sometimes yielded results which are discordant with behavioral pharmacology, often attributed to compensatory mechanisms that may develop in genetically modified animals (Kieffer and Gaveriaux-Ruff, 2002; Portugal and Gould, 2008). Altogether however, data analysis across the literature allows identification of potential endogenous receptor/peptide systems operating in drug reinforcement processes, and reveals differing mechanisms across the distinct classes of drugs of abuse (Figures 2 and 3).
