Effect of glutamine synthetase inhibition on astrocyte swelling and altered astroglial protein expression during hyperammonemia in rats.
Inhibition of glutamine synthesis reduces astrocyte swelling and associated physiological abnormalities during acute ammonium acetate infusion in anesthetized rats. We tested the hypothesis that inhibition of glutamine accumulation during more prolonged ammonium acetate infusion in unanesthetized rats reduces cortical astrocyte swelling and immunohistochemical changes in astrocytic proteins. Rats received a continuous i.v. infusion of either sodium acetate or ammonium acetate for 24 h to increase plasma ammonia from about 30-400 mumol/l. Cohorts were pretreated with vehicle or l-methionine-S-sulfoximine (MSO; 0.83 mmol/kg). MSO reduced glutamine synthetase activity by 57% and glutamine synthetase immunopositive cell number by 69%, and attenuated cortical glutamine accumulation by 71%. Hyperammonemia increased the number of swollen astrocytes in cortex and MSO reduced this increase to control values. The number of glial fibrillary acidic protein immunopositive cells in cortex was greater in hyperammonemic rats and the increase in superficial cortical layers was attenuated by MSO. Immunoreactivity for the gap junction protein connexin-43 in the neuropil, assessed by optical density, was greater in the hyperammonemic group compared with controls, but this increase was not attenuated by MSO. No changes in the optical density of GLT1 glutamate transporter immunoreactivity in cortex were detected in any group. We conclude that glutamine synthetase inhibition reduces astrocyte swelling and ameliorates some of the reactive astroglial cytoskeletal alterations seen at 24 h of hyperammonemia, but that gap junction changes in astrocytes occur independently of glutamine accumulation and swelling.