The vesicular glutamate transporter VGLUT3 synergizes striatal acetylcholine tone

Three subtypes of vesicular transporters accumulate glutamate into synaptic vesicles to promote its vesicular release. One of the subtypes, VGLUT3, is expressed in neurons, including cholinergic striatal interneurons, that are known to release other classical transmitters. Here we showed that disruption of the Slc17a8 gene (also known as Vglut3) caused an unexpected hypocholinergic striatal phenotype. Vglut3 –/– mice were more responsive to cocaine and less prone to haloperidol-induced catalepsy than wild-type littermates, and acetylcholine release was decreased in striatum slices lacking VGLUT3. These phenotypes were associated with a colocalization of VGLUT3 and the vesicular acetylcholine transporter (VAChT) in striatal synaptic vesicles and the loss of a synergistic effect of glutamate on vesicular acetylcholine uptake. We propose that this vesicular synergy between two transmitters is the result of the unbalanced bioenergetics of VAChT, which requires anion co-entry for continuing vesicular filling. Our study reveals a previously unknown effect of glutamate on cholinergic synapses with potential functional and pharmacological implications. Before exocytotic release, classical (nonpeptide) neurotransmitters, such as L-glutamate, are stored into synaptic vesicles by proton-driven membrane transporters. Either of two vesicular glutamate transporters, VGLUT1 and VGLUT2, is expressed in classical glutamatergic nerve terminals establishing asymmetrical contacts 1 . It was therefore unexpected to discover that the third isoform, VGLUT3, with transport properties similar to those of VGLUT1 and VGLUT2, is expressed in discrete populations of neurons releasing other nonpeptide transmitters 2–5 , including cholinergic interneurons from the dorsal and ventral striatum 3,6 , serotoninergic neurons of the medial and dorsal raphe nuclei and subsets of GABAergic interneurons in the hippocampus and cerebral cortex 3,7,8 . The function of VGLUT3 in these neurons remains largely unknown. To address this issue, we focused on striatal cholinergic interneurons. The striatum is the main input structure of the basal ganglia and is involved in adaptive control of behavior and in disorders such as Parkinson’s disease and Huntington’s disease. It receives afferents from the entire cerebral cortex, thalamic nuclei, substantia nigra and dorsal raphe, and participates in multiple corticobasal ganglia loops that control movement, as well as cognitive, motivational and emotional aspects of behavior. In rodents, 95% of its neurons are GABAergic and densely covered with dendritic spines, which project either directly or indirectly to the globus pallidus and substantia nigra 9 . These spiny