Two Types of Thalamocortical Projections from the Motor Thalamic Nuclei of the Rat: a Single Neuron-tracing Study Using Viral Vectors

The axonal arborization of single motor thalamic neurons was examined in rat brain using a viral vector expressing membrane-targeted palmitoylation site-attached green fluorescent protein (palGFP). We first divided the ventral anterior-ventral lateral motor thalamic nuclei into 1) the rostromedial portion, which was designated inhibitory afferent-dominant zone (IZ) with intense glutamate decarboxylase immunoreactivity and weak vesicular glutamate transporter 2 immunoreactivity, and 2) the caudolateral portion, named excitatory subcortical afferent-dominant zone (EZ) with the reversed immunoreactivity profile. We then labeled 38 motor thalamic neurons in 29 hemispheres by injecting a diluted palGFP-Sindbis virus solution and isolated 10 IZ and EZ neurons for reconstruction. All the reconstructed IZ neurons widely projected not only to the cerebral cortex but also to the neostriatum, whereas the EZ neurons sent axons almost exclusively to the cortex. More interestingly, 47-66% of axon varicosities of IZ neurons were observed in layer I of cortical areas. In contrast, only 2-15% of varicosities of EZ neurons were found in layer I, most varicosities being located in middle layers. These results suggest that 2 forms of information from the basal ganglia and cerebellum are differentially supplied to apical and basal dendrites, respectively, of cortical pyramidal neurons and integrated to produce a motor execution command.