Some of the most common neurochemical findings among the rat lines are alterations in central 5-HT and DA levels, or their metabolites, which is usually manifest as a deficit in the HAD and P rat lines, although both the AA and sP rat lines generally show similar or greater levels of DA and/or metabolites than their low alcohol-consuming counterparts. However, when DA receptors or its transporter were examined high alcohol-consuming rats consistently had similar or less protein or mRNA levels than their low alcohol-consuming counterparts. Similar differences have been reported for the 5-HT system with HAD, P and sP rat lines having lower 5-HT and 5-HIAA levels than LAD, NP and sNP rat lines, respectively. When levels of 5-HT receptors have been examined, the findings have been mixed for all three of these line-pairs. As with the DAergic system, AA rats either had similar or greater levels of 5-HT, its metabolites and receptors than ANA rats. To a large extent the AA vs. ANA rat line-pair has been the primary focus of research on the endogenous opioid system. While there
