Mu/pEnk signaling seems involved in nicotine dependence. Withdrawal signs of chronically nicotine-treated pEnk (Berrendero et al., 2005) and mu KO (Berrendero et al., 2002) mice were attenuated, while no difference with wild-type controls was observed for pDyn (Galeote et al., 2009), delta (Berrendero et al., 2012) and βend (Trigo et al., 2009) KO mice. Finally, the mu receptor also contributes to nicotine-induced locomotor sensitization (Yoo et al., 2005; Yoo et al., 2004), see Table 6).
