MECHANISMS UNDERLYING THE RAPID DEPOLARIZATION PRODUCED BY DEPRIVATION OF OXYGEN AND GLUCOSE IN RAT HIPPOCAMPAL CA1 NEURONS IN-VITRO

Intracellular recordings were made to investigate the mechanism, site, and ionic basis of generation of the rapid depolarization induced by superfusion with ischemia-simulating medium in hippocampal CA1 pyramid al neurons of rat tissue slices. Superfusion with ischemia-simulating medium produced a rapid depolarization after similar to 6 min of expos ure. When oxygen and glucose were reintroduced, the membrane potential did not repolarize but depolarized further, reaching 0 mV similar to 5 min after reintroduction. Simultaneous recordings of changes in cyto plasmic Ca2+ concentration ([Ca2+](i)) and membrane potential recorded from ofuranyloxy]-2-(2-amino-5-methylphenoxy)-ethane-N, N, N', N'-tet raacetic acid pentaacetoxymethyl ester (Fura-2/AM) loaded slices revea led a rapid increase in [Ca2+](i) in all CA1 layers corresponding to t he rapid depolarization of the soma membrane. The result suggests that the rapid depolarization is generated not only in the soma but also i n the apical and basal dendrites. Application of 6-cyano-7-nitroquinox aline-2,3-dione (CNQX), DL-2-amino-4-phosphonobutyric acid, and DL-2-a mino-3-phosphonopropionic acid or bicuculline did not affect the ampli tude and the maximal slope. Reduction in the concentration of extracel lular Ca2+ or addition of CNQX or DL-2-amino-5-phosphonopentanoic acid delayed the onset of the rapid depolarization. The amplitude of the r apid depolarization recorded with Cs acetate electrodes in tetraethyla mmonium-containing medium had a linear relationship to the membrane po tential between -50 and 20 mV. The reversal potential was shifted in t he hyperpolarizing direction by a decrease in either [Na+](o) or [Ca2](o), whereas the reversal potential was shifted in the depolarizing d irection by a decrease in [Cl-](o) or using CsCl electrodes. An increa se or decrease in [K+](o) did not affect the reversal potential. These results indicate that the rapid depolarization is Na+, Ca2+, and Cl- dependent. The lack of effects of changes in [Kt], is probably due to the accumulation of interstitial K+ before generating the rapid depola rization. Prolonged application of ouabain (30 mu M) caused an initial small hyperpolarization, a subsequent slow depolarization, and a rapi d depolarization. In summary, the present study has demonstrated that the rapid depolarization is voltage-independent and is probably due to a nonselective increase in permeability to all participating ions, wh ich may occur only in pathological conditions. The underlying conducta nce change is primarily the result of inhibition of Na,K-ATPase activi ty in the recorded neuron.