Synaptic characteristics between cortical cells in the rat prefrontal cortex and axon terminals from the ventral tegmental area that utilizes different neurotransmitters

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Projections from the ventral tegmental area (VTA) have been demonstrated to terminate in the prefrontal cortex (PFC) as dopaminergic and/or GABAergic, forming a neural circuit implicated in certain memory and cognitive processes. However, it has not been determined if these two neurotransmitters are localized in what types of axon terminals from the VTA to the PFC. In order to address synaptic characteristics made by postsynaptic prefrontal cortical structures and mesoprefrontal fibers utilizing either GABA or dopamine, we performed a double labeling method for electron microscopy that combined peroxidase markers for anterograde tract-tracing with postembedding immunogold labeling for tyrosine hydroxylase (TH), dopamine and GABA. The anterograde tract-tracing studies showed that tegmentocortical fibers from the VTA terminated as both symmetric and asymmetric axon terminals with the predominant symmetric synaptic type in the prefrontal cortex of the rat. Further, a combined study that used anterograde tract-tracing and postembedding immunocytochemistry indicated that tegmentocortical axon terminals forming symmetric synapses were either GABAergic or dopaminergic, while a small fraction of tegmentocortical terminals ending as asymmetric synapses was not immunoreactive for dopamine or GABA.These findings indicate that the tegmentocortical projections to the PFC exerts not only an inhibitory influence via symmetric synapses using dopamine and GABA on spontaneous activity of cells in the PFC but also unknown effects via asymmetric synapses using other neurotransmitters on that.