While repeated exposure to a single drug can produce behavioral sensitization, sometimes cross-sensitization between drugs is observed. In this type of experiment, animals are repetitively treated with a particular drug for a period of time and then challenged with a different drug after a defined drug-free period. Although the animal has experienced a different drug, locomotor sensitivity to the challenge drug is observed, indicating a common molecular substrate. For example, caffeine, cocaine, and amphetamine have all been shown to produce cross-sensitization to nicotine-induced hyperlocomotion (Collins and Izenwasser, 2004; Celik et al., 2006; Santos et al., 2009). Others studies have demonstrated cocaine and ethanol exhibit cross-sensitization of locomotor effects (Itzhak and Martin, 1999). The findings for nicotine and ethanol are mixed, with some studies reporting no cross-sensitization (Watson and Little, 1999; Darbra et al., 2004) while others report observing this phenomenon (Biala and Weglinska, 2004; Biala and Budzynska, 2010). There are, however, other behavioral measures that clearly illustrate a common molecular interaction between these two substances. In this respect, rats with prior exposure to nicotine show increased ethanol consumption (Blomqvist et
