EP 91073 prevents EP 80661-induced penile erection: new evidence for the existence of specific EP peptide receptors mediating penile erection

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.

Future prospects in the treatment of erectile dysfunction: focus on avanafil

The treatment of erectile dysfunction (ED) has been revolutionized in the last 15 years with the introduction of type 5 phosphodiesterase (PDE5) inhibitors. Their efficacy, safety, and ease of administration have made them first-line treatment for ED. This article reviews the current therapies available for ED, and the new PDE5 inhibitors that are being investigated. Furthermore, it examines all the current ED treatment options that are in different phases of development (including oral and topical pharmacotherapy, gene therapy, and tissue engineering). A special emphasis is on avanafil, a new PDE5 inhibitor that has been studied extensively in Phase I and II clinical trials and has undergone several Phase III trials. Avanafil is a promising medication for ED due to its favorable pharmacokinetics, safety, and efficacy.

Evidence for a role of the GHS-R1a receptors in ghrelin inhibition of gastric acid secretion in the rat

Ghrelin, the endogenous ligand of the GH secretagogue receptor (GHS-R) has been previously shown to inhibit gastric acid secretion in pylorus-ligated rats. Two isoforms of GHS-R have been identified: GHS-R(1a) and GHS-R(1b). The present study aimed: (i) to characterise the type of GHS-R involved in the central gastric inhibitory activity of ghrelin by using des-octanoyl ghrelin, and synthetic GHS-R(1a) agonist (EP1572) and antagonist (D-Lys(3)-GHRP-6) and (ii) to investigate the relationship between ghrelin and cortistatin (CST) in the control of gastric acid secretion by using the natural neuropeptide CST-14 and the synthetic octapeptide CST-8. The specific interactions of all the compounds with GHS-R(1a) were determined by comparing their ability to displace labelled ghrelin or somatostatin from its receptors on rat hypothalamic membranes or on rat cardiomyocyte, respectively. Intracerebroventricular administration of 0.01 and 1 nmol/rat des-octanoyl ghrelin did not affect gastric acid secretion in pylorus-ligated rats, whereas EP1572 either i.c.v. (0.01-1 nmol/rat) or i.p. (10 and 20 nmol/kg) inhibited acid gastric secretion. Preteatment with D-Lys(3)GHRP-6 (3 nmol/rat, i.c.v.) was able to remove the inhibitory action of ghrelin (0.01 nmol/rat, i.c.v.) on gastric acid volume and acid output, thus indicating that the type 1a GHS-R likely mediates the gastric inhibitory action of ghrelin. This is supported by binding data showing that D-Lys(3)GHRP-6, but not des-octanoyl ghrelin, binds to hypothalamic GHS-R. CST-14 (1 nmol/rat, i.c.v.) did not affect either basal or ghrelin inhibition of gastric acid secretion. CST-8 (1 nmol/rat, i.c.v.) was able to counteract the gastric ghrelin response. The observation that CST-14 binds both GHR-S and somatostatin receptors, whereas CST-8 specifically displaces only ghrelin binding, indicates that CST-8 behaves as a GHS-R(1a) antagonist.

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.

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.

Activation of GABAA and opioid receptors reduce penile erection induced by hexarelin peptides

The effect of muscimol, a GABA(A) receptor agonist, and of morphine, an opioid receptor agonist, on penile erection induced by the hexarelin analogue peptide EP 80661 (GAB-D-Trp(2-Me)-D-Trp(2-Me)-LysNH(2)) and on the increase in the concentration of NO(2)(-) and NO(3)(-) that occurs concomitantly in the dialysate obtained from the paraventricular nucleus (PVN) of the hypothalamus by intracerebral microdialysis, was studied in male rats. Muscimol (50, 100 and 200 ng) and morphine (0.1, 0.5, 1 and 5 microg) given into the PVN dose-dependently reduced penile erection induced by EP 80661 (1 microg) injected into the PVN. The reduction of penile erection was parallel to a decrease of the concomitant NO(2)(-) and NO(3)(-) increase that occurs in the paraventricular dialysate in these experimental conditions. Muscimol and morphine effects on EP 80661-induced penile erection and NO(2)(-) increase were prevented by the prior administration into the PVN of bicuculline (250 ng) and naloxone (5 microg), respectively. The present results show that the activation of GABA(A) receptors and of opioid receptors in the PVN reduces penile erection induced by hexarelin analogue peptides by reducing the increase in NO activity that occurs in this hypothalamic nucleus in these experimental conditions.

Ghrelin injected into the paraventricular nucleus of the hypothalamus of male rats induces feeding but not penile erection

The effect of ghrelin, a recently characterized endogenous receptor agonist for growth hormone (GH) secretagogue receptors, on feeding and penile erection was compared with that of EP 80661, a peptide analogue of the GH secretagogue hexarelin, previously identified for its pro-erectile activity when injected into the paraventricular nucleus of the hypothalamus of male rats. Ghrelin (0.01-1 microg), but not EP 80661 (0.02-1 microg), was found to be particularly effective in enhancing feeding. The minimal effective dose of ghrelin was 0.1 microg, which increased food intake by 88%, while the maximal response (355% above control values) was found with 1 microg of the peptide. The enhancing effect of ghrelin on feeding was prevented by the prior administration of the neuropeptide Y Y5 receptor antagonist (DTyr(2), DThr(32)) neuropeptide Y (NPY, 10 microg), but not by the GH-RH receptor antagonist MZ-4-71 (10 microg), or by EP 91073, a hexarelin analogue that antagonizes the pro-erectile effect of EP 80661 (10 microg), given into the lateral ventricles. In contrast, ghrelin failed to induce penile erection at all doses tested, while EP 80661 induced penile erection in a dose-dependent manner. The pro-erectile effect of EP 80661 was prevented by EP 91073 (10 microg), but not by (DTyr(2), DThr(32)) NPY (10 microg) or by the GH-RH receptor antagonist MZ 4-71 (10 microg), given into the lateral ventricles. The present results provide further support to the hypothesis that the GH secretagogue receptors mediating feeding are different from those mediating penile erection and activated by pro-erectile EP peptides.