Glutamate-associated plasticity in the ventral tegmental area is necessary for conditioning environmental stimuli with morphine.
We sought to determine if plasticity in the ventral tegmental area (VTA) of the midbrain is involved in learning to associate morphine exposure with a specific environment. For this, we tested whether activation of glutamate receptors and protein kinase A is needed for the acquisition and expression of a morphine-conditioned place preference (CPP). Rats received bilateral microinjections of either the NMDA antagonist AP5 (0.48 nmol/0.3 microl), the AMPA antagonist CNQX (0.21 nmol/0.3 microl), or vehicle into the VTA prior to each of three morphine-conditioning sessions. Both the AMPA and NMDA receptor antagonists blocked the development of morphine CPP when given into the VTA but not when given outside the VTA. In similar studies the protein kinase A (PKA) inhibitor, Rp-cAMPS (13 nmol/0.3 microl), blocked the acquisition of morphine CPP when given into the VTA immediately after morphine conditioning. In separate experiments, glutamate antagonists, or Rp-cAMPS, immediately prior to the preference test blocked the expression of morphine CPP when microinjected into the VTA. These data indicate that the VTA is an important site for synaptic modifications involved in the learning and memory of environmental cues predicting reward, and that glutamate input and PKA activation are crucial to this process.