Alcohol and tobacco are drugs of abuse which are legal to sell and consume in most western societies. Addiction to these two substances has major social and health implications worldwide. The brain structure known to mediate addictive behaviour is the dopaminergic mesocorticolimbic system. Dopaminegic neurons arise from the ventral tegmental area, project to the nucleus accumbens and interact with the amygdala and the prefrontal cortex. Chronic alcoholism elicits marked damage in the prefrontal cortex with significant loss of neurons and glia. The key components of addiction, tolerance and dependence, are thought to be the result of semipermanent adaptive changes in gene expression. Gene expression profiling of the mesocorticolimbic system from human alcoholics and alcohol-dependent animals has revealed highly region-specific alterations. How these molecular changes result in the development of alcohol dependence in humans is not fully understood. Complicating factors in human alcoholism include a high comorbidity with smoking, socioeconomic factors and the prevalence of underlying psychological pathologies. Gene expression profiling of the prefrontal cortex of six alcoholics and six controls resulted in the identification of functional gene groups sensitive to alcoholism. Mitochondrial function was found down regulated while mRNA levels of genes involved in stress response and cell protection were elevated. These results correlate with the pathology of the prefrontal cortex in chronic alcoholism. Some of the control cases used for gene expression profiling were later identified as chronic smokers, while all of the alcoholics were heavy smokers. To date the heavy co-morbidity of alcoholism with smoking has not been taken into account. Thus the expression of selected genes were investigated by realtime PCR in an extended case set of non-smoking alcoholics, smoking alcoholics, smoking non-alcoholics and non-smoking, non-alcoholics. This study revealed that alcoholism itself had a significant impact on the expression of midkine, the high affinity glial glutamate transporter, member 1 and the tissue inhibitor of the metalloproteinase 3. Heavy smoking itself led to a small but significant elevation of MDK mRNA levels as well as an increase in variation of excitatory amino acid transporter 1 and metalloproteinase inhibitor, member 3 expression. Apolipoprotein D however was induced by chronic smoking but not by alcohol dependence. These results highlight the need of careful case selection in future studies on gene expression in the human alcoholic brain. Peptide antibodies were produced to midkine and a polyclonal antibody against the excitatory amino acid transporter 1 was obtained from a collaborating laboratory. Western blots utilizing these antibodies revealed a marked increase in midkine and excitatory amino acid transporter 1 protein in alcoholics compared to non-smoking and non-drinking controls. In coronal sections of human prefrontal cortex of alcoholics and non-smoking non-drinking controls, immunofluorescence of midkine was obtained from nuclei throughout the layers of the cortex and from the cell bodies of a distinct set of astrocytes in cortical layer II. Double staining with glial fibrillary acidic protein revealed that a portion of midkine-positive nuclei were localised in glial cells. There was no difference in immunostaining of alcohol and control sections with midkine. In summary these results indicate that midkine protein is induced in the prefrontal cortex of the chronic alcoholic. However, this increase in protein may not be strong enough to be visualised by immunohistochemistry. Midkine induction may be reflective of reparative processes in the prefrontal cortex of the chronic alcoholic. Excitatory amino acid transporter 1 staining in non-alcoholic, non-smoking control cases were obtained as a confluent band in cortical layer II and sparsely in deeper cortical layers. Excitatory amino acid transporter 1 immunoreactivity overlapped partially with glial fibrillary acidic protein labelling. In chronic alcoholics, excitatory amino acid transporter 1 staining in the area between the cortical layer II and VI was significantly increased. At withdrawal, glutamate levels may reach toxic levels in the cortex. The increase in cells expressing excitatory amino acid transporter 1 throughout the cortical layers may indicate a protective measure of this brain region in the chronic alcoholic. Additionally, layer specific expression of midkine and excitatory amino acid transporter 1 in the prefrontal cortex of the healthy individual may implicate a specialised role of these astrocytes.
| Identifer | oai:union.ndltd.org:ADTP/291204 |
| Creators | Flatscher-Bader, Traute |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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