Melanotrope secretory cycle is regulated by physiological inputs via the hypothalamus

Pituitary hormone α-MSH promotes tumor-induced myelopoiesis and immunosuppression

The hypothalamic-pituitary (HP) unit can produce various hormones to regulate immune responses and some of its downstream hormones or effectors are elevated in cancer patients. We show that HP unit can promote myelopoiesis and immunosuppression to accelerate tumor growth. Subcutaneous implantation of tumors induced hypothalamus activation and pituitary α-melanocyte stimulating hormone (α-MSH) production in mice. α-MSH acted on bone marrow progenitors to promote myelopoiesis, myeloid cell accumulation, immunosuppression and tumor growth through its receptor MC5R. MC5R peptide antagonist boosted antitumor immunity and anti-programmed cell death protein 1 (anti-PD-1) immunotherapy. Serum α-MSH concentration was elevated and correlated with circulating myeloid-derived suppressor cells (MDSCs) in cancer patients. Our results reveal a neuroendocrine pathway which suppresses tumor immunity, and suggest MC5R as a potential target for cancer immunotherapy.

Orexin-B-like immunoreactivity in pituitary αMSH-producing cells and median eminence GnRH-containing fibres of the flat-tailed house gecko

We examined the distribution of the orexin-like peptides in the pituitary and median eminence of the flat-tailed house gecko (Hemidactylus platyurus) using immunohistochemistry. Orexin-B-like, but not orexin-A-like, immunoreactivity was detected in the pituitary, specifically in the pars intermedia, and these cells corresponded to alpha-melanocyte-stimulating hormone (αMSH)-producing cells. Orexin-B and αMSH secreted from pars intermedia may modulate secretion of adenohypophyseal cells in the pars distalis. In the median eminence, orexin-B-immunoreactive puncta and fibres were observed, and these structures corresponded to gonadotropin-releasing hormone (GnRH)-immunoreactive puncta and fibres. Orexin-B secreted from GnRH-containing neurons in the hypothalamus may affect thyrotropin-releasing hormone-containing neurons resulting in modulation of αMSH secretion of melanotrophs in the pars intermedia.

Thyrotropin-releasing hormone selectively stimulates human hair follicle pigmentation

In amphibians, thyrotropin-releasing hormone (TRH) stimulates skin melanophores by inducing secretion of α-melanocyte-stimulating hormone in the pituitary gland. However, it is unknown whether this tripeptide neurohormone exerts any direct effects on pigment cells, namely, on human melanocytes, under physiological conditions. Therefore, we have investigated whether TRH stimulates pigment production in organ-cultured human hair follicles (HFs), the epithelium of which expresses both TRH and its receptor, and/or in full-thickness human skin in situ. TRH stimulated melanin synthesis, tyrosinase transcription and activity, melanosome formation, melanocyte dendricity, gp100 immunoreactivity, and microphthalmia-associated transcription factor expression in human HFs in a pituitary gland-independent manner. TRH also stimulated proliferation, gp100 expression, tyrosinase activity, and dendricity of isolated human HF melanocytes. However, intraepidermal melanogenesis was unaffected. As TRH upregulated the intrafollicular production of "pituitary" neurohormones (proopiomelanocortin transcription and ACTH immunoreactivity) and as agouti-signaling protein counteracted TRH-induced HF pigmentation, these pigmentary TRH effects may be mediated in part by locally generated melanocortins and/or by MC-1 signaling. Our study introduces TRH as a novel, potent, selective, and evolutionarily highly conserved neuroendocrine factor controlling human pigmentation in situ. This physiologically relevant and melanocyte sub-population-specific neuroendocrine control of human pigmentation deserves clinical exploration, e.g., for preventing or reversing hair graying.

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

TRH acts as a multifunctional hypophysiotropic factor in vertebrates

Thyrotropin-releasing hormone (TRH) is the first hypothalamic hypophysiotropic neuropeptide whose sequence has been chemically characterized. The primary structure of TRH (pGlu-His-Pro-NH(2)) has been fully conserved across the vertebrate phylum. TRH is generated from a large precursor protein that contains multiple repeats of the TRH progenitor tetrapeptide Gln-His-Pro-Gly. In all tetrapods, TRH-expressing neurons located in the hypothalamus project towards the external zone of the median eminence while in teleosts they directly innervate the pars distalis of the pituitary. In addition, in frogs and teleosts, a bundle of TRH-containing fibers terminate in the neurointermediate lobe of the pituitary gland. Although TRH was originally named for its ability to trigger the release of thyroid-stimulating hormone (TSH) in mammals, it later became apparent that it exerts multiple, species-dependent hypophysiotropic activities. Thus, in fish TRH stimulates growth hormone (GH) and prolactin (PRL) release but does not affect TSH secretion. In amphibians, TRH is a marginal stimulator of TSH release in adult frogs, not in tadpoles, and a major releasing factor for GH and PRL. In birds, TRH triggers TSH and GH secretion. In mammals, TRH stimulates TSH, GH and PRL release. In fish and amphibians, TRH is also a very potent stimulator of alpha-melanocyte-stimulating hormone release. Because the intermediate lobe of the pituitary of amphibians is composed by a single type of hormone-producing cells, the melanotrope cells, it is a suitable model in which to investigate the mechanism of action of TRH at the cellular and molecular level. The occurrence of large amounts of TRH in the frog skin and high concentrations of TRH in frog plasma suggests that, in amphibians, skin-derived TRH may exert hypophysiotropic functions.

Neuronal, Neurohormonal, and Autocrine Control ofXenopusMelanotrope Cell Activity

Amphibian pituitary melanotropes are used to investigate principles of neuroendocrine translation of neural input into hormonal output. Here, the steps in this translation process are outlined for the melanotrope cell of Xenopus laevis, with attention to external stimuli, neurochemical messengers, receptor dynamics, second-messenger pathways, and control of the melanotrope secretory process. Emphasis is on the pathways that neurochemical messengers follow to reach the melanotrope. The inhibitory messengers, dopamine, gamma-aminobutyric acid, and neuropeptide Y, act on the cells by synaptic input from the suprachiasmatic nucleus, whereas the locus coeruleus and raphe nucleus synaptically stimulate the cells via noradrenaline and serotonin, respectively. Autoexcitatory actions are exerted by acetylcholine, brain-derived neurotrophic factor (BDNF), and the calcium-sensing receptor. At least six messengers released from the pituitary neural lobe stimulate melanotropes in a neurohormonal way: corticotropin-releasing hormone, thyrotropin-releasing hormone, BDNF, urocortin, mesotocin, and vasotocin. They all are produced by the magnocellular nucleus and coexist in various combinations in two types of neurohemal axon terminal. Most of the relevant receptors of the melanotropes have been elucidated. Apparently, the neural lobe has a dominant role in activating melanotrope secretory activity. The intracellular mechanisms translating the various inputs into cellular activities like biosynthesis and secretion constitute the adenylyl cyclase-cAMP pathway and Ca(2+) in the form of periodic changes of the intracellular Ca(2+) concentration, known as Ca(2+) oscillations. It is proposed that the pattern of these oscillations encodes specific regulatory information and that it is set by first messengers that control, for example, via G proteins and cAMP-related events, specific ion channel-mediated events in the membrane of the melanotrope cell.

Melanotrope cells as a model to understand the (patho)physiological regulation of hormone secretion

Regulation of hormone secretion is a complex process that comprises the sequential participation of numerous subcellular mechanisms. Hormone secretion is dictated by extracellular stimuli that are transduced intracellularly into activation/deactivation of different mechanisms, such as hormone expression, processing and exocytosis, which will ultimately determine the precise availability of hormone to be secreted. Malfunction in any of these steps may result in deficient or excessive hormone release and the subsequent appearance of endocrine disorders. Given the complexity of this system, it is difficult to find appropriate cellular models wherein to investigate the multiple components of the secretory process in a physiologically relevant, experimentally manipulable setting. In this review, we present recent evidence on the use of the intermediate lobe (IL) of the pituitary as a powerful tool to understand different aspects of the regulated secretory pathway. IL is composed of a single endocrine cell type, alpha-melanocyte stimulating hormone (alpha-MSH)-producing melanotropes, a fact that greatly facilitates its study. Furthermore, melanotropes can be separated using classic cell separation techniques into two cell subtypes showing opposite morphophysiological phenotypes of hypo- and hypersecretory cells. Comparison of their gene expression fingerprints has unveiled the existence of certain genes preferentially expressed in each melanotrope subtype. Because of their direct participation in the secretory pathway, we postulate that characterization of these gene products in an endocrine cell type may represent novel and useful markers for reliably determining the general secretory status in an endocrine gland, as well as a valuable new tool to further investigate this complex process.

Plasma alpha-MSH and acetylated beta-endorphin levels following stress vary according to CRH sensitivity of the pituitary melanotropes in common carp, Cyprinus carpio

Pituitary melanotropes release alpha-melanocyte-stimulating hormone (alpha-MSH) and acetylated beta-endorphin (NAc beta-end) during stress responses. However, effects of stressors on plasma concentrations of these hormones are highly inconsistent among fish species. Here, we show that also within a species, the common carp (Cyprinus carpio), fish sometimes respond with elevated alpha-MSH and NAc beta-end plasma levels, and at other times not. The origin of this variable response was investigated by (1) studying the effects of corticotropin-releasing hormone (CRH) on alpha-MSH and NAc beta-end release in vitro, (2) establishing where in the second messenger pathway coupled to CRH receptors melanotrope responsiveness is determined, and (3) testing modulatory actions of other hypothalamic factors (here opioid beta-endorphin). Melanotropes were in a high or low responsive state to CRH in vitro, which was especially evident when tissue was tested from fish kept at higher ambient water temperatures, and this correlates with the variability in alpha-MSH and NAc beta-end responses in vivo. Relative rates of alpha-MSH and NAc beta-end release following stimulation with CRH in vitro match plasma level changes in vivo, and this indicates that the CRH pathway does act in vivo. cAMP did not stimulate melanotropes in the low responsive state to release hormones in vitro. Thus, the mechanism that determines the cell status, occurs downstream of cAMP accumulation. Opioid beta-endorphin differentially modulated the actions of CRH, as NAc beta-end, but not alpha-MSH, release was inhibited. This response was not observed in the stress paradigms studied. We conclude that the variation in alpha-MSH and NAc beta-end stress responses in vivo correlates with many CRH responses in vitro; whether a cell is in a high or low responsive state to CRH is determined downstream of accumulation of the second messenger. We propose that melanotropes have to be in the high responsive state to be activated by CRH during stress in carp and other teleosts.

Mice lacking the thyrotropin-releasing hormone gene: what do they tell us?

Thyrotropin-releasing hormone (TRH) is localized in the brain hypothalamus and stimulates the secretion and synthesis of pituitary thyrotropin (TSH). Although TRH deficiency caused by artificial hypothalamic destructions has been reported to result in significant decreases in TSH secretion in rodents, clinical observations from the patients with possible TRH deficiency did not entirely agree with these animal results. Because of its ubiquitous distribution throughout the brain and in the peripheral tissues, TRH has been suggested to possess a wide variety of functions in these regions. However, the neurobehavioral and peripheral actions of TRH still remains to be established. It has been, therefore, anticipated that detailed analysis of TRH-knockout mice might provide insight into the physiological significance of endogenous TRH. The present review focuses on the phenotypic findings of mice deficient in TRH.