A Role for Peripheral 5-HT2 Receptors in Serotonin-Induced Facilitation of the Expulsion Phase of Ejaculation in Male Rats

Recent advances in understanding adverse effects associated with drugs targeting the serotonin receptor, 5-HT GPCR

It has been acknowledged that more women suffer from adverse effects of drugs than men globally. A group of drugs targeting serotonin [5-hydroxytryptamine] (5-HT) binding G-protein-coupled receptors (GPCRs) have been reported to preferentially affect women more than men, causing adverse effects such as breast cancer and infertility. 5-HT GPCR-targeted drugs in the central nervous system (CNS) manage psychiatric conditions, such as depression or bipolar and in the peripheral nervous system (PNS) treat migraines. Physiological characteristics such as specific types of hormones, higher body fat density and smaller body mass in women result in disparities in pharmacodynamics of drugs, thus explaining sex-related differences in the observed adverse effects. In this review, we discuss the side effects of drugs targeting 5-HT GPCRs based on serotonin's roles in the CNS and PNS. We have systematically reviewed adverse effects of drugs targeting 5-HT GPCR using information from the Food and Drug Administration and European Medicines Agency. Further information on drug side effects and receptor targets was acquired from the SIDER and DrugBank databases, respectively. These drugs bind to 5-HT GPCRs in the CNS, namely the brain, and PNS such as breasts, ovaries and testes, potentially causing side effects within these areas. Oestrogen affects both the biosynthesis of 5-HT and the densities of 5-HT GPCRs in given tissues and cells. 5-HT GPCR-targeting drugs perturb this process. This is likely a reason why women are experiencing more adverse effects than men due to their periodic increase and the relatively high concentrations of oestrogen in women and, thus a greater incidence of the oestrogen-mediated 5-HT system interference. In addition, women have a lower concentration of serotonin relative to men and also have a relatively faster rate of serotonin metabolism which might be contributing to the former. We discuss potential approaches that could mitigate at least some of the adverse effects experienced by women taking the 5-HT GPCR-targeting drugs.

Serotonergic paraneurones in the female mouse urethral epithelium and their potential role in peripheral sensory information processing

AIM

The mechanisms underlying detection and transmission of sensory signals arising from visceral organs, such as the urethra, are poorly understood. Recently, specialized ACh-expressing cells embedded in the urethral epithelium have been proposed as chemosensory sentinels for detection of bacterial infection. Here, we examined the morphology and potential role in sensory signalling of a different class of specialized cells that express serotonin (5-HT), termed paraneurones.

METHODS

Urethrae, dorsal root ganglia neurones and spinal cords were isolated from adult female mice and used for immunohistochemistry and calcium imaging. Visceromotor reflexes (VMRs) were recorded in vivo.

RESULTS

We identified two morphologically distinct groups of 5-HT+ cells with distinct regional locations: bipolar-like cells predominant in the mid-urethra and multipolar-like cells predominant in the proximal and distal urethra. Sensory nerve fibres positive for calcitonin gene-related peptide, substance P, and TRPV1 were found in close proximity to 5-HT+ paraneurones. In vitro 5-HT (1 μm) stimulation of urethral primary afferent neurones, mimicking 5-HT release from paraneurones, elicited changes in the intracellular calcium concentration ([Ca2+ ]i ) mediated by 5-HT2 and 5-HT3 receptors. Approximately 50% of 5-HT responding cells also responded to capsaicin with changes in the [Ca2+ ]i . In vivo intra-urethral 5-HT application increased VMRs induced by urethral distention and activated pERK in lumbosacral spinal cord neurones.

CONCLUSION

These morphological and functional findings provide insights into a putative paraneurone-neural network within the urethra that utilizes 5-HT signalling, presumably from paraneurones, to modulate primary sensory pathways carrying nociceptive and non-nociceptive (mechano-sensitive) information to the central nervous system.

Topographic distribution of serotonin-immunoreactive urethral endocrine cells and their relationship with calcitonin gene-related peptide-immunoreactive nerves in male rats

We investigated the topographic distribution and morphology of serotonin (5-HT)-immunoreactive endocrine cells in the urethra of male rats, and focused on their relationship with peptidergic nerve fibers immunoreactive for calcitonin gene-related peptide (CGRP). Urethral endocrine cells immunoreactive for 5-HT were densely distributed in the epithelial layers of the prostatic part, but were sparsely distributed in the membranous and spongy parts of urethra. Distribution of urethral endocrine cells with 5-HT immunoreactivity in the prostatic part was restricted from the internal urethral orifice to the region of seminal colliculus. 5-HT-immunoreactive endocrine cells were also observed in the ductal epithelial layers of coagulating glands, prostatic glands, and seminal vesicles. 5-HT-immunoreactive endocrine cells were triangular or flask in shape and possessed an apical projection extending toward the urethral lumen, and basal or lateral protrusions intruding between other epithelial cells were also detected in some cells. Double immunolabeling for 5-HT and CGRP revealed that CGRP-immunoreactive nerve fibers attached to urethral endocrine cells with 5-HT immunoreactivity in the prostatic part. These results suggest that urethral endocrine cells may release 5-HT in response to luminal stimuli, and that these cells and CGRP-immunoreactive nerves may regulate each other by an axon reflex mechanism.

Physiology and Pharmacology of Ejaculation

Ejaculation is the final stage of coitus in mammalian male and is mandatory for natural procreation. Two synchronized phases, emission and expulsion, form the ejaculatory response and involve specific organs and anatomical structures. The peripheral events leading to ejaculation are commanded by autonomic (sympathetic and parasympathetic) and somatic divisions of the nervous system. The autonomic and somatic motor efferents originate in spinal nuclei located in thoracolumbar and lumbosacral segments. Co-ordinated activation of autonomic and somatic spinal nuclei is orchestrated by a group of lumbar spinal interneurons defined as the spinal generator of ejaculation. The generator of ejaculation together with the autonomic and somatic spinal nuclei constitutes a spinal network that is under the strong influence of stimulating or inhibiting genital sensory and supraspinal inputs. A brain circuitry dedicated to ejaculation has been delineated that is part of a more global network controlling other aspects of the sexual response. This circuitry includes discrete neuronal populations distributed in all divisions of the brain. The corollary to the expanded CNS network is the variety of neurotransmitter systems participating in the ejaculatory process. Among them, serotonin neurotransmission plays a key role and its targeting led to the development of the first registered pharmacological treatment of premature ejaculation in human beings. Critical gaps remain in the understanding of neurophysiopharmacology of ejaculation and management of ejaculatory disorders in human beings needs improvement. Because the ejaculatory response in laboratory animals and in human beings shares many similarities, the use of animal models will certainly provide further advances in the field.

Ejaculatory physiology and pathophysiology: assessment and treatment in male infertility

Azoospermia is a heterogeneous condition with multiple etiologies and a variety of treatments. In this chapter we present a summary of retrograde ejaculation and anejaculation, both of which are characterized by an absence of antegrade semen propulsion through the male reproductive tract. Each of these affects fertility, but is pathophysiologically distinct disorders with differing evaluation and treatment. Retrograde ejaculation has a myriad of well-characterized causes, from pharmacologic disruption to interference of neural mechanisms by surgical intervention for a variety of diseases. Medication is the mainstay of treatment, although only a minority responds and develops antegrade ejaculation. For the men who are not responders to medical therapy, but still have fertility goals, there are a variety of sperm retrieval techniques to assist their reproductive abilities. Failure of emission is characterized by an absence of the emission phase and no antegrade or retrograde expulsion of ejaculatory products. If fertility is desired, these men must rely on assisted ejaculatory procedures, and treatment choice is guided by etiology and response. Ultimately, retrograde ejaculation and failure of emission are in a spectrum of ejaculatory disorders which impair male fertility.