The efferent projections of the ventral pallidum in the rat were studied using anterograde tracing of Phaseolus vulgaris-leucoagglutinin and retrograde tracing of choleratoxin subunit B. The main aim of this study was to determine the degree of topographical organization in the outputs of the ventral pallidum. In the telencephalon, ventral pallidal fibers reach the prefrontal cortex, the ventral striatum, the lateral septum, the basolateral, lateral, and central amygdaloid nuclei, and the lateral entorhinal area. Diencephalic targets of ventral pallidal fibers are the lateral hypothalamus, the reticular nucleus of the thalamus, the mediodorsal thalamic nucleus, the dorsomedial part of the subthalamic nucleus, the medial part of the parafascicular nucleus and the lateral habenula. In the mesencephalon, ventral pallidal fibers terminate in the ventral tegmental area, the substantia nigra, the retrorubral area, the median raphe nucleus, the nucleus raphe magnus, the peribrachial area, the ventromedial part of the central gray substance and the locus coeruleus.

The results of the experiments in which retrograde tracers were injected in different nuclei in the mesencephalon allow the distinction of two main areas in the ventral pallidum. Deposits of retrograde tracers in the substantia nigra, pars reticulata result in labeling of cells in the dorsolateral part of the ventral pallidum, located immediately ventral to the anterior limb of the anterior commissure. Retrograde tracer injections in other targets of the ventral mesencephalon, i.e. the dopaminergic cell groups A10, A9 or A8, or nuclei in the peribrachial area result in labeling of neurons in an extensive ventromedial and ventrolateral zone of the ventral pallidum. The medial part of this ventral pallidal zone projects to the ventral tegmental area, whereas ventral and lateral parts connect with more lateral and caudal mesencephalic targets.

The projections from the ventral pallidum to the ventral striatum, the subthalamic nucleus and adjacent lateral hypothalamic area, and the mediodorsal thalamic nucleus are distinctly topographically organized. The ventral pallidostriatal projections preserve a medial-to-lateral, a dorsal-to-ventral and, to a lesser degree, a rostral-to-caudal topography. With respect to the subthalamic region, the dorsolateral part of the ventral pallidum projects to the dorsomedial part of the subthalamic nucleus, whereas the ventromedial and ventrolateral parts of the ventral pallidum are topographically connected with the area of the lateral hypothalamus medially adjacent to the subthalamic nucleus. The ventral pallidal projections to the mediodorsal thalamic nucleus are arranged such that dorsal-to-ventral and medial-to-lateral gradients in the ventral pallidum correspond to medial-to-lateral and rostral-to-caudal gradients in the mediodorsal nucleus, respectively.

This topography in the ventral pallidal-mediodorsal thalamic pathways fits in with the parallel arrangement of a number of circuits that involve distinct parts of the prefrontal cortex, the ventral striatum, the ventral pallidum, and the mediodorsal thalamic nucleus. Such a parallel arrangement of the connections of the ventral parts of the basal ganglia with the prefrontal cortex and the mediodorsal thalamus is consistent with the more general concept of parallel, functionally segregated basal ganglia-thalamocortical circuits.