GABA-containing neurons in the ventral tegmental area project to the nucleus accumbens in rat brain.

The ventral tegmental area receives a gamma-aminobutyric acid (GABA) innervation from the nucleus accumbens and contains GABA immunoreactive neurons believed to be interneurons. We combined the immunocytochemical detection of retrogradely transported Fluoro-Gold (FG) from the nucleus accumbens (Acb) with the detection of GABA within the same section of tissue in the ventral tegmental area (VTA) of the rat brain to determine whether there might also be reciprocal GABAergic projections in the mesolimbic pathway. Immunoperoxidase labeling for FG and immunogold-silver labeling for GABA were most readily distinguished within perikarya and dendrites in sections examined by electron microscopy. Ultrastructural observations indicated that 36% (n = 110) of the FG-labeled perikarya and dendrites also contained GABA immunoreactivity. The present results provide the first evidence that GABA is contained in a subpopulation of neurons in the mesolimbic pathway from the VTA to the Acb. The reciprocity of this circuitry may provide an important feedback loop thus facilitating inhibition of motor activity.