Activation of sexual behavior by implantation of testosterone propionate and estradiol benzoate into the preoptic area of the male Japanese quail (Coturnix japonica)

Intracranial implantation of minute pellets of gonadal steroids was performed to determine neuroanatomical loci at which steroids activate sexual behavior in the Japanese quail (Coturnix japonica). In this species, systemic treatment of castrated males with either testosterone propionate (TP) or estradiol benzoate (EB) restores male-typical copulatory behavior (head grabbing, mounting, and cloacal contact movements). In addition, EB activates female-typical receptive behavior (crouching). Adult male castrated quail were implanted intracranially with 300-micrograms pellets containing TP, EB, or cholesterol (CHOL) and behavior was tested with intact males and females. Either TP or EB pellets in the preoptic area (POA) activated male-typical copulatory behavior. Mounting was specifically activated without concomitant activation of other steroid-sensitive sexual and courtship behaviors. TP and EB implants in adjacent nuclei containing receptors for these steroids and CHOL implants in POA had no effect on male-typical copulatory behavior. Eighteen percent of all males tested for female-typical receptivity crouched, but no specific effect of EB was seen at any site. The similarity of the POA sites for activation of mounting by TP and EB is consistent with the hypothesis that cells within the POA aromatize testosterone to estrogens, which directly stimulate the cellular processes leading to behavioral activation.     

Testosterone implanted in the preoptic area of male Japanese quail must be aromatized to activate copulation

Intracranial implantation of minute pellets of gonadal steroids was combined with aromatase inhibitor treatment to determine if aromatization within the preoptic area (POA) is necessary for androgens to activate sexual behavior in the Japanese quail (Coturnix japonica). In this species, implantation of pellets of testosterone propionate (TP) or estradiol benzoate (EB) in the POA of castrated males restores male-typical copulatory behavior. In Experiment 1, adult male castrated quail were implanted intracranially with 200-micrograms pellets of equimolar mixtures of crystalline TP + cholesterol (CHOL), TP + 1,4,6-androstatriene-3,17-dione (ATD, an aromatase inhibitor), EB + ATD, or CHOL and behavior-tested with intact males and females. Copulation was stimulated by POA implants containing TP or EB (three of six CHOL + TP males and two of seven ATD + EB males copulated vs zero of four CHOL males), but copulation was not inhibited by combining ATD with TP (three of four ATD + TP males copulated). In Experiment 2, adult male castrated quail were injected systemically with ATD or oil for 6 days prior to and 14 days after intracranial implantation of 200-micrograms pellets containing the same amounts of TP or EB as in Experiment 1. The ATD injections completely blocked copulatory behavior in males with TP implants in the POA such that ATD/TP and Oil/TP mount frequencies differed significantly, but failed to block copulation in males with EB implants in the POA (proportions of males copulating were ATD/EB, 6/8; ATD/TP, 0/6; Oil/TP, 4/7). The cloacal foam gland, an androgen-sensitive secondary sex character, was unaffected by the dose of ATD used. We conclude that activation of copulatory behavior by TP implants in the POA is not due to nonspecific effects of high local testosterone concentrations but rather to aromatization. These results support the hypothesis that cells within the POA aromatize testosterone to estrogens, which directly stimulate the cellular processes leading to activation of male-typical copulatory behavior.     

Androgen- and estrogen-induced copulatory behavior and inhibition of luteinizing hormone (LH) secretion in the male rat

The purpose of the present investigation was to determine if estrogen, aromatizable androgen or nonaromatizable androgen is capable of (1) inducing copulatory behavior and (2) inhibiting the postcastration rise in plasma LH. Castrate male rats were injected daily with either 1 mg testosterone (T), androstenedione (A), dihydrotestosterone (DHT), or 25 μg estradiol benzoate (EB) or oil and tested weekly for masculine behavior and for lordosis behavior after 38 days of steroid treatment. On day 40 blood was collected for radioimmunoassay of plasma LH. At least 89% of the males treated with T, A, or EB and 55% of those treated with DHT displayed ejaculatory behavior whereas none of the oil-treated males showed male copulatory behavior. Only estrogen-treated males displayed lordosis behavior. T and to a lesser extent A treatment reduced high levels of plasma LH; however, DHT and EB further reduced plasma LH to undectable levels. The relative potency of the steroid effect in stimulating accessory sex tissues followed the order: DHT > T > A > EB = oil. Significant dissociation was observed between the effects of these steroids on peripheral morphology, negative feedback, and mating behavior. These results indicate that masculine behavior is facilitated to the greatest extent, although not exclusively, by centrally acting aromatizable androgen or estrogen, whereas under the present conditions only estrogen stimulates feminine behavior.     

Induction of male mating behavior in androgen-insensitive (tfm) and Normal (King-Holtzman) male rats: effect of testosterone propionate, estradiol benzoate, and dihydrotestosterone

Castrated androgen-insensitive rats exhibited mounting and intromission patterns in response to testosterone propionate (TP), estradiol benzoate (EB), or EB combined with dihydrotestosterone (DHT) treatment in adulthood. Treatment with DHT alone was ineffective in stimulating male mating behavior in the mutant rats. Since androgen-insensitive rats, like normal males, have the potential to show mounting behavior following hormone treatment in adulthood, the neural substrate underlying this behavior must be masculinized during development. The effectiveness of gonadal hormones in activating the entire copulatory sequence in castrated littermate males (King-Holtzman) was also examined. TP treatment induced mating behavior in the control rats. DHT also stimulated the complete copulatory pattern, although it was not as effective as TP. The administration of EB, however, did not induce ejaculation in control rats. These results do not support the hypothesis that the activation of male mating behavior by testosterone requires its metabolite estrogen (aromatization hypothesis).     

Steroidal control of sexual behavior in the rough-skinned newt (Taricha granulosa): effects of testosterone, estradiol, and dihydrotestosterone

Adult, sexually mature, male rough-skinned newts (Taricha granulosa) obtained from a wild population were castrated and received Silastic capsules containing testosterone (T), estradiol (E), or 5 alpha-dihydrotestosterone (DHT). The newts received three capsules of T, either one or three capsules of E or DHT, or combined treatments with these two steroids. When tested for sexual responsiveness after 32 and 34 days of steroid treatment, no group differed from the castrated controls (C). After 74 and 75 days of treatment, more T-implanted than C newts were sexually responsive, but the newts treated with E, DHT, or these two steroids in combination did not differ behaviorally from the C group. The diameter of the vas deferens was greater in the T- and DHT-treated males than in the C males, indicating that the implants adequately replaced testicular androgens. Together with other studies on this and other species, these results confirm the participation of testosterone in the regulation of sexual behaviors in male amphibians. Furthermore, these results indicate that in this amphibian, the behavioral effects of T are mediated directly by this steroid, not indirectly by enzymatic conversion to DHT or E.     

The role of aromatization in androgen stimulation of gerbil scent marking

The androgen dependent scent marking behavior of male Mongolian gerbils (Meriones unguiculatus) can be stimulated after castration by either testosterone or estrogen, but not by dihydrotestosterone (DHT). To determine if DHT fails to evoke scent marking because it cannot be aromatized to form an estrogen, two other nonaromatizable androgens, 1α-methyltestosterone and 6α-fluorotestosterone, were studied. 6α-Fluorotestosterone and its propionate ester stimulated scent marking in castrated male gerbils as effectively as testosterone and its ester did. Hence, an androgen's aromatizability does not determine its ability to influence gerbil scent marking behavior.     

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.     

Estradiol Mediates Effects of Testosterone on Vasotocin Immunoreactivity in the Adult Quail Brain

In adult male quail, the activation of sexual behavior by testosterone (T) is mediated at the cellular level by the interaction of T metabolites with intracellular steroid receptors. In particular, the aromatization of T into an estrogen plays a key limiting role. Nonaromatizable androgens such 5alpha-dihydrotestosterone (DHT) synergize with estradiol (E2) to activate the behavior. Given that the density of vasotocin (VT) immunoreactive structures is increased by T in adult male quail and that VT injections affect male behavior, we wondered whether the expression of VT is also affected by T metabolites such as E2 and DHT. We analyzed here, in castrated male quail, the effects of a treatment with T, E2, DHT, or E2 + DHT on sexual behavior and brain VT immunoreactivity. The restoration by T of the VT immunoreactivity in the medial preoptic nucleus, bed nucleus striae terminalis, and lateral septum of castrated male quail could be fully mimicked by a treatment with E2. The androgen DHT had absolutely no effect on the VT immunoreactivity in these conditions and, at the doses used here, DHT did not synergize with E2 to enhance the density of VT immunoreactive structures. These effects of T metabolites in the brain were not fully correlated with their effects on the activation of male copulatory behavior, suggesting that the increase in VT expression in the brain does not represent a necessary step for the activation of behavior. Although VT expression in the medial preoptic nucleus and bed nucleus striae terminalis is often tightly correlated with the expression of male copulatory behavior, VT presumably does not represent simply one step in the biochemical cascade of events that is induced by T in the brain and leads to the expression of male sexual behavior.     

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.     

Induction of pseudopregnancy in the mongolian gerbil (Meriones unguiculatus) by vaginal stimulation.

In rats, pseudopregnancy has been induced by mating with vasectomized males, by mechanical stimulation of the uterine cervix with a glass rod or vibrator, and by stimulation of the vagina with a tampon. On the other hand, no practical data are available in reports on the induction of pseudopregnancy in Mongolian gerbils. Pseudopregnancy of gerbils has been induced by mating with vasectomized males. But this method was uncertain because the incidence of pseudopregnancy was lower than that obtained in rats by other means. In the present study, two experiments were undertaken as follows. 1) Copulatory behavior of gerbils was observed for one hour to determine the most effective stimulation interval. 2) From the results of Experiment 1, female gerbils in estrus were mechanically stimulated to test the effectiveness of inducing pseudopregnancy by vaginal stimulation at various time intervals. The results of these experiments indicated that, although the frequency of copulatory behavior varied among individuals, on average the most effective method for inducing pseudopregnancy was stimulation of 5 min duration and at 20 or 30 min intervals. Because the incidence of pseudopregnancy induced by such mechanical stimulation (83.3%) was higher than that induced by mating with vasectomized males (30.0%), this method might be useful in inducing pseudopregnancy in Mongolian gerbils.     

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.     

Androgenic regulation of chemoinvestigatory behaviors in male and female hamsters

In male hamsters, chemosensory responsiveness to sexually relevant female odors is facilitated by testosterone (T). Some evidence suggests that this is not a sexually dimorphic response in that adult females can respond similarly to males following administration of T. This was evaluated and additionally, the hypothesis that facilitation of chemosensory responsiveness by T might be mediated by the conversion of T to aromatized or 5 alpha-reduced metabolites was tested. In 2-min tests, we measured the time adult males or females investigated female hamster vaginal secretion (FHVS). These animals were gonadectomized and administered T, 5 alpha-dihydrotestosterone (DHT), estradiol (E2), or a combination of DHT and E2, by subcutaneous implantation of Silastic capsules. FHVS tests were conducted either 2 and 4 weeks, or 4 and 6 weeks subsequent to gonadectomy and hormone treatment. Comparisons among groups receiving different hormone doses indicated that (1) males and females are not equally responsive to the attractant properties of FHVS, and that (2) neither DHT, E2, nor their combination, can duplicate the effects of T in facilitating responsiveness to FHVS in either sex. The copulatory behavior of males under the hormone conditions described was also tested and it was found that variations in the rate at which the test males sniffed or licked the receptive female's anogenital region correlated with variations in measures of the males' sexual performance.     

Sexual responses of the male rat medial preoptic area and medial amygdala to estrogen II: site specific effects of selective estrogenic drugs

In the medial preoptic area (MPO) and medial amygdala (MEA), estradiol (E(2)) aromatized from testosterone (T) may act via either estrogen receptor (ER) α or ERβ to mediate mating in male rats. We tested the hypothesis that, in the MPO, ERα exclusively mediates sexual responses to E(2) by monitoring mating in four groups of castrated male rats administered dihydrotestosterone (DHT) subcutaneously and MPO implants delivering either: cholesterol, E(2), propyl pyrazole triol (PPT, ERα-agonist) or diarylpropionitrile (DPN, ER β-agonist); a fifth group of intact males served as DPN toxicity control, receiving DPN MPO implants. In a follow-up study, either 1-methyl-4-phenyl pyridinium (MPP, ERα-antagonist) or blank MPO cannulae were implanted in castrated male rats receiving T subcutaneously, whereas intact MPP toxicity controls received MPP MEA implants. PPT or E(2) MPO implants maintained mating, but cholesterol or DPN MPO implants did not. Moreover, MPP MPO implants interfered with T reinstatement of mating suggesting that, in the MPO, ERα is necessary and sufficient for mating in androgen-maintained male rats and ERβ is not sufficient. Because it is unknown which ER subtype(s) mediate sexual responses of the MEA to E(2), we examined mating following MEA implants of cholesterol, E(2), PPT or DPN in four groups of castrated male rats administered DHT subcutaneously. E(2) MEA implants maintained mounting but mating was significantly decreased in groups receiving PPT, DPN or cholesterol MEA implants suggesting that, unlike the MPO where ERα alone is essential, sexual responses of the MEA to E(2) require simultaneous interactions among multiple ER subtypes.     

Differential maintenance of penile responses and copulatory behavior by gonadal hormones in castrated male rats

Sexually experienced male rats were castrated and immediately received implants of Silastic tubing containing either testosterone (T), dihydrotestosterone (DHT), estradiol (E), or nothing (blank). The ability of these hormone treatments to maintain precastration levels of copulatory behavior and ex copula penile responses was assessed for 40 days after castration. Throughout the study T- and E-treated males, but not males with DHT or blank implants, maintained normal copulatory behavior. In contrast males treated with T and DHT, but not E or blanks, maintained penile responses ex copula. In blank-treated males, penile-response latencies increased more rapidly than did intromission latencies. These results, together with those of previous studies, appear to rule out a role for estradiol and reinforce the role of androgens in the activation of rats' penile-response potential ex copula. Similarly, the results support the conclusion that in castrated male rats estradiol treatment is sufficient for the activation of masculine copulatory behavior, and that the penile actions necessary for intromission are not dependent on androgen. Thus, the evocability of penile actions and their relative androgen dependence are context sensitive.     

Hormonal specificity and activation of sexual behavior in male zebra finches

Castrated zebra finches receiving one of six hormone treatments were given three weekly tests with different females and their sexual behavior was contrasted with that of two control groups consisting of intact or castrated males given implants of cholesterol. The six hormone treatments were: two aromatizable androgens, testosterone (T) and androstenedione (AE); two nonaromatizable androgens, androsterone (AN) and dihydrotestosterone (DHT); an estrogen, estradiol (E); or a combination of E + DHT. Half the males receiving DHT received the 5α-isomer, half received the 5β-isomer. Castration significantly reduced the proportion of males which courted females, total courtship displays, high-intensity courtship displays, beak wiping activity, and significantly increased the latencies to show these behaviors compared to intact males. Castrated males never attempted to mount a female. All of these measures of courtship and copulatory behavior were restored to normal levels only by treatments providing both estrogenic and α-androgenic metabolites (i.e., T, AE, E + αDHT). AE was clearly the most effective of these, raising behavior significantly above normal on several measures. AN treatment was more effective than αDHT on all measures and not significantly different from intact birds on some. Treatment with E, αDHT, βDHT, or E + βDHT was totally ineffective. Surprisingly, females only solicited males whose hormone treatments provided estrogenic metabolites. Not only did they solicit males given aromatizable androgens, which showed high rates of courtship activity, they also solicited males given E or E + βDHT, some of which never even courted. Castration and hormone treatment also affected body and syringeal weight, but in opposite directions. Castration increased body weight while decreasing syringeal weight. Hormone treatments providing α-androgenic metabolites decreased body weight and increased syrinx weight. Treatments supplying estrogen as well were slightly more effective.     

Hypertrophy of gonadotropin releasing hormone-containing neurons after castration in the teleost, Haplochromis burtoni.

In the African cichlid fish, Haplochromis burtoni, males are either territorial or nonterritorial. Territorial males suppress reproductive function in the nonterritorial males, and have larger gonads and larger gonadotropin-releasing hormone- (GnRH) containing neurons in the preoptic area (POA). We describe an experiment designed to establish the causal relationship between large GnRH neurons and large testes in these males by determining the feedback effects of gonadal sex steroids on the GnRH neurons. Territorial males were either castrated or sham-operated, 4 weeks after which they were sacrificed. Circulating steroid levels were measured, and the GnRH-containing neurons were visualized by staining sagittal sections of the brains with an antibody to salmon GnRH. The soma areas of antibody-stained neurons were measured with a computer-aided imaging system. Completely castrated males had markedly reduced levels of circulating sex steroids [11-ketotestosterone (11KT) and testosterone (T)], as well as 17 beta-estradiol (E2). POA GnRH neurons in castrates showed a significant increase in mean soma size relative to the intact territorial males. Hence, in mature animals, gonadal steroids act as a brake on the growth of GnRH-containing neurons, and gonadal products are not responsible for the large GnRH neurons characteristic of territorial males.     

Effect of forebrain implants of testosterone or estradiol on scent-marking and sexual behavior in male and female rabbits

Chinning consists of rubbing the chin against an object, thereby depositing secretions from the submandibular glands. As mating, chinning is stimulated in male and female rabbits by testosterone and estradiol, respectively. To investigate the brain sites where steroids act to stimulate chinning and mating we implanted into the ventromedial hypothalamus (VMH) or the medial preoptic area (MPOA) of gonadectomized male and female rabbits testosterone propionate (TP; males) or estradiol benzoate (EB; females) and quantified chinning and sexual behavior. EB implants into the VMH or MPOA reliably stimulated chinning in females. Most of those implanted into the VMH and around half of the ones receiving EB into MPOA or diagonal band of Broca (DBB) showed lordosis. Chinning, but not sexual behavior, was stimulated in males by TP implants into the MPOA or DBB. Neither chinning nor mounting were reliably displayed by males following TP implants into the VMH. Results indicate that, in females, the VMH is an estrogen-sensitive brain area that stimulates both chinning and lordosis while the MPOA seems to contain subpopulations of neurons involved in either behavior. In males, androgen-sensitive neurons of the MPOA, but not the VMH, are involved in chinning stimulation but it is unclear if these areas also participate in the regulation of copulatory behavior.     

Effect of forebrain implants of testosterone or estradiol on scent-marking and sexual behavior in male and female rabbits

Chinning consists of rubbing the chin against an object, thereby depositing secretions from the submandibular glands. As mating, chinning is stimulated in male and female rabbits by testosterone and estradiol, respectively. To investigate the brain sites where steroids act to stimulate chinning and mating we implanted into the ventromedial hypothalamus (VMH) or the medial preoptic area (MPOA) of gonadectomized male and female rabbits testosterone propionate (TP; males) or estradiol benzoate (EB; females) and quantified chinning and sexual behavior. EB implants into the VMH or MPOA reliably stimulated chinning in females. Most of those implanted into the VMH and around half of the ones receiving EB into MPOA or diagonal band of Broca (DBB) showed lordosis. Chinning, but not sexual behavior, was stimulated in males by TP implants into the MPOA or DBB. Neither chinning nor mounting were reliably displayed by males following TP implants into the VMH. Results indicate that, in females, the VMH is an estrogen-sensitive brain area that stimulates both chinning and lordosis while the MPOA seems to contain subpopulations of neurons involved in either behavior. In males, androgen-sensitive neurons of the MPOA, but not the VMH, are involved in chinning stimulation but it is unclear if these areas also participate in the regulation of copulatory behavior.     

Regulation of urine marking in male and female mice: effects of sex steroids

Effects of sex steroids on urine-marking activity were studied in male, female, and neonatally androgenized female mice. Urine marking was estimated by suspending ceramic tubes that were connected in a horizontal row with a steel rod into the home cage of an isolated mouse. Intact males showed high marking activity, which was diminished after castration. Both testosterone propionate (TP) and estradiol benzoate (EB) were effective in restoring the marking activity of castrated males, while 5-alpha-dihydrotesterone (DHT) did not have any stimulative effects. Intact normal females showed quite low marking activity and ovariectomy further depressed it. TP and DHT enhanced the marking of ovariectomized females, but EB restored the activity only to the preovariectomy level. In intact females which were neonatally androgenized, the marking activity was much higher than that of normal females. The pattern of the change induced by gonadectomy and hormone treatment in these females resembled that in males. Thus, ovariectomy reduced the activity and both TP and EB restored the level. These results indicate that the sexual dimorphism in the urine marking in mice is primarily determined by hormonal environment during early postnatal age. Hormonal control of scent marking is discussed in relation to the studies in other rodents.     

Both estrogen receptors and androgen receptors contribute to testosterone-induced changes in the morphology of the medial amygdala and sexual arousal in male rats

In male rats, a steroid-sensitive circuit in the forebrain regulates mating behavior. The masculine phenotype in one component of the circuit, the posterodorsal nucleus of the medial amygdala (MePD), depends on the level of circulating androgens in the adult. To investigate which gonadal steroid receptor(s) mediate sexual arousal and MePD plasticity, adult male rats were castrated and given Silastic capsules containing the nonaromatizable androgen 5alpha-dihydrotestosterone (DHT), 17beta-estradiol (E2), both steroids, or nothing. A fifth group was sham-castrated and treated with blank capsules. DHT treatment was necessary and sufficient to maintain the expression of noncontact penile erections and ultrasonic vocalizations in castrates. E2 had no significant effect on these measures. Both DHT and E2 increased olfactory investigation ("nosepokes") during the noncontact penile erection test. E2, but not DHT, maintained intromission patterns, while either steroid, alone or in combination, maintained ejaculatory behavior. Regional volume and cell soma size of the MePD both decreased following castration. Additionally, MePD cell size was lateralized, with left hemisphere neurons larger than those on the right, an effect that appeared independent of steroid manipulations. DHT and E2 each maintained neuronal soma size. E2 maintained MePD regional volume more effectively in the left MePD than in the right, which may have been due to a greater sensitivity of the left to both castration and hormone treatment. Thus, both androgen receptors and estrogen receptors appear to participate in sexual behaviors that may be mediated by the MePD in adult rats, and both receptors contribute to the steroid-regulated structural plasticity in this brain region.