Alpha-melanocyte stimulating hormone induces gonadotropin release

Melanocortin 4 receptor agonism enhances sexual brain processing in women with hypoactive sexual desire disorder

BACKGROUND

Hypoactive sexual desire disorder (HSDD) is characterized by a persistent deficiency of sexual fantasies and desire for sexual activity, causing marked distress and interpersonal difficulty. It is the most prevalent female sexual health problem globally, affecting approximately 10% of women, but has limited treatment options. Melanocortin 4 receptor (MC4R) agonists have emerged as a promising therapy for women with HSDD, through unknown mechanisms. Studying the pathways involved is crucial for our understanding of normal and abnormal sexual behavior.

METHODS

Using psychometric, functional neuroimaging, and hormonal analyses, we conducted a randomized, double-blinded, placebo-controlled, crossover clinical study to assess the effects of MC4R agonism compared with placebo on sexual brain processing in 31 premenopausal heterosexual women with HSDD.

RESULTS

MC4R agonism significantly increased sexual desire for up to 24 hours after administration compared with placebo. During functional neuroimaging, MC4R agonism enhanced cerebellar and supplementary motor area activity and deactivated the secondary somatosensory cortex, specifically in response to visual erotic stimuli, compared with placebo. In addition, MC4R agonism enhanced functional connectivity between the amygdala and the insula during visual erotic stimuli compared with placebo.

CONCLUSION

These data suggest that MC4R agonism enhanced sexual brain processing by reducing self-consciousness, increasing sexual imagery, and sensitizing women with HSDD to erotic stimuli. These findings provide mechanistic insight into the action of MC4R agonism in sexual behavior and are relevant to the ongoing development of HSDD therapies and MC4R agonist development more widely.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04179734.

FUNDING

This is an investigator-sponsored study funded by AMAG Pharmaceuticals Inc., the Medical Research Council (MRC) (MR/T006242/1), and the National Institute for Health Research (NIHR) (CS-2018-18-ST2-002 and RP-2014-05-001).

Metabolic control of teleost reproduction by leptin and its complements: Understanding current insights from mammals

Reproduction is expensive. Hence, reproductive physiology is sensitive to an array of endogenous signals that provide information on metabolic and nutritional sufficiency. Although metabolic gating of reproductive function in mammals, as evidenced by studies demonstrating delayed puberty and perturbed fertility, has long been understood to be a function of energy sufficiency, an understanding of the endocrine regulators of this relationship have emerged only within recent decades. Peripheral signals including leptin and cortisol have long been implicated in the physiological integration of metabolism and reproduction. Recent studies have begun to explore possible roles for these two hormones in the regulation of reproduction in teleost fishes, as well as a role for leptin as a catabolic stress hormone. In this review, we briefly explore the reproductive actions of leptin and cortisol in mammals and teleost fishes and possible role of both hormones as putative modulators of the reproductive axis during stress events.

Gonadal ERα/β, AR and TRPV1 gene expression: modulation by pain and morphine treatment in male and female rats

The results of several studies strongly indicate a bidirectional relationship among gonadal hormones and pain. While gonadal hormones play a key role in pain modulation, they have been found to be affected by pain therapies in different experimental and clinical conditions. However, the effects of pain and pain therapy on the gonads are still not clear. In this study, we determined the long-lasting (72 h) effects of inflammatory pain (formalin test) and/or morphine on estrogen receptor (ER), androgen receptor (AR) and TRPV1 gene expression in the rat testis and ovary. The animals were divided into groups: animals receiving no treatment, animals exposed only to the experimental procedure (control group), animals receiving no pain but morphine (sham/morphine), animals receiving pain and morphine (formalin/morphine), and animals receiving only formalin (formalin/saline). Testosterone (T) and estradiol (E) were determined in the plasma at the end of the testing. In the sham/morphine rats, there were increases of ERα, ERβ, AR and TRPV1 mRNA expression in the ovary; in the testis, ERα and ERβ mRNA expression were reduced while AR and TRPV1 expression were unaffected by treatment. T and E plasma levels were increased in morphine-treated female rats, while T levels were greatly reduced in morphine-treated and formalin-treated males. In conclusion, both testicular and ovarian ER (ERα and ERβ) and ovarian AR and TRPV1 gene expression appear to be affected by morphine treatment, suggesting long-lasting interactions among opioids and gonads.

Kisspeptin directly excites anorexigenic proopiomelanocortin neurons but inhibits orexigenic neuropeptide Y cells by an indirect synaptic mechanism

The neuropeptide kisspeptin is necessary for reproduction, fertility, and puberty. Here, we show strong kisspeptin innervation of hypothalamic anorexigenic proopiomelanocortin (POMC) cells, coupled with a robust direct excitatory response by POMC neurons (n > 200) to kisspeptin, mediated by mechanisms based on activation of a sodium/calcium exchanger and possibly opening of nonselective cation channels. The excitatory actions of kisspeptin on POMC cells were corroborated with quantitative PCR data showing kisspeptin receptor GPR54 expression in the arcuate nucleus, and the attenuation of excitation by the selective kisspeptin receptor antagonist, peptide 234. In contrast, kisspeptin inhibits orexigenic neuropeptide Y (NPY) neurons through an indirect mechanism based on enhancing GABA-mediated inhibitory synaptic tone. In striking contrast, gonadotropin-inhibiting hormone (GnIH and RFRP-3) and NPY, also found in axons abutting POMC cells, inhibit POMC cells and attenuate the kisspeptin excitation by a mechanism based on opening potassium channels. Together, these data suggest that the two central peptides that regulate reproduction, kisspeptin and GnIH, exert a strong direct action on POMC neurons. POMC cells may hypothetically serve as a conditional relay station downstream of kisspeptin and GnIH to signal the availability of energy resources relevant to reproduction.

Activation of melanocortin receptors accelerates the gonadotropin-releasing hormone pulse generator activity in goats

The present study aims to elucidate whether the central melanocortin receptors [melanocortin-3 and -4 receptors (MC3/4-R)] are involved in regulating GnRH pulse generator activity in female goats. The GnRH pulse generator activity was electrophysiologically assessed at the intervals of characteristic increases in multiple-unit activity (MUA volleys) in the mediobasal hypothalamus. In ovariectomized goats, all doses (0.02, 0.2 and 2 nmol) of MT II, an MC3/4-R agonist, injected into the lateral ventricle significantly shortened MUA volley intervals. The duration of the period during which MT II accelerated MUA volleys was positively correlated with the dose of MT II injected. The stimulatory effect of MT II on the GnRH pulse generator activity was attenuated in the presence of estrogen. Intracerebroventricular injection of SHU9119, an MC3/4-R antagonist, significantly prolonged MUA volley intervals at 1 nmol. MT II (0.2 nmol)-induced acceleration of MUA volleys was partially blocked by the antagonism of MC3/4-R with pre-administered SHU9119 (1 nmol). The present findings demonstrate that MC3/4-R are involved in maintaining GnRH pulse generator activity in goats.

Leptin directly acts within the hypothalamus to stimulate gonadotropin-releasing hormone secretion in vivo in rats

It is still not known whether leptin, an adipocyte-derived hormone, acts directly within the hypothalamus to stimulate the gonadotropin-releasing hormone (GnRH)-luteinizing hormone (LH) system. In order to address this question, the present study examined the effects of direct intrahypothalamic perfusions with leptin on the in vivo release of GnRH in ovarian steroid-primed ovariectomized rats utilizing the push-pull perfusion technique. Both alpha-melanocyte-stimulating hormone (alpha-MSH) and neuropeptide Y were also measured in the hypothalamic perfusates. In normally fed animals, the leptin infusion was without effect on the release of these three hypothalamic peptides and also without effect on plasma LH and prolactin (PRL), whether leptin was infused into the medial preoptic area (where the majority of GnRH neuronal cell bodies exist) or the median eminence-arcuate nucleus complex (where axon terminals of GnRH neurons are located). In contrast, in 3-day fasted rats leptin was effective in stimulating the secretion of GnRH, alpha-MSH, and LH, regardless of the site of perfusion. These three hormones were increased in a temporal order of alpha-MSH, GnRH and LH. Irrespective of the site of perfusion, leptin was without effect on the release of neuropeptide Y. Only when leptin was infused into the median eminence-arcuate nucleus complex was PRL secretion also stimulated, although its onset was 1 h behind that of LH. The leptin-induced elevations of GnRH, alpha-MSH, LH and PRL were all dose-dependently stimulated by subnormal (1.0 ng ml(-1)) and normal (3.0 ng ml(-1)) concentrations of leptin, but at higher concentrations (10 ng ml(-1)) it did not produce additional effects. Leptin infusion into the anterior hypothalamic area, a control site equidistant from both the medial preoptic area and the median eminence-arcuate nucleus complex, did not produce a significant change in any of the hormones in either the fed or fasted rats. These results demonstrate for the first time that leptin can act at both the cell bodies and axon terminals of GnRH neurons to stimulate the release of the neurohormone in vivo, and they also suggest that alpha-MSH may play a significant intermediary role in linking leptin and GnRH secretion.

Melanocortin-3 receptor mRNA expression in pro-opiomelanocortin neurones of the rat arcuate nucleus

The melanocortins alpha- and gamma-melanocyte-stimulating hormones (alpha- and gamma-MSH) derive from the pro-opiomelanocortin (POMC) precursor. Melanocortins exert a wide range of biological activities in the brain through activation of at least three distinct melanocortin receptor (MC-R) subtypes. In order to determine whether POMC neurones can modulate their own activity, we looked for the possible expression of the MC3-R gene in POMC-positive cell bodies in the rat hypothalamus. In situ hybridization experiments revealed that the density of MC3-R mRNA is particularly high in the arcuate nucleus which contains the main population of POMC neurones in the brain. The occurrence of MC3-R mRNA in POMC-positive cell bodies was demonstrated using a double-labelling in situ hybridization technique. The proportion of POMC neurones expressing MC3-R mRNA was significantly higher in the most rostral (43.5%) than in the most posterior part of the arcuate nucleus (8.2%). These results indicate that melanocortins likely exert a direct regulatory feedback on POMC neurones through activation of MC3-R receptors. Our data also suggest that MC3-R may be involved in the neuroendocrine responses induced by centrally administered melanocortins.

Evidence for an interaction between alpha-MSH and opioids in the regulation of gonadotropin secretion in man

Gonadotropin secretion is inhibited by the endogenous opioids and stimulated by their antagonist naloxone. LH secretion is stimulated by alpha-MSH, a tridecapeptide derived from the post-translational processing of POMC. The possibility that alpha-MSH interacts with the opioids, as suggested by the experimental evidence, was investigated in 7 normal males aged 24-29 through the performance of seven tests: naloxone (0.8 mg i.v. bolus, followed by infusion of 1.6 mg/h for 120'); alpha-MSH (2.5 mg i.v. bolus); naloxone + alpha-MSH (2.5 mg i.v. 15' after commencement of the naloxone infusion); naloxone + GnRH (100 micrograms i.v. 15' after commencement of the naloxone infusion); alpha-MSH + GnRH (respectively 2.5 mg and 100 micrograms at time 0), GnRH alone (100 micrograms at time 0), placebo (150 nmol/l NaCl solution). The LH AUCs during both naloxone (30.3 +/- 2.7 mIU/ml.min-1) and alpha-MSH test (32.9 +/- 4.6 mIU/ml.min-1) were significantly greater (p < 0.005) than that observed during placebo (16.9 +/- 3.6 mIU/ml.min-1). The LH AUC during alpha-MSH + naloxone (37.6 +/- 2.6 mIU/ml.min-1) was not significantly different from that recorded during their separate administration. GnRH injected alone, during the naloxone infusion and with alpha-MSH produced similar increases in LH, that were significantly higher than that observed during the other tests (AUCs: GnRH 89.4 +/- 10.6, GnRH + naloxone 100.5 +/- 9.1, GnRH + alpha-MSH 94.6 +/- 7.9 mIU/ml.min-1, p < 0.001). Significant increase in FSH (p < 0.001) was only observed during GnRH, GnRH + naloxone and GnRH + aMSH tests (AUCs: placebo 13.3 +/- 1.7; naloxone 14.7 +/- 2.5; alpha-MSH 15.5 +/- 2.3; alpha-MSH + naloxone 16.9 +/- 1.9; GnRH 19.1 +/- 1.1; GnRH + alpha-MSH 20.7 +/- 1.3; GnRH + naloxone 21.2 +/- 1.8 mIU/ml.min-1). These results are in line with the possibility of an interaction between alpha-MSH and the opioids in the regulation of gonadotropin secretion, perhaps with opposing effects on a final common pathway.

beta-Endorphin and gonadotropin-releasing hormone synaptic input to gonadotropin-releasing hormone neurosecretory cells in the male rat

Physiological and pharmacological evidence has suggested that both endogenous opiates and gonadotropin-releasing hormone (GnRH) itself can act centrally to exert a tonic inhibition on gonadotropin secretion via an inhibition of the neurosecretion of GnRH. To determine if the effects of these two peptides might be mediated via a direct synaptic input to the GnRH neuron, we undertook a double label ultrastructural study. We were able to localize in the same tissue section beta-endorphin and GnRH. Analysis of serial sections through GnRH perikarya and dendrites in the male rat diagonal band/preoptic area revealed that almost 10% of the synapses impinging on the GnRH neuron contained beta-endorphin; an additional 10% of the terminals contained GnRH. These data provide anatomical evidence in support of both a direct modulation of GnRH release by opiates and of the presence of an ultrashort feedback loop.

Seasonal rhythm of the plasma level of alpha-melanocyte stimulating hormone

The present results indicate the presence of a seasonal rhythm of immunoreactive alpha-melanocyte stimulating hormone (alpha-MSH) in 20- to 40-year-old subjects of skin type I (light color of skin and eyes, red hair, no tanning after sun exposure) and skin type II (light color of skin, eyes, and hair, rare tanning) with raised levels of alpha-MSH in summer and low levels in winter. With increasing age of the investigated subjects, the seasonal rhythm seems to be lost. In subjects with skin type III (light skin, brown eyes and black hair, strong pigmentation after sun exposure) alpha-MSH shows only insignificant variations over the whole year. A seasonal rhythm of ACTH could not be demonstrated. A diurnal rhythm could be seen for ACTH, but not for alpha-MSH. To summarize, one can suppose that the seasonal rhythm of alpha-MSH is controlled by a varying UV exposure of the integument which is different over the whole year.

Suppression of basal and stress-induced prolactin release and stimulation of luteinizing hormone secretion by alpha-melanocyte-stimulating hormone

alpha-MSH and beta-endorphin, both synthesized from a common precursor, have opposite behavioral actions. In order to determine if these peptides have opposite effects on pituitary function, basal LH secretion and basal and stress-induced prolactin release were studied in adult male rats after intraventricular injection of alpha-MSH. Each rat also received intraventricular saline in order to serve as its own control. 18 micrograms alpha-MSH stimulated plasma LH from 16.5 +/- 2.5 (SEM) ng/ml to a peak of 27.2 +/- 4.0 and 26.0 +/- 4.9 ng/ml at 5 and 10 min, and suppressed prolactin from 3.5 +/- 0.7 ng/ml to 1.3 +/- 0.1 and 1.2 +/- 0.1 ng/ml at 15 and 30 min. Intraventricular alpha-MSH also significantly blunted the prolactin rise associated with the stress of swimming. 10 and 20 min after the onset of swimming, prolactin levels in rats pretreated with alpha-MSH were significantly diminished: 7.4 +/- 1.5 and 6.5 +/- 2.0 ng/ml vs 23.8 +/- 3.6 and 15.2 +/- 2.8 after normal saline. Similarly, des-acetyl alpha-MSH which is the predominant form of alpha-MSH in the hypothalamus, diminished the stress-induced prolactin rise from 18.4 +/- 5.3 and 11.2 +/- 3.4 ng/ml at 10 and 20 min to 10.0 +/- 2.4 and 5.5 +/- 1.6 ng/ml. We conclude that centrally administered alpha-MSH stimulates LH and suppresses basal and stress-induced prolactin release in male rats. These actions are opposite to those previously shown for beta-endorphin and suggest that alpha-MSH may antagonize the effects of beta-endorphin on pituitary function.