alpha-MSH and ACTH4-9 analogue Org 2766 induce a cAMP increase in cultured rat spinal cord cells

The modulatory role of alpha-melanocyte stimulating hormone administered spinally in the regulation of blood glucose level in d-glucose-fed and restraint stress mouse models

Alpha-melanocyte stimulating hormone (α-MSH) is known as a regulator of the blood glucose homeostasis and food intake. In the present study, the possible roles of α-MSH located in the spinal cord in the regulation of the blood glucose level were investigated in d-glucose-fed and immobilization stress (IMO) mouse models. We found in the present study that intrathecal (i.t.) injection with α-MSH alone did not affect the blood glucose level. However, i.t. administration with α-MSH reduced the blood glucose level in d-glucose-fed model. The plasma insulin level was increased in d-glucose-fed model and was further increased by α-MSH, whereas α-MSH did not affect plasma corticosterone level in d-glucose-fed model. In addition, i.t. administration with glucagon alone enhanced blood glucose level and, i.t. injection with glucagon also increased the blood glucose level in d-glucose-fed model. In contrasted to results observed in d-glucose-fed model, i.t. treatment with α-MSH caused enhancement of the blood glucose level in IMO model. The plasma insulin level was increased in IMO model. The increased plasma insulin level by IMO was reduced by i.t. treatment with α-MSH, whereas i.t. pretreatment with α-MSH did not affect plasma corticosterone level in IMO model. Taken together, although spinally located α-MSH itself does not alter the blood glucose level, our results suggest that the activation of α-MSH system located in the spinal cord play important modulatory roles for the reduction of the blood glucose level in d-glucose fed model whereas α-MSH is responsible for the up-regulation of the blood glucose level in IMO model. The enhancement of insulin release may be responsible for modulatory action of α-MSH in down-regulation of the blood glucose in d-glucose fed model whereas reduction of insulin release may be responsible for modulatory action of α-MSH in up-regulation of the blood glucose in IMO model.

Melanocortins and the brain: from effects via receptors to drug targets

The lack of specific receptors (and antagonists) has hampered the research on the neural mechanism of action of adrenocorticotropic hormone (ACTH)- and melanocyte-stimulating hormone (MSH)-like peptides. Yet the original observations in the 1970s already pointed to cAMP as a possible mediator of ACTH/MSH effects in neurons. The cloning of melanocortin receptors since 1992, the identification of at least two subtypes (melanocortin MC(3) and MC(4) receptors) that are present in neural tissue and the development of selective and potent agonists as well as antagonists have markedly furthered the position of melanocortins as important neuropeptides. In this paper we discuss the role of especially the receptor subtype melanocortin MC(4) in various behaviors including grooming behavior and feeding behavior and consider new insights in the interaction between the opioid and the melanocortin system at the level of the spinal cord (i.e. pain perception). Finally, based on new data obtained in molecular pharmacological studies on brain melanocortin receptors, we suggest a general concept for selective receptor-ligand interaction: ligand residues outside the peptide core-sequence may direct the conformation of the residues in the ligand core-sequence that interact directly with the receptor-binding pocket and thereby determine selectivity.

Receptor-Mediated Modulation of Murine Mast Cell Function by α-Melanocyte Stimulating Hormone

The proopiomelanocortin (POMC)-derived neuropeptide α-melanocyte stimulating hormone (α-MSH) is known to modulate some aspects of inflammation through direct effects on T cells, B cells, and monocytes. To determine whether α-MSH might similarly influence mast cell responsiveness, mast cells were examined to see if they expressed the receptor for α-MSH, melanocortin-1 (MC-1), and whether α-MSH altered mast cell function. We thus first identified MC-1 on bone marrow cultured murine mast cells (BMCMC) and a murine mast cell line (MCP-5) employing flow cytometry and through detection of specific binding. Subsequent treatment of mast cells with α-MSH increased the cAMP concentration in a characteristic biphasic pattern, demonstrating that α-MSH could affect intracellular processes. We next examined the effect of α-MSH on mediator release and cytokine expression. IgE/DNP-human serum albumin-stimulated histamine release from mast cells was inhibited by ∼60% in the presence of α-MSH. Although activation of BMCMC induced the expression of mRNAs for the inflammatory cytokines IL-1β, IL-4, IL-6, TNF-α, and the chemokine lymphotactin, mRNAs for IL-1β, TNF-α, and lymphotactin were down-modulated in the presence of α-MSH. Finally, IL-3-dependent proliferative activity of BMCMC was slightly but significantly augmented by α-MSH. Taken together, these observations suggest that α-MSH may exert an inhibitory effect on the mast cell-dependent component of a specific inflammatory response.

Melanocortin receptors mediate alpha-MSH-induced stimulation of neurite outgrowth in neuro 2A cells

Melanocortins (MC), neuropeptides derived from pro-opiomelanocortin, have been implicated in enhancing neurite outgrowth via an as yet unknown mechanism. Recently, five MC receptors have been identified, three of which, the MC3-R, the MC4-R and the MC5-R, are expressed in the nervous system. In this study, alpha-MSH and the melanocortin analog [D-Phe7]ACTH (4-10) were able to stimulate neurite outgrowth in the neuroblastoma cell line Neuro 2A. ACTH (4-10), gamma2-MSH and ORG2766 were inactive. In addition, the MC4-R antagonist [D-Arg8]ACTH (4-10), inhibited the alpha-MSH effect, indicating that the MC4-R mediated stimulation of neurite outgrowth by alpha-MSH. Indeed, the presence of MC4-R mRNA in Neuro 2A cells was demonstrated by a RNase protection assay. Heterologous expression of the MC5-R in Neuro 2A cells lead to the recruitment of a responsiveness to gamma2-MSH, but did not increase the effect of alpha-MSH on neurite outgrowth. This finding indicated that the function of MC4-R can also be exerted by another MC receptor, suggesting that the coupling to Gs, which they have in common, plays an essential role in the neurite outgrowth promoting effect. This was further substantiated by the fact that forskolin treatment per se induced neurite outgrowth in a similar fashion. These data imply that the neurotrophic properties of alpha-MSH are likely to result from Gs-coupled MC receptor activity in neuronal cells.

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.

Protection by an ACTH4-9 analogue against the toxic effects of cisplatin and taxol on sensory neurons and glial cells in vitro

Sensory neuropathy is a serious side effect of anti-tumour drugs such as cisplatin and taxol. There are indications that an analogue of the adrenocorticotrophic hormone 4-9 fragment (ACTH4-9: Met(O2)-Glu-His-Phe-D-Lys-Phe) can prevent these neurotoxic effects. We studied the potential protective effects of this analogue in cultures of chick dorsal root ganglia and rat Schwann cells treated with cisplatin or taxol to gain insight into the mode of action and characteristics of this neuroprotection. Neurite outgrowth of sensory neurons in vitro was dose-dependently inhibited by cisplatin and taxol; after 48 hr, 10 micrograms/ml cisplatin reduced outgrowth from 431 +/- 17 microns to 220 +/- 6 microns and 0.01 micrograms/ml taxol from 344 +/- 3 microns to 200 +/- 43 microns. Co-treatment of 10 micrograms/ml cisplatin with the ACTH4-9 analogue (0.1 nM-1 nM) resulted in about 35% more outgrowth than cisplatin alone. In contrast, the analogue could not prevent taxol neurotoxicity. Migration of neurons and satellite cells from the DRG-body is completely inhibited by 10 micrograms/ml cisplatin. Taxol had no effect on the migration of these cells. In addition, cisplatin was more toxic to Schwann cells than taxol; 3-10 micrograms/ml cisplatin significantly reduced their laminin content, total protein, 2',3'-cyclic nucleotide 3'-phosphodiesterase activity, and cell division. The ACTH4-9 analogue (0.01 nM-100 nM) had no effect on the migration of cells out of the DRGs and could not prevent the toxic effect on the Schwann cells. These data support our hypothesis that the neuroprotective effect of ACTH4-9 analogue is brought about by a direct action on neurons, possibly by replacing a Schwann-/satellite-cell derived trophic factor.