DIFFERENTIAL SYNAPTIC INNERVATION OF STRIATOFUGAL NEURONS PROJECTING TO THE INTERNAL OR EXTERNAL SEGMENTS OF THE GLOBUS-PALLIDUS BY THALAMIC AFFERENTS IN THE SQUIRREL-MONKEY

It is well established that the centromedian nucleus (CM) is the major source of thalamic afferents to the sensorimotor territory of the str iatum in monkeys. However, the projection sites of striatal neurones c ontacted by thalamic afferents still remain to be determined. We there fore carried out an anatomical study aimed at elucidating the hodology of striatal neurones that receive input from the CRI in squirrel monk eys. Our approach was to combine the anterograde transport of Phaseolu s vulgaris-leucoagglutinin (PHA-L) or biocytin from the CM with the re trograde transport of biotinylated dextran-amine (bio-dex) or PKA-L fr om the internal (GPi) or external (GPe) segments of the globus pallidu s.Following CM injections, rich plexuses of anterogradely labelled, th in varicose fibres aggregated in the form of bands that were confined to the postcommissural region of the putamen. On the other hand, injec tions into the GPe or GPi led to profuse retrograde labelling of a mul titude of medium-sized spiny neurones. In cases where the injections i nvolved the caudoventral two-thirds of the GPe or GPi, the retrogradel y labelled striatopallidal cells and the anterogradely labelled thalam ostriatal fibres occurred in the sensorimotor territory of the putamen . After injections into either pallidal segments, clusters of retrogra dely labelled cells were in register with bands of anterogradely label led thalamic fibres. However, electron microscopic analysis of striata l regions containing both anterogradely labelled thalamic afferents an d retrogradely labelled cells revealed that terminals from the CM freq uently form asymmetric synapses with dendritic shafts and spines of st riato-GPi cells but rarely with those of striato-GPe cells. In conclus ion, our findings demonstrate that thalamic afferents from the CM inne rvate preferentially striatopallidal neurones projecting to the GPi in monkeys. These results indicate that the striatopallidal neurones con tributing to the ''direct'' and ''indirect'' output pathways are diffe rentially innervated by thalamic afferents in primates. (C) 1996 Wiley -Liss, Inc.