Prolactinergic and dopaminergic mechanisms underlying sexual arousal and orgasm in humans

Growth hormone, prolactin, and sexuality

GH and PRL, although not considered as 'classi cal' sexual hormones, could play a role in the endocrine control of sexual function both in men and women. Physiologically, PRL seems to be involved in the central control of sexual behavior and activity, by modulating mainly the effects of dopaminergic and serotoninergic systems on sexual function. Indeed, circulating PRL levels increase after orgasm and may potentially play a role in the acute regulation of further sexual arousal following orgasm both in men and women. On the other hand, either short-term or long-term PRL in crease can modulate central nervous system areas involved in the control of sexual function and, peripherally, can directly influence mechanisms of penile erection in men, and presently only as an hypothesis, mechanisms related to the sexual response of genitalia in women. Furthermore, chronic hyperprolactinemia is classically associated with hypogonadotropic hypogonadism and sexual dysfunction in both sexes. Successful treatment of chronic hyperprolactinemia generally restores normal sexual function both in men and women although this effect is not only related to relapse of gonadal function. Hypoprolactinemia is recently recognised as a possible risk factor of arteriogenic erectile dysfunction while a possible role on female sexual function is not known. The physiological role of GH on sexual function is not fully elucidated. GH is an important regulator of hypothalamus-pituitary-gonadal axis and seems to participate in the regulation of the sexual response of genitalia in men, and potentially also in women. Sexual function in men and women with GH deficiency (GHD) and GH excess, particularly in acromegaly, is scantily studied and GH- or IGF-I-dependent effects are difficult to quantify. Nevertheless, a decrease of desire and arousability both in men and women, together with an impairment of erectile function in men, have been described both in patients with GHD and acromegaly, although it is not clear whether they are dependent directly on the hormone defect or excess or they are consequence of the hypogonadism or the different clinical complications or the physical disfigurement and psychological imbalance, which are associated with the diseases, and are potentially affecting sexual function. Data on beneficial effects of GH replacement therapy and specific surgical or pharmacological approach for acromegaly are far to be fully elucidated although restoring normal GH/IGF-I levels have been associated to improvement of sexual function.

Sex, drugs, and rock 'n' roll: hypothesizing common mesolimbic activation as a function of reward gene polymorphisms

The nucleus accumbens, a site within the ventral striatum, plays a prominent role in mediating the reinforcing effects of drugs of abuse, food, sex, and other addictions. Indeed, it is generally believed that this structure mandates motivated behaviors such as eating, drinking, and sexual activity, which are elicited by natural rewards and other strong incentive stimuli. This article focuses on sex addiction, but we hypothesize that there is a common underlying mechanism of action for the powerful effects that all addictions have on human motivation. That is, biological drives may have common molecular genetic antecedents, which if impaired, lead to aberrant behaviors. Based on abundant scientific support, we further hypothesize that dopaminergic genes, and possibly other candidate neurotransmitter-related gene polymorphisms, affect both hedonic and anhedonic behavioral outcomes. Genotyping studies already have linked gene polymorphic associations with alcohol and drug addictions and obesity, and we anticipate that future genotyping studies of sex addicts will provide evidence for polymorphic associations with specific clustering of sexual typologies based on clinical instrument assessments. We recommend that scientists and clinicians embark on research coupling the use of neuroimaging tools with dopaminergic agonistic agents to target specific gene polymorphisms systematically for normalizing hyper- or hypo-sexual behaviors.

Oxytocin: an emerging regulator of prolactin secretion in the female rat

In the female rat, a complex interplay of both stimulatory and inhibitory hypothalamic factors controls the secretion of prolactin. Prolactin regulates a large number of physiological processes from immunity to stress. Here, we have chosen to focus on the control of prolactin secretion in the female rat in response to suckling, mating and ovarian steroids. In all three of these states, dopamine, released from neurones in the mediobasal hypothalamus, is a potent inhibitory signal regulating prolactin secretion. Early research has determined that the relief of dopaminergic tone is not sufficent to account for the full surge of prolactin secretion observed in response to the suckling stimulus, launching a search for possible prolactin-releasing factors. This research has subsequently broadened to include searching for prolactin-releasing factors controlling prolactin secretion after mating or ovarian steroids. A great deal of literature has suggested that this prolactin-releasing factor may include oxytocin. Oxytocin receptors are present on lactotrophs. These oxytocin receptors respond to exogenous oxytocin and antagonism of endogenous oxytocin inhibits lactotroph activity. In addition, the pattern of oxytocin neuronal activity and oxytocin release correlate with the release of prolactin. Here, we suggest not only that oxytocin is stimulating prolactin secretion, but also that prolactin secretion is controlled by a complex network of positive (oxytocin) and negative (dopamine) feedback loops. We discuss the available literature and attempt to describe the circuitry we believe may be responsible for controlling prolactin secretion.

Is there a significance of histamine in the control of the human male sexual response?

Although histamine has been suggested to be involved in the control of male sexual function, including the induction of penile erection, its role in the human corpus cavernosum penis is still poorly understood. The aim of our study was to evaluate the course of histamine plasma levels through different stages of sexual arousal in the systemic and cavernous blood of healthy male subjects. Thirty four (34) healthy men were exposed to erotic stimuli to elicit penile erection. Blood was aspirated from the corpus cavernosum and a cubital vein during the penile conditions flaccidity, tumescence, rigidity and detumescence. Blood was also collected in the post-ejaculatory period. Plasma levels of histamine (ng ml(-1)) were determined by means of a radioimmunoassay. Histamine slightly decreased in the cavernous blood when the penis became tumescent. During rigidity, histamine decreased further but remained unaltered in the phase of detumescence and after ejaculation. In the systemic circulation, no alterations were observed with the initiation or termination of penile erection, whereas a significant drop was registered following ejaculation. Results are not in favour of the hypothesis of an excitatory role of histamine in the control of penile erection. Nevertheless, the amine might mediate biological events during the post-ejaculatory period.

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Therapeutic targets for premature ejaculation

Premature ejaculation (PE) is the most common male sexual complaint, and may exert a profound negative impact on the man's life and partnership. Using currently available treatment alternatives (e.g., selective serotonin uptake inhibitor, agents acting locally on the penis), PE can be treated in most, but not all patients. However, since long term success rates have been disappointing, and the only approved treatment so far is the short-acting selective serotonin re-uptake inhibitor dapoxetine, there is currently an intensive search for new treatment modalities. Selection of the most promising therapeutic targets from a host of current and potential candidates depends heavily on their roles in the pathophysiology of PE. Possible central nervous targets that will be discussed are serotonin transporters, and CNS receptors for 5-HT(IA) and 5-HT(1B), dopamine, oxytocin, opioids, neurokinin-1, and glutamate. Putative peripheral targets include α(1)-adrenoceptors, phosphodiestrase enzymes, Rho kinases, purinergic (P2X) receptors, and penile sensory nerves. It is clear that exploiting the full therapeutic potential of these targets will require additional basic and clinical research.

Role of salsolinol in the regulation of pituitary prolactin and peripheral dopamine release

(R)-Salsolinol (SAL), a dopamine (DA)-related tetrahydroisoquinoline, has been found in extracts of the neuro-intermediate lobes (NIL) of pituitary glands and in the median eminence of the hypothalamus obtained from intact male rats and from ovariectomized and lactating female rats. Moreover, analysis of SAL concentrations in NIL revealed parallel increases with plasma prolactin (PRL) in lactating rats exposed to a brief (10 min) suckling stimulus after 4-h separation. SAL is sufficiently potent in vivo to account for the massive discharge of PRL that occurs after physiological stimuli (i.e. suckling). At the same time, it was without effect on the secretion of other pituitary hormones. It has been also shown that another isoquinoline derivative, 1-methyldihydroisoquinoline (1MeDIQ), which is a structural analogue of SAL, can dose-dependently inhibit the in-vivo PRL-releasing effect of SAL. Moreover, 1MeDIQ can inhibit the elevation of plasma PRL induced by physiological stimuli, for example suckling, or in different stressful situations also. 1MeDIQ also has a psycho-stimulant action, which is fairly similar to the effect of amphetamine, i.e. it induces an increase in plasma catecholamine concentrations. It is clear from these data that this newly discovered endogenous compound could be involved in regulation of pituitary PRL secretion. It has also been observed that SAL is present in peripheral, sympathetically innervated organs, for example the atrium, spleen, liver, ovaries, vas deferens, and salivary gland. Furthermore, SAL treatment of rats results in dose-dependent and time-dependent depletion of the DA content of the organs listed above without having any effect on the concentration of norepinephrine. More importantly, this effect of SAL can be completely prevented by amphetamine and by 1MeDIQ pretreatment. It is clear there is a mutual interaction between SAL, 1MeDIQ, and amphetamine or alcohol, not only on PRL release; their interaction with catecholamine "synthesis/metabolism" of sympathetic nerve terminals is also obvious.

Prolactin secretion patterns: basic mechanisms and clinical implications for reproduction

Prolactin (PRL) is one of the most versatile hormones in the mammalian body affecting reproductive, sexual, metabolic, immune, and other functions. It is therefore not surprising that the neural control of PRL secretion is complex, involving the coordinated actions of several hypothalamic nuclei. A plethora of experimental data exists on the hypothalamic control of hormone secretion under various physiological stimuli. There have been even mathematical models and computer studies published, which help to understand the complex hypothalamic–pituitary network. Nevertheless, the putative role of PRL for human reproduction still has to be clarified. Here, we review data on the underlying mechanisms controlling PRL secretion using both experimental and mathematical approaches. These investigations primarily focus on rhythmic secretion in rats during early pregnancy or pseudopregnancy, and they point to the important role of oxytocin as a crucial PRL-releasing factor. Recent data on human studies and their theoretical and clinical implications are reviewed as well. In particular, studies demonstrating a sustained PRL surge after sexual climax in males and females are presented, indicating possible implications for both sexual satiation and reproductive functions. Taking these data together, there is evidence for the hypothesis that the PRL surge induced by sexual activity, together with the altered PRL rhythmic pattern, is important for successful initialization of pregnancy not only in rodents but also possibly in humans. However, further investigations are needed to clarify such a role in humans.

Female sexual dysfunction

Sexual dysfunctions diminish the quality of life for many women, frequently causing enough distress to warrant the diagnosis of a sexual disorder. Problems with sexual function can occur in any stage of the sexual response cycle. Dysfunction is further influenced by a variety of factors: medical, psychiatric, cultural, and stage of life. A variety of treatment modalities exist, though current research has not yet provided Food and Drug Administration-approved therapies for sexual disorders in women.

The endocannabinoid system and nondrug rewarding behaviours

Rewarding behaviours such as sexual activity, eating, nursing, parenting, social interactions, and play activity are conserved strongly in evolution, and they are essential for development and survival. All of these behaviours are enjoyable and represent pleasant experiences with a high reward value. Remarkably, rewarding behaviours activate the same brain circuits that mediate the positive reinforcing effects of drugs of abuse and of other forms of addiction, such as gambling and food addiction. Given the involvement of the endocannabinoid system in a variety of physiological functions of the nervous system, it is not surprising that it takes part in the complex machinery that regulates gratification and perception of pleasure. In this review, we focus first on the role of the endocannabinoid system in the modulation of neural activity and synaptic functions in brain regions that are involved in natural and nonnatural rewards (namely, the ventral tegmental area, striatum, amygdala, and prefrontal cortex). Then, we examine the role of the endocannabinoid system in modulating behaviours that directly or indirectly activate these brain reward pathways. More specifically, current knowledge of the effects of the pharmacological manipulation of the endocannabinoid system on natural (eating, sexual behaviour, parenting, and social play) and pathological (gambling) rewarding behaviours is summarised and discussed.

Postorgasm illness syndrome--a spectrum of illnesses

INTRODUCTION

We describe two men with marked symptoms following orgasm. In each case, the symptoms were consistent with those found in postorgasm illness syndrome (POIS).

AIM

Further elucidation of the cause of the patients' symptoms.

METHODS

Both cases were investigated for causes of POIS with biochemical, hormonal, neurological, autonomic, cardiological, and psychological workup.

RESULTS

Extensive investigation did not reveal a major organic cause for these patients' symptoms. Detailed history revealed likely differing etiologies in each case. In one case, the symptom picture suggested cytokine release, and, in fact, the patient subjectively improved by 80% on taking nonsteroidal anti-inflammatory drugs just prior to and for a day or two after orgasm. The other case appeared to have an ethnic/cultural etiology that was associated with the "Dhat" syndrome.

CONCLUSION

The apparent differing etiologies/clinical associations of these cases highlight the need for careful history, examination, and investigations in patients presenting with POIS. We recommend that each case needs individual consideration and investigation, and treatment needs to be tailored to the likely cause. It seems likely that POIS represents a spectrum of syndromes of differing etiologies. Further research into the neurobiochemical sequelae of orgasm will be useful in understanding the pathological processes in these cases.

Restoration of sexual activity in patients with chronic hydrocephalus after shunt placement

BACKGROUND

Chronic (normotensive or low pressure) hydrocephalus is characterized clinically by gait disturbance, cognitive and urinary impairment, known as Hakim's triad. Nothing has been reported about impairment in sexual function, which could involve both the patient and the patient's partner.

METHODS

Out of 97 patients undergoing shunt placement for chronic hydrocephalus, 28 male patients (28.8%) referenced sexual dysfunction before operation. In these cases, we performed a preoperative and postoperative survey of sexual activity.

RESULTS

In the preoperative period, all 28 patients reported having no sexual activity or arousal, from 2 to 4 years before the operation. Following shunt placement, 22/28 (78.5%) of patients regained variable sexual desire within a period ranging from 3 to 8 weeks, affording normal sexual activity with their partner.

CONCLUSIONS

Sexual dysfunction can be part of the very early clinical background in patients with Hakim's triad and neuroradiological imaging compatible with chronic hydrocephalus. Restoration of sexual ability and arousal should be considered among the postoperative goals in these cases, together with improvements in cognition, gait, and urinary continence.

Sexual dysfunction in Parkinson's disease

Sexual dysfunction is one of the more disabling and poorly investigated aspects of PD. Several variables should be considered when evaluating sexual dysfunction in a disease in which physical, psychological, neurobiological and pharmacological features merge and are not easily distinguishable. Although sexual dysfunction is common in Parkinson's disease, the development of hypersexuality and aberrant sexual behaviour, probably due to dopamine replacement therapy, calls into question the role of dopamine in sexual behaviour. This paper reviews studies that have investigated sexual behaviour and dysfunction in PD patients, paying particular attention to the effect of dopamine replacement therapy.

A contextual definition of male sexual arousal

Sexual arousal is a construct without a widely shared definition. Historically, sexual arousal has usually referred to a central physiological state, but there has been much less agreement on its relation to motivation, emotion, and - for males - penile erection and ejaculation. Many behavioral and physiological measures have been used as operational definitions of sexual arousal, but the relation of the measure to arousal is often assumed rather than tested. For men, penile erection in the presence of erotic stimuli has been considered the most reliable and valid indicator of sexual arousal. The adoption of analogous criteria is recommended for research on other male mammals in order to establish a minimal basis for inferring that they are sexually aroused. That is, sexual arousal should be inferred only when penile erection is observed in a sexual context. A sexual context is provisionally defined as an environment that tends in most reproductively active males of the species to provoke further sexual stimulation, e.g., copulation or self-stimulation to ejaculation. Erection occurring outside of a sexual context, as during REM sleep or from injection of drugs, is not grounds for inferring arousal. Conversely, males engaging in behavior directed toward estrous females may be sexually motivated, but in the absence of erection, the males should not be assumed to be sexually aroused. Implications of other erection-context interactions are also considered. Adoption of these more conservative criteria for inferring sexual arousal may promote greater precision in identifying the physiological systems mediating this hypothetical construct.

The role of prolactin levels in the sexual activity of married men with erectile dysfunction

OBJECTIVE

To evaluate the effects of risk factors for erectile dysfunction (ED) including anthropometry, hormones, metabolic profiles and lifestyle, on sexual activity in married men with ED.

PATIENTS AND METHODS

The study included 261 men (mean age 53.7 years, range 23-80), who were evaluated for anthropometry, hormone levels, metabolic profiles and lifestyle factors. Erectile function was evaluated using the self-administered International Index of Erectile Function. Patients were classified into two groups based on the six-item erectile-function domain, as those with sexual activity and those without.

RESULTS

Of all patients, 62.5% (163 of 261) had no sexual activity (erectile-function domain score <6). There was a significant difference in mean (sem) prolactin level between patients with and with no sexual activity, at 4.8 (0.4) vs 6.8 (0.7) (P = 0.013). Of all patients, 73.7% (42 of 57) with diabetes had no sexual activity, while 59.3% (121 of 204) without diabetes had (P = 0.048). In a multivariate model, a higher prolactin level was associated with a greater likelihood of sexual inactivity (odds ratio 1.094; 95% confidence interval, 1.010-1.185; P = 0.028) but diabetes lost its statistical significance. The area under the receiver-operating characteristic curve for prolactin was 60.5% (95% confidence interval, 52.9-68.1%; P = 0.009) for sexual inactivity. No other factors were significant in this regard.

CONCLUSION

Our findings suggest that prolactin levels might play a role in sexual activity in men with ED.

Serial neurochemical measurement of cerebrospinal fluid during the human sexual response cycle

Recent studies examining the neuroendocrine response pattern underlying the human sexual response cycle revealed transient activation of the sympathoadrenal system and a substantial, long-lasting increase in plasma prolactin concentrations following orgasm in men and women. Prolactin has been discussed as being part of a feedback mechanism that signals centers in the central nervous system, such as the dopaminergic system controlling sexual arousal. To further elucidate the central role of neuropeptides, biogenic monoamines and neurotransmitters in human sexual behavior, a serial cerebrospinal fluid (CSF)-sampling technique was implemented using a previously established experimental paradigm for sexual activity in a laboratory setting. In parallel with peripheral endocrine measures, lumbar CSF was drawn via an indwelling spinal catheter during the sexual response cycle in 10 healthy males and 10 age-matched controls, and analysed for prolactin, oxytocin, biogenic monoamines and/or their metabolites as well as inhibitory and excitatory neurotransmitter concentrations. Parallel to raised peripheral sympathetic activity, norepinephrine also increased in CSF during audiovisual, masturbation-induced sexual arousal and orgasm, and remained elevated for the remainder of the session (F(4,72) = 8.79, P = 0.000). In contrast, none of the other measures, in particular prolactin and dopamine or its metabolites, reflected significant alteration. In conclusion, the human sexual response cycle is characterized by an increase in sympathetic activity in plasma and CSF, and by pronounced secretion of plasma prolactin after orgasm. However, alterations in dopaminergic or peptidergic activity are not found in lumbar CSF, possibly due to local and restricted release in diencephalic and mesencephalic brain regions.