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Ultrastructural single‐ and double‐label immunohistochemical studies of substance P‐containing terminals and dopaminergic neurons in the substantia nigra in pigeons

The vast majority of striatonigral projection neurons in pigeons contain substance P (SP), and the vast majority of SP‐containing fibers terminating in the substantia nigra arise from neurons in the striatum. To help clarify the role of striatonigral projection neurons, we conducted electron microscopic single‐ and double‐label immunohistochemical studies of SP+ terminals and/or dopaminergic neurons (labeled with either anti‐dopamine, DA, or anti‐tyrosine hydroxylase, TH) in pigeons to determine: (1) the synaptic organization of SP+ terminals, (2) the synaptic organization of TH+ perikarya and/or dendrites, and (3) the synaptic relationship between SP+ terminals and TH+ neurons in the substantia nigra.