Induction of argininosuccinate synthetase in rat brain glial cells after striatal microinjection of immunostimulants

The enzyme argininosuccinate synthetase (ASS) initiates the metabolic pathw ay leading from L-citrulline to L-arginine, the only physiological substrat e of all isoforms of nitric oxide synthases. The presence of ASS in glial c ells in vivo was investigated by immunohistochemical methods in a model of rat brain inflammation. Phosphate-buffered saline or a mixture of bacterial lipopolysaccharide and interferon-gamma was injected into the left striatu m, and animals were killed 24 hours later. Ipsilateral and contralateral si des of brain sections were incubated with an antiserum against ASS or antib odies against cell-specific markers. In the three areas examined, striatum, corpus callosum, and cortex, a strong induction of ASS immunoreactivity wa s observed in glial cells after injection of immunostimulants. A detailed q uantitative analysis of double-stained sections revealed that ASS was almos t exclusively expressed in reactive, ED1-positive microglial cells/brain ma crophages in immunostimulant- or sham-injected ipsilateral sides of the sec tions. Furthermore, ASS/ED1 costaining was observed in perivascular cells. Colocalization of ASS with astroglial marker glial fibrillary acidic protei n was given only occasionally after immunostimulation. ASS-positive neurons were detected in control and experimental animals; staining intensity was comparable in both cases. The results suggest that neurons express ASS cons titutively, whereas the enzyme is induced in glial cells in response to pro inflammatory stimuli. This finding is the first demonstration of an inducti on of a pathway auxiliary to generation of nitric oxide in brain in respons e to immunostimulants and provides new insight into neural arginine metabol ism.