Glutamatergic inputs to GABAergic interneurons in the motor thalamus of control and parkinsonian monkeys

The primate ventral motor thalamus contains a large number of GABAergic interneurons of poorly understood function and anatomical connectivity. Glutamatergic inputs to these cells arise predominantly from corticothalamic (in both basal ganglia- and cerebellar-receiving ventral motor thalamic territories; BGMT and CBMT, respectively) and cerebellothalamic terminals (in CBMT). In Parkinson's disease patients and animal models, neuronal activity is abnormal within both BGMT and CBMT. Historically, such motor thalamic dysregulation has been largely attributed to changes in inhibitory tone from the basal ganglia output nuclei, ignoring the potential role of other thalamic inputs in such processes, particularly within the CBMT, which is largely devoid of direct basal ganglia afferents. We have recently reported changes in the abundance and structural morphology of corticothalamic terminals in BGMT of parkinsonian monkeys. In this study, we assessed potential changes in the prevalence of cortical (vesicular glutamate transporter 1-positive, vGluT1-positive) and subcortical (vGluT2-positive) glutamatergic inputs in contact with GABAergic interneurons in BGMT and CBMT of MPTP-treated parkinsonian monkeys. Our findings revealed that interneurons represent a major target of both sets of glutamatergic terminals. In both BGMT and CBMT of control and parkinsonian monkeys, 29%-38% of total asymmetric axodendritic synapses (putative glutamatergic) were formed by vGluT1-positive terminals and 11%-17% of total vGluT1-positive terminals targeted dendrites of GABAergic interneurons. In CBMT, 16%-18% of asymmetric synaptic inputs on interneurons involved vGluT2-containing terminals. No major differences in the extent of glutamatergic innervation of thalamic GABAergic interneurons were found between control and parkinsonian monkeys.