Nitric oxide is involved in male sexual behavior of rats

Erectile Dysfunction: Treatments, Advances and New Therapeutic Strategies

Erectile dysfunction (ED) is the inability to get and maintain an adequate penile erection for satisfactory sexual intercourse. Due to its negative impacts on men's life quality and increase during aging (40% of men between 40 and 70 years), ED has always attracted researchers of different disciplines, from urology, andrology and neuropharmacology to regenerative medicine, and vascular and prosthesis implant surgery. Locally and/or centrally acting drugs are used to treat ED, e.g., phosphodiesterase 5 inhibitors (first in the list) given orally, and phentolamine, prostaglandin E1 and papaverine injected intracavernously. Preclinical data also show that dopamine D4 receptor agonists, oxytocin and α-MSH analogues may have a role in ED treatment. However, since pro-erectile drugs are given on demand and are not always efficacious, new strategies are being tested for long lasting cures of ED. These include regenerative therapies, e.g., stem cells, plasma-enriched platelets and extracorporeal shock wave treatments to cure damaged erectile tissues. Although fascinating, these therapies are laborious, expensive and not easily reproducible. This leaves old vacuum erection devices and penile prostheses as the only way to get an artificial erection and sexual intercourse with intractable ED, with penile prosthesis used only by accurately selected patients.

Erectile Function and Sexual Behavior: A Review of the Role of Nitric Oxide in the Central Nervous System

Nitric oxide (NO), the neuromodulator/neurotransmitter formed from l-arginine by neuronal, endothelial and inducible NO synthases, is involved in numerous functions across the body, from the control of arterial blood pressure to penile erection, and at central level from energy homeostasis regulation to memory, learning and sexual behavior. The aim of this work is to review earlier studies showing that NO plays a role in erectile function and sexual behavior in the hypothalamus and its paraventricular nucleus and the medial preoptic area, and integrate these findings with those of recent studies on this matter. This revisitation shows that NO influences erectile function and sexual behavior in males and females by acting not only in the paraventricular nucleus and medial preoptic area but also in extrahypothalamic brain areas, often with different mechanisms. Most importantly, since these areas are strictly interconnected with the paraventricular nucleus and medial preoptic area, send to and receive neural projections from the spinal cord, in which sexual communication between brain and genital apparatus takes place, this review reveals that central NO participates in concert with neurotransmitters/neuropeptides to a neural circuit controlling both the consummatory (penile erection, copulation, lordosis) and appetitive components (sexual motivation, arousal, reward) of sexual behavior.

Oxytocin, Erectile Function and Sexual Behavior: Last Discoveries and Possible Advances

A continuously increasing amount of research shows that oxytocin is involved in numerous central functions. Among the functions in which oxytocin is thought to be involved are those that play a role in social and sexual behaviors, and the involvement of central oxytocin in erectile function and sexual behavior was indeed one of the first to be discovered in laboratory animals in the 1980s. The first part of this review summarizes the results of studies done in laboratory animals that support a facilitatory role of oxytocin in male and female sexual behavior and reveal mechanisms through which this ancient neuropeptide participates in concert with other neurotransmitters and neuropeptides in this complex function, which is fundamental for the species reproduction. The second part summarizes the results of studies done mainly with intranasal oxytocin in men and women with the aim to translate the results found in laboratory animals to humans. Unexpectedly, the results of these studies do not appear to confirm the facilitatory role of oxytocin found in male and female sexual behavior in animals, both in men and women. Possible explanations for the failure of oxytocin to improve sexual behavior in men and women and strategies to attempt to overcome this impasse are considered.

Mechanisms, development, and comparative perspectives on experience-dependent plasticity in social behavior

Revealing the mechanisms underlying experience-dependent plasticity is a hallmark of behavioral neuroscience. While the study of social behavior has focused primarily on the neuroendocrine and neural control of social behaviors, the plasticity of these innate behaviors has received relatively less attention. Here, we review studies on mating-dependent changes to social behavior and neural circuitry across mammals, birds, and reptiles. We provide an overview of species similarities and differences in the effects of mating experiences on motivational and performative aspects of sexual behaviors, on sensory processing and preferences, and on the experience-dependent consolidation of sexual behavior. We also discuss recent insights into the neural mechanisms of and developmental influences on mating-dependent changes and outline promising approaches to investigate evolutionary parallels and divergences in experience-dependent plasticity.

Neuronal nitric oxide synthase activity mediates Lycium barbarum polysaccharides-enhanced sexual performance without stimulating noncontact erection in rats

RATIONALE

Lycium barbarum polysaccharide (LBP) is known to promote reproductive functions. However, its role in noncontact erection (NCE) of penis initiated by brain regions including medial preoptic area (MPOA) and paraventricular nucleus (PVN) regions responsible for sexual behavior has not been investigated.

OBJECTIVES

Therefore, this study initially investigated the effects of LBP on male sexual function, and subsequently, the mechanistic insight was investigated through assessing the expression of neuronal nitric oxide synthase (nNOS) in the MPOA and PVN.

METHODS

The adult male rats were treated with 100 mg/kg of LBP or vehicle by oral gavage. Before and after 14 days of treatment, copulatory behavior and noncontact erection (NCE) were recorded. After the last behavioral test, the brain was isolated to measure nNOS expression in the MPOA and PVN.

RESULTS

Data showed that LBP treatment significantly increased both the frequencies of intromission as well as ejaculation, compared to the control group. Whereas, a reduced post-ejaculatory interval was observed compared to same group on day 0. Furthermore, the treatment led to an increased intromission ratio, inter-intromission interval, and the number of MPOA nNOS-immunoreactive cells (nNOS-ir). Additionally, a significantly positive correlation between ejaculation frequency and MPOA nNOS-ir cells was recorded. Of note, LBP treatment had no effects on NCE and PVN nNOS-ir expression.

CONCLUSION

These findings suggest that LBP enhances sexual behavior through increased nNOS expression in the MPOA in male rats.

Aphrodisiac Activity of the Aqueous Crude Extract of Purple Corn ( Zea mays) in Male Rats

In the present study, the aphrodisiac properties of the purple corn (Zea mays) in male rats were analyzed. The aqueous crude extract of purple corn (at 25, 50, and 75 mg/kg) was administered to (a) copulating male rats and (b) anesthetized and spinal cord transected male rats. Behavioral parameters of copulatory behavior and parameters of the genital motor pattern of ejaculation previous to its inhibition, under the influence of the purple corn extract, are described. Administration of the aqueous crude extract of purple corn significantly facilitates the arousal and execution of male rat sexual behavior without significant influences on the ambulatory behavior. In addition, purple corn extract elicit a significant increase in the number of discharges of the ejaculatory motor patterns and in the total number of genital motor patterns evoked in spinal rats. The present findings show that the aqueous crude extract of purple corn possesses aphrodisiac activity.

Sexual experience influences mating-induced activity in nitric oxide synthase-containing neurons in the medial preoptic area

Nitric oxide (NO) acts in the medial preoptic area (mPOA) of the hypothalamus to facilitate the expression of male sexual behavior and has also been widely implicated in mechanisms of experience, learning, and memory. Using immunohistochemistry for Fos, as a marker for neural activity, and nitric oxide synthase (NOS), the enzyme that catalyzes the production of nitric oxide (NO), we examined whether sexual activity and sexual experience influence Fos co-expression in NOS-containing neurons in the mPOA of male rats. Consistent with previous findings, results indicate that mating increased activity in the mPOA, and that sexual experience facilitated the expression of sexual behaviors, together with increased mating-induced Fos and NOS in the mPOA. Results also indicate that mating increased co-expression of Fos in NOS-containing neurons, and that this increase was highest in animals undergoing their first sexual encounter, indicating that initial sexual experience increases NO production in the mPOA of male rats.

Anethum graveolens Linn. (dill) extract enhances the mounting frequency and level of testicular tyrosine protein phosphorylation in rats

OBJECTIVE

To investigate the effect of Anethum graveolens (AG) extracts on the mounting frequency, histology of testis and epididymis, and sperm physiology.

METHODS

Male rats induced by cold immobilization before treating with vehicle or AG extracts [50, 150, and 450 mg/kg body weight (BW)] via gastric tube for consecutive 1, 7, and 14 d were examined for mounting frequency, testicular phosphorylation level by immunoblotting, sperm concentration, sperm acrosome reaction, and histological structures of testis and epididymis, respectively.

RESULTS

AG (50 mg/kg BW) significantly increased the mounting frequency on Days 1 and 7 compared to the control group. Additionally, rat testis treated with 50 mg/kg BW AG showed high levels of phosphorylated proteins as compared with the control group. In histological analyses, AG extract did not affect the sperm concentration, acrosome reaction, and histological structures of testis and epididymis.

CONCLUSIONS

AG extract enhances the aphrodisiac activity and is not harmful to sperm and male reproductive organs.

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.

Differential effects of site-specific knockdown of estrogen receptor α in the medial amygdala, medial pre-optic area, and ventromedial nucleus of the hypothalamus on sexual and aggressive behavior of male mice

Testosterone is known to play an important role in the regulation of male-type sexual and aggressive behavior. As an aromatised metabolite of testosterone, estradiol-induced activation of estrogen receptor α (ERα) may be crucial for the induction of these behaviors in male mice. However, the importance of ERα expressed in different nuclei for this facilitatory action of testosterone has not been determined. To investigate this issue, we generated an adeno-associated virus vector expressing a small hairpin RNA targeting ERα to site-specifically knockdown ERα expression. We stereotaxically injected either a control or ERα targeting vector into the medial amygdala, medial pre-optic area (MPOA), or ventromedial nucleus of the hypothalamus (VMN) in gonadally intact male mice. Two weeks after injection, all mice were tested biweekly for sexual and aggressive behavior, alternating between behavior tests each week. We found that suppressing ERα in the MPOA reduced sexual but not aggressive behavior, whereas in the VMN it reduced both behaviors. Knockdown of ERα in the medial amygdala did not alter either behavior. Additionally, it was found that ERα knockdown in the MPOA caused a parallel reduction in the number of neuronal nitric oxide synthase-expressing cells. Taken together, these results indicate that the testosterone facilitatory action on male sexual behavior requires the expression of ERα in both the MPOA and VMN, whereas the testosterone facilitatory action on aggression requires the expression of ERα in only the VMN.

Effect of vardenafil on nitric oxide synthase expression in the paraventricular nucleus of rats without sexual stimulation

Vardenafil hydrochloride (HCl) is a potent and selective phosphodiesterase type-5 (PDE-5) inhibitor that enhances nitric oxide (NO)-mediated relaxation of human corpus cavernosum and NO-induced rabbit penile erection, and enhances erectile function in patients. In the present study, the effect of vardenafil on nitric oxide synthase (NOS) and neuronal NOS expressions in the paraventricular nucleus (PVN) of rats without sexual stimulation was investigated using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal NOS (nNOS) immunohistochemistry and western blot analysis. The present results showed that NOS and nNOS expression in the PVN was increased by vardenafil treatment as the dose- and duration-dependently without sexual stimulation. The phosphodiesterase type-5 inhibitor, vardenafil, augmented NOS expression in the brain without sexual stimulation. The present study suggests that sexual behaviour can be directly modulated by neurotransmitters such as nitric oxide.

Ginkgo biloba treatment increases copulation but not nNOS activity in the medial preoptic area in male rats

Nitric oxide (NO) is an important messenger in the central nervous system to mediate male copulatory behavior. EGb 761, a standardized extract of Gingko biloba, has been reported to facilitate male copulation in rats. The present study is to determine the effects of neuronal nitric oxide synthase (nNOS) in the medial preoptic area (MPOA) on copulation in male rats following EGb 761 treatment. Adult male rats were treated with 50mg/kg of EGb 761 or distilled water by oral gavage for 14 consecutive days. The animals were sacrificed approximately 14h after the last behavioral test and MPOA brain tissues were collected for nNOS immunohistochemistry. EGb 761 treatment for 14 days significantly increased the intromission frequency compared to the vehicle-treated controls on day 14. An increase in ejaculation frequency was also seen in the EGb 761-treated group compared to the vehicle-treated controls on day 14 and to the same group on day 0. However, EGb 761 treatment did not influence the number of nNOS-immunoreactive cells in the MPOA. These results suggest that enhanced male copulatory performance in sexually experienced rats administered EGb 761 may not be related to central nNOS activity in the MPOA.

The ventral premammillary nucleus links metabolic cues and reproduction

The amount of body fat and the energy balance are important factors that influence the timing of puberty and the normal reproductive function. Leptin is a key hormone that conveys to the central nervous system information about the individual energy reserve and modulates the hypothalamus-pituitary-gonad (HPG) axis. Recent findings suggest that the ventral premammillary nucleus (PMV) mediates the effects of leptin as a permissive factor for the onset of puberty and the coordinated secretion of luteinizing hormone during conditions of negative energy balance. In this review, we will summarize the existing literature about the potential role played by PMV neurons in the regulation of the HPG axis.

Role of nitric oxide in pheromone-mediated intraspecific communication in mice

Nitric oxide is known to take part in the control of sexual and agonistic behaviours. This is usually attributed to its role in neural transmission in the hypothalamus and other structures of the limbic system. However, socio-sexual behaviours in rodents are mainly directed by chemical signals detected by the vomeronasal system, and nitric oxide is abundant in key structures along the vomeronasal pathway. Thus, here we check whether pharmacological treatments interfering with nitrergic transmission could affect socio-sexual behaviour by impairing the processing of chemical signals. Treatment with an inhibitor of nitric oxide synthesis (Nomega-Nitro-l-arginine methyl ester hydrochloride, L-NAME, 100mg/kg) blocks the innate preference displayed by female mice for sexual pheromones contained in male-soiled bedding, with a lower dose of the drug (50mg/kg) having no effect. Animals treated with the high dose of L-NAME show no reduction of olfactory discrimination of male urine in a habituation-dishabituation test, thus suggesting that the effect of the drug on the preference for male pheromones is not due to an inability to detect male urine. Alternatively, it may result from an alteration in processing the reinforcing value of pheromones as sexual signals. These results add a new piece of evidence to our understanding of the neurochemistry of intraspecific chemical communication in rodents, and suggest that the role of nitric oxide in socio-sexual behaviours should be re-evaluated taking into account the involvement of this neuromodulator in the processing of chemical signals.

Estrous cycle influences the expression of neuronal nitric oxide synthase in the hypothalamus and limbic system of female mice

BACKGROUND

Nitric oxide plays an important role in the regulation of male and female sexual behavior in rodents, and the expression of the nitric oxide synthase (NOS) is influenced by testosterone in the male rat, and by estrogens in the female. We have here quantitatively investigated the distribution of nNOS immunoreactive (ir) neurons in the limbic hypothalamic region of intact female mice sacrificed during different phases of estrous cycle.

RESULTS

Changes were observed in the medial preoptic area (MPA) (significantly higher number in estrus) and in the arcuate nucleus (Arc) (significantly higher number in proestrus). In the ventrolateral part of the ventromedial nucleus (VMHvl) and in the bed nucleus of the stria terminalis (BST) no significant changes have been observed. In addition, by comparing males and females, we observed a stable sex dimorphism (males have a higher number of nNOS-ir cells in comparison to almost all the different phases of the estrous cycle) in the VMHvl and in the BST (when considering only the less intensely stained elements). In the MPA and in the Arc sex differences were detected only comparing some phases of the cycle.

CONCLUSION

These data demonstrate that, in mice, the expression of nNOS in some hypothalamic regions involved in the control of reproduction and characterized by a large number of estrogen receptors is under the control of gonadal hormones and may vary according to the rapid variations of hormonal levels that take place during the estrous cycle.

Estrous cycle influences the expression of neuronal nitric oxide synthase in the hypothalamus and limbic system of female mice

BACKGROUND

Nitric oxide plays an important role in the regulation of male and female sexual behavior in rodents, and the expression of the nitric oxide synthase (NOS) is influenced by testosterone in the male rat, and by estrogens in the female. We have here quantitatively investigated the distribution of nNOS immunoreactive (ir) neurons in the limbic hypothalamic region of intact female mice sacrificed during different phases of estrous cycle.

RESULTS

Changes were observed in the medial preoptic area (MPA) (significantly higher number in estrus) and in the arcuate nucleus (Arc) (significantly higher number in proestrus). In the ventrolateral part of the ventromedial nucleus (VMHvl) and in the bed nucleus of the stria terminalis (BST) no significant changes have been observed. In addition, by comparing males and females, we observed a stable sex dimorphism (males have a higher number of nNOS-ir cells in comparison to almost all the different phases of the estrous cycle) in the VMHvl and in the BST (when considering only the less intensely stained elements). In the MPA and in the Arc sex differences were detected only comparing some phases of the cycle.

CONCLUSION

These data demonstrate that, in mice, the expression of nNOS in some hypothalamic regions involved in the control of reproduction and characterized by a large number of estrogen receptors is under the control of gonadal hormones and may vary according to the rapid variations of hormonal levels that take place during the estrous cycle.

Sexual experience modulates neuronal activity in male Japanese quail

After an initial increase, repeated exposure to a particular stimulus or familiarity with an event results in lower immediate early gene expression levels in relevant brain structures. We predicted that similar effects would occur in Japanese quail after repeated sexual experience within brain areas involved in sexual behavior, namely, the medial preoptic nucleus (POM), the bed nucleus of stria terminalis (BST), and the nucleus taeniae of the amygdala (TnA), an avian homolog of medial amygdala. High experience subjects copulated with a female once on each of 16 consecutive days, whereas low experience subjects were allowed to copulate either once or twice. Control subjects were never exposed to a female. High experience subjects were faster to initiate sexual interaction, performed more cloacal contacts, and completed each cloacal contact faster than low experience subjects. Low experience subjects showed an increase in egr-1 (ZENK) expression, an immediate early gene product used as marker of neural activation in birds, in the areas of interest. In contrast, in high experience animals, egr-1 expression in the POM, BST, and the periaqueductal gray (PAG) was not different than the level of expression in unmated controls. These results show that experience modulates the level of immediate early gene expression in the case of sexual behavior. Our results also indicate that immediate early gene expression in specific brain areas is not necessarily related to behavioral output but depends on the behavioral history of the subjects.

Sexual behavior in male rodents

The hormonal factors and neural circuitry that control copulation are similar across rodent species, although there are differences in specific behavior patterns. Both estradiol (E) and dihydrotestosterone (DHT) contribute to the activation of mating, although E is more important for copulation and DHT for genital reflexes. Hormonal activation of the medial preoptic area (MPOA) is most effective, although implants in the medial amygdala (MeA) can also stimulate mounting in castrates. Chemosensory inputs from the main and accessory olfactory systems are the most important stimuli for mating in rodents, especially in hamsters, although genitosensory input also contributes. Dopamine agonists facilitate sexual behavior, and serotonin (5-HT) is generally inhibitory, though certain 5-HT receptor subtypes facilitate erection or ejaculation. Norepinephrine agonists and opiates have dose-dependent effects, with low doses facilitating and high doses inhibiting behavior.

Morphometrical and neurochemical changes in the anteroventral subdivision of the rat medial amygdala during estrous cycle

The anteroventral subdivision of the medial amygdala (MeAV) is one of the vomeronasal structures involved in the control of hormonally dependent behaviors such as sexual and agonistic behaviors in rats. The present study investigates some anatomical and neurochemical parameters of this nucleus (volume, number of neurons, number of glial elements, and of NADPH-diaphorase-positive neurons) in females in two estrous cycle phases (diestrous and estrous) and in males. We also investigate the possible existence of adult neurogenesis in this nucleus in the females. Results showed that volume and estimated number of Nissl-stained neurons in the MeAV vary with the estrous cycle phase: estrous females have greater values than diestrous females. As a consequence of these variations, there is a transient sex difference between males and diestrous females. Two subpopulations of NADPH-diaphorase-positive neurons were detected: intensely stained and medium stained. The intensely stained neurons were more numerous in the estrous than the diestrous females. Neither BrdU nor GFAP inmunostaining revealed significant differences between the two groups, suggesting that adult cell generation, i.e., increases in the number of glial elements, has no significant role in the changes detected in the number of Nissl-stained sections. In conclusion, the MeAV shows functional diergism, due to plastic changes in the female rat brain probably linked to the increase of estradiol during estrous. Finally, these changes are probably functionally related to changes in the behaviors that are controlled through this nucleus.

Involvement of nitric oxide in cocaine-induced erections and ejaculations after paradoxical sleep deprivation

OBJECTIVES

As nitric oxide (NO) is involved in penile erectile (PE) function and also influences the sleep-wake cycle, we speculated that NO could play a role in PE and ejaculation of paradonical sleep deprivation (PSD) rats.

METHODS

Animals were pretreated with N(G)-nitro-L-arginine methyl ester (L-NAME, ip) and L-arginine (ip and icv) prior to saline or cocaine injection.

RESULTS

Cocaine-induced PE in 90% of PSD rats, 60% of which ejaculated. L-NAME reduced the frequency of erection, but had no effect in the proportion of PSD-cocaine-injected rats displaying this response. L-NAME had no effect in saline groups. L-Arginine in PSD-saline rats reduced the proportion of animals displaying PE at the highest dose and reduced the frequency of PE at all doses in both saline and cocaine groups. The icv administration of L-arginine reduced PE only in PSD-cocaine rats. Results indicate that common to both drugs, whether it was NO synthase (NOS) inhibitor or NO precursor, was their capacity to strongly reduce PE frequency in cocaine-treated rats. Moreover, L-arginine (ip) played a relevant inhibitory role in the erection displayed by PSD rats.

CONCLUSIONS

Our findings suggest that the stimulating effects of PSD associated or not with cocaine on erection can be modified by alterations in the NO system.

Testosterone induction of male-typical sexual behavior is associated with increased preoptic NADPH diaphorase and citrulline production in female whiptail lizards

In rodents, male-typical copulatory behavior is generally dependent on gonadal sex steroids such as testosterone, and it is thought that the mechanism by which the hormone gates the behavior involves the gaseous neurotransmitter nitric oxide. According to one model, testosterone induces an up-regulation of nitric oxide synthase (NOS) in the preoptic area, increasing nitric oxide synthesis following exposure to a sexual stimulus. Nitric oxide in turn, possibly through its effect on catecholamine turnover, influences the way the stimulus is processed and enables the appropriate copulatory behavioral response. In whiptail lizards (genus Cnemidophorus), administration of male-typical levels of testosterone to females induces the display of male-like copulatory responses to receptive females, and we hypothesized that this radical change in behavioral phenotype would be accompanied by a large change in the expression of NOS in the preoptic area. As well as comparing NOS expression using NADPH diaphorase histochemistry between testosterone-treated females and controls, we examined citrulline immunoreactivity (a marker of recent nitric oxide production) in the two groups, following a sexual stimulus and following a nonsexual stimulus. Substantially more NADPH diaphorase-stained cells were observed in the testosterone-treated animals. Citrulline immunoreactivity was greater in testosterone-implanted animals than in blank-implanted animals, but only following exposure to a sexual stimulus. This is the first demonstration that not only is NOS up-regulated by testosterone, but NOS thus up-regulated is activated during male-typical copulatory behavior.

Effects of alpha-melanocyte-stimulating hormone on magnocellular oxytocin neurones and their activation at intromission in male rats

The peptides alpha-melanocyte-stimulating hormone (alpha-MSH) and oxytocin have very similar effects on several behaviours, including male sexual behaviour. Both induce penile erection and enhance copulatory behaviour when given centrally, suggesting that their central actions are not independent. Here, we used intromission as a physiological stimulus to investigate whether some central effects of alpha-MSH during male sexual behaviour are mediated by oxytocin neurones. We used the expression of the immediate-early gene product Fos to investigate oxytocin neurone activation at intromission and after intracerebroventricular (i.c.v.) administration of alpha-MSH (1 microg/5 microl) and studied the effects of i.c.v. administration of a MC4 receptor antagonist on Fos expression and on the latency of male rats to exhibit sexual behaviour in the presence of a receptive female. In rats that showed intromission, Fos was expressed in magnocellular oxytocin neurones in both the paraventricular nucleus (PVN) and the supraoptic nucleus (SON), but there was no significant activation of parvocellular oxytocin neurones of the PVN. Similarly, alpha-MSH increased Fos expression in magnocellular oxytocin neurones but had little or no effect in parvocellular oxytocin neurones. In male rats that achieved intromission, central injection of a MC4 receptor antagonist significantly attenuated the increase in Fos expression in magnocellular oxytocin neurones in both the PVN and the SON and increased mount and intromission latencies compared to vehicle-injected controls. Together, the results indicate that magnocellular oxytocin neurones are involved in the central regulation of male sexual behaviour, and that some of the central effects of alpha-MSH are likely to be mediated by magnocellular oxytocin neurones.

Age-related changes of the GABA-B receptor in the lumbar spinal cord of male rats and penile erection

Dorsal horn neurons of lumbosacral spinal cord innervate penile vasculature and regulate penile erection. GABAergic system is involved in the regulation of male sexual behavior. Because aging is frequently accompanied by a progressive decline in erectile function, the aim of this work was to examine age-related changes of the GABA-B receptor in the lumbar spinal cord. Sprague-Dawley rats of 10 and 21 days old, 3, 9 and 20 months old were used. GABA-B receptors were evaluated by quantitative autoradiography using [3H]-Baclofen as ligand with or without GABA (10 microM) to determine the non-specific binding. Ten days after birth a homogeneous neuroanatomical distribution pattern was found in the gray matter, however at 20-day-old adult distribution emerged becoming heterogeneous with the highest binding values at layers II-III and X. In dorsal layers a significant decrease was observed in 9-month-old rats while layer X showed an earlier decrease (21-day-old). GABA-B receptor affinity showed significant age-dependent and regional increase. The GABA-B receptor decrease in aged rats seems not to be related to this receptor inhibitory function in penile erection. Moreover the changes found in GABA-B receptor binding anatomical distribution may indicate its role in the morphological development of the lumbar spinal cord rather than in the decline of the erectile function.

The cannabinoid receptor antagonist SR-141716A induces penile erection in male rats: Involvement of paraventricular glutamic acid and nitric oxide

The cannabinoid CB1 receptor antagonist SR141716A (0.5, 1 and 2 microg) induces penile erection when injected into the paraventricular nucleus of male rats. The pro-erectile effect of SR 141716A occurs concomitantly with an increase in the concentration of NO2- and NO3- in the paraventricular dialysate obtained by means of intracerebral microdialysis. Both penile erection and NO2- increase induced by SR 141716A were reduced by the prior injection into the PVN of the cannabinoid CB1 agonists WIN 55,212-2 (5 microg) or HU 210 (5 microg), given into the paraventricular nucleus at doses unable to induce penile erection or to modify NO2- concentration. SR 141716A responses were also reduced by nitro-L-arginine methylester (20 microg), a non-selective NO synthase inhibitor, S-methyl-L-thiocitrulline (20 microg), a selective neuronal NO synthase inhibitor, the excitatory amino acid NMDA receptor antagonist dizocilpine ((+)MK 801) (1 microg), or the GABAA receptor agonist muscimol (0.2 microg) injected into the PVN 15 min before SR 141716A. In contrast, the inducible NO synthase inhibitor L-N(6)-(1-iminoethyl)lysine (20 microg), the GABAB receptor agonist baclofen (0.2 microg), the mixed dopamine receptor antagonist cis-flupenthixol (10 microg), and the oxytocin receptor antagonist d(CH2)5Tyr(Me)-Orn8 -vasotocin (1 microg), were ineffective. Despite its inability to reduce penile erection and NO2- increase induced by SR 141716A when injected into the PVN, d(CH2)5Tyr(Me)-Orn8 -vasotocin (1 microg) reduced almost completely penile erection without reducing paraventricular NO2- increase when injected into the lateral ventricles 15 min before SR 141716A. The present results show that SR 141716 induces penile erection by a mechanism (possibly activation of excitatory amino acid neurotransmission), which causes the activation of neuronal NO synthase in paraventricular oxytocinergic neurons mediating penile erection.

Effects of gonadal hormones on central nitric oxide producing systems

Nitric oxide-containing neurons are widely distributed within the CNS, including regions involved in the control of reproduction and sexual behavior. The expression of neuronal nitric oxide synthase is influenced by testosterone in male rat, and by estrogens in female. Moreover, nitric oxide synthase may co-localize with gonadal hormones' receptors. Gonadal hormones may influence nitric oxide synthase expression in adulthood as well as during the development. In fact, in mice knockout for estrogen receptor alpha, the nitric oxide synthase-expressing population is deeply reduced in specific regions. In physiological conditions, the female in mammalian species is exposed to short-term changes of gonadal hormones levels (estrous cycle). Our recent studies, performed in the rat vomeronasal system and in mouse hypothalamic and limbic systems reveal that, in rodents, the expression of nitric oxide synthase-producing elements within regions relevant for the control of sexual behavior is under the control of gonadal hormones. The expression of nitric oxide synthase may vary according to the rapid variations of hormonal levels that take place during the estrous cycle. This seems in accordance with the hypothesis that gonadal hormone activation of nitric oxide-cyclic guanosine-monophosphate pathway is important for lordosis behavior, as well as that this system is activated during mating behavior. Finally, comparative data available for other vertebrates suggest that class-specific and species-specific differences occur in the nitric oxide synthase system of hypothalamus and limbic structures. Therefore, particular caution is needed to generalize data obtained from studies in rodents.

The nitric oxide synthase inhibitor l-NAME suppresses androgen-induced male-like pseudocopulatory behavior in whiptail lizards

The synthesis of nitric oxide by the enzyme nitric oxide synthase (NOS) is involved in the androgen-dependent gating of male-typical copulatory behavior, both centrally, particularly in the preoptic area, and peripherally, notably through its role in penile erection. In the all-female whiptail lizard species Cnemidophorus uniparens, individuals display copulatory behaviors indistinguishable from males of similar species if gonadectomized and treated with testosterone. In this experiment, androgenized individuals were treated with a NOS inhibitor, which eliminated male-like behavior in half the individuals, suggesting that the central role of nitric oxide synthesis is conserved in this species. The deficit was principally in mounting, suggesting that sexual motivational systems were affected, rather than consummatory mechanisms.

Central control of penile erection: Role of the paraventricular nucleus of the hypothalamus

The paraventricular nucleus of the hypothalamus is an integration centre between the central and peripheral autonomic nervous systems. It is involved in numerous functions from feeding, metabolic balance, blood pressure and heart rate, to erectile function and sexual behaviour. In particular, a group of oxytocinergic neurons originating in this nucleus and projecting to extra-hypothalamic brain areas (e.g., hippocampus, medulla oblongata and spinal cord) control penile erection in male rats. Activation of these neurons by dopamine and its agonists, excitatory amino acids (N-methyl-D-aspartic acid) or oxytocin itself, or by electrical stimulation leads to penile erection, while their inhibition by gamma-amino-butyric acid (GABA) and its agonists or by opioid peptides and opiate-like drugs inhibits this sexual response. The activation of these neurons is secondary to the activation of nitric oxide synthase, which produces nitric oxide. Nitric oxide in turn causes, by a mechanism that is as yet unidentified, the release of oxytocin in extra-hypothalamic brain areas. Other compounds recently identified that facilitate penile erection by activating central oxytocinergic neurons are peptide analogues of hexarelin, a growth hormone releasing peptide, pro-VGF-derived peptides, endogenous peptides that may be released by neuronal nerve endings impinging on oxytocinergic cell bodies, SR 141716A, a cannabinoid CB1 receptor antagonist, and, less convincingly, adrenocorticotropin-melanocyte-stimulating hormone (ACTH-MSH)-related peptides. Paraventricular oxytocinergic neurons and similar mechanisms are also involved in penile erection occurring in physiological contexts, namely noncontact erections that occur in male rats in the presence of an inaccessible receptive female, and during copulation. These findings show that the paraventricular nucleus of the hypothalamus plays an important role in the control of erectile function and sexual activity. As the male rat is a model of sexual behaviour and penile physiology, which has largely increased in the last years our knowledge of peripheral and central mechanisms controlling erectile function (drugs that induce penile erection in male rats usually do so also in man), the above results may have great significance in terms of a human perspective for the treatment of erectile dysfunction.

Effects of testosterone metabolites on copulation, medial preoptic dopamine, and NOS-immunoreactivity in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. Dopamine (DA) is released in the MPOA of male rats shortly before and during copulation. In a previous study, we identified 17beta-estradiol (E(2)) as the metabolite of testosterone (T) that maintains MPOA basal extracellular DA levels. However, the presence of dihydrotestosterone (DHT), an androgenic metabolite of T, is required for the female-induced increase in MPOA DA observed during copulation. Recently, we reported that assays of MPOA tissue DA content showed that castrates actually had more stored DA than did gonadally intact males. Therefore, the reduction in extracellular levels in castrates was not due to decreased availability of DA; most likely it was due to decreased release. Furthermore, T upregulates neuronal nitric oxide synthase (nNOS) in the MPOA. NO has been implicated in the regulation of DA release in the MPOA. It is not known, however, which metabolite(s) of T regulate(s) tissue stores of DA and/or nNOS in the MPOA of male rats. The present experiments were designed to test the following: (1) whether E(2), DHT, or the combination of the two influences MPOA DA tissue levels, an indication of stored DA, in male rat castrates; and (2) whether E(2), DHT, or the combination of the two influences NOS-ir in the MPOA of castrated male rats. The results indicate that E(2) up-regulates nNOS-ir in the MPOA and maintains tissue content of DA at levels similar to those in T-treated rats. DHT did not influence nNOS-ir, while attenuating the effect of castration on tissue DA content.

Central nitric oxide synthase inhibition disrupts maternal behavior in the rat

Blocking nitric oxide (NO) production, by 3rd ventricle administration of a nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 250 microg/5 microl, postpartum [pp]) decreased milk ejections in Day 10 pp rats. On Day 4 pp, L-NAME treatment eliminated pup retrieval and at both stages of lactation suppressed maternal aggression. Fewer rats treated with L-NAME on Day 10 pp retrieved 4-day-old pups than controls, although all nursed older litters. Following exposure to a mobile intruder, Fos expression was lower in the medial preoptic area and the bed nucleus of the stria terminalis in L-NAME-treated rats than in controls but was lower in the medial amygdala only following exposure to an anaesthetized intruder. Thus, the elevated levels of NO observed in lactation may contribute to the mechanism(s) that mediate maternal behavior and aggression.

Regulation of Oxytocin Secretion

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.

The role of oxytocin and the paraventricular nucleus in the sexual behaviour of male mammals

The paraventricular nucleus of the hypothalamus contains the cell bodies of a group of oxytocinergic neurons projecting to extrahypothalamic brain areas and to the spinal cord, which are involved in the control of erectile function and copulation. In male rats, these neurons can be activated by dopamine, excitatory amino acids, nitric oxide (NO), hexarelin analogue peptides and oxytocin itself to induce penile erection and facilitate copulation, while their inhibition by gamma-aminobutyric acid (GABA) and GABA agonists and by opioid peptides and opiate-like drugs inhibits sexual responses. The activation of paraventricular oxytocinergic neurons by dopamine, oxytocin, excitatory amino acids and hexarelin analogue peptides is apparently mediated by the activation of nitric oxide (NO) synthase. NO in turn activates, by a mechanism that is as yet unidentified, the release of oxytocin from oxytocinergic neurons in extrahypothalamic brain areas. Paraventricular oxytocinergic neurons and mechanisms similar to those reported above are also involved in the expression of penile erection in physiological contexts, namely, when penile erection is induced in the male by the presence of an inaccessible receptive female, which is considered a model for psychogenic impotence in man, as well as during copulation. These findings show that paraventricular oxytocinergic neurons projecting to extrahypothalamic brain areas and to the spinal cord and the paraventricular nucleus play an important role in the control of erectile function and male sexual behaviour in mammals.

A Nitric Oxide Synthesis Inhibitor in the Medial Preoptic Area Inhibits Copulation and Stimulus Sensitization in Male Rats

Dopamine in the medial preoptic area (MPOA) facilitates copulation in male rats, and nitric oxide (NO) regulates basal and female-stimulated MPOA dopamine release. Microinjection of L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor) into the MPOA blocked copulation in naive rats and impaired copulation in sexually experienced males. In other naive rats, L-NAME or saline was microinjected into the MPOA before each of 7 daily exposures to a receptive female placed over their cage. In a drug-free test on Day 8, copulation by L-NAME-treated rats was similar to that of unexposed controls and was impaired relative to saline-treated males. Therefore, NO in the MPOA is important for copulation and stimulus sensitization in male rats.

NADPH-diaphorase activity increases during estrous phase in the bed nucleus of the accessory olfactory tract in the female rat

We investigated the presence of nitric oxide in the bed nucleus of the accessory olfactory tract (BAOT) in males, diestrous females and estrous females using NADPH-diaphorase. Our results demonstrate a significant increase in the density of the medium-stained cells in the estrous female rats suggesting that during estrous a specific subpopulation of nitrinergic cells are activated in the BAOT. This might be related to the physiological and behavioral changes that occurs in estrous.

Preventing L-NAME inhibitory effects on rat sexual behavior with hydralazine, isradipine or captopril co-treatment

The effects of the chronic oral treatment with N(G)-nitro-L-arginine methyl ester (L-NAME), separately or in combination with isradipine, captopril or hydralazine, on standard and temporal patterning sexual behavior of male rats were evaluated. L-Arginine and filtered water were used as control. L-NAME treatment decreased the copulatory rate and hit rate factors of sexual behavior. However, the initiation factor and temporal patterning were less modified by the drug. After 14 days of L-NAME treatment suspension the male rat sexual response was recovered. The three antihypertensive agents were able to reverse partially or totally the inhibitory effects of L-NAME, suggesting that the chronic oral treatment with L-NAME induces penile erection dysfunction by peripheral mechanisms. The present results suggest that chronic oral treatment with nitric oxide (NO) synthase inhibitor can be a relevant and powerful peripheral erectile dysfunction model to evaluate the effects of drugs on erectile function of male rats.

Extra-cellular dopamine increases in the paraventricular nucleus of male rats during sexual activity

Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations were measured in the dialysate obtained with vertical microdialysis probes implanted into the paraventricular nucleus of the hypothalamus of sexually potent male rats. Animals showed noncontact erections when put in the presence of, and copulated with a receptive (ovarietomized oestrogen and progesterone primed) female rat. Dopamine and DOPAC concentrations in the paraventricular dialysate increased 140% and 19%, respectively, above baseline values during exposure to the receptive female and 280% and 31%, respectively, during copulation. No changes in dopamine and DOPAC concentrations were detected in the paraventricular dialysate when sexually potent male rats were exposed to nonreceptive (ovariectomized not oestrogen plus progesterone primed) female rats. These results confirm the involvement of the paraventricular nucleus in control of erectile function and copulatory behaviour and show for the first time that dopamine neurotransmission is increased in this hypothalamic nucleus when erection occurs in physiological contexts.

Roles of estrogen receptor alpha and androgen receptor in the regulation of neuronal nitric oxide synthase

In brain and peripheral tissues, steroid hormones regulate nitric oxide synthase (nNOS). We asked whether estrogen receptor-alpha (ERalpha) and/or androgen receptor (AR) regulated nNOS immunoreactivity in mouse brain. First, we quantified cells singly labeled for nNOS immunoreactivity or labeled dually with ERalpha-immunoreactive (-ir) or AR-ir cells in the nucleus accumbens (Acb), preoptic area (POA), bed nucleus of the stria terminalis (BNST), posterior dorsal and posterior ventral regions of the medial amygdala (MePD and MePV, respectively), and paraventricular nucleus (PVN). The POA and MePD contained the greatest number of double-labeled cells. More nNOS-ir cells were colabeled with ERalpha immunoreactivity compared with AR immunoreactivity. Next, by using a double mutant mouse in which males lacked functional ERalpha, AR, or both, we investigated the roles of these steroid receptors in nNOS-ir cell numbers and immunoreactive area staining under testosterone (T) and estradiol (E2) conditions. Our data show that functional ERalpha is correlated with more nNOS-ir cells under T conditions and more immunoreactive area staining in the POA under both T and E2 conditions. However, ERalpha decreases nNOS-ir cell number in the BNST under E2 treatment. In summary, the data suggest that AR has organizational actions on nNOS-ir cell numbers in the MePV, that interactions between ERalpha and AR genes occur in PVN, and that sex differences in nNOS-ir area staining are limited to the POA. Thus, we show that ERalpha and AR interact to regulate nNOS in male and female brain in a site-specific manner.

Mating-activated nitric oxide-producing neurons in specific brain regions in the female rat

Nitric oxide (NO)-containing neurons have been localized in various parts of the central nervous system including the hypothalamus. NO plays an important role in the regulation of reproductive activities including sexual behavior and pituitary hormone secretion. To test the hypothesis that NO-containing neurons in specific brain areas may respond to the stimulus of mating and participate in integrating the tactile information in the hypothalamus, this study used Fos as a marker of neuronal activity. Proestrous rats receiving intromissions (mated group) from males or mounts-without-intromission (mounted group) were sacrificed along with rats taken directly from their home cage (control group) 90 min after the beginning of mating or mounting. NOergic neurons were labeled by histochemical reaction for nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d). The presence of activated NO-producing (double-stained NADPH-d/Fos) neurons was quantitatively assessed in several brain areas before and after mating. The results showed that mating-with-intromissions induced a significant increase in the percentage of NADPH-d/Fos colabeled neurons in the medial preoptic area (mPOA) and the magnocellular component of the paraventricular nucleus (PVNm) compared to mounts-without-intromission or control treatment. Both mating and mounting induced Fos expression in NADPH-d-positive cells in the ventromedial nucleus of hypothalamus (VMN). In contrast, the expression of Fos in the NADPH-d-positive neurons in the supraoptic nucleus (SON) and the parvocellular portion of the paraventricular nucleus (PVNp) was not influenced by either mating or mounting although abundant NO-containing neurons were found in the two brain areas. The second experiment of the study examined whether NOergic neurons in these brain areas are influenced directly by estrogen by determining the number of NADPH-d-positive neurons that contained the estrogen receptor alpha (ERalpha), the classical ER. Double labeled NADPH-d/ERalpha neurons were observed in several brain areas including the mPOA and VMN while few, if any, NADPH-d-positive neurons in the SON, PVNm or PVNp contained ERalpha. The results suggest that the activated NOergic neurons in these brain areas may be involved in processing and integrating the mating stimulus. Further investigation is required to determine the physiological role of the mating-activated NOergic activity in specific mating-induced changes in reproductive neuroendocrinology.

Reduction of drug-induced yawning and penile erection and of noncontact erections in male rats by the activation of GABAA receptors in the paraventricular nucleus: involvement of nitric oxide

The effect of muscimol, a GABAA receptor agonist, injected into the paraventricular nucleus (PVN) of the hypothalamus on drug-induced (apomorphine, oxytocin and NMDA) yawning and penile erection, and on the increase in the concentration of NO2- and NO3- occurring in the paraventricular dialysate in these experimental conditions, was studied in male rats. Muscimol (50, 100 and 200 ng) reduced, in a dose-dependent manner, penile erection and yawning induced by apomorphine (50 ng), oxytocin (30 ng) and NMDA (50 ng) delivered into the PVN. The reduction of penile erection and yawning was parallel to a reduction of the concomitant NO2- and NO3- increase that occurs in the paraventricular dialysate in this experimental condition. In contrast, baclofen (200 ng), a GABAB receptor agonist, was ineffective. The muscimol effects on drug-induced penile erection, yawning and NO2- increase were prevented by the prior administration of bicuculline (250 ng into the paraventricular nucleus). Muscimol (200 ng) but not baclofen (200 ng), injected into the PVN, reduced both noncontact erections in male rats placed in the presence of an inaccessible receptive female, and also the NO2- increase that occurs in the paraventricular dialysate in this experimental condition. As found with drug-induced penile erection, the muscimol reduction of noncontact erections and of NO2- increase was prevented by bicuculline. The present results show that the activation of GABAA receptors in the PVN reduces yawning and penile erection induced by drugs or physiological stimuli by reducing the increase in NO activity that occurs in this hypothalamic nucleus in these experimental conditions.

An alternative promoter of the human neuronal nitric oxide synthase gene is expressed specifically in Leydig cells

Neuronal nitric oxide synthase (nNOS) plays a modulatory role in the biology of a variety of neuroendocrine tissues and is especially relevant to gonadal function. We have previously reported the cloning and characterization of a variant of the nNOS protein, termed testis nNOS (TnNOS), the mRNA for which was restricted in expression to male gonadal tissues. To examine the cell-specificity of the testis-specific NOS regulatory regions we defined patterns of beta-galactosidase expression of an insertional transgene in which the reporter gene lacZ was under the transcriptional control of the human TnNOS promoter. beta-galactosidase activity was detected exclusively in the interstitial cells of the testis in transgenic mice. These cells also evidenced positive staining for nNOS protein and were identified as androgen-producing Leydig cells by staining with the Leydig cell marker, P(450)scc. Expression of the promoter was absent in cells of the seminiferous tubules, specifically germline cells of different stages and Sertoli cells. In contrast to the male gonad, beta-galactosidase activity was not detected in ovaries of adult female mice. Activity was also not evident in organs known to express full-length nNOS, such as skeletal muscle, kidney, or cerebellum. The same pattern of beta-galactosidase staining was observed in independent transgenic founders and was distinct from that observed for an endothelial NOS promoter/reporter transgene. In the testis of male adult eNOS promoter-reporter transgenic mice, beta-galactosidase activity was expressed only in endothelial cells of large- and medium-sized arterial blood vessels. Transcriptional activity of the human TnNOS promoter could not be detected in a variety of cell types, including Leydig cells, using episomal promoter-reporter constructs suggesting that a nuclear environment and higher order genomic complexity are required for appropriate promoter function. The restricted expression pattern of an nNOS variant in Leydig cells of the male gonad suggests an important role in the regulation of testosterone release and represents an intriguing model with which to dissect the molecular basis of Leydig cell-specific gene expression.

Muscarinic mediation of the urethro genital reflex in spinal cord-transected rats

The effects of muscarinic receptor stimulation were tested on the urethro genital reflex (UGR) in anesthetized and spinal cord-transected rats. Drugs were applied directly to the spinal cord. The electromyographic activity (EMG) of the bulbospongiosus (BS) muscle was used for recording UGR. In six animals BS as well as soleus, posterior biceps or peroneus tertius muscle EMG was recorded simultaneously. Muscarine (5, 10, 20, 50 and 100 microg) was applied in 22 animals after cutting L6-S1 dorsal roots. Some observations were made on another six animals, to which an extensive bilateral dorsal rhizotomy (L3-S2) was performed. Rhythmic bursts of similar frequency and size to those seen during UGR were found in BS muscle a few minutes after muscarine application. No rhythmic bursting was found on the hindlimb muscles, but exclusively on BS muscles. The effects of homatropine (25, 50, 100 and 200 microg), an acetylcholine muscarinic receptor antagonist, were tested in 21 rats after UGR was elicited three times at low stimulation intensity (7 mm Hg). Homatropine produced two effects: (i) A significant increase in the latency of UGR. (ii) A facilitation of UGR inhibition. In view of these results it can be speculated that muscarinic receptor stimulation is involved in the elicitation of UGR.

Effect of excitatory amino acid, dopamine, and oxytocin receptor antagonists on noncontact penile erections and paraventricular nitric oxide production in male rats

In male rats, noncontact erections occur concomitantly with an increase in NO2- and NO3- in the paraventricular nucleus of the hypothalamus (PVN). In the present study, both responses were reduced by the blockade of PVN excitatory amino acid receptors by dizocilpine, (+)-MK-801(1 and 5 microg), but not by 6-cyano-7-nitro-quinoxaline-2,3-dione (5 microg) or (+)-2-amino-4-phosphono-butanoic acid (5 microg). Also ineffective when injected into the PVN were the dopamine antagonists SCH 23390 (5 microg), S(+)-raclopride (10 microg), and cis-flupenthixol (10 microg), and the oxytocin antagonist d(CH2)5Tyr(Me)2-Om8-vasotocin (1 microg). However, when the last was given into the lateral ventricles, it reduced noncontact erections without modifying NO2- and NO3- increases. These results suggest that excitatory amino acid transmission increases in the PVN during noncontact erections. This may contribute to increased NO production in the PVN, and it may activate oxytocin neurons mediating this sexual response.

Reduction in libido and fertility of male rats by administration of the nitric oxide (NO) synthase inhibitor N-nitro-L-arginine methyl ester

The role of nitric oxide (NO) in libido and fertility of male rats was investigated by administration of the NO synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) (25 or 50 mg/kg/day). L-NAME caused marked reduction of precoital sexual behaviour, and a failure of most rats to mount or ejaculate during the test interval. In most matings that were successful, recovered spermatozoa were present in normal numbers. In mating experiments, L-NAME profoundly reduced the fertility of male rats. In those animals that did succeed in mating, the quantal pregnancy and the number of implants were reduced. After cessation of treatment with L-NAME, the fertility parameters returned close to normal. The inactive stereoisomer, D-NAME, caused none of the above effects when administered to rats. The results suggest that NO is essential for the expression of normal libido and fertility in male rats. It is likely that NO is required both in the male reproductive tract and in the brain.

The role of glutamate and nitric oxide in the reproductive neuroendocrine system

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.

Key words: glutamate, nitric oxide, GnRH, postsynaptic density, hypothalamus.