Male sexual behavior is associated with LHRH neuron number in middle-aged rats

Regional and subtype-specific loss of GnRH neurons is associated with diminished mating behavior in middle-aged male rats

The current study was to examine the relationship between the number of gonadotropin-releasing hormone (GnRH) neurons and male sexual behavior in middle-aged rats. Based on their sexual performance, middle-aged male rats (18-19 months) were assigned to three groups: (i) Group MIE (showing mounts, intromissions, and ejaculation), (ii) Group MI (displaying mounts and intromissions, but no ejaculation), and (iii) Group NC (showing no copulatory behavior). The brains of these middle-aged animals and of sexually active, young controls were collected and then examined for immunohistochemical localization of GnRH neurons. The numbers of two subtypes of GnRH neurons, smooth (s-GnRH) and irregular (i-GnRH), in the medial septum (MS), organum vasculosum of the lamina terminalis (OVLT), preoptic area (POA), and anterior hypothalamus (AH), were determined under a light microscope. As compared to young controls, an age-related decrease in the number of s-GnRH neurons was found in the MS of MIE rats. Among three groups of middle-aged rats, Groups MIE and MI had more s-GnRH neurons in the POA and i-GnRH neurons in the OVLT and POA than Group NC. In addition, loss of s-GnRH and i-GnRH neurons in the MS was observed in Groups MI and NC and Group NC, respectively. Our results suggest that a decrease in GnRH neuron subtypes occurring in different brain regions might be critical for the loss of specific components of male rat sexual behavior during aging.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

Somatostatin inhibition of GnRH neuronal activity and the morphological relationship between GnRH and somatostatin neurons in rats

In rodents, GnRH neurons are diffusely distributed from the medial septum through to the medial preoptic area and control gonadal functions through the pituitary. The activity of GnRH neurons is regulated by a variety of bioactive substances, including the inhibitory peptide somatostatin. In the present study, we focused on somatostatin because intracerebroventricular injection of somatostatin inhibits the LH surge in rats and reduces LH secretion in ewes. Somatostatin also decreases GnRH release from rat hypothalamic slices. In mice, somatostatin is also thought to suppress GnRH neuronal activity through contact on the soma of GnRH neurons. However, similar data are missing in rats. Moreover, rat GnRH neurons receive only a few synaptic inputs. In this study, we assessed the morphological relationship between GnRH and somatostatin neurons. Confocal microscopy on the sections from the medial septum through medial preoptic area revealed about 35 close contacts per rat between the GnRH and somatostatin neuronal fibers in the organum vasculosum of the lamina terminalis region. No contact of somatostatin fibers on the GnRH neuronal somata was observed. Multicell RT-PCR for somatostatin receptor mRNA in rat GnRH neurons was also performed, which revealed moderate expression of somatostatin receptor subtypes 1-5. In addition, patch clamp experiments were carried out in acute slice preparations. Somatostatin suppressed neuronal firing in cells recorded in a cell-attached configuration and also induced whole-cell outward currents in GnRH neurons. These findings suggest that somatostatin directly inhibits the activity of rat GnRH neurons through volume transmission in the organum vasculosum of the lamina terminalis region.

Age-dependent expression of AVT and its oxytocic-like receptor VT3 in the shell gland of Japanese quail, Coturnix coturnix japonica

The avian neurohypophyseal hormone AVT is an important regulatory hormone involved in many physiological processes including oviposition; an age-related phenomenon, through its action on the shell gland. In this study, immunohistochemistry and in situ hybridization was performed to study the expression of immunoreactive (ir) AVT and its oxytocic-like receptor VT3 transcript in the ovary/shell gland of Japanese quail representing sexually immature, mature and old condition. Our results indicate that ir-AVT is present in the ovary of sexually active adult only, but in the shell gland it is observed in both sexually active adult and sexually quiescent old quail. Further, VT3 gene transcript although not detected in the shell gland of sexually immature birds, has been found abundantly in the myometrium of shell gland of sexually active adult quail with a slight decrease in old birds. It is concluded that in addition to the ovarian function and shell gland activity, the expression of AVT and VT3 receptor in the shell gland also varies with the age dependent reproductive/egg laying performance of the Japanese quail. Our findings also suggest that (i) local synthesis of AVT and the expression of its oxytocic-like VT3 receptors is estrogen dependent and (ii) shell gland AVT upregulates its VT3 receptor (paracrine role) in advance to facilitate the role of neurohypophyseal AVT during oviposition.

Monoamine levels in the nucleus accumbens correlate with male sexual behavior in middle-aged rats

The correlation between monoamine levels in the nucleus accumbens (NAcc) and male sexual behavior was studied in middle-aged rats. Male rats (18-19months) were assigned to three groups: (1) Group MIE consisted of rats showing mounts, intromissions, and ejaculations; (2) Group MI was composed of rats showing mounts and intromissions, but no ejaculation; and (3) Group NC were non-copulators showing no sexual behavior. Young adult rats (4-5months), displaying complete copulatory behavior, were used as the control group. Levels of dopamine (DA), serotonin, and norepinephrine and their metabolites in the NAcc were measured by high-pressure liquid chromatography with electrochemical detection. No difference was seen in DA levels between MIE rats and young controls, whereas DA levels in NC rats were significantly lower than those in both MIE and MI rats. Serotonin levels in NC rats were significantly higher than those in MIE and MI rats. Conversely, norepinephrine levels in NC rats were lower than those in MIE rats. These results suggest that monoamine levels in the NAcc correlate with sexual performance in male rats and that changes in NAcc monoamine levels might affect male sexual behavior in middle-aged rats.

The Japanese quail: a model for studying reproductive aging of hypothalamic systems

During aging, the decline of neuroendocrine, endocrine, and behavioral components of reproduction ultimately leads to reproductive failure. These studies considered both neuroendocrine and behavioral aspects of reproductive aging in Japanese quail, using chronological age and reproductive status to separate animals into experimental groups. In Study I, age-related changes in the gonadotropin releasing hormone (GnRH-I) system were investigated and a sharp decrease was observed in GnRH-I concentration in the median eminence of aging animals of both sexes, whereas preoptic-lateral septal region GnRH-I concentrations declined only in aging males. Immunohistochemistry confirmed these findings since aging females retained, whereas males lost GnRH-I cells. Functional changes were assessed by in vitro incubation of parasaggittal hypothalamic slices collected from young and old inactive males and females. Results showed reduced baseline GnRH-I release and diminished response to norepinephrine (NE). Deteriorating fertility also correlated with decreased male sexual behavior and loss of aromatase immunoreactive (AROM-ir) neurons in the medial, but not lateral preoptic nucleus (POA). Sexual behavior and AROM-ir were restored with exogenous testosterone, which was associated with increased cell size in the medial POA. Comparison of cell size and number of AROM-ir cells showed that aged sexually active males had fewer, larger AROM-ir cells when compared to young males, suggesting neuroplasticity of specific neural systems and a critical role of estradiol in maintaining reproductive function.

Age-related effects on genital reflexes induced by paradoxical sleep deprivation and cocaine in rats

Although sexual function often decreases with age, a recent study demonstrated that paradoxical sleep deprivation (PSD) was effective in inducing penile erection (PE) in 60% of 22-month-old PSD rats after acute cocaine injection, whereas this behavior was absent in non-PSD control rats. The present study sought to compare alterations in genital reflexes (PE and ejaculation-EJ) induced by PSD followed by acute cocaine or saline at different points in the life span of male rats. Rats of different ages (3, 6, 10, 12, 18, 22 and 28 months), were given a single injection of either saline or cocaine (7 mg/kg, i.p.) after a 4-day period of PSD, and then evaluated for genital reflexes. Results indicated that genital reflexes in PSD rats given saline become less frequent with age and disappear after the age of 18 months. However, cocaine potentiated and prolonged these behaviors until the age of 22 months. Although a number of factors are involved in such a complex phenomenon as PE, we suggest that the previously documented dopamine receptor supersensitivity induced by PSD may be an important contributor to the potentiation by cocaine of genital reflexes after sleep deprivation.

Penile neuronal nitric oxide synthase and its regulatory proteins are present in hypothalamic and spinal cord regions involved in the control of penile erection

Control of penile erection requires the coordination of the hypothalamus and the L6-S1 region of the spinal cord. Erection requires the activation of neuronal nitric oxide synthase (nNOS), which is tightly regulated. Because variants of nNOS (penile nNOS: PnNOS) and the N-methyl-D-aspartate receptor (truncated NMDAR subunit 1: NMDAR1-T) as well as protein inhibitor of NOS (PIN) have all been located in the pelvic ganglia and penile nerves, this work aims to determine whether these proteins are also present in the hypothalamus. It was found that PnNOS, the brain-type nNOS, and PIN, were expressed in the hypothalamus. In contrast, NMDAR1-T was expressed only in the penis, whereas the brain-type NMDAR1 was present in the brain and sacral spinal cord and not in the penis. PnNOS was found in the media preoptic area, posterior magnocellular, and the parvocellular regions of the paraventricular nucleus, supraoptic nucleus, septohypothalamic nucleus, medial septum, cortex, and in some of the nNOS staining neurons throughout the brain. It was absent in the organum vasculosum of the lamina terminalis. PIN staining was present in neurons of the medial preoptic area, paraventricular nucleus, medial septum, and cortex, but not in the supraoptic nucleus, septohypothalamic nucleus, or organum vasculosum of the lamina terminalis. Colocalization between PnNOS and PIN was found in the medial preoptic area, medial septum, and cortex, and less in the paraventricular nucleus. PnNOS and oxytocin were colocalized in the paraventricular nucleus and supraoptic nucleus. In hypothalamic extracts, recombinant PIN-GST protein bound to PnNOS in the extracts and partially inhibited NOS activity. These results indicate that both nNOS variants, and their respective regulatory proteins are present and colocalize in the hypothalamic and spinal cord regions involved in penile erection.

Neuropeptides and sexual behaviour

Many neuropeptides are involved in the control of sexual behaviour at the central level. Among these, the most studied are adrenocorticotropin, alpha-melanocyte stimulating hormone, oxytocin and opioid peptides. This attempt to review old and new neuropharmacological, biochemical and psychobiological studies in this field, shows that all these neuropeptides apparently facilitate sexual behaviour, except for opioid peptides, which inhibit sexual performance, in most of the species studied so far (rats, mice, monkeys and humans). However, gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin, cholecystokinin, substance P and vasoactive intestinal peptide may be also involved in the control of sexual behaviour. Apparently, corticotropin releasing factor, neuropeptide Y and cholecystokinin inhibit, while substance P and vasoactive intestinal peptide facilitate, sexual behaviour. In contrast, gonadotropin-releasing hormone has been reported to exert a facilitative, inhibitory or no effect at all on sexual behaviour. Galanin was also shown either to facilitate or inhibit sexual behaviour. The above-mentioned putative role of the neuropeptides in sexual behaviour derives mainly from studies done in rats. In these studies, neuropeptides, their antisera or drugs that act as agonists or antagonists of neuropeptide receptors, were tested for their effect on sexual behaviour after systemic, intracerebroventricular, or intracerebral administration. The latter were infused into brain areas relevant for sexual behaviour, such as the medial preoptic area, and the ventromedial and paraventricular nuclei of the hypothalamus. The above studies show that little information is available on the mechanisms by which neuropeptides influence sexual behaviour. Also unclear is whether the above neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except for opioid peptides. New information about the role of neuropeptides may come from the application of molecular biology and genetic manipulation techniques to the study of sexual behaviour. Of these, FOS protein determination, antisense oligonucleotides aimed at the neutralisation of neuropeptide and/or neuropeptide receptor mRNAs in specific brain areas, and gene ablation seem the most promising. Although still in the early stages, it is likely that these methodologies will provide new insights into the role of neuropeptides in the control of sexual behaviour.