Potentiation of penile erection and yawning responses to apomorphine by cannabinoid receptor antagonist in rats

Mechanisms of penile erection and basis for pharmacological treatment of erectile dysfunction

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, both autonomic and somatic, and supraspinal influences from visual, olfactory, and imaginary stimuli. Several central transmitters are involved in the erectile control. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropin/α-melanocyte-stimulating hormone, have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. The balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa (CC) and determines the functional state of the penis. Noradrenaline contracts both CC and penile vessels via stimulation of α₁-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and CC. The role of other mediators, released from nerves or endothelium, has not been definitely established. Erectile dysfunction (ED), defined as the "inability to achieve or maintain an erection adequate for sexual satisfaction," may have multiple causes and can be classified as psychogenic, vasculogenic or organic, neurologic, and endocrinologic. Many patients with ED respond well to the pharmacological treatments that are currently available, but there are still groups of patients in whom the response is unsatisfactory. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including oral phosphodiesterase inhibitors and intracavernosal injections of prostaglandin E₁. Irrespective of the underlying cause, these drugs are effective in the majority of cases. Drugs with a central site of action have so far not been very successful. There is a need for therapeutic alternatives. This requires identification of new therapeutic targets and design of new approaches. Research in the field is expanding, and several promising new targets for future drugs have been identified.

Anatomy, physiology, and pathophysiology of erectile dysfunction

INTRODUCTION

Significant scientific advances during the past 3 decades have deepened our understanding of the physiology and pathophysiology of penile erection. A critical evaluation of the current state of knowledge is essential to provide perspective for future research and development of new therapies.

AIM

To develop an evidence-based, state-of-the-art consensus report on the anatomy, physiology, and pathophysiology of erectile dysfunction (ED).

METHODS

Consensus process over a period of 16 months, representing the opinions of 12 experts from seven countries.

MAIN OUTCOME MEASURE

Expert opinion was based on the grading of scientific and evidence-based medical literature, internal committee discussion, public presentation, and debate.

RESULTS

ED occurs from multifaceted, complex mechanisms that can involve disruptions in neural, vascular, and hormonal signaling. Research on central neural regulation of penile erection is progressing rapidly with the identification of key neurotransmitters and the association of neural structures with both spinal and supraspinal pathways that regulate sexual function. In parallel to advances in cardiovascular physiology, the most extensive efforts in the physiology of penile erection have focused on elucidating mechanisms that regulate the functions of the endothelium and vascular smooth muscle of the corpus cavernosum. Major health concerns such as atherosclerosis, hyperlipidemia, hypertension, diabetes, and metabolic syndrome (MetS) have become well integrated into the investigation of ED.

CONCLUSIONS

Despite the efficacy of current therapies, they remain insufficient to address growing patient populations, such as those with diabetes and MetS. In addition, increasing awareness of the adverse side effects of commonly prescribed medications on sexual function provides a rationale for developing new treatment strategies that minimize the likelihood of causing sexual dysfunction. Many basic questions with regard to erectile function remain unanswered and further laboratory and clinical studies are necessary.

Cannabinoid CB1 receptors in the paraventricular nucleus and central control of penile erection: Immunocytochemistry, autoradiography and behavioral studies

[N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide] (SR 141716A), a selective cannabinoid CB1 receptor antagonist, injected into the paraventricular nucleus of the hypothalamus (PVN) of male rats, induces penile erection. This effect is mediated by the release of glutamic acid, which in turn activates central oxytocinergic neurons mediating penile erection. Double immunofluorescence studies with selective antibodies against CB1 receptors, glutamic acid transporters (vesicular glutamate transporters 1 and 2 (VGlut1 and VGlut2), glutamic acid decarboxylase-67 (GAD67) and oxytocin itself, have shown that CB1 receptors in the PVN are located mainly in GABAergic terminals and fibers surrounding oxytocinergic cell bodies. As GABAergic synapses in the PVN impinge directly on oxytocinergic neurons or on excitatory glutamatergic synapses, which also impinge on oxytocinergic neurons, these results suggest that the blockade of CB1 receptors decreases GABA release in the PVN, increasing in turn glutamatergic neurotransmission to activate oxytocinergic neurons mediating penile erection. Autoradiography studies with [(3)H](-)-CP 55,940 show that chronic treatment with SR 141716A for 15 days twice daily (1 mg/kg i.p.) significantly increases the density of CB1 receptors in the PVN. This increase occurs concomitantly with an almost twofold increase in the pro-erectile effect of SR 141716A injected into the PVN as compared with control rats. The present findings confirm that PVN CB1 receptors, localized mainly in GABAergic synapses that control in an inhibitory fashion excitatory synapses, exert an inhibitory control on penile erection, demonstrating for the first time that chronic blockade of CB1 receptors by SR 141716A increases the density of these receptors in the PVN. This increase is related to an enhanced pro-erectile effect of SR 141716A, which is still present 3 days after the end of the chronic treatment.

Rimonabant: Just an Antiobesity Drug? Current Evidence on Its Pleiotropic Effects

The advent of the highly selective cannabinoid receptor (CB1) antagonist, rimonabant (SR141716; Acomplia) can revolutionize the ability of the clinicians to manage obesity. Large-scale clinical trials have demonstrated that rimonabant therapy can reduce obesity. Although, the precise mechanisms of action of rimonabant have to be further dissected, it is emerging, from both preclinical and clinical research, that not only is rimonabant an antiobesity drug, but also its pleiotropic functions affect a broad range of diseases, from obesity-related comorbidities to drug dependence and cancer. Here we review recent data from the literature and discuss the full pharmacological potential of this drug.

The cannabinoid CB1 receptor antagonist SR 141716A induces penile erection by increasing extra-cellular glutamic acid in the paraventricular nucleus of male rats

The cannabinoid CB1 receptor antagonist SR 141716A (0.1, 0.5 and 1 microg) induces penile erection when injected into the paraventricular nucleus of the hypothalamus of male rats. The pro-erectile effect of SR 141716A occurs concomitantly with an increase in the concentration of glutamic acid in the paraventricular dialysate obtained by means of intra-cerebral microdialysis. Glutamic acid increase and penile erection did not occur when SR 141716A was given after tetrodotoxin, a voltage-dependent Na(+) channel blocker. Both penile erection and glutamic acid increases were also reduced by the cannabinoid CB1 receptor agonists WIN 55,212-2 or HU 210 given into the paraventricular nucleus before SR 141716A at doses unable to induce penile erection or to modify glutamic acid. In contrast, dizocilpine ((+)MK-801), an antagonist of excitatory amino acid receptors of the N-methyl-d-aspartic acid (NMDA) subtype, given into the paraventricular nucleus reduced penile erection, but was ineffective on the glutamic acid increase induced by the CB1 receptor antagonist. 6-Cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and (+/-)-2-amino-4-phosphono-butanoic acid (AP(4)), antagonists of the excitatory amino acid receptors of the AMPA subtype and of the metabotropic subtype, respectively, were ineffective on both penile erection and glutamic acid increase. SR 141716A responses were also reduced by muscimol, a GABA(A) receptor agonist, but not by baclofen, a GABA(B) receptor agonist, given into the paraventricular nucleus before SR 141716A. The present results show that SR 141716A induces penile erection by activating glutamic acid neurotransmission, which causes in turn the activation of paraventricular oxytocinergic neurons mediating penile erection.

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.

Keynote review: Medicinal chemistry strategies to CB1 cannabinoid receptor antagonists

The proven clinical efficacy of the CB(1) cannabinoid receptor antagonist rimonabant in both obesity and smoking cessation and its therapeutic potential in other disorders has given a tremendous impetus to the discovery of novel CB(1) antagonists. The number of disclosed patents wherein novel chemical entities having CB(1) antagonistic or inverse agonistic properties have been claimed has exploded. Besides novel compound classes that were identified in screening, rational medicinal chemistry approaches such as conformational constraint and scaffold hopping have been successfully applied. CB(1) receptor modelling has provided insight into crucial receptor-ligand interaction points thereby leading to a general CB(1) inverse agonist pharmacophore model.

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.

Antagonism of cannabinoid CB1 receptors in the paraventricular nucleus of male rats induces penile erection

The effect of cannabinoid CB1 receptor agonists and antagonists on penile erection was studied in male rats when injected into the paraventricular nucleus of the hypothalamus. The CB1 receptor antagonist SR 141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide] (0.5-5 microg) induced penile erection in a dose-dependent manner. The minimal effective dose was 1 microg, while the maximal response was found with 5 microg of the compound. In contrast, the CB1 receptor agonists WIN 55,212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1-I,j]quinolin-6-one] (0.5-5 microg) and CP 55,940 [1alpha,2beta-(R)-5alpha]-5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxy-propyl)cyclohexyl]phenol (0.5-5 microg) were ineffective at all the doses tested. Nevertheless, both compounds reduced the enhancing effect of SR 141716A on penile erection when given into the paraventricular nucleus at the above doses before SR 141716A. The pro-erectile effect of SR 141716A was also reduced by the non-competitive NMDA receptor antagonist dizolcipine (MK-801) (0.2 microg) and by the NO synthase inhibitor NG-nitro-l-arginine methylester (L-NAME) (20 microg) but not by the dopamine receptor antagonist cis-flupenthixol (10 microg) or the oxytocin receptor antagonist d(CH2)5Tyr(Me)2-Orn8-vasotocin (0.1 microg), when given into the paraventricular nucleus. In spite of its inability to prevent the pro-erectile effect of SR 141716A when given in the paraventricular nucleus, d(CH2)5Tyr(Me)2-Orn8-vasotocin) (1 microg) reduced almost completely SR 141716A-induced penile erection when given into the lateral ventricles. The present results show that cannabinoid CB1 receptors present in the paraventricular nucleus may influence erectile function and sexual activity by modulating paraventricular oxytocinergic neurons mediating erectile function.