Drug-induced sexual dysfunction in rats

The endogenous cannabinoid system modulates male sexual behavior expression

The endocannabinoid system (ECS) plays a key neuromodulatory role in the brain. Main features of endocannabinoids (eCBs) are that they are produced on demand, in response to enhanced neuronal activity, act as retrograde messengers, and participate in the induction of brain plasticity processes. Sexual activity is a motivated behavior and therefore, the mesolimbic dopaminergic system (MSL) plays a central role in the control of its appetitive component (drive to engage in copulation). In turn, copulation activates mesolimbic dopamine neurons and repeated copulation produces the continuous activation of the MSL system. Sustained sexual activity leads to the achievement of sexual satiety, which main outcome is the transient transformation of sexually active male rats into sexually inhibited animals. Thus, 24 h after copulation to satiety, the sexually satiated males exhibit a decreased sexual motivation and do not respond to the presence of a sexually receptive female with sexual activity. Interestingly, blockade of cannabinoid receptor 1 (CB1R) during the copulation to satiety process, interferes with both the appearance of the long-lasting sexual inhibition and the decrease in sexual motivation in the sexually satiated males. This effect is reproduced when blocking CB1R at the ventral tegmental area evidencing the involvement of MSL eCBs in the induction of this sexual inhibitory state. Here we review the available evidence regarding the effects of cannabinoids, including exogenously administered eCBs, on male rodent sexual behavior of both sexually competent animals and rat sub populations spontaneously showing copulatory deficits, considered useful to model some human male sexual dysfunctions. We also include the effects of cannabis preparations on human male sexual activity. Finally, we review the role played by the ECS in the control of male sexual behavior expression with the aid of the sexual satiety phenomenon. Sexual satiety appears as a suitable model for the study of the relationship between eCB signaling, MSL synaptic plasticity and the modulation of male sexual motivation under physiological conditions that might be useful for the understanding of MSL functioning, eCB-mediated plasticity and their relationship with motivational processes.

Effect of Selective Dorsal Neurectomy on Erectile Function in Rats

OBJECTIVE

This study aimed to investigate the effect of penile selective dorsal neurectomy (SDN) on erectile function in rats.

METHODS

Twelve adult male Sprague-Dawley rats (15 weeks old) were divided into three groups (n=4 per group): in control group, rats received no treatment; in sham group, rats underwent a sham operation; in SDN group, rats underwent SDN with half of the dorsal penile nerve severed. The mating test was performed, and the intracavernous pressure (ICP) assessed six weeks after the surgical treatment.

RESULTS

At postoperative six weeks, the mating test revealed no significant difference in mounting latency and mounting frequency among the three groups (P>0.05), while the ejaculation latency (EL) was significantly longer and ejaculation frequency (EF) lower in the SDN group than in the control and sham groups (P<0.05). There were no significant differences in preoperative and postoperative ICP and ICP/mean arterial blood pressure (MAP) among the three groups (P>0.05).

CONCLUSION

SDN does not adversely affect the erectile function and sexual desire of rats, and at the same time it can reduce EL and EF, providing an application basis for SDN in the clinical treatment of premature ejaculation.

Fadogia agrestis (Schweinf. Ex Hiern) Stem Extract Restores Selected Biomolecules of Erectile Dysfunction in the Testicular and Penile Tissues of Paroxetine-Treated Wistar Rats

Inadequate release of nitric oxide (NO) by the penile tissue impacts negatively on penile erection causing erectile dysfunction (ED). Fadogia agrestis has been implicated in the management of ED without information on key biomolecules associated with ED in male rats. Therefore, this study evaluated the influence of aqueous extract of Fadogia agrestis stem (AEFAS) on key biomolecules associated with ED in the penile and testicular tissues of male Wistar rats induced with ED by paroxetine. Thirty male rats were assigned into 6 groups (I, II, III, IV, V and VI) of 5. Group I (sham control, without ED) was administered distilled water orally. Paroxetine-induced ED rats in groups II (negative control), III (positive control), IV, V and VI received distilled water, sildenafil citrate (SC, 50 mg/kg body weight) and AEFAS at 18, 50 and 100 mg/kg body weight respectively. Paroxetine lowered/reduced (p < 0.05) the MF, IF, EF, NO, cGMP, catalase, SOD, T-SH, GSH and GST whilst it prolonged/increased ML, IL, EL, PEI, AChE, PDE5, arginase, ACE, TBARS and H₂O₂. Contrastingly, AEFAS like sildenafil citrate increased (p < 0.05) the penile and testicular NO, cGMP, catalase, SOD, T-SH, GSH and GST and reduced AChE, PDE5, arginase, ACE, TBARS and H₂O₂ to levels that compared favourably (p > 0.05) with those of sham control. The study concluded that AEFAS restored the NO/cGMP pathway and ED-associated key enzymes in the penile and testicular tissues of male rats via antioxidant means. The study recommended the use of aqueous extract of Fadogia agrestis stem in managing ED after clinical trials.

The Rare Phenomenon of Consecutive Ejaculations in Male Rats

Mounting, intromission and ejaculation are commonly reported sexual behaviours in male rats. In a mating session, they can have several copulatory series with post-ejaculatory intervals in between ejaculations before they reach sexual satiety. Here, we describe a phenomenon where male rats displayed consecutive ejaculations (CE) with a short inter-ejaculatory interval (IEI). Male rats were daily mated with a sexually receptive female rat. Two out of 15 rats displayed CE in one of their mating tests. The first rat had CE at 9.9 and 10.1 min (IEI = 16.3 s) after the start of the test. The second rat showed CE at 28.1 and 28.5 min (IEI = 18.7 s) after the test onset. During the IEI, the rats did not show any mounting or intromission.

Pharmacological Studies on the Role of 5-HT1 A Receptors in Male Sexual Behavior of Wildtype and Serotonin Transporter Knockout Rats

Brain serotonin (5-HT) neurotransmission plays an important role in male sexual behavior and it is well established that activating 5-HT_{1 A} receptors in rats facilitate ejaculatory behavior. However, the relative contribution of 5-HT_{1 A} somatodendritic autoreceptors and heteroreceptors in this pro-sexual behavior is unclear. Moreover, it is unclear whether the contribution of somatodendritic 5-HT_{1 A} autoreceptors and postsynaptic 5-HT_{1 A} heteroreceptors alter when extracellular 5-HT levels are chronically increased. Serotonin transporter knockout (SERT^-/-) rats exhibit enhanced extracellular 5-HT levels and desensitized 5-HT_{1 A} receptors. These rats model neurochemical changes underlying chronic SSRI-induced sexual dysfunction. We want to determine the role of presynaptic versus postsynaptic 5-HT_{1 A} receptors in the pro-sexual effects of 5-HT_{1 A} receptor agonists in SERT^+/+ and in SERT^-/- rats. Therefore, acute effects of the biased 5-HT_{1 A} receptor agonists F-13714, a preferential 5-HT_{1 A} autoreceptor agonist, or F-15599, a preferential 5-HT_{1 A} heteroreceptor agonist, and S15535 a mixed 5-HT_{1 A} autoreceptor agonist/heteroreceptor antagonist, on male sexual behavior were assessed. A clear and stable genotype effect was found after training where SERT^+/+ performed sexual behavior at a higher level than SERT^-/- rats. Both F-15599 and F-13714 induced pro-sexual activity in SERT^+/+ and SERT^-/- animals. Compared to SERT^+/+, the F13714-dose-response curve in SERT^-/- rats was shifted to the right. SERT^+/+ and SERT^-/- rats responded similar to F15599. Within both SERT^+/+ and SERT^-/- rats the potency of F-13714 was much stronger compared to F-15599. S15535 had no effect on sexual behavior in either genotype. In SERT^+/+ and SERT^-/- rats that were selected on comparable low sexual activity (SERT^+/+ 3 or less ejaculations and SERT^-/- 5 or less ejaculations in 10 weeks) S15535 also did not influence sexual behavior. The two biased compounds with differential effects on 5-HT_{1 A} auto- and hetero-receptors, exerted pro-sexual activity in both SERT^+/+ and SERT^-/- rats. Applying these specific pharmacological tools has not solved whether pre- or post-synaptic 5-HT_{1 A} receptors are involved in pro-sexual activity. Moreover, the inactivity of S15535 in male sexual behavior in either genotype was unexpected. The question is whether the in vivo pharmacological profile of the different 5-HT_{1 A} receptor ligands used, is sufficient to differentiate pre- and/or post-synaptic 5-HT_{1 A} receptor contributions in male rat sexual behavior.

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.

Differential effects of vilazodone versus citalopram and paroxetine on sexual behaviors and serotonin transporter and receptors in male rats

RATIONALE

Sexual side effects are commonly associated with selective serotonin reuptake inhibitor (SSRI) treatment. Some evidence suggest that activation of 5-HT1A receptors attenuates SSRI-induced sexual dysfunction.

OBJECTIVE

This study in male rats compared the effects of vilazodone, an antidepressant with SSRI and 5-HT1A receptor partial agonist activity, with other prototypical SSRIs (citalopram and paroxetine) on sexual behaviors and 5-HT receptors (5-HT1A and 5-HT2A) and transporter (5-HTT) levels in select forebrain regions of the limbic system using quantitative autoradiography.

METHODS

Rats received vilazodone (1, 3, and 10 mg/kg), citalopram (10 and 30 mg/kg), or paroxetine (10 mg/kg) treatment for 14 days. Sexual behaviors (frequency and latency of mounts, intromissions, and ejaculations) were measured in the presence of an estrous female rat on days 1 (acute), 7 (subchronic), and 14 (chronic).

RESULTS

Vilazodone-treated rats exhibited no sexual dysfunction compared with controls; in contrast, the citalopram- and paroxetine-treated rats exhibited impaired copulatory and ejaculatory behaviors after subchronic and chronic treatments. Chronic vilazodone treatment markedly decreased 5-HT1A receptor levels in cortical and hippocampal regions, while the SSRIs increased levels of this receptor in similar regions. All chronic treatments reduced 5-HTT levels across the forebrain; however, the magnitude of the decrease was considerably smaller for vilazodone than for the SSRIs.

CONCLUSIONS

The current studies showed that chronic treatment with vilazodone, in contrast to citalopram and paroxetine, was not associated with diminished sexual behaviors in male rats, which may be related to the differential effects of vilazodone on 5-HT1A receptor and 5-HTT levels relative to conventional SSRIs.

Aqueous root extract of Lecaniodiscus cupanioides restores nitric oxide/cyclic guanosine monophosphate pathway in sexually impaired male rats

ETHNOPHARMACOLOGICAL RELEVANCE

Aqueous root extract of Lecaniodiscus cupanioides is widely used in the management of sexual dysfunction in Nigeria. The effect of aqueous root extract of L. cupanioides root on the concentrations of penile cyclic Guanosine Monophosphate (cGMP) and plasma nitric oxide in paroxetine-induced sexually impaired male rats was evaluated.

METHODS

Thirty (30) albino rats were assigned into six groups (A, B, C, D, E and F) of five rats each such that animals in Group A (control) received distilled water while those in Groups B, C, D, E and F which were induced into sexual dysfunction (p.o 10mg/kg of paroxetine hydrochloride suspension in Tween-80) and in addition received distilled water, 7.14 mg/kg body weight of a reference herbal drug (PowmaxM), 25, 50 and 100mg/kg body weight of the extract respectively, orally, once daily for five days.

RESULTS

Administration of paroxetine significantly reduced the levels of penile cyclic Guanosine Monophosphate (cGMP) and plasma nitric oxide. These decreases were dose dependently reversed by the aqueous extract of L. cupanioides root. The reversal by the 25 and 50mg/kg body weight of the extract compared favorably with the PowmaxM, whereas the 100mg/kg body weight of the extract compared favorably with the non-sexually impaired distilled water treated control animals.

CONCLUSION

The results of this study show that aqueous extract of L. cupanioides root restored the levels of cGMP and nitric oxide in sexually impaired rats. This study further lends credence to the use of aqueous root extract of L. cupanioides in the management of sexual dysfunction in Nigeria.

Sexual side effects of serotonergic antidepressants: mediated by inhibition of serotonin on central dopamine release?

Antidepressant-induced sexual dysfunction adversely affects the quality of life of antidepressant users and reduces compliance with treatment. Animal models provide an instructive approach for examining potential sexual side effects of novel drugs. This review discusses the stability and reproducibility of our standardized test procedure that assesses the acute, subchronic and chronic effects of psychoactive compounds in a 30 minute mating test. In addition, we present an overview of the effects of several different (putative) antidepressants on male rat sexual behavior, as tested in our standardized test procedure. By comparing the effects of these mechanistically distinct antidepressants (paroxetine, venlafaxine, bupropion, buspirone, DOV 216,303 and S32006), this review discusses the putative mechanism underlying sexual side effects of antidepressants and their normalization. This review shows that sexual behavior is mainly inhibited by antidepressants that increase serotonin neurotransmission via blockade of serotonin transporters, while those that mainly increase the levels of dopamine and noradrenaline are devoid of sexual side effects. Those sexual disturbances cannot be normalized by simultaneously increasing noradrenaline neurotransmission, but are normalized by increasing both noradrenaline and dopamine neurotransmission. Therefore, it is hypothesized that the sexual side effects of selective serotonin reuptake inhibitors may be mediated by their inhibitory effects on dopamine signaling in sex brain circuits. Clinical development of novel antidepressants should therefore focus on compounds that simultaneously increase both serotonin and dopamine signaling.

Differences in sexual behaviour in male and female rodents: role of serotonin

Serotonin plays an important role in both male and female sexual behaviour. In general, reduction of 5-HT function facilitates, whereas enhancement inhibits sexual behaviour. Most fundamental research on the involvement of 5-HT in sex has been performed in rats. Selective serotonin reuptake inhibitors (SSRIs) have comparable effects on male and female sexual behaviour in rats; they inhibit it but only after chronic administration. Activation of the 5-HT(1A) receptor facilitates sexual behaviour in male rats but inhibits sexual behaviour in female rats, suggesting a differential role for 5-HT(1A) receptors in male and female rats. Research on sexual behaviour in rats with null mutations in the serotonin transporter (SERT) indicated also a differential role for 5-HT(1A) receptors in male and female sexual behaviour. Evidence exists that different pools of 5-HT(1A) receptors have differential roles in various parts of the cascade of sexual events occurring during sexual interactions. Roles for other 5-HT receptors are less well defined although 5-HT(1B), 5-HT(2A/B) and 5-HT(7) receptors seem to be involved. Identification of putative differential or comparable roles in female and male sexual activities requires more research.