Effects of Intracavernosal Botulinum Toxin A Injection on Erectile Function and Cavernosal Tissue in a Diabetic Rat Model

Project no. 31929 · Status: Kapanmış (Yayın bekleniyor) · 2024

Summary

Erectile dysfunction (ED) is among the most common causes of male sexual dysfunction. Its aetiology is complex and heterogeneous, and impairment in any organic or psychogenic factor may lead to ED. Both non-endocrine (neurogenic, vasculogenic, and iatrogenic) and endocrine pathways contribute to the pathophysiology. Organic causes include obesity, diabetes mellitus (DM), hypertension, cardiovascular disease, neuroendocrine disorders, atherosclerosis, and neurological diseases, whereas psychogenic causes encompass stress, anxiety, depression, relationship problems, performance anxiety, and prior sexual trauma. ED negatively affects quality of life; many patients report depressive and anxiety symptoms related to sexual performance, which in turn influence the partner’s sexual experience and the couple’s overall well-being.DM is one of the most prevalent chronic diseases and is characterised by impaired carbohydrate metabolism. Its incidence and prevalence have risen steadily, and its complications pose a major health burden. Long-term complications of DM include macroangiopathy, microangiopathy, and neuropathy. DM is a significant cause of sexual dysfunction (SD) in both men and women; in men, ED is the predominant manifestation of SD associated with DM.Rat models are frequently employed to induce ED in the context of DM, and numerous examples exist in the literature. In diabetic ED (DMED) pathophysiology, penile samples from affected men show pathological changes in cavernosal arteries, ultrastructural alterations in cavernosal smooth muscle, and impairment of endothelium-dependent relaxation of corporal smooth muscle.Minimally invasive treatments for ED include lifestyle modification, oral pharmacotherapy, intracavernosal injection of vasodilator agents, and vacuum erection devices. Surgical options are reserved for patients with contraindications to, or failure of, these conservative approaches, and in those with penile fibrosis or vascular insufficiency.Botulinum toxin type A (BoNT-A) has multiple therapeutic uses in medicine and can produce a sustained and potent relaxation of smooth muscle. Produced by bacterial strains within the genus Clostridia, botulinum toxin exists as seven serotypes (A, B, C, D, E, F, G); among these, serotype A is the most potent and the most widely used for therapeutic purposes.Intracavernosal BoNT-A injection reduces α-adrenergic activation by inhibiting norepinephrine release from sympathetic nerve terminals, thereby promoting smooth muscle relaxation and erection. BoNT-A also blocks acetylcholine release from parasympathetic cholinergic neurons, which can reduce subendothelial nitric oxide (NO) production; consequently, cavernosal smooth-muscle expansion becomes predominantly dependent on NO released from non-adrenergic, non-cholinergic (NANC) nerves. In addition, BoNT-A has been explored in peripheral nerve injury, peripheral neuropathies, and ischaemic damage. Although the precise mechanisms facilitating recovery from peripheral nerve injury remain uncertain and differ from the well-known paralytic effects on skeletal/smooth muscle, proposed mechanisms include enhanced Schwann-cell activation/proliferation, inhibition of vasoconstrictor neurons, improved blood flow, and increased angiogenesis. The latter is consistent with the central role of angiogenesis in wound healing and with reports that BoNT-A may alleviate vasospasm in conditions such as frostbite, Raynaud’s phenomenon, or ischaemic injury of the digits.Guided by these considerations, the present study investigates the effects of intracavernosal BoNT-A on erectile function and cavernosal tissue in a diabetic rat model of ED. To our knowledge, this work represents the first study to evaluate intracavernosal botulinum toxin therapy in DMED, addressing a highly prevalent clinical problem.We plan to randomise 28 adult male Wistar albino rats into four groups: Control (non-diabetic, healthy), DMED, DMED+saline, and DMED+BoNT-A. Erectile function will be assessed using the apomorphine test (100 μg/kg; subcutaneous injection at the dorsal neck, with the number of erections observed over 30 minutes). In the Control group, penectomy will be performed under general anaesthesia at baseline, followed by euthanasia. In the diabetic cohorts, DM will be induced by two intraperitoneal doses of streptozotocin (STZ; 25 mg/kg) dissolved in citrate buffer (pH 7.0; 100 mM citric acid, 200 mM disodium phosphate) given two weeks apart. Seventy-two hours after the second STZ dose, tail-vein blood samples will be obtained under aseptic conditions; animals with glucose >300 mg/dL will be included in the DM groups, whereas those with glucose ≤300 mg/dL will be excluded and replaced by repeating the same protocol in a new animal. Eight weeks later, erectile capacity will be reassessed via apomorphine testing (erections observed for 30 minutes post-injection). After confirming ED, the DMED+BoNT-A group will receive 10 IU intracavernosal BoNT-A, whereas the saline group will receive an equal volume of intracavernosal isotonic saline; the remaining DMED group will receive no injection. On day 8 after these interventions, apomorphine testing will be repeated, followed by penectomy under general anaesthesia and euthanasia; cavernosal tissues from all penectomy specimens will undergo histomorphological evaluation.Original Contribution: Intracavernosal BoNT-A may enhance smooth-muscle relaxation, increase NO bioavailability (a key neurotransmitter in erectile physiology), contribute to neuronal regeneration, mitigate endothelial injury, and promote angiogenesis, thereby offering potential therapeutic benefit in DMED, where endothelial and neuronal mechanisms are central. Given the suboptimal response rates to oral pharmacotherapy in DMED, the limited acceptance and persistence with intracavernosal vasodilator injections, and the destructive, complication-prone nature of third-line penile prosthesis implantation, a therapeutic gap remains. We aim to demonstrate the potential disease-modifying effects of intracavernosal BoNT-A via these mechanisms, thereby addressing unmet needs in DMED treatment and providing a preclinical rationale for future clinical studies. If our findings support improved responsiveness to oral pharmacotherapy, subsequent studies may test whether reliance on invasive intracavernosal injections can be reduced. As intracavernosal BoNT-A has not previously been investigated in this widely prevalent patient group, our results may help fill a gap in the literature and guide future research.Broader Impact/Added Value: Demonstrating benefit of intracavernosal BoNT-A in a DMED rat model could contribute substantially to the literature on this common condition. In DMED, first-line therapies frequently fail and second-line options are often poorly accepted, leading to increased use of costly penile prosthesis implantation. In this population, the immunosuppressive effects of diabetes and heightened infection susceptibility raise the risk of prosthesis infection, further magnifying costs. Compared with other vasodilator injections, intracavernosal BoNT-A may be administered at longer intervals and may possess not only symptomatic but also disease-modifying potential, potentially reducing the need for prosthesis surgery-thereby decreasing morbidity and overall treatment costs. An additional anticipated benefit is to lay the groundwork for studies assessing whether BoNT-A can enhance response to oral phosphodiesterase-5 inhibitors (PDE5i) in DMED. If efficacy and safety are demonstrated in this model, a subsequent step would be to examine PDE5i responsiveness post-BoNT-A within the same disease context. Successful outcomes could also translate into improvements in psychosocial domains—such as major depression, guilt, low self-esteem, and anxiety-commonly associated with DMED.

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