Gamma-aminobutyric acid in the medial rat nucleus accumbens: ultrastructural localization in neurons receiving monosynaptic input from catecholaminergic afferents.

Neurons containing gamma-aminobutyric acid (GABA) in the medial portion of the adult rat nucleus accumbens were characterized with respect to their ultrastructure, sites of termination, and catecholaminergic input. Antisera against GABA-conjugates and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), were localized within single sections by means of peroxidase-antiperoxidase (PAP) and immunoautoradiographic labeling methods. Peroxidase reaction product indicating GABA-like immunoreactivity (GABA-LI) was seen in medium-size (15-20 microns) perikarya containing either round and unindented or invaginated nuclear membranes. The cells with invaginated nuclei were few in number and usually exhibited more intense peroxidase reaction product in sections collected at the same distance from the surface of the tissue. Reaction product for GABA was also detected in proximal (1.5-3.0 microns) dendrites, axons, and terminals. Terminals with GABA-LI formed symmetric junctions on perikarya, proximal dendrites, and dendritic spines of neurons that usually lacked detectable immunoreactivity. Many of the GABAergic terminals also were apposed directly to other unlabeled terminals and to terminals exhibiting either peroxidase labeling for GABA or immunoautoradiographic labeling for TH. Many of the unlabeled terminals associated with the GABAergic axons formed asymmetric junctions on dendritic spines. From 138 TH-labeled, principally dopaminergic terminals that were examined in the medial nucleus accumbens, 4% were associated with the somata of GABAergic neurons and another 14% formed symmetric junctions with proximal dendrite showing GABA-LI. The remaining TH-immuno-reactive terminals either lacked recognizable densities or formed symmetric synapses on unlabeled dendrites and spines. A few of the unlabeled dendrites, as well as those containing GABA-LI, received symmetric synapses from both catecholaminergic and GABAergic terminals. We conclude that in the medial portion of the rat nucleus accumbens, GABA is localized to two morphologically distinct types of neurons, one or both of which receive monosynaptic input from catecholaminergic afferents, and that GABAergic terminals form symmetric synapses on other principally non-GABAergic neurons. The results also support earlier physiological evidence showing that GABA may modulate the output of other GABAergic and non-GABAergic neurons through presynaptic associations.