Cocaine-induced locomotion was also investigated in delta KO mice, showing an increased response to cocaine in these mutant animals (Chefer et al., 2004). Locomotion stimulation upon cocaine administration was maintained or increased (Chefer et al., 2005) in kappa KO animals depending on the dose, and maintained (Bailey et al., 2007) or decreased (Chefer and Shippenberg, 2006) in pDyn KO mice, indicating contrasting effects of the kappa/dynorphin system in this response. Similarly, locomotor sensitization was abolished in kappa KO mice (Chefer et al., 2005), and increased in pDyn KO animals (Bailey et al., 2007), suggesting a dissociation of kappa receptors and dynorphins in the locomotor stimulant effect of cocaine.
