COMPARTMENTAL ORIGIN OF THE STRIATO-ENTOPEDUNCULAR PROJECTION IN THE RAT

The mammalian neostriatum is divisible into neurochemically and cytoar chitectonically distinct striosome and matrix compartments. This compa rtmentalization is respected by many afferent and efferent projections of the striatum. The distribution of distinct types of neuroactive su bstances and receptors and the unique connections of the striosome and matrix suggest a functional segregation between these compartments. T he present study examines the organization of efferent projections fro m each of the striatal compartments to the entopeduncular nucleus (EPN ), a major output center of the basal ganglia. The fluorescent retrogr ade tracer fluorogold, or rhodamine-conjugated dextran, was injected i nto the lateral habenula or the ventrolateral nucleus of the thalamus of adult Wistar rats to identify the topographical organization of EPN -habenular and EPN-thalamic neurons. Fluorogold was then placed into t he rostral or caudal parts of the EPN, identified from the previous ex periment as areas containing predominantly EPN-habenular or EPN-thalam ic neurons, respectively. Sections containing retrogradely labeled neu rons in the neostriatum were simultaneously immunolabeled for calbindi n-D28kDa, a calcium-binding protein found exclusively in the projectio n neurons of the matrix. The results indicate that the striatal projec tion to the EPN-habenular and EPN-thalamic parts of the EPN originates from striosome and matrix neurons, respectively. The duality of stria tal outflow involving the EPN suggests a mechanism whereby the strioso me is integrated into subcortical pathways that modulate the activity of the basal ganglia via the ascending serotoninergic projection from the dorsal raphe nucleus, whereas the matrix is involved in a loop tha t includes the thalamus and the cerebral cortex.