Contribution of ATP-Sensitive Potassium Channels to Hypoxic Hyperpolarization in Rat Hippocampal CA1 Neurons In Vitro. Fujimura, N., E. Tanaka, S. Yamamoto, M. Shigemoria and H. Higashi. Departments of Physiology and Neurosurgery, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830, Japan.
APStracts 3:0204N, 1996.
ABSTRACT
In order 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. Re-oxygenation produced a further hyperpolarization, with corresponding outward current, followed by a recovery to the pre-exposure level. The amplitude of the post-hypoxic 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, using 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 mM) reduced the amplitude of the hypoxic hyperpolarization, in a concentration-dependent manner by up to approximately 70 %. The glibenclamide or tolbutamide concentrations producing half-maximal inhibition of the hypoxic hyperpolarization were 6 and 12 mM, respectively. The chord conductance of the membrane potential between -80 to -90 mV in the absence of glibenclamide (30 mM) or tolbutamide (100 mM) 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 approximately 40 % of CA1 neurons, diazoxide (100 mM) 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 non-hydrolysable ATP analogue, AMP-PNP, reduced the amplitude of the hypoxic hyperpolarization. Furthermore, application of dinitrophenol (10 mM) 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 KATP channels with concomitant hyperpolarization.

Received 21 June 1996; accepted in final form 12 September 1996.
APS Manuscript Number J492-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996