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 are reports of AA rats having greater or lesser levels of opioid peptides and their receptors relative to ANA rats, it is surpising that very few studies have reported similar levels between these rat lines. A handful of studies have been published with the P vs. NP and sP vs. sNP rat lines regarding the endogenous opioid system, and the general finding has been that high alcohol-consumers have lower levels of opioid peptides and receptors than low alcohol-consumers. Other than the endogenous opioid system, most of the research on innate differences in neuropeptide systems has been conducted in the P vs. NP line-pair. In general, P rats display deficits in the CRF and tachykinin systems relative to NP rats. Moreover, brain-derived neurotrophic factor (BDNF) and activity-regulated cytoskeleton-associated protein (ARC) have been implicated in excessive ethanol intake as well as its modulation by the NPY system and P rats display deficits in these systems relative to NP rats as well.
