FACTORS THAT REVERSE THE PERSISTENT DEPOLARIZATION PRODUCED BY DEPRIVATION OF OXYGEN AND GLUCOSE IN RAT HIPPOCAMPAL CA1 NEURONS IN-VITRO

In CA1 pyramidal neurons in rat hippocampal tissue slices, superfusion with ischemia-simulating medium produced a rapid depolarization after 6 min of exposure. The membrane potential eventually reached 0 after 5 min (a persistent depolarization), even when oxygen and glucose were reintroduced. The role of various ions in the reversal of this persis tent depolarization after reintroduction of oxygen and glucose was inv estigated. The peak of the persistent depolarization was decreased in solutions containing reduced Na+ or Ca2+ and in solutions containing C o2+ Or Ni2+. In contrast, the depolarization was not affected by reduc tion of external K+ or Cl- or by addition of tetrodetoxin (TTX), fluna rizine, or nifedipine. These results suggest that sustained Na+ and Ca 2+ influxes produce the persistent depolarization. The membrane potent ial recovered after reintroduction of oxygen and glucose in low Ca2+, low Cl-, or K+-rich medium and in ?TX-or tetraethylammonium-containing medium, but not in low Na+ or low K+ medium and in flunarizine-or nif edipine-containing medium. Either reduction in extracellular Ca2+ or a ddition of Co2+ was the most effective in promoting recovery from the persistent depolarization, suggesting that Ca2+ influx has a key role in causing the membrane dysfunction. The peak of the persistent depola rization was reduced by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), D L-2-amino-5-phosphonopentanoic acid (AP3), DL-amino-4-phosphonopropion ic acid (AP3): or DL-amino-4-phosphonobutyric acid, suggesting that ac tivation of non-N-methyl-D-aspartate (non-NMDA), NMDA, and metabotropi c glutamate (Glu) receptors is involved in the generation and maintena nce of the persistent depolarization. Among these Glu receptor antagon ists, only CNQX or AP5 was able to reduce dose dependently the level o f depolarization, suggesting that Ca 2+ influx via both pha-amino-3-hy droxy-5-methyl-4-isoxazole-propionic acid/kainate type II receptors an d NMDA receptors contributes to the membrane dysfunction. trans-1-amin ocyclopentane-1,3-dicarboxylic acid (t-ACPD) did not affect the peak p otential of the persistent depolarization, but it dose-dependently res tored the membrane potential. AP3 antagonized the protective action of t-ACPD. The membrane potential also recovered after reintroduction wh en the slice was pretreated by 1,2-bis(2-aminophenoxy) ethane-N, N,N', N'tetraacetic acid tetraacetoxymethyl ester, ryanodol 3-(1H-pyrrole-2- carboxylate), 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochlo ride, and procaine, suggesting that raised [Ca2+](i) from Ca2+- induce d Ca2+ release pool contributes to the membrane dysfunction. It, there fore, is concluded that raised [Ca2+](i) has a dominant role in causin g irreversible changes. The increase in [Ca2+](i) during the persisten t depolarization may be the result of Ca2+ entry via both a leaky memb rane and Glu-activated receptor channels as well as Ca2+ released from internal stores.