Synaptic endings and their postsynaptic targets in neostriatum: synaptic specializations revealed from analysis of serial sections.

Neostriatal tissue from rat brain was examined in ribbons of serial sections, using tissue from subjects in which the synaptic marker 5-hydroxydopamine had been injected into the lateral ventricle. A total of 440 synaptic terminals, identified by the presence of a population of vesicles, was studied, including those labeled by 5-hydroxydopamine and all those unlabeled in the surrounding neuropil. Three categories of synaptic profiles innervating neostriatal dendrites could be discerned. One category contained small rounded or slightly pleomorphic vesicles and consisted of a sample of 375 endings, 53 of which exhibited light to heavy label inside the synaptic vesicles. All of these, labeled and unlabeled, showed evidence of membrane specializations and other conventional criteria of synaptic contact, mostly in the form of asymmetric synaptic contacts with the heads of dendritic spines. A second category of 40 terminals, 3 of which were lightly labeled, contained large rounded or slightly pleomorphic vesicles and made predominantly symmetric contacts with the shafts of dendritic spines or spine-free portions of dendritic membrane. None of these synaptic endings appeared to lack synaptic contact with postsynaptic targets in the neostriatum. A third category of 25 terminals, 3 of which were labeled, contained small flattened vesicles, and these endings also invariably made synaptic contact, mostly onto spine-free dendritic membrane, and were characterized by symmetric membrane thickenings at the point of apposition. Our evidence supports the view that dopaminergic and other synaptic terminals in the rat caudate-putamen make synaptic contact with postsynaptic targets in the neostriatum and, at least in the adult, do not exist in appreciable numbers in the form of terminals that are not apposed to membranes of postsynaptic targets in the neostriatum.