Cholinergic axon terminals in the ventral tegmental area target a subpopulation of neurons expressing low levels of the dopamine transporter
Cholinergic activation of dopaminergic neurons in the ventral tegmental area (VTA) is thought to play a major role in cognitive functions and reward. These dopaminergic neurons differentially project to cortical and limbic forebrain regions, where their terminals differ in levels of expression of the plasmalemmal dopamine transporter (DAT). This transporter selectively identifies dopaminergic neurons, whereas the vesicular acetylcholine transporter (VAchT) is present only in the neurons that store and release acetylcholine. We examined immunogold labeling for DAT and immunoperoxidase localization of VAchT antipeptide antisera in single sections of the rat VTA to determine whether dopaminergic somata and dendrites in this region differ in their levels of expression of DAT and/or input from cholinergic terminals. VAchT immunoreactivity was prominently localized to membranes of small synaptic vesicles in unmyelinated axons and axon terminals. VAchT-immunoreactive terminals formed almost exclusively asymmetric synapses with dendrites. Of 159 dendrites that were identified as cholinergic targets, 35% contained plasmalemmal DAT, and 65% were without detectable DAT immunoreactivity. The DAT-immunoreactive dendrites postsynaptic to VAchT-labeled terminals contained less than half the density of gold particles as seen in other dendrites receiving input only from unlabeled terminals. These results suggest selective targeting of cholinergic afferents in the VTA to non-dopaminergic neurons and a subpopulation of dopaminergic neurons that have a limited capacity for plasmalemmal reuptake of dopamine, a characteristic of those that project to the frontal cortex.