Trophic effects of melanocortins on neuronal cells in culture

Melanocortins, Melanocortin Receptors and Multiple Sclerosis

The melanocortins and their receptors have been extensively investigated for their roles in the hypothalamo-pituitary-adrenal axis, but to a lesser extent in immune cells and in the nervous system outside the hypothalamic axis. This review discusses corticosteroid dependent and independent effects of melanocortins on the peripheral immune system, central nervous system (CNS) effects mediated through neuronal regulation of immune system function, and direct effects on endogenous cells in the CNS. We have focused on the expression and function of melanocortin receptors in oligodendroglia (OL), the myelin producing cells of the CNS, with the goal of identifying new therapeutic approaches to decrease CNS damage in multiple sclerosis as well as to promote repair. It is clear that melanocortin signaling through their receptors in the CNS has potential for neuroprotection and repair in diseases like MS. Effects of melanocortins on the immune system by direct effects on the circulating cells (lymphocytes and monocytes) and by signaling through CNS cells in regions lacking a mature blood brain barrier are clear. However, additional studies are needed to develop highly effective MCR targeted therapies that directly affect endogenous cells of the CNS, particularly OL, their progenitors and neurons.

Melanocortin receptor agonist ACTH 1-39 protects rat forebrain neurons from apoptotic, excitotoxic and inflammation-related damage

Patients with relapsing-remitting multiple sclerosis (RRMS) are commonly treated with high doses of intravenous corticosteroids (CS). ACTH 1-39, a member of the melanocortin family, stimulates production of CS by the adrenals, but melanocortin receptors are also found in the central nervous system (CNS) and on immune cells. ACTH is produced within the CNS and may have direct protective effects on glia and neurons independent of CS. We previously reported that ACTH 1-39 protected oligodendroglia (OL) and their progenitors (OPC) from a panel of excitotoxic and inflammation-related agents. Neurons are the most vulnerable cells in the CNS. They are terminally differentiated, and sensitive to inflammatory and excitotoxic insults. For potential therapeutic protection of gray matter, it is important to investigate the direct effects of ACTH on neurons. Cultures highly enriched in neurons were isolated from 2-3 day old rat brain. After 4-7 days in culture, the neurons were treated for 24h with selected toxic agents with or without ACTH 1-39. ACTH 1-39 protected neurons from death induced by staurosporine, glutamate, NMDA, AMPA, kainate, quinolinic acid, reactive oxygen species and, to a modest extent, from rapidly released NO, but did not protect against kynurenic acid or slowly released nitric oxide. Our results show that ACTH 1-39 protects neurons in vitro from several apoptotic, excitotoxic and inflammation-related insults.

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

The proopiomelanocortin system

POMC (31,000 MW) is localized to the pituitary, brain, skin, and other peripheral sites. The particular enzyme profile present within a cell dictates the nature of the hormonal ligand (melanocortin) synthesized and secreted: melanotropic peptides (alpha-MSH beta-lipotropin, lambda-MSH), corticotropin (ACTH), several endorphins (e.g., met-enkephalin). These POMC-derived peptides mediate their actions through typical seven-spanning membrane receptors (MCRs; MCR1, 2, 3, 4, and 5). A specific melanocortin acting on a specific MCR regulates a particular biological response; for example, alpha-MSH on MCR1 increases melanogenesis within melanocytes, ACTH on MCR2 increases cortisol production within adrenal zona fasciculata cells. Within the brain melanocortins regulate satiety (MCR4) and erectile activity (MCR?). MCRs have been localized by melanocortin macromolecular probes, for example, fluorescent to human epidermal melanocytes and also to keratinocytes, suggesting that systemic melanocortins or localized POMC products might regulate these integumental cellular elements in synchrony to enhance skin pigmentation and/or immunological responses. Superpotent, prolonged acting melanotropic peptides have been synthesized and their application in clinical medicine has been demonstrated. MCR antagonists have been used to discover and further delineate other roles of melanocortin ligands. For example, melanocortin-induced satiety can be antagonized by a melanocortin antagonist. Defects in melanocortin ligand biosynthesis, secretion, and melanocortin receptor function can lead to a diverse number of pathological states.

Regulation of ischemic cell death by glucocorticoids and adrenocorticotropic hormone

Transient global ischemia results in delayed selective neuronal death of hippocampal CA1 pyramidal cells. Glucocorticoids increase and adrenalectomy decreases the rate of neuronal death; however, they also produce changes in brain temperature, serum glucose and adrenocorticotropic hormone levels. In order to understand the role of glucocorticoids in regulating ischemic cell death, we studied RU 38486, a glucocorticoid receptor blocker, and Org 2766, a non-steroidogenic adrenocorticotropic hormone 4-9 analog. Male Mongolian gerbils were subjected to 5 min of bilateral carotid artery occlusion under a controlled temperature environment (37.0-38.0 degrees C). Animals were injected with either physiological saline, Org 2766 (10 microg/kg/24 h) or RU 38486 (50 mg/kg/8 h), beginning just prior to the occlusion until killing at either day 4 or 7. Blood was collected for serum glucose and cortisol analysis. Damage was evaluated by blinded counts of CAI neurons. Both RU 38486 and Org 2766 treatment significantly (P<0.004) reduced hippocampal CA1 damage at day 4, but not on day 7. While RU 38486 raised serum cortisol and adrenocorticotropic hormone levels, neither treatment affected temperature or serum glucose. The fact that RU 38486 mimicked adrenalectomy without changing temperature suggests that the decreased rate of cell death resulted from either removal of glucocorticoids or increases in adrenocorticotropic hormone. The ability of Org 2766 to affect this rate strongly suggests that adrenocorticotropic hormone is the active regulatory hormone rather than glucocorticoids. While both RU 38486 and Org 2766 prolong the survival of CA1 neurons after transient global ischemia, only RU 38486, which is available and tested in both animals and humans, can block the detrimental effects of post-ischemia glucocorticoid elevations. Thus, the administration of RU 38486 may be a practical adjunct to other neuroprotective agents for victims of cardiac arrest, anesthetic accidents or drowning.