DISINHIBITION AS A BASIC PROCESS IN THE EXPRESSION OF STRIATAL FUNCTIONS .2. THE STRIATO-NIGRAL INFLUENCE ON THALAMOCORTICAL CELLS OF THE VENTROMEDIAL THALAMIC NUCLEUS

Besides the nigro-collicular pathway, the nigro-thalamic projection to the ventromedial thalamic nucleus represents another efferent system of the basal ganglia through which striatum can influence motor centers. Since the striato-nigrothalamic circuit is composed of 2 successive GABAergic inhibitory links and the SNR [substantia nigra pars reticulata] is tonically active, tested by inhibiting nigrothalamic cells, the striatum may exert a facilitatory influence on VM[ventromedial nucleus of the thalamus]-thalamocortical cells. A transitory block of SNR firing (induced by either intranigral application of GABA or by stimulating the inhibitory striato-nigral pathway), causes a perfectly time-locked increase of activity in a large number of VM cells projecting to motor cortex. The striatally evoked excitation of VM-thalamocortical cells requires the functional integrity of the GABAergic striato-nigral link. The double inhibitory striato-nigrothalamic pathway acts on VM-thalamocortical cells through a disinhibitory mechanism. The functional implication of such a mechanism is discussed.