Immunocytochemical localization of the quinolinic acid synthesizing enzyme, 3-hydroxyanthranilic acid oxygenase, in the rat substantia nigra.

Quinolinic acid, an endogenous excitatory amino acid receptor agonist, may play a role in several brain diseases. In the present study, the immunocytochemical localization of 3-hydroxyanthranilic acid oxygenase (3HAO), the enzyme responsible for the synthesis of quinolinic acid, was examined in the adult rat substantia nigra at the light and electron microscopic levels. 3HAO-immunoreactivity was detected exclusively in astrocytes. Labeling was present in cell bodies and in fine glial processes, which frequently encircled capillaries and partially enveloped neuronal somata. Notably, 3HAO-labeled processes were in close contact with several types of synaptic profiles. Often, they partially engulfed asymmetric synapses, characteristic of excitatory neurotransmission. In addition, they were found in apposition to putative dopaminergic cell bodies. These data provide an anatomical basis for the idea that functional interactions may occur between glial processes which synthesize quinolinic acid, and synaptic profiles, many of which presumably utilize excitatory neurotransmitters.