Excitotoxic lesions of the nucleus paragigantocellularis facilitate male sexual behavior but attenuate female sexual behavior in rats

Acute caffeine reverses the disruptive effects of chronic fluoxetine on the sexual behavior of female and male rats

RATIONALE

Sexual side effects of chronic treatment with selective serotonin reuptake inhibitors (SSRIs) in humans include anorgasmia and loss of sexual desire and/or arousal which interferes with treatment compliance. There are few options at present to reduce these effects. Because orgasm and desire are mediated in part by activation of sympathetic arousal, we asked whether the sympathomimetic effects of acute caffeine treatment could reverse these effects.

OBJECTIVE

The present study examined whether acute treatment with caffeine (CAF; 10 or 20 mg/kg, ip) versus vehicle could ameliorate the disruption of appetitive and consummatory measures of copulatory behavior produced by chronic fluoxetine (10 mg/kg, sc) in adult, sexually active female or male rats.

METHODS

Sexually experienced female or male rats received daily injections of FLU over a 24-day period and were tested for sexual behaviors five times at 4-day intervals during this period in bilevel pacing chambers. Females had been ovariectomized and given hormone replacement with estradiol benzoate and progesterone prior to each test. Males were left gonadally intact. Four days after the final FLU test, rats were randomly assigned to one of the three doses of CAF and received ip injections of CAF or the saline vehicle 60 min before testing.

RESULTS

Chronic FLU reduced solicitations and lordosis over time in females and reduced the number of ejaculations in males. Both doses of CAF restored solicitations and lordosis in females and ejaculations in males. On their own, both doses of CAF increased females' pacing behavior and the number of mounts and intromissions in the males.

CONCLUSIONS

Stimulation of sympathetic outflow by CAF may constitute a readily accessible on-demand treatment for the sexual side-effects of SSRIs.

Brain neuronal activation induced by flibanserin treatment in female rats

RATIONALE

Flibanserin, a 5-HT1A agonist and 5-HT2A antagonist, is developed for the treatment of hypoactive sexual desire disorder in women, and its efficacy has been evidenced in several clinical studies. Flibanserin prosexual effects have been also evidenced in preclinical animal models. However, the mechanism of action of flibanserin remains not fully understood.

OBJECTIVE

The aim of the present study was to examine brain neuronal activation in female rats treated with flibanserin, using single immunocytochemical labeling of Fos protein, a marker of neuronal activation, and co-localization of Fos and catecholaminergic marker.

METHOD

Six groups of female rats received either acute or chronic administrations of vehicle, flibanserin 15 mg/kg or flibanserin 45 mg/kg. The brains were collected and processed for immunocytochemical labeling.

RESULTS

Acute flibanserin increased levels of Fos immunoreactivity in the nucleus accumbens, arcuate hypothalamic nucleus, locus coeruleus, lateral paragigantocellular nucleus, and nucleus of the solitary tract. Chronic 22-day treatment with flibanserin increased Fos expression in the medial preoptic area and arcuate nucleus of the hypothalamus, ventral tegmental area, locus coeruleus, and lateral paragigantocellular nucleus. Both acute and chronic flibanserin increased the density of activated catecholaminergic neurons in the ventral tegmental area but not in the locus coeruleus.

CONCLUSION

Altogether, our results showed that flibanserin, at the dose known to enhance female sexual motivation, preferentially activated the brain regions belonging to the mesolimbic dopaminergic pathway and hypothalamic structures involved in the integration of sexual cues related to sexual motivation.

Somatic genital reflexes in rats with a nod to humans: anatomy, physiology, and the role of the social neuropeptides

Somatic genital reflexes such as ejaculation and vaginocervical contractions are produced through the striated muscles associated with the genitalia. The coordination of these reflexes is surprisingly complex and involves a number of lumbosacral spinal and supraspinal systems. The rat model has been proven to be an excellent source of information regarding these mechanisms, and many parallels to research in humans can be drawn. An understanding of the spinal systems involving the lumbosacral spinal cord, both efferent and afferent, has been generated through decades of research. Spinal and supraspinal mechanisms of descending excitation, through a spinal ejaculation generator in the lumbar spinal cord and thalamus, and descending inhibition, through the ventrolateral medulla, have been identified and characterized both anatomically and physiologically. In addition, delineation of the neural circuits whereby ascending genitosensory information regarding the regulation of somatic genital reflexes is relayed supraspinally has also been the topic of recent investigation. Lastly, the importance of the "social neuropeptides" oxytocin and vasopressin in the regulation of somatic genital reflexes, and associated sociosexual behaviors, is emerging. This work not only has implications for understanding how nervous systems generate sexual behavior but also provides treatment targets for sexual dysfunction in people.

Serotonergic lesions of the periaqueductal gray, a primary source of serotonin to the nucleus paragigantocellularis, facilitate sexual behavior in male rats

While selective serotonin reuptake inhibitors (SSRIs) are widely used to treat anxiety and depression, they also produce profound disruptions of sexual function including delayed orgasm/ejaculation. The nucleus paragigantocellularis (nPGi), a primary source of inhibition of ejaculation in male rats, contains receptors for serotonin (5-HT). The ventrolateral periaqueductal gray (vlPAG) provides serotonin to this region, thus providing an anatomical and neurochemical basis for serotonergic regulation of the nPGi. We hypothesize that 5-HT acting at the nPGi could underlie the SSRI-induced inhibition of ejaculation in rodents. To this end, we produced 5-HT lesions of the source of 5-HT to the nPGi (the vlPAG) and examined sexual behavior. Removing the source of 5-HT to the nPGi facilitated genital reflexes, but not other aspects of sexual behavior, consistent with our hypothesis. Namely, 5-HT lesions produced a significant increase in the mean number of ejaculations and a significant decrease in ejaculation latency as compared to sham lesioned animals, while latency to mating and the post-ejaculatory interval did not differ. These data suggest that the serotonergic vlPAG-nPGi pathway is an important regulatory mechanism for the inhibition of ejaculation in rats and supports the hypothesis that this circuit contributes to SSRI-induced inhibition of ejaculation.