of progesterone (P) ring A-reduced metabolites into the medial preoptic area (MPOA) and ventromedial hypothalamus (VMH) induces facilitation of female sexual behaviour in ovariectomized (ovx) rats pretreated with estrogen. Present studies were designed to explore the possibility that ring-A reduced progesterone metabolites might play a role in controlling the duration of estrous behavior. To this aim ovariectomized (ovx) Sprague Dawley rats implanted with guide cannulae directed towards the VMH or the MPOA were submitted to a systemic hormonal treatment to provoke P-induced sequential inhibition (estradiol benzoate (EB) at time 0 + P at 44 h + P at 68 h). The second dose of P was administered simultaneously with the ic implantation of one of the following P metabolites: 3β-hydroxy-5β-pregnan-20-one (5β,3α P), 3α-hydroxy-5β-pregnan-20-one (5β,3α P) or 3β-hydroxy-5βpregnan-20-one (5α,3β P) into the MPOA or VHM. Lordosis behavior was evaluated by the lordosis quotient (LQ = number of lordosis/10 male mount × 100) and by the percentage of responding subjects. Results show that 5β,3βP implanted into the VMH or MPOA counteracted the sequential inhibitory effect induced by systemic administration of P. 5α,3β P was also able to counteract sequential inhibition, but with less potency and only in the VMFI. Results show that P-induced sequential inhibition can be counteracted by intrabrain administration of ring-A reduced progestins in both the VMH and MPOA. Data are discussed in terms of a putative physiological role of naturally occurring P metabolites in P-mediated female sexual behavior expression.     

Studies on feminine sexual behavior in the male rat: Influence of olfactory stimuli

The aim of this study was to determine if the display of lordosis behavior in the male rat could be influenced by the olfactory environment. Unexperienced adult male rats were orchidectomized (ORCH). They were primed with 75 μg estradiol benzoate and 1 mg progesterone was injected at an interval of 39 hr following long-term (LT = 3 weeks) or short-term (SHT = 8 hr 30 min) exposure to the odor of male or female urine. For 10 min they were placed in the presence of a “stimulus” male of proven sexual vigor 9 hr 30 min ± 1 hr after progesterone injection. Both LT and SHT exposure to the odor of male urine caused a significant increase in the number of ORCH rats which showed lordosis response to male mounts compared to either the ORCH rats exposed to the odor of female urine or to the controls. Following complete olfactory bulb removal (COBR), no difference was observed in the occurrence of lordosis behavior between the ORCH rats whether or not exposed to the odor of urine. For the ORCH-COBR rats exposed to male urine the proportion of animals responding to mounts did not differ from that of their nonbulbectomized counterparts. In comparing the effects of COBR vs anterior olfactory bulb removal (AOBR) lordosis behavior occurred more frequently in COBR than in AOBR-ORCH rats. The lordosis quotient (LQ) was not affected by exposure to the odor of male urine in the nonbulbectomized ORCH rats. In contrast, it appeared to be higher in both COBR and AOBR animals than in their nonbulbectomized counterparts. The olfactory bulbs were then concluded to inhibit the display of lordosis behavior in the male rat. It was also thought that the olfactory stimuli originating from male urine were capable of releasing the hypothalamic structures involved in the control of lordosis behavior of the male rat from an olfactory inhibitory influence.     

Inhibition of Estrogen-Induced Sexual Receptivity by Androgens: Role of the Androgen Receptor

Both naturally occurring and synthetic androgens have been shown to inhibit estrogen-induced sexual receptivity when administered to ovariectomized (OVX) rats. The mechanisms by which androgens exert these effects, however, remain unclear. Experiments were conducted to determine the role of the androgen receptor in the inhibition of estrogen-induced sexual receptivity in OVX rats by using flutamide, an androgen receptor antagonist. In each experiment, OVX Long–Evans rats received 6 consecutive days of estradiol benzoate (EB; 2.0 μg/day) followed by 15 days of EB concurrent with flutamide (10.0 mg/kg; twice daily) or the vehicle and one of the following androgens or the vehicle: dihydrotestosterone propionate (7.5 mg/kg), 3α-androstanediol (3.75 mg/kg), 17α-methyltestosterone (7.5 mg/kg), stanozolol (7.5 mg/kg), or nandrolone decanoate (7.5 mg/kg). On Day 15, all female rats received progesterone (P; 1.0 mg/rat) 4 h before testing. Tests for sexual receptivity were conducted on Days 3, 6, 14, and 15 of androgen/flutamide treatment. Each androgen inhibited sexual receptivity as expected, and concurrent treatment with flutamide reversed the inhibitory effects of all androgens on sexual receptivity on all test days. High levels of sexual receptivity were displayed in response to P on Day 15, regardless of experimental treatment. These results suggest that naturally occurring and synthetic androgens act at the androgen receptor to inhibit estrogen-induced sexual receptivity in OVX rats.     

Medroxyprogesterone acetate acutely facilitates and sequentially inhibits sexual behavior in female rats

Medroxyprogesterone acetate (MPA), a synthetic progestin commonly used in contraception and hormone replacement therapy, appears to inhibit libido in women, but little is known about the mechanisms through which it may exert this effect. We compared the acute and sequential actions of MPA and natural progesterone (P4) on sexual behavior in female rats to test the hypothesis that MPA inhibits sexual behavior, at least in part, by acting as a potent progesterone receptor (PR) agonist. Ovariectomized females were placed in one of three dose groups (high, mid, or low), and each subject was tested under three different conditions (MPA, P4, and vehicle). The order of progestin treatment was balanced among subjects, and within each dose group equimolar quantities of MPA and P4 were administered. During each trial, females were injected with estradiol benzoate (EB, 4 mug) followed by one of three progestin treatments (MPA, P4, or vehicle) at +44 h, and behavioral testing at +48 h. On the next day, all females were given a standard 500-microg injection of P4 at +68 h and were tested again for sexual behavior at +72 h. On the first day of behavioral testing, both MPA and P4 induced a pronounced rise in receptive and proceptive behavior at the mid and high doses, but at the lowest dose MPA had a much greater effect in comparison to P4. On the second day of behavioral testing, MPA attenuated the expression of proceptive and receptive behavior at both the mid and high doses, whereas P4 only attenuated the expression of lordosis and only did so at the highest dose. These findings illustrate that MPA and P4 have a similar impact on sexual behavior in female rats and suggest that the inhibitory effects of MPA may be attributable, at least in part, to its potent effects at the progesterone receptor.     

Inhibition of guinea pig lordosis behavior by the phenylethanolamine N-methyltransferase (PNMT) inhibitor SKF-64139: mediation by alpha noradrenergic receptors

Experiments were undertaken to determine whether the steroid-dependent lordosis response of female guinea pigs is under adrenergic control. In initial experiments, treatment with the centrally active phenylethanolomine N-methyltransferase (PNMT; the enzyme catalyzing methylation of norepinephrine to epinephrine) inhibitor SKF-64139 inhibited lordosis behavior induced by estradiol-17 beta benzoate plus progesterone. SKF-29661, a PNMT inhibitor that does not cross the blood-brain barrier, did not affect lordosis. However, no detectable epinephrine was found in brain or spinal cord of drug- or vehicle-treated guinea pigs. This suggests that epinephrine neuronal systems do not exist in the guinea pig CNS. In agreement with this idea, the inhibitory effects of SKF-64139 on lordosis were found to be primarily attributable to the blockade of alpha noradrenergic receptors rather than to PNMT inhibition. Two lines of evidence support this conclusion. First, using in vitro receptor binding techniques, SKF-64139 was found to have a relatively high affinity for alpha 1 and particularly alpha 2 receptors in guinea pig forebrain. Second, presumably through competitive inhibition of SKF-64139 binding to alpha receptors, treatment with clonidine (an alpha receptor agonist) overrode the inhibitory effects of SKF-64139 on lordosis. Taken together, these findings indicate the possible absence of epinephrine neuronal systems in guinea pig brain and reemphasize the importance of alpha receptors in regulating steroid-dependent lordosis behavior in this species.     

Vaginocervical stimulation induces Fos in glutamate neurons in the ventromedial hypothalamus: Attenuation by estrogen and progesterone

Vaginocervical stimulation (VCS) induces the immediate-early gene product Fos in the ventromedial hypothalamus (VMH) of female rats. However, this induction is lower in ovariectomized rats that receive estradiol benzoate (EB) and progesterone (P) relative to an oil vehicle. We have observed that a substantial proportion of cells activated in the VMH by VCS stain for glutamate, and infusions of glutamate or its selective receptor agonists to the VMH inhibit both appetitive and consummatory sexual behaviors in females. This raises the possibility that VCS activates an inhibitory glutamate system in the VMH, and that ovarian steroids blunt the activation, although it is not known whether EB or P, alone or in combination, lead to this effect. The present experiment examined the ability of VCS to induce Fos in glutamate neurons in the VMH of ovariectomized rats under 4 hormonal regimens: oil, EB alone, P alone, or EB + P, following 1 or 50 distributed VCSs administered with a lubricated glass rod over the course of 1 h. Treatment with EB or P alone significantly reduced the number of glutamate neurons activated by 1 VCS, with P being more effective than EB. Treatment with EB + P also produced a significant reduction, but not to the extent of EB or P alone. Although EB and P work in synergy to activate sexual behavior in female rats, actions of EB or P alone are sufficient to blunt the ability of VCS to activate glutamate neurons in the VMH. It thus appears that ovarian steroids may “disinhibit” sexual responding, in part, by dampening the ability of VCS to activate glutamate neurons in the VMH. In turn, this may allow females to receive a sufficient number of intromissions for the activation of sexual reward and the facilitation of pregnancy.     

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.     

Nonluteal source of lordosis-promoting progesterone secretion in guinea pigs

Prepubertal (21-24 days of age), intact female guinea pigs treated sequentially with estradiol benzoate and LH or FSH displayed lordosis behavior. The gonadotropins apparently caused release of progesterone from the ovaries, because lordosis behavior in guinea pigs is activated by sequential action of estrogen and progesterone. These data demonstrate that immature ovaries, completely devoid of corpus luteum tissue, are capable of secreting behaviorally significant concentrations of progesterone when stimulated by gonadotropins. Therefore, the luteal compartment of the guinea pig ovary is not essential for the preovulatory surges of progesterone that coincide with expression of lordosis behavior in adulthood. Likely candidates for sources of preovulatory progesterone in prepubertal females are antral follicle and interstitial gland tissue.     

The inhibitory actions of progesterone: effects on male and female sexual behavior of the hamster

A series of three experiments compared the inhibitory effects of progesterone on estrogen- or androgen-induced sexual behavior in male and female hamsters. In the first experiment chronic progesterone treatment was found to have no effect on male copulatory behavior maintained after castration with testosterone propionate or estradiol benzoate. However, testosterone propionate was more effective at maintaining male behavior than estradiol benzoate. In the second experiment progesterone was found to have a slight inhibitory effect on the rate of the restoration of the intromission response after androgen treatment in males which had been castrated for 8 weeks. In the final experiment, chronic progesterone treatment markedly inhibited sexual receptivity in male and female hamsters which had been given 4 weeks of androgen or estrogen treatment and a single pretest injection of progesterone. Thus, progesterone was shown to be a potent inhibitor of androgen- or estrogen-induced estrus in both male and female hamsters. Due to the large difference in effectiveness on these two behavioral systems, we suggest that progesterone affects steroid-induced male copulatory behavior and female receptivity by different mechanisms of action.     

Chelicerae as male grasping organs in scorpions: sexual dimorphism and associated behaviour

Specialised structures that enable males to grasp females during sexual interactions are highly susceptible to selection and thus diverge relatively rapidly over evolutionary time. These structures are often used to test hypotheses regarding sexual selection such as sexually antagonistic co-evolution and sexual selection by female choice. In the present study, we determine whether there is a relationship between a novel record of scorpion sexual dimorphism, the sexual dimorphism of chelicerae (CSD), and the presence of the mating behaviour termed “cheliceral grip” (CG). The presence of both traits in the order Scorpiones is also reviewed from a phylogenetic perspective. The results confirm a strong relationship between CSD and the presence of CG. The morphological and behavioural patterns associated with “CSD–CG” are opposed to the predictions postulated by the hypothesis of sexually antagonistic co-evolution. However, if the female shows resistance after the deposition of the spermatophore, the possibility that the male exerts pressure as a “cryptic form” of coercion to prevent the interruption of mating cannot be ruled out completely. Female choice by “mechanical fit” could be another explanation for some aspects of the CG's contact zone. The possibility that the “CG–CSD” complex has evolved under natural selection in order to ensure sperm transfer is also considered.     

Further evidence for masculinization of female rats by males located caudally in utero

The morphology and behavior of female rodents is partially masculinized as a result of residence near males in the same uterine horn (Clemens effect). Two hypothetical mechanisms have been proposed to account for this effect. In the first hypothesis (“contiguity”) androgens secreted by males in utero are proposed to diffuse across the amniotic membrane, reaching adjacent fetuses. In the second hypothesis (“caudal male”) androgens are transported via the cervical-to-ovarian blood flow and may diffuse directly between closely apposed uterine veins and arteries. This study was designed to test directly which of these mechanisms appears more influential in masculinizing the morphology of female rats. Pregnant Sprague-Dawley rats were decapitated early on Day 22 of gestation and pups were Caesarean delivered. Their anogenital distance and body weight were recorded, location in utero coded by means of footpad tatooing, and each litter fostered to a maternal female. Measurements were taken again when the animals were weaned. Statistical analysis revealed that the presence of one or more males caudal to a female in the uterine horn has a more critical influence on that female's morphology than contiguity per se. Such a mechanism may result in partial masculinization of dimorphic behaviors later in life.     

Effects of prostaglandin E2 and indomethacin on sexual behavior in the female rat

Prostaglandin E₂ (PGE₂) 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 PGE₂ 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.     

Progesterone does not inhibit lordosis through interference with estrogen priming

Progesterone facilitated sexual receptivity in two experimental paradigms using estrogen-primed, ovariectomized rats. Methysergide can facilitate sexual receptivity in estrogen-primed rats and is able to overcome the inhibitory effects of progesterone. Our results suggest that the inhibitory action of progesterone on sexual receptivity cannot be due to a simple interference with the actions of estrogen on sexual behavior.     

Use of genital inspection and female urine tests to detect oestrus in captive Asian elephants

Captive Asian elephant (Elephas maximus) populations are decreasing due to low birth rates compared to wild elephants. Improving oestrous detection in female elephants is required to ensure successful mating in captive and semi-captive herds. Responsive behaviours of eight semi-captive bull elephants to the uro-genital area (genital inspection test) or urinary pheromones (urine test) of 14 female elephants throughout the oestrous cycle were evaluated. Weekly blood samples were collected for 27 consecutive months (14 months for the genital inspection test and 13 months for the urine test) from female elephants to characterize the patterns of circulating progestagen. Responsive behaviours of bulls were compared between females in the follicular versus the luteal phase of the cycle. The sensitivity and specificity of the genital inspection test were 65% and 68%, while those of the urine test were 52% and 61%, respectively. The bulls showed significantly higher “genital inspection”, “flehmen from genital area” and “trunk on back” behaviours during the genital inspection test, and “flehmen” behaviours during the urine test in oestrous than in non-oestrous females. In sum, this study showed that monitoring sexual behaviours of Asian elephant bulls towards females or their urine can be used to detect the oestrous period. Although the sensitivity and specificity of both tests were not as high as expected, still, these methods appear to be more efficient at detecting oestrous than traditional methods based on mahout estimations of female receptivity. The use of genital inspection and urine tests may lead to more successful matings and thus to creating self-sustaining populations of captive elephants in range countries.     

Feminine sexual behavior in rats enhanced by prenatal inhibition of androgen aromatization

Male and female rats were exposed to the aromatization inhibitor 1,4,6-androstatriene-3, 17-dione (ATD) in utero via prenatal injections to the pregnant mother. In adulthood, lordosis behavior was measured in response to ovarian hormones. Males and females exposed prenatally to ATD showed enhanced lordosis behavior in response to estrogen alone and in response to estrogen plus progesterone when compared to controls. These data lend further support to the idea of a prenatal, androgen-sensitive phase of sexual differentiation in which defeminization normally occurs in both male and female rats. Further, these data support the concept that androgen aromatization is an important process in this defeminization.     

Differential hormonal control of aggression and sexual behavior in female Syrian hamsters

These experiments were designed to test the effects of chronic estradiol treatment on aggression and sexual behavior in female hamsters. Isolated female hamsters were ovariectomized and tested for their behavioral responses to a group-housed, ovariectomized female hamster (aggression test) and a group-housed, intact male hamster (sexual behavior test). Following these baseline tests, the experimental females were implanted sc with Silastic capsules containing different concentrations of estradiol (100, 25, 10, or 0%) diluted with cholesterol and retested 3, 7, 10, and 14 days after implantation. High levels of aggression were observed on the baseline test, with no changes in aggression toward an intruder female observed for any implant group on subsequent tests. Despite these high levels of aggression toward another female, most of the estradiol-treated females (80% at 14 days) were sexually responsive in the presence of a male. There was no effect of Silastic estradiol concentration on sexual behavior, even though a range of serum estradiol levels (39–105 pg/ml) resulted. Lordosis latencies decreased and lordosis durations increased over the extent of estradiol treatment. Seventeen days after Silastic implantation, all females were injected with progesterone and retested. Estradiol-treated females showed an extreme reduction in aggression toward a stimulus female, as well as a further stimulation of sexual behavior after progesterone treatment. High levels of aggression in cholesterol-treated females (0% estradiol) were maintained even after progesterone injection, and these females never displayed any sexual responsivity. These results suggest that sexual behavior in the female hamster is sensitive to estradiol alone, whereas the inhibition of aggression requires the combination of estradiol plus progesterone.