Characterization of melanocortin receptor ligands on cloned brain melanocortin receptors and on grooming behavior in the rat.

Since the melanocortin MC3 and melanocortin MC4 receptors are the main melanocortin receptor subtypes expressed in rat brain, we characterized the activity and affinity of nine melanocortin receptor ligands using these receptors in vitro, as well as their activity in a well-defined melanocortin-induced behavior in the rat: grooming behavior. We report here that [D-Tyr4]melanotan-II and RMI-2001 (Ac-cyclo-[Cys4, Gly5, D-Phe7, Cys10]alpha-MSH-NH2) have significantly higher affinity and potency on the rat melanocortin MC4 receptor as compared to the rat melanocortin MC3 receptor. Nle-gamma-MSH (melanocyte-stimulating hormone) was the only ligand with higher affinity and potency on the rat melanocortin MC3 receptor. The potency order of melanocortin MC4 receptor agonists, but not that of melanocortin MC3 receptor agonists, fitted with the potency of these ligands to stimulate grooming behavior, when administered intracerebroventricularly. SHU9119 (Ac-cyclo-[Nle4, Asp5, D-Nal(2)7, Lys10]alpha-MSH-(4-10)-NH2) and RMI-2005 (Ac-cyclo-[Cys4, Gly5, D-Na](2)7, Nal(2)9, Cys10]alpha-MSH-(4-10)-NH2) were able to inhibit alpha-MSH-induced melanocortin receptor activity in vitro, as well as alpha-MSH-induced grooming behavior. Melanotan-II, [Nle4-D-Phe7]alpha-MSH and RMI-2001 were also effective in inducing grooming behavior when administered intravenously. In the absence of purely selective melanocortin MC(3/4) receptor ligands, we demonstrated that careful comparison of ligand potencies in vitro with ligand potencies in vivo, could identify which melanocortin receptor subtype mediated alpha-MSH-induced grooming behavior. Furthermore, blockade of novelty-induced grooming behavior by SHU9119 demonstrated that this physiological stress response is mediated via activation of the melanocortin system.

Brain as a unique antisense environment.

During the last few years, antisense oligodeoxyribonucleotides (asODN) have become a commonly used tool for blocking of gene expression in the mammalian central nervous system. Successful gene inhibition has been reported for such diverse targets as those encoding neurotransmitter receptors, neuropeptides, trophic factors, transcription factors, cytokines, transporters, ion channels, and others. This review presents a discussion of recent studies on ODN in the brain, with a focus on specific approaches taken by the researchers in this field and especially on peculiar features of this organ as a milieu for asODN action. It is concluded that from the presented literature survey no coherent view on how to rationally design ODN for brain studies has emerged.

Plasticity- and neurodegeneration-linked cyclic-AMP responsive element modulator/inducible cyclic-AMP early repressor messenger RNA expression in the rat brain.

In order to explore the role of CREM (cyclic-AMP responsive element modulator) gene expression in the function of the central nervous system, the gene transcripts were investigated in the rat brain in several conditions linked to increased neuronal activity. Up-regulation of CREM messenger RNA levels in the hippocampus was found to follow intraperitoneal administration of kainate (10 mg/kg). This increase was observed in both the dentate gyrus and hippocampus proper (CA subfields) and reached its maximum at 6 h after the treatment. Intrahippocampal injection of N-methyl-D-aspartate (200 nmol) resulted in elevated CREM messenger RNA expression as well. A similar increase of the messenger RNA abundance was also observed in the retrosplenial cortex after treating the female rats with a high dose (5 mg/kg) of dizocilpine maleate, an N-methyl-D-aspartate receptor antagonist. All these conditions are linked to neuronal excitation and neurodegeneration. However, an increase in CREM messenger RNA accumulation was also observed in the visual cortex after exposure of dark-adapted animals to the light, a procedure linked to neuronal plasticity. In the latter condition, it was found that CREM messenger RNA reached its highest levels at 6 h, i.e. later than the maximal increase of expression of immediate early genes such as c-fos, jun B and zif268, observed 45 min following the onset of visual stimulation. The ICER (inducible cyclic-AMP early repressor) form of CREM messenger RNA was identified to be induced by the light exposure. Finally, it was also found that cycloheximide, an inhibitor of protein synthesis, overinduces CREM/ICER gene expression. Together, these data suggest that CREM/ICER may be responsive to neuronal activation. Furthermore, given that CREM products have been shown previously to down-regulate expression of immediate early genes in vitro, they suggest that ICER may function as a molecular switch involved in down-regulation of immediate early gene expression in the rat brain.

Kainate-evoked modulation of gene expression in rat brain.

Kainate is a glutamate analog that produces neuronal excitation resulting in seizures within hours following its intraperitoneal injection into adult rats. Then, at 2-3 days after the treatment, neurodegeneration of apoptotic character can be observed in limbic system. As a consequence, plastic reorganization and glial reactivation phenomena occur. These physiological and pathological responses are reflected by specific changes in gene expression, that can be dissected according to their spatio-temporal patterns. The early phase of gene expression observed in all hippocampal subfields appears to reflect a sudden burst of spiking activity. Changes in mRNA levels restricted to dentate gyrus are suggestive of a link to neuronal plasticity. The late gene expression response implies its correlation either to neuronal cell death or glial reactivation, depending on cellular localization of gene products. Thus analysis of the temporal and spatial gene expression pattern in the hippocampus after kainate treatment may provide clues revealing specific phenomena to which gene expression could be attributed.