Dopaminergic influences on male sexual behavior of rhesus monkeys: Effects of dopamine agonists

From pathology to pleasure: Reframing mechanistic studies on same-sex sexual behavior in primates

Same-sex sexual behaviors (SSB) in primates have historically been studied as sexual perversions, evolutionary paradoxes, and hormone-driven pathologies. Researchers in recent decades have challenged these perspectives, yet some of the original biases still linger. In this paper, we examine how the study of endocrinological mechanisms in SSB has been influenced by the historical framework of pathology. Societal attitudes and cultural conceptions of human sexuality have led researchers to study SSB in primates as the outcome of "abnormal" processes of "feminization" or "masculinization" of sexual behavior. Here, we argue for a renewed attention to other areas of inquiry regarding the relationship between hormones and SSB, such as the role of pleasure. We briefly review how current knowledge on the neuroendocrinology of pleasure in nonhuman primates may relate to the expression of SSB and highlight oxytocin and dopamine as potentially fruitful areas for future research. We argue that future studies on SSB in primates would benefit from 1) acknowledging how the historical study of SSB as a pathology has shaped mechanistic studies and 2) studying SSB with the same holistic approach as is taken with different-sex sexual behavior (DSB).

Dopamine, Erectile Function and Male Sexual Behavior from the Past to the Present: A Review

Early and recent studies show that dopamine through its neuronal systems and receptor subtypes plays different roles in the control of male sexual behavior. These studies show that (i) the mesolimbic/mesocortical dopaminergic system plays a key role in the preparatory phase of sexual behavior, e.g., in sexual arousal, motivation and reward, whereas the nigrostriatal system controls the sensory-motor coordination necessary for copulation, (ii) the incertohypothalamic system is involved in the consummatory aspects of sexual behavior (penile erection and copulation), but evidence for its role in sexual motivation is also available, (iii) the pro-sexual effects of dopamine occur in concert with neural systems interconnecting the hypothalamus and preoptic area with the spinal cord, ventral tegmental area and other limbic brain areas and (iv) D₂ and D₄ receptors play a major role in the pro-sexual effects of dopamine. Despite some controversy, increases or decreases, respectively, of brain dopamine activity induced by drugs or that occur physiologically, usually improves or worsens, respectively, sexual activity. These findings suggest that an altered central dopaminergic tone plays a role in mental pathologies characterized by aberrant sexual behavior, and that pro-erectile D₄ receptor agonists may be considered a new strategy for the treatment of erectile dysfunction in men.

Fos responses of dopamine neurons to sociosexual stimuli in male zebra finches

Dopamine (DA) is produced in numerous brain areas and influences a wide variety of social behaviors, but very few data are available to establish the socially-relevant response properties of most DA populations, which comprise eight cell groups numbered A8-A15. Anatomically, these DA populations are evolutionarily conserved, and all have been identified in both birds and mammals. We now report the Fos responses of tyrosine hydroxylase-immunoreactive (TH-ir; putatively dopaminergic) neurons in the A8-A15 cell groups of male zebra finches following exposure to a control condition or one of six different social stimuli: a heterospecific male, conspecific male, fighting in a mate competition paradigm (which includes both male and female stimuli), a courtship interaction without physical contact, a courtship interaction with physical contact but no mounting, and a courtship interaction with mounting. We found that the DA cell groups exhibit distinctive profiles of responsiveness to social stimuli. Fos induction in A8, A9, A10 and midbrain A11 neurons increased significantly in response to a variety of conspecific stimuli, but not heterospecific stimuli. In contrast, Fos induction in the preoptic A14 neurons was observed specifically in response to sexual interactions, and Fos induction in hypothalamic A11 neurons appears to primarily reflect the performance of courtship singing. Infundibular A12 neurons, which may be involved in stress-related processes, showed the highest level of TH+Fos colocalization in control subjects. This colocalization decreased in response to all conspecific stimuli except fighting, and did not decrease following exposure to a heterospecific male.

Female Hypoactive Sexual Desire Disorder: History and Current Status

INTRODUCTION

Hypoactive sexual desire disorder (HSDD) is a common problem that is often treatment refractory. This refractoriness to treatment is reflective of our lack of knowledge concerning the determinants of sexual libido in women.

AIM

To review the development and current status of information concerning the diagnosis and treatment of HSDD.

METHODOLOGY

Literature on HSDD published between 1950 and 2005 was reviewed.

RESULTS

Historically, HSDD was considered to be a psychiatric disorder. Thus, the diagnostic criteria utilized in studies of interventions for this disorder are based on the Diagnostic and Statistical Manual of the American Psychiatric Association. This system was never designed to encompass organic causes of sexual dysfunction and has evolved by expert opinion. If the syndrome is poorly defined by these diagnostic criteria, this could limit progress in this field. Epidemiological studies have found that approximately 24-43% of women complain of low sexual desire in the preceding year. The percentage of the population meeting diagnostic criteria for HSDD is probably much lower. There has been considerable progress in the development of psychometrically sound instruments for the assessment of libido. The development of approaches to treatment was reviewed. Approaches to treatment have evolved in three major areas: psychological, hormonal, and use of psychopharmacological agents. There is some evidence of efficacy utilizing all three approaches. The major evidence of efficacy concerns the use of testosterone therapy. Long-term safety data concerning this treatment are absent.

CONCLUSION

There is a rapidly expanding knowledge base concerning the diagnosis and treatment of HSDD. However, the contemporary clinician is faced with the absence of an approved treatment for this disorder and the lack of clear guidelines concerning the indications and safety of the use of non-approved agents.

AC pulsed electromagnetic fields-induced sexual arousal and penile erections in Parkinson's disease

Sexual dysfunction is common in patients with Parkinson's disease (PD) since brain dopaminergic mechanisms are involved in the regulation of sexual behavior. Activation of dopamine D2 receptor sites, with resultant release of oxytocin from the paraventricular nucleus (PVN) of the hypothalamus, induces sexual arousal and erectile responses in experimental animals and humans. In Parkinsonian patients subcutaneous administration of apomorphine, a dopamine D2 receptor agonist, induces sexual arousal and penile erections. It has been suggested that the therapeutic efficacy of transcranial administration of AC pulsed electromagnetic fields (EMFs) in the picotesla flux density in PD involves the activation of dopamine D2 receptor sites which are the principal site of action of dopaminergic pharmacotherapy in PD. Here, 1 report 2 elderly male PD patients who experienced sexual dysfunction which was recalcitrant to treatment with anti Parkinsonian agents including selegiline, levodopa and tolcapone. However, brief transcranial administrations of AC pulsed EMFs in the picotesla flux density induced in these patients sexual arousal and spontaneous nocturnal erections. These findings support the notion that central activation of dopamine D2 receptor sites is associated with the therapeutic efficacy of AC pulsed EMFs in PD. In addition, since the right hemisphere is dominant for sexual activity, partly because of a dopaminergic bias of this hemisphere, these findings suggest that right hemispheric activation in response to administration of AC pulsed EMFs was associated in these patient with improved sexual functions.

Dopamine and sexual behavior in the male rabbit

Male rabbits were treated with the dopamine releasing drug amphetamine or the dopamine D1/D2 receptor antagonist cis(Z)-flupenthixol. Amphetamine, 1 to 4 mg/kg, had no effect on sexual behavior. Flupenthixol, 2 mg/kg, reduced the proportion of rabbits that ejaculated and the number of ejaculations per test. Lower doses were ineffective. Castrated males were treated with both drugs at two intervals after castration, 19-21 and 27-29 days postcastration, respectively. Amphetamine was without effect while flupenthixol, 1 mg/kg, reduced sexual behavior at the test 19-21 days postcastration. At the second test, sexual behavior was almost completely absent in control animals. Therefore, no further reduction could be observed after treatment with flupenthixol. Another group of animals was castrated and given androgen replacement. Testosterone decanoate was injected once weekly at a dose of 3 mg/kg. This treatment maintained a stable, low sexual activity. In these animals, amphetamine was again ineffective whereas flupenthixol, 1 mg/kg, inhibited sexual behavior. Gross motor function was evaluated in a water escape test. Amphetamine was inactive, and the effective dose of flupenthixol was 10 mg/kg. This dose is far above the dose required for inhibiting sexual behavior. In sum, facilitated dopaminergic transmission does not seem to affect on sexual behavior in the male rabbit, whereas reduced dopaminergic activity disrupts this behavior.

Effects of acute treatment with dopaminergic drugs on open field behavior of adult monkeys treated with lead during the first year postpartum

A pharmacological challenge of nonhuman primate open field behavior, similar to that which previously assessed the cholinergic system (10), was used here to measure potential lead-induced alterations in the dopaminergic system. Monkeys that had been treated with lead during the first year postpartum were assessed at 7 years of age in the open field after acute intramuscular injection of apomorphine (0.0-0.3 mg/kg) or haloperidol (0.0-30 micrograms/kg). Duration of environmental exploration indicated a possible greater responsivity to 0.2 mg/kg apomorphine in the lead-treated monkeys; however, in all other behaviors, lead-treated monkeys responded to both drugs similarly to controls. Regardless of lead treatment, apomorphine administration decreased duration of inactivity and increased environmental exploration; the latter possibly included an increase in stereotypical behavior that might have been recorded as environmental exploration. Haloperidol had no significant effects on open field behavior over the dose range tested. Open field behavioral alterations previously reported for these monkeys at 4-6 years of age were no longer strongly exhibited by the lead-treated monkeys in any portion of the current study. Latency to enter the open field was marginally increased in the lead-treated group but levels of environmental exploration were comparable to controls. These results indicate an attenuation of lead-related effects with maturity and/or familiarity with the open field.

Effect of cocaine on sexual behaviour in male stumptail macaques (Macaca arctoides)

The effect of cocaine (0.01-1.0 mg/kg) on sexual behaviour was studied in four male stumptail macaques (Macaca arctoides). Following drug-saline control administration, the behaviour of the male monkey with a female was observed for 30 min in two different behavioural conditions; in one of the conditions the baseline sexual activity was low, and in the other it was high (partial or complete separation of the male and the female between the sessions, respectively). The reversal of the cocaine-induced effects was attempted by haloperidol (0.003-0.01 mg/kg), a dopamine-2-receptor antagonist. Cocaine (0.1-1.0 mg/kg) produced a highly significant dose-dependent suppression in the number of ejaculations. The cocaine-induced suppression of ejaculatory behaviour was completely reversed by haloperidol. Haloperidol at the dose range used did not in itself influence ejaculatory behaviour. The effect of cocaine on grooming, nonejaculatory mounting, aggression, or perineal investigations did not reach statistical significance. The possibility that cocaine at very low doses (0.01-0.1 mg/kg) might increase sexual activity was excluded in the behavioural condition with a low basal sexual activity. The results indicate that cocaine dose-dependently suppresses ejaculatory behaviour as a result of dopamine-2-receptor-mediated mechanisms. The cocaine-induced suppression of ejaculatory behaviour might be explained by the strong rewarding effect of cocaine alone.

Dopamine and sexual behavior

Among central neurotransmitters involved in the control of sexual behavior, dopamine is certainly one of the most extensively studied. Our attempt to review old and recent neuropharmacological, biochemical, electrophysiological, and psychobiological studies performed so far only in rats, monkeys, and humans, provides evidence that dopamine through its different neuronal systems and receptor subtypes plays different roles in the control of several aspects of sexual behavior. In fact, while the nigrostriatal system is necessary for the control of the sensory-motor coordination required for copulation, the mesolimbic-mesocortical system plays a key role in the preparatory phase of the behavior, mainly in sexual arousal, motivation and possibly reward. Conversely, the incertohypothalamic system plays a major role in the consummation of the behavior, mainly in seminal emission and erectile performance, but evidence for its involvement in sexual motivation also exists. The dopaminergic receptors playing the major role in the control of male sexual behavior belong to the D2 receptor subtype. However a D1/D2 receptor interaction is well established and an opposite role for D1 and D2 receptors in the preoptic area suggested. Despite some differences, most studies show that treatments that increase or decrease, respectively, brain dopaminergic activity improve or worsen, respectively, several parameters of copulatory activity, supporting a facilitatory role of dopamine in male sexual behavior. In contrast, no conclusion can be deduced from the available studies on the role of central dopaminergic systems in the control of proceptivity and receptivity, the two main components of female sexual behavior.

Impairment of male copulatory behavior in rhesus monkeys following acute administration of cocaine

Although numerous studies in nonhuman primates have demonstrated an influence of cocaine on behavior, no studies have yet examined whether cocaine affects sexual behavior in nonhuman primates. The objective of the present study was to examine the acute effects of cocaine on male copulatory behavior of rhesus monkeys. Administration of cocaine produced dose-dependent effects on male copulatory behavior, with monkeys taking significantly longer to initiate copulation (mount latency) and achieve an ejaculation (ejaculation latency) after receiving 200-800 micrograms/kg cocaine. Male copulatory behavior was not affected by cocaine at doses below 200 micrograms/kg. These results indicate that cocaine can acutely impair sexual behavior performance of male rhesus monkeys. Further study is needed to determine the possible long-term consequences of chronic cocaine administration on male sexual behavior.

5-HT1A and 5-HT1C/1D receptor agonists produce reciprocal effects on male sexual behavior of rhesus monkeys

Research has indicated that serotonin (5-HT) is involved in regulating male sexual behavior in rodent, as well as primate species. The present study was designed to further characterize 5-HT influences on male sexual behavior of rhesus monkeys. Experiment 1 examined the effects of 5-HT1A and 5-HT1C/1D receptor stimulation on penile erections and yawning behavior. Administration of the 5-HT1C/1D receptor agonist, m-chlorophenylpiperazine (m-CPP, 0.8 and 3.0 mg/kg), facilitated the occurrence of penile erection, and at doses greater than 0.2 mg/kg stimulated yawning. By contrast, the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.01-0.2 mg/kg) did not significantly influence penile erections or yawning behavior. Experiment 2 evaluated the effects of m-CPP and 8-OH-DPAT on the behavior of male monkeys in the presence of a sexually receptive female monkey which the males could see, hear and smell, but not physically contact. Administration of m-CPP along with presentation of a receptive female stimulated penile erections to a greater extent than they were stimulated by either one of these manipulations alone. Administration of 8-OH-DPAT (0.1 and 0.2 mg/kg) produced a decrease in the percent of monkeys exhibiting penile erections in the presence of the female. In this experiment, yawning was affected in opposite directions, with m-CPP stimulating and 8-OH-DPAT decreasing the frequency of yawning. Experiment 3 assessed the effects of m-CPP on male copulatory behavior of rhesus monkeys. Administration of m-CPP (0.8-3.0 mg/kg) produced a dose-dependent decline in the percent of males initiating copulation and achieving ejaculation.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonergic influences on male sexual behavior of rhesus monkeys: effects of serotonin agonists

Although numerous studies in rats have demonstrated an influence of serotonin (5-HT) on male copulation, no studies have yet to demonstrate whether such a relationship exists in primate species. The present study sought to characterize 5-HT influences on male copulatory behavior of rhesus monkeys by using three different 5-HT agonists: a full 5-HT1A agonist, 8-hydroxy-2-(din-propylamino) tetralin (8-OH-DPAT); a partial 5-HT1A agonist, ipsapirone; and a 5-HT 1C/ID agonist, m-chlorophenylpiperazine (m-CPP). 8-OH-DPAT had a biphasic effect upon ejaculation latency, with low doses (5-10 micrograms/kg) producing a shortening of ejaculation latency (time from initiation of copulation to ejaculation), and the highest dose (100 micrograms/kg) producing a lengthening of ejaculation latency. Intromission frequency (number of intromissions preceding ejaculation) was affected only at 10 micrograms/kg 8-OH-DPAT with monkeys requiring fewer intromissions to ejaculation at this dose. Ipsapirone administration led to a shortening of ejaculation latency at all doses tested (50-800 micrograms/kg), and a reduction in intromission frequency at 200-800 micrograms/kg ipsapirone. Administration of the 5-HT 1C/1D agonist, m-CPP, resulted in an increase in ejaculation latency at 200-400 micrograms/kg m-CPP and mount latency at 400 micrograms/kg m-CPP, but did not affect intromission frequency. In summary, stimulation of 5-HT1A receptors lowered the ejaculatory threshold of the monkeys, while stimulation of 5-HT 1C/1D receptors interfered with copulatory behavior and raised the ejaculatory threshold.(ABSTRACT TRUNCATED AT 250 WORDS)