Sexual behavior in male rats after radiofrequency or dopamine-depleting lesions in nucleus accumbens

Sexual behavior and sex-associated environmental cues activate the mesolimbic system in male rats

The mesolimbic system plays an important role in the regulation of both pathological behaviors such as drug addiction and normal motivated behaviors such as sexual behavior. The present study investigated the mechanism by which this system is endogenously activated during sexual behavior. Specifically, the effects of sexual experience and sex-related environmental cues on the activation of several components of the mesolimbic system were studied. The mesolimbic system consists of a dopaminergic projection from the ventral tegmental area (VTA) to the nucleus accumbens (NAc). Previous studies suggest that these neurons are under tonic inhibition by local GABA interneurons, which are in turn modulated by mu opioid receptor (MOR) ligands. To test the hypothesis that opioids are acting in the VTA during sexual behavior, visualization of MOR internalization in VTA was used as a marker for ligand-induced activation of the receptor. Significant increases in MOR internalization were observed following copulation or exposure to sex-related environmental cues. The next goal was to determine if sexual behavior activates dopamine neurons in the VTA, using tyrosine hydroxylase as a marker for dopaminergic neurons and Fos-immunoreactivity as a marker for neuronal activation. Significant increases in the percentage of activated dopaminergic neurons were observed following copulation or exposure to sex-related environmental cues. In addition, mating and sex-related cues activated a large population of nondopaminergic neurons in VTA as well as neurons in both the NAc Core and Shell. Taken together, our results provide functional neuroanatomical evidence that the mesolimbic system is activated by both sexual behavior and exposure to sex-related environmental cues.

Opposing roles of the nucleus accumbens and anterior lateral hypothalamic area in the control of sexual behaviour in the male rat

Opposing roles have been implicated for the nucleus accumbens (NAc) and anterior portion of the lateral hypothalamic area (aLHA) in the regulation of sexual behaviour in male rats based on in vivo neurochemical correlates. The present study provides functional evidence supporting this hypothesis by examining the effects of lesions to these structures on copulation, noncontact erection and receptive female preference. Sexually naïve male Long-Evans rats received either bilateral 1.0- micro L injections of NMDA (10 micro g/ micro L/side) or vehicle (shams) into either the aLHA or the NAc. During repeated tests of copulation most of the sham-lesioned males, but few of the aLHA-lesioned and NAc-lesioned males, copulated to ejaculation. Most of the NAc-lesioned males also failed to intromit, whereas the majority of the aLHA-lesioned males intromitted repeatedly. During exposure to an inaccessible receptive female behind a wire-mesh screen, aLHA-lesioned males displayed facilitation of noncontact erections, whereas NAc-lesioned males displayed impaired noncontact erections. Conversely, during simultaneous exposure to inaccessible receptive and nonreceptive females in different compartments, all males spent more time in the proximity of the receptive female. These findings indicate that the aLHA plays an inhibitory role in the regulation of sexual arousal and an excitatory role in the regulation of ejaculation. Conversely, the NAc plays an excitatory role in the regulation in sexual arousal.

Extra-cellular dopamine increases in the paraventricular nucleus of male rats during sexual activity

Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations were measured in the dialysate obtained with vertical microdialysis probes implanted into the paraventricular nucleus of the hypothalamus of sexually potent male rats. Animals showed noncontact erections when put in the presence of, and copulated with a receptive (ovarietomized oestrogen and progesterone primed) female rat. Dopamine and DOPAC concentrations in the paraventricular dialysate increased 140% and 19%, respectively, above baseline values during exposure to the receptive female and 280% and 31%, respectively, during copulation. No changes in dopamine and DOPAC concentrations were detected in the paraventricular dialysate when sexually potent male rats were exposed to nonreceptive (ovariectomized not oestrogen plus progesterone primed) female rats. These results confirm the involvement of the paraventricular nucleus in control of erectile function and copulatory behaviour and show for the first time that dopamine neurotransmission is increased in this hypothalamic nucleus when erection occurs in physiological contexts.

Androgen implants in medial amygdala briefly maintain noncontact erection in castrated male rats

Castration of male rats causes a rapid loss of their normal erectile response to inaccessible estrous females. Previous studies had demonstrated that these noncontact erections (NCEs), a putative sign of sexual arousal, could be restored by systemic treatment with testosterone (T) or dihydrotestosterone (DHT), but not estradiol (E). We examined whether androgen delivered to the medial amygdala (MeA) of castrated rats would maintain NCE. In Experiment 1, males received bilateral cannulae filled with T, DHT, or E directed at the MeA. Control males had the same hormone-filled cannulae implanted subcutaneously and blank cannulae in the MeA, or they received T in the anterior forebrain. During the 2 weeks after surgery, males were tested twice for NCE and copulation. About half the males with androgens in the MeA had NCEs 1 week after castration, but few responded a week later. Closer proximity of androgen implants to the posterodorsal MeA (MeApd) predicted shorter NCE latencies. No males with subcutaneous androgen had NCEs in either test, and few anterior forebrain-implanted males did. Some males receiving E in MeA or subcutaneously had NCE in each test. In copulation tests, the type of steroid treatment did not affect the incidence of ejaculation or most measures of copulation, and the proximity of cannulae to MeApd predicted only the time from ejaculation to the occurrence of NCE during the postejaculatory interval. Experiment 2 showed that NCEs displayed by males with androgen in MeA occurred in response to estrous females, not spontaneously. The results suggest that androgens, perhaps augmented by estrogen, act in the posterodorsal MeA to facilitate NCE and its associated arousal.

Sublingual apomorphine for the treatment of erectile dysfunction

Erectile dysfunction (ED) is a prevalent problem in the world and wreaks havoc on the psychosocial well-being of sufferers as well as partners. First-line treatment for ED is oral therapy, however, the currently available oral erectogenic agent is not uniformly successful and requires approximately 1 h to exert its effect. Apomorphine is a novel, oral erectogenic agent taken sublingually. It exerts its effect on hypothalamic centres involved in the triggering of the erection cascade. It dissolves rapidly and results in an erection in responders in approximately 20 min. By avoiding first pass metabolism through its sublingual administration, apomorphine avoids any significant interaction with food or other drugs. In trials in over 4000 subjects representative of the general ED population, beneficial effects were seen in a significant percentage of men. Its exact position in the market remains to be seen, however, its ability to permit spontaneous sexual relations will prove to be of significant benefit to many men.

Central neuropharmacological agents and mechanisms in erectile dysfunction: the role of dopamine

Central nervous system processes are fundamental to sexual function. Considerable progress has been made in our understanding of the neuroanatomical and neuropharmacological bases for erection. Based largely on rat models, there is adequate understanding presently of the general anatomical areas of the brain that relate to sexual function, including the medial amygdala, medial preoptic area, paraventricular nucleus, the periaqueductal gray, ventral tegmentum and others. There is also a burgeoning body of evidence implicating nitric oxide, dopamine, serotonin and oxytocin as critical central neurotransmitters involved in various aspects of sexual function. The role of dopamine, in particular, appears fundamental in the mediation of erectile responses in both animals and man. Additionally, clinical research with apomorphine, a D1/D2 agonist, has shown significant promise in improving erections in men with a wide range of erectile difficulties. Finally, a new classification matrix has been proposed for existing treatments for erectile dysfunction based upon the putative site and mechanism of action. Implications for the further development of neuropharmacological agents in this area are discussed.

Contextual approaches to the physiology and classification of erectile function, erectile dysfunction, and sexual arousal

This paper offers a reexamination of some long-held beliefs relating to the physiology of erectile function and dysfunction, including the idea that there is a singular physiology of erection. Rather, there appear to be plural neural, neurochemical, and endocrine mechanisms whose participation in erectile function depends on the behavioral context in which erection occurs. The best examples of this context-dependent physiology come from research on rats. For example, the medial amygdala is essential for noncontact erection in response to inaccessible estrous females, but not for erection during copulation. Also, androgen is necessary for touch-based and noncontact erection, but not for erection during copulation. Even the specific dopamine receptors important to erection may differ, depending on the context. If there is not a singular physiology of erection, then it follows that the physiology of erectile dysfunction may also vary from context to context. Thus, some disorders of the central nervous system may not be manifested in sleep-related erection, and therefore may be misinterpreted as "psychogenic" erectile dysfunction. This term belies the axiom that all psychological processes have a somatic basis; therefore, there can be no psychogenic dysfunction that does not involve organic processes which may respond to pharmacotherapy. A revised classification of erectile dysfunction based on this premise is offered. Finally, closer attention to erectile context may also illuminate male "sexual arousal" and its relation to "sexual motivation". The former term has so many meanings in current usage as to impede research, especially into the physiology of sexual arousal, which depends on comparisons between animals and humans. It is proposed that attention be given to two variables: whether or not erection occurs and whether or not the context is sexual. The occurrence of penile erection within a sexual context is viewed as the only case in which sexual arousal may be inferred unambiguously.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

The role of nucleus accumbens dopamine in motivated behavior: a unifying interpretation with special reference to reward-seeking

Studies addressing behavioral functions of dopamine (DA) in the nucleus accumbens septi (NAS) are reviewed. A role of NAS DA in reward has long been suggested. However, some investigators have questioned the role of NAS DA in rewarding effects because of its role in aversive contexts. As findings supporting the role of NAS DA in mediating aversively motivated behaviors accumulate, it is necessary to accommodate such data for understanding the role of NAS DA in behavior. The aim of the present paper is to provide a unifying interpretation that can account for the functions of NAS DA in a variety of behavioral contexts: (1) its role in appetitive behavioral arousal, (2) its role as a facilitator as well as an inducer of reward processes, and (3) its presently undefined role in aversive contexts. The present analysis suggests that NAS DA plays an important role in sensorimotor integrations that facilitate flexible approach responses. Flexible approach responses are contrasted with fixed instrumental approach responses (habits), which may involve the nigro-striatal DA system more than the meso-accumbens DA system. Functional properties of NAS DA transmission are considered in two stages: unconditioned behavioral invigoration effects and incentive learning effects. (1) When organisms are presented with salient stimuli (e.g., novel stimuli and incentive stimuli), NAS DA is released and invigorates flexible approach responses (invigoration effects). (2) When proximal exteroceptive receptors are stimulated by unconditioned stimuli, NAS DA is released and enables stimulus representations to acquire incentive properties within specific environmental context. It is important to make a distinction that NAS DA is a critical component for the conditional formation of incentive representations but not the retrieval of incentive stimuli or behavioral expressions based on over-learned incentive responses (i.e., habits). Nor is NAS DA essential for the cognitive perception of environmental stimuli. Therefore, even without normal NAS DA transmission, the habit response system still allows animals to perform instrumental responses given that the tasks take place in fixed environment. Such a role of NAS DA as an incentive-property constructor is not limited to appetitive contexts but also aversive contexts. This dual action of NAS DA in invigoration and incentive learning may explain the rewarding effects of NAS DA as well as other effects of NAS DA in a variety of contexts including avoidance and unconditioned/conditioned increases in open-field locomotor activity. Particularly, the present hypothesis offers the following interpretation for the finding that both conditioned and unconditioned aversive stimuli stimulate DA release in the NAS: NAS DA invigorates approach responses toward 'safety'. Moreover, NAS DA modulates incentive properties of the environment so that organisms emit approach responses toward 'safety' (i.e., avoidance responses) when animals later encounter similar environmental contexts. There may be no obligatory relationship between NAS DA release and positive subjective effects, even though these systems probably interact with other brain systems which can mediate such effects. The present conceptual framework may be valuable in understanding the dynamic interplay of NAS DA neurochemistry and behavior, both normal and pathophysiological.

Sexual motivation--an inquiry into events determining the occurrence of sexual behavior

For the individual engaged in it, sexual behavior has no finality or purpose other than its own execution. Data are presented showing that the execution of sexual reflexes can promote learning, i.e. it functions as reinforcement. Furthermore, positive affect is generated. Based on these principles, a model of sexual motivation has been elaborated. The conceptual framework is the incentive motivation theory previously proposed by Bindra D, A motivational view of learning, performance, and behavior modification, Psychol Rev 1974: 81:199-213; A Theory of Intelligent Behavior, New York: Wiley, 1976; How adaptive behavior is produced: a perceptual-motivational alternative to response reinforcement, Behav Brain Sci 1978; 1:41-52. Although the model is intended for application to most mammals, the rat is used as example. Essentially, sexual approach behaviors are activated by appropriate incentives (conditioned in the male, unconditioned in the female). Approach is, in the inexperienced male, followed by the execution of copulatory reflexes as a consequence of accidentally obtained tactile stimulation of the perineal region. In the female, copulatory acts are activated by tactile stimulation of the flanks and hinds provided by the mounting male. The role of conditioning for the execution of copulatory reflexes and for the acquisition of incentive value of neutral stimuli is analyzed. It is also shown that the incentive properties of sexual acts are not substantially different from those of other incentives. Sexual exhaustion is suggested to be either a case of negative alliesthesia or of stimulus habituation and the Coolidge effect is, in consequence, an example of dishabituation. Studies in women and men support this proposal. It is emphasized that sexual behavior is best understood as being entirely mechanistic albeit not deterministic.

Pharmacological treatment of premature ejaculation

Ejaculatory dysfunction is the most common male sexual disorder and premature ejaculation the most common presentation of ejaculatory dysfunction. Convincing data are lacking from controlled clinical studies to support sustainable long term efficacy of psychosexual counselling in the management of premature ejaculation. The pharmacological treatment of premature ejaculation is now receiving increased attention from both physicians with an interest in sexual medicine and from the pharmaceutical industry.

Lateral hypothalamic serotonin inhibits nucleus accumbens dopamine: implications for sexual satiety

Dopamine (DA) is released in several brain areas, including the nucleus accumbens (NAcc), before and during copulation in male rats. DA agonists administered into this area facilitate, and DA antagonists inhibit, numerous motivated behaviors, including male sexual behavior. Serotonin (5-HT) is generally inhibitory to male sexual behavior. We reported previously that 5-HT is released in the anterior lateral hypothalamic area (LHA(A)) and that a selective serotonin reuptake inhibitor microinjected into that area delayed and slowed copulation. Our present results, using high temporal resolution microdialysis, (1) confirm previous electrochemical evidence that extracellular levels of DA increase in the NAcc during copulation and decrease during the postejaculatory interval (PEI) and (2) reveal that LHA(A) 5-HT can inhibit both basal and female-elicited DA release in the NAcc. These findings suggest that the neural circuit promoting sexual quiescence during the PEI includes serotonergic input to the LHA(A), which in turn inhibits DA release in the NAcc. These findings may also provide insights concerning the inhibitory control of other motivated behaviors activated by the NAcc and may have relevance for understanding the sexual side effects common to antidepressant medications.

Neuronal Fos activation in olfactory bulb and forebrain of male rats having erections in the presence of inaccessible estrous females

Volatile odors from estrous female rats are necessary and sufficient to induce non-contact penile erections in male rats. It is not known whether these pheromones are detected by the accessory as opposed to the main olfactory system or whether they are processed by forebrain regions that receive olfactory inputs. Using nuclear Fos immunoreactivity as a marker of neuronal activation, we asked how the detection and processing of distal cues from inaccessible estrous females, which elicited non-contact penile erections, compared with the processing of sensory cues from soiled estrous bedding which did not elicit non-contact penile erections. In Experiment 1, groups of sexually experienced males were given one of five treatments. A control group was placed on clean bedding. A second group displayed non-contact penile erections when exposed to the smell, sight and sound of an estrous female restrained behind a permeable barrier. A third group was exposed to the same stimuli as the second (an estrous female) but failed to exhibit non-contact penile erections during the first hour of testing. A fourth group was placed on soiled estrous bedding, and a fifth group was allowed two ejaculations with an estrous female. All males were perfused with 4% paraformaldehyde 2 h after the onset of these respective treatments, and their brains were later processed for Fos immunoreactivity. Non-contact penile erections were observed in males that were exposed to distal cues from an estrous female but not in males exposed to soiled estrous bedding. Males that displayed non-contact penile erections or that were exposed to estrous bedding showed significantly more neuronal Fos immunoreactivity than clean-bedding controls in the nucleus accumbens core and shell, anterior and posterior medial amygdala, bed nucleus of the stria terminalis and the medial preoptic nucleus. Even greater neuronal Fos responses occurred in these regions in mated males. In Experiment 2 these same treatments were given to another cohort of sexually experienced males. Increased neuronal Fos immunoreactivity was observed in the granule and mitral cell layers of the accessory olfactory bulb of males that were either mated or exposed to estrous bedding, but not in males that displayed non-contact penile erections in response to distal cues from an estrous female. The volatile odors which presumably caused non-contact penile erections failed to stimulate significant neuronal Fos immunoreactivity in five main olfactory bulb sites examined. Even so, it seems likely that these pheromones are detected via the main olfactory system and are subsequently processed by the same projection circuit that responds to other pheromones present in estrous bedding that are incapable of eliciting non-contact penile erections.

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.

Morphine injected into the paraventricular nucleus of the hypothalamus prevents noncontact penile erections and impairs copulation: involvement of nitric oxide

Male rats show four to six penile erection episodes when put in the presence of an inaccessible receptive female for 80 min. These noncontact erections occur concomitantly with an increase in nitric oxide production in the paraventricular nucleus of the hypothalamus. This is shown by the increases in the NO2- and NO3- concentrations in the paraventricular dialysate obtained from these males by in vivo microdialysis. The NO2- concentration increased from 0.75 +/- 0. 10 microm to 2.89 +/- 0.39 microm and that of NO3- from 4.13 +/- 0. 58 microm to 9.5 +/- 1.2 microm. Morphine (0.5, 1 and 5 microg), given unilaterally into the paraventricular nucleus 15 min before the introduction of the receptive female, prevented the NO2- and NO3- increases, and noncontact erections, dose-dependently. In contrast, the kappa opioid receptor agonist U-69 593 (5 microg) was ineffective. The effects of morphine on NO2- and NO3-, and on noncontact erections, were prevented by the opiate receptor antagonist naloxone (10 microg) injected into the paraventricular nucleus 15 min before morphine. The NO2- and NO3- concentrations were also increased in the paraventricular dialysate of male rats during copulation, i.e. when in copula penile erections occurred. As found with noncontact erections, morphine, but not U-69 593, injected into the paraventricular nucleus prevented the NO2- and NO3- increases and impaired copulatory behaviour, and naloxone prevented these responses when given before morphine. Although some diffusion of the opiate to surrounding brain areas cannot be completely ruled out, the present results suggest that morphine acts through mu receptors in the paraventricular nucleus to impair noncontact erections and copulation. These effects of morphine are apparently mediated by a prevention of the increased nitric oxide production that occurs in the paraventricular nucleus of the hypothalamus of male rats during sexual activity.