Since morphine-induced psychomotor sensitization and conditioned place preference were observed in the GluA1−/− mice, these behaviors can be dissociated from the glutamate receptor neuroplasticity in the VTA DA neurons. This is supported by the results from a transgenic mouse line with GluA1 subunits lacking from the DA neurons, which show both unaltered place preference and sensitization in the absence of drug-induced plasticity in VTA DA cells [36]. However, it is possible that drug-induced glutamate plasticity in VTA DA neurons, as expressed by the elevation in AMPA/NMDA ratio, is linked to state-dependency, since the drug-state seems not to be detectable as synaptic plasticity of the VTA DA neurons in GluA1-deficient mice.
