Direct contacts between fibers from the ventrolateral thalamic nucleus and frontal cortical neurons projecting to the...

The aim of the present study was to improve our knowledge of the synaptic organization of one major motor loop, the thalamo-fronto-striate system. We examined, within the motor and sensorimotor (Fr1-3, FL, HL) cortices, the organization of the synaptic articulation between thalamic terminals and fronto-striatal neurons. An anterograde tracer (Phaseolus vulgaris leuco-agglutinin) was used to label the fibers from the ventrolateral (VL) thalamic nucleus and a retrograde tracer (subunit b of the cholera toxin) was employed to label the neurons of the frontal cortex projecting to the dorso-lateral quadrant of the caudate-putamen. Our findings indicated that anterogradely labeled fibers arising from the VL formed appositions, presumably synaptic contacts, with retrogradely labeled corticostriate neurons within layer V and, to a lesser degree, within layer III of the frontal cortex. These direct contacts were axo-somatic or axo-dendritic. These findings suggest that some aspects of the motor control mediated by the thalamo-fronto-striate loop could be partly achieved by monosynaptic circuits.