Melanocortin Receptors and Erectile Function

European Urology

European Urology 45 (2004) 706–713

Review

Melanocortin Receptors and Erectile Function William J. Martin*, D. Euan MacIntyre Department of Pharmacology, Merck Research Laboratories, P.O. Box 2000, Rahway, NJ, USA Accepted 6 January 2004 Available online 28 January 2004

Abstract Objective: Review the historical and current evidence that suggests that activation of melanocortin receptors modulates erectile activity. Methods: The available literature was reviewed. Results: Melanocortin peptides derived from the pro-opiomelanocortin (POMC) precursor protein exert a host of diverse physiological effects in the periphery and in the CNS through interactions with one or more of the five cloned melanocortin receptors. Natural and synthetic melanocortin peptide agonists influence erectile and sexual function in a range of preclinical species. Emerging clinical evidence now suggests that the proerectile effects observed in preclinical species are evident in man as well. Conclusions: Preclinical and clinical results support the involvement of melanocortins in the modulation of erectile and sexual function. Current evidence indicates that the melanocortin 4 receptor subtype contributes to the proerectile effects observed with pan-receptor agonists. However, the putative receptor subtypes, pathways and mechanisms implicated in mediating the proerectile effects of melanocortins remain to be fully elucidated. # 2004 Elsevier B.V. All rights reserved. Keywords: Erectile dysfunction; MT-II; MC4 receptor; Oxytocin; Pharmacotherapy; Central regulation

1. Introduction In the 1950s and 1960s, Ferrari and colleagues showed that direct central administration of alphamelanocyte stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) induce sexual excitement in a range of experimental species including dogs, rabbits, monkeys, and cats (see [1] for review). Over the ensuing decades, investigators determined that alpha-MSH and ACTH derive from the pro-opiomelanocortin (POMC) gene and that the behavioral effects of these peptides are attributable to actions at one or more of the five cloned melanocortin receptors (MC15R; see Table 1) [2]. However, the link between proerectile activity in preclinical species and erectogenesis in man emerged only after dermatologist Norman Levine noted that men receiving an experimental medicine designed to cause tanning (Melanotan II or *

Corresponding author. Fax: 732.594.3841. E-mail address: [email protected] (W.J. Martin).

MT-II) presented with unexpected erections [3]. The observation of enhanced erectile activity led to the formal study of MT-II in men with erectile dysfunction.

2. Human clinical studies with pan-melanocortin agonists MT-II, a cyclic peptide analog of alpha-MSH, exhibits agonist activity at 4 of the 5 known melanocortin receptors, namely MC1R, MC3R, MC4R and MC5R (Table 1) [4]. In a double-blind crossover study, Wessells and colleagues demonstrated that, compared to placebo controls, subcutaneous administration of MT-II significantly increased the number of erectile events in men with psychogenic erectile dysfunction (ED) [5]. Erectile activity was quantitated by measuring RigiScan activity. Eight of ten men achieved a tip rigidity of greater than 80% which lasted, on average, for 38 minutes at doses as low as 0.025 mg/kg. The mean time of onset to the first erection was just over

0302-2838/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.eururo.2003.03.001

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713

707

Table 1 Pharmacological tools used to study the contribution of melanocortin receptors to erectile function Ligand

Type

MCR binding profilea

Reference

Alpha-MSH ACTH (1–24) MT-II PT-141 THIQ SHU 9119

endogenous peptide agonist endogenous peptide agonist synthetic peptide agonist synthetic peptide agonist synthetic small molecule agonist synthetic peptide agonist

MC1R, MC1R, MC1R, MC1R, MC4R MC1R,

[55] [56] [4] [9] [15] [56]

SHU 9119 HS014 AgRP (87–132) MBP10

synthetic peptide antagonist synthetic peptide antagonist endogenous peptide antagonist synthetic peptide antagonist

MC3R, MC4R MC1R, MC3R, MC4R, MC5R MC3R, MC4R MC4R

a

MC3R, MC2R, MC3R, MC3R,

MC4R, MC5R MC3R MC4R, MC5R MC4R, MC5R MC4R

MC5R

[56] [22] [35] [36]

MCR binding profile based on a minimum of >100-fold selectivity over other subtypes.

two hours (range 15–270 minutes). Results from questionnaires corroborated the objective measurements of increased rigidity, i.e. patients subjectively reported enhanced erectile activity. A study of MT-II in men with organic ED confirmed the observations reported for men with psychogenic ED. In this group of ten patients with organic ED, whose risk factors included hypercholesterolemia, obesity and hypertension, nine responded to at least one dose of MT-II [6]. By comparison, erectogenesis was observed in only one of ten patients who received placebo. The MT-II responders reported a mean rigidity score of 6.9 (on a scale of 0 to 10) and achieved a tip rigidity of greater than 80% for an average of 45 minutes. The most commonly reported side effects associated with MT-II administration were yawning, decreased appetite and nausea. The nausea was considered severe in 15% of the patients and was accompanied by vomiting in one patient [7]. In only one of the two studies (organic ED) [6] did patients report increased sexual desire according to the international index of erectile function (IIEF) questionnaire [8]. Whether this effect is an inherent feature of melanocortin receptor activation or is secondary to the presence of potent, long-lasting erections in men who have ED remains to be determined. From a clinical perspective, the potential utility of MT-II is limited by its route of administration (subcutaneous) and onset of action (>90 min). PT-141, a novel synthetic analog of alpha-MSH, may overcome these limitations [9]. Intranasal administration of PT141 (20 mg) reaches peak concentrations in plasma after approximately 30 minutes which corresponds to its onset of proerectile activity (range 34–63 minutes) in healthy volunteers. In 24 patients with mild to moderate erectile dysfunction (clinical diagnosis >6 months), PT-141 dose-dependently increased the duration and extent of penile rigidity in the absence of any reported side effects. Unlike the previous studies with

MT-II, the proerectile effects of PT-141 were evaluated during visual sexual stimulation. The increased erectile activity was mostly confined to the period of visual sexual stimulation which suggests that, at least based on this limited study, PT-141 facilitated rather than initiated erectile activity in the patients with ED. However, the design of the study in which two videos were shown 10–40 and 70–100 min after drug administration, within the time frame of erectogenesis in healthy volunteers, and the presence of tonic mechanical stimulation from the RigiScan transducers makes it difficult to interpret whether PT-141 initiated or facilitated erectile activity.

3. Melanocortin-induced erectogenesis in preclinical species In one of the first systematic investigations of the mechanisms that underlie melanocortin-induced erectogenesis, Ferrari and colleagues demonstrated that intracerebroventricular injections of ACTH-like peptides caused yawning, stretching and penile erections in rabbits [10]. These investigators dissociated the effects of the melanocortin peptides on penile erection and stretching/yawning and further showed that these proerectile effects were mediated by cAMP, were specific for ACTH and alpha-MSH, and only occurred in the presence of testosterone. They also suggested that ACTH and alpha-MSH directly target certain brain regions that regulate sexual activity and, in this way, act independently of the adrenal gland. These observations and the hypotheses that emerged from them set the stage for subsequent mechanistic studies on the behavioral effects of melanocortin peptides. The unraveling of the biological actions of melanocortins underwent a resurgence following the cloning of the five melanocortin receptors in the 1990s (see [2] for review).

708

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713

The discovery of the five distinct melanocortin receptors permitted the confirmation that these receptors signal via the cAMP transduction pathway and revealed the subtypes of melanocortin receptors activated by ACTH (MC1R, -2R, -3R, -4R, -5R) and alphaMSH (all except MC2R) (see Table 1). More importantly, the identification of the melanocortin receptor subtypes enabled the localization of each subtype in specific brain regions, facilitated the identification of selective agonists and antagonists and led to the generation of mutant mice which lacked one or more subtype. Collectively, receptor expression studies, the discovery of novel pharmacological tools and use of receptor knockout mice provided a framework within which to investigate the neural pathways and receptors involved in melanocortin-induced modulation of erectile activity.

4. Receptor subtype Since ACTH, alpha-MSH, MT-II and PT-141 do not discriminate amongst MC1R, MC3R, MC4R and MC5R, the melanocortin receptor subtype(s) through which these peptides produce erectogenic effects is unknown. Only MC3R and MC4R are expressed in regions known to participate in the modulation of erectile function [11–13]. In anesthetized rabbits, SHU 9119, an antagonist to MC3R and MC4R, abolished MT-II-induced increases in intracavernosal pressure [14]. To further distinguish the contribution of MC3R and MC4R to erectogenesis, we carried out a series of studies targeting the MC4R using rodent models of erectile function. Using a mouse model in which increased intracavernosal pressure (ICP) serves as an index of erectile activity we tested a selective MC4R agonist, with and without electrical stimulation of the cavernous nerve, to determine whether selective activation of MC4R promotes erectile activity. We used a tetrahydroisoquinoline (THIQ) MC4R agonist which is a full, high affinity agonist for MC4R (mouse Mc4r EC50 ¼ 0:3 nM) and is >500-fold selective for MC4R in binding assays (>1000-fold selective in functional assays) compared to other MCR subtypes (see Table 2) [15]. THIQ enhanced electrically evoked increases in ICP in wildtype mice but not in Mc4r null mice [16]. In a behavioral model of sexual function, the absence of Mc4r led to diminished copulatory behavior (mounting and intromission latency); whereas activation of MC4R agonist improved sexual function in wild type mice along both motivation (latency to begin mounting behavior) and performance (latency to first ejaculation)

Table 2 Affinity and selectivity of melanocortin agonists and antagonists at human MC4R Ligand

Potency (Ki)

Selectivity ratio (MC3R/MC4R affinity)

Reference

Alpha-MSH ACTH (1–24) MT-II PT-141 THIQ SHU 9119 HS014 AgRP (87–132) MBP10

900 nM 755 nM 6.6 nM 10.0 nM 1.2 nM 0.4 nM 3.2 nM 2.6 nM 6.2 nM

0.04 0.04 5.2 10?a 634 3.3 17.2 1.3 125

[56] [55] [56] [9] [15] [56] [22] [35] [36]

a

No Ki reported for PT-141 against MC3R.

parameters. Thus, MC4R is an important determinant of erectile activity and copulatory behavior in mice. We sought to further characterize the involvement of MC4R in erectogenesis using a rat model in which the penile sheath is retracted to apply tonic pressure at the base of the glans [17]. These touch-evoked penile erections, which occur outwith the context of copulation (ex copula), are associated with, and presumably mediated by changes in intracavernosal pressure. In addition to counting the number of penile erections, we recorded telemetrically intracavernosal pressure to quantitatively assess erectile function [18,19]. The MC4R agonist THIQ did not reduce the time to observe significant increases in intracavernous pressure or the number of transient rises in intracavernous pressure (<15 s duration). However, THIQ tended to increase the magnitude of these responses as well as the number of episodes in which intracavernous pressure remained above a pre-determined threshold for more than 15 s (‘intervals’), a requirement for a penile erection. The duration of each interval recorded following THIQ treatment was significantly longer than that observed after vehicle administration. In agreement with our findings with intracavernous pressure, THIQ consistently increased the total number of penile erections, scored visually in dorsally recumbent animals following retraction of the preputial sheath, but did not affect time of onset to the first erection. This profile was distinct from other centrally-acting erectogenic agents like apomorphine [20] which decreased the latency to the first erection and increased the number of erections, and from the PDE5 inhibitor sildenafil which affected neither the onset nor the total number of erections. These findings in mouse and rat, coupled with the fact that the affinities of the clinically effective MT-II and PT-141 are highest for MC4R (see Table 2) [9,21], clearly illustrate the importance of MC4R in erectile activity. However, results from other preclinical studies

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713

implicate MC3R, not MC4R, in the proerectile effects of non-subtype selective melanocortin agonists. For example, several studies found that administration of a melanocortin MC4R preferring antagonist [22], HS014, failed to block penile erections produced by ACTH and alpha-MSH [23,24] or influence sexual behavior in male rats [25]. It is difficult to reconcile these conflicting results. Clearly, the work with the Mc4r null mutant mouse [16] argues that activation of the melanocortin MC4R is required for the proerectile effects of melanocortins. It is possible that incomplete blockade of MC4R by HS014 accounts for the failure to block penile erections in rat. Moreover, we cannot exclude the possibility that concomitant activation of MC3R, MC4R and MC5R increases erectile activity more than activation of the MC4R subtype alone.

5. Sites of action The unique profile of melanocortin-induced erectogenesis suggests that the sites of action may be distinct from other centrally and peripherally acting erectogenic agents. The generation and modulation of penile erection requires the integration of information of peripheral, supraspinal and spinal origin. Receptor localization studies and targeted pharmacological approaches indicate that melanocortin receptors in the periphery as well as in the brain and spinal cord likely contribute to the proerectile effects of melanocortin agonists. 5.1. Peripheral Studies in vivo indicate that MC4R agonists (THIQ) and nonselective MCR agonists (alpha-MSH) elicit changes in intracavernosal pressure in rodents [26,27]; however, neither THIQ (rat) nor MT-II (rabbit) directly relax corpus cavernosum strips [14,16] and direct intracavernosal injection of MT-II fails to alter intracavernosal pressure in rats [28]. The absence of direct relaxant effects in isolated or intact erectile tissue preparations prompted a series of anatomical studies to determine if any of the proerectile effects of melanocortins could be ascribed to a peripheral locus of action and, if so, by what mechanism. Peripheral (IV) administration to rats of [125]I-MT-II, which has a similar pharmacological profile to MT-II, displays extremely limited brain penetration and predominantly labels circumventricular organs [29]. Membrane binding studies with [125]I-MT-II in rat provided evidence that MCR are localized to the penis and suggested a locus of action not previously postu-

709

lated for MCR [16]. Inhibition of the specific binding of 125I-MTII by an MC4R agonist indicates that MC4R appears to be the predominant melanocortin receptor subtype expressed in the penis. Consistent with the functional studies described above, transcripts encoding MC4R were not detected in primary corpus cavernosum cells which suggested that smooth muscle cells were not the source for MCR expression in rat penis. However, MC4R mRNA was detected in the penis and pelvic ganglion, a major autonomic relay center to the penis. In situ hybridization and immunohistochemistry further revealed that MC4R was localized to nerve fibers in corpus and glans of human penis. In the glans, MC4R mRNA was detected in sensory receptors [16] that resembled clusters of free nerve endings characteristic of end bulbs of Krause, as well as Ruffini-type nerve endings [30]. Localization of MC4R to Ruffini nerve endings, a type of cutaneous mechanoreceptor readily excited by dermal stretch [31], suggests a role for melanocortins in the modulation of glandular sensitivity. Though MC4R was associated with neuronal processes and peripheral nerves, hybridization signals did not co-localize with nitric oxide synthase. Collectively these data indicate that MCR modulation of erectile activity in the periphery is not mediated via a direct action on cavernosal smooth muscle, but rather occurs through activation of peripheral proerectile pathways. The neurochemistry of the nerve fibers that express MC4R remains to be determined and the peripheral pathways within which MCR agonists modulate erectogenic activity await further investigation. 5.2. Supraspinal While the contribution of peripheral erectogenic pathways to the actions of melanocortins is provocative, converging lines of evidence underscore the importance of central pathways within the brain and spinal cord in melanocortin-induced erectogenesis. Far more is known about the suprapsinal effects of melanocortins than their spinal actions. In rats, Vergoni and colleagues showed that intracerebral ventricular (ICV) injections of ACTH and alpha-MSH recapitulate the stretching, yawning and penile erection syndrome first described by Ferrari and colleagues [23]. Indeed, binding studies using a radiolabeled analog of alpha-MSH ([125I]NDP-MSH) illustrated the potential for multiple sites of action for melanocortins in the brain [32]. Receptor subtype specific studies revealed that only MC3R [11] and MC4R [13] are expressed in CNS regions known to participate in the modulation of erectile function [12]. Of these regions, MCR expression was rich in the hypothalamus where MC4R expression was particularly dense [33].

710

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713

Consistent with MCR localization, microinjections of ACTH and alpha-MSH into specific hypothalamic regions, including the paraventricular nucleus (PVN), reproduce the behavioral syndrome observed with intracerebroventricular administration [24]. That the hypothalamus is one potential locus of action of melanocortin peptides is supported by the finding that intranasal administration of proerectile doses of PT141 induces immediate early gene expression (Fos), a measure of neuronal activity, in the PVN and supraoptic nucleus (SON) of the hypothalamus [9]. Furthermore, track tracing studies in which pseudorabies virus is injected directly into the corpus cavernosum demonstrated transsynaptic labeling of neurons in the PVN [34]. ICV administration of MT-II or PT-141 elicited erections in rats at 100–1000-fold lower doses than those required to produce erectogenesis when given systemically [9,28]. Mizusawa and co-workers showed that direct injection of another pan-MCR agonist, alphaMSH (3 mg), elevated intracavernosal pressure (ICP) in anesthetized rats and that these proerectile effects, which lasted approximately 3 minutes, were blocked by pre-administration of 100 mg L-NG-nitro-Arginine Methyl Ester (L-NAME), indicating that the supraspinally-mediated erectogenic effects of melanocortin agonists may require nitric oxide [27]. Interestingly, ICV administration of the dual MC3/4R antagonist SHU-9119 inhibited penile erections at doses of 300 ng [28]. Thus, tonic activation of MC3R and/or MC4R may confer a level of basal erectile activity. We showed that ICV injection of the endogenous nonsubtype specific melanocortin receptor antagonist, agouti-related protein (AgRP) [35] or of a melanocortin MC4R preferring antagonist (MBP10) [36] blocked the penile erections induced by intravenous administration of the MC4R selective agonist THIQ. These results illustrate that melanocortin receptors in brain can modulate penile erection and further implicate the MC4R subtype in erectogenesis. 5.3. Spinal The spinal cord as a locus of erectogenesis is a relatively new concept [37,38]. Consequently, much less is known about the effects of melanocortins on penile erection at the level of the spinal cord. Wessells and co-workers showed that intrathecal administration of MT-II (1 mg) elicited more erections than an equal dose given directly into the lateral ventricles [28]. The proerectile effects of MT-II were blocked by intrathecal administration of the dual MC3/4R antagonist, SHU-9119. By contrast, Mizusawa and colleagues found that intrathecal injection of alpha-MSH (3 mg)

did not affect intracavernosal pressure in anesthetized rats [27], an effect that might be attributable to the 10–100-fold lower affinity for MC4R compared with MT-II. Considerably more work will be required to better understand that contribution of melanocortin receptors and receptor subtypes to erectogenesis at the level of the spinal cord.

6. Mechanisms of melanocortin receptor-mediated erectogenesis The ability of MCR agonists to modulate erectile activity at peripheral, supraspinal and spinal levels raises several questions with respect to potential interactions of this receptor system with the biochemical and pharmacological pathways that control erectile activity. For example, we know that peripherally-acting agents such as sildenafil (as well as vardenafil and tadalafil) enhance erectile function by modulating the nitric oxide (NO)-cGMP pathway [39] and that neuronal nitric oxide synthase (nNOS) is a critical isoform for the pro-erectile effects of PDE5 inhibitors [40,41]; yet the involvement of the peripheral NO-cGMP pathway or any other peripheral mediators in MCR-induced erectogenesis is unknown. By contrast, several studies have attempted to define the involvement of melanocortins in the central mechanisms of penile erection such as the aforementioned role of central nitric oxide in alpha-MSH-induced proerectile activity [27] (see above). However, most studies have focused primarily on the role of oxytocin. 6.1. Role of oxytocin Oxytocin plays an essential role in the expression of penile erection and is itself a viable target for the treatment of erectile dysfunction [42]. Oxytocin receptor antagonists inhibit not only penile erections induced by oxytocin but also those induced by sexual stimuli or other centrally acting proerectile agents (see [43] for review). The presence of melanocortin receptors in hypothalamic regions [44] where oxytocin is produced [45] raises the possibility that oxytocin release is important for the expression of melanocortin-induced erectogenesis. To this end, Mizusawa and colleagues reported that ICV administration of alphaMSH induced penile erections and increased intracavernous pressure in anesthetized rats, but that these effects were not blocked by an oxytocin antagonist, l-deamino, 2-D-Tyr(Oet), 4-Thr, 8-Orn-OT [27]. Similarly, penile erections produced by ACTH administration were not reduced by a potent oxytocin antagonist (d(CH2)5-Tyr(Me)-[Orn8]vasotocin which effectively

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713

blocked the penile erections produced by apomorphine [46]. In contrast to these findings, we found that systemic and central administration of an oxytocin receptor antagonist, L-368899, significantly reduced the number of penile erections produced by a systemically administered MC4R agonist [26]. The use of a MC4R selective agonist vs. alpha-MSH and/or the use of different oxytocin antagonists could account for the contradictory findings. These discrepancies notwithstanding, the finding that ICV administration of an erectogenic dose of MT-II (1 mg) increases the number of penile erections [28] and induces Fos expression in hypothalamic nuclei [29] from which oxytocin is released [47] suggests that oxytocinergic mechanisms may participate in melanocortin receptor-mediated erectogenesis at supraspinal levels. Oxytocin [48,49] and melanocortins [28] also modulate erectile activity at the level of the spinal cord, but to our knowledge the interactions between these two systems have not been studied at this level. 6.2. Dopaminergic system Apomorphine, a dopaminergic agonist, is currently the only centrally acting agent approved for the treatment of ED [50]. In contrast to oxytocin, no studies have directly assessed the role of the dopaminergic system in melanocortin-induced erectogenesis. Therefore, inferences about the potential differences and similarities between these two pharmacological systems must be drawn from indirect comparisons. One potential point of commonality of apomorphine and melanocortins derives from their sites of action. For example, apomorphine [51,52] and ACTH [24] both elicit penile erection when injected directly into the PVN. However, lesioning experiments suggest that the PVN is required only for the expression of penile erection induced by apomorphine (and oxytocin), not that triggered by ACTH [53]. Apomorphine and melanocortin agonists may exhibit a similar degree of overlap, with subtle distinction, at the level of the spinal cord. Specifically, apomorphine [54] and MTII [28] each increase the number of penile erections when injected intrathecally. Comparatively, however, apomorphine is more efficacious after systemic or ICV administration [54]. By contrast, MT-II induces significantly more penile erections following spinal administration than it does after i.c.v. administration [28] though it is unclear if this finding broadly applies to all melanocortins since alpha-MSH apparently increases ICP more effectively after ICV administration [27]. Taken together, the available data suggest that apomorphine and melanocortin agonists each promote erectile activity. However, at least in animal

711

studies, the profile of this effect with respect to onset of penile erection differs between apomorphine (reduces latency) and MCR agonists (no effect on latency). Therefore, although these two systems may modulate erectile activity via actions within overlapping central pathways, the relative contributions of brain and spinal cord and the specific mechanisms (e.g. oxytocin, see above) through which pro-erectile activity is induced may be distinct and will require further study.

7. Summary and conclusion The pan-melanocortin receptor agonists, MT-II and PT-141, augment erectile activity in men with and without erectile dysfunction. These clinical findings are consistent with the proerectile effects of melanocortins in animals. While many questions remain with respect to sites and mechanisms of action of melanocortin agonists, the evidence reviewed herein favors the hypothesis that activation of melanocortin receptors in the periphery, in the brain and in the spinal cord participate in the proerectile effects of melanocortins. Furthermore, activation of MC4R, and the ensuing sequelae of events, appears sufficient for modulating erectile activity. In animal species, melanocortins do not directly relax cavernosal smooth muscle, directly augment intracavernosal pressure or reduce the latency to touch-evoked reflexogenic penile erections. These findings suggest that melanocortins facilitate erectile activity in response to sexual stimulation rather than induce it in the absence of stimulation. The difference between the number of erections elicited following systemic administration of melanocortins in freely behaving rats (1–2 erections more than vehicle-treated rats over 30 min time period) [9,28] compared to rats in which the retracted preputial sheath applies tonic pressure on the base of the penis (10–20 erections more than vehicletreated rats over 15 min time period) [26] exemplifies the facilitatory effect of melanocortins. Whether melanocortins initiate or facilitate erectile activity in men is complicated by the fact that the clinical results reported thus far with PT-141 and MT-II were obtained in the presence of ongoing tactile input provided by the RigiScan device. Beyond tactile stimuli, this strict dichotomy may also be more difficult to draw in men in part because visual, olfactory and imaginative stimuli themselves may provide a degree of ongoing arousal that is subject to amplification by pharmacological agents acting within proerectile pathways.

712

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713

References [1] Bertolini A, Gessa GL. Behavioral effects of ACTH and MSH peptides. J Endocrinol Invest 1981;4(2):241–51. [2] Adan RA, Gispen WH. Melanocortins and the brain: from effects via receptors to drug targets. Eur J Pharmacol 2000;405(1–3):13– 24. [3] Gura T. Having it all. Science 2003;299(5608):850. [4] Hruby VJ, Lu D, Sharma SD, Castrucci AL, Kesterson RA, al-Obeidi FA, et al. Cyclic lactam alpha-melanotropin analogues of Ac-Nle4cyclo[Asp5, D-Phe7,Lys10] alpha-melanocyte-stimulating hormone(4–10)-NH2 with bulky aromatic amino acids at position 7 show high antagonist potency and selectivity at specific melanocortin receptors. J Med Chem 1995;38(18):3454–61. [5] Wessells H, Fuciarelli K, Hansen J, Hadley ME, Hruby VJ, Dorr R, et al. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction: double-blind, placebo controlled crossover study. J Urol 1998;160(2):389–93. [6] Wessells H, Gralnek D, Dorr R, Hruby VJ, Hadley ME, Levine N. Effect of an alpha-melanocyte stimulating hormone analog on penile erection and sexual desire in men with organic erectile dysfunction [In Process Citation]. Urology 2000;56(4):641–6. [7] Wessells H, Levine N, Hadley ME, Dorr R, Hruby V. Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with melanotan II [In Process Citation]. Int J Impot Res 2000;12(Suppl 4):S74–79. [8] Rosen RC, Riley A, Wagner G, Osterloh IH, Kirkpatrick J, Mishra A. The International Index of Erectile Function (IIEF): a multidimensional scale for assessment of erectile dysfunction. Urology 1997;49(6):822–30. [9] Molinoff PB, Shadiack AM, Earle D, Diamond LE, Quon CY. PT141: a melanocortin agonist for the treatment of sexual dysfunction. Ann N Y Acad Sci 2003;994:96–102. [10] Bertolini A, Vergoni W, Gessa GL, Ferrari W. Induction of sexual excitement by the action of adrenocorticotrophic hormone in brain. Nature 1969;221(181):667–9. [11] Lindblom J, Schioth HB, Larsson A, Wikberg JE, Bergstrom L. Autoradiographic discrimination of melanocortin receptors indicates that the MC3 subtype dominates in the medial rat brain. Brain Res 1998;810(1–2):161–71. [12] McKenna KE. Some proposals regarding the organization of the central nervous system control of penile erection. Neurosci Biobehav Rev 2000;24(5):535–40. [13] Gantz I, Miwa H, Konda Y, Shimoto Y, Tashiro T, Watson SJ, et al. Molecular cloning, expression, and gene localization of a fourth melanocortin receptor. J Biol Chem 1993;268(20):15174– 9. [14] Vemulapalli R, Kurowski S, Salisbury B, Parker E, Davis H. Activation of central melanocortin receptors by MT-II increases cavernosal pressure in rabbits by the neuronal release of NO. Br J Pharmacol 2001;134(8):1705–10. [15] Sebhat IK, Martin WJ, Ye Z, Barakat K, Mosley RT, Johnston DB, et al. Design and pharmacology of N-[(3R)-1,2,3,4-tetrahydroisoquinolinium- 3-ylcarbonyl]-(1R)-1-(4-chlorobenzyl)- 2-[4-cyclohexyl-4-(1H-1,2,4-triazol-1-ylmethyl)piperidin-1-yl]-2-oxoethylamine (1), a potent, selective, melanocortin subtype-4 receptor agonist. J Med Chem 2002;45(21):4589–93. [16] Van der Ploeg LH, Martin WJ, Howard AD, Nargund RP, Austin CP, Guan X, et al. A role for the melanocortin 4 receptor in sexual function. Proc Natl Acad Sci USA 2002;99(17):11381–6. [17] Sachs BD. Contextual approaches to the physiology and classification of erectile function, erectile dysfunction, and sexual arousal. Neurosci Biobehav Rev 2000;24(5):541–60. [18] Bernabe J, Rampin O, Giuliano F, Benoit G. Intracavernous pressure changes during reflexive penile erections in the rat. Physiol Behav 1995;57(5):837–41.

[19] Bernabe J, Rampin O, Sachs BD, Giuliano F. Intracavernous pressure during erection in rats: an integrative approach based on telemetric recording. Am J Physiol 1999;276(2 Pt 2):R441–449. [20] Andersson KE. Pharmacology of penile erection. Pharmacol Rev 2001;53(3):417–50. [21] Wikberg JE, Muceniece R, Mandrika I, Prusis P, Lindblom J, Post C, et al. New aspects on the melanocortins and their receptors. Pharmacol Res 2000;42(5):393–420. [22] Schioth HB, Mutulis F, Muceniece R, Prusis P, Wikberg JE. Discovery of novel melanocortin4 receptor selective MSH analogues. Br J Pharmacol 1998;124(1):75–82. [23] Vergoni AV, Bertolini A, Mutulis F, Wikberg JE, Schioth HB. Differential influence of a selective melanocortin MC4 receptor antagonist (HS014) on melanocortin-induced behavioral effects in rats. Eur J Pharmacol 1998;362(2–3):95–101. [24] Argiolas A, Melis MR, Murgia S, Schioth HB. ACTH- and alphaMSH-induced grooming, stretching, yawning and penile erection in male rats: site of action in the brain and role of melanocortin receptors. Brain Res Bull 2000;51(5):425–31. [25] Vergoni AV, Bertolini A, Guidetti G, Karefilakis V, Filaferro M, Wikberg JE, et al. Chronic melanocortin 4 receptor blockage causes obesity without influencing sexual behavior in male rats. J Endocrinol 2000;166(2):419–26. [26] Martin WJ, McGowan E, Cashen DE, Gantert LT, Drisko JE, Hom GJ, et al. Activation of melanocortin MC(4) receptors increases erectile activity in rats ex copula. Eur J Pharmacol 2002;454(1):71–9. [27] Mizusawa H, Hedlund P, Andersson KE. alpha-Melanocyte stimulating hormone and oxytocin induced penile erections, and intracavernous pressure increases in the rat. J Urol 2002;167(2 Pt 1): 757–60. [28] Wessells H, Hruby VJ, Hackett J, Han G, Balse-Srinivasan P, Vanderah TW. Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH2 induces penile erection via brain and spinal melanocortin receptors. Neuroscience 2003;118(3):755–62. [29] Trivedi P, Jiang M, Tamvakopoulos CC, Shen X, Yu H, Mock S, et al. Exploring the site of anorectic action of peripherally administered synthetic melanocortin peptide MT-II in rats. Brain Res 2003;977(2):221–30. [30] Munger BL, Ide C. The structure and function of cutaneous sensory receptors. Arch Histol Cytol 1988;51(1):1–34. [31] Horch KW, Tuckett RP, Burgess PR. A key to the classification of cutaneous mechanoreceptors. J Invest Dermatol 1977;69(1):75–82. [32] Tatro JB, Entwistle ML. Heterogeneity of brain melanocortin receptors suggested by differential ligand binding in situ. Brain Res 1994;635(1–2):148–58. [33] Mountjoy KG, Mortrud MT, Low MJ, Simerly RB, Cone RD. Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain. Mol Endocrinol 1994;8(10):1298–308. [34] Marson L, Platt KB, McKenna KE. Central nervous system innervation of the penis as revealed by the transneuronal transport of pseudorabies virus. Neuroscience 1993;55(1):263–80. [35] Yang YK, Thompson DA, Dickinson CJ, Wilken J, Barsh GS, Kent SB, et al. Characterization of Agouti-related protein binding to melanocortin receptors. Mol Endocrinol 1999;13(1):148–55. [36] Bednarek MA, MacNeil T, Kalyani RN, Tang R, van der Ploeg LH, Weinberg DH. Selective, high affinity peptide antagonists of alphamelanotropin action at human melanocortin receptor 4: their synthesis and biological evaluation in vitro. J Med Chem 2001;44(22):3665–72. [37] Giuliano F, Rampin O. Central neural regulation of penile erection. Neurosci Biobehav Rev 2000;24(5):517–33. [38] Giuliano F, Allard J, Rampin O, Droupy S, Benoit G, Alexandre L, et al. Spinal proerectile effect of apomorphine in the anesthetized rat. Int J Impot Res 2001;13(2):110–5.

W.J. Martin, D.E. MacIntyre / European Urology 45 (2004) 706–713 [39] Boolell M, Allen MJ, Ballard SA, Gepi-Attee S, Muirhead GJ, Naylor AM, et al. Sildenafil: an orally active type 5 cyclic GMPspecific phosphodiesterase inhibitor for the treatment of penile erectile dysfunction. Int J Impot Res 1996;8(2):47–52. [40] Burnett AL, Nelson RJ, Calvin DC, Liu JX, Demas GE, Klein SL, et al. Nitric oxide-dependent penile erection in mice lacking neuronal nitric oxide synthase. Mol Med 1996;2(3):288–96. [41] Cashen DE, MacIntyre DE, Martin WJ. Effects of sildenafil on erectile activity in mice lacking neuronal or endothelial nitric oxide synthase. Br J Pharmacol 2002;136(5):693–700. [42] Melis MR, Argiolas A. Central oxytocinergic neurotransmission: a drug target for the therapy of psychogenic erectile dysfunction. Curr Drug Targets 2003;4(1):55–66. [43] Argiolas A. Neuropeptides and sexual behaviour. Neurosci Biobehav Rev 1999;23(8):1127–42. [44] Adan RA, Gispen WH. Brain melanocortin receptors: from cloning to function. Peptides 1997;18(8):1279–87. [45] Swaab DF, Pool CW, Nijveldt F. Immunofluorescence of vasopressin and oxytocin in the rat hypothalamo-neurohypophypopseal system. J Neural Transm 1975;36(3–4):195–215. [46] Argiolas A, Melis MR, Vargiu L, Gessa GL. d(CH2)5Tyr(Me)[Orn8]vasotocin, a potent oxytocin antagonist, antagonizes penile erection and yawning induced by oxytocin and apomorphine, but not by ACTH-(1–24). Eur J Pharmacol 1987;134(2):221–4. [47] Sabatier N, Caquineau C, Dayanithi G, Bull P, Douglas AJ, Guan XM, et al. Alpha-melanocyte-stimulating hormone stimulates oxytocin release from the dendrites of hypothalamic neurons while inhibiting oxytocin release from their terminals in the neurohypophysis. J Neurosci 2003;23(32):10351–8. [48] Veronneau-Longueville F, Rampin O, Freund-Mercier MJ, Tang Y, Calas A, Marson L, et al. Oxytocinergic innervation of autonomic

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

713

nuclei controlling penile erection in the rat. Neuroscience 1999;93(4): 1437–47. Giuliano F, Bernabe J, McKenna K, Longueville F, Rampin O. Spinal proerectile effect of oxytocin in anesthetized rats. Am J Physiol Regul Integr Comp Physiol 2001;280(6):R1870–1877. Uckert S, Stief CG, Jonas U. Current and future trends in the oral pharmacotherapy of male erectile dysfunction. Expert Opin Investig Drugs 2003;12(9):1521–33. Melis MR, Argiolas A, Gessa GL. Apomorphine-induced penile erection and yawning: site of action in brain. Brain Res 1987; 415(1):98–104. Chen KK, Chan JY, Chang LS. Dopaminergic neurotransmission at the paraventricular nucleus of hypothalamus in central regulation of penile erection in the rat. J Urol 1999;162(1):237–42. Argiolas A, Melis MR, Mauri A, Gessa GL. Paraventricular nucleus lesion prevents yawning and penile erection induced by apomorphine and oxytocin but not by ACTH in rats. Brain Res 1987;421(1–2): 349–52. Hsieh GC, Hollingsworth PR, Martino B, Chang R, Terranova MA, O’Neill AB, et al. Central Mechanisms Regulating Penile Erection in Conscious Rats: The Dopaminergic Systems Related to the Pro-erectile Effect of Apomorphine. J Pharmacol Exp Ther 2003; 20:20. Schioth HB, Bouifrouri AA, Rudzish R, Muceniece R, Watanobe H, Wikberg JE, et al. Pharmacological comparison of rat and human melanocortin 3 and 4 receptors in vitro. Regul Pept 2002;106(1–3): 7–12. Schioth HB, Muceniece R, Mutulis F, Prusis P, Lindeberg G, Sharma SD, et al. Selectivity of cyclic [D-Nal7] and [D-Phe7] substituted MSH analogues for the melanocortin receptor subtypes. Peptides 1997;18(7):1009–13.