The ventral striatopallidal parts of the basal ganglia in the rat--II. Compartmentation of ventral pallidal efferents.

This paper describes the results of experiments designed to address whether neuron populations giving rise to different ventral pallidal efferent projections are segregated in a manner concordant with the recently described immunohistochemical compartmentation of ventral pallidum. The retrograde transport of horseradish peroxidase conjugated to wheatgerm agglutinin following injections in the ventral tegmental area of Tsai, medial substantia nigra and subthalamic nucleus was charted in relationship to the patterns of immunohistochemical staining in the forebrain following incubation of sections in antisera against substance P, neurotensin or leucine-enkephalin. In some cases the retrograde labeling and immunohistochemical protocols were combined in the same experiment. As a supplement, the electron microscope was utilized to investigate the ultrastructure and synaptic input of projecting cells making up populations of ventral pallidum neurons that project to different efferent targets. The results indicated that the immunocytochemical compartmentation of ventral pallidum observed in our earlier study is reflected in the organization of neurons from which ventral pallidal efferents originate. Thus, axons destined to terminate in the medial parts of substantia nigra and subthalamic nucleus project from neurons located in the lateral, neurotensin immunoreactivity-deficient part of ventral pallidum and the globus pallidus. Fibers en route to the ventral tegmental area originate in neurotensin immunoreactivity-rich medial parts of the ventral pallidum as part of a large cluster of retrogradely labeled neurons that invades several forebrain structures, including the bed nucleus of the stria terminalis, preoptic regions, and the nuclei of the diagonal band. The electron microscopic results provided additional evidence distinguishing the medial and lateral compartments of ventral pallidum. Whereas projection neurons located in both compartments display similar cytologic features, the density of synaptic input to retrogradely labeled perikarya and proximal dendrites following injections in the subthalamic nucleus is significantly greater than that seen following injections in the ventral tegmental area. Although no attempt was made to examine more distal parts of labeled dendrites in the present study, the observation that most dendritic profiles in the medial part of ventral pallidum were less contacted by boutons than their counterparts in the lateral district suggests that the level of innervation of projection neurons is generally lesser in medial ventral pallidum. This hypothesis is further supported by the presence in the medial ventral pallidum of significant numbers of "glial blockades," a cytologic configuration that is clearly capable of preventing functional interactions and is rarely observed in the lateral ventral pallidum...