Preproenkephalin Gene and mRNA : Studies of Structure, Function, Cocaine Responses in an Animal Model, and Genetic Association with Human Opiate Addiction

Sammanfattning: The endogenous opioid enkephalin neuropeptides are mediators of pain perception and have been implicated in human addictions. The preproenkephalin gene and its mRNA have also provided many examples of tissue- and species-specific variations in mRNA structure produced through a variety of transcriptional and post-transcriptional mechanisms. Resultant differences in mRNA structure, in several cases, have impact on translation of enkephalin prepropeptide. The reports and discussion presented herein describe studies of the preproenkephalin gene and mRNA structure in the guinea pig, an animal that may have specific advantages for modeling the human endogenous opioid system. A guinea pig brain cDNA library was constructed and screened for clones of preproenkephalin and preprodynorphin, which were then sequenced. These studies confirmed the predicted mRNA structure that had been previously proposed based on homology with gene sequences and other methods. Multiple transcription initiation sites for each of these prepropeptide genes were also identified. Studies were conducted in the guinea pig to evaluate the effects of the administration of cocaine in a “binge” paradigm for two and seven days on preproenkephalin mRNA levels in several brain regions. “Binge” cocaine administration for seven (but not two) days resulted in differential changes in mRNA levels in different brain regions. Decreases were observed in the nucleus accumbens and hypothalamus, and increases in the frontal cortex, amygdala and hippocampus. These findings differ from those of previous rodent studies and suggest that this species may provide a useful alternative model for the study of the effects of cocaine on preproenkephalin gene expression in the human brain. Human genetic studies were also conducted in opioid-dependent (formerly heroin-addicted) and control subjects to test the hypothesis that the preproenkephalin gene is associated with heroin addiction. In two separate studies, we obtained evidence that this gene may be associated with the development of human heroin addiction.