CONTRIBUTION OF ATP-SENSITIVE POTASSIUM CHANNELS TO HYPOXIC HYPERPOLARIZATION IN RAT HIPPOCAMPAL CA1 NEURONS IN-VITRO

To investigate the mechanism of generation of the hypoxia-induced hype rpolarization (hypoxic hyperpolarization) in hippocampal CA1 neurons i n rat tissue slices, recordings were made in current-clamp mode and si ngle-electrode voltage-clamp mode. Superfusion with hypoxic medium pro duced a hyperpolarization and corresponding outward current, which wer e associated with an increase in membrane conductance. Reoxygenation p roduced a further hyperpolarization, with corresponding outward curren t, followed by a recovery to the preexposure level. The amplitude of t he posthypoxic hyperpolarization was always greater than that of the h ypoxic hyperpolarization. In single-electrode voltage-clamp mode, it w as difficult to record reproducible outward currents in response to re peated hypoxic exposure with the use of electrodes with a high tip res istance. 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 mu M) reduced the amplitude of the hypoxic hyperpolarization in a con centration-dependent manner by up to similar to 70%. The glibenclamide or tolbutamide concentrations producing half-maximal inhibition of th e hypoxic hyperpolarization were 6 and 12 mu M, respectively. The chor d conductance of the membrane potential between -80 and -90 mV in the absence of glibenclamide (30 mu M) or tolbutamide (100 mu M) was 2-3 t imes 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 tolbutam ide or glibenclamide. In similar to 40% of CA1 neurons, diazoxide (100 mu M) or nicorandil (1 mM) mimicked the hypoxic hyperpolarization and pretreatment of these drugs occluded the hypoxic hyperpolarization. W hen ATP was injected into the impaled neuron, hypoxic exposure could n ot produce a hyperpolarization. The intracellular injection of the non hydrolyzable ATP analogue 5'-adenylylimidodiphosphate lithium salt red uced the amplitude of the hypoxic hyperpolarization. Furthermore, appl ication of dinitrophenol (10 mu M) mimicked the hypoxic hyperpolarizat ion, 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 i ntracellular ATP. It is therefore concluded that in the majority of hi ppocampal CA1 neurons, exposure to hypoxic conditions resulting in a r eduction in the intracellular level of ATP leads to activation of ATP- sensitive potassium channels with concomitant hyperpolarization.