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The neurotoxic effects of activated microglia in neurodegenerative diseases are well established. We recently provided evidence that chromogranin A (CGA), a multifunctional protein localized in dystrophic neurites and in senile plaques, induces an activated phenotype and secretion of neurotoxins by rat microglia in culture. In the present study, we focused on the mechanisms underlying neuronal degeneration triggered by CGAactivated microglia. We found that neuronal death exhibits apoptotic features, characterized by the externalization of phosphatidylserine and the fragmentation of DNA. Microglial neurotoxins markedly stimulate the phosphorylation and activity of neuronal p38 mitogenactivated protein kinase and provoke the release of mitochondrial cytochrome c, which precedes apoptosis. Inhibition of p38 kinase with SB 203580 partially protects neurons from death induced by CGA-activated microglia. Furthermore, neurons are also protected by Fas-Fc, which antagonizes the interactions between the death receptor Fas and its ligand FasL and by cell-permeable peptides that inhibit caspases 8 and 3. Thus, CGA triggers the release of microglial neurotoxins that mobilize several death-signaling pathways in neurons. Our results further support the idea that CGA, which is upregulated in many neuropathologies, represents a potent endogeneous inflammatory factor possibly responsible for neuronal degeneration.

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 caudateputamen. Using the polyethyleneimine delivery system, an active protein was expressed by injecting a plasmid vector containing the human PKG-I␣ 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 cocaineinduced 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.

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