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The anatomical organization and electrophysiological characteristics of a projection from the nucleus accumbens to anteroventral parts of the globus pallidus and to a subpallidal region that includes the substantia innominata (SI), the lateral preoptic area (LPO), and anterior parts of the lateral hypothalamic area (LHA) were investigated in the rat. Autoradiographic experiments, with injections of 3H-proline into different sites in the nucleus accumbens and adjacent caudoputamen, indicate that the descending fibers are organized topographically along both mediolateral and dorsoventral gradients, although labeled fibers from adjacent regions of the nucleus accumbens overlap considerably in the ventral globus pallidus and subpallidal region. Injections confined to the caudoputaman only labeled fibers in the globus pallidus. Retrograde transport experiments with the marker true blue confirmed that only the nucleus accumbens projects to the subpallidal region and that the caudoputamen projects upon the glubus pallidus in a topographically organized manner. In electrophysiological recording experiments single pulse stimulation (0.1 to 0.7 mA; 0.15 msec duration) of the nucleus accumbens changed the discharge rate of single neurons in the ventral globus pallidus and in the SI, LPO, and LHA. Typically, the responses were inhibition of neuronal discharge with latencies of 6 to 18 msec. Single pulse stimulation of the dorsolateral caudoputamen altered the discharge rate of single neurons in dorsal regions of the globus pallidus, with inhibition being the most frequently observed response. The results of these anatomical and electrophysiological experiments are complementary and indicate that fibers from the nucleus accumbens innervate the anteroventral region of the globus pallidus as well as the subpallidal region, while most fibers of the caudoputamen innervate the globus pallidus but not the subpallidal region. It appears, therefore, that these two components of the striatum have different output connections. The possible functional significance of these findings is discussed in relation to the projections of the subpallidal region, which may include an output to the mesencephalic locomotor region, and in relation to the nucleus accumbens afferents from the amygdala and hippocampal formation.

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