A novel mouse model for assessment of male sexual function

Serotonin neurons in mating female mice are activated by male ejaculation

Sexual stimulation triggers changes in female physiology and behavior, including sexual satiety and preparing the uterus for pregnancy. Serotonin (5-HT) is an important regulator of reproductive physiology and sexual receptivity, but the relationship between sexual stimulation and 5-HT neural activity in females is poorly understood. Here, we investigated dorsal raphe 5-HT neural activity in female mice during sexual behavior. We found that 5-HT neural activity in mating females peaked specifically upon male ejaculation and remained elevated above baseline until disengagement. Artificial intravaginal mechanical stimulation was sufficient to elicit increased 5-HT neural activity but the delivery of ejaculatory fluids was not. Distal penis expansion ("penile cupping") at ejaculation and forceful expulsion of ejaculatory fluid each provided sufficient mechanical stimulation to elicit 5-HT neuron activation. Our study identifies a female ejaculation-specific signal in a major neuromodulatory system and shows that intravaginal mechanosensory stimulation is necessary and sufficient to drive this signal.

The elusive concept of sexual motivation: can it be anchored in the nervous system?

Sexual motivation is an abstract concept referring to the mechanisms determining the responsivity to sexually relevant stimuli. This responsivity determines the likelihood of producing a sexual response and the intensity of that response. Both responsivity to stimuli and the likelihood of making a response as well as the intensity of response are characteristics of an individual. Therefore, we need to assume that the concept of sexual motivation materializes in physiological mechanisms within the individual. The aim of the present communication is to analyze the requisites for the endeavor to materialize sexual motivation. The first requisite is to provide an operational definition, making the concept quantifiable. We show that parameters of copulatory behavior are inappropriate. We argue that the intensity of sexual approach behaviors provides the best estimate of sexual motivation in non-human animals, whereas the magnitude of genital responses is an exquisite indicator of human sexual motivation. Having assured how to quantify sexual motivation, we can then proceed to the search for physiological or neurobiological underpinnings. In fact, sexual motivation only manifests itself in animals exposed to appropriate amounts of gonadal hormones. In female rats, the estrogen receptor α in the ventrolateral part of the ventromedial nucleus of the hypothalamus is necessary for the expression of sexual approach behaviors. In male rats, androgen receptors within the medial preoptic area are crucial. Thus, in rats sexual motivation can be localized to specific brain structures, and even to specific cells within these structures. In humans, it is not even known if sexual motivation is materialized in the brain or in peripheral structures. Substantial efforts have been made to determine the relationship between the activity of neurotransmitters and the intensity of sexual motivation, particularly in rodents. The results of this effort have been meager. Likewise, efforts of finding drugs to stimulate sexual motivation, particularly in women complaining of low sexual desire, have produced dismal results. In sum, it appears that the abstract concept of sexual motivation can be reliably quantified, and the neurobiological bases can be described in non-human animals. In humans, objective quantification is feasible, but the neurobiological substrate remains enigmatic.

Serotonin neurons in mating female mice are activated by male ejaculation

Sexual stimulation triggers changes in female physiology and behavior, including sexual satiety and preparing the uterus for pregnancy. Serotonin is an important regulator of reproductive physiology and sexual receptivity, but the relationship between sexual stimulation and serotonin neural activity in females is poorly understood. Here, we investigated dorsal raphe serotonin neural activity in females during sexual behavior. We found that serotonin neural activity in mating females peaked specifically upon male ejaculation, and remained elevated above baseline until disengagement. Artificial intravaginal mechanical stimulation was sufficient to elicit increased 5-HT neural activity but the delivery of ejaculatory fluids was not. Distal penis erectile enlargement ("penile cupping") at ejaculation and forceful expulsion of ejaculatory fluid each provided sufficient mechanical stimulation to elicit serotonin neuron activation. Our study identifies a female ejaculation-specific signal in a major neuromodulatory system and shows that intravaginal mechanosensory stimulation is necessary and sufficient to drive this signal.

European Society for Sexual Medicine Consensus Statement on the Use of the Cavernous Nerve Injury Rodent Model to Study Postradical Prostatectomy Erectile Dysfunction

INTRODUCTION

Rodent animal models are currently the most used in vivo model in translational studies looking into the pathophysiology of erectile dysfunction after nerve-sparing radical prostatectomy.

AIM

This European Society for Sexual Medicine (ESSM) statement aims to guide scientists toward utilization of the rodent model in an appropriate, timely, and proficient fashion.

METHODS

MEDLINE and EMBASE databases were searched for basic science studies, using a rodent animal model, looking into the consequence of pelvic nerve injury on erectile function.

MAIN OUTCOME MEASURES

The authors present a consensus on how to best perform experiments with this rodent model, the details of the technique, and highlight possible pitfalls.

RESULTS

Owing to the specific issue-basic science-Oxford 2011 Levels of Evidence criteria cannot be applied. However, ESSM statements on this topic will be provided in which we summarize the ESSM position on various aspects of the model such as the use of the Animal Research Reporting In Vivo Experiments guideline and the of common range parameter for nerve stimulation. We also highlighted the translational limits of the model.

CONCLUSION

The following statements were formulated as a suggestive guidance for scientists using the cavernous nerve injury model. With this, we hope to standardize and further improve the quality of research in this field. It must be noted that this model has its limitations. Weyne E, Ilg MM, Cakir OO, et al. European Society for Sexual Medicine Consensus Statement on the Use of the Cavernous Nerve Injury Rodent Model to Study Postradical Prostatectomy Erectile Dysfunction. Sex Med 2020;8:327-337.

Telemetric monitoring of penile pressure during mating in rats after chronic spinal cord injury

In men with a spinal cord injury (SCI), erectile function, ejaculation, and fertility are severely impaired. The present study utilized a telemetric pressure transducer implanted into the corpus cavernosum of the penis to examine sexual function during awake mating behavior in a rat contusion model with a range of Infinite Horizon Impactor forces distributed between 150 and 215 kdyn. The mating behavior paradigm included examination of the counts, average pressure, and average duration for mounts, intromissions, and ejaculations. Male Wistar rats were mated with receptive females in 30-min sessions preinjury (sexual acclimation) and once per week for 6 wk beginning after a 2-wk recovery period post-SCI. All SCI animals had significant deficits in sexual function in the parameters measured. These deficiencies were more prevalent in a subset having less than 20% white matter sparing, likely a reflection of the extent of bilateral spino-bulbo-spinal sexual circuitry disruption at the lesion epicenter. The resulting discoordination of the autonomic and somatic reflex control of erection and ejaculation recorded using telemetry devices in an awake, behaving animal model provides an effective means of gauging sexual function deficits after SCI and could have utility for quantifying recovery after a therapeutic intervention.

Voluntary urination control by brainstem neurons that relax the urethral sphincter

Voluntary urination ensures that waste is eliminated when safe and socially appropriate, even without a pressing urge. Uncontrolled urination, or incontinence, is a common problem with few treatment options. Normal urine release requires a small region in the brainstem known as Barrington's nucleus (Bar), but specific neurons that relax the urethral sphincter and enable urine flow are unknown. Here we identify a small subset of Bar neurons that control the urethral sphincter in mice. These excitatory neurons express estrogen receptor 1 (Bar^ESR1), project to sphincter-relaxing interneurons in the spinal cord and are active during natural urination. Optogenetic stimulation of Bar^ESR1 neurons rapidly initiates sphincter bursting and efficient voiding in anesthetized and behaving animals. Conversely, optogenetic and chemogenetic inhibition reveals their necessity in motivated urination behavior. The identification of these cells provides an expanded model for the control of urination and its dysfunction.

Erectile dysfunction in a murine model of sleep apnea

RATIONALE

Erectile dysfunction (ED) is frequent in obstructive sleep apnea syndrome (OSAS). Chronic intermittent hypoxia (CIH), one of the hallmarks of OSAS, could mediate ED.

OBJECTIVES

To determine whether intermittent hypoxia during sleep affects erectile dysfunction in mice.

METHODS

Three groups of C57BL/6 mice were exposed to CIH for 5 or 24 weeks. Sexual function was evaluated by in vivo telemetry of corpus spongiosum pressure. Spontaneous erections, sexual activity during mating, and noncontact tests were assessed after 5 weeks of CIH and after treatment with tadalafil. Plasma testosterone was measured after 8 and 24 weeks of CIH, and the expression of nitric oxide synthase (NOS) isoforms was examined in penile tissue.

MEASUREMENTS AND MAIN RESULTS

Noncontact, spontaneous, and contact sexual activity in the mice was suppressed after CIH. Spontaneous erection counts decreased after the first week of CIH by 55% (P < 0.001) and remained unchanged thereafter. Recovery of erectile activity during normoxia for 6 weeks was incomplete. Compared with control mice, latencies for mounts and intromissions increased by 60- and 40-fold, respectively (P < 0.001), and the sexual activity index decreased sixfold. Tadalafil treatment significantly attenuated these effects. Immunoblot analyses of NOS proteins in the erectile tissue showed decreased expression of endothelial NOS after CIH (P < 0.01), with no changes in plasma testosterone levels after 8 and 24 weeks of CIH.

CONCLUSIONS

CIH during sleep is associated with decreased libido in mice. The decreased expression of endothelial NOS protein in erectile tissue and the favorable response to tadalafil suggest that altered nitric oxide mechanisms underlie CIH-mediated ED. No changes in testosterone emerge after intermittent hypoxia.