Differential effects of testosterone plus dihydrotestosterone on male courtship of castrated newts, Taricha granulosa

Sexually responsive Taricha granulosa males were castrated and implanted with testosterone (T), dihydrotestosterone (DHT), T plus DHT, or no steroid. The incidence of sexual behavior was highest in castrates implanted with T plus DHT. Newts implanted with T or DHT exhibited sexual behavior more frequently than did the untreated castrates, which exhibited none during the last 5 days of testing. A second experiment was conducted using sexually unresponsive males. The occurrence of sexual behavior remained low in castrates implanted with T plus DHT, untreated castrates, and intact control males. These results support the hypothesis that for this species of amphibian the presence of adequate levels of testicular androgen is a necessary, but not sufficient, condition for the manifestation of sexual behavior; the appearance of these behaviors requires, in addition, the presence of some nontesticular hormone.     

Facilitation of Male Sexual Behavior in Syrian Hamsters by the Combined Action of Dihydrotestosterone and Testosterone

Background

Testosterone (T) controls male Syrian hamster sexual behavior, however, neither of T''s primary metabolites, dihydrotestosterone (DHT) and estradiol (E₂), even in highly supraphysiological doses, fully restores sexual behavior in castrated hamsters. DHT and T apparently interact with androgen receptors differentially to control male sexual behavior (MSB), but whether these two hormones act synergistically or antagonistically to control MSB has received scant experimental attention and is addressed in the present study.

Methodology/Principal Findings

Sexually experienced male Syrian hamsters were gonadectomized and monitored 5 weeks later to confirm elimination of the ejaculatory reflex (week 0), at which time they received subcutaneous DHT-filled or empty capsules that remained in situ for the duration of the experiment. Daily injections of a physiological dose of 25 µg T or vehicle commenced two weeks after capsule implantation. MSB was tested 2, 4 and 5 weeks after T treatment began. DHT capsules were no more effective than control treatment for long-term restoration of ejaculation. Combined DHT + T treatment, however, restored the ejaculatory reflex more effectively than T alone, as evidenced by more rapid recovery of ejaculatory behavior, shorter ejaculation latencies, and a greater number of ejaculations in 30 minute tests.

Conclusions/Significance

DHT and T administered together restored sexual behavior to pre-castration levels more rapidly than did T alone, whereas DHT and vehicle were largely ineffective. The additive actions of DHT and T on MSB are discussed in relation to different effects of these androgens on androgen receptors in the male hamster brain mating circuit.     

Paternal behavior in the Mongolian gerbil (Meriones unguiculatus): Estrogenic and androgenic regulation

Here, we analyzed the effects of testosterone (T) and its metabolites, estradiol (E₂) and dihydrotestosterone (DHT), on the onset of paternal behavior in virgin male Mongolian gerbils (Meriones unguiculatus). We hypothesized that T and E₂, but not DHT, would facilitate the onset of paternal behavior. Seventy males displaying aggression toward pups were selected through a paternal behavior screening test. Forty males were bilaterally castrated. Of them, 10 were implanted with T, 10 with E₂, and 10 with DHT, and 10 received no treatment. Another 30 males underwent a sham procedure. In these gerbils, T, E₂ and DHT were measured to obtain the basal levels of these hormones. After treatment, the paternal behavior test was conducted again. Blood samples were obtained immediately after the administration of the test for the quantification of T, E₂ and DHT by radioimmunoassay. Surprisingly, 100% of the males that received T, E₂ and DHT implants stopped being aggressive and became paternal. Castrated and sham-operated males displayed no changes in their aggressive behaviors. This is the first report that T and its metabolites are involved in neuroendocrine mechanisms that inhibit aggression toward pups and facilitate paternal behavior in virgin male Mongolian gerbils. In addition, this is the first report of regulation of paternal behavior in a rodent by estrogenic and androgenic pathways.     

A single injection of 17 beta-estradiol facilitates sexual behavior in castrated male mice

Sexual behavior was assessed in castrated adult CD-1 male mice given exogenous steroids under various treatment regimens. Castrated mice maintained on 20 μg testosterone (T) daily for 1 week, but given 250 μg testosterone propionate (TP) on the day of testing showed higher levels of copulatory activity than intact mice or the males receiving an additional dose of 20 μg T on the test day, although plasma testosterone levels were not different at the time of behavioral testing. Castrated males given 50, 125, or 250 μg TP for 1 week including the day of testing showed higher levels of sexual behavior than males receiving the same doses of TP only once, on the test day. A single injection of 17β-estradiol (E₂) completely restored the male copulatory pattern, including ejaculation, in castrated mice under every condition examined. Testosterone and dihydrotestosterone (DHT) were less effective than E₂, as was the combination of E₂ and DHT. The relative efficacy of a single dose of T, DHT, and E₂ plus DHT was dependent upon factors such as the delay between steroid administration and testing, as well as whether or not the castrated mice received androgen replacement prior to testing. Estradiol benzoate (E₂B) was not capable of restoring sexual behavior in castrated mice in this study. The comparison of results obtained with TP, T, E₂, and E₂B suggests that an appreciable, but not necessarily sustained, elevation of E₂ levels in the brain may be critical in the facilitation of male copulatory behavior in mice.     

A single injection of 17β-estradiol facilitates sexual behavior in castrated male mice

Sexual behavior was assessed in castrated adult CD-1 male mice given exogenous steroids under various treatment regimens. Castrated mice maintained on 20 μg testosterone (T) daily for 1 week, but given 250 μg testosterone propionate (TP) on the day of testing showed higher levels of copulatory activity than intact mice or the males receiving an additional dose of 20 μg T on the test day, although plasma testosterone levels were not different at the time of behavioral testing. Castrated males given 50, 125, or 250 μg TP for 1 week including the day of testing showed higher levels of sexual behavior than males receiving the same doses of TP only once, on the test day. A single injection of 17β-estradiol (E₂) completely restored the male copulatory pattern, including ejaculation, in castrated mice under every condition examined. Testosterone and dihydrotestosterone (DHT) were less effective than E₂, as was the combination of E₂ and DHT. The relative efficacy of a single dose of T, DHT, and E₂ plus DHT was dependent upon factors such as the delay between steroid administration and testing, as well as whether or not the castrated mice received androgen replacement prior to testing. Estradiol benzoate (E₂B) was not capable of restoring sexual behavior in castrated mice in this study. The comparison of results obtained with TP, T, E₂, and E₂B suggests that an appreciable, but not necessarily sustained, elevation of E₂ levels in the brain may be critical in the facilitation of male copulatory behavior in mice.     

Androgens are not major down-regulators of androgen receptor levels during growth of the immature rat penis

This study was undertaken to investigate the prevalent hypothesis that androgens are responsible for the organ-specific down-regulation of penile androgen receptors (ARs) and decline of penile growth in the rat during sexual maturation. Sexually immature male rats (21 days old) were castrated and treated for 3 days (“short-term”), with high doses of: (a) testosterone and the -reductase inhibitor finasteride (T/F); (b) dihydrotestosterone (DHT); or (c) finasteride alone (F). Intact and castrate controls received vehicle only. PolyA + RNA was analysed by Northern blot hybridization and ARs were estimated in the penis and ventral prostates by (3-H)R-1881 binding in the cytosol. Short-term castration, with or without F, increased penile AR mRNA, whereas high doses of T/F and DHT reduced it considerably. Although penile cytosol AR concentration in the control castrates, with or without F, paralleled the AR mRNA rise, treatment with androgens left cytosol AR content per organ and AR concentration above those of the intact rat penis despite the drop in AR mRNA. A “long-term” treatment (10 days) on 19-day-old rats with either medium or high doses of T/F and DHT also failed to down-regulate penile cytosol ARs below the intact controls. Western blot analysis of penile cytosol AR levels confirmed these results. Block of pituitary FSH and LH release by a GnRH antagonist in castrates receiving T/F or DHT at high doses did not modify the response. In the case of intact rats, high doses of T/F or DHT actually increased penile cytosol AR content. No difference was observed between T/F and DHT effects. In contrast to what occurs during sexual maturation, the prostate ARs and growth rate responded to all treatments in a similar way to what was observed in the penis. Our results suggest that increases in serum T or DHT are not major factors in the physiological down-regulation of ARs and androgen-dependent growth in the rat corpora cavernosa.     

Androgens and estrogens induce seasonal‐like growth of song nuclei in the adult songbird brain

In seasonally breeding songbirds, the brain regions that control song behavior undergo dramatic structural changes at the onset of each annual breeding season. As spring approaches and days get longer, gonadal testosterone (T) secretion increases and triggers the growth of several song control nuclei. T can be converted to androgenic and estrogenic metabolites by enzymes expressed in the brain. This opens the possibility that the effects of T may be mediated via the androgen receptor, the estrogen receptor, or both. To test this hypothesis, we examined the effects of two bioactive T metabolites on song nucleus growth and song behavior in adult male white‐crowned sparrows. Castrated sparrows with regressed song control nuclei were implanted with silastic capsules containing either crystalline T, 5α‐dihydrotestosterone (DHT), estradiol (E₂), or a combination of DHT+E₂. Control animals received empty implants. Song production was highly variable within treatment groups. Only one of seven birds treated with E₂ alone was observed singing, whereas a majority of birds with T or DHT sang. After 37 days of exposure to sex steroids, we measured the volumes of the forebrain song nucleus HVc, the robust nucleus of the archistriatum (RA), and a basal ganglia homolog (area X). All three steroid treatments increased the volumes of these three song nuclei when compared to blank‐implanted controls. These data demonstrate that androgen and estrogen receptor binding are sufficient to trigger seasonal song nucleus growth. These data also suggest that T's effects on seasonal song nucleus growth may depend, in part, upon enzymatic conversion of T to bioactive metabolites. © 2003 Wiley Periodicals, Inc. J Neurobiol 57:130–140, 2003     

Hormonal regulation of male reproductive behavior in the lizard Anolis sagrei: a test of the aromatization hypothesis

This study examined the hypothesis that aromatization of testosterone (T) to estradiol (E) is required to activate reproductive behavior in castrated male lizards (Anolis sagrei). Adult, reproductively active males were assigned to an intact control group or to one of four treatment groups. Treatment males were castrated and 1 week later three of the four castrated groups were implanted with subcutaneous pellets containing either 0.05 mg of E, 0.5 mg of T, or 0.5 mg of dihydrotestosterone (DHT). Two weeks after pellet implantation, males were tested with stimulus males, and 2 days later were tested with stimulus females. Behavioral tests were of 15-min duration and were videotaped. Significantly fewer E-treated castrates erected a crest in tests with stimulus males than did intact males. In tests with stimulus females, significantly fewer E-treated castrates displayed, neck-gripped, and intromitted than did intact males. Estradiol-treated castrates also showed significantly less display behavior than did intact males. However, aggressive and sexual behavior of DHT-treated castrates was not significantly different from that of intact males. The same was true for T-treated castrates with the exception that display behavior in tests with stimulus females was reduced compared to that of intact males. The results suggest that aromatization of T to E is not required for induction of androgen-dependent reproductive behavior in this lizard.     

Lack of a synergistic effect between estradiol and dihydrotestosterone in the masculinization of the zebra finch song system

Previous studies have suggested that both major active metabolites of testosterone, estradiol (E₂) and dihydrotestosterone (DHT), are needed for complete masculinization of the brain regions that control song in passerine birds. However, DHT treatment of hatchling female zebra finches has only small masculinizing effects on the song system. To assess whether E₂ and DHT have a synergistic effect on the masculinization of the zebra finch song system, female zebra finches were given Silastic implants of E₂ on the day of hatching (day 1) either without any additional hormone treatment or in combination with DHT on days 1, 14, or 70. At 105 to 110 days of age, we measured the volumes of Area X, higher vocal center (HVC), robust nucleus of the archistriatum (RA), soma sizes in HVC, RA, and the lateral magnocellular nucleus of the neostriatum (lMAN), and neuron density and number in RA. E₂ masculinized all of the measures in the song system with the exception of the number of neurons in RA. DHT did not synergize with E₂ to produce any additional masculinization of the attributes measured. These data demonstrate that the combination of E₂ and DHT did not result in the complete masculinization of the song control nuclei and argue against the importance of androgen in sexual differentiation of the song system. © 1995 John Wiley & Sons, Inc.     

Sexual incentive motivation in male rats requires both androgens and estrogens

In Experiment 1 castrated male rats were implanted with a Silastic capsule containing either E or cholesterol (CHOL) 35 days after castration. They were then tested for sexual incentive motivation and copulatory behaviors every 5th day for 3 weeks. None of the treatments affected sexual incentive motivation. After the last test, all subjects were implanted with DHT-containing Silastic capsules, and tests continued for another 3 weeks. While E + DHT enhanced sexual incentive motivation and copulatory behavior, DHT alone failed to do so. In Experiment 2 the aromatase inhibitor fadrozole (F) was combined with testosterone (T). T restored all behaviors to the level seen in intact rats, and F significantly reduced these effects. In fact, T + F was not different from DHT. T and DHT restored the weight of the prostate and seminal vesicles to levels close to those of intact rats. In Experiment 3 a lower dose of E was employed. Also this dose of E failed to affect sexual incentive motivation while E + DHT restored it to the level of intact animals. Castration enhanced the serum concentrations of LH and FSH. E alone caused a marked reduction, and E + DHT brought both gonadotropins back to the level of intact animals. It was concluded that the doses of E and DHT employed in these experiments were within or close to the physiological range, and that such doses of E completely fail to enhance sexual incentive motivation in castrated animals. DHT has small or no effects. It appears that sexual incentive motivation and copulation require simultaneous stimulation of androgen and estrogen receptors.     

Effects of sex steroids on calling,locomotor activity,and sexual behavior in castrated male Japanese quail

Castrated male Japanese quail were implanted with Silastic capsules containing testosterone (T), estradiol-17β (E₂), 5β-dihydrotestosterone (5β-DHT), Δ₄-androstenedione (Δ₄) 5α-androstanedione (A), 5α-dihydrotestosterone (5α-DHT) or with empty capsules. Calling, monitored continuously and automatically, was induced significantly by T and Δ₄. Locomotor activity, also monitored continuously by floor deflection, was enhanced by T, Δ₄, and E₂. Additional data concerning heterosexual and homosexual behavior were obtained from castrated quails after implantation of T, Δ₄, E₂, or 5α-DHT. T and Δ₄ restored hetero- and homosexual behavior as did E₂ but to a lesser extent. 5α-DHT did not induce either sexual behavior. Growth of the cloacal protrusion was induced in birds implanted with T, Δ₄, A, and 5α-DHT but not with 5β-DHT and E₂. These results indicate that calling and locomotor activity enhancement (including sexual behavior) are two different components of reproductive behavior which require different androgens or their metabolites to be activated.     

A role for ATP-sensitive potassium channels in male sexual behavior

ATP-sensitive potassium (K⁺_ATP) channels regulate cell excitability and are expressed in steroid-responsive brain regions involved in sexual behavior, such as the preoptic area (POA) and medial basal hypothalamus (MBH). We hypothesized that K⁺_ATP channels serve as a mechanism by which testosterone can control the electrical activity of neurons and consequently elicit male sexual responsiveness. RT-PCR analysis indicated that castration induces, while testosterone inhibits, mRNA expression of the K⁺_ATP channel subunit Kir6.2 in both the POA and MBH of adult male rats. Intracerebral infusion of the pharmacological K⁺_ATP channel inhibitor tolbutamide increased the proportion of long-term castrates displaying sexual behavior and restored mount frequency, intromission frequency, and copulatory efficacy to values observed in testes-intact animals. Infusions of tolbutamide, but not vehicle, also decreased latencies to mount and intromit in castrated males. Unilateral tolbutamide infusion directly into the POA significantly reduced mount latency of castrates; however, it did not affect other copulatory measures, suggesting that blockade of K⁺_ATP channels in additional brain regions may be necessary to recover the full range of sexual behavior. These data indicate that blockade of K⁺_ATP channels is sufficient to elicit the male sexual response in the absence of testosterone. Our observations are consistent with the hypothesis that testosterone modulates male sexual behavior by regulating K⁺_ATP channels in the brain. Decreased channel expression or channel blockade may increase the excitability of androgen-target neurons, rendering them more sensitive to the hormonal, chemical, and somatosensory inputs they receive, and potentially increase secretion of neurotransmitters that facilitate sexual behavior.     

The effects of testosterone, estradiol, and dihydrotestosterone on male mouse (Mus musculus) ultrasonic vocalizations

Two experiments examined the effects of the free forms of testosterone (T), dihydrotestosterone (DHT), and estradiol (E₂) upon male mouse (Mus musculus) courtship vocalizations and seminal vesicle weight. In the first experiment, administration of T to castrated males was associated with a large number of vocalizations and large seminal vesicle weights, DHT was associated with large seminal vesicle weights but very few vocalizations, whereas E₂ was associated with low measures of both vocalization and seminal vesicle weight. In the second experiment, T and DHT had effects highly similar to those of the previous experiment; however, in contrast to the previous experiment, both low and high dosages of E₂ were associated with large numbers of ultrasonic vocalizations but small seminal vesicles. A mixture of E₂ and DHT was very similar to T in its effect upon both measures. We suggest from these results that hormonal mechanisms similar to those reported for rat sex behavior may interact with situational variables to determine the expression of male mouse courtship.     

Behavioral characteristics of freemartins administered estradiol, estrone, testosterone, and dihydrotestosterone

Eighteen genetic females born co-twin with males and diagnosed as being sterile intersexes (freemartins) were studied from birth to 79 weeks of age. Testosterone (T) and estrone (EI) were administered in Silastic capsules of two groups from birth to 50 weeks of age and other animals were left untreated. At 50 weeks the two treated groups had larger implants installed and the untreated animals were assigned to a new estrone (EII) and estradiol (E2) treatment. Later a dihydrotestosterone (DHT) group was formed in comparison with new E2 and testosterone propionate-enanthate (TP-TE) groups, plus untreated controls. Vulvar interest, Flehmen lip curl, mounting, and agonistic behavior were recorded daily for 30 min while animals were allowed social interaction. Agonistic behavior, interest in the genital area, and mounting were induced or stimulated by T, TP-TE, and E2, but not by DHT or estrone (EI or EII). Also, only animals in the T, TP-TE, and E2 groups induced to mount displayed the standing type of behavioral estrus. Flehmen lip curl was stimulated only by T or TP-TE. The evidence is interpreted to indicate that T, per se, evokes the lip curl, but it probably stimulates other responses at the neural level by conversion to E2. Also, the freemartin response, the response of castrates to steroid hormones, and current knowledge of circulating steroid hormones in male and female cattle could be interpreted to indicate that the neural tissue responsible for sexual behavior in both sexes of this species may respond similarly in several respects.     

Arginine vasotocin induces sexual behavior of newts by acting on cells in the brain

To investigate whether arginine vasotocin (AVT) acts on target cells in the brain of Taricha granulosa (a urodele amphibian), the behavioral effects of intracerebroventricular (ICV) and intraperitoneal (IP) injections of AVT were compared. Male newts exhibited the greatest sexual activity (amplectic clasping) following an ICV injection of 0.1 μg AVT. Another study showed that nanogram quantities of AVT, administered ICV, stimulated the behavior. An ICV injection of an antagonist to arginine vasopressin, d(CH₂)₅Tyr(Me)AVP, or an anti-AVT immune serum significantly inhibited the sexual behavior. Intracranial implants of 17β-estradiol (E₂) or 5α-dihydrotestosterone (DHT) in castrated males maintained the behavioral response to an injection of AVT. Another study found that an IP injection of DHT or E₂ did not increase the incidence of newt sexual behavior during the 8 hours following the injection.     

Effects of testosterone metabolites on copulation, medial preoptic dopamine, and NOS-immunoreactivity in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. Dopamine (DA) is released in the MPOA of male rats shortly before and during copulation. In a previous study, we identified 17beta-estradiol (E(2)) as the metabolite of testosterone (T) that maintains MPOA basal extracellular DA levels. However, the presence of dihydrotestosterone (DHT), an androgenic metabolite of T, is required for the female-induced increase in MPOA DA observed during copulation. Recently, we reported that assays of MPOA tissue DA content showed that castrates actually had more stored DA than did gonadally intact males. Therefore, the reduction in extracellular levels in castrates was not due to decreased availability of DA; most likely it was due to decreased release. Furthermore, T upregulates neuronal nitric oxide synthase (nNOS) in the MPOA. NO has been implicated in the regulation of DA release in the MPOA. It is not known, however, which metabolite(s) of T regulate(s) tissue stores of DA and/or nNOS in the MPOA of male rats. The present experiments were designed to test the following: (1) whether E(2), DHT, or the combination of the two influences MPOA DA tissue levels, an indication of stored DA, in male rat castrates; and (2) whether E(2), DHT, or the combination of the two influences NOS-ir in the MPOA of castrated male rats. The results indicate that E(2) up-regulates nNOS-ir in the MPOA and maintains tissue content of DA at levels similar to those in T-treated rats. DHT did not influence nNOS-ir, while attenuating the effect of castration on tissue DA content.     

Liraglutide Improves Hypertension and Metabolic Perturbation in a Rat Model of Polycystic Ovarian Syndrome

Polycystic ovarian syndrome (PCOS) is the most common endocrine disorder in women of reproductive age, with a prevalence of 5–8%. Type 2 diabetes and cardiovascular disease (CVD) are its long-term complications. Targeted therapies addressing both these complications together are lacking. Glucagon like peptide-1 (GLP-1) agonists that are used to treat type 2 diabetes mellitus have beneficial effects on the cardiovascular system. Hence we hypothesized that a GLP-1 agonist would improve both cardiovascular and metabolic outcomes in PCOS. To test this hypothesis, we used an established rat model of PCOS. Prepubertal female Sprague Dawley rats were sham-implanted or implanted s.c. with dihydrotestosterone (DHT) pellets (90 day release; 83μg/day). At 12 wks of age, sham implanted rats received saline injections and the DHT treated animals were administered either saline or liraglutide (0.2mg/kg s.c twice daily) for 4 weeks. Subgroups of rats were implanted with telemeters between 12-13 weeks of age to monitor blood pressure. DHT implanted rats had irregular estrus cycles and were significantly heavier than the control females at 12 weeks (mean± SEM 251.9±3.4 vs 216.8±3.4 respectively; p<0.05) and 4 weeks of treatment with liraglutide in DHT treated rats significantly decreased body weight (mean± SEM 294.75 ±3.2 in DHT+ saline vs 276.25±2.7 in DHT+ liraglutide group respectively; p<0.01). Liraglutide treatment in the DHT implanted rats significantly improved glucose excursion during oral glucose tolerance test (area under the curve: DHT+ saline 28674±310 vs 24990± 420 in DHT +liraglutide p <0.01). DHT rats were hypertensive and liraglutide treatment significantly improved mean arterial pressure. These results suggest that GLP-1 treatment could improve DHT–induced metabolic and blood pressure deficits associated with PCOS.     

Effect of estradiol and dihydrotestosterone on hypergravity-induced MAPK signaling and occludin expression in human umbilical vein endothelial cells

Female astronauts have been reported to have a higher incidence of post-flight orthostatic intolerance (POI) compared with that of their male counterparts. POI may result from increased permeability of the endothelial cell (EC) layer in the vasculature. The goal of this study has been to determine whether estradiol (E₂) and dihydrotesterone (DHT) alter human umbilical vein ECs (HUVECs) responses to short term (10 min) hypergravity (1–3 g) mimicking the g force experienced by astronauts during liftoff. E₂ and DHT rapidly (within 5 min) activated MAPK (mitogen-activated protein kinase) in HUVEC at 1 g in a receptor-dependent manner. Liftoff inhibited MAPK phosphorylation, and rapid E₂ and DHT activation of MAPK was blocked. Liftoff simulation or brief (5–90 min) treatment with E₂ or DHT at 1 g had no effect on the expression of the EC tight-junction protein occludin. However, 24-h pre-treatment of HUVECs with E₂ and DHT prior to liftoff simulation significantly increased occludin expression, and hypergravity exposure did not alter this increase. These data provide evidence for a possible protective effect of E₂ and DHT on EC function as indicated by increased occludin; this may help maintain the integrity of EC tight junction and could thus retard or reduce the incidence of POI.This work was supported by NASA grant NAG5-12874 to C.M.K. and R.S.K. and by American Heart Association Ohio Valley Affiliate grant 0555270B to C.M.K. The American Heart Association Ohio Valley supported W.K.S. with an affiliate post-doctoral fellowship (0425431B).     

Hormonal control of sexual and scent marking behaviors of male gerbils in relation to the sexually dimorphic area of the hypothalamus

The sexual and scent marking behaviors of male gerbils are stimulated by testosterone (T) action in the preoptic area (POA) of the hypothalamus. The sexually dimorphic area (SDA) in the posterior POA, which also responds to T, is implicated in this process. This research studied the sensitivities of mating, marking, and the SDA to T metabolites and other steroids. Experiment 1 focused on mating. Male gerbils were implanted at castration with 2-mm Silastic capsules containing T, dihydrotestosterone (DHT), 19-nortestosterone (19-nor T), estradiol (E), or no hormone and were tested 3-7 weeks later. T, E, and 19-nor T maintained intromissions, but E-treated males rarely ejaculated. Controls and DHT-treated males stopped mounting. Experiment 2 compared the ability of these steroids to reinstate marking and mating using the same dose and a larger one (5 mm). Androstenedione, 19-hydroxytestosterone (19-OHT), and E plus DHT were studied as well. Volumes of the SDA and SDA pars compacta (SDApc) were also measured. Only T, 19-nor T, E, and E + DHT reinstated sexual behavior, but all steroids except 19-OHT stimulated marking. E and DHT synergized to elicit mating. For marking, they were no more effective together than alone. Steroid-treated males had larger SDAs than controls. Moreover, steroids that stimulated sexual activity produced larger SDAs than steroids that did not. SDA size correlated with copulatory rate, but not with copulatory efficiency. SDApc size correlated with copulatory efficiency, but not with copulatory rate. Like copulatory rate and efficiency, sizes of the SDA and SDApc did not correlate with each other.     

Induction of sexual receptivity in ovariectomized female rats by subcutaneous implants of estradiol-17β

Ovariectomized female rats were implanted with estradiol-17β (E₂)-filled Silastic capsules at various times and the display of sexual receptivity in response to progesterone (P) treatment, 42 hr after the initiation of E₂ treatment, was observed. Exposure to E₂ for 24 hr or less was insufficient to prepare the animals to respond to P treatment. Implantation of E₂-filled capsules for 48 hr, however, brought all animals into sexual receptivity. Anti-estrogen treatment, both at the time of implantation of E₂-filled capsules and 24 hr after implantation, partially inhibited the effect of a 48-hr exposure to E₂ on the display of sexual behavior. Continued presence of e₂ may be necessary for display of sexual receptivity by Ovariectomized rats.