Effects of denervation and local 6-hydroxydopamine injection on testicular growth in rats

Neurotransmitters, neuropeptides and calcium in oocyte maturation and early development

A primary reason behind the high level of complexity we embody as multicellular organisms is a highly complex intracellular and intercellular communication system. As a result, the activities of multiple cell types and tissues can be modulated resulting in a specific physiological function. One of the key players in this communication process is extracellular signaling molecules that can act in autocrine, paracrine, and endocrine fashion to regulate distinct physiological responses. Neurotransmitters and neuropeptides are signaling molecules that renders long-range communication possible. In normal conditions, neurotransmitters are involved in normal responses such as development and normal physiological aspects; however, the dysregulation of neurotransmitters mediated signaling has been associated with several pathologies such as neurodegenerative, neurological, psychiatric disorders, and other pathologies. One of the interesting topics that is not yet fully explored is the connection between neuronal signaling and physiological changes during oocyte maturation and fertilization. Knowing the importance of Ca²⁺ signaling in these reproductive processes, our objective in this review is to highlight the link between the neuronal signals and the intracellular changes in calcium during oocyte maturation and embryogenesis. Calcium (Ca²⁺) is a ubiquitous intracellular mediator involved in various cellular functions such as releasing neurotransmitters from neurons, contraction of muscle cells, fertilization, and cell differentiation and morphogenesis. The multiple roles played by this ion in mediating signals can be primarily explained by its spatiotemporal dynamics that are kept tightly checked by mechanisms that control its entry through plasma membrane and its storage on intracellular stores. Given the large electrochemical gradient of the ion across the plasma membrane and intracellular stores, signals that can modulate Ca²⁺ entry channels or Ca²⁺ receptors in the stores will cause Ca²⁺ to be elevated in the cytosol and consequently activating downstream Ca²⁺-responsive proteins resulting in specific cellular responses. This review aims to provide an overview of the reported neurotransmitters and neuropeptides that participate in early stages of development and their association with Ca²⁺ signaling.

Building the mammalian testis: origins, differentiation, and assembly of the component cell populations

Development of testes in the mammalian embryo requires the formation and assembly of several cell types that allow these organs to achieve their roles in male reproduction and endocrine regulation. Testis development is unusual in that several cell types such as Sertoli, Leydig, and spermatogonial cells arise from bipotential precursors present in the precursor tissue, the genital ridge. These cell types do not differentiate independently but depend on signals from Sertoli cells that differentiate under the influence of transcription factors SRY and SOX9. While these steps are becoming better understood, the origins and roles of many testicular cell types and structures-including peritubular myoid cells, the tunica albuginea, the arterial and venous blood vasculature, lymphatic vessels, macrophages, and nerve cells-have remained unclear. This review synthesizes current knowledge of how the architecture of the testis unfolds and highlights the questions that remain to be explored, thus providing a roadmap for future studies that may help illuminate the causes of XY disorders of sex development, infertility, and testicular cancers.

Peptidergic not monoaminergic fibers profusely innervate the young adult human testis

Numerous studies have reported that intratesticular nerves exert important regulatory effects on the functions of the male gonad; however, as yet little is known about their distribution in the young adult human testis. The purpose of this study was to explore whether peptidergic and adrenergic nerves occur in the male gonad of this age, and, if present, to depict their distribution further. Thirty testes were collected from 15 reproductively healthy donors aged 21-32 years. Antibodies against protein gene product 9.5 (PGP 9.5), neuropeptide Y (NPY), C-terminal flanking peptide of NPY (CPON) and vasoactive intestinal peptide (VIP) were employed for immunohistochemical detection of intratesticular peptidergic nerves, and those against dopamine-beta-hydroxylase (DBH) and 5-hydroxytryptamine (5-HT) for monoaminergic ones. The testicular parenchyma exhibited a rich innervation by PGP 9.5-positive fibers, mainly associated with Leydig cell nests, blood vessels, and seminiferous tubules. Numerous NPY- and CPON-immunoreactive (IR) nerves also appeared in the gonads, but the vast majority were confined to blood vessels. A small number of VIP-IR fibers were detected in some arterioles. By contrast, however, no fibers displaying DBH or 5-HT immunoreactivity were observed within the testis. Additionally, expression of PGP-9.5, NPY, CPON, VIP, DBH and 5-HT was found in Leydig cells, PGP 9.5 in spermatogonia, and NPY and CPON in peritubular myoid cells. Our results suggest that the young adult human testis is devoid of monoaminergic nerves but profusely innervated by peptidergic fibers, which may serve as major neuronal regulators for testicular functions at this age.

Central nervous system structures labelled from the testis using the transsynaptic viral tracing technique

In the present study, the transneuronal transport of neurotrophic virus technique was used to identify cell groups of the spinal cord and the brain that are transsynaptically connected with the testis. Pseudorabies virus was injected into the testis and after survival times of 3-6 days, the spinal cord and brain were processed immunocytochemically using a polyclonal antibody against the virus. Virus-infected perikarya were detected in the preganglionic neurones of the spinal cord (T10-L1, L5-S1) and in certain cell groups and areas of the brain stem, the hypothalamus and the telencephalon. In the brain stem, the cell groups and areas in which labelled neurones were present included, among others, the nucleus of the solitary tract, the caudal raphe nuclei, the locus coeruleus and the periaqueductal grey of the mesencephalon. In the hypothalamus, virus infected perikarya were observed in the paraventricular nucleus and in certain other cell groups. Telencephalic structures containing labelled neurones included the preoptic area, the bed nucleus of the stria terminalis, the central amygdala and the insular cortex. These data identify a multisynaptic circuit of neurones in the spinal cord and in the brain which may be involved in the control of testicular functions.

Testicular denervation-induced nuclear changes in Leydig cell of hemicastrated adult rats

In the adult male rat, hemicastration (HC) induces a remarkable enhancement of testosterone secretion from the Leydig cells of the remaining testis. We have observed previously that the denervation of inferior spermatic nerves inhibits this enhancement. The present experiments were designed to assess morphometrically whether or not this change had a cytological correlate in Leydig cells. At least five testes from each group (denervation plus HC, sham denervation plus HC and intact rats) were prepared for both optical and electron microscopy studies. The results showed that after 24 h of denervation, the nuclear volume of the Leydig cells in denervation plus HC rats was smaller (p < 0.01) than those of sham denervation plus HC (196.56 +/- 16.53 vs. 280.71 +/- 13.37), whereas both the volume density of heterochromatin (19.84 +/- 3.14 vs. 10.03 +/- 2.47%) and the heterochromatin index (expressed as periphery heterochromatin area divided by nuclear perimeter, 0.149 +/- 0.046 vs. 0.094 +/- 0.026) were significantly (p < 0.01) higher in denervation plus HC rats than in its sham groups. No changes in Leydig cell numbers or cytoplasmic organelles were detected. The results suggest that some nuclear and heterochromatin-associated cellular activity might be inhibited by testicular denervation in hemicastrated rats.

Neuroendocrine asymmetry

The information available at present clearly indicates that asymmetry exists from the level of elementary particles to the human cerebral cortex, the latest stage of evolution. Cerebral lateralization is one of the well-known asymmetries. This paper summarizes the data published in the past decades on the asymmetry of the neuroendocrine system. The information on the sided-differences between the gonads, adrenals, and thyroid lobes and that on the lateralization of hypothalamic, limbic, and other brain structures participating in the control of the endocrine glands as well as relevant clinical observations are reviewed here. The innervation of the peripheral endocrine glands is also briefly summarized because the afferent and efferent fibers of these glands may represent one part of the pathway involved in neuroendocrine asymmetry. The data reviewed clearly indicate that some kind of asymmetry (morphological, biochemical, physiological, pathological) is evident at different levels of the neuroendocrine system (at limbic, hypothalamic, peripheral endocrine glands and their innervation) and there are species, sex, and age differences. Most of the information accumulated deals with the CNS-gonadal system. A majority of the observations suggest that in both male and female rats there is a predominance of the right half of brain structures controlling gonadal function. The asymmetry, however, is not restricted to CNS structures: it also exists at the level of the gonads, including their innervation. It appears that the characteristic pattern of the CNS-gonadal system becomes fixed only after sexual maturation. Very few reports are available suggesting some kind of asymmetry of the CNS-adrenal cortex and the CNS-thyroid system. There are convincing findings consistent with the view that in addition to the hypothalamo-adenohypophyseal system acting via the general circulation on the peripheral endocrine glands, there is also a pure neural link between the CNS and the gonads, the CNS and the adrenal gland, and also between the CNS and the thyroid. This link contains afferent and efferent pathways and is able to modulate the functional activity or the responsiveness of the gland. It may also serve as a neural reflex arc. It is assumed that the neuroendocrine asymmetry expresses itself through (i) hypophysiotrophic neurohormones and hormones of the peripheral endocrine glands, (ii) neural pathways, or (iii) a combination of (i) and (ii). The authors hope that this publication, in addition to providing an overview, will also stimulate research, both basic and clinical, in this exciting area of neuroendocrinology.

Effects of guanethidine-induced sympathectomy on the spermatogenic and steroidogenic testicular functions of prepubertal to mature rats

Selective chemical sympathectomy of the internal genital organs of prepubertal to mature male Wistar rats was performed by chronic treatment with low doses of guanethidine. Sympathetic denervation caused an increase in intratesticular progesterone levels in prepubertal and early pubertal rats in addition to a decrease in androstenedione and testosterone levels in prepubertal animals, thus indicating a decrease in the conversion of progesterone into androgen, probably by blocking the steroidogenic enzymatic pathway at the 17 alpha-hydroxylase/17, 20 desmolase level. A lower degree of testicular maturation, probably related to reduced androgen activity, was observed in prepubertal and early pubertal sympathectomized rats. Concentration of spermatozoa, on the other hand, was increased in the enlarged cauda epididymidis of late pubertal and mature denervated animals. This result is discussed in terms of the impairment of epididymal mechanisms of seminal emission, fluid resorption and spermatozoal disposal.

Morphometric evaluation of the rat testis, epididymis and vas deferens following chemical sympathectomy with guanethidine

Selective chemical sympathectomy of the internal sex organs of adult male rats was undertaken by long term administration of low doses of guanethidine. The spermatogenic activity of the testis was unaffected by treatment. Examination of the vas deferens using morphometric methods revealed a marked increase in luminal area in contrast to a decrease in muscle layer area and in epithelial height. This is morphological evidence of sperm accumulation caused by a disorder in ductal contractile activity. No structural changes were observed in the epididymis. However, the concentration of spermatozoa in the sperm suspension stored in the cauda epididymidis was significantly increased in denervated rats. This result is discussed in terms of a sympathetic control of resorption mechanisms in the epididymis.

The effect of long-term variation in sympathetic activity on testicular morphology in immature rats

In order to investigate the effect of increased and decreased sympathetic activity on testicular development, immature male rats were injected with isoproterenol or 6-hydroxydopamine from birth up to 49 days of age. Isoproterenol treatment resulted in a marked Leydig cell hypertrophy but the development of the spermatogenic epithelium was unaffected compared to that in saline treated controls. 6-hydroxydopamine treatment in a dose inhibiting the development of the parotid gland did not influence testicular development.

The effect of neonatal vasectomy on testicular function

The effect of unilateral or bilateral vasectomy on testicular weight and basal testosterone production in vitro was studied in neonatal intact and hemicastrated rats. Five days after right-side vasectomy ipsilateral to vasectomy testicular weight increased, and basal testosterone production decreased. Ten days postvasectomy the changes were opposite, and affected both testes. In hemicastrated animals hemivasectomy did not interfere with compensatory hypertrophy but induced a significant decrease in basal testosterone production. The data of the present study suggest that in neonatal animals intact ductus deferens bundle(s) is also required for the full control of testicular weight and basal testosterone secretion.

Possible mechanisms of TSH--independent thyroid growth

Considerable evidence has been accumulated which indicates the participation of the autonomic nervous system in the growth of adrenals, ovaries, testes and thyroid lobes. Results have been gathered indicating that the pituitary is not required for the growth of the thyroid, adrenals and ovaries; interest is currently focused on the involvement of the pineal gland in the control of growth not only of the gonads, but also of the thyroid. This paper summarizes the data currently available on the concepts of thyroid hypertrophic and hyperplastic mechanisms, which occur independently of thyrotropin (TSH), and which suggest the existence of a reciprocal relationship between the pineal and the thyroid.