Mesoaccumbens Dopamine Neuron Synapses Reconstructed in Vitro Are Glutamatergic

The mesoaccumbens projection, formed by ventral tegmental area dopamine neurons synapsing on nucleus accumbens gamma-aminobutyric acid neurons, has been implicated in the pathogenesis of schizophrenia and drug addiction. Despite intensive study, the nature of the signal conveyed by dopamine neurons has not been fully resolved. In addition to several slower, dopamine-mediated, modulatory actions, several lines of evidence suggest that dopamine neurons have fast excitatory actions. To test this, we placed dopamine neurons together with accumbens neurons in microcultures. Surprisingly, most dopamine neurons made excitatory recurrent connections (autapses), which provided a basis for their identification; accumbens gamma-aminobutyric acid neurons were identified by their distinctive size. In 75% of mesoaccumbens cell pairs, stimulation of the dopamine neuron evoked a glutamate-mediated, excitatory synaptic response in the accumbens neuron. Immunostaining revealed dopamine neuron varicosities that were predominantly dopaminergic, ones that were predominantly glutamatergic, and ones that were both dopaminergic and glutamatergic. Despite close appositions of both glutamatergic and dopaminergic varicosities to the dendrites of accumbens neurons, only glutamatergic synaptic responses were seen. In the majority of cell pairs, pharmacologic activation of D2-type dopamine receptors inhibited glutamatergic responses, presumably via immunocytochemically-visualized presynaptic D2 receptors. In some cell pairs, the evoked autaptic and synaptic responses were discordant, suggesting that D2 receptors may be differentially trafficked to different presynaptic varicosities.Thus, dopamine neurons appear to mediate both slow dopaminergic and fast glutamatergic actions via separate sets of synapses. Together with evidence for glutamate cotransmission in serotonergic raphe neurons and noradrenergic locus coeruleus neurons, these results add a new dimension to monoamine neuron signaling that may have important implications for neuropsychiatric disorders.