High-frequency afferent stimulation induces long-term potentiation of field potentials in the ventral tegmental area

Excitatory synapses on dopamine neurons in the VTA can undergo both long-term potentiation and depression. Additionally, drug-induced plasticity has been found at VTA synapses, and is proposed to play a role in reward-related learning and addiction by modifying dopamine cell firing. LTP at these synapses is difficult to generate experimentally in that it requires an undisturbed intracellular milieu and is often small in magnitude. Here, we demonstrate the induction of LTP as a property of evoked field potentials within the VTA. Excitatory field potentials were recorded extracellularly from VTA neurons in acute horizontal midbrain slices. Using extracellular and intracellular recording techniques, we found that evoked field potentials originate within the VTA itself and are largely composed of AMPA receptor-mediated EPSPs and action potentials triggered by activation of glutamatergic synapses on both dopamine and GABA neurons. High-frequency afferent stimulation (HFS) induced LTP of the field potential. The induction of this LTP was blocked by application of the NMDAR antagonist, d-APV, prior to HFS. As reported previously, glutamatergic synapses on GABA neurons did not express LTP while those on dopamine neurons did. We conclude that the potentiation of glutamatergic synapses on dopamine neurons is a major contributor to NMDA receptor-dependent LTP of the field potential. Field potential recordings may provide a convenient approach to explore the basic electrophysiological properties of VTA neurons and the development of addiction-related processes in this brain region.