INTERCONNECTED PARALLEL CIRCUITS BETWEEN RAT NUCLEUS-ACCUMBENS AND THALAMUS REVEALED BY RETROGRADE TRANSYNAPTIC TRANSPORT OF PSEUDORABIES VIRUS

One of the primary outputs of the nucleus accumbens is directed to the mediodorsal thalamic nucleus (MD) via its projections to the ventral pallidum (VP), with the core and shell regions of the accumbens projec ting to the lateral and medial aspects of the VP, respectively. In thi s study, the multisynaptic organization of nucleus accumbens projectio ns was assessed using intracerebral injections of an attenuated strain of pseudorabies virus, a neurotropic alpha herpesvirus that replicate s in synaptically linked neurons. Injection of pseudorabies virus into different regions of the MD or reticular thalamic nucleus (RTN) produ ced retrograde transynaptic infections that revealed multisynaptic int eractions between these areas and the basal forebrain. Immunohistochem ical localization of viral antigen at short postinoculation intervals confirmed that the medial MD (m-MD) receives direct projections from t he medial VP, rostral RTN, and other regions previously shown to proje ct to this region of the thalamus. At longer survival intervals, injec tions confined to the m-MD resulted in transynaptic infection of neuro ns in the accumbens shell but not in the core. Injections that also in cluded the central segment of the MD produced retrograde infection of neurons in the lateral VP and the polymorph (pallidal) region of the o lfactory tubercle (OT) and transynaptic infection of a small number of neurons in the rostral accumbens core. Injections in the lateral MD r esulted in retrograde infection in the globus pallidus (GP) and in tra nsynaptic infection in the caudate-putamen. Viral injections into the rostroventral pole of the RTN infected neurons in the medial and later al VP and at longer postinoculation intervals, led to transynaptic inf ection of scattered neurons in the shell and core. Injection of virus into the intermediate RTN resulted in infection of medial VP neurons a nd second-order infection of neurons in the accumbens shell. Injection s in the caudal RTN or the lateral MD resulted in direct retrograde la beling of cells within the GP and transynaptic infection of neurons in the caudate-putamen. These results indicate that the main output of V P neurons receiving inputs from the shell of the accumbens is heavily directed to the m-MD, whereas a small number of core neurons appear to influence the central MD via the lateral VP. Further segregation in t he flow of information to the MD is apparent in the organization of VP and GP projections to subdivisions of the RTN that give rise to MD af ferents. Collectively, these data provide a morphological basis for th e control of the thalamocortical system by ventral striatal regions, i n which parallel connections to the RTN may exert control over activit y states of cortical regions.