GABA axons in synaptic contact with dopamine neurons in the substantia nigra: double immunocytochemistry with biotin-peroxidase and protein A-colloidal gold.

Two different antigens in the same ultrathin section of brain tissue can be revealed by 'double immunocytochemistry' in which one antigen is detected by horseradish peroxidase and the other by silver intensification of colloidal gold (SIG) adsorbed to Protein A. By means of this procedure it has been possible to show that GABAergic axon terminals (containing glutamate decarboxylase) are in synaptic contact with the cell bodies and dendrites of dopaminergic neurons (containing tyrosine hydroxylase) in the substantia nigra of the rat. Thus, several of the physiological and pharmacological effects of GABA and GABAergic drugs in this part of the brain are likely to be mediated by a direct action via postsynaptic GABAergic receptors located on dopaminergic nigrostriatal neurons.