Immunocytochemical localization of tyrosine hydroxylase in the human striatum: a postmortem ultrastructural study

An electron microscopic evaluation of tyrosine hydroxylase (TH) immunocytochemistry was used to describe the synaptic organization of dopamine innervation of the striatum in postmortem human brain tissue. TH immunoreactivity was qualitatively and quantitatively similar in the caudate and putamen. TH immunoreactivity was present mainly in unmyelinated axons and occasionally in myelinated axons. Both TH-immunoreactive (TH-i) varicosities (0.75-1.5 microm) and intervaricose segments (0.2-0.3 microm) formed synapses with spines and dendrites. Most synapses formed by TH-i profiles were symmetric axospinous (57-62%) or symmetric axodendritic (33-35%). An occasional asymmetric axodendritic or asymmetric axospinous synapse was observed. Approximately 35-50% of all symmetric axospinous and axodendritic synapses were formed by TH-i boutons. Synapses formed by TH-i profiles were short in length (0.226 microm) and had nonperforated postsynaptic densities. TH-i profiles formed synapses with both the head (40%) and the neck (60%) of spines. Typically, the TH-i bouton was apposed to both a spine and a nonlabeled terminal which formed an asymmetric synapse with that spine. Direct, nonsynaptic appositions were often seen between TH-labeled and nonlabeled boutons forming asymmetric synapses. The general pattern ofTH immunoreactivity was similar to that of other species except for the presence of TH-i myelinated axons and the observation that the majority of TH-i synapses were formed with spines rather than with dendritic shafts.