Two types of projection neurons in the internal pallidum of primates: Single-axon tracing and three-dimensional reconstruction

The axonal projections of the internal pallidum. (GPi) in cynomolgus monkey s (Macaca fascicularis) were studied by labeling small pools of neurons wit h biotinylated dextran amine. Fifty-two axons were reconstructed entirely f rom serial sections with a camera lucida. Two types of projection neurons w ere identified in the GPi on the basis of their target sites. The abundant and centrally located type I neurons gave rise to a long axonal branch that descended directly to the pedunculopontine tegmental nucleus, where it arb orized discretely. Other branches ascended to the thalamus and broke into 1 0-15 thinner collaterals that ran through most of the ventral anterior nucl eus, where they terminated as typical plexuses. About half of these axons g ave rise to collaterals that arborized in both components of the centre med ian/parafascicular thalamic complex. The less numerous and peripherally loc ated type II neurons had an axon that climbed the rostral thalamic pole, co ursed along the stria medullaris, and arborized profusely within the latera l habenular nucleus, which stood out as the most densely innervated pallida l target. Some type II axons provided collaterals to the anterior thalamic nuclei. A small proportion of axons of both types had branches that crossed the midline and terminated in contralateral GPi target structures. Three-d imensional reconstruction showed that type I axons arborized principally al ong the sagittal plane. These data reveal that GPi neurons of type I act th rough a widely distributed axonal network upon thalamic and brainstem premo tor neurons, whereas type II neurons act in a much more focused manner upon lateral habenular neurons. (C) 2001 Wiley-Liss, Inc.