In vivo measurement of somatodendritic release of dopamine in the ventral tegmental area

The ventral tegmental area (VTA), the locus of mesolimbic dopamine cell bodies, contains dopamine. Experiments in brain slices have demonstrated that VTA dopamine can be released by local electrical stimulation. Measurements with both push-pull cannula and microdialysis in intact animals have also obtained evidence for releasable dopamine. Here we demonstrate that dopamine release in the VTA can be evoked by remote stimulations of the medial forebrain bundle (MFB) in the anesthetized rat. In initial experiments, the MFB was electrically stimulated while a carbon-fiber electrode was lowered to the VTA, with recording by fast-scan cyclic voltammetry. While release was not observed with the carbon fiber 4-6 mm below dura, a voltammetric response was observed at 6-8 mm below dura, but the voltammogram was poorly defined. At lower depths, in the VTA, dopamine release was evoked. Immunohistochemistry experiments with antibodies for tyrosine hydroxylase (TH) confirmed that dopamine processes were primarily found below 8 mm. Similarly, tissue content determined by liquid chromatography revealed serotonin but not dopamine dorsal to 8 mm with both dopamine and serotonin at lower depths. Evaluation of the VTA signal by pharmacological means showed that it increased with inhibitors of dopamine uptake, but release was not altered by D2 agents. Dopamine release in the VTA was frequency dependent and could be exhausted by stimulations longer than 5 s. Thus, VTA dopamine release can be evoked in vivo by remote stimulations and it resembles release in terminal regions, possessing a similar uptake mechanism and a finite releasable storage pool.