Properties and plasticity of excitatory synapses on dopaminergic and GABAergic cells in the ventral tegmental area

Excitatory inputs to the ventral tegmental area (VTA) influence the activity of both dopaminergic (DA) and GABAergic (GABA) cells, yet little is known about the basic properties of excitatory synapses on these two cell types. Using a midbrain slice preparation and whole-cell recording techniques, we found that excitatory synapses on DA and GABA cells display several differences. Synapses on DA cells exhibit a depression in response to repetitive activation, are minimally affected by the GABAB receptor agonist baclofen, and express NMDA receptor-dependent long-term potentiation (LTP). In contrast, synapses on GABA cells exhibit a facilitation in response to repetitive activation, are depressed significantly by baclofen, and do not express LTP. The relative contribution of NMDA and non-NMDA receptors to the synaptic currents recorded from the two cell types is the same as is the depression of synaptic transmission elicited by the application of adenosine, serotonin, or methionine enkephalin (met-enkephalin). The significant differences in the manner in which excitatory synaptic inputs to DA and GABA cells in the VTA can be modulated have potentially important implications for understanding the behavior of VTA neurons during normal behavior and during pathological states such as addiction.