NMDA RECEPTOR ACTIVATION INHIBITS NEURONAL VOLUME REGULATION AFTER SWELLING-INDUCED BY VERATRIDINE-STIMULATED NA+ INFLUX IN RAT CORTICAL CULTURES

Neurons and glia experience rapid fluctuations in transmembrane solute and water fluxes during normal brain activity, Cell volume must be re gulated under these conditions to maintain optimal neural function. Al most nothing is known, however, about how brain cells respond to volum e challenges induced by changes in transmembrane solute flux, As such, we characterized the volume-regulatory mechanisms of cultured cortica l neurons swollen by veratridine-stimulated Na+ influx: Exposure of co rtical neurons to 100 mu M veratridine for 10-15 min caused a 1,8- to 2-fold increase in cell volume that persisted for at least 90 min. Thi s volume increase was blocked by extracellular Na+ removal or by expos ure to 5 mu M tetrodotoxin, indicating that swelling is a result of Na + entry via Na+ channels. Treatment of cells with veratridine together with various NMDA receptor antagonists had no effect on the magnitude of swelling. NMDA receptor antagonist-treated cells, however, underwe nt nearly complete volume recovery within 50-70 min after veratridine exposure. This recovery suggests that NMDA receptor activation disrupt s neuronal osmoregulatory pathways. Volume regulation was blocked by B a2-. quinidine, or 5-nitro-2-(3-phenylpropylamino) benzoic acid, indic ating that swelling activates volume regulatory K+ and Cl- channels, V eratridine also caused a rapid, transient increase in intracellular Ca 2+. Extracellular Ca2+ removal or intracellular Ca2+ chelation prevent ed or dramatically reduced veratridine-induced increases in intracellu lar Ca2+ and completely blocked volume recovery. These findings indica te that increases in Ca2+ during cell swelling induced by Na+ influx a re required for activation of neuronal volume-regulatory pathways.