EFFERENT CONNECTIONS OF THE INTERNAL GLOBUS-PALLIDUS IN THE SQUIRREL-MONKEY .2. TOPOGRAPHY AND SYNAPTIC ORGANIZATION OF PALLIDAL EFFERENTS TO THE PEDUNCULOPONTINE NUCLEUS

The first objective of the present study was to verify whether project ions from regions of the internal pallidum (GPI) that receive inputs f rom different functional areas of the striatum remain segregated at th e level of the pedunculopontine nucleus (PPN) in squirrel monkeys. Sec ond, we analyzed the ultrastructural features and synaptic organizatio n of pallidal terminals in contact with PPN neurons. This was achieved by performing iontophoretic injections of biotinylated dextran amine (BDA) in different regions of the GPi. The animals were pooled into th ree groups on the basis of the location of the injection sites and the resulting distribution of retrogradely labelled striatal neurons. The experimental groups were divided as follows: group 1: injections in t he dorsal one-third of the GPi, retrograde labelling in the head and b ody of the caudate nucleus (''associative striatum''); group 2: inject ions in the ventrolateral two-thirds of the GPi, retrograde labelling in the postcommissural region of the putamen (''sensorimotor striatum' '); and group 3: injections in the rostromedial pole of the GPi, retro grade labelling in the ventral striatum (''limbic striatum''). These i njections led to the anterograde labelling of varicose fibers that arb orized profusely in common regions of the PPN dorsal to the brachium c onjunctivum. The fields of fibers that arose from the dorsal one-third and the rostromedial pole of the GPi were more widely spread than the afferents from the ventrolateral two-thirds of the GPi. Small numbers of retrogradely labelled cells were encountered in the PPN after each injection in the GPi. Some of them were tightly surrounded by large, BDA-containing varicosities, which implies that the connections betwee n the GPi and the PPN are partly reciprocal. In sections processed for the simultaneous localization of beta-nicotinamide adenine dinucleoti de phosphate (NADPH)-diaphorase (a marker of cholinergic cells in the PPN) and BDA, the anterogradely labelled fibers largely avoided the de nse aggregate of NADPH-diaphorase-containing neurons in the PPN pars c ompacta (PPNc) but, rather, established contacts with unlabelled neuro ns in the pars dissipata (PPNd). In the electron microscope, the GPi t erminals were large (1.0-5.0 mu m in diameter), contained many mitocho ndria and pleomorphic vesicles, and formed symmetric synapses predomin antly with proximal dendrites of PPN cells. In conclusion, our data su ggest that the noncholinergic neurons of the PPNd are potential target s for the integration of information arising from different functional territories of the GPi in primates. The PPNd is thus in a position to act as an interface between motivational, cognitive, and motor inform ation transmitted along the pallidotegmental projection in primates. J . Comp. Neurol. 382:348-363, 1997. (C) 1997 Wiley-Liss, Inc.