Two transpallidal pathways originating in the rat nucleus accumbens.

The striatopallidal projection originating in the nucleus accumbens was investigated by using anterograde transport of PHA-L in combination with peptide immunohistochemistry in order to localize the injection sites and transported lectin with respect to neurochemically defined subterritories in the nucleus accumbens and subcommissural ventral pallidum. The results reported here supplement our previous observations, which indicated that the subcommissural ventral pallidum of the rat comprises two immunohistochemically defined subterritories (Zahm and Heimer, '88: J. Comp. Neurol., 272:516-535) which give rise to dichotomous downstream projection systems (Zahm, '89: Neuroscience, 30:33-50). The present data indicate that the neurotensin immunoreactivity-rich ventromedial district of ventral pallidum receives its accumbal input almost exclusively from the shell district of the nucleus accumbens. The accumbal core, alternatively, projects to the dorsolateral ventral pallidal subterritory that lacks appreciable neurotensin immunoreactivity and in many other respects more resembles the adjoining striatopallidal components of the caudate-putamen. In addition to direct topographic relationships in the frontal plane among the accumbal injection sites and ventral pallidal terminations, it was observed that more caudally placed core injections resulted in patches of striatopallidal terminations that were more caudally located in ventral pallidum. Shell injections, in contrast, produced columns of terminations that extended continuously from the rostralmost level that they appeared to the caudal end of ventromedial ventral pallidum. The accumbal shell, its exclusive projection to the ventromedial subterritory in the subcommissural ventral pallidum, and the previously reported, almost exclusive projection of that pallidal subdistrict to the mesencephalic ventral tegmental area are discussed in terms of a number of other neurochemical and hodological features that serve to distinguish them sufficiently to suggest that they represent a uniquely specialized part of the basal ganglia.