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 electr on microscopic levels. 3HAO-immunoreactivity was detected exclusively in astrocytes. Labeling was present in cell bodies and in fine glial p rocesses, which frequently encircled capillaries and partially envelop ed neuronal somata. Notably, 3HAO-labeled processes were in close cont act with several types of synaptic profiles. Often, they partially eng ulfed asymmetric synapses, characteristic of excitatory neurotransmiss ion. In addition, they were found in apposition to putative dopaminerg ic cell bodies. These data provide an anatomical basis for the idea th at functional interactions may occur between glial processes which syn thesize quinolinic acid, and synaptic profiles, many of which presumab ly utilize excitatory neurotransmitters.