ORGANIZATION OF THE OUTPUT OF THE VENTRAL STRIATOPALLIDAL SYSTEM IN THE RAT - VENTRAL PALLIDAL EFFERENTS

The efferent projections of the ventral pallidum in the rat were studi ed using anterograde tracing of Phaseolus vulgaris-leucoagglutinin and retrograde tracing of choleratoxin subunit B. The main aim of this st udy was to determine the degree of topographical organization in the o utputs 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 thal amus, the mediodorsal thalamic nucleus, the dorsomedial part of the su bthalamic nucleus, the medial part of the parafascicular nucleus and t he lateral habenula. In the mesencephalon, ventral pallidal fibers ter minate in the ventral tegmental area, the substantia nigra, the retror ubral area, the median raphe nucleus, the nucleus raphe magnus, the pe ribrachial area, the ventromedial part of the central gray substance a nd the locus coeruleus. The results of the experiments in which retrog rade tracers were injected in different nuclei in the mesencephalon al low the distinction of two main areas in the ventral pallidum. Deposit s of retrograde tracers in the substantia nigra, pars reticulata resul t in labeling of cells in the dorsolateral part of the ventral pallidu m, located immediately ventral to the anterior limb of the anterior co mmissure. Retrograde tracer injections in other targets of the ventral mesencephalon, i.e. the dopaminergic cell groups A10, A9 or A8, or nu clei in the peribrachial area result in labeling of neurons in an exte nsive ventromedial and ventrolateral zone of the ventral pallidum. The medial part of this ventral pallidal zone projects to the ventral teg mental area, whereas ventral and lateral parts connect with more later al 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 d istinctly topographically organized. The ventral pallidostriatal proje ctions preserve a medial-to-lateral, a dorsal-to-ventral and, to a les ser degree, a rostral-to-caudal topography. With respect to the subtha lamic region, the dorsolateral part of the ventral pallidum projects t o the dorsomedial part of the subthalamic nucleus, whereas the ventrom edial and ventrolateral parts of the ventral pallidum are topographica lly connected with the area of the lateral hypothalamus medially adjac ent to the subthalamic nucleus. The ventral pallidal projections to th e mediodorsal thalamic nucleus are arranged such that dorsal-to-ventra l and medial-to-lateral gradients in the ventral pallidum correspond t o medial-to-lateral and rostral-to-caudal gradients in the mediodorsal nucleus, respectively. This topography in the ventral pallidal-mediod orsal thalamic pathways fits in with the parallel arrangement of a num ber of circuits that involve distinct parts of the prefrontal cortex, the ventral striatum, the ventral pallidum, and the mediodorsal thalam ic nucleus. Such a parallel arrangement of the connections of the vent ral parts of the basal ganglia with the prefrontal cortex and the medi odorsal thalamus is consistent with the more general concept of parall el, functionally segregated basal ganglia-thalamocortical circuits.