Injections of N-methyl-D-aspartate into the ventral hippocampus increase extracellular dopamine in the ventral tegmental area and nucleus accumbens.

The nucleus accumbens septi receives inputs from dopaminergic neurons of the ventral tegmental area (VTA) and glutamatergic neurons of the ventral subiculum (VS). The convergence of these inputs in the NAS is important for the normal expression of exploratory locomotion; stimulation of the VS by injection of the glutamate receptor agonist N-methyl-D-aspartate (NMDA) causes dopamine-dependent increases in locomotion. In the present study, in vivo microdialysis in conjunction with high-performance liquid chromatography and electrochemical detection (HPLC-EC) was used to estimate changes in extracellular dopamine in the VTA and NAS in response to intra-VS injections of NMDA (0.074, 0.28, 0.74 microg). NMDA injections caused dose-dependent elevations in extracellular dopamine in each region. Each dose of NMDA clearly increased extracellular dopamine in the NAS, whereas only the two higher doses increased dopamine significantly in the VTA. The highest dose of NMDA elevated extracellular dopamine to approximately 180% of baseline in each region. Whereas elevations in NAS dopamine might be induced by impulse-independent local mechanisms, elevations of dopamine in the VTA are presumed to reflect increased somatodendritic release associated with increased impulse flow through dopamine neurons. Thus, the present study suggests that the modulation of dopaminergic neurotransmission by the ventral subiculum results from a trans-synaptic activation of dopamine cell bodies in the VTA.