The organization of the efferent projections and striatal afferents of the entopeduncular nucleus and adjacent areas in the rat.

Multiple retrograde fluorescent tracing was employed to investigate the organization of the rat entopeduncular nucleus projections to the lateral habenula, ventral anterior-ventral lateral thalamus, parafasicular-centre median complex, and tegmenti pedunculopontis area of the brain stem. The results indicate that neurons in the rostral 2/3 of the entopeduncular nucleus project to the lateral habenula. In contrast, neurons in the caudal 1/3 of the entopeduncular nucleus project to the ventral anterior-ventral lateral thalamus, parafasicular-centre median complex, and tegmenti pedunculopontis area of the brain stem. The majority of neurons in the caudal 1/3 of the entopeduncular nucleus are retrogradely double-labeled from various combinations of tracer injections into the 3 termination areas to which they projected. Little or no retrograde labeling in the entopeduncular nucleus was produced by tracer injections in the substantia nigra or subthalamic nucleus. Only large injections of tracers in the tegmenti pedunculopontis are and the surrounding brain stem produced retrograde labeling in the entopeduncular nucleus. These brain stem injections also labeled a band of cells surrounding the entopeduncular nucleus in the zona incerta, lateral hypothalamus, of the ansa lenticularis and central nucleus of the amygdala. [3H]Leucine injections in the head of the caudate-putamen complex (but not in the cortex or globus pallidus) produced dense accumulations of silver grains over both the rostral and caudal portions of the entopeduncular nucleus. [3H]Leucine injections in the caudal body of the caudate-putamen complex produced accumulations of silver grains over a "ventral entopeduncular nucleus area' in the nucleus of the ansa peduncularis. It was suggested that the head of the striatum projects both to the neurons of the rostral "limbic' portion of the entopeduncular nucleus, which project to the lateral habenula, and to the neurons of the caudal "motor' portion of the entopeduncular nucleus, which project primarily by way of axon collaterals to the ventral anterior-ventral lateral thalamus, parafascicular-centre median complex, and tegmenti pedunculopontis area of the brain stem.