Contribution of ATP-Sensitive Potassium Channels to Hypoxic Hyperpolarization in Rat Hippocampal CA1 Neurons In Vitro

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Abstract
Fujimura, N., E. Tanaka, S. Yamamoto, M. Shigemori, and H. Higashi. Contribution of ATP-sensitive potassium channels to hypoxic hyperpolarization in rat hippocampal CA1 neurons in vitro. J. Neurophysiol. 77: 378–385, 1997. To investigate the mechanism of generation of the hypoxia-induced hyperpolarization (hypoxic hyperpolarization) in hippocampal CA1 neurons in rat tissue slices, recordings were made in current-clamp mode and single-electrode voltage-clamp mode. Superfusion with hypoxic medium produced a hyperpolarization and corresponding outward current, which were associated with an increase in membrane conductance. Reoxygenation produced a further hyperpolarization, with corresponding outward current, followed by a recovery to the preexposure level. The amplitude of the posthypoxic hyperpolarization was always greater than that of the hypoxic hyperpolarization. In single-electrode voltage-clamp mode, it was difficult to record reproducible outward currents in response to repeated hypoxic exposure with the use of electrodes with a high tip resistance. The current-clamp technique was therefore chosen to study the pharmacological characteristics of the hypoxic hyperpolarization. In 60–80% of hippocampal CA1 neurons, glibenclamide or tolbutamide (3–100 μM) reduced the amplitude of the hypoxic hyperpolarization in a concentration-dependent manner by up to ∼70%. The glibenclamide or tolbutamide concentrations producing half-maximal inhibition of the hypoxic hyperpolarization were 6 and 12 μM, respectively. The chord conductance of the membrane potential between −80 and −90 mV in the absence of glibenclamide (30 μM) or tolbutamide (100 μM) was 2–3 times greater than that in the presence of glibenclamide or tolbutamide. In contrast, the reversal potential of the hypoxic hyperpolarization was approximately −83 mV in both the absence and presence of tolbutamide or glibenclamide. In ∼40% of CA1 neurons, diazoxide (100 μM) or nicorandil (1 mM) mimicked the hypoxic hyperpolarization and pretreatment of these drugs occluded the hypoxic hyperpolarization. When ATP was injected into the impaled neuron, hypoxic exposure could not produce a hyperpolarization. The intracellular injection of the nonhydrolyzable ATP analogue 5′-adenylylimidodiphosphate lithium salt reduced the amplitude of the hypoxic hyperpolarization. Furthermore, application of dinitrophenol (10 μM) mimicked the hypoxic hyperpolarization, and the dinitrophenol-induced hyperpolarization was inhibited by either pretreatment of tolbutamide or intracellular injection of ATP, indicating that the hypoxic hyperpolarization is highly dependent on intracellular ATP. It is therefore concluded that in the majority of hippocampal CA1 neurons, exposure to hypoxic conditions resulting in a reduction in the intracellular level of ATP leads to activation of ATP-sensitive potassium channels with concomitant hyperpolarization.