Yohimbine and naloxone: effects on male rat sexual behavior

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.

Detection of Effects on Male Reproduction—A Literature Survey

The 1CH (International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use) Guideline for Detection of Toxicity to Reproduction for Medicinal Products, adopted at the Second ICH Conference in Orlando, FL, U.S.A., emphasized the need for research into the suitability of various methods for the detection of effects on fertility in males. The current project was undertaken to compare the efficiency of methods by evaluating reports in the open literature. The results of the examination of 117 substances or substance classes support the view that histopathology and organ weight analysis provide the best general-purpose means of detecting substances with the potential to affect male fertility. Examinations at up to 4 weeks of treatment appear to be as effective as examinations conducted at later times. Mating with females for detection of effects unrelated to interference with sperm production appears to provide an optimal combination because adding other methodologies does not materially improve the detection rate. As to the timing of the mating trial, a 2-week premating period is as efficient as mating at 4 weeks and apparently more efficient than mating after prolonged premating treatment.

Inhibitory and disinhibitory effects of psychomotor stimulants and depressants on the sexual behavior of male and female rats

Drugs of abuse comprise several pharmacological classes, including psychomotor stimulants, such as amphetamine and cocaine, and CNS depressants, such as morphine and alcohol. Few studies have examined the effects of those drugs systematically on human sexual behavior, although substantial clinical and epidemiological literatures suggest that drugs in both classes either inhibit sexual responding or can be "prosexual" in certain situations, thereby increasing the potential of risky sexual activity and the spread of sexually transmitted diseases. This paper reviews original data in rats showing that both classes of drug inhibit or disinhibit sexual behavior depending on the animal's baseline level of sexual responding, hormonal status, whether the drug is given acutely or chronically, and whether the animal has learned to inhibit sexual responding toward nonreceptive partners or in the presence of conditioned olfactory cues that predict sexual nonreward.

The adrenergic α2 receptor and sexual incentive motivation in male rats

The purpose of the present series of experiments was to determine whether drugs acting at the alpha2-adrenoceptor modify unconditioned sexual incentive motivation in the male rat. To that end a highly specific agonist, dexmedetomidine, a corresponding antagonist, atipamezole, and a less specific antagonist, yohimbine, were administered to groups of sexually inexperienced male rats. The subjects were tested in a large rectangular arena, where a sexually receptive female and an intact male were employed as incentives. The incentive animals were confined behind a wire mesh in opposite corners of the arena. The animals could see, hear and smell each other, but no sexual interaction was possible. Approach to the incentives constituted the measure of incentive motivation. In addition, the test provided data on ambulatory activity and general arousal. Dexmedetomidine, at a dose of 8 microg/kg, produced a slight reduction of sexual incentive motivation. Ambulatory activity and general arousal were also inhibited. Atipamezole, in doses of 0.1 and 0.3mg/kg enhanced the positive incentive properties of the receptive female. A high dose of 1mg/kg did not have any significant effect. Ambulatory activity was slightly reduced by the two larger doses of atipamezole. Yohimbine had a slight stimulatory effect on sexual incentive motivation at a dose (4 mg/kg) that also reduced ambulatory activity and general arousal. It is concluded that blockade of the adrenergic alpha2 receptor stimulates sexual incentive motivation in the male rat whereas stimulation of it has the opposite effect. At present it is not clear if these drug effects are caused by pre- or postsynaptic actions of the drugs, and the importance of secondary changes in other neurotransmitter systems remains unknown.

Metachlorophenylpiperazine (m-CPP) induced intracavernous pressure responses in anaesthetized rats

Here we have recorded the effects of metachlorophenylpiperazine (m-CPP) on intracavernous pressure (ICP) in anesthetized rats pretreated with various pharmacological agents in an attempt to determine the mechanism and relevance of the m-CPP induced ICP response to other models of erection. m-CPP elicited consistent and significantly greater increases in ICP (71.5+/-6.6 mmHg) compared with the mixed 5-HT(2a/2c) agonists trifluoromethylphenylpiperazine (3.4+/-1.3 mmHg) and quipazine (10.9+/-1.8 mmHg). Blockade of 5-HT(2a) receptors with ketanserin failed to unmask any stimulatory effect of quipazine (7.2+/-1.0 mmHg). m-CPP induced ICP responses (71+/-7.0 mmHg) were unaffected in the presence of mianserin (63+/-5 mmHg) and ketanserin (51+/-12 mmHg). Spiperone significantly reduced the m-CPP induced increase in ICP (8.0+/-1.0 mmHg). Naloxone, yohimbine and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) failed to elicit increases in ICP on their own. All three drugs significantly reduced the latency to the first m-CPP induced ICP response compared to saline. Yohimbine increased the duration of m-CPP induced ICP responses whereas 8-OHDPAT increased the mean number of m-CPP induced ICP responses compared to saline. The effects of m-CPP on ICP in anesthetized rats may not be mediated by 5-HT(2c) receptors and appears to be similar to erection in copula, but not erection elicited by other drugs or penile sheath retraction.

Effects of yohimbine and apomorphine on the male sexual behaviour pattern of the golden hamster (Mesocricetus auratus)

It has been reported that the copulatory pattern of male hamsters differs from that displayed by most rodents. Besides mount, intromission and ejaculatory patterns, male hamsters display a peculiar copulatory pattern known as long intromission (LI). This peculiar behavioural pattern emerges after the male has been allowed to ejaculate repeatedly. Although LIs have been linked to sexual exhaustion, their functional meaning and their pharmacological regulation have not yet been elucidated. In this study, the sexual behaviour pattern of male golden hamsters was analysed after the administration of yohimbine and apomorphine, drugs that selectively acts on the noradrenergic and dopaminergic system, respectively. Both drugs have proved effective in inducing facilitation of masculine sexual behaviour in several species, including rodents. Results showed that, as in rats, the administration of yohimbine and apomorphine in male hamsters seems to have a stimulatory effect on masculine sexual behaviour, although their effects differ in characteristics and in intensity. In particular, after yohimbine administration, the onset of LIs appears sooner than in control subjects and it seems that they are linked to the number of ejaculations. In addition, sexual activity seems increased after the onset of LIs, including an increase in ejaculations and in the number of LIs. On the other hand, apomorphine administration induced just a slight stimulatory effect limited to ejaculatory latency and postejaculatory interval. Concerning LIs, apomorphine induced a complete disappearance of LIs in 60% of the subjects. The full significance of these findings remains to be elucidated.

Effects of antisense to the (alpha)2A-adrenoceptors administered into the region of the locus ceruleus on behaviors in plus-maze and sexual behavior tests in sham-operated and castrated male rats

Clinical and experimental findings have implicated brain alpha2-adrenoceptors in the regulation of many physiological functions, including sexual activity and stress-related behavior. However, which subtypes of the three alpha2-adrenoceptors that have now been cloned (alpha2A, alpha2B, and alpha2C) are involved in these controls have yet to be established. Here, we investigated the contribution of alpha2A-adrenoceptors of the locus ceruleus, the principal source of brain noradrenaline, to exploratory and sexual behaviors. Using administration of antisense oligodeoxynucleotide to inhibit the receptor expression, we found that reductions in brainstem alpha2A-adrenoceptor mRNA levels and alpha2-adrenoceptor densities induced by antisense treatment were not accompanied by any changes in the major characteristics of male sexual activity, such as mount latencies and numbers of mounts. However, in sexual behavior tests, antisense-treated male rats had decreased numbers of rearings and thus have higher percentages of behaviors positively correlated with sexual activity. Besides, antisense-treated animals had decreased anxiety in plus-maze tests. The data demonstrate that inhibition of alpha2A-adrenoceptor expression in the region of the locus ceruleus has an anxiolytic-like effect and facilitates male's attention to female in sexual behavior test.

Pharmacological features of masculine sexual behavior in an animal model of depression

Neonatal treatment with clomipramine induces behavioral alterations during adulthood that resemble symptoms observed in human depression. Therefore, it has been proposed as an animal model of depression. Impairment of male sexual performance is one of the main effects of this treatment. Using this model of depression, we evaluated the effects of drugs that stimulate sexual performance by acting selectively on the adrenergic, serotonergic, or cholinergic system. Yohimbine, a selective antagonist of the alpha-2 receptors; 8-OH-DPAT, a selective agonist of the 5-HT1A receptors; and oxotremorine, a muscarinic agonist, were administered to male rats neonatally treated with clomipramine that showed sexual behavior impairments. Yohimbine and oxotremorine induced only a slight improvement of sexual deficiencies. 8-OH-DPAT not only restored sexual behavior to normal levels, but induced facilitation in most of the copulatory parameters. These results suggest that neonatal treatment with clomipramine induces sexual deficits acting mainly on the adrenergic and cholinergic systems, while the serotoninergic system seems to be preserved.

Comparative sexual side effects of bupropion, fluoxetine, paroxetine, and sertraline

OBJECTIVE

To investigate patient reported prosexual side effects of the aminoketone antidepressant bupropion (INN, amfebutamone) and to compare directly the sexual side effects of bupropion and the selective serotonin reuptake inhibitor (SSRI) antidepressants fluoxetine, paroxetine, and sertraline.

METHODS

One hundred seven psychiatric outpatient respondents receiving current treatment with one of the above antidepressants anonymously completed questionnaires that allowed reporting of both decreases and increases in sexual function. The main outcome measures were antidepressant-associated changes in libido, arousal, duration of time from arousal to orgasm, intensity of orgasm, and duration of orgasm relative to that experienced before the onset of the patients' psychiatric illnesses.

RESULTS

Bupropion-treated patients reported significant increases in libido, level of arousal, intensity of orgasm, and duration of orgasm beyond levels experienced premorbidly. The three SSRIs to an equal degree significantly decreased libido, arousal, duration of orgasm, and intensity of orgasm below levels experienced premorbidly. Overall, 27% of the SSRI-treated patients had no adverse sexual side effects; in contrast, 86% of patients treated with bupropion had no adverse sexual effects, and 77% of bupropion-treated patients reported at least one aspect of heightened sexual functioning.

CONCLUSIONS

SSRI-induced adverse sexual effects appear to be the rule rather than the exception and may be substantially underreported unless patients are specifically asked about the effects of these medications on various aspects of sexual function. In contrast, prosexual effects were reported by the majority of patients treated with bupropion. The findings are reviewed in light of the neurochemistry of these agents and the sexual response.

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.

Monoaminergic influences on temporal patterning of sexual behavior in male rats

We evaluated the effects of the serotonin (5-HT) presynaptic uptake blocker fluoxetine (FLX) and the dopamine (DA)/noradrenaline (NE) releaser amantadine (AMA), separately and in combination, on the temporal patterning of male rat sexual behavior. FLX alone increased intermount-bout intervals, time-outs, grooming time, ejaculation latency, number of mounts per mount bout, and number of mount bouts per ejaculation. AMA alone had the opposite effect on these measures. Additionally, AMA, when given in combination with FLX, completely reversed the FLX-induced deficits in copulatory behavior. We interpret our results as suggesting an interaction between 5-HT and catecholamines in the temporal patterning of male rat copulatory behavior.

Lesions of the nucleus paragigantocellularis: effects on mating behavior in male rats

We evaluated the effects of bilateral radio-frequency lesions of the paragigantocellular (PGi) reticular nucleus in the ventral medulla on male rat copulatory behavior. In Experiment 1, sexually naive male rats with such lesions were more likely than sham-operated controls to copulate to ejaculation during their first exposure to an estrous female. Additionally, among the rats that copulated to ejaculation, those with lesions demonstrated a reduction in mount frequency (MF), intromission frequency (IF), and ejaculation latency (EL), and an increase in copulatory efficiency (CE). In Expt. 2, sexually experienced male rats were allowed to mate to sexual exhaustion. Males with PGi lesions showed an increased latency to sexual exhaustion and an increased number of ejaculations prior to exhaustion. Additionally, rats with PGi lesions displayed reductions in IF, EL, and post-ejaculatory interval (PEI) as they approached sexual exhaustion. Our results provide further evidence that the PGi is a supraspinal locus of descending inhibitory influence on spinal nuclei mediating ejaculatory reflexes in the male rat.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.