Induction of copulatory behavior in sexually inactive rats by naloxine

“Death drive” scientifically reconsidered: Not a drive but a collection of trauma-induced auto-addictive diseases

Over the last 102 years, a lot of discussion was being held about the psychoanalytic conception of the “death drive,” but still with inconclusive results. In this paper, we start with a brief review of Freud’s conception, followed by a comprised overview of its subsequent support or criticisms. The core of our argument is a systematic review of current biochemical research about two proposed manifestations of the “death drive,” which could hopefully move the discussion to the realm of science. It was already established that drive satisfaction leads to the secretion of beta-endorphins, and research evidence also shows that the same biochemical mechanisms get activated in the case of masochism and the gambling disorder but only if they are preceded by chronic frustration of the essential drives. We conclude that the actual situation is more complex than Freud hypothesized, and that a fundamental revision of the psychoanalytic drive theory is necessary.

Behavioral, Neural, and Molecular Mechanisms of Conditioned Mate Preference: The Role of Opioids and First Experiences of Sexual Reward

Although mechanisms of mate preference are thought to be relatively hard-wired, experience with appetitive and consummatory sexual reward has been shown to condition preferences for partner related cues and even objects that predict sexual reward. Here, we reviewed evidence from laboratory species and humans on sexually conditioned place, partner, and ejaculatory preferences in males and females, as well as the neurochemical, molecular, and epigenetic mechanisms putatively responsible. From a comprehensive review of the available data, we concluded that opioid transmission at μ opioid receptors forms the basis of sexual pleasure and reward, which then sensitizes dopamine, oxytocin, and vasopressin systems responsible for attention, arousal, and bonding, leading to cortical activation that creates awareness of attraction and desire. First experiences with sexual reward states follow a pattern of sexual imprinting, during which partner- and/or object-related cues become crystallized by conditioning into idiosyncratic “types” that are found sexually attractive and arousing. These mechanisms tie reward and reproduction together, blending proximate and ultimate causality in the maintenance of variability within a species.

Roles of β-Endorphin in Stress, Behavior, Neuroinflammation, and Brain Energy Metabolism

β-Endorphins are peptides that exert a wide variety of effects throughout the body. Produced through the cleavage pro-opiomelanocortin (POMC), β-endorphins are the primarily agonist of mu opioid receptors, which can be found throughout the body, brain, and cells of the immune system that regulate a diverse set of systems. As an agonist of the body's opioid receptors, β-endorphins are most noted for their potent analgesic effects, but they also have their involvement in reward-centric and homeostasis-restoring behaviors, among other effects. These effects have implicated the peptide in psychiatric and neurodegenerative disorders, making it a research target of interest. This review briefly summarizes the basics of endorphin function, goes over the behaviors and regulatory pathways it governs, and examines the variability of β-endorphin levels observed between normal and disease/disorder affected individuals.

Motivational Drive in Non-copulating and Socially Monogamous Mammals

Motivational drives guide behaviors in animals of different species, including humans. Some of these motivations, like looking for food and water, are crucial for the survival of the individual and hence for the preservation of the species. But there is at least another motivation that is also important for the survival of the species but not for the survival of the individual. Undoubtedly, sexual motivation is important for individuals to find a mate and reproduce, thus ensuring the survival of the species. In species with sexual reproduction, when males find a female in the appropriate hormonal conditions, they will display sexual behavior. However, some healthy males do not mate when they have access to a sexually receptive female, even though they are repeatedly tested. These non-copulating (NC) individuals have been reported in murine, cricetid and ungulates. In humans this sexual orientation is denominated asexuality. Asexual individuals are physically and emotionally healthy men and women without desire for sexual intercourse. Different species have developed a variety of strategies to find a mate and reproduce. Most species of mammals are polygamous; they mate with one or several partners at the same time, as occur in rats, or they can reproduce with different conspecifics throughout their life span. There are also monogamous species that only mate with one partner. One of the most studied socially monogamous species is the Prairie vole. In this species mating or cohabitation for long periods induces the formation of a long-lasting pair bond. Both males and females share the nest, show a preference for their sexual partner, display aggression to other males and females and display parental behavior towards their pups. This broad spectrum of reproductive strategies demonstrates the biological variability of sexual motivation and points out the importance of understanding the neurobiological basis of sexual motivational drives in different species.

Sexual interaction is essential for the transformation of non-copulating rats into sexually active animals by the endocannabinoid anandamide

The endocannabinoid anandamide (AEA) transforms half of the population of previously non-copulating (NC) rats into sexually active animals in a long-lasting manner. The aim of this work was to explore the nature of this transformation. We identified the dose range in which AEA induces mating behavior in previously NC rats, which evidenced a dose-based, biphasic profile for AEA to induce the transformation of NC rats. We demonstrate that the sexual interaction with a receptive female, involving at least an intromission, is essential for AEA to induce the transformation of NC rats. This AEA-induced conversion is centrally mediated and involves the activation of CB1 receptors. Results indicate that the sexual impairment of this population of NC rats relies on their incapacity to initiate sexual activity and that an unidentified brain inhibitory influence on sexual behavior expression is removed by AEA treatment, allowing previously NC rats to show copulatory behavior in a long-lasting manner. The inhibitory influence is not removed by AEA treatment when animals are not allowed to have sexual contact with the female immediately after AEA injection. The same result was found for the opioid receptor antagonist naloxone, the other treatment reported to induce copulation in rats classified as NC. These data suggest that sexual behavior expression could depend on two different neural mechanisms at two different moments: one involved in the display of the first copulatory response and another responsible for maintaining subsequent sexual behavior responding.

Tramadol's Inhibitory Effects on Sexual Behavior: Pharmacological Studies in Serotonin Transporter Knockout Rats

Tramadol is an effective pharmacological intervention in human premature ejaculation (PE). To investigate whether the inhibitory action of tramadol is primarily caused by its selective serotonin reuptake inhibitory (SSRI) effects we tested the dose–response effects of tramadol on sexual behavior in serotonin transporter wild type (SERT^+/+), heterozygous (SERT^+/-), and knockout (SERT^-/-) rats. To investigate whether other mechanisms contribute to the inhibitory effects, WAY100,635, a 5-HT_1A receptor antagonist and naloxone, a μ-opioid receptor antagonist, were tested on sexual behavior together with tramadol. Tramadol dose-dependently decreases sexual activity in all genotypes. In all studies, SERT^+/- rats did not respond differently from SERT^+/+ rats. WAY100,635 did not affect sexual activity in SERT^+/+, but dose-dependently reduced sexual activity in SERT^-/- rats. WAY100,635 (0.3 mg/kg) combined with tramadol (20 mg/kg) significantly reduced sexual activity in SERT^+/+ and even stronger in SERT^-/- rats. Naloxone did not affect sexual behavior consistently in SERT^+/+ rats, while in SERT^-/- rats all doses reduced ejaculation frequency mildly. Combining naloxone (20 mg/kg) and tramadol (20 mg/kg) decreased ejaculation frequencies in both genotypes. Interestingly, combining tramadol (20 mg/kg), WAY100,635 (0.3 mg/kg) and naloxone (20 mg/kg) led to complete elimination of all sexual activity in both SERT^+/+ and SERT^-/- rats. These findings suggest that the inhibitory effects of tramadol on male sexual behavior in SERT^+/+ rats is mainly, if not exclusively, due to SERT inhibition, with an important role for 5-HT_1A receptors, although influence of other systems (e.g., noradrenergic) cannot be excluded. As SSRIs exert their sexual inhibition after chronic administration, tramadol may be therapeutically attractive as “on demand” therapy for PE.

Hormonal systems, human social bonding, and affiliation

Which hormones are implicated in human social bonding and affiliation? And how does field research speak to this issue? We begin by laying out a broad view of how endocrine hormones in general modulate life history allocations of energy and other resources, and the ways in which their neuromodulatory functions must be understood within a broader conceptualization of how they have been shaped to affect allocations. We then turn to four specific hormones or hormone families that have received much attention: oxytocin, opioids, prolactin, and progesterone. Each plays a role in regulating psychological capacities and propensities that underlie individuals' interactions with important social targets. Yet in no case is it clear exactly what regulatory roles these hormones play. We suggest several directions for future research.

Aphrodisiac activity of Phlegmariurus saururus in copulating and noncopulating male rats

BACKGROUND

Phlegmariurus saururus is popularly known in Argentina as aphrodisiac. For this reason, it was previously investigated and determined that the decoction of this plant elicits pro-ejaculatory activity and increases the ejaculatory potency in the Fictive Ejaculation Model. HYPOTHESIS/PURPOSE: the decoction of P. saururus facilitates sexual behavior in sexually experienced male rat and induces copulatory behavior in non-copulating male rats.

METHODS

The extraction method (decoction) was validated through Selectivity, Accuracy and Precision, by identification of the majority alkaloids, expressed as sauroxine. Male (sexually experienced and noncopulating) and female (receptive) Wistar rats were used to determine sexual behavior. Sildenafil was used as positive control. The following variables were evaluated: Mount Latency, Intromission Latency, Ejaculation Latency, Post Ejaculatory Interval, as well as the Mounts and Intromissions Number.

RESULTS

In sexually experienced male rats, P. saururus decoction stimulates sexual arousal and facilitates sexual execution. In noncopulating male rats, this decoction induces copulation with behavioral characteristics similar to sexually experienced animals.

CONCLUSION

P. saururus possesses aphrodisiac activity in copulating and noncopulating male rats.

Sexual performance and precontact 50-kHz ultrasonic vocalizations in WAG/Rij rats: effects of opioid receptor treatment

WAG/Rij rats are genetically selected animals that model absence epilepsy in rats. Ultrasonic vocalizations and sexual behavior - both ethologically relevant markers of reward system functioning - are poorly described in this strain. The aim of our experiment was to investigate reward-dependent precontact 50-kHz vocalizations (PVs) and copulatory behavior as well as the effects of opioid receptor treatment on such behaviors in sexually experienced WAG/Rij males and rats from two control strains: Sprague-Dawley and Crl: Han Wistar. We analyzed the effects of the opioid receptor antagonist naltrexone (3 mg/kg) and the agonist morphine (1 mg/kg) administration. Additionally, we analyzed the initiation of copulation in sexually naïve males before drug treatment. A significantly lower number of sexually naïve WAG/Rij rats initiated copulation. Sexually experienced WAG/Rij males differed at the control session (after physiological saline treatment) compared with Sprague-Dawley rats: WAG/Rij rats displayed more 50-kHz precontact vocalizations and had longer mount and intromission latencies, longer ejaculation latency, longer postejaculatory latency to exploration, longer 22-kHz vocalization duration after ejaculation, and longer postejaculatory intromission latency. Compared with Crl: Han Wistar rats, WAG/Rij males displayed longer mount latency and shorter 22-kHz vocalization duration. Neither naltrexone nor morphine affected PVs in all groups. On the other hand, opioid receptor treatment differently influenced the number of intromissions required to achieve ejaculation and 22-kHz postejaculatory vocalization duration in WAG/Rij rats than in both control groups. This suggests functional differences in the opioid system in this strain. As a result of the number of males that initiated copulation as well as the number of intromissions to ejaculation and 22-kHz postejaculatory vocalizations which all depend on D1 receptor activation, we suggest that the proportion of opioid receptor to D1 receptors in WAG/Rij rats is different when compared with the control strains. The reward system of Wag/Rij rats with absence epilepsy is sensitive to social rewards (high level of precontact 50-kHz ultrasounds) although this strain displays a lower level of sexual motivation (longer mount latency) compared with other control strains. A lower number of sexually naïve rats initiating copulation and longer mount latency in sexually experienced males could suggest a moderate depressive-like syndrome in this strain of rats.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

Anandamide transforms noncopulating rats into sexually active animals

INTRODUCTION

Noncopulating (NC) male rats are apparently normal and healthy animals that will not mate despite repeated exposure to sexually receptive females. Several lines of evidence suggest the involvement of endogenous opioids in this sexual inhibitory state. Endogenous opioids and endocannabinoids are neuromodulators of neurotransmitter release, although through different mechanisms.

AIM

To establish if the endocannabinoid anandamide was able to induce sexual behavior expression in male rats classified as noncopulators.

METHODS

NC male rats were intraperitoneally (i.p.) injected with anandamide or vehicle and tested for copulatory behavior with a receptive female during 120 minutes. Fourteen days after anandamide or vehicle injection, the animals were subjected to a second sexual behavior test during 60 minutes.

MAIN OUTCOME MEASURES

The percentage of rats showing male sexual behavior responses: mount, intromission, ejaculation, and copulation resumption after ejaculation and the specific sexual behavior parameters were quantified.

RESULTS

Anandamide injection induced sexual behavior expression in 50% of previously NC rats, while the NC animals injected with vehicle did not show sexual behavior. The responding animals executed several successive ejaculatory series and were still capable of showing sexual behavior 14 days after anandamide injection. Copulation in these rats (the first copulatory series) was characterized by a large number of mounts and intromissions preceding ejaculation, as well as by statistically significant increases in the latencies to mount, intromit, and ejaculate when compared with the sexual performance of sexually naïve animals copulating for the first time.

CONCLUSION

The endocannabinoid anandamide transforms previously NC rats into sexually active animals, capable of showing sexual behavior in a long-lasting manner. Only half of the NC population responds to anandamide injection, suggesting that different mechanisms underlie the sexual inhibition of NC rats. The endocannabinoid system seems to play a role in the regulation of male rat sexual behavior expression.

Opioid mediation of learned sexual behavior

Identifying the role of opioids in the mediation of learned sexual behaviors has been complicated by the use of differing methodologies in the investigations. In this review addressing multiple species, techniques, and pharmaceutical manipulations, several features of opioid mediation become apparent. Opioids are differentially involved in conditioned and unconditioned sexual behaviors. The timing of the delivery of a sexual reinforcer during conditioning trials, especially those using male subjects, acutely influences the role that opioids have in learning. Opioids may be particularly important in the maintenance of conditioned sexual behaviors during periods of non-reinforcement. This appears to be true both for probe trials and procedures designed explicitly to extinguish a sexual conditioned response. These features of opioid mediation of learning do not appear to be restricted to sexual conditioning paradigms. This suggests that, as for other aspects of sexual learning that despite distinctive features conform to underlying behavioral principles, the mediation of conditioned sexual behavior by opioids relies on processes common across reinforcement systems.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Basic science and neurobiological research: potential relevance to sexual compulsivity

A unique challenge posed by advancing scientific knowledge about the biology of human behavior is how to integrate that understanding with the desire to hold ourselves--and one another--morally accountable. As human beings, we are something more than just passive agents whose behavior is the sum product of biologic determinism. Because of the existence of the mind, we are also active agents with the capacity to influence, at least to some extent, our own destinies. Behavior may be determined, but it is not predetermined. We are one of its determinants. Misconduct by a person of sound mind should not be attributed improperly to brain pathology. On the other hand, suffering, legitimate mental disorder, and associated impairments should not be trivialized. Historically, persons who once were labeled "lazy" are often more appropriately understood by modern standards as clinically depressed. Frequently they are more in need of pharmacologic treatments that alter brain chemistry than "a kick in the behind." Gluttony, one of the original cardinal sins, is often more properly understood as morbid obesity, a condition that deserves appropriate medical care. Persons who have alcoholism, once judged morally as "bums in the gutter," are more frequently referred to treatment facilities, such as The Betty Ford Clinic. One should not approach the issue of human sexual behavior without at least some appreciation of moral values and scientific research. Although clearly some persons choose to act in a sexually selfish and self-indulgent fashion with wanton disregard, others seem to be more genuinely burdened and struggle to integrate their sexual desires into an otherwise healthy and fully responsible lifestyle. When a person, whether male or female, seems to be so driven that it becomes difficult to master erotic desires and he or she experiences difficulty serving his or her own best longterm interests, the concept of sexual compulsivity seems to be relevant. Ultimately, a better understanding of any associated neuropathologies may help to facilitate future treatments and public acceptance. The possibility exists, at least in some instances, that a sexually compulsive individual is less an example of a bad person deserving of punishment than a "broken mind" in need of repair. In time, increased knowledge about the precise workings of the brain in reciprocally initiating and sustaining the sexual interests of the mind may facilitate a much clearer appreciation of the issues at hand.

Psychopharmacology of male rat sexual behavior: modeling human sexual dysfunctions?

Most of our current understanding of the neurobiology, neuroanatomy and psychopharmacology of sexual behavior and ejaculatory function has been derived from preclinical studies in the rat. When a large population of male rats is tested on sexual activity during a number of successive tests, over time individual rats display a very stable sexual behavior that is either slow, normal or fast as characterized by the number of ejaculations performed. These sexual endophenotypes are postulated as rat counterparts of premature (fast rats) or retarded ejaculation (slow rats). Psychopharmacology in these endophenotypes helps to delineate the underlying mechanisms and pathology. This is illustrated by the effects of serotonergic antidepressants and serotonergic compounds on sexual and ejaculatory behavior of rats. These preclinical studies and models contribute to a better understanding of the neurobiology of ejaculation and boost the development of novel drug targets to treat ejaculatory disorders such as premature and retarded ejaculation.

Perinatal ethanol exposure alters met-enkephalin levels of male and female rats

This study used a rat model of Fetal Alcohol Syndrome to investigate whether combined prenatal and postnatal ethanol exposure affects met-enkephalin levels in the brains of male and female Long-Evans adult rats. Intragastric ethanol was administered to a group of rats (ET) from gestational day (GD) 1 through 22 and from postnatal day (PD) 2 through 10. The control groups consisted of a nontreated control group (NTC) and an intubated control group (IC) that received the intragastric intubation procedure but no exposure to ethanol. We measured met-enkephalin levels in the prefrontal cortex, nucleus accumbens, hypothalamus, central and basolateral nucleus of amygdala and ventral tegmental area. Met-enkephalin levels in the hypothalamus of male and female ET animals were significantly higher than those in either the NTC or IC animals. Met-enkephalin levels in the central nucleus of the amygdala of male and female ET animals were significantly lower than the levels in the NTC animals. Met-enkephalin levels in the nucleus accumbens of ET females were significantly greater than those in the IC females. These results demonstrate that the combination of prenatal and postnatal ethanol exposure affects basal met-enkephalin levels in specific regions in a sex-specific manner. These changes in met-enkephalin levels may explain how early ethanol exposure affects opioid-regulated behaviors such as social play, sexual behavior, and other social behaviors.

Effect of small doses of naloxone on sexual exhaustion in White New Zealand male rabbits

The objective of the present study was to determine the effect of small doses of naloxone on sexual exhaustion in White New Zealand male rabbits. Twelve young and 12 adult male rabbits 6-12 months old and 14-20 months of age, respectively, were selected from a commercial farm. Each male rabbit was housed individually in galvanized cages (90 cm x 60 cm x 40 cm). The rabbits were housed in an open shed exposed to natural photoperiod (12 L 12 D, 19 degrees N). Daily temperature fluctuated through the year from 28 to 16 degrees C. Humidity was 45+/-5%. Water and food (rabbit chow PMI) was supplied ad libitum. After sexual behaviour for each studied group was established, the males were given a 6-day rest, and 3 days before next trial, six males of each group (treated) received a subcutaneous implant of 8 mg of naloxone in a crystalline nitrocellulose pellet formulated to be completely absorbed in 15 days. The remaining six males were sham-treated (control). At the end of the resting period as previously described, the sexual behavior of each group was studied and compared using a Mann-Whitney statistical U-test. The effect of naloxone on sexual behavior was analyzed with a Wilcoxon test for correlated samples. With regard to sexual activity between young and adult rabbits, it was observed that there was a significant difference between groups (P=0.00275, Z=2.8823, adjusted Z=2.99.43) showing that younger rabbits mounted/ejaculated from 9 to 10 females compared with 6 to 8 mounted/ejaculated by older rabbits. When naloxone was administered to both groups, there was a significant difference when comparing sexual behavior before and after administration of naloxone (table first and second trial). Young rabbits treated with naloxone mounted/ejaculated 11-12 females while older rabbits mounted nine females before reaching sexual exhaustion. A significant difference was observed when comparing the number of estrous females that were mounted/ejaculated between groups. Environmental photoperiod and temperature changes were not considered. It was concluded that endogenous opioids are important modulators of behavioral and hormonal interactions related to sexual behavior.

Individual differences in male rat ejaculatory behaviour: searching for models to study ejaculation disorders

In addition to investigating sexual function in rats that display normal ejaculatory behaviour, studying rats that are either 'hyposexual' or 'hypersexual' may provide important insights into the aetiology of ejaculatory dysfunctions in men, such as premature and retarded ejaculation. To this end, rats were matched into groups of 'sluggish', 'normal' and 'rapid' ejaculators based on their ejaculation frequencies displayed in a series of weekly sexual behaviour tests. Selecting rats on this parameter revealed large and stable differences in other parameters of sexual behaviour as well, including ejaculation latency and mount frequency but not intromission frequency and mount latency, putative indices of sexual motivation. Neuroanatomically, Fos immunoreactivity as a measure of neuronal activation was increased in rapid ejaculators compared with sluggish ejaculators in ejaculation-related brain areas, presumably associated with the differences in ejaculatory behaviour. Although the total number of oxytocin neurones within subregions of the hypothalamus did not differ between groups, in the supraoptic nucleus of the hypothalamus more oxytocin neurones were activated in rapid ejaculators compared with the other groups. Apart from the differences observed in ejaculatory behaviour, groups did not differ with respect to their locomotor activity and approach-avoidance behaviour as measured in the elevated plus-maze. Finally, apomorphine-induced stereotypy was similar in sluggish and rapid ejaculators, suggesting no large differences in dopamine susceptibility. Altogether, the present results suggest stable differences in male rat ejaculatory behaviour. Further exploring the neurobiological mechanisms underlying these differences may be a promising approach to gain insights into the aetiology of sexual dysfunctions such as premature, retarded or an-ejaculation.

Retarded ejaculation in men: an overview of psychological and neurobiological insights

Disorders of orgasm and ejaculation are erroneously mixed up in the DSM-IV classification system. Male Orgasmic Disorder to denote "delayed ejaculation" is inadequate as orgasm and ejaculation represent clinical expressions of different neurobiological phenomena. Unfortunately, the DSM-IV criteria for delayed ejaculation were accepted regardless of any research with appropriate methodology and design. The psychological approach and associated psychotherapy to solve this problem is rather disappointing. The neurobiological approach, which started with animal studies, has demonstrated various neurotransmitters with the potency to inhibit ejaculation. Indeed, several experimental drugs have been tested in rats, showing the successful acceleration of ejaculation. We propose that human research should start with the development of an operational definition of delayed ejaculation. To achieve this goal, we propose unselected epidemiological stopwatch studies which also provide information on the prevalence and incidence of delayed ejaculation in men. Currently, no effective and safe drugs are available to accelerate ejaculation time in men. The best way to treat lifelong delayed ejaculation is, thus far, to inform the patients about biological and psychological inhibiting factors which they need to avoid, and to remain critical about unrealistic expectations from psychotherapy. Psychotherapy may be useful in subgroups, particularly in the absence of effective and safe drugs.

Testosterone and luteinizing hormone responses to naloxone help predict sexual performance in rams1

The first objective of this study was to determine whether LH and testosterone respond differently to a naloxone injection in relation to varying sexual performance in rams. If differences occurred, the second objective was to determine whether differences would predict variation in sexual performance. From a group of 1.5- to 3-yr-old rams, 20 sexually active and 39 sexually inactive rams were selected based on previously observed sexual behavior with estrual ewes. Each ram was exposed to three estrual ewes for 18 30-min sexual performance tests, and those found to be inactive were given two 30-min sexual partner preference tests. The final distribution was 28 sexually active, 22 inactive, and nine male-oriented rams. Rams were treated with 1.5 mg of naloxone/kg BW in December of Year 1 and again with either 0.75 or 1.5 mg of naloxone/kg BW in November of Year 2. Plasma concentrations of LH and testosterone were evaluated with mixed model analyses for repeated measures separately for each year to coincide with logistic procedures for modeling the probability that rams were sexually active. For Year 1, a sexual activity x age x time interaction for LH after naloxone was observed (P < 0.03). For testosterone, there was a sexual activity x time interaction (P < 0.03), with a similar, early increase for sexually active female-and male-oriented rams compared with a delayed, minimal increase for inactive rams. For Year 2, when all rams were over 2.5 yr of age, a sexual activity x time interaction for both LH and testosterone (P < 0.02) seemed more related to an earlier increase of both hormones for sexually active rams than the increase observed for inactive rams. In addition, sexually active rams had a greater increase in testosterone than inactive rams. No significant difference was observed between 0.75 and 1.50 mg of naloxone/kg BW. Testosterone and LH were used as explanatory variables and sexual activity was used as the response variable in logistic procedures. In Year 1, greatest prediction accuracy was 73.5% using testosterone at 60 min after naloxone injection. In Year 2, the greatest prediction accuracy was 85% using LH at 15 min multiplied by testosterone at 60 min after naloxone. Test repeatability for both years on the same rams was 76%. In conclusion, pattern and magnitude of naloxone-induced changes in endocrine function may facilitate identification of sexually active and inactive rams during the breeding season. Prediction accuracy of the naloxone-based test was 69 to 85%.

Effects of morphine or naloxone on cocaine-induced genital reflexes in paradoxical sleep-deprived rats

The involvement of opioidergic neurotransmission in the modulation of genital reflexes induced by paradoxical sleep deprivation (PSD) and cocaine in rats was the aim of the present study. Morphine (0, 1, 5 and 10 mg/kg) and naloxone (0, 0.3, 3 and 30 mg/kg) were administered prior to saline or cocaine to rats that had been deprived of sleep and the incidence of penile erections (PE) and ejaculations (EJ) was measured. PSD alone induced PE in 50% and EJ in 20% of the rats, but these behaviors were not influenced by morphine or naloxone. Cocaine potentiated the incidence of genital reflexes in PSD rats to 90% (PE) and 70% (EJ). Morphine and not naloxone significantly reduced the percentage of rats displaying this response at the highest doses. Morphine also significantly reduced PE and EJ frequencies at 10 mg/kg. Furthermore, this inhibitory effect of morphine on genital reflexes was prevented by the prior injection of naloxone. Although a number of factors are involved in such a complex phenomenon as PE and EJ, our data show that activation of the opioidergic systems by the agonist morphine reduces genital reflexes-induced by cocaine in PSD males while the antagonist, naloxone, did not have any significant effect. The findings suggest that the stimulating effects of cocaine in potentiating genital reflexes in PSD rats can be unidirectionally modified by opioidergic systems.

Effects of central administration of naloxone during the extinction of appetitive sexual responses

Several studies indicate that opioids are involved in the control of consummatory sexual behavior in male Japanese quail. Naloxone has been reported to increase copulatory responses. In the current study, the effect of naloxone on appetitive sexual behaviors was assessed during extinction test trials. Naloxone was found to substantially reduce appetitive responding, suggesting that opioids differentially affect anticipatory and contact components of sexual behavior.

Aphrodisiac properties of Montanoa tomentosa aqueous crude extract in male rats

Cihuapatli, the Mexican zoapatle (Montanoa tomentosa) has an extensive ethnomedical history of use as a traditional remedy for reproductive impairments. During the study of the ejaculatory function in rats and by testing a set of Mexican plants with medicinal properties, we observed that crude extracts of M. tomentosa facilitated ejaculation. Thus, we decided to analyze the possibility that this plant possessed sexual stimulant properties. To that aim, copulatory behavior of sexually active male rats receiving doses of 38, 75 and 150 mg/kg of the aqueous crude extract of M. tomentosa, as it is prepared in traditional medicine, was assessed. In addition, we evaluated the effect of the 75-mg/kg dose of the extract on males with anesthetization of the genital area and on sexual behavior of sexually inactive male rats (noncopulators). Results showed that acute oral administration of crude extracts of M. tomentosa facilitates expression of sexual behavior in sexually active male rats, significantly increases mounting behavior in genitally anesthetized animals and induces the expression of sexual behavior in noncopulating males. Altogether, these data reveal a facilitatory action of this extract on sexual activity and particularly on sexual arousal. Present findings provide experimental evidence that the crude extract preparation of M. tomentosa, used as a traditional remedy, possesses aphrodisiac properties.

Endomorphins and related opioid peptides

Opioid peptides and their G-protein-coupled receptors (delta, kappa, mu) are located in the central nervous system and peripheral tissues. The opioid system has been studied to determine the intrinsic mechanism of modulation of pain and to develop uniquely effective pain-control substances with minimal abuse potential and side effects. Two types of endogenous opioid peptides exist, one containing Try-Gly-Gly-Phe as the message domain (enkephalins, endorphins, dynorphins) and the other containing the Tyr-Pro-Phe/Trp sequence (endomorphins-1 and -2). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has high mu receptor affinity (Ki = 0.36 nM) and remarkable selectivity (4000- and 15,000-fold preference over the delta and kappa receptors, respectively), was isolated from bovine and human brain. In addition, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), isolated from the same sources, exhibited high mu receptor affinity (Ki = 0.69 nM) and very high selectivity (13,000- and 7500-fold preference relative to delta and kappa receptors, respectively). Both opioids bind to mu-opioid receptors, thereby activating G-proteins, resulting in regulation of gastrointestinal motility, manifestation of antinociception, and effects on the vascular systems and memory. To develop novel analgesics with less addictive properties, evaluation of the structure-activity relationships of the endomorphins led to the design of more potent and stable analgesics. Opioidmimetics and opioid peptides containing the amino acid sequence of the message domain of endomorphins, Tyr-Pro-Phe/Trp, could exhibit unique binding activity and lead to the development of new therapeutic drugs for controlling pain.

Sexual deviancy: diagnostic and neurobiological considerations

Individuals who engage in sexual offenses may be afflicted with a paraphilic disorder or sexual deviation syndrome. Paraphilias are psychiatric disorders characterized by deviant and culturally non-sanctioned sexual fantasies, thoughts, and/or behaviors. A proportion of these individuals may also suffer from symptoms of mental illness that can go unrecognized. Although the etiology and pathophysiology of paraphilic disorders continue to be under investigation, data from empirical, biomedical, and psychopharmacological studies suggest abnormalities at a biological level. This article will discuss and review clinical and neurobiological characteristics of the paraphilias. To this end, we will begin with a general exploration and overview of basic principles that are germane to the subject matter and will conclude with an examination of the most recent relevant research findings.

Morphine administration selectively facilitates social play in common marmosets

Common marmosets (Callithrix jacchus) form extended families, and several cohorts of young may reside together. Play is shown extensively among the offspring. We hypothesized that opiate activity modulates social play, and predicted that administration of morphine (0.5 mg/kg) would facilitate social play, whereas pretreatment with naloxone (0.5 mg/kg) would block morphine's effects. Morphine administration was associated with significantly increased social play, and the effect of morphine was attributable to the focal subject, since play initiated by others was unaffected by treatment. Non-social categories of play, such as object manipulation and locomotor play, and affiliative behaviors, such as time spent huddled, were unaffected by treatment. Twittering and play face, behaviors used by young during play, also increased after morphine administration. Pretreatment with naloxone blocked morphine's effects. Total activity was significantly increased by morphine administration. We conclude that social play is specifically facilitated by opiate activation, whereas other categories of play behavior and social behavior were unaffected by morphine. Thus, social play represents a distinct category of social behavior in juvenile common marmosets with regulatory processes that are unique from other types of social behavior.

Evidence for changes in brain enkephalin contents associated to male rat sexual activity

Indirect evidence suggests that ejaculation might activate endogenous opioid systems, which exert an inhibitory influence on male rat sexual behaviour. The objective of the present study was to search for putative long-term changes in the contents of immunoreactive (IR) Met-enkephalin (IR-Met), Leu-enkephalin (IR-Leu) and opioid octapeptide Met--Arg(6)--Gly(7)--Leu(8) (IR-Oct) in specific brain areas, after the execution of different amounts of sexual activity. Additionally, basal contents of these enkephalins were compared between sexually active (SA) and persistent sexually inactive (SI) rats. Immunoreactivity to enkephalins was determined by radioimmunoanalysis, in the frontal cortex, the hypothalamus and midbrain of SA and SI rats, as well as 24 or 48 h after males had one ejaculation or copulated to exhaustion. Twenty-four hours after sexual activity, there was a generalised increase in enkephalin contents that returned to control values at the 48 h measurement in all brain areas, but the hypothalamus, where IR-Met and IR-Oct remained elevated. No differences in the magnitude of the changes were found between rats that ejaculated once and sexually satiated males. IR-Oct concentration in the hypothalamus of SI rats appeared significantly higher than in SA animals, with no differences in IR-Met and IR-Leu. Results give direct evidence of the activation of endogenous opioid systems by male rat sexual activity. The occurrence of long lasting increases in the contents of IR-Met and IR-Oct in the hypothalamus of rats that copulated was detected. Finally, an intrinsically elevated octapeptide concentration in the hypothalamus of SI rats was found.

Naltrexone-induced augmentation of sexual response in men

BACKGROUND

To ascertain the role of endogenous opioids in sexual response, naltrexone, an opiate receptor antagonist, was administered to men, and its effect on selected self-report measures of sexual response to masturbation was recorded.

METHODS

The data are based on results from 20 healthy, sexually active (alone or with a partner) men, aged 20-29 years, who ingested naltrexone (25 mg/day x 3) or placebo in a randomized, double-blind crossover design. There was at least a 14-day interval between drug and placebo treatment. Between 18 and 22 h after the most recent dose of drug or placebo, subjects viewed sexually explicit videos in privacy for 2 h. They were instructed to masturbate and have as many orgasms as desired. The following three different self-report measures of their responses were recorded: number of orgasms; intensity of sexual arousal, and orgasmic intensity.

RESULTS

Under the naltrexone condition, the volunteers experienced a significantly greater mean number of orgasms (3.4 +/- 0.2 SEM) than under the placebo condition (2.6 +/- 0.3). The total number of orgasms was 67 under the naltrexone condition and 51 under the placebo condition. At the first orgasm, the measure of intensity of arousal was significantly greater in the naltrexone (3.9 +/- 0.2) than placebo (3.4 +/- 0.2) condition, and the measure of orgasmic intensity was significantly greater in the naltrexone (3.7 +/- 0.2) than in the placebo (3.0 +/- 0.3) condition.

CONCLUSIONS

The present study provides evidence that endogenous opioids modulate orgasmic response and the perceived intensity of sexual arousal and orgasm in men. The findings suggest that naltrexone could be clinically useful in cases of inhibited sexual desire and erectile dysfunction.

Neuropeptides and sexual behaviour

Many neuropeptides are involved in the control of sexual behaviour at the central level. Among these, the most studied are adrenocorticotropin, alpha-melanocyte stimulating hormone, oxytocin and opioid peptides. This attempt to review old and new neuropharmacological, biochemical and psychobiological studies in this field, shows that all these neuropeptides apparently facilitate sexual behaviour, except for opioid peptides, which inhibit sexual performance, in most of the species studied so far (rats, mice, monkeys and humans). However, gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin, cholecystokinin, substance P and vasoactive intestinal peptide may be also involved in the control of sexual behaviour. Apparently, corticotropin releasing factor, neuropeptide Y and cholecystokinin inhibit, while substance P and vasoactive intestinal peptide facilitate, sexual behaviour. In contrast, gonadotropin-releasing hormone has been reported to exert a facilitative, inhibitory or no effect at all on sexual behaviour. Galanin was also shown either to facilitate or inhibit sexual behaviour. The above-mentioned putative role of the neuropeptides in sexual behaviour derives mainly from studies done in rats. In these studies, neuropeptides, their antisera or drugs that act as agonists or antagonists of neuropeptide receptors, were tested for their effect on sexual behaviour after systemic, intracerebroventricular, or intracerebral administration. The latter were infused into brain areas relevant for sexual behaviour, such as the medial preoptic area, and the ventromedial and paraventricular nuclei of the hypothalamus. The above studies show that little information is available on the mechanisms by which neuropeptides influence sexual behaviour. Also unclear is whether the above neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except for opioid peptides. New information about the role of neuropeptides may come from the application of molecular biology and genetic manipulation techniques to the study of sexual behaviour. Of these, FOS protein determination, antisense oligonucleotides aimed at the neutralisation of neuropeptide and/or neuropeptide receptor mRNAs in specific brain areas, and gene ablation seem the most promising. Although still in the early stages, it is likely that these methodologies will provide new insights into the role of neuropeptides in the control of sexual behaviour.

Sexual impotence is associated with a reduced production of oxytocin and with an increased production of opioid peptides in the paraventricular nucleus of male rats

Oxytocin plays a physiological stimulatory role on sexual behavior. Conversely, opioid neuropeptides play a physiological inhibitory role. Here we show that in sexually impotent rats there is a reduced expression of oxytocin mRNA and an increased expression of proenkephalin and pro-dynorphin mRNA in the paraventricular nucleus of hypothalamus (PVN), a brain structure of key importance for sexual behavior. These data suggest that an imbalance in the production of oxytocin and of opioid peptides in the PVN, with prevalence of opioid peptides, may underlie a condition of sexual impotence.

Differential sexual activity of isolated and group-housed male mice: lack of substantial influence of acute or chronic naloxone administration

Influences of naloxone upon male sexual behavior were examined using two different baseline activity levels: individually and group-housed mice. In Experiment 1, single injections of 0,12.5, or 50 micrograms per animal were administered before testing. Isolated mice showed more sexual activity than did grouped mice; naloxone failed to alter those differences. In Experiment 2, a similar result was obtained despite administration of 50 or 150 micrograms per animal of naloxone. In Experiment 3,0 or 50 micrograms of naloxone was administered to isolated or grouped males daily on the 5 days before testing. Isolated mice showed performance superior to that of grouped mice, but there was no effect of the drug. In Experiment 4, doses of 0. 12.5, or 50 micrograms of naloxone were given to isolated or grouped males twice daily for 7 days prior to testing, producing little effect. These results suggest that the influences of prior social condition on male sexual activity are robust in the face of naloxone administration.

Magnetic resonance imaging of NMDA-induced lesion of the medial preoptic area and changes in sleep, temperature and sex behaviour

Destruction of the medial preoptic area (mPOA) neurons of rat brain, induced by intracerebral injection of N-methyl D-aspartic acid (NMDA), has been studied by employing the non-invasive Magnetic Resonance Imaging (MRI) technique. Changes in the MRI images are compared and correlated with the functional changes after the mPOA lesion. The progress of the lesion at the injected site has been monitored (using MRI) from 15 min to 1 month after the stereotaxic microinfusion of NMDA (5 micrograms in 0.2 microliter). This study shows that the localised hyperintense (bright) area starts appearing at the mPOA from 3 h after NMDA injection, and the brightness increases progressively for about 2 days. The size and brightness of hyperintense area decrease thereafter. It has not been possible to locate the lesion site after 3 days, using MRI, except in one rat where a vacuole-like area was seen at the NMDA injected site on postmortem histological examination. The reduction in sleep after the mPOA lesion does not show any correlation with the changes in MRI, as it persists throughout the 3 weeks of recording. On the other hand, the initial drastic reduction in male sex behaviour and the increase in body temperature correlated to some extent with the increased brightness in MRI at the site of lesion. The size and location of the hyperintense area, observed during the first 2 days, match with the lesioned area which was histologically identified after 1 month of NMDA administration. Control administration of normal saline into the mPOA did not produce any alteration in the brightness of the MRI image and practically no loss of neurons at the injected site. Though some functional changes have correlation with the alteration in MRI, this cannot be used to interpret the changes in all the physiological parameters. This study also demonstrates that the disappearance of the brightness in MRI should not be taken to indicate a positive prognosis. Though the lesion could not be seen in MRI within 2 hours, its detection after 3-4 h (but within 3 days) after NMDA lesion would give very valuable information for long term studies.

Opioid antagonists and the sexual satiation phenomenon

This study evaluates the effects of the IP injection of naloxone (0.3, 3 and 30 mg/kg) and naltrexone (0.2, 2 and 20 mg/kg) on the sexual satiation phenomenon. It was found that both antagonists exert a dose-based biphasic effect on the proportion of sexually exhausted rats displaying copulation. The intermediate doses of both opioid antagonists were more effective than the low and high doses in increasing the percentage of animals engaged in copulation. The analysis of the specific sexual behaviour parameters revealed that naloxone produces a slight inhibitory effect at the lowest dose, evidenced as an increase in the intromission number. The higher doses of this compound facilitated copulation reflected as a shortening of the ejaculation latency and the interintromission interval (III) and an increase in the copulatory rate. Naltrexone treatment had only facilitatory effects at the lower doses by reducing the III. The higher doses of naloxone (3 and 30 mg/kg) and the intermediate dose of naltrexone (2 mg/kg) decreased the spontaneous ambulatory behaviour of sexually satiated rats without impairing sexual behaviour execution. Data suggest a participation of the endogenous opioid systems in the sexual inhibition resulting from sexual exhaustion.

Effect of morphine and naloxone on LH response and sexual behavior of rams (Ovis aries)

Effects of the opiate agonist, morphine, and antagonist, naloxone, on LH release, courtship behavior and ejaculation frequency of mature, sexually active or sexually inactive rams were investigated. Plasma LH concentrations were monitored from blood samples collected every 15 min for 10 hr (0800 to 1800 hr) from eight rams that were isolated from or in contact with estrous females. Plasma LH concentration was higher (P < .05) in sexually active rams exposed to receptive females compared with hormone concentration of rams isolated from ewes. Intravenous infusion of morphine sulphate (1 mg/kg) into rams 4 and 6 hr after exposure to ewes reduced (P < .05) plasma LH concentration as compared to rams given saline. Morphine did not affect (P > .05) courtship behavior (investigatory sniff, mount attempt, foreleg kick, flehmen, vocalization) but diminished (P < .05) number of ejaculations. In another trial, LH concentrations were higher (P < .05) in seven sexually active rams given naloxone iv or when given to three rams through an intracerebroventricular cannula (icv) as compared to LH response of sexually inactive rams. LH did not differ (P > .05) in seven sexually inactive rams before or after administration of naloxone. Investigatory sniffs by sexually active rams were increased (P < .03) after treatment with the opiate antagonist. Four of the seven sexually active rams had more ejaculations after naloxone compared with the pretreatment period, but mean ejaculation frequency after treatment did not differ (P = .31). Naloxone did not stimulate courtship behavior of sexually inactive males.(ABSTRACT TRUNCATED AT 250 WORDS)

mu-Opioid receptor mRNA expression in the rat CNS: comparison to mu-receptor binding

The distribution of cells expressing mu-receptor mRNA and mu-receptor binding sites were compared in brain and spinal cord tissue sections using a combination of in situ hybridization and receptor autoradiographic techniques. mu-Receptor mRNA was visualized with a 35S-labeled cRNA probe directed to transmembrane III-VI of the rat mu-receptor, while mu-receptor binding sites were labeled with the mu-selective ligand [3H]DAMGO. A high correspondence between the mu-receptor mRNA and receptor binding distributions was observed in the nucleus of the accessory olfactory bulb, anterior olfactory nuclei, striatal patches of the nucleus accumbens and caudate-putamen, endopiriform nucleus, claustrum, diagonal band of Broca, globus pallidus, ventral pallidum, bed nucleus of stria terminalis, most thalamic nuclei, medial and posteriocortical medial amygdala, lateral, dorsomedial, posterior and mammillary nuclei of the hypothalamus, presubiculum, subiculum, rostral interpeduncular nucleus, median raphe, inferior colliculus, parabrachial nucleus, locus coeruleus, central grey, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis, nucleus cuneatus, and the dorsal motor nucleus of vagus. Differences in mu-receptor mRNA and receptor binding distributions were observed in several regions, including the olfactory bulb, cortex, hippocampus, superior colliculus, spinal trigeminal nucleus, cochlear nucleus and spinal cord, and may be due to mu-receptor transport to presynaptic terminals.