Activation of the cGMP pathway in dopaminergic structures reduces cocaine-induced EGR-1 expression and locomotor activity.

Nitric oxide (NO) and the C-type natriuretic peptide (CNP) exert their action on brain via the cGMP signaling pathway. NO, by activating soluble guanylyl cyclase, and CNP, by stimulating membrane-bound guanylyl cyclase, cause intracellular increases of cGMP, activating cGMP-dependent protein kinases (PKGs). We show here that injection of CNP into the rat ventral tegmental area strongly reduced cocaine-induced egr-1 expression in the nucleus accumbens in a dose-dependent manner. The effect of CNP was reversed by the previous injection of a selective PKG inhibitor, KT5823. Activation of PKG by 8-bromo-cGMP reduced, like CNP, cocaine-induced gene transcription in dopaminergic structures. To confirm the involvement of PKG, this was overexpressed in either the mesencephalon or the caudate-putamen. Using the polyethyleneimine delivery system, an active protein was expressed by injecting a plasmid vector containing the human PKG-Ialpha cDNA. PKG was overexpressed in dopaminergic and GABAergic neurons when the plasmid was injected in the ventral tegmental area, whereas overexpression was observed in medium spiny GABAergic neurons and in both cholinergic and GABAergic interneurons when the PKG vector was injected into the caudate-putamen. Activation of the overexpressed PKG reduced cocaine-induced egr-1 expression in dopaminergic structures and affected behavior (i.e., locomotor activity). These effects were again reversed by previous injection of the selective PKG inhibitor. The current data suggest that NO and the neuropeptide CNP are potential regulators of cocaine-related effects on behavior.

Acute or repeated cocaine administration generates reactive oxygen species and induces antioxidant enzyme activity in dopaminergic rat brain structures.

Either a single (acute) or repeated daily (chronic) injections (1 injection/day) of 20 mg/kg cocaine for 10 days to rats was found to increase reactive oxygen species production in two dopaminergic brain structures, the frontal cortex and the striatum. We found that the mitochondrial genome was down-regulated after acute cocaine injection. Hydroperoxide and lipid peroxide generation was correlated with an increase in mitochondrial hydrogen peroxide generation and with a reduced functioning of mitochondrial complex I in response to cocaine. As judged from the measurement of caspase-3 activity and TUNEL labeling, neither acute nor chronic cocaine treatment has been found to induce apoptosis in any of the structures examined. This differs dramatically from what has been described for methamphetamine. Cocaine-induced radical formation was accompanied by the induction of the antioxidant enzymes superoxide dismutase and glutathione peroxidase, after both acute and chronic cocaine treatment. In addition, proteasome chymotrypsin-like activity was enhanced following a single cocaine injection in both cortex and striatum. It is proposed that the compensatory mechanisms to oxidative stress occurring in response to cocaine were effective in scavenging reactive oxygen species and in preventing subsequent cellular damage, thus explaining why no significant cell death was found in these brain structures.

Expression of the methyl-CpG-binding protein MeCP2 in rat brain. An ontogenetic study.

Rett syndrome (RS) is caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2) and is characterized by arrested postnatal neurodevelopment. We followed the expression of MeCP2 protein in various brain structures of normal rat from birth to 2 years of age. By measuring the amount of protein using the Western blot technique, or by determining the percentage of immunoreactive cells, significant heterogeneity in MeCP2 distribution among various brain areas was observed. Highest expression was found in olfactory bulb and in frontal cortex. In contrast, little expression was detected in caudate-putamen, septum and hippocampus. Except in the olfactive nuclei, very few cells showed detectable MeCP2 protein at birth. The number increased during the first week of age, especially in cortex and nucleus accumbens. Rather than playing a global role in gene transcription, the heterogeneous distribution of MeCP2 transcription factor favors the idea that it has a specialized function in neurons.