The molecular processes underlying alcohol dependence are not fully understood. Many characteristic behaviours result from neuroadaptations in the mesocorticolimbic system. In addition, alcoholism is associated with a distinct neuropathology. To elucidate the molecular basis of these features, we compared the RNA expression profile of the nucleus accumbens and prefrontal cortex of human brain from matched individual alcoholic and control cases using cDNA microarrays. Approximately 6% of genes with a marked alcohol response were common to the two brain regions. Alcohol-responsive genes were grouped into 11 functional categories. Predominant alcohol-responsive genes in the prefrontal cortex were those encoding DNA-binding proteins including transcription factors and repair proteins. There was also a down-regulation of genes encoding mitochondrial proteins, which could result in disrupted mitochondrial function and energy production leading to oxidative stress. Other alcohol-responsive genes in the prefrontal cortex were associated with neuroprotection/apoptosis. In contrast, in the nucleus accumbens, alcohol-responsive genes were associated with vesicle formation and regulation of cell architecture, which suggests a neuroadaptation to chronic alcohol exposure at the level of synaptic structure and function. Our data are in keeping with the previously reported alcoholism-related pathology characteristic of the prefrontal cortex, but suggest a persistent decrease in neurotransmission and changes in plasticity in the nucleus accumbens of the alcoholic.

No abstract

Drugs of abuse including nicotine and alcohol elicit their effect by stimulating the mesocorticolimbic dopaminergic system. There is a high incidence of nicotine dependence in alcoholics. To date only limited data is available on the molecular mechanism underlying the action of alcohol and nicotine in the human brain. This study utilized gene expression screening to identify genes sensitive to chronic alcohol abuse within the ventral tegmental area (VTA) of the human brain. Alcohol-responsive genes encoded proteins primarily involved in structural plasticity and neurotransmitter transport and release. In particular, genes involved with brain-derived neurotrophic factor signalling and glutamatergic transmission were found to be affected. The possibility that glutamate transport was a target of chronic alcohol and/or tobacco abuse was further investigated in an extended case set by measurement of mRNA and protein expression. Expression levels of vesicular glutamate transporters SLC17A6 and SLC17A7 were robustly induced by smoking, an effect that was reduced by alcohol co-exposure. Glutamatergic transmission is vital for the control of the VTA and may also be critical to the weighting of novelty and importance of a stimulus, an essential output of this brain region. We conclude that enduring plasticity within the VTA may be a major molecular mechanism for the maintenance of smoking addiction and that alcohol, nicotine and co-abuse have distinct impacts on glutamatergic transmission with important implications for the control of this core mesolimbic structure.

The mesocorticolimbic system is the reward centre of the brain and the major target for drugs of abuse including alcohol. Neuroadaptive changes in this region are thought to underlie the process of tolerance and dependence. Recently, several research groups have searched for alcohol-responsive genes using high-throughput microarrays and well-characterized human post-mortem material. Comparison of data from these studies of cortical regions highlights the differences in experimental approach and selection of cases. However, alcoholresponsive gene sets associated with transcription, oxidative stress and energy production were common to these studies. In marked contrast, alcohol-responsive genes in the nucleus accumbens and the ventral tegmental area are primarily associated with changes in neurotransmission and signal transduction. These data support the concept that, within cortical regions, changes in gene expression are associated with alcoholism-related pathology. In the dopaminergic tract of the mesocorticolimbic system, alcohol-responsive gene sets suggest long-term neuroplastic changes in synaptic transmission.

Midkine promotes neuronal outgrowth and survival. The up-regulation of MDK protein expression may indicate the induction of reparative processes. The amino acid transporter is vital for the removal of glutamate from the synaptic cleft. At alcohol withdrawal, extracellular glutamate is thought to reach excitotoxic concentrations. Up-regulation of EAAT1 throughout the cortical layers may indicate an attempt to combat elevated glutamate concentrations. The predominant expression of the two proteins in layer II of the cortex implies a region-specific role of astrocytes.