Melanocortins stimulate proliferation and induce morphological changes in cultured rat astrocytes by distinct transducing mechanisms

Neuroprotective effect of ACTH on collagenase-induced peri-intraventricular hemorrhage in newborn male rats

Peri-intraventricular hemorrhage (PIVH) is a common and serious prematurity-related complication in neonates. Adrenocorticotropic hormone (ACTH) has neuroprotective actions and is a candidate to ameliorate brain damage following PIVH. Here, we tested the efficacy of ACTH1-24 on a collagenase-induced lesion of the germinal matrix (GM) in newborn male rats. Animals received microinjection of the vehicle (PBS, 2 µl) or collagenase type VII (0.3 IU) into the GM/periventricular tissue on postnatal day (PN) 2. Twelve hours later pups received microinjection of either the agonist ACTH1-24 (0.048 mg/kg), or the antagonist SHU9119 (antagonist of MCR3/MCR4 receptors, 0.01 mg/kg), or their combination. Morphological outcomes included striatal injury extension, neuronal and glial cells counting, and immunohistochemical expression of brain lesion biomarkers ipsilateral and contralateral to the hemorrhagic site. Data were evaluated on PN 8. Collagenase induced PIVH and severe ipsilateral striatal lesion. ACTH1-24 dampened the deleterious effects of collagenase-induced hemorrhage in significantly reducing the extension of the damaged area, the striatal neuronal and glial losses, and the immunoreactive expression of the GFAP, S100β, and NG2-glia biomarkers in the affected periventricular area. SHU9119 blocked the glial density rescuing effect of ACTH1-24. ACTH1-24 could be further evaluated to determine its suitability for preclinical models of PVH in premature infants.

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.

New Implications for the Melanocortin System in Alcohol Drinking Behavior in Adolescents: The Glial Dysfunction Hypothesis

Alcohol dependence causes physical, social, and moral harms and currently represents an important public health concern. According to the World Health Organization (WHO), alcoholism is the third leading cause of death worldwide, after tobacco consumption and hypertension. Recent epidemiologic studies have shown a growing trend in alcohol abuse among adolescents, characterized by the consumption of large doses of alcohol over a short time period. Since brain development is an ongoing process during adolescence, short- and long-term brain damage associated with drinking behavior could lead to serious consequences for health and wellbeing. Accumulating evidence indicates that alcohol impairs the function of different components of the melanocortin system, a major player involved in the consolidation of addictive behaviors during adolescence and adulthood. Here, we hypothesize the possible implications of melanocortins and glial cells in the onset and progression of alcohol addiction. In particular, we propose that alcohol-induced decrease in α-MSH levels may trigger a cascade of glial inflammatory pathways that culminate in altered gliotransmission in the ventral tegmental area and nucleus accumbens (NAc). The latter might potentiate dopaminergic drive in the NAc, contributing to increase the vulnerability to alcohol dependence and addiction in the adolescence and adulthood.

Astrocytes: new targets of melanocortin 4 receptor actions

Astrocytes exert a wide variety of functions with paramount importance in brain physiology. After injury or infection, astrocytes become reactive and they respond by producing a variety of inflammatory mediators that help maintain brain homeostasis. Loss of astrocyte functions as well as their excessive activation can contribute to disease processes; thus, it is important to modulate reactive astrocyte response. Melanocortins are peptides with well-recognized anti-inflammatory and neuroprotective activity. Although melanocortin efficacy was shown in systemic models of inflammatory disease, mechanisms involved in their effects have not yet been fully elucidated. Central anti-inflammatory effects of melanocortins and their mechanisms are even less well known, and, in particular, the effects of melanocortins in glial cells are poorly understood. Of the five known melanocortin receptors (MCRs), only subtype 4 is present in astrocytes. MC4R has been shown to mediate melanocortin effects on energy homeostasis, reproduction, inflammation, and neuroprotection and, recently, to modulate astrocyte functions. In this review, we will describe MC4R involvement in anti-inflammatory, anorexigenic, and anti-apoptotic effects of melanocortins in the brain. We will highlight MC4R action in astrocytes and discuss their possible mechanisms of action. Melanocortin effects on astrocytes provide a new means of treating inflammation, obesity, and neurodegeneration, making them attractive targets for therapeutic interventions in the CNS.

ACTH protects mature oligodendroglia from excitotoxic and inflammation-related damage in vitro

Corticosteroids (CS) are widely employed to treat relapses in multiple sclerosis (MS). Endogenous ACTH is a 39-amino acid peptide that, among other functions, stimulates CS production. Exogenous ACTH 1-39 is used to treat MS relapses, presumably by stimulating endogenous CS production. However, unlike CS, ACTH binds to melanocortin receptors, found in the central nervous system (CNS) as well as on inflammatory cells. Since glia are implicated in MS and other neurodegenerative diseases, and oligodendroglia (OL) are more sensitive to injury than other glia, we characterized the protective effects of ACTH on OL in vitro without the confounding effects of CS. Rat brain cultures containing OL, astrocytes (AS), and microglia (MG) were incubated for 1 day with potentially cytotoxic agents with or without preincubation with ACTH 1-39. The cytotoxic agents killed 55-70% of mature OL, but caused little or no death of AS or MG at the concentrations used. ACTH protected OL from death induced by staurosporine, AMPA, NMDA, kainate, quinolinic acid, or reactive oxygen species, but did not protect against kynurenic acid or nitric oxide. The protective effects of ACTH were dose dependent, and decreased OL death induced by the different agents by 30-60% at 200 nM ACTH. We show for the first time that melanocortin 4 receptor is expressed on OL in addition to MG and AS. In summary, ACTH 1-39 protects OL in vitro from several excitotoxic and inflammation-related insults. ACTH may be activating melanocortin receptors on OL or alternately on AS or MG to prevent OL death.

Development of the hypothalamic melanocortin system

The melanocortin system is a critical component of the forebrain and hindbrain regulatory systems involved in energy balance. This system is composed of pro-opiomelanocortin (POMC) neurons that act, in part, through the melanocortin-4 receptor (MC4R). Although the importance of the melanocortin system in controlling feeding has been established for two decades, the understanding of the developmental substrates underlying POMC and MC4R neuron development and function has just begun to emerge. The formation of the melanocortin system involves several discrete developmental steps that include the birth and fate specification of POMC- and MC4R-containing neurons and the extension and guidance of POMC axons to their MC4R-expressing target nuclei. Each of these developmental processes appears to require specific sets of genes and developmental cues that include perinatal hormones. Recent evidence has also highlighted the importance of perinatal nutrition in controlling the ultimate architecture of the melanocortin system.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Identification of differential melanocortin 4 receptor agonist profiles at natively expressed receptors in rat cortical astrocytes and recombinantly expressed receptors in human embryonic kidney cells

Using cAMP accumulation as a functional readout, we pharmacologically characterized the response of native melanocortin receptors in cultured rat astrocytes, and found this response to be mediated by the melanocortin 4 receptor (MC4R). Melancortin agonists stimulate cAMP in a concentration-dependent manner in both astrocytes and human embryonic kidney cells recombinantly expressing rat MC4R (HEK-rMC4R), however, the relative potency and intrinsic activity of both small molecule and peptide agonists are reduced in the native system. As such, the small molecules THIQ, NBI-702 and MB243 display 43, 30 and 18% of the maximal response elicited by alpha-MSH in astrocytes. Likewise, the peptides MTII and ACTH display 55 and 72% of the maximal response elicited by alpha-MSH in these cells. In contrast, all of these compounds elicit full agonist responses with similar intrinsic activity to alpha-MSH in HEK-rMC4R cells. MC4R mRNA was detected in astrocytes, however radioligand binding experiments failed to detect measurable MC4R in astrocyte membranes, in contrast to membranes from HEK-rMC4R cells that display a binding site density of 18.1+/-1.5 fmol/mg. We propose that the divergent observations in functional activity between the cell types reflect differences in receptor expression and that caution should be exercised when interpreting agonist activity in over-expression systems for the purposes of drug discovery.

Accelerating sensory recovery after sciatic nerve crush: non-selective versus melanocortin MC4 receptor-selective peptides

Melanocortin receptor ligands accelerate functional recovery after peripheral nerve crush. It is not known which mechanism is involved or via which melanocortin receptor this effect occurs, albeit indirect evidence favours the melanocortin MC4 receptor. To test whether the melanocortin MC4 receptor is involved in the effects of melanocortins on functional recovery, we used melanocortin compounds that distinguish the melanocortin MC4 receptor from the melanocortin MC1, MC3 and MC5 receptors on basis of selectivity and agonist/antagonist profile. Activation and binding studies indicated that the previously described peptides JK1 (Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH2) and [D-Tyr4]melanotan-II ([D-Tyr4]MTII. Ac-Nle-c[Asp-His-D-Tyr-Arg-Trp-Lys]NH2) are selective for the rat melanocortin MC4 receptor as compared to the rat melanocortin MC3 and MC5 receptors, but are also potent on the melanocortin MC1 receptor. Both peptides did not accelerate sensory recovery in rats with a sciatic nerve crush, whereas the non-selective melanocortin agonist melanotan-II (MTII, Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]NH2) was effective. The melanocortin MC3/MC4 receptor antagonist SHU9119 (Ac-Nle-c[Asp-His-D-Nal(2)-Arg-Trp-Lys]NH2) also enhanced sensory recovery. This effect was probably not due to interaction with the melanocortin MC4 receptor, since JK46 (Ac-Gly-Lys-His-D-Nal(2)-Arg-Trp-Gly-NH2), a selective melanocortin MC4 receptor antagonist, was ineffective. Taken together, these data suggest that melanocortins do not accelerate sensory recovery via interaction with the melanocortin MC4 receptor. From the known melanocortin receptors, only the involvement of the melanocortin MC5 receptor in acceleration of recovery could not be excluded.

Inhibition of muscarinic receptor-induced proliferation of astroglial cells by ethanol: mechanisms and implications for the fetal alcohol syndrome

In utero exposure to ethanol is deleterious to fetal brain development. Children born with the fetal alcohol syndrome (FAS) display a number of abnormalities, the most significant of which are central nervous system (CNS) dysfunctions, such as microencephaly and mental retardation. An interaction of ethanol with glial cells, particularly astrocytes, has been suggested to contribute to the developmental neurotoxicity of this alcohol. At low concentrations (10-100 mM) ethanol inhibits the proliferation of astroglial cells in vitro, particularly when stimulated by acetycholine through muscarinic M3 receptors. Of the several signal transduction pathways activated by these receptors in astrocytes or astrocytoma cells, which are involved in mitogenic signaling, only some (e.g. protein kinase C (PKC) zeta, p70S6 kinase) appear to be targeted by ethanol at the same low concentrations which effectively inhibit proliferation. Inhibition of astroglial proliferation by ethanol may contribute to the microencephaly seen in FAS.

ACTH(1-24) down-regulates expression of ciliary neurotrophic factor mRNA in cultured rat astrocyte

We examined the effects on astrocytes of ACTH, which is used to treat West syndrome. We stimulated cultured rat astrocytes with ACTH(1-24), corticotropin-releasing factor, and dexamethasone, and examined changes in neurotrophic factor mRNAs by reverse transcription-PCR. Down-regulation of ciliary neurotrophic factor mRNA expression was observed by stimulation with ACTH(1-24), but the expression of nerve growth factor, brain-derived neurotrophic factor, and nerotrophin-3 mRNAs was unaffected. Northern blot analysis revealed that the decrease in ciliary neurotrophic factor mRNA occurred 4 h after stimulation with more than 10 nM of ACTH(1-24). Up-regulation of nerotrophin-3 mRNA expression was found after stimulation with 1 mM dexamethasone. These results suggest that ACTH(1-24) administrated in West syndrome may influence the expression of neurotrophic factors in astrocytes in vivo.

Marked distributional difference of alpha-melanocyte-stimulating hormone (alpha-MSH)-like immunoreactivity in the brain between two elasmobranchs (Scyliorhinus torazame and Etmopterus brachyurus): an immunohistochemical study

Application of alpha-MSH immunohistochemistry to the brain of elasmobranchs (Scyliorhinus torazame and Etmopterus brachyurus) demonstrated a marked species difference concerning the distribution of the alpha-MSH-like molecule in the brain. In S. torazame, alpha-MSH-like immunoreactive cells were present in the hypothalamus, mainly in the tuberculum posterius and the nucleus lateralis tuberis, and also in the distal and neurointermediate lobes of the hypophysis. Labeled varicose fibers were densely distributed in the hypothalamus, but they were sparse or absent in other portions of the brain. In striking contrast to the results for S. torazame, the immunoreactivity in the E. brachyurus brain was associated exclusively with the glial system, represented by astrocytes and tanycytes, throughout the central nervous system; no immunoreactivity was found in the neuronal elements. In the E. brachyurus hypophysis, the labeled cells were present in the distal and intermediate lobes, similarly to their presence in S. torazame, but in the intermediate lobe the immunoreactivity was confined to the peripheral cell cord closely adjacent to the neural lobe. The present findings are the first as regards the occurrence of alpha-MSH-like immunoreactivity in the glial system of the central nervous system of vertebrates and suggest diversity of expression and/or processing of proopiomelanocortin in the brain of elasmobranchs.

Modulation of DNA synthesis by muscarinic cholinergic receptors

Acetylcholine muscarinic receptors are a family of five G-protein-coupled receptors widely distributed in the central nervous system and in peripheral organs. Activation of certain subtypes of muscarinic receptors (M1, M3, M5) has been found to modulate DNA synthesis in a number of cell types. In several cell types acetylcholine, by activating endogenous or transfected muscarinic receptors, can indeed elicit cell proliferation. In other cell types, however, or under different experimental conditions, activation of muscarinic receptors has no effect, or inhibits DNA synthesis. A large number of intracellular pathways are being investigated to define the mechanisms involved in these effects of muscarinic receptors; these include among others, phospholipase D, protein kinases C and mitogen-activated-protein kinases. The ability of acetylcholine to modulate DNA synthesis through muscarinic receptors may be relevant in the context of brain development and neoplastic growth.

Fourier analysis of light scattered by elongated scatterers

Biological stimulation of living cells is sometimes associated with morphological changes. A practical method is developed to monitor cell stimulation by means of their conformational changes through interpretation of the pattern of light scattered from a cell population. For this purpose a mathematical model is suggested that predicts the power spectrum from a population of elliptic objects with a given eccentricity. A computer simulation of that model is presented together with supporting experimental results of the simulation. The predicted and the measured spectra are in good agreement. This technique was applied to elongated cells that become circular on exposure to a human hormone, indicating the potential applicability of the method in biology and medicine. The method and the apparatus presented in this study could be applied to bioassays of cell systems that respond to a variety of stimulants and to trace quantitatively the structural changes that occur during biological processes.

Muscarinic cholinergic receptor signal transduction as a potential target for the developmental neurotoxicity of ethanol

Central nervous system dysfunctions (most notably mental retardation and microcephaly) are among the most significant effects of in utero exposure to ethanol. Ethanol has been shown to cause alterations of both neuronal and glial cells, including cell loss, and changes in their migration and maturation. Here, we propose that one of the potential targets for the developmental neurotoxicity of ethanol may be represented by the signal transduction systems activated by cholinergic muscarinic receptors. Ethanol has been shown to inhibit second messenger systems activated by various G-protein-coupled receptors, including certain subtypes of muscarinic receptors. Although the roles of muscarinic receptors in brain development have not been fully elucidated, two potentially relevant effects have been discovered in the past few years. By activating muscarinic receptors coupled to phospholipid metabolism, acetylcholine can induce proliferation of glial cells, and act as a trophic factor in developing neurons by preventing apoptotic cell death. Ethanol has been shown to inhibit both actions of acetylcholine in vitro. These effects of ethanol may lead to a decreased number of glial cells and to a loss of neurons, which have been observed following in vivo alcohol exposure. In turn, these may be the basis of microencephaly and cognitive disturbances in children diagnosed with Fetal Alcohol Syndrome.

Neurotrophic ACTH4-9 analogue therapy normalizes electroencephalographic alterations in chronic experimental allergic encephalomyelitis

Chronic experimental allergic encephalomyelitis (CEAE) is an established experimental model for multiple sclerosis (MS). The demyelinating lesions in the white matter of the central nervous system observed in CEAE and in MS are accompanied by various neurophysiological alterations. Among the best defined electrophysiological abnormalities are the changes in event-related potentials, in particular evoked potentials involving the spinal cord, i.e. motor and sensory evoked potentials. Less familiar are the changes observed in the electroencephalogram of CEAE-affected animals, which are also encountered in the human equivalent, MS. In the present experiment we evaluated the therapeutic value of a neurotrophic peptide treatment [H-Met(O2)-Glu-His-Phe-D-Lys-Phe-OH, an ACTH4-9 analogue] and its effect on the delayed flash visual evoked potentials (VEP) and power spectra of the electroencephalogram, during a 17-week follow-up of CEAE. CEAE animals treated with the neurotrophic peptide were protected against the development of neurological symptoms during the course of the demyelinating syndrome. VEPs of animals suffering from CEAE showed a delay of the latencies of the late components which was significantly counteracted by peptide treatment. The peak-to-peak amplitude of the VEP afterdischarge recorded from CEAE animals was significantly increased during the course of CEAE and correlated closely with the progression of the myelinopathy. Furthermore, CEAE animals showed an increase of electroencephalogram (EEG) beta activity of up to 500% as compared with the age-matched control group. This increase in beta power mainly consisted of a prevailing 20-21 Hz peak, a frequency that normally is not dominant in control EEG recordings of the rat during passive wakefulness. All these electrophysiological phenomena were absent in ACTH4-9 analogue-treated animals. The present findings underscore the potential importance of a neurotrophic peptide treatment in the pharmacotherapy of central demyelinating syndromes, and possibly of MS.

The role of protein kinase C alpha and epsilon isozymes in DNA synthesis induced by muscarinic receptors in a glial cell line

Acetylcholine has been shown to induce proliferation of human astrocytoma cells by activating muscarinic receptors, particularly the m3 subtype. In the present study the role of protein kinase C in DNA synthesis induced by carbachol has been investigated. Carbachol-induced [methyl-3H]thymidine incorporation was inhibited by the protein kinase C inhibitors GF 109203X and staurosporine. However, carbachol-induced DNA synthesis was only partially reduced by protein kinase C down-regulation by phorbol 12-myristate 13-acetate (PMA), and maximal concentrations of carbachol and PMA had an additive effect on [methyl-3H]thymidine incorporation. Exposure for 24 h to maximally effective concentrations of carbachol did not induce down-regulation of protein kinase C alpha, and caused a small but significant down-regulation of protein kinase C epsilon; cells exposed for 24 h to carbachol were still able to respond with protein kinase C translocation to PMA stimulation. Carbachol caused a significant increase of phorbol ester binding, but did not stimulate protein kinase C alpha translocation, while it caused a short-lasting translocation of protein kinase C epsilon; however, protein kinase C epsilon translocation was not correlated with the time-course of carbachol-induced increase in [methyl-3H]thymidine incorporation. On the other hand, the time-course of translocation/down-regulation of protein kinase C alpha and protein kinase C epsilon induced by PMA was in good correlation with the time-course of PMA-induced [methyl-3H]thymidine incorporation. These results suggest that protein kinase C alpha may not be involved in DNA synthesis induced by muscarinic receptors stimulation in 132-1N1 astrocytoma cells, while protein kinase C epsilon appears to play a role in the initial exit from G0/G1 phase, though it cannot be considered the major determinant for sustained proliferation.

Melanocortin and MCH precursor-derived NEI effects on striatum-midbrain co-cultures

The possibility of developmental effects of POMC-derived melanocortins and analogs on neurons of fetal rat brain regions exhibiting marked developmental melanocortin receptor expression, was studied in serum-free co-cultures of gestational day 18 striatal and mesencephalic cells, and compared with NEI and NGE. These two peptide fragments of the melanin concentrating hormone precursor, occurring in brain areas devoid of POMC terminals, cross-react with alpha-MSH antibodies; NEI elicits grooming similar to alpha-MSH. Neurofilament protein (NF), growth-associated protein (GAP-43) and synaptophysin of the synaptosomal fraction were determined by ELISA as markers for neuritogenesis, growth cones, and nerve terminal differentiation. Cell survival was analyzed by MTT assay, proportions of major cell types by immunocytochemistry. alpha-Melanocyte-stimulating hormone (alpha-MSH, effective concentration 250-2500 nM), the analog Nle4-, D-Phe7-alpha-MSH (NDP, 3.1-750 nM), and NEI (250 nM) increased NF in 3 day cultures by 11%, 17%, and 22%, respectively, whereas ACTH(1-24) and ACTH(1-39) (25 2500 nM) were ineffective. In 11 day cultures, alpha-MSH (250-750 nM), but not NDP, ACTH(1-24) or ACTH(1-39), increased synaptosomal synaptophysin by 11%. GAP-43 and cell survival remained unaffected. These data indicate that selected melanocortins as well as NEI can influence differentiation of neural processes in brain neurons.

Association of Ad4BP/SF-1 transcription factor with steroidogenic activity in oncogene-transformed granulosa cells

Adrenal binding protein 4 (Ad4BP) known also as steroidogenic factor 1 (SF-1) is a cell specific transcription factor regulating all steroidogenic P450 genes and is present exclusively in steroidogenic tissues. In this study, we examined whether Ad4BP expression is affected by oncogene-induced cell transformation. Using a gel shift assay we report here that nuclear extracts of steroidogenic granulosa cell lines, transformed by SV40 DNA and the Ha-ras oncogene show specific binding activity towards an Ad4 recognition sequence oligonucleotide. In contrast, nuclear extracts obtained from granulosa cells transformed with SV40 alone, which lost their steroidogenic activity, did not exhibit any binding to the Ad4 oligonucleotide. Using a specific antibody to Ad4BP, it was demonstrated that only the steroidogenic cell lines, i.e. transfected with SV40 + Ha-ras, expressed significant amount of the protein. No binding activity to the Ad4 oligonucleotide was evident in fibroblasts transformed with the same oncogenes (SV40 + Ha-ras). Steroidogenic activity in SV40 + Ha-ras transformed granulosa cells was markedly elevated following forskolin or follice stimulating factor (FSH) and further augmented by incubation of the cells with dexamethasone. However, no change in Ad4BP expression and binding activity was observed following such stimulations. It is suggested that Ha-ras expression in SV40 transformed granulosa cells can play an important role in restoring Ad4BP expression and activity, which are required for their steroidogenic function. Thus, expression of Ad4BP is essential for steroidogenesis both in primary and in oncogene transformed granulosa cells.

Establishment of an in vitro bioassay and radio receptor assay for LH/CG in human sera using immortalized granulosa cells transfected with LH/CG receptor

Levels of gonadotropic hormones in human sera or urine are routinely measured by radioimmunoassay or by enzyme-linked immunoassay (ELISA), which determine the immunoactivity of the hormone, but not its biological activity. We have utilized immortalized stable steroidogenic granulosa cells, which express 5-10 times more of the luteinizing hormone/chorionic gonadotropin (LH/CG) receptors than the primary cells, to develop a biological assay and radioreceptor assay for this hormone. We found that stimulation of cells expressing LH/CG receptor with increasing doses of human LH or human CG resulted in a dose-dependent increase of cAMP and progesterone with an ED(50) of 30 and 57 mlU/mL, respectively. These dose-response data served as calibration curves for measuring the gonadotropin bioactivity in human serum samples at concentrations as low as 1-5 mlU/mL. We found a close correlation between LH levels measured by enzyme immunoassay (EIA) and the in vitro bioassay in normal cycling and menopausal women, as well as in normal adult men. Also, a close correlation was found between the EIA and the in vitro biological assay of hCG in pregnant women. In addition, we have developed a radioreceptor assay (RRA) for this hormone using enriched cell membranes of the appropriate cell line, which corresponds well to both the EIA and the bioassay in human sera. Deglycosylated hCG was fully active in RRA, but failed to activated cAMP response in these cells, demonstrating the importance of the bioassay in the biologically inactive form of gonadotropins. We believe this novel in vitro bioassay of gonadotropic hormones will serve as a useful tool for a more comprehensive set of assays that will determine not only the amount, but also the possible modulation in bioactivity of the gonadotropin associated with gonadal failure and miscarriage.

Fourier analysis of differential light scattering for the quantitation of FSH response associated with structural changes in immortalized granulosa cells

We have established granulosa cell lines which express constitutively the rat FSH receptors by cotransfection of primary granulosa cells obtained from preovulatory follicles with SV40 DNA, Ha-ras oncogene and a plasmid expressing FSH receptors. These cells respond specifically to ovine and human FSH by cell rounding, intracellular cAMP accumulation, and progesterone secretion in a dose-dependent manner. A new method for the demonstration and quantitation of changes in cell shape-Small Angle Laser Light Scattering (SALLS) analysis-has been utilized for measurement of cell rounding in response to FSH stimulation in these cells. When cells were incubated with increasing doses of either ovine or human FSH, partial rounding of cells was observed at FSH concentrations as low as 24 pM, while complete rounding of cells was observed at a range of 0.24-2.4 nM of FSH. Following aldehyde fixation, hormone-treated cells were examined using the method of SALLS analysis. Histograms obtained by applying SALLS analysis on FSH stimulated GFSHR-17 cells were a reflection of the structural changes induced by the hormone. FSH- and forskolin-incubated cells yielded structured distributions with defined mean size and standard deviations. Moreover, the increase in sharpness of dominant peak in the histogram was correlated with elevated concentration of FSH in a dose dependent manner. In conclusion, cellular response to FSH is correlated with a specific pattern of light scattered in immortalized granulosa cells expressing functional FSH receptors. Therefore, SALLS analysis may serve as a useful tool for in vitro bioassay of the gonadotropic hormone. Moreover, this method may lend itself to in vitro bioassay of any hormone that induces specific morphological changes in target cells.

Functional and neurophysiological evidence of the efficacy of trophic pharmacotherapy using an adrenocorticotrophic hormone4-9 analog in experimental allergic encephalomyelitis, an animal model of multiple sclerosis

Chronic experimental allergic encephalomyelitis (CEAE) is a well-established animal model for the human syndrome, multiple sclerosis. CEAE has striking histological, electrophysiological and clinical analogies with multiple sclerosis and is a valuable animal model for the preclinical pharmacotherapeutical development of new putative therapeutic agents. In this paper, we describe a neurotrophic repair approach in Lewis rats suffering from CEAE. The neurotrophic peptide used is a degradation resistant adrenocorticotrophic hormone4-9 analog. The development of CEAE was examined using a combination of clinical, functional and electrophysiological parameters including somatosensory and motor evoked potentials. The latencies and amplitudes of the various evoked potentials can provide quantitative, objective data regarding the involvement of different nerve tracts in CEAE and the effectiveness of the neurotrophic peptide. Repeated subcutaneous injections of the neurotrophic peptide suppressed the development of CEAE-related clinical symptoms, markedly improved motor performance and reduced the reaction time upon thermal stimulation as compared to saline-treated CEAE animals during a 17 week follow-up study. Prolonged onset latencies of corticomotor evoked potentials and peak latencies of somatosensory evoked potentials due to the demyelination were normalized upon peptide treatment. In addition, peptide treatment substantially prevented total blocking of the corticomotor pathway in CEAE-animals and reduced the attenuation of sensory evoked potentials-related peak amplitudes as compared to saline-treated animals. The functional and electrophysiological improvements observed in CEAE-animals treated with the adrenocorticotrophic hormone4-9 analog, suggest that a neurotrophic repair approach could be of great value to promote the restoration of function in a disabling demyelinating disorder.

Acetylcholine as a mitogen: muscarinic receptor-mediated proliferation of rat astrocytes and human astrocytoma cells

The mitogenic effect of muscarinic receptor agonists in glial cells has been characterized in rat cortical astrocytes and human 132 1N1 astrocytoma cells. The muscarinic receptor agonist carbachol caused a dose- and time-dependent increase in proliferation, as measured by [3H]thymidine incorporation. The mitogenic effect was mimicked by several muscarinic, but not nicotinic receptor agonists, and was blocked by muscarinic receptor antagonists. Reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated the presence of m2, m3 and to a lesser degree, m5 muscarinic receptor mRNA in both astrocytes and astrocytoma cells. Proliferation experiments with subtype-specific muscarinic receptor antagonists suggest that carbachol-induced proliferation is due to activation of muscarinic M3 receptors. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) also stimulated glial cell proliferation. Down-regulation of protein kinase C, or the protein kinase C antagonist 1,5-(isoquinolynsulfanyl)-2-methylpiperazine dihydrochloride (H7) blocked proliferation induced by either TPA or carbachol. Of other neurotransmitters tested, histamine caused glial cell proliferation, norepinephrine and gamma-aminobutyric acid were ineffective, while serotonin and glutamate inhibited basal or serum-stimulated proliferation.

Activation of FSH-responsive adenylate cyclase by staurosporine: role for protein phosphorylation in gonadotropin receptor desensitization

Prolonged stimulation of gonadotropin receptors in granulosa cells leads to desensitization of the cellular response to gonadotropic hormones which is evident by decrease in cAMP formation. In order to explore the mechanism of desensitization and to examine whether protein phosphorylation may play a role in this phenomenon, we have studied the effect of various stimulators and inhibitors of protein phosphorylation on FSH-induced cAMP formation in the FSH-responsive cell line, GFSHR-17, recently established in our laboratory. Both ovine and human FSH activated the hormone sensitive adenylate cyclase in a dose-dependent manner with an ED50 of 0.5 nM. This stimulation was followed by a sharp decrease in cAMP formation after 30 min incubation of the cell with the hormone. When cells were preincubated for 60 min with staurosporine, cAMP accumulation during 20 min of FSH stimulation was elevated about 500%, compared to cells stimulated by FSH alone. Staurosporine alone showed a negligible effect on cAMP accumulation in these cells. In cells stimulated with forskolin, a non-specific activator of adenylate cyclase, or with cholera toxin (CT), an inhibitor of GTPase activity associated with Gs of adenylate cyclase, preincubation with staurosporine increased cAMP formation in these cells by only 50-70 or 80-120%, respectively. Preincubation of cells with the protein kinase C (PKC) inhibitors chelerythrine and GF109203X increased FSH-stimulated accumulation of cAMP by 50 and 30%, respectively. These drugs exhibit a similar effect on forskolin-stimulated cells. Preincubation of cells for 60 min with a PKC stimulator, TPA, suppressed FSH-mediated cAMP response in these cells by 40%. Tyrosine kinase inhibitors such as AG18, AG33 and genistein exhibit a modest inhibitory effect of up to 20% on FSH-stimulated cAMP accumulation. All the above results were obtained both in the presence and absence of IBMX, a potent inhibitor of the cellular phosphodiesterases. Upon prolonged incubation with FSH (3 h) cells pretreated with staurosporine exhibited a much slower rate of decline in intracellular cAMP levels. Moreover, in desensitized cells, following 1 or 2 h of continuous stimulation with FSH, staurosporine could markedly enhance cAMP formation in the presence of FSH. Our data suggest that staurosporine-sensitive phosphorylation of serine or threonine in the FSH receptor-cyclase system may be responsible for desensitization of the FSH coupled activation of cAMP formation, while reactivation of the system can be achieved by protein dephosphorylation at these specific sites. Because specific inhibition of PKC could not mimic the staurosporine effect on FSH-stimulated cAMP formation, nor could activation of kinase C antagonize it, it is suggested that a specific staurosporine-sensitive receptor kinase may be responsible for modulation of the coupling between the gonadotropin receptor and the adenylate cyclase system.

Developmental changes in purine nucleotide metabolism in cultured rat astroglia

The present study was conducted in order to clarify the role of the glia in brain purine metabolism. This, in connection with the clarification of the etiology of the neurological manifestations associated with some of the inborn errors of purine metabolism in man. Purine nucleotide content, the capacity for de novo and salvage purine synthesis and the activity of several enzymes of purine nucleotide degradation, were assayed in primary cultures of rat astroglia in relation to culture age. The capacity of the intact cells to produce purine nucleotides de novo exhibited a marked decrease with the culture age, but the activity of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), catalyzing salvage nucleotide synthesis, increased. Aging was also associated with a marked increase in the activity of the degradation enzymes AMP deaminase, purine nucleoside phosphorylase (PNP) and guanine deaminase (guanase). The activity of adenosine deaminase and of AMP-5'-nucleotidase, increased markedly during the first 17 days in culture, but decreased thereafter. The results indicate that purine nucleotide metabolism in the cultured astroglia is changing with aging to allow the cells to maintain their nucleotide pool by reutilization of preformed hypoxanthine, rather than by de-novo production of new purines. Aging is also associated with increased capacity for operation of the adenine nucleotide cycle, contributing to the homeostasis of adenine nucleotides and to the energy charge of the cells. In principle, the age-related alterations in purine metabolism in the astroglia resemble those occurring in the maturating neurons, except for the capacity to produce purines de novo, which exhibited inverse trends in the two tissues. However, in comparison to the neurons, the cultured astroglia possess the capacity for a more intensive metabolism of purine nucleotides.

Thy-1+ dendritic cells express truncated form of POMC mRNA

The present study was designed to examine the expression of proopiomelanocortin (POMC) and its related derivative peptide adrenocorticotropic hormone (ACTH) in murine derived Thy-1+ dendritic cells. Immunostaining using a polyclonal antibody specific to ACTH and parent POMC molecule indicated the presence of POMC and its derivative peptide, ACTH, in cultures of Thy-1+ dendritic cells. To explore whether the POMC peptide is present as a reservoir or synthesized de novo in Thy-1+ dendritic cells. Northern blot analysis using 30-mer oligonucleotide probe for alpha-MSH/ACTH precursor POMC was carried out in total RNA from these cells. Northern blot analysis revealed the presence of POMC like mRNA transcript. However, the observed size of transcript was smaller (approximately 0.9 kb) than that expressed by murine AtT20 cells (approximately 1.2 kb), an anterior pituitary tumor cell line used as a positive control. These observations suggest that the epidermal Thy-1+ lymphocytes, like thymic lymphocytes, might serve the epidermis as one source for the synthesis of POMC. The synthesis and presence of POMC in the epidermis may be related to some of the pigmentary anomalies observed in many mucocutaneous disorders.

Interaction of ethanol and anoxia with muscarinic receptor--stimulated phosphoinositide metabolism during brain development

The mechanism(s) by which ethanol induces alterations in brain development may involve direct actions (e.g. changes in specific biochemical pathways), or indirect effects, such as cerebral hypoxia resulting from ethanol - induced circulatory changes. Since both ethanol and hypoxia are known to affect the metabolism of phosphoinositides, which has been suggested as a possible target for ethanol's developmental neurotoxicity, in the present study we have investigated the in vitro effects of both severe hypoxia (anoxia) and ethanol (alone or in combination) on muscarinic receptor-stimulated phosphoinositide metabolism in cerebral cortex slices from neonatal rats. Anoxia markedly inhibited carbachol - stimulated phosphoinositide metabolism in adult rats (67%), but only slightly (10%) in neonatal animals. Reoxygenation reversed the effect of anoxia at both ages. On the other hand, ethanol's inhibitory effect was pronounced in neonatal rats only, and was additive to that of anoxia. The presence of ethanol did not affect the recovery of carbachol - stimulated phosphoinositide metabolism following anoxia and reoxygenation. These results indicate that ethanol and anoxia differently and independently affect muscarinic receptor - stimulated phosphoinositide metabolism and may mutually contribute to the CNS effects observed following developmental ethanol exposure.

Regulation of GFAP expression in glial-like cells of the rat pituitary intermediate lobe by lactation, salt-loading, and adrenalectomy

Glial-like cells in rat pituitary intermediate lobe were localized and characterized by immunohistochemistry with antisera against glial fibrillary acidic protein (GFAP) and S-100. Individual GFAP immunoreactive (IR) cells possessed several processes that often branched into secondary and tertiary processes, terminating with end-feet. The GFAP-immunopositive cell population was distributed in specific rostrocaudal and dorsoventral patterns. The distribution and numbers of cells differed between male and female rats. Examination of altered physiological states, e.g., adrenalectomy, lactation, and salt-loading, revealed state-specific changes in the appearance and distribution of GFAP-IR cells. Adrenalectomy and lactation increased GFAP-IR glial-like cell numbers, whereas salt-loading decreased their numbers and the typical pattern of distribution. By contrast, S-100-expressing cells were evenly distributed in male and female rats, and its expression was not affected by the experimental conditions. Double-label immunocytochemistry indicated that GFAP-IR cells are a subpopulation of S-100-IR cells. These results suggest that cells normally expressing only S-100 may be induced to express GFAP under altered physiologic conditions.

Increased intracellular cyclic AMP levels suppress the mitogenic responses of human astrocytoma cells to growth factors

It has been shown that the intracellular cAMP levels were decreased in human malignant astrocytomas. On the other hand, various growth factors and their receptors were found to be overexpressed in these tumors. It is therefore intriguing as to whether there is interplay between the two phenomena in the modulation of the astrocytoma cell growth. In a basal medium consisting of 75% DMEM, 25% Ham's F-12 supplemented with 2% FBS, we show that the mitogenic effects of platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and epidermal growth factor (EGF) on human astrocytoma cells were suppressed by dibutyryl-cAMP. Dibutyryl-cAMP alone neither potentiated nor inhibited the tumor cell growth. Further studies show that PDGF-induced receptor autophosphorylation in human astrocytoma cells is suppressed by increased intracellular cAMP levels as measured by immunoprecipitation with anti-PDGF receptor and antiphosphotyrosine antibodies. Our results indicate that there is antagonistic interplay between the receptor tyrosine kinase pathway and cAMP-dependent protein kinase pathway in the control of the malignantly transformed glial cells. A reduced cAMP level seen in many human astrocytoma cells may favor their response to growth factor mitogenesis.

Melanocortin analogue Org2766 binds to rat Schwann cells, upregulates NGF low-affinity receptor p75, and releases neurotrophic activity

Binding of the stable melanocortin(4-9) analogue, Org2766 [Met(O2)-Glu-His-Phe-D-Lys-Phe] to cultured rat sciatic nerve Schwann cells was demonstrated using a biotinylated derivative in semiquantitative histochemical and CELISA assays. Org2766 bound to Schwann cells, but not to fibroblasts, and was displaced maximally by unlabeled Org2766, alpha-MSH and ACTH(1-24). Displacement of Org2766 from the binding sites was considerably reduced by N- and C-truncation of the peptide. Specific binding of Org2766 was also demonstrated in the immortal rat Schwann cell line SCL4.1/F7 and was more pronounced in cells displaying a differentiated morphology. Org2766 and alpha-MSH increased cyclic AMP content of Schwann cells but neither stimulated DNA synthesis when applied alone. However, in the presence of a priming (subthreshold) concentration of the mitogen, cholera toxin, Org2766 and alpha-MSH caused a delayed increase in DNA synthesis. Org2766 did not modulate the expression of several differentiation-related Schwann cell markers. However, Org2766 increased immunoreactivity for p75 low-affinity NGF receptor on Schwann cells and evoked the release of neurotrophic factor(s) that synergized with NGF in stimulating neurite outgrowth in rat DRG neurons. The results indicate that Schwann cells are a primary target for the action of Org2766 and provide evidence for an indirect mechanism by which melanocortins might stimulate neurite sprouting in regenerating peripheral nerve axons.

ACTH-related peptides: receptors and signal transduction systems involved in their neurotrophic and neuroprotective actions

ACTH-related peptides are promising neurotrophic and neuroprotective agents, as demonstrated in many in vivo and in vitro studies. They accelerate nerve repair after injury, improving both sensor and motor function. Furthermore, ACTH-related peptides have neuroprotective properties against cisplatin- and taxol-induced neurotoxicity, they improve neuronal function in animals with neuropathy due to experimental diabetes, and they prevent degeneration of myelinated axons in rats suffering from experimental allergic neuritis, a model of peripheral demyelinating neuropathy. Studies in neuronal cultures have corroborated these clinical observations and serve to investigate the mechanism of action of the ACTH-related peptide effects. This paper reviews both in vitro and in vivo effects and emphasizes the mechanism of action. Recent data on melanotrophic receptors and signal transduction systems will be discussed in this context.

The role of calcium and cAMP in the mechanism of action of two melanocortins: alpha MSH and the ACTH4-9 analogue Org 2766

Melanocortins accelerate functional recovery after nerve crush and enhance neurite outgrowth in vitro. To get more insight in the mechanism of action of melanocortins, we studied the effects of two neurotrophic peptides: alpha-melanocyte-stimulating hormone (alpha MSH) and an adrenocorticotropin4-9 analogue Org 2766 on second messengers in cultures of spinal cord (SC), dorsal root ganglion (DRG) and Schwann cells. alpha MSH (10 microM) enhanced the forskolin-induced cAMP production in SC- (45%) and in DRG-cells (35%). Org 2766 (1 microM) induced an increase in cAMP only in SC-cells (39%). The peptides did not affect the cAMP levels in Schwann cells. Neither peptide evoked significant changes in the intracellular free calcium concentration ([Ca2+]i) in batch-measurements of all cell types, however, Ca(2+)-imaging revealed an infrequent occurrence of large [Ca2+]i-elevations in individual SC-neurons. The results indicate that SC- and DRG-cells are targets for both peptides, while Schwann cells are not or exploit different pathways. We observed for alpha MSH that cAMP production always coincides with outgrowth stimulation, whereas for Org 2766 cAMP production and outgrowth stimulation appear not causally related. These differences in second messenger stimulation could be explained by receptor heterogeneity. We suggest that alpha MSH and Org 2766 act through different receptors, each with its own signalling pathways.

Molecular cloning, functional expression and pharmacological characterization of a mouse melanocortin receptor gene

We describe the cloning of the mouse HGMP01A gene that encodes a melanocortin receptor functionally distinct from the adrenal cortex corticotropin (adrenocorticotrophic hormone; ACTH) receptor and the melanocyte-stimulating hormone (MSH) receptor expressed in melanoma. The gene encodes a protein of 323 amino acids with a calculated molecular mass of 35,800 Da, displaying potential sites for N-linked glycosylation and phosphorylation by protein kinase C. An RNAase protection assay detected weak expression in the brain, but not in adrenal gland, skin, or any of the other tissues tested. Stable CHO cell lines expressing over 100,000 receptors per cell were generated. The recombinant receptor binds iodinated [Nle4,D-Phe7]alpha-MSH (NDP-MSH) with an apparent Kd of 700 pM. Displacement of the ligand by a variety of pro-opiomelanocortin-derived peptides revealed a pharmacological profile distinct from that of the classical ACTH and MSH receptors. NDP-MSH was the most powerful competitor (IC50 1.4 nM), followed by gamma-MSH (IC50 7 nM). alpha-MSH, beta-MSH and ACTH-(1-39) were significantly less potent, with IC50 values of 30, 19 and 21 nM respectively. ACTH-(4-10) was poorly active (IC50 2.4 microM), while corticotropin-like intermediate lobe peptide (CLIP) and beta-endorphin were totally ineffective. The recombinant receptor was found to stimulate adenylate cyclase. The potency order of the agonists in this assay was consistent with that of the binding displacement assays. This receptor represents the orthologue of the human melanocortin 3 receptor reported recently. The growing family of melanocortin receptors constitute the molecular basis for the variety of actions of melanocortins that have been described over the years. The availability of functionally expressed receptors from the melanocortin family will allow the development of a specific pharmacology, and a better understanding of the function of the pro-opiomelanocortin-derived peptides.

Developmental neurotoxicity of ethanol: further evidence for an involvement of muscarinic receptor-stimulated phosphoinositide hydrolysis

Various lines of evidence suggest that muscarinic receptor-stimulated phosphoinositide hydrolysis during postnatal development in the rat brain may play a relevant role in glial cell proliferation and neuronal differentiation. We have previously shown that administration of ethanol to developing rats during the brain growth spurt causes microencephaly and selectively decreases muscarinic receptor-stimulated phosphoinositide hydrolysis. In the present study we have investigated the sensitivity of the phosphoinositide system coupled to muscarinic receptors to ethanol inhibition during distinct stages of the brain growth spurt. Different groups of rats were treated for 3 days with ethanol (4 g/kg per day) on postnatal days 2-4 (initial), 6-8 or 10-12 (peak), 13-15 (final stage of the brain growth spurt). The results show that the period of maximal sensitivity to ethanol of muscarinic receptor-stimulated phosphoinositide hydrolysis coincides with the peak of the brain growth spurt and with the period of maximal efficacy of muscarinic receptor agonists to induce inositol phosphates accumulation. Interestingly, only when muscarinic receptor-stimulated phosphoinositide hydrolysis was inhibited, a significant reduction of brain weight was observed. The close parallel between inhibition of this second messenger response and reduction of brain weight suggests that the phosphoinositide system coupled to muscarinic receptors may represent a target for the neurotoxic effects of ethanol during this stage of brain development.

Adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides modulate adenylate cyclase activity in rat brain slices: evidence for an ACTH/MSH receptor-coupled mechanism

The regulation of adenylate cyclase activity by adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides was investigated in rat brain slices using a superfusion method. Adenylate cyclase activity was concentration-dependently increased by ACTH-(1-24), alpha-MSH (EC50 values 16 and 6 nM, respectively), and [Nle4,D-Phe7]alpha-MSH (EC50 value 1.6 nM), in the presence of forskolin (1 microM, optimal concentration). 1-9-Dideoxyforskolin did not augment the response of adenylate cyclase to ACTH-(1-24). Various peptide fragments were tested for their ability to enhance [3H]cyclic AMP production. [Nle4,D-Phe7]alpha-MSH increased [3H]cyclic AMP formation with a maximal effect of 30% and was more potent than ACTH-(1-24), ACTH-(1-16)-NH2, alpha-MSH, ACTH-(1-13)-NH2, [MetO4]alpha-MSH, [MetO2(4),D-Lys8,Phe9]ACTH-(4-9), ACTH-(7-16)-NH2, ACTH-(1-10), and ACTH-(11-24), in order of potency. This structure-activity relationship resembles that found for the previously described peptide-induced display of excessive grooming. ACTH-(1-24) stimulated adenylate cyclase activity in both striatal (maximal effect, approximately 20%) and septal slices (maximal effect, approximately 40%), but not in hippocampal or cortical slices. Lesioning of the dopaminergic projections to the striatum did not result in a diminished effect of [Nle4,D-Phe7]alpha-MSH on [3H]cyclic AMP accumulation, which indicates that the ACTH/MSH receptor-stimulated adenylate cyclase is not located on striatal dopaminergic terminals. ACTH-(1-24) did not affect the dopamine D1 or D2 receptor-mediated modulation of adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Signaling mechanisms controlled by melanocortins in melanoma, lacrimal, and brain astroglial cells

Melanocortins appear to be involved as regulators in an ever growing number of physiological processes in cells and tissues of diverse functions. While such trends are apparent also in the case of other peptide hormones, it appears that melanocortin receptors can be regarded as unique among G-protein-linked receptors due to their special need for extracellular Ca2+ which may relate to some, yet undetermined selectivity of their actions. The physiological role that Ca2+ may be playing and the diverse signaling mechanisms regulated, as well as the nature of the cell-specific responses elicited in melanocortin-sensitive cells/tissues, have yet to be elucidated. Likewise, it will be of interest to establish the relationship of melanocortins to processes like growth and differentiation of cells, as well as to higher, more complex processes such as those regulated in the CNS.

Mechanism of action of melanocortin peptides. Possible role in astrocyte regulation

Melanocortin peptides exert pleiotropic effect in numerous cell types, controlling processes ranging from adrenal steroidogenesis and melanocyte pigmentation to lacrimation and nerve regeneration. The binding of melanocortins to specific cell surface receptors initiates cellular responses via GTP binding proteins (G-proteins). The affinity of these peptides to the receptor is modulated by extracellular Ca2+ ions, a property unique to melanocortin receptors. In astrocyte cultures derived from the rat brain, melanocortin stimulation elevates cAMP levels that appear to induce morphological changes. However, a transient proliferative response to melanocortins in these cells appears to be cAMP independent. The presence of melanocortin receptors in brain tissue and their unique Ca2+ dependence are discussed in relation to their putative role as regulators of astrocytes.