SYNAPTOLOGY OF THE NIGROSTRIATAL PROJECTION IN RELATION TO THE COMPARTMENTAL ORGANIZATION OF THE NEOSTRIATUM IN THE RAT

The patch-matrix organization of the striatal complex, which is fundam ental to the structural and functional organization of the basal gangl ia, is characterized on the basis of both connections and neurochemist ry. In order to determine whether differences in the connections and n eurochemistry are reflected in differences in synaptic organization, w e examined the synaptology of the dopaminergic nigrostriatal projectio n in the patch-matrix complex of the rat. Three approaches were used. First, deposits of the anterograde tracer, biotinylated dextran amine, were placed in the substantia nigra. Sections of perfuse-fixed neostr iatum were then processed to reveal anterogradely-labelled nigrostriat al axons and calbindin-D28k immunoreactivity, a marker for the patch-m atrix complex. Secondly, sections of perfuse-fixed neostriatum were im munolabelled to reveal both tyrosine hydroxylase, a marker for dopamin ergic structures and calbindin-D28k. Labelled axons in the patches and the matrix were examined at both the light and the electron microscop ic levels. Finally, in order to test for the presence of fixed GABA in sub-types of anterogradely-labelled terminals in the neostriatum, ult rathin sections were immunolabelled by the post-embedding immunogold m ethod. Based on morphological analysis, anterogradely-labelled nigrost riatal axons were divided into two types (Type I and Type II). The den sity of tyrosine hydroxylase labelling in the neostriatum prevented th e classification of immunolabelled nigrostriatal axons. The Type I ant erogradely-labelled axons and tyrosine hydroxylase-positive axons were found both in the patches and in the matrix. They both formed symmetr ical synapses with spines, dendrites and occasionally somata. The morp hology, dimensions, type of synaptic specialization and the distributi on of postsynaptic targets of axons labelled by both methods were simi lar in the patches and the matrix. The Type I anterogradely-labelled a xons were immunonegative for GABA. The Type II anterogradely-labelled axons were GABA-immunopositive, were found only in the matrix and were only present in those animals in which retrograde labelling was obser ved in the globus pallidus, they are thus not part of the dopaminergic nigrostriatal projection. It is concluded that although the patch-dir ected and matrix-directed dopaminergic projections from the Ventral me sencephalon arise from different populations of dopaminergic neurons, their innervation of neurons in the patches and matrix is similar. The anatomical substrate, and therefore probably also the mechanism, for dopaminergic modulation of the how of cortical information through the striatal complex is essentially the same in the patch and in the matr ix sub-divisions of the striatal complex. (C) 1997 IBRO. Published by Elsevier Science Ltd.