Studies elaborating state-dependency with drug-induced place conditioning are sparse (see [26]), but some other methods have been used, e.g. operant food-reinforced system [5] and passive avoidance [27]. The data presented here show similar pattern of state-dependency as these previous observations using other methods, that is, the greatest responding is acquired in the same drug state while out-of-state responding is lower, but not non-existent [5], [27]. Morphine-induced state-dependency has been shown to rely on selectivity of opioid receptors, and on various other neurotransmitter systems, such as the DA system, in the brain [5], [27], [28]. Our data point to a significant role of AMPA-type glutamate receptors in state-dependency.
