The effect of sildenafil on electrostimulation-induced erection in the rat model

Intracavernosal OnabotulinumtoxinA Exerts a Synergistic Pro-Erectile Effect When Combined With Sildenafil in Spontaneously Hypertensive Rats

BACKGROUND

Botulinum toxin A (BTX-A) has a variety of uses in medicine. Some evidence suggests that intracavernosal (ic) BTX-A injection administered in addition to phosphodiesterase type 5 inhibitors (PDE5-Is) could effectively treat erectile dysfunction (ED) in insufficient responders to PDE5-Is.

AIM

To provide experimental pharmacological evidence for the use of onabotulinumtoxinA ic alone or in combination with PDE5-Is for difficult-to-treat ED. We thus compared the effects of BTX-A ic alone and BTX-A ic combined with PDE5-I iv, and a placebo treatment ic or iv.

METHODS

Erectile function was evaluated following cavernous nerve electrical stimulation (6 V, 1-millisecond pulse, 45-second duration) at different frequencies (0, 2, 3, 4, 5, 7.5, and 10 Hz) in 4 groups (n = 8 / group) of anesthetized, spontaneously hypertensive rats, a robust animal model of ED of vascular origin. Rats were treated by onabotulinumtoxinA 10U or saline ic 1 week prior to erectile function testing and sildenafil (0.3 mg/kg) or saline iv 4 minutes prior to testing. Frequency-response curves were compared with a 2 way ANOVA.

OUTCOMES

Both onabotulinumtoxinA ic, and sildenafil iv significantly improved erectile responses in spontaneously hypertensive rats, however the effect was greatly amplified when the treatments were combined.

RESULTS

Intracavernosal pressure and/or mean arterial pressure ratios were significantly increased by sildenafil and onabotulinumtoxinA ic versus the control condition. OnabotulinumtoxinA 10U ic combined with sildenafil iv significantly potentiated erectile responses. Area under the curve and/or mean arterial pressure ratio increased by 19% with sildenafil iv, by 15% with onabotulinumtoxinA ic and by 58% with the combined treatment following cavernous nerve electrical stimulation at 6V, 1 ms, 10 Hz: these stimulation parameters elicited the maximal erectile response.

CLINICAL TRANSLATION

These data provide a pharmacological rationale for the combined administration of onabotulinumtoxinA ic and sildenafil iv since the effects of both treatments were potentiated when their administration was combined.

STRENGTHS & LIMITATIONS

First evidence of a synergistic pro-erectile effect of BTX-A combined with PDE5-I, however the mechanism behind the pro-erectile effect of BTX-A ic remains hypothetical.

CONCLUSIONS

These results support further studies into the mechanisms behind the pro-erectile effect of BTX-A ic, as well as multicenter randomized control trials to evaluate the safety and efficacy of BTX-A ic combined with sildenafil for difficult-to-treat ED. Giuliano F., Joussain C., Denys P., et al. Intracavernosal OnabotulinumtoxinA Exerts a Synergistic Pro-Erectile Effect When Combined With Sildenafil in Spontaneously Hypertensive Rats. J Sex Med 2022;19:899-906.

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.

Animal models of erectile dysfunction

Animal models have contributed to a great extent to understanding and advancement in the field of sexual medicine. Many current medical and surgical therapies in sexual medicine have been tried based on these animal models. Extensive literature search revealed that the compiled information is limited. In this review, we describe various experimental models of erectile dysfunction (ED) encompassing their procedures, variables of assessment, advantages and disadvantages. The search strategy consisted of review of PubMed based articles. We included original research work and certain review articles available in PubMed database. The search terms used were “ED and experimental models,” “ED and nervous stimulation,” “ED and cavernous nerve stimulation,” “ED and central stimulation,” “ED and diabetes mellitus,” “ED and ageing,” “ED and hypercholesteremia,” “ED and Peyronie's disease,” “radiation induced ED,” “telemetric recording,” “ED and mating test” and “ED and non-contact erection test.”

Bladder and erectile dysfunctions in the Type 2 diabetic Goto-Kakizaki rat

Despite the fact that urogenito-sexual complications significantly impact the quality of life of diabetic patients, a robust in vivo experimental model is lacking. Bladder and erectile function in the Type 2 diabetic Goto-Kakizaki (GK) rat and responses to standard-of-care treatments for each disorder have been assessed. GK rats (n = 25, 18-wk-old, GK/Par colony) and age-matched Wistar rats (n = 23), characterized for their metabolic parameters, were used. Bladder function was assessed by cystometry in conscious rats treated by intravenous solifenacin (1 mg/kg). Subsequently, erectile function was assessed under anesthesia following electrical stimulation of the cavernous nerve in presence of intravenous sildenafil (0.3 mg/kg). GK rats displayed detrusor overactivity with a significant increase in frequency/amplitude of nonvoiding contractions during the filling phase, together with an increase in bladder capacity, intercontraction interval, voided volume, and maximal pressure of voiding contraction. Solifenacin significantly decreased parameters characterizing voiding contractions without modifying voiding efficiency. Erectile function in GK rats was markedly impaired and remained so after sildenafil treatment despite a significant improvement. GK rats display both bladder and erectile dysfunctions and respond at least partially to standard-of-care treatments for each disorder, thus representing a suitable model to investigate the pathophysiology and assess the efficacy of new therapeutic agents for Type 2 diabetes-associated bladder and erectile complications.

Change of erectile function and responsiveness to phosphodiesterase type 5 inhibitors at different stages of streptozotocin-induced diabetes in rats

INTRODUCTION

It has been suggested that risk of erectile dysfunction (ED) increases with duration of diabetes and phosphodiesterase type 5 inhibitors (PDE5I) are not as effective in treatment of diabetes-associated ED. However, few studies have investigated time-dependent change in erectile function during the course of diabetes.

AIM

To investigate time-dependent change in erectile function and responsiveness to PDE5I in streptozotocin-induced diabetic rats and to understand the pathophysiology of diabetic ED.

MAIN OUTCOME MEASURES

At 6, 8, 10, 12, and 14 weeks after diabetic induction, erectile function was assessed by cavernous nerve stimulation before and after administration of DA-8159, a novel PDE5I. Penile tissue was assessed for apoptosis with immunohistochemistry. Protein expression of Rho-kinase 2 (ROCK2), myosin phosphatase targeting subunit 1 (MYPT1), and endothelial nitric oxide synthase (eNOS) was evaluated by Western blot.

METHODS

Streptozotocin was injected into 50 8-week-old male Sprague-Dawley rats, which were then classified into five diabetic groups according to the observation period.

RESULTS

Diabetic rats maintained normal erectile responses until 6 weeks of diabetes. Following 8 weeks, the rats showed lower erectile responses at higher frequencies of nerve stimulation, which were normalized to control by administration of DA-8159. In contrast, erectile responses were significantly decreased in 10-week diabetic rats, and administration of DA-8159 resulted in partial recovery of normal responses. At more than 12 weeks, rats demonstrated severe deterioration of erectile function, which did not fully respond to PDE5I. Corporal apoptosis was significantly increased after 10 weeks. Upregulation of ROCK2 was found at 6 weeks, and was followed by an increase of MYPT1 phosphorylation. Phosphorylation of eNOS showed marked suppression at 6 weeks and remained lower during the experimental period.

CONCLUSIONS

Impairment of erectile function was followed by decreased responsiveness to PDE5I during the course of diabetes. The RhoA/ROCK pathway played an important role in diabetes-associated ED.

Yohimbine enhances the effect of sildenafil on erectile process in rats

Combining the centrally acting drug yohimbine with the peripheral conditioner sildenafil might be an approach to erectile dysfunction cases in which sildenafil alone failed. This work aimed to investigate the effect of yohimbine on sildenafil-induced facilitation of erectile process. Erectile responses to electrical stimulation of the cavernous nerve in anesthetized male rats were recorded. Intracavernosal pressure/systemic arterial pressure (ICP/SAP) was calculated, 1 and 5 min after intravenous administration of sildenafil, yohimbine or a combination of both. Changes in sexual arousal and copulatory performance indices were compared before and after these injections using behavioral mating experiments. It was shown that systemic administration of sildenafil produced a significant increase in ICP/SAP than control at doses >or=10 micromol kg(-1). Yohimbine alone failed to potentiate erectile responses but yohimbine (1 micromol kg(-1)) significantly potentiated the effect of sildenafil 1-10 micromol kg(-1) and 1 mmol kg(-1), 1 and 5 min after injection. Potentiation of ICP/SAP induced by their combination was greater than the sum of the effects of the corresponding doses of either drug at the same time interval. A nonsignificant additional decrease in SAP than sildenafil-induced was observed if administered with yohimbine. Addition of sildenafil to yohimbine significantly enhanced the effect of the latter on intromission frequency, intercopulatory interval and the number of ejaculations per session. It is concluded that yohimbine may enhance and prolong the effect of sildenafil on erectile process without additional hypotension. Sildenafil may enhance the central effects of yohimbine on erection; it amplifies the effect of yohimbine on male copulatory performance but not on sexual motivation. The potential beneficial effect of the combination was found to be more pronounced on the central component than on the peripheral component of the erectile process.

Animal models in urological disease and sexual dysfunction

There are several conditions associated with dysfunction of the lower urinary tract or which result in a reduction in the ability to engage in satisfactory sexual function and result in significant bother to sufferers, partners and/or carers. This review describes some of the animal models that may be used to discover safe and effective medicines with which to treat them. While alpha adrenoceptor antagonists and 5-alpha-reductase inhibitors deliver improvement in symptom relief in benign prostatic hyperplasia sufferers, the availability of efficacious and well-tolerated medicines to treat incontinence is less well served. Stress urinary incontinence (SUI) has no approved medical therapy in the United States and overactive bladder (OAB) therapy is limited to treatment with muscarinic antagonists (anti-muscarinics). SUI and OAB are characterised by high prevalence, a growing ageing population and a strong desire from sufferers and physicians for more effective treatment options. High patient numbers with low presentation rates characterizes sexual dysfunction in men and women. The introduction of Viagra in 1998 for treating male erectile dysfunction and the success of the phosphodiesterase type 5 inhibitor class (PDE5 inhibitor) have indicated the willingness of sufferers to seek treatment when an effective alternative to injections and devices is available. The main value of preclinical models in discovering new medicines is to predict clinical outcomes. This translation can be established relatively easily in areas of medicine where there are a large number of drugs with different underlying pharmacological mechanisms in clinical usage. However, apart from, for example, the use of PDE5 inhibitors to treat male erectile dysfunction and the use of anti-muscarinics to treat OAB, this clinical information is limited. Therefore, current confidence in existing preclinical models is based on our understanding of the biochemical, physiological, pathophysiological and psychological mechanisms underlying the conditions in humans and how they are reflected in preclinical models. Confidence in both the models used and the pharmacological data generated is reinforced if different models of related aspects of the same disorder generate confirmatory data. However, these models will only be fully validated in retrospect once the pharmacological agents they have helped identify are tested in humans.

Butea superba Roxb. enhances Penile erection in rats

This study aimed to investigate the effects of ethanol extracts of Butea superba in increasing intracavernous pressure (ICP) in vivo. The extracts were prepared from fresh and dried root cores and fresh and dried root barks. Penile erection was induced in aged rats by electrical stimulation of the cavernous nerve. Cavernous smooth muscle relaxation was also observed in vitro in the presence of the extract, cGMP or isobutyl-methylxanthine (IBMX) alone or the extract together with cGMP or IBMX. The dried root core extract from Phrae was the most effective in increasing the ICP. The dose-response relationship study revealed a bell-shape curve with the maximum effective dose at 1 mg/kg. The ICP of the control and 1 mg/kg extract-treated animals were 45.3 +/- 2.5 and 100.9 +/- 14.0 mmHg, respectively. The extract, cGMP and IBMX alone induced dose dependent muscle relaxation. B. superba significantly enhanced the effects of cGMP and IBMX. The results suggest that ethanol extracts of B. superba are effective in enhancing penile erection. The dried root core extract from Phrae is the most effective part with a maximal dose of 1 mg/kg. The results also suggest that B. superba may act through cAMP/cGMP pathways.

In vivo analysis of chronic phosphodiesterase-5 inhibition with sildenafil in penile erectile tissues: no tachyphylaxis effect

PURPOSE

Conflicting information exists regarding the long-term efficacy of phosphodiesterase-5 (PDE5) inhibitor therapy for erectile dysfunction, particularly in regard to whether therapeutic resistance occurs. We investigated the erectile response, and cavernous PDE5 expression and activity after continuous long-term administration of sildenafil at verified therapeutic plasma concentrations, applying an in vivo rat model of age related erectile dysfunction.

MATERIALS AND METHODS

Male Fisher 344 young (4 months old) and aged (19 months old) rats (National Institute of Aging, Bethesda, Maryland) were injected with sildenafil mesylate (20 mg/kg) or saline subcutaneously every 8 hours for 3 weeks. After a 10 to 18-hour washout period electrical stimulation of the cavernous nerve was performed to assess penile erection. Penes were excised to measure PDE5 protein expression and activity, and blood was collected for sildenafil measurement. Responses were compared with those determined 30 minutes after a single sildenafil injection.

RESULTS

Chronic sildenafil treatment increased the detumescence phase in young and aged rats (p <0.05), although aged rats showed a greater increase than young rats. Baseline cavernous PDE5 expression and activity were greater in aged vs young rats (p <0.05). After chronic sildenafil treatment cavernous PDE5 expression was increased in young (p <0.05) but not in aged rats. Chronic and acute sildenafil treatment similarly inhibited PDE5 activity in the penis of young and aged rats (p <0.05), coincident with its free plasma concentrations equivalent to clinically therapeutic ranges.

CONCLUSIONS

Pharmacological PDE5 inhibitor therapy with sildenafil chronically does not result in treatment resistance. Rather, therapeutic efficacy is maintained and apparently more pronounced with erectile impairment than with normal erectile ability.

Effects of sildenafil citrate on ejaculation latency, detumescence time, and refractory period: placebo-controlled, double-blind, crossover laboratory setting study

OBJECTIVES

To evaluate whether sildenafil citrate (SC) prolongs ejaculation latency and detumescence time and shortens the refractory period in a laboratory setting.

METHODS

Two successive double-blind, placebo-controlled, crossover laboratory studies were performed with 30 different healthy volunteers in each study (total of 60). In the first study, the subject ingested placebo or SC. Real-time penile tumescence and rigidity monitoring and audiovisual sexual stimulation was performed. When the subject had his best erection, he applied vibratory stimulation until he ejaculated, and then audiovisual sexual stimulation was stopped. Monitoring was continued until he lost rigidity. The test was repeated with the second medication in 7 to 15 days. In the second study, another group of 30 volunteers were tested, as in the first study, and audiovisual sexual stimulation was continued for an additional hour after ejaculation.

RESULTS

In the first study, the time to ejaculation with vibratory stimulation was 2.23 and 3.89 minutes (P = 0.01) and the time to minimal tip rigidity after ejaculation was 1.93 and 3.1 minutes (P <0,001) in the placebo and SC groups, respectively. In the second study, the time to ejaculation with vibratory stimulation was 2.23 and 4.91 minutes (P = 0.006), the time to best tip rigidity after ejaculation was 19.10 and 15.66 minutes (P = 0.242), and the area under the curve of tip rigidity in 3 minutes after ejaculation was 73.61 and 144.05 (P <0.001) in the placebo and SC groups, respectively.

CONCLUSIONS

In this laboratory setting, SC seemed to prolong the ejaculation latency time. The detumescence time was also longer, with better quality. However, we did not show that SC shortens the refractory period after ejaculation.

Erection Capability Is Potentiated by Long-Term Sildenafil Treatment: Role of Blood Flow-Induced Endothelial Nitric-Oxide Synthase Phosphorylation

Despite demonstrated clinical efficacy of sildenafil for the temporary treatment of erectile dysfunction, the possibility that sildenafil used long-term durably augments erectile ability remains unclear. We investigated whether continuous long-term administration of sildenafil at clinically relevant levels to aged rats "primes" the penis for improved erectile ability and involves nitric oxide (NO) or RhoA/Rho-kinase signaling pathways. In aged, but not young rats, sildenafil prolonged erection and increased the protein expressions of phosphorylated endothelial NO synthase (eNOS) at serine-1177 and phosphorylated Akt at serine-473 in penes. Only in the young rat penis, protein expressions of phosphodiesterase-5 and phosphomyosin phosphatase target subunit 1, a marker of Rho-kinase activity, were increased by sildenafil. Sildenafil inhibited phosphodiesterase-5 activity in penes of young and aged rats coincident with assayed free plasma levels of the drug equivalent to clinically therapeutic measurements. We conclude that erectile ability can be enhanced under preconditions of erectile impairment by long-term inhibition of phosphodiesterase-5 and that the effect is mediated by Akt-dependent eNOS phosphorylation. The lack of erectile ability enhancement in young rats by long-term phosphodiesterase-5 inhibition may relate to restrained NO signaling by phosphodiesterase-5 up-regulation, lack of incremental Akt and eNOS phosphorylation, and heightened Rho-kinase signaling in the penis.

Penile erectile responses to electric stimulation are enhanced by a new phosphodiesterase type-5 inhibitor

AIM

This study was conducted to investigate the effect of DA-8159, a new phosphodiesterase type-5 (PDE5) inhibitor, on electrostimulation-induced penile erection in rats.

METHODS

Intracavernous pressure (ICP) and arterial blood pressure (BP) were simultaneously recorded through electric pelvic-ganglion stimulation (2-10 Hz) after the oral administration of DA-8159 (3 or 10 mg/kg) in normal and streptozotocin-induced diabetic rats. Statistical analysis was performed on the maximal intracavernous pressure (ICP), detumescence time, maximal intracavernous pressure/blood pressure (ICP/BP) ratio, and the area under the curve (AUC) of the ICP/BP ratio.

RESULTS

In normal and diabetic rats, electrical stimulation of the pelvic ganglion induced a frequency- and dose-dependent increase in the intracavernous pressure. The ICP/BP ratio and the corresponding AUC values were also significantly and dose-dependently increased after DA-8159 administration. In addition, the detumescence time significantly increased after DA-8159 administration compared to that of the controls.

CONCLUSIONS

These results show that the DA-8159 significantly increased the intracavernous pressure response and prolonged the decay period induced by electrical stimulation of the pelvic ganglion, and suggest that DA-8159 might be a potential therapeutic agent for the treatment of erectile dysfunction.