Complementary Distribution of Glutamatergic Cerebellar and GABAergic Basal Ganglia Afferents to the Rat Motor Thalamic Nuclei

Motor thalamic nuclei, ventral anterior (VA), ventral lateral (VL) and ventral medial (VM) nuclei, receive massive glutamatergic and GABAergic afferents from the cerebellum and basal ganglia, respectively. In the present study, these afferents were characterized with immunoreactivities for glutamic acid decarboxylase of 67 kDa (GAD67) and vesicular glutamate transporter (VGluT)2, and examined by combining immunocytochemistry with the anterograde axonal labeling and neuronal depletion methods in the rat brain. VGluT2 immunoreactivity was intense in the caudodorsal portion of the VA-VL, whereas GAD67 immunoreactivity was abundant in the VM and rostroventral portion of the VA-VL. The rostroventral VA-VL and VM contained two types of GAD67-immunopositive varicosities (large and small), but the caudodorsal VA-VL comprised small ones alone. VGluT2-immunopositive varicosities were much larger in the caudodorsal VA-VL than those in the rostroventral VA-VL and VM. When anterograde tracers were injected into the basal ganglia output nuclei, the vast majority of labeled axon varicosities were large and distributed in the rostroventral VA-VL and VM, showing immunoreactivity for GAD67, but not for VGluT2. Only the large GAD67-immunopositive varicosities were mostly abolished by kainic acid depletion of substantia nigra neurons. In contrast, large to giant axon varicosities derived from the deep cerebellar nuclei were distributed mostly in the caudodorsal VA-VL, displaying VGluT2 immunoreactivity. The VGluT2-positive varicosities disappeared from the core portion of the caudodorsal VA-VL by depletion of cerebellar nucleus neurons. Thus, complementary distributions of large VGluT2- and GAD67-positive terminals in the motor thalamic nuclei are considered to reflect glutamatergic cerebellar and GABAergic basal ganglia afferents, respectively.