Oxytocinergic neurotransmission at the hippocampus in the central neural regulation of penile erection in the rat

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

The pathophysiology of lifelong premature ejaculation

For many decades it has been thought that lifelong premature ejaculation (PE) is only characterized by persistent early ejaculations. Despite enormous progress of in vivo animal research, and neurobiological, genetic and pharmacological research in men with lifelong PE, our current understanding of the mechanisms behind early ejaculations is far from complete. The new classification of PE into four PE subtypes has shown that the symptomatology of lifelong PE strongly differs from acquired PE, subjective PE and variable PE. The phenotype of lifelong PE and therefore also the pathophysiology of lifelong PE is much more complex. A substantial number of men with lifelong PE not only have PE, but also premature erection and premature penile detumescence as part of an acute hypertonic or hypererotic state when engaged in an erotic situation or when making love. As both erectio praecox, ejaculatio praecox, detumescentia praecox, and the hypererotic state are part of the phenotype lifelong PE, it is argued that lifelong PE is not only a disturbance of the timing of ejaculation but also a disturbance of the timing of erection, detumescence and arousal. Since 1998, the pathophysiology of lifelong PE was thought to be mainly mediated by the central serotonergic system in line with genetic polymorphisms of specific serotonergic genes. However, by accepting that lifelong PE is characterized by the reversible hypertonic state the hypothesis of mainly serotonergic dysfunction is no longer tenable. Instead, it has been postulated that the pathophysiology of lifelong PE is mediated by a very complex interplay of central and peripheral serotonergic, dopaminergic, oxytocinergic, endocrinological, genetic and probably also epigenetic factors. Progress in research of lifelong PE can only be accomplished when a stopwatch is used to measure the IELT and the cut-off point of 1 minute for the definition of lifelong PE is maintained. Current use of validated questionnaires, neglect of stopwatch research, clinically inexperienced investigators and inclusion of anonymous men in a study performed by the Internet endanger the continuation of objective research of lifelong PE.

Oxytocin: recent developments

Oxytocin is a neurohypophyseal hormone that is produced centrally by neurons in the paraventricular nucleus and supraoptic nucleus of the hypothalamus. It is released directly into higher brain centres and into the peripheral circulation where it produces a multitude of effects. Classically, oxytocin is known for inducing uterine contractions at parturition and milk ejection during suckling. Oxytocin also acts in a species and gender specific manner as an important neuromodulator. It can affect behaviours associated with stress and anxiety, as well social behaviours including sexual and relationship behaviours, and maternal care. Additionally, oxytocin has been shown to have a variety of physiological roles in peripheral tissues, many of which appear to be modulated largely by locally produced oxytocin, dispelling the notion that oxytocin is a purely neurohypophyseal hormone. Oxytocin levels are altered in several diseases and the use of oxytocin or its antagonists have been identified as a possible clinical intervention in the treatment of mood disorders and pain conditions, some cancers, benign prostatic disease and osteoporosis. Indeed, oxytocin has already been successful in clinical trials to treat autism and schizophrenia. This review will report briefly on the known functions of oxytocin, it will discuss in depth the data from recent clinical trials and highlight future targets for oxytocinergic modulation.

The role of oxytocin in male and female reproductive behavior

Oxytocin (OT) is a nonapeptide with an impressive variety of physiological functions. Among them, the 'prosocial' effects have been discussed in several recent reviews, but the direct effects on male and female sexual behavior did receive much less attention so far. As our contribution to honor the lifelong interest of Berend Olivier in the control mechanisms of sexual behavior, we decided to explore the role of OT in the present review. In the successive sections, some physiological mechanisms and the 'pair-bonding' effects of OT will be discussed, followed by sections about desire, female appetitive and copulatory behavior, including lordosis and orgasm. At the male side, the effects on erection and ejaculation are reviewed, followed by a section about 'premature ejaculation' and a possible role of OT in its treatment. In addition to OT, serotonin receives some attention as one of the main mechanisms controlling the effects of OT. In the succeeding sections, the importance of OT for 'the fruits of labor' is discussed, as it plays an important role in both maternal and paternal behavior. Finally, we pay attention to an intriguing brain area, the ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl), apparently functioning in both sexual and aggressive behavior, which are at first view completely opposite behavioral systems.

Oxytocin-messages via the cerebrospinal fluid: behavioral effects; a review

The cerebrospinal fluid (CSF) usually is considered as a protective 'nutrient and waste control' system for the brain. Recent findings suggest, however, that the composition of CSF is actively controlled and may play an influential role in the changes in brain activity, underlying different behavioral states. In the present review, we present an overview of available data concerning the release of oxytocin into the CSF, the location of the oxytocin-receptive brain areas and the behavioral effects of intracerebroventricular oxytocin. About 80% of the oxytocin-receptive areas are located close to the ventricular or subarachnoid CSF, including the hypothalamic 'Behavior Control Column' (L.W.Swanson, 2003). As a conclusion we suggest that 'CSF-oxytocin' contributes considerably to the non-synaptic communication processes involved in hypothalamic-, brainstem- and olfactory brain areas and behavioral states and that the flowing CSF is used as a 'broadcasting system' to send coordinated messages to a wide variety of nearby and distant brain areas.

Perirhinal cortex lesions delay ejaculation in rats

Afferents from the perirhinal cortex (PRh) form a major input to the hippocampal formation, which is known to be involved in sexual behavior in rodents. But there is a lacuna in literature regarding the role of the PRh in sexual behavior. Bilateral neurotoxic lesions of the PRh delayed the ejaculation latency and prolonged the mean inter-intromission interval significantly, suggesting a facilitatory role of the PRh in male rat sex behavior.

Influence of chronic cocaine treatment and sleep deprivation on sexual behavior and neurogenesis of the male rat

The present study investigated the influence of chronic cocaine treatment on genital reflexes associated with paradoxical sleep deprivation (PSD), and possible alterations in hippocampus neurogenesis of the male rat. At 21 days of age, the rats were distributed into two groups and injected with saline or cocaine (7 mg/kg, three times a week for 12 weeks). At age 90 days, they were submitted to a four-day period of PSD (PSD groups) or maintained in home-cages (control groups), challenged with saline or cocaine administration, and placed in observation cages to assess genital reflexes. Two additional groups were used to quantify neurogenesis. PSD rats treated chronically with cocaine and challenged with saline did not differ from their respective control groups. The association of PSD with cocaine potentiated penile erection (PE) when compared to PSD-saline (saline challenged) rats, and these effects were similar to those observed in long-term cocaine treated rats. The bromodeoxyuridine (BrdU) assay indicated a reduction in BrdU-positive cells in the adult hippocampus after chronic cocaine treatment. These findings show that long-term cocaine treatment from brain development through adulthood had a marked effect on sexual responses and neuronal proliferation.

Oxytocin Involvement in SSRI-Induced Delayed Ejaculation: A Review of Animal Studies

INTRODUCTION

Selective serotonin reuptake inhibitors (SSRIs) differ in the severity of induced ejaculation delay. Various studies indicate that oxytocin is involved in sexual behavior.

AIM

To review and evaluate the involvement of oxytocin in SSRI-induced ejaculation delay.

MAIN OUTCOME MEASURES

Oxytocine release, 5-hydroxytryptamine (5-HT) neurotransmission, and desensitization of 5-HT(1A) receptors.

METHODS

A review and critical analysis of animal studies investigating the interaction of serotonergic and oxytocinergic neurotransmission in relation to the ejaculation process.

RESULTS

Although acute treatment with the SSRIs fluoxetine and paroxetine immediately causes increased serotonin levels, delayed ejaculation does not occur. The increased serotonin levels induce oxytocin release via activation of 5-HT(1A) receptors, and this might compensate for the inhibitory actions of serotonin on sexual behavior. Chronic treatment with fluoxetine and paroxetine desensitizes 5-HT(1A) receptors on oxytocin neurons, and that might in part determine the onset of delayed ejaculation. Desensitization of 5-HT(1A) receptors is less strong following chronic treatment with the SSRIs fluvoxamine or citalopram, which may attenuate the degree of delayed ejaculation.

CONCLUSIONS

Preliminary data suggest that the severity of chronic SSRI treatment-induced delayed ejaculation and the differences between the various SSRIs in inducing ejaculation delay is related to gradual desensitization of 5-HT(1A) receptors on oxytocin neurons.

Analysis of direct hippocampal cortical field CA1 axonal projections to diencephalon in the rat

The hippocampal formation is generally considered essential for processing episodic memory. However, the structural organization of hippocampal afferent and efferent axonal connections is still not completely understood, although such information is critical to support functional hypotheses. The full extent of axonal projections from field CA1 to the interbrain (diencephalon) is analyzed here with the Phaseolus vulgaris-leucoagglutinin (PHAL) method. The ventral pole of field CA1 establishes direct pathways to, and terminal fields within, the anterior hypothalamic nucleus, ventromedial hypothalamic nucleus, lateral hypothalamic and lateral preoptic areas, medial preoptic area, and certain other hypothalamic regions, as well as particular midline thalamic nuclei. These results suggest that hippocampal field CA1 modulates motivated or goal-directed behaviors, and physiological responses, associated with the targeted hypothalamic neuron populations.

Central Nervous System Agents in the Treatment of Erectile Dysfunction

In the last two decades, a better understanding of the mechanisms governing erectile function and the pathophysiologies underlying erectile dysfunction (ED) have led re-searchers to investigate novel treatment concepts. Selective type-5 phosphodiesterase inhibitors are recommended as first-line therapy because of their high efficacy, but 30% to 40% of patients who have ED do not respond adequately to these agents and require alternative methods. The central nervous system plays a fundamental role in sexual behavior. Animal models have advanced our understanding of the neuroanatomic and neuropharmacologic basis of centrally induced penile erections. Clinical research with apomorphine has demonstrated efficacy in men who have a range of ED. Recent interest has focused on other centrally acting agents for ED treatment, including the melanocortin receptor agonists.

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.

Effect of excitatory amino acid receptor agonists on penile erection after administration into paraventricular nucleus of hypothalamus in the rat

OBJECTIVES

To investigate whether the excitatory amino acid receptor agonists can activate the paraventricular nucleus of the hypothalamus (PVN) and induce penile erections in the rat.

METHODS

Male adult Sprague-Dawley rats were used. A 26-gauge needle was inserted into the corpus cavernosum to monitor the intracavernous pressure (ICP) simultaneously with the systemic arterial pressure and heart rate. The study was divided into seven parts: stereotaxic delivery of N-methyl-d-aspartic acid (NMDA) (50 ng/500 nL) into the PVN; administration of the NMDA noncompetitive antagonist MK-801 (100 ng/250 nL) and NMDA (50 ng/250 nL) into the PVN; administration of the NMDA competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 100 ng/250 nL) and NMDA (50 ng/250 nL) into the PVN; microinjection of (+/-)-alpha-(amino)-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 100 ng/500 nL) into the PVN; microinjection of trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD; 100 ng/500 nL) into the PVN; saline 500 nL into the PVN; and intracavernous administration of NMDA (50 ng/100 microL).

RESULTS

On administration of NMDA into the PVN, a significant increase occurred in the ICP from a resting 8.3 +/- 1.8 mm Hg to a peak at 59.0 +/- 8.4 mm Hg. No change occurred in the resting ICP after administration of either the mixture of MK-801 and NMDA or CPP and NMDA into the PVN. Microinjection of AMPA, ACPD, or saline into the PVN and intracavernous administration of NMDA were all ineffective to induce an increase in ICP.

CONCLUSIONS

The results of this study suggest that ionotropic excitatory amino acid (NMDA) may have an effect on inducing penile erection through activation of the PVN in the rat.

Involvement of L-arginine/nitric oxide pathway at the paraventricular nucleus of hypothalamus in central neural regulation of penile erection in the rat

The objective of this study is to investigate whether the L-arginine/nitric oxide pathway is involved in the neurotransmission of paraventricular nucleus of hypothalamus (PVN) activation-induced penile erection in the rat. Male adult Sprague-Dawley rats anesthetized with pentobarbital were used. The femoral artery was cannulated to measure systemic and mean arterial pressure (SAP and MAP), and heart rate (HR). A 26-gauge needle was inserted into corpus cavernosum to measure the intracavernous pressure (ICP) simultaneously with SAP, MAP and HR on a polygraph. Four groups of study were arranged: (1) stereotaxically delivery of L-arginine (500 nmol/500 nl) into PVN; (2) administration of a mixture (1 microl) containing N(G)-Nitro-L-arginine methyl ester (L-NAME) 500 nmol and L-arginine 500 nmol into PVN; (3) microinjection of saline 500 nl into PVN as a vehicle control; and (4) intracavernous injection of L-arginine (100 nmol/50 microl). The ICP, SAP, MAP and HR were monitored for at least 2 h after each administration of the experimental agents. Upon administration of L-arginine into PVN, there was a significant increase of ICP from resting 9.6+/-2.5 mmHg to peaked at 64.4+/-9.8 mmHg after a latency of 3016.0+/-1749.7 s and with a duration of 27.6+/-15.8 min. There was no change of resting ICP after administration of the mixture of L-NAME and L-arginine into PVN. Application of saline to PVN and intracavernous injection of L-arginine failed to increase ICP. Based on elicitation of penile erection upon administration of L-arginine into PVN, and elimination of this L-arginine induced penile erection by co-administration of L-NAME with L-arginine, the results of this study suggest that L-arginine/nitric oxide pathway may be involved in the neurotransmission of PVN activation-induced penile erection in the rat.

Immunolesion of hindbrain catecholaminergic projections to the medial hypothalamus attenuates penile reflexive erections and alters hypothalamic peptide mRNA

The central mechanisms underlying diabetes-associated impotence are currently unknown. This study utilized immunolesion techniques to eliminate hindbrain catecholaminergic projections to the medial hypothalamus which have been reported to be glucoresponsive. The immunolesioned male rats had an attenuated feeding response to glucoprivic challenge. Furthermore, these lesioned rats had significantly attenuated penile reflexes. Northern blot analyses of hypothalamic oxytocin mRNA expression showed a significant increase; however, neuropeptide Y mRNA expression did not. These results suggest that hindbrain catecholaminergic neurones may alter the expression of hypothalamic neuropeptides that stimulate penile erections based upon glucoregulatory signals from the periphery.