A recent study has also examined innate differences in gene expression within the VTA across the AA vs. ANA, HAD1 vs. LAD1, HAD2 vs. LAD2, P vs. NP and sP vs. sNP rat line-pairs (McBride et al., 2012). This study was based on three premises (McBride et al., 2012). First, that many genes not related to alcohol abuse and dependence have been fixed during the selective breeding programs discussed above. By extension, if multiple rat line-pairs that have been subjected to the same selection criteria are studied under the same methodological conditions, then the detection of significant differences in gene expression that are not directly associated with the phenotype should be minimized. Second, any common genes that are identified across the rat line-pairs should have a high probability of mediating the high, or low, alcohol consumption phenotype. Finally, the fact that there are genotypic and phenotypic differences between the line-pairs suggests that common gene networks/pathways may be identified across the line-pairs rather than common individual genes. The study revealed that no single gene was significant across all five line-pairs, although
