Postnatal development of the gastrin-releasing peptide system in the lumbosacral spinal cord controlling male reproductive function in rats

Neuropeptidergic control circuits in the spinal cord for male sexual behaviour: Oxytocin-gastrin-releasing peptide systems

The neuropeptidergic mechanisms controlling socio-sexual behaviours consist of complex neuronal circuitry systems in widely distributed areas of the brain and spinal cord. At the organismal level, it is now becoming clear that "hormonal regulations" play an important role, in addition to the activation of neuronal circuits. The gastrin-releasing peptide (GRP) system in the lumbosacral spinal cord is an important component of the neural circuits that control penile reflexes in rats, circuits that are commonly referred to as the "spinal ejaculation generator (SEG)." Oxytocin, long known as a neurohypophyseal hormone, is now known to be involved in the regulation of socio-sexual behaviors in mammals, ranging from social bonding to empathy. However, the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system remains unclear. Oxytocin is known to be synthesised mainly in hypothalamic neurons and released from the posterior pituitary into the circulation. Oxytocin is also released from the dendrites of the neurons into the hypothalamus where they have important roles in social behaviours via non-synaptic volume transmission. Because the most familiar functions of oxytocin are to regulate female reproductive functions including parturition, milk ejection, and maternal behaviour, oxytocin is often thought of as a "feminine" hormone. However, there is evidence that a group of parvocellular oxytocin neurons project to the lower spinal cord and control male sexual function in rats. In this report, we review the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system and effects of these neuropeptides on male sexual behaviour. Furthermore, we discuss the finding of a recently identified, localised "volume transmission" role of oxytocin in the spinal cord. Findings from our studies suggest that the newly discovered "oxytocin-mediated spinal control of male sexual function" may be useful in the treatment of erectile and ejaculatory dysfunction.

Chronic corticosterone exposure evokes itch hypersensitivity and sexual dysfunction in male rats: relationship between the two distinct gastrin-releasing peptide systems in the spinal cord

In today's society, people are subjected to many social stressors, and excessive chronic stress causes functional disruption of the neuroendocrine system and many diseases. Although the exacerbation of atopic dermatitis with symptoms of itching and erectile dysfunction is induced by chronic stress, the details of the mechanisms are unknown. Here, we examined the effects of chronic stress on itch sensation and male sexual function at the behavioral and molecular levels, focusing on two distinct gastrin-releasing peptide (GRP) systems that independently regulate itch transmission, i.e., the somatosensory GRP system, and male sexual function, i.e., the lumbosacral autonomic GRP system, in the spinal cord. In a rat model of chronic stress induced by chronic corticosterone (CORT) administration, we observed increased plasma CORT concentrations, decreased body weight, and increased anxiety-like behavior, similar to that observed in humans. Chronic CORT exposure induced hypersensitivity to itch and increased the Grp mRNA level in the spinal somatosensory system, but there was no change in pain or tactile sensitivity. Antagonists of the somatosensory GRP receptor, an itch-specific mediator, suppressed itch hypersensitivity induced by chronic CORT exposure. In contrast, chronic CORT exposure decreased male sexual behavior, ejaculated semen volume, vesicular gland weight, and plasma testosterone levels. However, there were no effects on the expression of Grp mRNA or protein in the lumbosacral autonomic GRP system, which regulates male sexual function. In summary, chronic stress model rats showed itch hypersensitivity and impaired sexual function in males, and the involvement of the spinal GRP systems was apparent in itch hypersensitivity.

Sexual Experience Induces the Expression of Gastrin-Releasing Peptide and Oxytocin Receptors in the Spinal Ejaculation Generator in Rats

Male sexual function in mammals is controlled by the brain neural circuits and the spinal cord centers located in the lamina X of the lumbar spinal cord (L3–L4). Recently, we reported that hypothalamic oxytocin neurons project to the lumbar spinal cord to activate the neurons located in the dorsal lamina X of the lumbar spinal cord (dXL) via oxytocin receptors, thereby facilitating male sexual activity. Sexual experiences can influence male sexual activity in rats. However, how this experience affects the brain–spinal cord neural circuits underlying male sexual activity remains unknown. Focusing on dXL neurons that are innervated by hypothalamic oxytocinergic neurons controlling male sexual function, we examined whether sexual experience affects such neural circuits. We found that >50% of dXL neurons were activated in the first ejaculation group and ~30% in the control and intromission groups in sexually naïve males. In contrast, in sexually experienced males, ~50% of dXL neurons were activated in both the intromission and ejaculation groups, compared to ~30% in the control group. Furthermore, sexual experience induced expressions of gastrin-releasing peptide and oxytocin receptors in the lumbar spinal cord. This is the first demonstration of the effects of sexual experience on molecular expressions in the neural circuits controlling male sexual activity in the spinal cord.

Oxytocin Influences Male Sexual Activity via Non-synaptic Axonal Release in the Spinal Cord

Oxytocinergic neurons in the paraventricular nucleus of the hypothalamus that project to extrahypothalamic brain areas and the lumbar spinal cord play an important role in the control of erectile function and male sexual behavior in mammals. The gastrin-releasing peptide (GRP) system in the lumbosacral spinal cord is an important component of the neural circuits that control penile reflexes in rats, circuits that are commonly referred to as the "spinal ejaculation generator (SEG)." We have examined the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system in rats. Here, we show that SEG/GRP neurons express oxytocin receptors and are activated by oxytocin during male sexual behavior. Intrathecal injection of oxytocin receptor antagonist not only attenuates ejaculation but also affects pre-ejaculatory behavior during normal sexual activity. Electron microscopy of potassium-stimulated acute slices of the lumbar cord showed that oxytocin-neurophysin-immunoreactivity was detected in large numbers of neurosecretory dense-cored vesicles, many of which are located close to the plasmalemma of axonal varicosities in which no electron-lucent microvesicles or synaptic membrane thickenings were visible. These results suggested that, in rats, release of oxytocin in the lumbar spinal cord is not limited to conventional synapses but occurs by exocytosis of the dense-cored vesicles from axonal varicosities and acts by diffusion-a localized volume transmission-to reach oxytocin receptors on GRP neurons and facilitate male sexual function.

Lumbar spinal cord neurons putatively involved in ejaculation are sexually dimorphic in early postnatal mice

A crucial role in ejaculation is thought to be played by a population of lumbar spino-thalamic neurons (LSt), which express galanin and other neuropeptides. In rats, these neurons are activated with ejaculation and their lesion selectively abolishes ejaculation but not other mating behaviors. Consistently with their role, in adult rats and humans, LSt neurons are sexually dimorphic, being more numerous in males. Here we examined whether sexual dimorphism arises early in development, using a transgenic mouse line in which the expression of fluorescent protein is driven by the galanin promoter. We focused on postnatal day 4, shortly after a transient perinatal androgen surge in males that could play an organizational role in LSt development. We found a population of brightly fluorescent neurons organized in bilateral columns dorsolateral to the central canal in segments L1-L5, the expected location of the LSt group. Their number was close to that of adult preparations and significantly greater in male than in female siblings (+19%; CI_95% : +13% to +27%; p < .01). This was not due to a generalized higher galanin expression in the male since fluorescent L4 DRG neurons, innervating the hindlimbs and lower back, were not significantly dimorphic (-4%; CI_95% : -10% to +8%; p = .92). Unexpectedly, we found in cervical segments a population of fluorescent neurons having a location relative to the central canal similar to the LSt. Thus, the LSt group is sexually dimorphic soon after birth. However, it is possible that only a subset of its neurons participate in the control of ejaculation.

Effects of Sex Steroids on the Spinal Gastrin-Releasing Peptide System Controlling Male Sexual Function in Rats

The gastrin-releasing peptide (GRP) system in the lumbosacral spinal cord controls male sexual function in rats. In contrast, in female rats, GRP neurons could scarcely be detected around puberty when circulating ovarian steroid hormones such as estradiol and progesterone levels are increasing. However, little information is available on feminizing or demasculinizing effects of ovarian steroids on the central nervous system in female puberty and adulthood. In this study, to visualize the spinal GRP neurons in vivo, we generated a GRP-promoter-Venus transgenic (Tg) rat line and studied the effects of the sex steroid hormones on GRP expression in the rat lumbar cord by examining the Venus fluorescence. In these Tg rats, the sexually dimorphic spinal GRP neurons controlling male sexual function were clearly labeled with Venus fluorescence. As expected, Venus fluorescence in the male lumbar cord was markedly decreased after castration and restored by chronic androgen replacement. Furthermore, androgen-induced Venus expression in the spinal cord of adult Tg males was significantly attenuated by chronic treatment with progesterone but not with estradiol. A luciferase assay using a human GRP-promoter construct showed that androgens enhance the spinal GRP system, and more strikingly, that progesterone acts to inhibit the GRP system via an androgen receptor-mediated mechanism. These results demonstrate that circulating androgens may play an important role in the spinal GRP system controlling male sexual function not only in rats but also in humans and that progesterone could be an important feminizing factor in the spinal GRP system in females during pubertal development.

A sexually dimorphic peptidergic system in the lower spinal cord controlling penile function in non-human primates

STUDY DESIGN

Experimental animal study.

OBJECTIVES

Although a population of gastrin-releasing peptide (GRP) neurons in the lumbar spinal cord has an important role in erection and ejaculation in rats, little information exists on this GRP system in primates. To identify the male-specific GRP system in the primate spinal cord, we studied the lumbosacral cord in macaque monkeys as a non-human primate model.

SETTING

University laboratory in Japan.

METHODS

To determine the gene sequence of GRP precursors, the rhesus macaque monkey genomic sequence data were searched, followed by phylogenetic analysis. Subsequently, immunocytochemical analysis for GRP was performed in the monkey spinal cord.

RESULTS

We have used bioinformatics to identify the ortholog gene for GRP precursor in macaque monkeys. Phylogenetic analysis suggested that primate prepro-GRP is separated from that of other mammalian species and clustered to an independent branch as primates. Immunocytochemistry for GRP further demonstrated that male-dominant sexual dimorphism was found in the spinal GRP system in monkeys as in rodents.

CONCLUSION

We have demonstrated in macaque monkeys that the GRP system in the lower spinal cord shows male-specific dimorphism and may have an important role in penile functions not only in rodents but also in primates.

SPONSORSHIP

Tissues of Nihonzaru (Japanese macaque monkeys) were provided in part by National Institutes of Natural Sciences (NINS) through the National Bio-Resource Project (NBRP) of the MEXT, Japan. This work was supported in part by KAKENHI from the Japan Society for the Promotion of Science (JSPS) (to KT; 15KK0343, 15J40220 and HS; 15K15202, 15KK0257, 15H05724).