Studies of Drug Abuse using Human Postmortem Tissue

National Institute On Drug Abuse

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Abstract

Two contrasting systems for studies of drug abuse, human postmortem tissue and cell line models, have until now been employed very minimally. Yet, both present opportunities for understanding drug abuse which are not shared by more traditional systems, e.g., animal models and imaging studies of human patients. The use of postmortem tissue, despite limitations of the resource and experimental conditions, allows for the direct measurement of biochemical and histopathological parameters, in the actual human condition of drug abuse. Brain imaging also allows for direct examination of human drug abuse, but gene expression and many other parameters cannot be measured directly, and for most studies experimental administration of drugs is required. Studies of drugs in cell lines have a contrasting set of potential advantages: Gene expression, proteins, and other biochemical parameters can be measured in a homogeneous cell type, thereby obviating interpretational problems that arise from studies using the hetereogeneous cell population which is present in brain tissue. Also, effects of drugs on human cells can be examined directly, and under experimentally-controlled conditions. The purpose of this project is to exploit human postmortem brain tissue specimens to contribute to an understanding of the problem of drug abuse. [unreadable] [unreadable] An initial postmortem study of cocaine abuse employed CNS-focused cDNA microarrays to examine the expression of 1152 transcripts in dorsolateral prefrontal cortex (dlPFC, Area 46) from individually age- and postmortem interval-matched male cocaine abusers and controls. Gas chromatography-mass spectrometry detected cocaine in the dlPFC of all abusing individuals. Sixty-five transcripts consistently displayed differential expression across multiple subject-control comparisons. Alterations in energy metabolism, mitochondria and oligodendrocyte function, cytoskeleton and related signaling, and neuronal plasticity were found. Especially, changes in transcripts encoding extracellular signal-regulated kinases (MEK/ERKs) and putative up- and downstream targets implicate the MEK/ERK pathway as a possible mediator of cocaine-related CNS effects. In addition, there was evidence for two distinct states of transcriptional regulation in cocaine abuse, differing according to the direction of changes in gene expression. The existence of this pattern was confirmed by quantitative PCR for select transcripts. Increases in gene expression predominated in subjects testing positive for a metabolite indicative of recent "crack" cocaine abuse, while decreased expression profiles predominated in subjects without evidence of recent crack abuse. These data suggest that cocaine abuse targets a distinct subset of genes in the dlPFC of cocaine abusers, resulting in either a state of acute activation in which increased gene expression predominates, or a relatively de-stimulated, refractory phase. The activation phase may be present shortly following drug administration, e.g., when "crack" metabolites are still present, whereas repression of gene expression and a de-activation phase may occur at longer periods following the most recent drug episode. [unreadable] [unreadable] A second larger postmortem study to characterize gene expression changes in the brains of human cases with a history of substance abuse, especially PCP, cocaine and marijuana has been completed. This study involved 42 substance abuse-cases and a similar number of controls, and includes extensive characterization by multiple toxicological tests and collection of pre-mortem data. Results of this larger study suggest that, similar to the first study, there exists more than one pattern of gene expression change in human drug abuse. Certain transcriptional changes are observed only in certain types of drug abuse. Some transcriptional changes, such as changes in the cholecystokinin receptor calmodulin-related transcripts, apolipoproteins, and Golgi-endoplasmic reticulum transcripts which occur in all or most drug abuse cases. Additional characterization has involved protein measurements and Q-PCR. We believe that the use of postmortem human brain for studies of drug abuse has the potential to delineate the gene expression changes which are seen in the bona fide condition of human drug abuse. This may lead to an improved understanding of the short and long-term changes in brain function which accompany drug dependence.