Developmental expression of penile reflexes and copulatory behavior in male rats.

First, pubertal development of the penile reflexes, e.g., erections, cups and flips in Wistar-Imamichi male rats was investigated following sheath retraction. Second, the penile reflexes and copulatory behavior in the above males were compared between 10 and 44 weeks of age. The penile reflexes in Wistar-Imamichi rats began to appear from day 26, and all males displayed full components of the reflexes on day 47. The occurrence rates and mean numbers of erections, cups and flips in aged adults were significantly low, compared with the young adults. Also, in the observation of copulatory behavior the occurrence rate of ejaculations, and mean numbers of intromissions and ejaculations in aged adults were significantly lower than that in young adults. These results of the present study may suggest that the decrease of copulatory behavior in male rats with age results from the dysfunction of the penile reflexes with age.     

Supraspinal influences on the penile reflexes of the male rat: a comparison of the effects of copulation, spinal transection, and cortical spreading depression.

The penile reflexes of the rat were observed on interruption of the copulatory behavior sequence after intromission and ejaculation in the initial ejaculatory series, after the penultimate series, during sexual exhaustion, and during recovery from sexual exhaustion 24 and 72 hr later. These were compared to the reflexes of the normal rat in control conditions, to those of the male rat after spinal transection, and to those of the sexually rested and sexually exhausted male rat under cortical spreading depression (CSD). It was concluded that (1) the stimuli associated with copulation evoke disinhibition of the penile reflexes, these showing the short reflex latencies observed in the spinal animal. The release of the spinal mechanisms is lost within 30 min of the last copulatory event. CSD further inhibits reflex responsivity. (2) Stimuli associated with intromission provoke acceleration of the normal rhythmic presentation of reflexes seen in the normal and spinal rat, resulting in a decrease in the duration of intervals between reflex clusters and an increase in reflex number. This excitation decays within about 15 min after intromission. (3) The increase in degree of penile extension and percentage of penile flips after spinal transection suggests tonic inhibition of reflex intensity in the normal rat. The decrease in capacity to attain full erection with the approach of sexual exhaustion suggests an increase in this inhibition. This does not recover during a rest period but instead intensifies. CSD effects did not mimic the effects of spinal transection but instead depressed reflex excitability. The relationship of these changes to the copulatory behavior pattern is discussed.     

Sexual behavior and penile reflexes of neonatally castrated male rats treated in infancy with estrogen and dihydrotestosterone

Male rats castrated neonatally and treated with a combination of 0.5 μg estradiol benzoate (EB) plus 50μg dihydrotestosterone propionate (DHTP) for the next 14 days displayed normal sexual behavior when injected with testosterone propionate (TP) in adulthood. Neither EB nor DHTP alone had this developmental effect inasmuch as only 20–25% of the neonatal castrates treated with just 0.1, 0.5, or 10 μg EB, or 50 μg DHTP, displayed ejaculatory responses. The periodic application of mildly painful electric shock, which has been previously shown to markedly facilitate ejaculatory responding in normal male rats, failed to improve sexual performance in these latter subjects. This was true even of the castrates treated neonatally with DHTP which frequently intromitted. Castrates treated with EB or DHTP alone neonatally were subjected to spinal transection (after testing of sexual behavior) for examination of penile reflexes. Those treated with DHTP showed normal reflexes, characterized by numerous erections and flips, indicating the presumably nonaromatizable DHTP has developmental effects on penile reflexes similar to those of testosterone. Subjects treated with EB, including four animals that had ejaculated at least once, displayed very few, if any, erections on reflex tests and no flips. These results show that sometimes intromissive and ejaculatory patterns can occur even though the animal appears to have little or no capacity for penile reflexes.     

Tactile stimulation during artificial rearing influences adult function and morphology in a sexually dimorphic neuromuscular system

Maternal licking of rat pups affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Maternal licking influences SNB motoneurons, with reductions in licking producing decreased SNB number, size, and dendritic length in adulthood. Reduced maternal licking also produces deficits in adult male copulatory behavior. In this experiment, we used an artificial rearing paradigm to assess the potential role of tactile stimulation in mediating the effects of maternal licking on the SNB neuromuscular system. During artificial rearing, pups were stroked with a paintbrush to mimic maternal licking, receiving low, medium, or high levels of daily stimulation. In adulthood, ex copula penile reflex behavior was tested and the morphology of SNB motoneurons assessed. SNB motoneurons were retrogradely labeled with cholera toxin-conjugated HRP and dendritic arbor was reconstructed in three dimensions. Animals that received low levels of stimulation showed deficits in penile reflexes relative to maternally reared controls, including a longer latency to erection, fewer cup erections, and fewer erection clusters. SNB dendritic morphology was also shaped by stimulation condition, with animals that received low or medium levels of stimulation showing an average 27% reduction in dendritic length. In addition, several reflex behaviors were significantly correlated with dendritic length, including latency to first erection, percent of cup erections, and number of erection clusters. These results suggest that tactile stimulation provided by maternal licking mediates some of the effects of maternal care on the development of male copulatory behavior.     

Effects of ATD on male sexual behavior and androgen receptor binding: a reexamination of the aromatization hypothesis

The aromatization hypothesis asserts that testosterone (T) must be aromatized to estradiol (E2) to activate copulatory behavior in the male rat. In support of this hypothesis, the aromatization inhibitor, ATD, has been found to suppress male sexual behavior in T-treated rats. In our experiment, we first replicated this finding by peripherally injecting ATD (15 mg/day) or propylene glycol into T-treated (two 10-mm Silastic capsules) or control castrated male rats. In a second experiment, we bilaterally implanted either ATD-filled or blank cannulae into the medial preoptic area (MPOA) of either T-treated or control castrated male rats. With this more local distribution of ATD, a lesser decline in sexual behavior was found, suggesting that other brain areas are involved in the neurohormonal activation of copulatory behavior in the male rat. To determine whether in vivo ATD interacts with androgen or estrogen receptors, we conducted cell nuclear androgen and estrogen receptor binding assays of hypothalamus, preoptic area, amygdala, and septum following treatment with the combinations of systemic T alone. ATD plus T, ATD alone, and blank control. In all four brain areas binding of T to androgen receptors was significantly decreased in the presence of ATD, suggesting that ATD may act both as an androgen receptor blocker and as an aromatization inhibitor. Competitive binding studies indicated that ATD competes in vitro for cytosol androgen receptors, thus substantiating the in vivo antiandrogenic effects of ATD. Cell nuclear estrogen receptor binding was not significantly increased by exposure to T in the physiological range. No agonistic properties of ATD were observed either behaviorally or biochemically. Thus, an alternative explanation for the inhibitory effects of ATD on male sexual behavior is that ATD prevents T from binding to androgen receptors.     

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.     

Characterization of p-chloroamphetamine-induced penile erection and ejaculation in anesthetized rats

Methodological shortcomings present in elicitation of male sexual reflexes in anesthetized animals. The present study has demonstrated, however, that intraperitoneal (i.p.) injection of p-chloroamphetamine (PCA), an indirect serotonin (5-HT) agonist, elicited simultaneously both penile erection and ejaculation in anesthetized rats. PCA (2.5-10.0 mg/kg, i.p.) caused an intermittent cluster of genital responses consisting of penile erection, glans erections, and penile cups, which closely resembles the response observed during the ex copula tests in unanesthetized rats. Measurements of intracavernous penile pressure showed that rhythmic changes in penile pressure were produced by PCA, together with glans erections and penile cups. PCA also caused a frequent ejaculations and the weighing of ejaculate accumulated over 0.5 hr was increased in a bell-shaped pattern, and the maximum effect was observed at 5.0 mg/kg. Pretreatment with p-chlorophenylalanine, a serotonin (5-HT)-synthesis inhibitor, significantly inhibited the expression of PCA-induced penile erection and ejaculation, while acute spinal transection at thoracic level did not affect the sexual responses. These results indicate that PCA-induced penile erection and ejaculation in anesthetized rats are mainly produced by the release of 5-HT, which is limited to the lower spinal cord and/or the peripheral sites. Furthermore, the sexual responses can be easily and reliably elicited by administration of PCA, which may be useful for the study of the mechanisms underlying male sexual functions.     

Effects of testosterone propionate and dihydrotestosterone on penile morphology and sexual reflexes of spinal male rats

Testosterone propionate (TP) has a quantitative influence on sexual reflexes mediated at the spinal level in male rats. The possibility that this influence reflects the direct action of androgen on neural elements in the cord, rather than on sensory receptors in the penis was examined indirectly by the use of dihydrotestosterone (DHT). Spinal castrated male rats maintained initially on TP and then switched to DHT showed a significant decline in sexual reflexes paralleling the decline of another group of spinal rats receiving no hormone after initial TP treatment. Yet the number of penile papillae and weight of the penile shaft for the DHT subjects were not significantly different from these measures of penile morphology in a third group of subjects receiving continuous TP and in which reflexes did not decline. These and other observations are consistent with the hypothesis that neural elements within the spinal cord, related to the mediation of the ejaculatory pattern in intact male rats, are directly influenced by gonadal androgen.     

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.     

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.     

Maternal care effects on the development of a sexually dimorphic motor system: The role of spinal oxytocin

Maternal licking in rats affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Reduced maternal licking results in decreased motoneuron number, size, and dendritic length in the adult SNB, as well as deficits in adult male copulatory behavior. Our previous findings that licking-like tactile stimulation influences SNB dendritic development and upregulates Fos expression in the lumbosacral spinal cord suggest that afferent signaling is changed by differences in maternal stimulation. Oxytocin afferents from the hypothalamus are a possible candidate, given previous research that has shown oxytocin is released following sensory stimulation, oxytocin modulates excitability in the spinal cord, and is a pro-erectile modulator of male sex behavior. In this experiment, we used immunofluorescence and immediate early gene analysis to assess whether licking-like tactile stimulation of the perineum activated parvocellular oxytocinergic neurons in the hypothalamus in neonates. We also used enzyme immunoassay to determine whether this same stroking stimulation produced an increase in spinal oxytocin levels. We found that stroking increased Fos immunolabeling in small oxytocin-positive cells in the paraventricular nucleus of the hypothalamus, in comparison to unstroked or handled control pups. In addition, 60 s of licking-like perineal stimulation produced a transient 89% increase in oxytocin levels in the lumbosacral spinal cord. Together, these results suggest that oxytocin afferent activity may contribute to the effects of early maternal care on the masculinization of the SNB and resultant male copulatory behavior.     

Erectile function and bulbospongiosus EMG activity in estrogen-maintained castrated rats vary with behavioral context.

Electromyographic (EMG) activity in the bulbospongiosus muscles (BS) was recorded to monitor potential castration-induced alterations in muscle activity during copulation and reflexive erections. EMG recordings were made from intact male rats and from castrated rats maintained from 7 to 50 days on estradiol benzoate (300 micrograms/day) or testosterone (200 micrograms/day). Despite a 40-50% postcastration reduction in the weight of the BS and accessory sexual glands in estrogen-treated rats, the pattern of EMG activity during copulation was similar across groups. In estradiol-treated males, the EMG burst frequency during mounts and burst duration during intromissions exceeded the parameters of intact males and of castrated males maintained on testosterone. Between intromissions, and following ejaculatory patterns, estrogen-treated males displayed spontaneous muscle bursts accompanied by visually confirmed erection of the glans penis, but these males quickly lost the capacity for reflexive erections. These data demonstrate that despite castration-induced atrophy of the penile muscles and, presumably, their spinal motor nuclei, the motor output to these muscles is maintained following androgen removal. The capacity for substantial penile erection is retained during copulation long after reflexive erections have diminished.     

Effect of some antiestrogens and aromatase inhibitors on androgen induced sexual behavior in castrated male rats

The effect of antiestrogens (MER-25, ICI-46474, and cis-clomiphene) and aromatase inhibitors (5-α-androstanedione, metopirone, and aminoglutethimide) on androgen induced copulatory behavior was tested in sexually inexperienced castrated male tats. Daily injections of 1 mg testosterone (T) for 21 days induced sexual activity in most subjects (61% mounting). Daily pretreatment with MER-25 or cis-clomiphene at three dose levels did not block the behavioral response to T. ICI-46474 at the high dose level (1 mg/kg) elicited a significant depressory effect on the sexual behavior of the T treated castrated rats. A single injection of 6 mg testosterone propionate (TP) induced mounting behavior in 56% of the tested rats within 120 hr. Treatment with metopirone or 5 α-androstanedione (injections every 12 hr for 96 hr) did not inhibit the response to TP. By contrast, aminoglutethimide (5 or 15 mg every 12 hr for 96 hr) abolished the behavioral response to androgen.     

Homotypical sexual behavior in gonadectomized female and male rats treated with 5α-19-hydroxytestosterone: Comparison with related androgens

Ovariectomized female rats were treated in turn over several weeks with estradiol benzoate (EB), testosterone (T), 19-hydroxytestosterone (19HT), dihydrotestosterone (DHT) and 5α-19-hydroxytestosterone (5α19HT). EB was given as a single dose, the androgens were given over 3 days, and progesterone (P) was given 48 hr after the last injection. Each week, rats were tested for lordosis behavior 4–6 hr after P. High levels of receptivity were seen after EB + P, 19HT + P and T + P. Rats treated with DHT + P or 5α19HT + P were unreceptive. Four groups of castrated male rats were treated with T, 19HT, DHT and 5α19HT for 4 weeks starting from castration. In weekly sexual behavior tests, only T and 19HT maintained normal copulatory performance throughout the experiment. 19HT and 5α19HT had negligible effects on peripheral androgen target organs. The failure of 5α19HT to stimulate sexual behavior in rats of either sex supports the view that this steroid does not undergo central aromatization.     

Androgen threshold to activate copulation differs in male rats prenatally exposed to alcohol, stress, or both factors.

Few male rats prenatally exposed to a combination of alcohol and stress copulate spontaneously. This study determined adult sensitivity to testosterone (T) in males prenatally exposed to alcohol, to stress, or to both factors. Sexually naive males were tested with receptive females following castration and implantation of 20-, 30-, or 45-mm Silastic T-filled capsules. Serum T levels provided by these implants were measured. The behavior shown by males exposed only to prenatal alcohol did not differ from untreated control animals at any T dosage. Prenatal stress alone diminished the copulatory potential below control levels only when the intermediate T dosage was provided. Few males exposed to both alcohol and stress copulated under the lowest or the intermediate dose of adult T replacement, but most ejaculated normally when the largest capsule was implanted. The threshold to the sexual behavior-activating-properties of adult T exposure was moderately raised by prenatal stress but was severely affected when prenatal stress was combined with alcohol. We conclude that a diminished sensitivity to androgen in adulthood underlies some copulatory deficits resulting from treatments that alter fetal T levels. Such deficits may be concealed when behavior is evaluated in gonadally intact animals.     

Regulation of Noncontact Erection in Rats by Gonadal Steroids

Male rats exhibit erections in the presence of inaccessible estrous females, and we investigated which gonadal steroids regulate these noncontact erections (NCEs). Sexually experienced Wistar males (n >/= 8/group) were tested for NCE four times (every 3 days) before castration, after castration, and after receiving subcutaneous implants of 10-mm Silastic capsules that were empty or filled with crystalline testosterone propionate (TP), dihydrotestosterone (DHT), estradiol benzoate (EB), or DHT + EB (10 mm each). Before castration, males responded with NCE in approximately 50% of tests. No males had NCEs after castration, beginning 3 days after surgery. Also, no males responded after treatment with EB or empty capsules. After receiving implants of TP, DHT, or DHT + EB, 50% of males had NCEs, beginning with the first test 3 days after treatment. On every measure of NCE, males treated with DHT or DHT + EB were indistinguishable from each other and from TP-treated males. Among the sexual responses of male rats, NCE appears to be more sensitive than other behaviors to changes in gonadal condition. In its profile of response to gonadal steroids (testosterone+, dihydrotestosterone+, estradiol-), NCE is similar to reflexive erection, for which spinal systems are sufficient, and unlike copulation (T+, DHT-, E+), which depends on discrete areas of the brain. We nonetheless conclude that NCE depends on androgen-sensitive systems in the brain, but androgen-sensitive neurons in the lumbosacral spinal cord may also play a role.     

Influence of androgen on the development of sexual behavior in rats : I. Time of administration and masculine copulatory responses, penile reflexes, and androgen receptors in females

The objective of the present study was to investigate the effect of the time of administration of androgen, during the neonatal period, on the development of masculine copulatory behavior in female rats. In addition, the influence of androgen, administered neonatally, on the development of penile reflexes and cytoplasmic androgen receptor levels in the hypothalamic-preoptic area (HPOA) was examined. Female rats were injected with 0.5 mg testosterone propionate (TP) at either 1, 8, or 24 hr after birth and again 24 hr after the first injection. Fifty percent of the females treated with TP at 1 and 8 hr after birth displayed the ejaculatory response when tested in adulthood. In contrast, 93 and 87.5% of oil-treated males and females, respectively, which were androgenized at 24 hr after birth exhibited this response. The results indicate that a considerable amount of masculinization occurs postnatally in the rat. However, none of the androgenized females displayed any penile reflexes even when tested following the display of an ejaculatory response. HPOA androgen receptor levels were somewhat higher in the oil-treated females than in males but were not correlated with the ability to exhibit ejaculation patterns.     

Aromatizable and 5alpha-reduced androgens: differentiation between central and peripheral effects on male rat sexual behavior.

Sexual behavior was observed in the 5 wk after castration in four separate experiments. In each, 50 male rats were allocated equally to the following treatment groups; intact controls; castrated controls; castrated + testosterone propionate, 75 μg/day; castrated + test steroid 1, 150 μg/day; castrated + test steroid 2, 150 μg/day. The following propionated compounds were used; dihydrotestosterone, androsterone, 3α- and 3β-androstanediols, androstenedione, androstanedione, 19-hydroxyandrostenedione, estradiol. With the exception of androstanedione, only aromatizable androgens or estrogen prevented post-castration increases in refractory period durations. However, aromatizable and 5α-reduced androgens stimulated penile spine growth. In each experiment there was a significant positive correlation between spine number and copulatory efficiency i.e., the ratio of intromission to mount frequencies.     

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.     

Comparing the relative amount of testosterone required to restore sexual arousal, motivation, and performance in male rats

It is well established that male rat reproductive behaviors including sexual arousal, motivation, and performance are dependent on circulating levels of testosterone (T). The present study was designed to (1) compare the relative amount of T required to restore these different aspects of behavior in castrated rats, and (2) create an animal model for clinical populations with sexual impairments. Twenty-nine male Long–Evans rats were tested before and after castration for sexual performance (copulation), motivation (partner preference), and arousal (50 kHz ultrasonic vocalizations; measured together with scent marking). Sexual arousal was also inferred from copulation data. Rats were then assigned to one of four groups, and T was re-introduced via Silastic capsule implants varying in length and content: No T (empty capsules), Low T (2 mm capsules), Medium T (5 mm capsules), or High T (two 10 mm capsules). The highest dose was intended to restore physiological levels. Results indicate that High T is required for 50 kHz vocalizations, while Medium T was sufficient for the restoration of copulation, partner preference, and scent marking. These data suggest that sexual arousal may be most sensitive to reductions in testosterone. The role of T levels in measures of generalized and specific (sexual) arousal is discussed in the context of other reproductive behaviors. Furthermore, because the Low T group showed impairments across all behaviors during post-implant tests, we propose that these animals may provide a good animal model for studying clinical conditions marked by reduced motivation and arousal, including Hypoactive Sexual Desire Disorder.