Ultrastructural morphology of dopaminergic nerve terminals and synapses in the striatum of the rat using tyrosine hydroxylase immunocytochemistry: a topographical study

Abstract

Structures immunoreactive for TH were examined in the rat striatum (including caudate-putamen, nucleus accumbens and globus pallidus) by electron microscopy using the indirect peroxidase-labeled antibody method. Axon profiles and nerve terminals were the only structures stained by DAB precipitates in the axoplasm. The reactive boutons frequently contained a population of large pleomorphic vesicles (40-60 nm in diameter) but their interiors remained free of reactions. The synaptic contacts formed belonged principally to the symmetric type 2 of Gray while asymmetric Gray's type 1 synapses were rarely observed. The former were mostly apposed to dendritic trunks (rarely to perikarya) and the latter to dendritic spines. In addition, numerous immunoreactive nerve terminals were often found in close contact with small structures identified as the neck of dendritic spines. The active zone of these presumed synapses was characterized by a prominent thickening of the presynaptic membrane but the post-synaptic thickening was lacking. For similar reasons, it was difficult to assert the existence of one axo-axonic synapse when a positive nerve terminal was closely apposed to another one (generally unreactive). The exact morphology of dopaminergic synapses, or even their existence, have not been firmly established owing to large discrepancies between previous reports. No synapses, or synaptic contacts of either asymmetric or symmetric type were described according to the technique used. Our work was undertaken to elucidate further this problem, and in particular, we thought that regional differences in the synaptic organization might explain the divergent data. However, regional quantitative analysis performed in this study did not show significant differences in the percentage of either kind of synapses in the various striatal regions.