Over the past decade or so, CART (cocaine- and amphetamine-regulated transcript) peptides have emerged as major neurotransmitters and hormones. CART peptides are widely distributed in the CNS and are involved in regulating many processes, including food intake and the maintenance of body weight, reward and endocrine functions. Recent studies have produced a wealth of information about the location, regulation, processing and functions of CART peptides, but additional studies aimed at elucidating the physiological effects of the peptides and at characterizing the CART receptor(s) are needed to take advantage of possible therapeutic applications.

Quinones are ubiquitous in nature and constitute an important class of naturally occurring compounds found in plants, fungi and bacteria. Human exposure to quinones therefore occurs via the diet, but also clinically or via airborne pollutants. For example, the quinones of polycyclic aromatic hydrocarbons are prevalent as environmental contaminants and provide a major source of current human exposure to quinones. The inevitable human exposure to quinones, and the inherent reactivity of quinones, has stimulated substantial research on the chemistry and toxicology of these compounds. From a toxicological perspective, quinones possess two principal chemical properties that confer their reactivity in biological systems. Quinones are oxidants and electrophiles, and the relative contribution of these properties to quinone toxicity is influenced by chemical structure, in particular substituent effects. Modification to the quinone nucleus also influences quinone metabolism. This review will therefore focus on the differences in structure and metabolism of quinones, and how such differences influence quinone toxicology. Specific examples will be discussed to illustrate the diverse manner by which quinones interact with biological systems to initiate and propagate a toxic response.

The selective serotonergic neurotoxicity of 3,4-methylenedioxyamphetamine (MDA) and 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) depends on their systemic metabolism. We have recently shown that inhibition of brain endothelial cell ␥-glutamyl transpeptidase (␥-GT) potentiates the neurotoxicity of both MDMA and MDA, indicating that metabolites that are substrates for this enzyme contribute to the neurotoxicity. Consistent with this view, glutathione (GSH) and N-acetylcysteine conjugates of ␣-methyl dopamine (␣-MeDA) are selective neurotoxicants. However, neurotoxic metabolites of MDMA or MDA have yet to be identified in brain. Using in vivo microdialysis coupled to liquid chromatography-tandem mass spectroscopy and a high-performance liquid chromatography-coulometric electrode array system, we now show that GSH and N-acetylcysteine conjugates of N-methyl-␣-MeDA are present in the striatum of rats administered MDMA by subcutaneous injection. Moreover, inhibition of ␥-GT with acivicin increases the concentration of GSH and N-acetylcysteine conjugates of N-methyl-␣-MeDA in brain dialysate, and there is a direct correlation between the concentrations of metabolites in dialysate and the extent of neurotoxicity, measured by decreases in serotonin (5-HT) and 5-hydroxyindole acetic (5-HIAA) levels. Importantly, the effects of acivicin are independent of MDMA-induced hyperthermia, since acivicin-mediated potentiation of MDMA neurotoxicity occurs in the context of acivicin-mediated decreases in body temperature. Finally, we have synthesized 5-(N-acetylcystein-S-yl)-N-methyl-␣-MeDA and established that it is a relatively potent serotonergic neurotoxicant. Together, the data support the contention that MDMA-mediated serotonergic neurotoxicity is mediated by the systemic formation of GSH and N-acetylcysteine conjugates of N-methyl-␣-MeDA (and ␣-MeDA). The mechanisms by which such metabolites access the brain and produce selective serotonergic neurotoxicity remain to be determined.Although the selectivity of (Ϯ)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) and (Ϯ)-3,4-methylenedioxyamphetamine (MDA) for the serotonergic system in rats and humans is firmly established, the mechanism(s) involved are not fully understood. In rats, MDMA is cleared mainly by hepatic metabolism by N-demethylation to form MDA. MDMA and MDA are further O-demethylenated to 3,4-dihydroxymethamphetamine (N-methyl-␣-methyldopamine; N-Me-␣-MeDA) and 3,4-dihydroxyamphetamine (␣-methyldopamine; ␣-MeDA), respectively. N-Me-␣-MeDA and ␣-MeDA are highly redox-unstable catechols and are conjugated with sulfate and glucuronic acid. Both catechols can also be rapidly oxidized to their corresponding orthoquinones and form adducts with glutathione (GSH) and other thiol-containing compounds (Lim and Foltz, 1988;Hiramatsu et al., 1990). Alternatively, N-Me-␣-MeDA and ␣-MeDA can be O-methylated to form 4-hydroxy-3-methoxymethamphetamine (3-O-Me-N-Me-␣-MeDA) or 4-hydroxy-3-methoxyamphetamine (3-O-Me-␣-MeDA), respectively.

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Dopamine (DA) oxidation and the generation of reactive oxygen species (ROS) may contribute to the degeneration of dopaminergic neurons underlying various neurological conditions. The present study demonstrates that DA-induced cytotoxicity in differentiated PC12 cells is mediated by ROS and mitochondrial inhibition. Because cyanide induces parkinson-like symptoms and is an inhibitor of the antioxidant system and mitochondrial function, cells were treated with KCN to study DA toxicity in an impaired neuronal system. Differentiated PC12 cells were exposed to DA, KCN, or a combination of the two for 12-36 h. Lactate dehydrogenase (LDH) assays indicated that both DA (100 -500 M) and KCN (100 -500 M) induced a concentration-and time-dependent cell death and that their combination produced an increase in cytotoxicity. Apoptotic death, measured by Hoechst dye and TUNEL (terminal deoxynucleotidyltransferase dUTP nick end-labeling) staining, was also concentration-and time-dependent for DA and KCN. DA plus KCN produced an increase in apoptosis, indicating that KCN, and thus an impaired system, enhances DA-induced apoptosis. To study the mechanism(s) of DA toxicity, cells were pretreated with a series of compounds and incubated with DA (300 M) and/or KCN (100 M) for 24 h. Nomifensine, a DA reuptake inhibitor, rescued nearly 60 -70% of the cells from DAand DA plus KCN-induced apoptosis, suggesting that DA toxicity is in part mediated intracellularly. Pretreatment with antioxidants attenuated DA-and KCN-induced apoptosis, indicating the involvement of oxidative species. Furthermore, buthionine sulfoximine, an inhibitor of glutathione synthesis, increased the apoptotic response, which was reversed when cells were pretreated with antioxidants. DA and DA plus KCN produced a significant increase in intracellular oxidant generation, supporting the involvement of oxidative stress in DA-induced apoptosis. The nitric oxide synthase inhibitor N G -nitro-L-arginine methyl ester and the peroxynitrite scavenger uric acid blocked apoptosis and oxidant production, indicating involvement of nitric oxide. These results suggest that DA neurotoxicity is enhanced under the conditions induced by cyanide and involves both ROS and nitric oxide-mediated oxidative stress as an initiator of apoptosis.