Organization of the Pallidal Projections of the Rostromedial Tegmental Nucleus of the Dog Brain

Abstract

The organization of the projections of the rostromedial tegmental nucleus (RMTN) to functionally diverse nuclei of the pallidum in the dog brain (n = 13) was studied by retrograde and anterograde transport of horseradish peroxidase. These studies showed that fibers formed by neurons in the limbic medial segments of the rostral and caudal divisions of the RMTN are directed to the limbic parts of the pallidum – the ventral zone of the globus pallidus and the ventral pallidum. Reciprocal connections were observed between the ventral pallidum and the medial part of the rostral RMTN. These data identify the potential for segregated conduction of limbic information via these projection systems. However, most pallidal structures showed convergence of projection fibers arising from neurons in functionally diverse parts of the RMTN. Thus, projection fibers from neurons in the motor lateral and limbic medial parts of the RMTN are directed to the limbic ventral segment of the globus pallidus and the entopeduncular nucleus, which are innervated by fibers formed by neurons in functionally diverse structures. The possible pathways for the conduction of functionally different information and its integration in the projection systems studied here are discussed.