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INSERM U. 29, Paris, France.

1. Current or voltage clamp recordings from CA3 neurones of the adult rat hippocampal slice were performed to study the inactivation properties of a slow outward K+ current identified as the delayed rectifier (IK). 2. In current clamp experiments, burst firing evoked from resting membrane potential by intracellular current injection was reduced or blocked by conditioning hyperpolarizing pre-pulses of 20-40 mV amplitude. This effect was inhibited by tetraethylammonium (TEA; 20 mM) but was unaffected by Cs+ (3 mM), 4-aminopyridine (4-AP; 2 mM), carbachol (30-50 microM), mast cell degranulating peptide (MCDP; 300 nM), thyrotrophin releasing hormone (TRH; 1 microM) or by a Ca(2+)-free solution containing Mn2+ or Co2+ (2 mM). 3. Single-electrode voltage clamp experiments were carried out on neurones superfused with Ca(2+)-free solution, containing tetrodotoxin (TTX; 1 microM), Mn2+ or Co2+ (2 mM), 4-AP (2 mM), Cs+ (3 mM) and carbachol (30 microM). Step depolarizations from a holding potential of -55 mV activated an outward current which reached a plateau after 200 ms, followed by an outward tail current. Such an outward current had the characteristics of IK. 4. The outward currents were significantly potentiated by conditioning hyperpolarizing pre-pulses suggesting the IK was reduced by a voltage-dependent inactivation process. Removal of inactivation was a function of the amplitude of the conditioning hyperpolarizing pre-pulse. At a holding potential of -55 mV removal of inactivation was time dependent with a time constant of 211 ms. High K+ (12.5 or 21.5 mM) solutions did not affect the inactivation characteristics of IK. 5. Tetraethylammonium (20 mM) or low concentrations of Ba2+ (0.1 mM) readily depressed the outward current without significantly affecting the inactivation process. Dendrotoxin (200 nM) also depressed such a slow current but, in addition, increased the inactivation process of IK. 6. It is suggested that removal of inactivation of IK by hyperpolarization can modulate cell excitability by fully restoring the ability of IK to inhibit burst firing of CA3 hippocampal neurones.