Neurochemical characterization of the cerebellar-recipient motor thalamic territory in the macaque monkey

The immunoarchitectonics of the macaque motor thalamus was analysed to look for a possible neurochemical characterization of thalamic territories, which were not definable cytoarchitectonically, associated with different functional pathways. Thalamic sections from 15 macaque monkeys were processed for visualization of calbindin (CB), parvalbumin (PV), calretinin (CR) and SMI-32 immunoreactivity (ir). PV-, CR- and SMI-32ir distributions did not show any clear correlation with known functional subdivisions. In contrast, CBir distribution reliably defined two markedly distinct motor thalamic territories, one characterized by high cell and neuropil CBir (CB-positive territory), the other by very low cell and neuropil CBir (CB-negative territory). These two neurochemically distinct compartments, the CB-negative and the CB-positive territories, appear to correspond to the cerebellar- and basal ganglia-recipient territories, respectively. To verify the possible correspondence of the CB-negative territory with the cerebellar-recipient sector of the motor thalamus, we compared the distribution of cerebello-thalamic projections with the distribution of CBir in two monkeys. The distribution of cerebellar afferent terminals was similar to that reported from previous reports and in line with the notion that in the motor thalamus the cerebellar-recipient territory does not respect cytoarchitectonic boundaries. Comparison with CB immunoarchitecture showed very close correspondence in the motor thalamus between the distribution of the anterograde labeling and the CB-negative territory, suggesting that the CB-negative territory represents the architectonic counterpart of the cerebellar-recipient territory. CB immunostaining may therefore represent a helpful tool for describing the association between thalamocortical projections and the basal ganglia or the cerebellar loops and for establishing possible homologies between the motor thalamus of non-human primates and humans.