CONTRALATERAL CORTICAL PROJECTION TO THE MEDIODORSAL THALAMIC NUCLEUS- ORIGIN AND SYNAPTIC ORGANIZATION IN THE RAT

The origin of the corticothalamic projections to the contralateral med iodorsal nucleus, the collateralization of cortical fibers and their s ynaptic organization in the ipsi-and contralateral mediodorsal nuclei were investigated in adult rats with double retrograde fluorescent and anterograde tracing. After tracer injections in the mediodorsal nucle i on either side, neurons were retrogradely labeled in all the areas o f the contralateral prefrontal cortex in which ipsilateral labeling wa s also observed. Contralateral corticothalamic cells accounted for 15% of the labeled neurons in the orbital and agranular insular areas, wh ile their proportion was lower (3%) in the anterior cingulate cortex. Up to 70% of the contralateral cortical neurons were double labeled by bilateral injections in the mediodorsal nuclei. At the electron micro scopic level, unilateral injections of biotinylated dextran-amine in t he orbitofrontal cortex resulted in anterograde labeling of small term inals and a few large boutons in the ipsilateral mediodorsal nucleus, while only small boutons were identified contralaterally. The diameter of postsynaptic dendritic profiles contacted by labeled small cortica l endings was significantly larger in the ipsilateral mediodorsal nucl eus than contralaterally. These findings demonstrate that dense contra lateral cortical projections to the mediodorsal nucleus derive from th e orbital and agranular insular areas, and that crossed corticothalami c afferents are mostly formed by collaterals of the ipsilateral connec tions. Our observations also point out the heterogeneity of corticotha lamic boutons in the rat mediodorsal nucleus and morphological differe nces in the synaptic organization of prefrontal fibers innervating the two sides, indicating that ipsilateral cortical afferents may be more proximally distributed than crossed cortical fibers on dendrites of m ediodorsal neurons. (C) 1998 IBRO. Published by Elsevier Science Ltd.