Localization of peripheral dopamine D1 and D2 receptors in rat corpus cavernosum

6-Nitrodopamine is an endogenous mediator of the rabbit corpus cavernosum relaxation

BACKGROUND

6-Nitrodopamine (6-ND) is a novel endogenous catecholamine that has a potent relaxant action on vascular smooth muscle in vitro.

OBJECTIVES

To evaluate the basal release of 6-ND and noradrenaline from rabbit-isolated corpus cavernosum (RbCC) and its relaxing action on this tissue.

METHODS

Rabbit corpus cavernosa were dissected and suspended in a 5-mL organ bath containing oxygenated Krebs-Henseleit's solution. 6-ND and noradrenaline release was quantified by liquid chromatography coupled to tandem mass spectrometry. The relaxant activity of 6-ND was assessed in RbCC strips pre-contracted with endothelin-1 (10 nM).

RESULTS

Rabbit corpus cavernosum presented basal release of both 6-ND (2.9 ± 0.8 ng/mL, n = 12) and noradrenaline (1.7 ± 1.3 ng/mL, n = 12). The 6-ND release was reduced by pre-treatment with Nω-nitro-l-arginine methyl ester (l-NAME) (100 µM), whereas that of noradrenaline was unaffected. Tetrodotoxin (TTX, 1 µM) abolished the noradrenaline release but had no effect on 6-ND release, indicating a non-neurogenic origin for 6-ND. 6-ND and the selective dopamine D2-agonist L-741,626 caused concentration-dependent RbCC relaxations (pEC50 of 11 ± 0.15 and 11.15 ± 0.28, respectively). Pre-treatment with either l-NAME or the soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-on (ODQ) (100 µM) caused a rightward shift of the concentration-response curve to 6-ND, without affecting the L-741,626 responses. In TTX (100 nM)-pre-treated preparations, neither l-NAME nor ODQ shifted the 6-ND concentration-response curve. Dopamine, noradrenaline, and adrenaline caused concentration-dependent RbCC contractions. Pre-incubation with 6-ND concentration-dependently inhibited the dopamine-induced contractions, without affecting those induced by either noradrenaline or adrenaline.

DISCUSSION AND CONCLUSION

6-Nitrodopamine is the most potent endogenous relaxant agent in RbCC ever described and represents a novel mechanism by which NO causes corpus cavernosum smooth muscle relaxation. The finding that 6-ND acts as a truly selective dopamine D2-receptor antagonist indicates that the balance of dopamine and 6-ND release/synthesis may be the main mechanism that modulates corpus cavernosum smooth muscle tonus in vivo.

A novel reuptake inhibitor, IP2015, induces erection by increasing central dopamine and peripheral nitric oxide release

BACKGROUND AND PURPOSE

An estimated 40% of patients with erectile dysfunction have a poor prognosis for improvement with currently available treatments. The present study investigated whether a newly developed monoamine transport inhibitor, IP2015, improves erectile function.

EXPERIMENTAL APPROACH

We investigated the effects of IP2015 on monoamine uptake and binding, erectile function in rats and diabetic mice and the effect on corpus cavernosum contractility.

KEY RESULTS

IP2015 inhibited the uptake of 5-HT, noradrenaline and dopamine by human monoamine transporters expressed in cells and in rat brain synaptosomes. Intracavernosal pressure measurement in anaesthetized rats revealed that IP2015 dose-dependently increased the number and the duration of spontaneous erections. Whereas pretreatment with the dopamine D2-like receptor antagonists, clozapine and (-)-sulpiride, or cutting the cavernosal nerve inhibited IP2015-induced erectile responses, the phosphodiesterase type 5 inhibitor sildenafil further enhanced the IP2015-mediated increase in intracavernosal pressure. IP2015 also increased the number of erections in type 2 diabetic db/db mice. Direct intracavernosal injection of IP2015 increased penile pressure, and in corpus cavernosum strips, IP2015 induced concentration-dependent relaxations. These relaxations were enhanced by sildenafil and blunted by endothelial cell removal, a nitric oxide synthase inhibitor, NG-nitro-l-arginine and a D1-like receptor antagonist, SCH23390. Quantitative polymerase chain reaction (qPCR) showed the expression of the dopamine transporter in the rat corpus cavernosum.

CONCLUSION AND IMPLICATIONS

Our findings suggest that IP2015 stimulates erectile function by a central mechanism involving dopamine reuptake inhibition and direct NO-mediated relaxation of the erectile tissue. This novel multi-modal mechanism of action could offer a new treatment approach to erectile dysfunction.

Effects of chronic l-DOPA administration on neurogenic and endothelium-dependent relaxation responses in rabbit corpus cavernosum

BACKGROUND

Dopamine is a crucial central neurotransmitter that plays a fundamental role in the autonomic and somatic components of penile reflexes in animals and humans. Similar to the erectile responses of dopamine, systemic administration of l-DOPA induces yawning and penile erection in some species. The possible effects of l-DOPA on nitric oxide (NO)-dependent and -independent non-adrenergic non-cholinergic (NANC) relaxation responses mediated by electrical field stimulation (EFS) and endothelium-dependent relaxation were investigated in this study.

METHODS

Thirty-two adult albino male rabbits, in two- and four-week-treatment groups, were divided into three subgroups: control group (saline-injected) (n=4), 3mg/kg/day (low dose) l-DOPA-injected groups (n=6) and 12mg/kg/day (high dose) l-DOPA-injected groups (n=6). After the intraperitoneal injection treatments, the corpus cavernosum tissues were placed in organ bath chambers. The EFS-mediated responses, and the concentration-response curve to carbachol, sodium nitroprusside (SNP), sildenafil were assessed.

RESULTS

The two-week treatment with high-dose l-DOPA decreased the NO-dependent NANC relaxation responses, while there was no change in the low-dose two- and four-week treatment groups. The NO-independent NANC relaxation responses in the two-week groups decreased, and the responses in the four-week groups were unchanged when compared to the controls. The relaxation responses to carbachol showed no differences among all groups except for the high-dose four-week l-DOPA group. The relaxation responses of SNP and sildenafil were increased in all of the treatment groups when compared to the controls.

CONCLUSIONS

The observed increases in SNP- and sildenafil-induced responses, along with the decreased EFS-mediated responses, suggest increased sensitivity in the NO-signalling pathway following l-DOPA administration.

Animal models of erectile dysfunction (ED): potential utility of non-human primates as a model of atherosclerosis-induced vascular ED

Erectile dysfunction (ED) is a prevalent medical condition affecting 18 million men and their sexual partners in the United States alone. In the majority of patients, ED is related to alterations in the flow of blood to or from the penis. Undeniably, significant progress has been made in understanding the multifactorial mechanisms that modulate erectile capacity and predispose one to ED, and this, in turn, has led to the availability of more effective treatment options. Nonetheless, all current therapies have untoward side effects, and moreover, there are still no satisfactory treatments for many patients with ED. Further enhancements in the treatment of ED would logically result from both early intervention and more detailed mechanistic insight into the characteristics of the disease process per se. This fact underscores the importance of improved understanding of the initiation, development and progression of ED. However, to do so requires longitudinal studies on animal models that more closely approximate the corresponding clinical features and time course of human disease. The goal of this report is twofold. First, to provide a brief general overview of the applicability of commonly used animal models for the study of ED. The second and primary goal is to highlight the scientific rationale for using non-human primates to evaluate the impact of atherosclerosis-induced vascular disease on the penile and systemic circulatory systems. This latter goal seems especially relevant in light of the recent literature documenting a link between ED and systemic vascular disease, a finding that has major implications in an aging US male population consuming a high fat diet.

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.

Future sexual medicine physiological treatment targets

INTRODUCTION

Sexual function in men and women incorporates physiologic processes and regulation of the central and peripheral nervous systems, the vascular system, and the endocrine system. There is need for state-of-the-art information as there is an evolving research understanding of the underlying molecular biological factors and mechanisms governing sexual physiologic functions.

AIM

To develop an evidence-based, state-of-the-art consensus report on the current knowledge of the major cellular and molecular targets of biologic systems responsible for sexual physiologic function.

METHODS

State-of-the-art knowledge representing the opinions of seven experts from four countries was developed in a consensus process over a 2-year period.

MAIN OUTCOME MEASURES

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

RESULTS

Scientific investigation in this field is needed to increase knowledge and foster development of the future line of treatments for all forms of biological-based sexual dysfunction. This article addresses the current knowledge of the major cellular and molecular targets of biological systems responsible for sexual physiologic function. Future treatment targets include growth factor therapy, gene therapy, stem and cell-based therapies, and regenerative medicine.

CONCLUSIONS

Scientific discovery is critically important for developing new and increasingly effective treatments in sexual medicine. Broad physiologic directions should be vigorously explored and considered for future management of sexual disorders.

The effect of chronic antipsychotic drug administration on nitric oxide synthase activity and gene expression in rat penile tissues

Antipsychotic drug treatment may be associated with common and problematic sexual dysfunction, especially impotence, which can diminish quality of life and lead to treatment noncompliance. Nitric oxide synthase (NOS) is an important cellular modulator of erectile function. We have therefore investigated the effect of antipsychotic drug on activity and gene expression of NOS in rat penile tissues. The activity of constitutive NOS was significantly suppressed below control by a 21 days administration of 1 mg/kg haloperidol, which also significantly decreased expression of endothelial NOS (eNOS) and neural NOS mRNA. Risperidone at 0.5 mg/kg also reduced eNOS mRNA expression. Haloperidol or risperidone did not change gene expression and activity of inducible NOS (iNOS). Quetiapine significantly increased activity and mRNA expression of iNOS with 20 and 40 mg/kg doses. These preliminary results have important implications for enhancing our understanding of mechanisms by which antipsychotic drugs induce sexual dysfunction.

The effects of dopamine receptor agonists on BK Ca channels and signal transduction mechanism in corpus cavernosal smooth muscle cells

In this study, we investigated the effect of dopamine receptor agonists on potassium channels' activity and their signal transduction pathway in corporal smooth muscle cells. We used cultured human corporal smooth muscle cells. The whole cell and cell-attached configuration of the patch-clamp technique were used for electrophysiological recordings, and enzyme immunoassay was used for measuring cyclic AMP (cAMP) and cyclic GMP levels. Extracellular application of 10 microM dopamine and apomorphine significantly increased whole-cell K(+) currents by 283.5+/-55.7% (at +60 mV; n=12, P<0.001), 292.4+/-58.8.0% (at +60 mV; n=9, P<0.005), respectively. We confirmed that the increase in whole-cell currents was mainly due to activation of the tetraethylammonium-sensitive large conductance Ca(2+)-activated K(+) channels (BK(Ca) channels). Enzyme immunoassay indicated that dopamine and apomorphine stimulates cAMP levels in corporal smooth muscle cells in a concentration-dependent fashion. The activation of BK(Ca) channels by dopamine receptor agonists in corporal smooth muscle cells might be one of the mechanisms in inducing penile erection.

Peripheral Modulation of Dopaminergic Receptors Affects Erectile Responses in Rats

Clinical observations have suggested that dopaminergic mechanisms are involved in the regulation of male sexual responses, including erection. Apomorphine was initially reported to exert its erectogenic effects by modifying central dopaminergic activity. This study aimed primarily at evaluating and investigating the effect of apomorphine on erectile functions in rats and its potential effects on the cardiovascular system, as well as the possible role of dopaminergic stimulation in the peripheral control of erection. Measurement of intracavernosal pressure changes elicited by electrical stimulation of the cavernosal nerve in anaesthetized rats and mating tests were used. SCH23390, the D1 receptor antagonist, attenuated penile response to electrical stimulation. Intravenous administration of apomorphine in low dose (100 microg/kg), but not in high dose, significantly potentiated erectile responses to electrical stimulation. Intracavernosally injected apomorphine (50 microg/kg) significantly potentiated the filling rate of the corpora cavernosa 5 min. after injection, and did not induce erection in absence of electrical stimulation. In addition, apomorphine amplified the male sexual and copulatory behaviour by reducing ejaculation, mount and intromission latencies, and significantly increasing the number of ejaculations per session. However, apomorphine produced rapid and long-lasting hypotension and potentiated the hypotension and tachycardia associated with nerve-stimulated penile erection. Our results suggest that D1-dopaminergic receptors may be functionally involved in the peripheral mediation of penile erection. Apomorphine may amplify sexual and copulatory behaviour and may also, by a complementary role, amplify neurogenically mediated erections by acting in the periphery.

Novel identification of peripheral dopaminergic D2 receptor in male germ cells

Dopamine is a recognized modulator in the central nervous system (CNS) and peripheral organ functions. The presence of peripheral dopamine receptors outside the CNS has suggested an intriguing interaction between the nervous system and other functional systems, such as the reproductive system. In the present study we analyzed the expression of D2R receptors in rat testis, rat spermatogenic cells and spermatozoa, in different mammals. The RT-PCR analysis of rat testis mRNA showed specific bands corresponding to the two dopamine receptor D2R (L and S) isoforms previously described in the brain. Using Western blot analysis, we confirmed that the protein is present in rat testis, isolated spermatogenic cells and also in spermatozoa of a range of different mammals, such as rat, mouse, bull, and human. The immunohistochemistry analysis of rat adult testis showed that the receptor was expressed in all germ cells (pre- and post-meiotic phase) of the tubule with staining predominant in spermatogonia. Confocal analysis by indirect immunofluorescence revealed that in non-capacitated spermatozoa of rat, mouse, bull, and human, D2R is mainly localized in the flagellum, and is also observed in the acrosomal region of the sperm head (except in human spermatozoa). Our findings demonstrate that the two D2 receptor isoforms are expressed in rat testis and that the receptor protein is present in different mammalian spermatozoa. The presence of D2R receptors in male germ cells implies new and unsuspected roles for dopamine signaling in testicular and sperm physiology.

Effects in vitro and in vivo by apomorphine in the rat corpus cavernosum

The study was performed to clarify if apomorphine at the level of the rat corpus cavernosum can produce erectile responses or interfere with nerve-induced penile erection. Apomorphine (10(-9)-10(-4) M) exhibited a 10-fold higher potency to relax phenylephrine (Phe)- than endothelin-1 (ET-1)-induced contractions. Relaxant effects of apomorphine in Phe-activated corpus cavernosum did not change tissue levels of cyclic nucleotides, and were unaffected by inhibition of the synthesis of nitric oxide, or by inhibition of the soluble guanylate cyclase. Relaxations by apomorphine of ET-1-contracted rat corpus cavernosum were not influenced by alpha2-adrenoceptor blockade (yohimbine, 10(-7) M), or by the dopamine D1-like receptor antagonist SCH 23390 (10(-6) M). Clozapine (10(-6) M), a proposed dopamine D2-like receptor antagonist, partly reduced apomorphine-induced relaxations, and significantly altered the -log IC50 value for apomorphine. Nerve-induced contractions of the rat corpus cavernosum were attenuated by apomorphine in a concentration-dependent and biphasic manner. Yohimbine (10(-7) M) abolished the biphasic concentration-response pattern. SCH 23390 (10(-6) M) attenuated the inhibitory effects of apomorphine on contractions, and significantly altered the -log IC50 value for the compound. In anesthetized rats (50 mg kg(-1) pentobarbital sodium, 10 mg kg(-1) ketamine), intracavernous apomorphine (100, 300, or 1000 nmol) did not have effects on basal cavernous pressure under resting conditions, and did not affect filling or emptying rates, or peak pressures of the rat corpus cavernosum during submaximal activation of the cavernous nerve. In awake rats, apomorphine produced a maximal number of erections at 300 nmol kg(-1). In the rat isolated corpus cavernosum, pre- and postjunctional effects of apomorphine appear to involve dopamine D1- and D2-like receptors, as well as alpha-adrenoceptors. At relevant systemic doses of apomorphine, peripheral effects of the compound are unlikely to contribute to its proerectile effects in rats.

Dopamine D5 receptor modulates male and female sexual behavior in mice

RATIONALE

Dopamine exerts its actions through at least five receptor (DAR) isoforms. In female rats, D5 DAR may be involved in expression of sexual behavior. We used a D5 knockout (D5KO) mouse to assess the role of D5 DAR in mouse sexual behavior. Both sexes of D5KO mice are fertile and exhibit only minor disruptions in exploratory locomotion, startle, and prepulse inhibition responses.

OBJECTIVE

This study was conducted to characterize the sexual behavior of male and female D5KO mice relative to their WT littermates.

METHODS

Female WT and D5KO littermates were ovariectomized and given a series of sexual behavior tests after treatment with estradiol benzoate (EB) and progesterone (P). Once sexual performance was optimal the dopamine agonist, apomorphine (APO), was substituted for P. Male mice were observed in pair- and trio- sexual behavior tests. To assess whether the D5 DAR is involved in rewarding aspects of sexual behavior, WT and D5KO male mice were tested for conditioned place preference.

RESULTS

Both WT and D5KO females can display receptivity after treatment with EB and P, but APO was only able to facilitate receptivity in EB-primed WT, not in D5KO, mice. Male D5KO mice display normal masculine sexual behavior in mating tests. In conditioned preference tests, WT males formed a conditioned preference for context associated with either intromissions alone or ejaculation as the unconditioned stimulus. In contrast, D5KO males only showed a place preference when ejaculation was paired with the context.

CONCLUSIONS

In females, the D5 DAR is essential for the actions of dopamine on receptivity. In males, D5 DAR influences rewarding aspects of intromissions. Taken together, the work suggests that the D5 receptor mediates dopamine's action on sexual behavior in both sexes, perhaps via a reward pathway.

Of mice and missing data: what we know (and need to learn) about male sexual behavior

With recent advances in molecular genetics, the popularity of mice as subjects for behavioral neuroscience is increasing at an exponential rate. Unfortunately, the existing body of knowledge on sexual behavior in male mice is not large and many basic gaps exist. The assumption that what is true of rats is also true of mice is a dangerous one that can misdirect and, in the worst case, impede progress. We summarize the current knowledge about the sexual behavior of male mice, with an emphasis on hormonal bases of these behaviors. Behavioral differences between strains, developmental actions of steroids, activational actions of steroids given peripherally and in the brain, and data generated in various receptor knockout and related mice are discussed. In addition, suggestions are made for the standardization of experimental protocols used in investigations of the sexual physiology and behavior of male mice in order to facilitate between-experiment and between-laboratory comparisons and to expedite the growth of knowledge in this area.