Intromission Patterns Research Articles

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 5α-dihydrotestosterone (DHT), 17β-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.

Effects of Androgens and Estrogens on Sexual Differentiation of Sex Behavior, Scent Marking, and the Sexually Dimorphic Area of the Gerbil Hypothalamus

Mating, marking and the sexually dimorphic area (SDA) were studied in male gerbils that were castrated or sham-operated on the day of birth and in females given testosterone (T) propionate or oil 1 day later. Other females received dihydrotestosterone (DHT), R1881, estradiol benzoate (EB), R2858, DHT and EB, or DHT and R2858. Females and control males were castrated as adults. Then half of each group was given T and tested for marking and male sex behavior. They were still exposed to T at perfusion. The rest were given EB and progesterone and were tested for marking and lordosis. They were perfused 1 month after hormone treatment ended. Neonatal castration eliminated mounting, decreased T-induced marking and increased lordosis in males. Neonatal exposure to TP decreased lordosis and increased mounting, intromission patterns and T-induced marking in females. DHT and R1881 did not affect marking or mating when given alone. EB and R2858 decreased lordosis and increased mounting. DHT enhanced the former effect but not the latter. Only combined treatments increased T-induced marking. Total SDA volume was not affected by neonatal treatments, but “dark volume” was largest in control males. The SDApars compacta(SDApc) was seldom seen in females unless they received steroid neonatally but was present in males that were castrated neonatally. SDApcs of neonatal castrates and steroid-treated females differed, though, from SDApcs of control males in that they were smaller in the absence of T and did not enlarge when exposed to T. Another cell group, the cmSDApc, was affected by T neonatally but was not seen consistently in either sex. SDApc volume correlated positively with T-induced marking and an index of male sex behavior. Both SDApc and cmSDApc volumes correlated negatively with lordosis. The correlations with marking and male sex behavior probably reflect constraints on sexual differentiation rather than causal relationships since these behaviors persist when the SDApc is gone. The effects of direct manipulations of the SDApc on lordosis have not been studied.

Electromyographic analysis of male rat perineal muscles during copulation and reflexive erections

Anatomical examination of the ventral bulbospongiosus (BS) muscle suggested that its proximal and distal portions may act during penile erection as a two-stage pump governing the intensity of glans erections. The coordination between these portions of the BS, and of the proximal BS with the ischiocavernosus (IC) muscle, was studied using electromyographic (EMG) recordings taken during copulation and reflexive erections. Mounts without intromission were accompanied by either strong IC activity with little or no proximal BS activity, or strong proximal BS activity preceding the onset of IC activity. Activity in the proximal BS during mounts was variable in both duration and amplitude but uniform in frequency. During mounts with intromission, EMG activity of the proximal BS consisted of two characteristic phases, an early phase of low-amplitude activity which was similar to proximal BS activity during nonintromissive mounts, followed by an intromissive phase of high-amplitude, high-frequency activity. During intromission patterns, IC activity reliably preceded proximal BS activity. Ejaculations were accompanied by stronger proximal BS activity than were other copulatory events and were followed by a series of proximal BS and IC bursts lasting for 10–20 seconds. During reflexive erections, EMG activity in the proximal BS was always fusiform and varied with the intensity of erection only in frequency. In contrast to the proximal BS, activity in the distal BS was similar in frequency and amplitude across copulatory and reflexive events. These findings suggest that: a) different motoneuron pools serve the different portions of the BS muscle; b) the distal BS does not differentially affect glans erection but may serve primarily to promote rigidity of the portion of the bulb that it surrounds, while the proximal BS acts as the variable aspect of a hypothetical two-stage pump, and c) activity in the IC must precede activity in the proximal BS to achieve intromission.

Deterioration of male sexual behavior in rats by the new prolactin-secreting tumor 7315b

The effects of hyperprolactinemia on male copulatory behavior in adult male and female rats were studied. Hyperprolactinemia was induced by the transplantable purely prolactin-secreting tumor 7315b. Male rats were castrated and received testosterone-filled capsules of different sizes which induced normal and subnormal testosterone levels. After sexual training the rats of the experimental groups were inoculated with tumor 7315b. Three weeks after tumor-inoculation high prolactin levels (2000–30000 ng/ml) were found. During this hyperprolactinemia ejaculation latency increased significantly, while the mount frequency and intromission frequency remained unchanged. Only 9 out of 22 rats ejaculated 19 days after inoculation. Moreover, it appeared that the inhibitory effect of the tumor was as strong in the presence of normal (2.33 ± 0.07 ng/ml) as in the presence of low (0.35 ± 0.01 ng/ml) testosterone levels. The inhibitory effect of tumor 7315b on copulatory behavior was not influenced by adrenalectomy. In gonadectomized female rats bearing testosterone-filled capsules tumor 7315b induced prolactin levels of about 2000 ng/ml and an almost complete cessation of mounts and intromission patterns 4 weeks after tumor-inoculation. It was concluded that tumor 7315b causes a strong inhibitory effect on male copulatory behavior in male and female rats and that this effect is not influenced by the presence of normal or low testosterone levels or removal of the adrenals, suggesting a direct effect of prolactin on brain functions.

Neonatal androgen influences sexual motivation but not the masculine copulatory motor pattern in the rat

Masculine sexual responses displayed by female rats, were compared to those of males. Twenty-five percent of females mounted and 19% showed intromission behavior, but none of them displayed the ejaculation pattern. Masculine sexual behavior was displayed in all stages of the estrous cycle. Accelerometric and spectrum frequency analysis of electrical signals generated by pelvic movements during mounting and intromission showed that these patterns were identical in both sexes excepting that mount duration in females was longer than in males. Neonatal androgenization of females increased the display of intromission patterns. Treatment of ovariectomized rats, androgenized or not, with either estradiol benzoate or testosterone propionate stimulated masculine sexual behavior. The ejaculatory pattern was only displayed by neonatally androgenized females. Mounting and intromission motor patterns of females under steroid treatment, and ejaculations of neonatally androgenized females, were similar to those of males. The results show that the organization of the movements involved in masculine sexual behavior in rats are identical in both sexes, thus suggesting that the neural circuits controlling these behaviors could be identical. Neonatal or postpubertal androgen in the rat influences the frequency with which male-like responses are displayed, but not their temporal (frequency, rhythm) or dynamic (acceleration, vigour) characteristics.

Group mating among Norway rats I. Sex differences in the pattern and neuroendocrine consequences of copulation

The copulatory pattern of groups of rats ( Rattus norvegicus) was studied in the laboratory in a seminatural environment. In a given mating session, every oestrous female copulated with each male; likewise, every male copulated with each oestrous female. While individual males and females experienced similar amounts of copulation, there were dramatic sex differences in sequence and temporal pattern. Males mated in a multiple intromission pattern and had more ejaculatory series when several females were in oestrus. In contrast, females received intromissions and ejaculations in a random order, not in the sequence of a male ejaculatory series. Males copulated at shorter intervals than females did, a temporal sex difference that was determined by the pattern of female solicitations and male approaches. These sex differences are used to discuss the different units of analysis that are appropriate for male and female sexual behaviour in this species. Furthermore, the sex differences in the temporal pattern of copulation which emerged during group mating parallel the known sex differences in the temporal parameters of the neuroendocrine reflexes which mediate successful reproduction in the domestic strain.

Effect of neonatal intrahypothalamic testosterone implants on cyclicity and adult sexual behavior in the female hamster.

To localize the neural sites of hormone action for sexual differentiation in the hamster, pellets containing 3 microgram testosterone were implanted directly in the preoptic area-anterior hypothalamus (POA-AH) and the ventromedial hypothalamus-arcuate regions of the hypothalamus of 3-day-old female hamsters. The POA-AH implants strongly enhanced the potential for ectopic mounts, rear mounts, and intromission patterns after adult testosterone treatment. 50% of the females with neonatal POA-AH implants demonstrated vaginal acyclicity in adulthood, suggesting a significant disruption of normal gonadotropin release. Tests for female sexual behavior revealed that implants in both POA-AH and ventromedial arcuate had no significant effect on latency to lordosis or total lordosis duration with estrogen and progesterone priming, but did reduce the longest single bout of lordosis. In conjunction with other recent investigations, these findings suggest that in the hamster, as in other species, androgenization involves several independent processes in terms of both location and sensitivity to hormone exposure.

Influence of androgen in the neonatal period on ejaculatory and postejaculatory behavior in the rat

The objective of the present report was to investigate the influence of androgen in the neonatal period on the development of ejaculatory and postejaculatory behavior. At birth, male rats were either castrated (neonatally castrated males), implanted with a Silastic tube of the aromatase inhibitor androsta-1,4,6-triene-3,17-dione for the first 10 days (ATD males), or left untreated (normal males). Female rats were either injected with 0.5 mg testosterone propionate (TP) on Days 1 (day of birth) and 2 (androgenized females) or left untreated (normal females). All gonadally intact animals were castrated at 60 days of age. Following TP administration, all animals were tested for ejaculatory and postejaculatory behavior under both shock and nonshock conditions. All animals were capable of showing the intromission pattern; however, the ejaculatory pattern was exhibited regularly only by those animals exposed to androgen at birth (normal males, androgenized females, and ATD males). The normal males required fewer intromissions to achieve ejaculation than the other two groups exhibiting this reflex. This result is discussed in terms of peripheral genital stimulation deficits and the differentiation of neural tissue responsible for masculine copulatory behavior. Androgenized females and ATD males displayed a refractory period, characterized by 22-kHz vocalizations, equal to or longer than that found in normal males. These results indicate that defeminization is not necessary for the display of normal ejaculatory and postejaculatory 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).

Copulatory performances of penile desensitized male rats as a function of prior social and sexual experience

Male rats were reared under one of the following conditions from 14 days of age: (1) Isolation (raised in individual cages); (2) Contact isolation (isolation except for Days 40–60 when two females lived close to the male, separated only by a screen); (3) Cohabition with males prepuberally (males together between Days 14 and 40); (4) Cohabition with males postpuberally (males together between Days 40 and 60); or (5) Cohabition with females (male together with females between Days 40 and 60). When 75 days old, the rats were subjected to penile deafferentation by pudendal nerve transection and tested for masculine sexual behavior. The Group 5 males showed more intromission patterns than those of Groups 1, 2, and 3 and more ejaculation patterns than those of Groups 1, 2, 3, and 4. It was concluded that postpuberal heterosexual experience controls the expression of sexual behavior in two ways: (a) by increasing the readiness to initiate mounting behavior and (b) by facilitating the display of a complete mating pattern.

Effects of p-chloroamphetamine and 5,7-dihydroxytryptamine on the sexual behavior of gonadectomized male and female rats

Treatment with the 5-HT neurotoxins p-chloroamphetamine (PCA, 2× 10 mg/kg) or 5,7-dihydroxytryptamine (5,7-DHT, 2×6 μg intracerebrally) stimulated the display of all aspects of sexual behavior, including ejaculations, by castrated male rats in the absence of testosterone (T) treatment and increased the behavioral sensitivity to a low level of T stimulation. The reduction of the ( 3H) 5-HT uptake after PCA treatment was more pronounced in the cortex than in the hypothalamus. 5,7-DHT treatment reduced the ( 3H) 5-HT uptake in the septum, hippocampus, amygdala, hypothalamus and cortex but the behavioral effects produced by the 5,7-DHT treatment could not be correlated to the biochemical effects in any of these brain areas. Since the behavioral effect of PCA appears to be stronger than that of 5,7-DHT, the 5-HT neurotoxins may exert their effect on sexual behavior in forebrain structures rather than in the hypothalamus. PCA treatment had a very small effect on mounting behavior but 5,7-DHT treatment stimulated the display of mounts and intromission patterns by ovariectomized female rats given no hormone treatment. Neither PCA nor 5,7-DHT had any effect on lordosis behavior tested before and after treatment with estradiol benzoate alone or in combination with progesterone. The observations support the conclusion that 5-HT is involved in the control by T of sexual behavior in male rats, but argue against a role of 5-HT in the neural control of lordosis behavior.

Independent masculinization of neuroendocrine systems by intracerebral implants of testosterone or estradiol in the neonatal female rat

Small pellets of testosterone (T) or estradiol (E 2), but not cholesterol (CH), when implanted into the brain of neonatal female rats on day 2 or day 5 of life, produce masculinization of the adult regulation of gonadotropic hormone (GTH) release, female sexual behavior or masculine sexual behavior, specific to the site and time of implantation and the hormone implanted. Site specificity: There appear to be specific neuronal sites where implantation of T or E 2 produces independent masculinization of GTH, female sexual behavior or masculine sexual behavior patterns. Implants of T or E 2 placed in the dorsal preoptic area (POA) perinatally increase the amount of masculine sexual behavior displayed by adults. On the other hand, the ventromedial hypothalamus (VMH) is the only area in which neonatal implants of T or E 2 produce GTH acyclicity in adults. Female sexual behavior is affected in opposite directions by hormonal implants in two different areas. Neonatal implants of T or E 2 in the POA increase adult behavioral responsiveness to estradiol benzoate (EB) alone, whereas implants in the VMH decrease adult responsiveness to EB plus progesterone therapy. Temporal speficity: The most effective time for augmenting masculine sexual behavior is before day 5, since hormone implants on that day produce marginal effects on male sexual behavior, whereas day 2 implants in the POA result in substantial increases in both mount and intromission patterns. GTH release is masculinized equally well by implants of hormones on either day 2 or day 5. Female sexual behavior is affected only by neonatal implants on day 2. Hormone specificity: Estradiol is as effective as T in masculinizing all three neuroendocrine parameters. In any particular neural site in which T implants produce an alteration in the neuroendocrine response, a similar effect is produced by E 2 implants in the same site. It is suggested that independent masculinization of GTH, female sexual behavior and masculine sexual behavior patterns is produced by the action of T and/or E 2 on separate neural areas, and that these neural areas may be susceptible to the action of hormones at different times.

Mounting in female hamsters: Effects of different hormone regimens

Adult female hamsters have substantial capacity to display male behavior without perinatal androgen administration and apparently without perinatal exposure to gonadal steroids. Seventy-two ovariectomized females were tested for male behavior during chronic treatment with either testosterone (T, N = 8); androstenedione (AD, N = 7); dihydrotestosterone (DHT, N = 8); estradiol (E 2, N = 7); T + E 2 (N = 9); AD + E 2 (N = 6); DHT + E 2 (N = 7); or no hormone (NoH, N = 19). Eighty-three percent of the females treated with T, T + E 2, AD + E 2, or DHT + E 2 mounted during at least one test, and 70% displayed the intromission pattern; there were no significant differences among these treatment conditions. Forty-two percent of E 2-treated females, 14% of AD-treated females, none of the DHT-treated females, and one of the NoH females mounted; these treatments were significantly less effective than T or any of the androgen-plus-estrogen combinations. Females treated with E 2 or AD + E 2 displayed lordosis responses when tested for female behavior, but females treated with T + E 2, DHT + E 2, T, DHT or NoH did not. In the hamster the capacity for the execution of male behavior develops independent of early androgen, and only the arousability of the system by hormonal and external stimulation is sex dimorphic.

Effects of genital deafferentation on mounting and intromission behavior in spayed female rats treated with estrogen

Thirty-five spayed female rats, treated daily with a low dose of estrogen, were tested for mounting and intromission behavior. The animals exhibiting intromission patterns were submitted to sham-operation or genital denervation by transecting both pudendal and genito-femoral nerves. In 2 postoperative mating tests it appeared that genital denervation completely prevented the occurrence of intromission patterns while mounting behavior was not affected. It is concluded that the occurrence of intromission patterns in the adult female rat depends on genital sensory input. The importance of eliminating the confounding influence of genital stimuli for the interpretation of sexual motivation is stressed.

Penile movements and the sensory control of copulation in the rat

Under certain conditions, male rats display extravaginal intromission patterns (XVIPs) and extravaginal ejaculations. In Experiment 1, videotape recordings of ventral views were used to describe the skeletal and penile actions occurring during these behavior patterns. XVIPs and extravaginal ejaculations appeared to be topographically and temporally identical to normal copulatory patterns, and the penile actions observed were inferred to be comparable to those likely to occur intravaginally during normal copulation. In Experiment 2, it was determined that the sensory feedback provided by mounts, XVIPs, and intromissions influenced the probability that clusters of sexual acts (mount bouts) would end, but did not affect the duration of the intervals between mount bouts.

Estrogen plus androgen induced mounting in adult female hamsters

This study demonstrated that the combined administration of estrogens and androgens activates the display of mounting by female hamsters. Forty-nine ovariectomized hamsters were injected daily with either estradiol benzoate (EB, N = 8); dihydrotestosterone propionate (DHTP, N = 7); testosterone propionate (TP, N = 6); androstenedione (AD, N = 9); EB plus DHTP ( N = 10); or estrone plus DHTP (E 1 + DHTP, N = 9). All androgens were administered at a dose of 1 mg per day for the first 24 days and at a dose of 2 mg per day for the last 14 days. The EB dose was 6 μg per day and the E 1 dose was 100 μg per day. Females were tested for male behavior once a week starting on Day 10 of injections and for female behavior on Day 39. One hundred percent of EB + DHTP treated females; 67% of the E 1 + DHTP treated females; 55% of the AD treated females; 33% of the TP treated females; 29% of the DHTP treated females; and none of the EB treated females mounted during at least one test. Only one of the E 1 + DHTP treated females showed the intromission pattern; otherwise most females which mounted displayed the intromission pattern. The median number of days preceding the onset of mounting ranged from 21 to 31 days and did not differ among treatment groups.

Male copulatory behavior induced in female rats by peripheral electric shock

Peripheral skin shock, like administration of testosterone propionate (TP), increased the display of male copulatory patterns in a limited number of ovariectomized female rats. Shock treatment alone led to moderate rates of mounting with thrusting and display of intromission patterns by 2 of the 3 responding females in this group. TP treatment alone resulted in fewer male responses than did shock treatment and failed to produce the intromission pattern. In a group in which both TP and shock were given, all females showed high rates of male mounting and all displayed the intromission pattern. Augmentation of nonspecific arousal by peripheral shock appears to facilitate the display of male sexual behavior in females as it does in males.

Increased mounting behavior in the female rat following a single neonatal injection of testosterone propionate

Female rats, injected with 1 mg testosterone propionate within 24 hr after birth, showed drastically enhanced mounting behavior in adulthood as compared to an oil-injected control group when tested with the ovaries in situ. Ovariectomy diminished but did not abolish mounting behavior. Subsequent estrogen treatment stimulated mounting behavior more in the androgenized than in the normal females, while no group difference was found in the frequency of intromission patterns. When injected with testosterone the androgenized females tended to show more mounts and intromission patterns than the normal females. Neonatally testosterone-treated females sniffed at the genital area of the stimulus female significantly more and climbed the stimulus female significantly less than did controls.

Mounting behavior in the female rat during the estrous cycle, after ovariectomy, and after estrogen or testosterone administration

Female rats displaying regular 4-day estrous cycles were tested for male sexual behavior with a receptive stimulus female during five consecutive estrous cycles, after ovariectomy, and after estrogen or testosterone treatment. Such behaviors as mounts (mount with pelvic thrusts), climbings (mount without pelvic thrusts), and sniffing at the genital area of the stimulus female varied systematically during the estrous cycle, being at minimal values when the rats were in heat. Intromission patterns (mount with pelvic thrusts accompanied by a final deep thrust and by genital grooming) were also displayed by most females but at relatively low rates. Ovariectomy decreased, but did not abolish masculine behavior. Estrogen in different doses (2, 10, and 50 μg/kg) stimulated the display of intromission patterns and mounts and depressed the frequency of climbings. The effects of various doses of testosterone (100, 500, and 2500 μg/kg) were similar to those of estrogen, i.e., testosterone stimulated the display of intromission patterns and mounts and depressed the frequency of climbings. No systematic dose-response relationships between amount of injected hormone and sniffing frequency or latencies to the first intromission pattern or mount could be detected. The results suggest that ovarian hormones may be involved in the regulation of male sexual behavior in the female rat.

Induction of target organ insensitivity to testosterone in the male guinea-pig with cyproterone.

SUMMARY Male guinea-pigs born to females injected with cyproterone acetate (CA) from day 28 to 58 of gestation developed non-tubular hypospadiac penes and stunted seminal vesicles and prostate glands. When adult animals were stimulated with exogenous or endogenous androgens, secretory failure and only minimal growth was observed in the accessory organs. The concentrations of endogenous testosterone in systemic plasma from intact CA males were not different from those in control males, but the uptake and distribution of [1,2-3H]testosterone in castrated CA males did not conform to control values. Deficiences in mounting, intromission and ejaculatory patterns were observed in CA males compared with controls. Lordosis, the primary behavioural response used as a criterion of 'feminization', was never observed in CA males even when large concentrations of oestrogens were administered. The results suggest that target organ insensitivity to testosterone in adulthood can be the result of deficient androgenic stimulation during embryological differentiation.