Independence of the differentiation of masculine and feminine sexual behavior in rats.

Male rats received Silastic implants of the aromatase inhibitor, 1,4,6-androstatriene-3, 17-dione (ATD), on days 2–10 of life. Controls received blank implants. There were no differences in the masculine sexual behavior of ATD and control males when they were tested as gonadally intact adults. In contrast, even without exogenous hormone treatment, nine of 14 ATD males exhibited lordosis behavior, whereas only one of 12 controls did so. In addition, during a sexual preference test in which access was provided to both a sexually receptive female and to a stud male, there was no difference in the proportions of ATD ($\text{11}\text{14}$) and control ($\text{7}\text{12}$) males that copulated with the stimulus female; however, seven of the ATD males also exhibited feminine sexual behavior including some instances of solicitation. Only one of the control males showed any lordosis behavior. In general, all animals spent more time with the stimulus female than with the stud male. At the termination of preference testing, all animals were castrated and then tested twice for feminine sexual behavior under exogenous estradiol benzoate and progesterone. All of the ATD males showed lordosis behavior with a mean lordosis quotient (LQ) of 85; and 11 of the 14 also showed solicitation behavior. Only five of 12 control males exhibited lordosis ($\text{X}\text{?}\text{LQ}\text{= 59}$) and only one showed solicitation behavior. These results indicate that the propensity of males to show feminine sexual behavior can be manipulated independently of the capacity for masculine sexual behavior. Moreover, our results suggest that the process of defeminization may occur primarily postnatally in rats since treatment during that period results in substantial increments in later feminine sexual behavior including solicitation behaviors.     

Diethylstilbestrol facilitates masculine and feminine copulatory behavior in female rats

Diethylstilbestrol (DES), which binds to estrogen receptor without crossreacting to androgen receptor, was tested for its ability to facilitate male-typical mounting behavior. The administration of either 30 or 90 μg of DES per day to adult ovariectomized female rats activated mounting behavior in 94% of the animals (last three tests). The response to the two dosages did not differ. The mount frequency response to 30 μg of DES of 6.8 (mean of last three tests) was only about half as high as the response to 100 μg of testosterone propionate. All animals receiving DES also responded with lordosis behavior when mounted. The lower dosage of DES depleted 91% of hypothalamic cytosol estrogen receptors, and the higher dosage 96%. Androgen receptors were not depleted. Thus the synthetic estrogen DES can facilitate masculine sex behavior by interacting with estrogen receptors without binding to or translocating androgen receptors.     

Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat

Oxytocin (OT) is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive function and behavior has been well established. The majority of pharmacological studies of the effects of OT on male sexual behavior have focused on the paraventricular nucleus (PVN), ventral tegmental area (VTA), hippocampus, and amygdala. Less attention has been given to the medial preoptic area (MPOA), a major integrative site for male sexual behavior. The present study investigated the effects of intra-MPOA administration of OT and (d(CH2)51, Tyr(Me)2, Thr4, Orn8, Tyr-NH29)-vasotocin, an OT antagonist (OTA), on copulation in the male rat. The relationship between OT receptor (OTR) binding levels in the MPOA and sexual efficiency was also explored. Microinjection of OT into the MPOA facilitated copulation in sexually experienced male rats, whereas similar injections of an OTA inhibited certain aspects of copulation but had no significant effect on locomotor activity in an open field. Contrary to expectation, sexually efficient males had lower levels of OTR binding in the rostral MPOA compared to inefficient animals. The present data suggest that OT activity in the MPOA is not necessary for the expression of male sexual behavior but is sufficient to facilitate copulatory behaviors and improve sexual efficiency in sexually experienced male rats. These data also suggest that OTR activity in the MPOA stimulates anogenital investigation, facilitates the initiation of copulation, and plays a role in the sensitization effect of the first ejaculation on subsequent ejaculations.     

Estrogen in the medial preoptic area of male rats facilitates copulatory behavior

Mating was studied in sexually experienced, gonadally intact male rats assigned to two surgical groups matched on the basis of mean mounting frequency during behavioral screening trials conducted prior to the study. Estradiol (E(2)) was delivered bilaterally into the medial preoptic area (MPO) of experimental males by means of hormone-coated implants, and fadrozole was given sc (0.25 mg/kg/day) via osmotic minipumps to block E(2) formation from testicular testosterone throughout the brain. Control males received blank bilateral implants in the MPO and sc fadrozole. After the completion of behavioral testing, immunocytochemical comparisons of the brains from experimental and control rats were made using the H222 antiestrogen receptor (ER) antibody, whose labeling is inhibited by the presence of E(2). The histology demonstrated that E(2) was confined exclusively to the MPO of experimental males but was absent throughout the brains of controls. In controls, mounting decreased significantly by the 7th day after surgery compared with presurgical levels and did not recover. In contrast, on all postsurgical days, the mounting frequency of the experimental group was significantly higher than that of controls. Although experimental males also showed an initial, significant postsurgical decline in mounting frequency, it recovered completely by the 28th postoperative day. Ejaculations declined significantly after surgery in both groups but, unlike in controls whose performance remained low, ejaculations in experimental males partially recovered and were significantly higher than in controls during the postoperative period. Results showed that ER-containing neurons in the MPO influence male rat copulatory behavior.     

Prenatal endogenous androgenic influences on masculine sexual behavior and genital morphology in male and female rats

Female rats located near a male during uterine development showed increased frequencies of male-like behavior as adults and virilization of genital morphology. These changes in behavior and morphology were blocked by prenatal treatment with the anti-androgen, Flutamide.     

Differential effects of two estrogen antagonists on the development of masculine and feminine sexual behavior in hamsters

To determine whether nonsteroidal antiestrogens can be used to investigate the role of aromatization in behavioral masculinization and defeminization, newborn male and female hamsters were administered 0.5 or 5.0 μg of the antiestrogens nafoxidine or tamoxifen on postnatal Days 1 and 2. Other females received 1.0 μg of the synthetic estrogen RU-2858 (RU) alone or in combination with 0.5 or 5.0 μg of nafoxidine or tamoxifen. All animals were tested for the display of masculine and feminine sexual behaviors in adulthood. Nafoxidine, tamoxifen, and RU all reduced lordosis behavior in adult females, indicating that the antiestrogens probably have some estrogenic properties. Nafoxidine had no effect on male mating behavior in female hamsters when given alone; when given to male hamsters or female hamsters receiving a partially masculinizing dose of RU, this compound effectively reduced the frequencies of masculine sexual behaviors (mounts and intromissions) displayed by the treated animals. However, nafoxidine-treated males had the same mating efficiency (ME = intromissions/ mounts) as control males. Tamoxifen, in contrast, facilitated the display of mounting behavior in females when given alone or in combination with RU. Male hamsters receiving 5.0 μg of tamoxifen had high mount frequencies and slightly reduced intromission frequencies, but their ME scores were only half of control levels. Thus nafoxidine itself simultaneously promoted defeminization and antagonized masculinization while tamoxifen appeared to facilitate both processes. The data support the hypothesis that estrogens derived via aromatization from androgens play an important role in both masculinization and defeminization, at least in hamsters. The differential effects of tamoxifen and nafoxidine suggest that these and other antiestrogens might serve as useful tools for dissociating and independently examining these two components of the sexual differentiation process.     

Changes in masculine sexual behavior, corticosterone and testosterone in response to acute and chronic stress in male rats

Chronic exposure to stressors increases HPA axis activity and concomitantly reduces HPG axis activity. This antagonistic relationship between both these axes has been proposed to underlie the inhibition of reproductive function due to stress. Sexual behavior in males may be the most vulnerable aspect of male reproduction to acute and chronic stress and it has been suggested that alterations in sexual behavior during stress are due to the antagonistic relationship between testosterone and corticosteroids. However, only in a few studies has a correlation between the levels of testosterone and corticosterone, and sexual behavior been made. In this study, we evaluated the effects of different stressors, applied both acute and chronically, on masculine sexual behavior and whether or not these effects on sexual behavior are accompanied by changes in plasma levels of corticosterone and testosterone. Additionally, we evaluated the effect of testosterone treatment on the effects of stress on sexual behavior. Sexually experienced male rats were exposed to one of the following stressors: immobilization (IMB), electric foot shocks (EFS) or immersion in cold water (ICW). Sexual behavior and plasma levels of testosterone and corticosterone were assessed on days 1, 5, 10, 15, and 20 of stress. In a second experiment, males were castrated, treated with 3 different doses of testosterone propionate (TP) and exposed to ICW for 20 consecutive days. Sexual behavior was assessed on days 1, 5, 10, 15, and 20 and steroids were evaluated on day 20. Parameters of masculine sexual behavior were modified depending on the characteristics of each stressor. Mount, intromission and ejaculation latencies increased significantly, the number of mounts increased, and ejaculations decreased significantly in males exposed to EFS and to ICW but not in males exposed to IMB. Associated with these effects, testosterone decreased in the EFS and ICW groups on days 1, 15, and 20. However, corticosterone increased only in males exposed to ICW. In castrated males, TP treatment failed to block the effects of stress by ICW on sexual behavior and corticosterone. These results indicate that the effects of stress on sexual behavior depend on the characteristics of each stressor, and these effects, as well as the decrease in testosterone are not necessarily associated with the increase in corticosterone. The fact that testosterone treatment did not prevent the effects of stress on sexual behavior suggests that other mediators could be involved in the alterations of sexual behavior caused by stress.     

Role of aromatization in anticipatory and consummatory aspects of sexual behavior in male rats

The present study was designed to test the hypothesis that aromatization is involved in the maintenance by testosterone of the appetitive component of male sexual behavior. We measured appetitive sexual behavior by administering behavioral tests in bilevel chambers and quantifying anticipatory level changes during a 5-min period prior to introduction of a stimulus female. In addition, we recorded standard measures of consummatory male sexual behavior after the female was introduced. Following 3 weekly tests, level-changing behavior reached a plateau and remained stable for up to 10 weeks. After 10 bilevel tests, rats were given subcutaneous testosterone capsules to clamp circulating androgen at physiological levels. Rats were tested and divided into two groups that were matched for measures of sexual behavior. One group was then treated with the nonsteroidal aromatase inhibitor, Fadrozole (2.5 mg/kg/day), given subcutaneously in beta-cyclodextrin and the other group was treated with vehicle. Within 1 week of Fadrozole treatment, the number of anticipatory levels changes was significantly reduced, but not the latency to begin searching. Fadrozole treatment also significantly reduced all measures of copulatory behavior over the period of treatment and increased latencies to first mount, intromission, and ejaculation. After 8 weeks, both treatment groups were given an additional Silastic capsule filled with estradiol and tested for 4 additional weeks. Estrogen treatment partially restored level-changing behavior, mounts, and intromissions but had little effect on ejaculations. These results support the view that aromatization is important for maintaining both the appetitive and the consummatory aspects of sexual behavior in male rats.     

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.     

Role of neurotransmitters in coordination of male sexual behavior

The review considers the published data and the results obtained by the author on the role of monoamines (serotonin, dopamine, and noradrenaline) in the control of male sexual behavior. In the above respect, the central mechanisms of action of the neurotransmitters are discussed.     

Bisexual behavior in the male rat: Influence of masculine sexual activity on the display of lordosis behavior

Sexually inexperienced male Wistar rats (strain WI in our colony) known to very infrequently display spontaneous lordosis behavior (Schaeffer et al., 1990b) were used. A first group was tested four times at 5-day intervals for lordosis with vigorous stimulus males (heterotypic sexual behavior), immediately following testing for masculine sexual activity with highly receptive females (homotypic sexual behavior). A small number of animals displayed lordosis during the first test, but more and more animals displayed this behavior from the first to the fourth test. There was no relationship between the degree of masculine sexual activity--intromission without ejaculation or ejaculation--and the occurrence of lordosis behavior. A second group was tested only once for both masculine sexual activity and lordosis behavior as above and afterwards three times at 5-day intervals for lordosis behavior in the absence of any previous testing for masculine sexual activity. A few animals displayed lordosis during their first test. As compared to the first group, the animals which had not displayed lordosis in the first test never showed lordosis responses in the following tests. It is concluded that both homotypic and heterotypic sexual interactions are required for the display of lordosis behavior in the strain of Wistar rats used in this study.     

Role of the medial preoptic area in sexual behaviour of the male rat: a study using repeated cycloheximide infusions

The effect of cycloheximide (CHX), an inhibitor of proteosynthesis, on sexual behaviour was studied in adult male rats in which it was infused into the medial preoptic area of the hypothalamus (MPOA). Sexual interaction took place under control and modification of the precopulatory behaviour of the female. Among the various bilaterally infused amounts of CHX--20 micrograms in 1 microliter, 40 micrograms in 1 microliter, 80 micrograms in 2 microliters--the effect of the largest dose corresponded to a hypothetical state of copulatory readiness of the males; 2 h after administration the males were not capable of initiating copulatory behaviour with a passively receptive female, but did so successfully with a highly soliciting female. The dependence of copulatory readiness of the males on the precopulatory behaviour of the females was confirmed repeatedly after this dose of CHX. At the same time, the precopulatory activity of the males towards a passively receptive female was unimpaired. The effect of CHX was reversible; 48 h after infusion the males displayed high copulatory readiness. Only half the males (n = 7) given bilateral infusions of 80 micrograms CHX fulfilled criterion of copulatory performance. Histological control demonstrated that the MPOA was affected bilaterally by CHX infusion in 10 males out of 14. The results are discussed from the aspect of participation of the MPOA in the regulation of male sexual behaviour. The method allows changes in sexual behaviour to be studied in the same individual in a chronic experiment.     

Possible role of cingulate cortex in regulating sexual behavior in male rats: effects of lesions and cuts.

The role of the cingulate cortex in regulating male sexual behavior was studied in testosterone propionate-treated castrated male rats. Males with lesions in the anterior part of the cingulate cortex showed lower levels of mount, intromission and ejaculation activities than sham-operated control males and males with lesions in the posterior part of the cingulate cortex or the frontal cortex. In male rats in which lateral connections of the anterior cingulate cortex were bilaterally interrupted by sagittal cuts, the sexual activity was much lower than in the control rats, being comparable to that of the anterior cingulate cortex lesion group, but transection of the anterior connections by a transverse cut made in the anterior part of the anterior cingulate had no effect. These results suggest that the anterior cingulate cortex and its lateral connections are critical in regulating male sexual behavior in male rats.     

Neonatal hormonal influences on the development of proceptive and receptive feminine sexual behavior in rats

The objective of this study was to examine the influence of androgen and of the inhibiting of aromatization of androgen to estrogen during the early neonatal period on the development of receptive (lordosis and acceptance of stimulus male mounting attempts) and proceptive (affiliation with and solicitation of stimulus males) feminine sexual behavior. Within 8 hr of birth, male rats were castrated or received subcutaneous implants of the aromatase inhibitor androst-1,4,6-triene-3, 17-dione (ATD) while females received injections of testosterone propionate (TP). At 90 days of age all treated animals and controls were tested for receptive and proceptive feminine sexual behavior. It was found that androgen present neonatally blocked proceptive as well as receptive behavior patterns in adult rats. The proceptive and receptive feminine sexual behavior patterns displayed by adult males deprived of the effects of androgen neonatally either by castration or by treatment with ATD were comparable to those of normal females.     

Plasma corticosterone levels during sexual behavior in male rats

The activity of the pituitary-adrenal (p-a) system, as reflected in plasma levels of corticosterone, was determined in 14 male Long Evans rats during copulation, exposure to an open field and in control conditions. Plasma corticosterone concentration during copulation was higher than in control conditions (13.3 ± 1.2 vs 9.4 ± 1.2 μm%, p < .01), but well below mean levels obtained in the open field (21.2 ± 0.8 μm%). Individual data indicated that some males gave no evidence of p-a activation during sexual activity. Furthermore, animals which showed increased steroid levels during copulation tended to have longer latencies to reinitiate copulation after ejaculation and were behaviorally less active in a subsequent open field test. It was suggested that neither sexual arousal nor copulatory performance necessarily activates the p-a system. Males showing p-a activation may be slow to habituate to a novel stimulus and thus the elevated steroid levels may reflect an insufficient number of habituation trials with the receptive female.