The GABA and substance P input to dopaminergic neurones in the substantia nigra of the rat.

In order to examine the synaptic input to dopaminergic neurones in the substantia nigra from GABAergic terminals and terminals that contain substance P, double and triple immunocytochemical studies were carried out at the light and electron microscopic levels in the rat. In a first series of experiments sections of the substantia nigra were incubated to reveal axon terminals containing either substance P or glutamate decarboxylase and then incubated to reveal dopaminergic neurones using tyrosine hydroxylase immunocytochemistry. Examination of this material in the light microscope revealed that many substance P- and glutamate decarboxylase-immunoreactive boutons were associated with the dopaminergic cells. In the electron microscope it was found that the perikarya and dendrites of the dopaminergic neurons received symmetrical synaptic input from terminals that displayed immunoreactivity for substance P or glutamate decarboxylase. A small proportion of the substance P-positive boutons formed asymmetrical synapses. In a second series of experiments sections of the substantia nigra were processed by the pre-embedding immunocytochemical technique for tyrosine hydroxylase and then the post-embedding immunogold technique for gamma-aminobutyric acid (GABA). Examination in the electron microscope revealed that the tyrosine hydroxylase-positive neurons received symmetrical synaptic input from many GABA-positive terminals. Quantitative analyses demonstrated that a minimum of 50-70% of all boutons afferent to the dopaminergic neurones display glutamate decarboxylase or GABA immunoreactivity. Triple immunocytochemical studies i.e. pre-embedding immunocytochemistry for tyrosine hydroxylase and substance P, combined with post-embedding immunogold staining for GABA, revealed that some of the substance P-immunoreactive boutons that were in contact with the dopaminergic neurones also displayed GABA immunoreactivity. In a third series of experiments the combination of anterograde transport of lectin-conjugated horseradish peroxidase or biocytin with post-embedding GABA immunocytochemistry demonstrated that at least one of the sources of GABA-containing terminals in the substantia nigra is the striatum. The results of the present study: (1) demonstrate that dopaminergic neurones in the substantia nigra receive symmetrical synaptic input from GABAergic and substance P-containing terminals, (2) show that a proportion of these terminals contain both substance P and GABA and (3) suggest that the major synaptic input to dopaminergic neurones is from GABAergic terminals and that a part of this innervation is derived from the striatum.