The orbital cortex in rats topographically projects to central parts of the caudate-putamen complex.

Disturbances of the orbitofrontal-striatal pathways in humans have been associated with several psychopathologies including obsessive-compulsive disorder and drug addiction. In nonhuman primates, different subareas of the orbitofrontal cortex project topographically to central and ventromedial parts of the striatum. Relatively little is known about the anatomical organization of the rat orbital cortex while there is a growing interest in this cortical area from a functional and behavioral point of view. The aim of the present neuroanatomical tracing study was to determine in rats the striatal target area of the projections of the orbital cortex as well as the topographical organization within these projections. To this end, anterograde tracers were injected in the different cytoarchitectonically distinct subareas of the orbital cortex. The results show that the individual orbital areas, i.e. medial orbital area, ventral orbital area, ventrolateral orbital area and lateral orbital area, project to central parts of the caudate-putamen, exhibiting a mediolateral and, to a lesser degree, rostrocaudal topographical arrangement. Orbital projections avoid the most dorsal, as well as rostral and caudal parts of the caudate-putamen. Terminal fields from cytoarchitectonically different areas show a considerable overlap. Superficial cortical layers project preferentially to the striatal matrix, deep layers to the patch compartment. The projections from the ventrolateral orbital area are strongest and occupy the most extensive striatal area. In addition to projections to the caudate-putamen, the ventrolateral, lateral and dorsolateral orbital areas have a scarce projection to the most lateral part of the nucleus accumbens shell in the ventral striatum. In contrast to nonhuman primates, the remainder of the rat nucleus accumbens is virtually free of orbital projections.