Endogenous opioid peptides in drug addiction

Abstract

Depending on the addictive drug the opioid peptides, enkephalin and ß-endorphin vary in their contribution to reward. Conditional Penk1 knockout mice were successfully generated which allow the cell- or tissue-specific deletion of enkephalin. The mutated Penk allele contains two loxP sites allowing cre recombinase-mediated deletion of the coding exons. A lacZ reporter gene resulting in ß-galactosidase expression under the enkephalin promoter after cre-mediated recombination was also inserted. In these mice, knockout of preproenkephalin results in ß-galactosidase activity in brain regions where enkephalin is known to be expressed. Thus, the functionality of both genetic modifications was demonstrated. <br /> Molecular analysis of the floxed Penk1 mice revealed a reduction in mRNA and peptide levels. As a consequence, these mice have a slightly anxiogenic phenotype. Although this weak phenotype has to be considered it does not impair the applicability of these mice. Because the floxed Penk1 allele always represent the control group, phenotypes measured after tissue- or cell-specific deletion can be attributed to the absence of enkephalin. <br /> Successful deletion of enkephalin in the targeted brain region was demonstrated by stereotactic injection of cre recombinase expressing viruses. Ongoing experiments targeting the nucleus accumbens and the VTA will provide new insights into the role of enkephalinergic neurons in mediating nicotine reward and reinforcement. Using this new mouse model, the contribution of enkephalin to a wide range of opioid-mediated phenotypes can be studied. Now brain regions involved in addiction-related behaviors or neuronal structures involved in pain relief can be assessed in detail. <br /> Additionally the contribution of variants in opioid peptide genes to human alcohol dependence was systematically analyzed. This lead to the identification of POMC variants contributing to human alcohol addiction. A systematic analysis of the genomic regions coding for all three endogenous opioid peptides was performed in two independent case-control samples. Association analysis illustrated the importance to replicate findings in an independent cohort to exclude potential false positive signals. The assumed role for POMC-encoded peptides in alcohol dependence was further substantiated by this work. Interestingly, this contribution was gender-specific. A POMC two-marker haplotype was significantly more frequent in female cases across two independent case-control cohorts. Subsequently, the role of POMC was confirmed by independent studies.