Effects of hypoxia on rat hippocampal neurones in vitro

1. The effects of hypoxia on the rat hippocampal CA1 neurones in tissue slices of the rat brain were studied in vitro by intracellular recording. 2. In response to superfusion of a hypoxic medium equilibrated with 95% N2-5% CO2, a majority of the neurones showed a hyperpolarization of 5-15 mV in amplitude and 4-12 min in duration. The hyperpolarization was, in turn, followed by a slow depolarization which within 20 min of hypoxic exposure reached a plateau level of about 25 mV above the pre-hypoxic resting potential. Both the initial hyperpolarization and subsequent depolarization were associated with a reduction in membrane resistance. 3. The hyperpolarization reversed in polarity at a membrane potential of -83 mV. There was an almost linear relationship between amplitude of the hyperpolarization and membrane potential. The hyperpolarization was markedly enhanced in potassium-free media and was depressed in high-potassium solutions. 4. The hyperpolarization was not significantly affected by low-chloride or low-sodium medium or by solution containing tetraethylammonium (10 mM), 4-aminopyridine (1.5 mM) or caesium (3 mM). Moreover, intracellular injection of ethyleneglycol-bis-(beta-aminoethylether)N,N-tetraacetic acid (EGTA) did not alter the hyperpolarization. On the other hand, barium (0.5 mM)-containing medium reduced the amplitude of the hyperpolarization by 20-40%. 5. Superfusion of ouabain (5-7 microM)-containing medium in normoxic conditions produced hyperpolarizing and depolarizing responses similar to those elicited by hypoxic exposure. The slow depolarization was also mimicked by elevation of the extracellular potassium concentration to 10-20 mM. 6. Evoked i.p.s.p.s were abolished within 4 min of hypoxic exposure while evoked e.p.s.p.s were maintained for about 20 min of hypoxic superfusion. Soma spikes of the neurones elicited by a depolarizing pulse were also well preserved. Their threshold was, however, raised, concomitant with a decrease in the peak amplitude. 7. When the slice was reoxygenated after 20-40 min of hypoxic exposure, the neurones immediately began to repolarize and showed a transient hyperpolarization of 5-10 mV in amplitude and 1-2 min in duration. The membrane potential, input resistance and action potential returned to the pre-hypoxic levels after 15-20 min of reoxygenation. The amplitude of the reoxygenation-induced hyperpolarization was not significantly changed when the membrane was hyperpolarized or depolarized. The hyperpolarization was eliminated by potassium-free medium or solution containing ouabain (1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)