Thalamic inputs to striatal interneurons in monkeys: synaptic organization and co-localization of calcium binding proteins

Recent studies indicate that extrinsic inputs from sensorimotor regions of the cerebral cortex and the centromedian intralaminar thalamic nucleus terminate preferentially upon specific subpopulations of striatal output neurons in monkeys. The objective of the present study was to verify whether this specificity of innervation also characterizes the synaptic interactions between thalamic inputs from the centromedian nucleus and the four major populations of striatal interneurons. This was achieved by double labelling techniques at the electron microscope level, combining the anterograde transport of biotinylated-dextran amine with the immunostaining for specific markers of striatal interneurons (somatostatin, parvalbumin, choline acetyltransferase and calretinin). Injections of biotinylated-dextran amine in the centromedian nucleus led to dense bands of anterograde labelling which, in double immunostained sections, largely overlapped with the four populations of interneurons in the post-commissural region of the putamen. In the electron microscope, biotinylated-dextran amine-containing terminals formed asymmetric axo-dendritic synapses with somatostatin-, parvalbumin-, and choline acetyltransferase-containing elements. However, synapses between anterogradely labelled terminals and calretinin-positive neurons were not found. In sections processed to localize biotinylated-dextran amine and parvalbumin or calretinin, double-labelled terminals (biotinylated-dextran amine/parvalbumin and biotinylated-dextran amine/calretinin), morphologically similar to thalamostriatal boutons, were found in the striatum indicating that calcium binding proteins may be expressed by thalamostriatal neurons. To test this possibility, we combined the retrograde transport of lectin-conjugated horseradish peroxidase from the putamen with parvalbumin and calretinin immunostaining and found that, indeed, most of the retrogradely labelled cells in the centromedian nucleus displayed parvalbumin and calretinin immunoreactivity. Moreover, co-localization studies revealed that calretinin and parvalbumin co-exist in single neurons of the centromedian nucleus. In conclusion, striatal interneurons immunoreactive for somatostatin, parvalbumin and choline acetyltransferase, but not those containing calretinin, receive strong inputs from the centromedian nucleus in monkeys. Moreover, our findings indicate that parvalbumin and calretinin co-exist in individual thalamostriatal neurons. In combination with our previous data, these results suggest that thalamic information may be conveyed to striatal projection neurons both, directly via excitatory synaptic inputs, or indirectly via striatal interneurons. The relative importance of those direct and indirect thalamic influences upon the activity of striatal output neurons remains to be established.