INDIRECT NUCLEUS-ACCUMBENS INPUT TO THE PREFRONTAL CORTEX VIA THE SUBSTANTIA-NIGRA PARS RETICULATA - A COMBINED ANATOMICAL AND ELECTROPHYSIOLOGICAL STUDY IN THE RAT

The nucleus accumbens is a major component of the ventral striatum thr ough which most of the limbic affiliated cortical areas gain access to the basal ganglia circuitry. In this study, the organization of the p athways linking the nucleus accumbens to the thalamus, via the substan tia nigra pars reticulata, was examined in the rat using anatomical an d electrophysiological methods. Use of anterograde and retrograde tran sport of wheatgerm agglutinin conjugated to horseradish peroxidase has established that the core of the nucleus accumbens innervates a dorsa l region of the substantia nigra pars reticulata which projects to sub fields of the mediodorsal and Ventral medial thalamic nuclei. These su bfields consist of the rostral pole of the mediodorsal nucleus with th e exception of its central segment and a region of the ventral medial nucleus, medial to the mammillothalamic tract. Confirming the existenc e of a nucleus accumbens nigrothalamic link, we have observed that ele ctrical or chemical stimulation of the nucleus accumbens induces an in hibition of the spontaneous discharges of the nigral cells which proje ct to the mediodorsal and ventral medial thalamic nuclei. Finally, the cortical projections of the thalamic subfields involved in the nucleu s accumbens nigrothalamic circuit were determined using the anterograd e and retrograde axonal transport of wheatgerm agglutinin conjugated w ith horseradish peroxidase. These subfields innervate mainly the preli mbic and to a lesser degree the orbital areas of the prefrontal cortex . The present data show that the substantia nigra pars reticulata is a major link beteen the core of the nucleus accumbens and the prefronta l cortex and provide further evidence for the concept of a parallel ar chitecture in the basal ganglia thalamocortical circuits of the ventra l striatum.