Stimulation of sexual behavior in the male rat by galanin-like peptide

Galanin-like peptide (GALP) is a recently described neuropeptide, which shares a partial sequence identity with galanin but is derived from a separate gene. Central injections of GALP stimulate the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and induce the expression of Fos in several brain areas known to regulate male sexual behavior in the rat. We postulated that GALP may also stimulate sexual behavior in concert with its stimulatory effect on the hypothalamic-pituitary-gonadal (HPG) axis. To test this hypothesis, we administered GALP, galanin, or the vehicle (artificial cerebrospinal fluid, aCSF) alone to sexually experienced male rats and assessed the effects of these agents on sexual behavior. We observed that compared to aCSF alone, GALP significantly increased all aspects of male-typical sexual behavior, whereas galanin inhibited all of these same behaviors. To examine whether the stimulatory effects of GALP on sexual behavior were mediated by GALP's stimulatory effects on the HPG axis, we castrated the same male rats and repeated the behavioral experiment. We found that GALP maintained its inductive action on male-typical sexual behaviors in the castrated animals, suggesting that the effects of GALP on sexual behavior are not the result of GALP's ability to stimulate testosterone secretion. These observations suggest that GALP neurons are part of the hypothalamic circuitry controlling sexual behavior in the male rat.     

Heterotypic sexual behavior in male rats after lesions in different amygdaloid nuclei

Previous studies showed that different amygdaloid nuclei are involved in the control of lordosis behavior in female rats. The objective of the present study was to determine whether these nuclei played a role in the control of lordosis behavior in the male rat. Lesions were placed into different amygdaloid nuclei in male rats castrated as adults and primed with ovarian hormones. Lesions in the corticomedial amygdaloid nucleus completely suppressed lordosis behavior as expressed by the number of animals displaying lordosis responses to male mounts. By contrast extended lesions placed into the lateral amygdaloid nucleus (LN) remained without effects. Lesions placed in the very posterior part of the LN produced "hypersexuality" with a rise in the number of animals displaying lordosis responses and high LQ values. Lesions in the anterior part of the LN induced a decrease in the number of animals showing lordosis responses. The amygdala was then concluded to represent a functionally heterogeneous structure with different regions exerting opposite effects on the display of lordosis behavior in the male rat.     

The endogenous androgen-regulated sialorphin modulates male rat sexual behavior

In sexually mature male rats, sialorphin is synthesized under androgenic control and its surge endocrine secretion is evoked in response to environmental acute stress. These findings led us to suggest that this signaling mediator might play a role in physiological and behavioral integration, especially reproduction. The present study investigates the effects induced by sialorphin on the male sexual behavior pattern. Intact male rats were treated in acute mode, with sialorphin at the 0.3, 1, and 3 microg/kg doses, before being paired with receptive female for 45 min. The data obtained show that sialorphin increased, in a dose-related manner, the occurrence of intromissions across the successive ejaculatory sequences. The rats treated with the highest 3 microg/kg dose significantly ejaculated less often compared to controls; however, 80% of them achieved up to three ejaculations. Further analyses of mount bouts for rats achieving three ejaculations reveal that there were significant stimulatory effects of sialorphin, at all doses, on the frequency of intromissions before ejaculation and on the propensity of males to engage in investigatory behavior directed to the female during the post-ejaculatory interval. Thus, sialorphin has the ability to modulate, at doses related to physiological circulating levels, the male rat mating pattern, that is, exerting a dual facilitative or inhibitory dose-dependent effect on the sexual performance, while stimulating the apparent sexual arousal or motivation. These findings led us to speculate that the endogenous androgen-regulated sialorphin helps modulate the adaptative balance between excitatory and inhibitory mechanisms serving appropriate male rat sexual response, depending on the context.     

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.     

Variation in male sexual behavior

Relaxation of natural selection on sexual performance traits in male ruminants has increased phenotypic variation in these heritable traits. Thus, males with sub-standard sexual performance continue to reproduce. This has created a "dud" phenomenon that is costly to animal agriculture. Identification and culling of these lesser performers at an early age and identification of high performing males are critical management goals that must be addressed, and for which greater research priority is needed.     

Inhibition of testosterone-induced sexual behavior in the castrated male rat by aromatase blockers

The effect of some aromatase inhibitors (aminoglutethimide, 1,4,6-androstatrien-3, 17-dione, and 4-hydroxy-androstenedione) on testosterone propionate (TP)-induced copulatory behavior was tested in sexually inexperienced castrated male rats. A single injection of 6 mg of TP induced mounting in 48% and ejaculatory pattern in 19% of the rats within 120 hr. Treatment with the aromatase inhibitors (injections every 12 hr for 108 hr) suppressed ejaculation in all but one rat and significantly reduced the number of rats mounting and intromitting. Concurrent administration of estradiol benzoate (EB, 1 or 3 μg every 12 hr) prevented the inhibitory effect of aromatase blockers. No inhibitory effect of the aromatization blockers was observed in rats in which sexual behavior was induced by dihydrotestosterone (1 mg/day) and EB (2.5 μg/day) for 20 days. The results support the concept that aromatization is an essential step for the induction of male sexual behavior by androgen in the rat.     

Estrogen-activated sexual behavior in male rats

Daily injections of 100 μg estradiol benzoate activated the whole pattern of sexual behavior in castrated sexually experienced male rats. If compared to rats treated daily with 100 μg testosterone propionate, the estrogen-treated males tended to have longer latencies and more mounts and intromissions prior to ejaculation. Fifty micrograms of estradiol benzoate stimulated the display of mounts and intromissions in prepuberally castrated male rats. No peripheral effects of the estrogen treatment were noted. These results suggest that estrogen has central “androgen-like” effects, but no such effects in the periphery. Estrogen treatment (5, 50, and 200 μg/kg for 3 weeks) of intact sexually experienced male rats resulted in testicular atrophy and loss of body weight, but had no significant effects on the sexual behavior.     

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.     

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.     

Patterns of sexual behavior in male macaques

One large social group of each of three species of macaques (Macaca mulatta, M. fascicularis, M. radiata), housed in half-acre field cages at the California Primate Research Center, were observed for a total of 150 h. Data on sexual behavior and dominance interactions were recorded by pairs of experienced observers using a focal animal technique. Single or multiple mount-to-ejaculation sequences, number of thrusts per mount, number of mounts per sequence, duration of mounts per sequence, duration of sequences, social rank and frequency of sexual activity were recorded for each adult male. M. mulatta used a multiple mount-to-ejaculation (MME) pattern in 91% of their copulations. M. radiata used a single mount-to-ejaculation (SME) pattern in 91% of their copulations. M. fascicularis used both patterns—53% MME and 47% SME. A positive correlation was found between rank and sexual activity in fascicularis and mulatta males. A negative correlation between rank and sexual activity was found in radiata males and also a positive correlation between rank and age indicating that the youngest and most subordinate radiata males were the most sexually active males. In reviewing the literature, a relationship between degree of intermale competition, intermale tolerance and type of mounting pattern was revealed. Macaque species that primarily use an SME pattern also show sa high degree of intermale tolerance and little interrnale competition. Macaque species that primarily use an MME pattern typically show a high degree of intermale competition and a low degree of intermale tolerance. Possible events leading to such relationships are discussed.     

Effect of newborn rat testes on the male sexual behavior of the testosterone-treated adult

Neonatal male rats were castrated either at 0, 6 or 24 hrs. after birth. As adults, testosterone was delivered by subcutaneous implantation of a Silastic capsule containing this hormone. The probability to display mounting behavior in presence of an estrous female was lower when the animals were castrated at 0 hr. than at 6 or 24 hrs. or when they received a subcutaneous injection of 1 microgram of testosterone propionate, at the time of castration at 0 hr. These results suggest that in the rat, during the 6 hrs. following birth, neonatal testes influence the sensitivity of the adult central nervous system to testosterone.     

Aging and primate male sexual behavior

Old male rhesus macaques display less sexual behavior than young and middle-aged males. The decrease in sexual activity occurs without a statistically significant decline in gonadal hormones or change in diurnal patterns of serum T, DHT, or LH. Levels of sexual activity are not increased by administering T to old intact males. However, the hormone is effective in increasing sexual behavior in old long-term-castrated males. Performance can be increased to levels observed in equally old untreated intact males. Readily detectable physical disabilities of old age have been observed to impair sexual performance, but the observed general decline in sexual activity cannot be accounted for by known physical disabilities. Novelty, as represented by a change in female partner or by a change in environment, has not increased sexual performance in old rhesus males. Only when old males were paired with empirically selected preferred females is their sexual behavior increased to levels displayed by young males. Drugs reported to increase levels of sexual behavior in rats have thus far been less effective in old rhesus males than powdered rhinoceros horn has been in man. The probable absence of a placebo effect in rhesus males should increase their usefulness as an animal model for the study of sexual behavior in aging men.     

X-chromosome dosage affects male sexual behavior

Sex differences in the brain and behavior are primarily attributed to dichotomous androgen exposure between males and females during neonatal development, as well as adult responses to gonadal hormones. Here we tested an alternative hypothesis and asked if sex chromosome complement influences male copulatory behavior, a standard behavior for studies of sexual differentiation. We used two mouse models with non-canonical associations between chromosomal and gonadal sex. In both models, we found evidence for sex chromosome complement as an important factor regulating sex differences in the expression of masculine sexual behavior. Counter intuitively, males with two X-chromosomes were faster to ejaculate and display more ejaculations than males with a single X. Moreover, mice of both sexes with two X-chromosomes displayed increased frequencies of mounts and thrusts. We speculate that expression levels of a yet to be discovered gene(s) on the X-chromosome may affect sexual behavior in mice and perhaps in other mammals.