THE PROJECTIONS FROM THE PARAFASCICULAR THALAMIC NUCLEUS TO THE SUBTHALAMIC NUCLEUS AND THE STRIATUM ARISE FROM SEPARATE NEURONAL POPULATIONS - A COMPARISON WITH THE CORTICOSTRIATAL AND CORTICOSUBTHALAMIC EFFERENTS IN A RETROGRADE FLUORESCENT DOUBLE-LABELING STUDY

The parafascicular thalamic nucleus projects to the subthalamic nucleu s and the striatum. Double-retrograde fluorescent tracing was used to determine whether these projections arise from the same neurons via ax on collaterals. True Blue was injected into the subthalamic nucleus an d Nuclear Yellow was injected into the striatum of each rat and the pa rafascicular thalamic nucleus was examined under the fluorescence ligh t-microscope. individual parafascicular neurons were not double-labell ed with the tracers. The True Blue- and Nuclear Yellow-labelled neuron s wee located in different parts of the parafascicular nucleus ipsilat eral to the injections. In the rostral part of the parafascicular nucl eus, True Blue-labelled neurons were located ventral to the fasciculus retroflexus, and in the caudal part of the nucleus. True Blue-labelle d neurons were located close to the medial and lateral borders of fasc iculus retroflexus. Nuclear Yellow-labelled neurons were found mainly to encircle the fasciculus retroflexus in the rostral part of the para fascicular nucleus and in the dorsolateral sector of the caudal part o f the parafascicular nucleus. Double-labelled neurons were, however, f ound in the cortex. The proportion of neurons projecting to both the s ubthalamic nucleus and the striatum accounted for 38% of the total num ber of corticosubthalamic neurons in the prefrontal cortex, 15.5% in t he cingulate cortex and 9% in the sensorimotor cortex. The present fin ding of an individualization between the parafascicular efferents to t he subthalamic nucleus and the striatum emphasize the importance of th is projection and provides further evidence of the associative functio ns attributable to the subthalamic nucleus.