Suppressive effect of carbachol on forskolin-stimulated neurite outgrowth in human neuroblastoma NB-OK1 cells

The role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception

The melanocortins are a family of bioactive peptides derived from proopiomelanocortin. Those peptides, included among hormones and comprising ACTH, alpha-MSH, beta-MSH and gamma-MSH, are best known mainly for their physiological effects, such as the control of skin pigmentation by alpha-MSH, and ACTH effects on pigmentation and steroidogenesis. Melanocortins are released in various sites in the central nervous system and in peripheral tissues, and participate in the regulation of multiple physiological functions. They are involved in grooming behavior, food intake and thermoregulation processes, and can also modulate the response of the immune system in inflammatory states. Research of the past decade provided evidence that melanocortins could elicit their diverse biological effects by binding to a distinct family of G protein-coupled receptors with seven transmembrane domains. To date, five melanocortin receptor genes have been cloned and characterized. Those receptors differ in their tissue distribution and in their ability to recognize various melanocortins. These advances have opened up new horizons for exploring the significance of melanocortins, their ligands and their receptors for a variety of important physiological functions. We reviewed the origin of MSH peptides, the function and distribution of melanocortin receptors and their endogenous and exogenous ligands and the role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception. Moreover, we analyzed their interaction with opioid peptides and finally, we discussed the postulated role of the melanocortin system in pain transmission at the spinal cord level.

Bimodal regulation of cyclic AMP by muscarinic receptors. Involvement of multiple G proteins and different forms of adenylyl cyclase

In membranes of rat olfactory bulb, muscarinic receptor agonists stimulate basal adenylyl cyclase activity . This response is inhibited by a number of muscarinic receptor antagonists with a rank order of potency suggesting the involvement of the M4 muscarinic receptor subtype. The stimulatory effect does not require Ca2+ and occurs independently of activation of phosphoinositide hydrolysis. Pretreatment with pertussis toxin completely prevents the muscarinic stimulation of adenylyl cyclase, indicating the participation of G proteins of the Gi/Go family. Immunological impairment of the G protein, Gs, also reduces the muscarinic response, whereas concomitant activation of Gs-coupled receptors by CRH or VIP results in a synergistic stimulation of adenylyl cyclase activity. Although these data suggest a role for Gs, a body of evidence indicates that the muscarinic receptors do not interact directly with this G protein. Moreover, the Ca2+/calmodulin (Ca2+/CaM)- and forskolin-stimulated enzyme activities are inhibited by muscarinic receptor activation in a pertussis toxin-sensitive manner and with a pharmacological profile similar to that observed for the stimulatory response. These data indicate that in rat olfactory bulb M4 muscarinic receptors exert a bimodal control on cyclic AMP formation through a sequence of events that may involve activation of Gi/Go proteins, synergistic interaction with Gs and differential modulation of Ca2+/CaM-independent and -dependent forms of adenylyl cyclase.

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.

Discrete activation of transduction pathways associated with acetylcholine m1 receptor by several muscarinic ligands

Activation of transfected muscarinic m1 acetylcholine receptors (m1AChR) has been linked to several signal transduction pathways which include phosphoinositide hydrolysis, arachidonic acid release and cAMP accumulation. In Chinese hamster ovary cells stably transfected with the rat m1AChR gene, carbachol elicited all three responses with EC50 values of 2.6, 3.8 and 76 microM, respectively. However, pilocarpine and the selective muscarinic agonist AF102B activated phosphoinositide hydrolysis (by 94 and 27% vs. carbachol, respectively), while antagonizing carbachol-mediated cAMP accumulation. Carbachol also activated (by 4-fold) adenylyl cyclase in membranes prepared from these cells, indicating independence of this signal from intracellular mediators. Moreover, carbachol and AF102B similarly elevated cytosolic Ca2+ in intact m1AChR-transfected cells. The ligand-selective cAMP accumulation, its independence from Ca2+ and the carbachol-activated adenylyl cyclase in membranes suggest that it represents an independent m1AChR-mediated signal, unrelated to phosphoinositide hydrolysis. Selective muscarinic ligands such as AF102B may independently activate distinct signalling pathways, which may be important for designing cholinergic replacement therapy for treating Alzheimer's disease.