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Kainate induces an intracellular Na⁺-activated K⁺ current in cultured embryonic rat hippocampal neurones

Qi-Ying Liu, Anne E. Schaffner and Jeffery L. Barker

1. In embryonic rat hippocampal neurones cultured for < 3 days, kainate induced an inward current at negative potentials that recovered to baseline levels immediately upon termination of agonist application. However, in neurones cultured for longer, the kainate-induced current was often followed by a long-lasting inward current that slowly recovered to baseline levels. The amplitude of the delayed current (I_delay) triggered by kainate was positively related both to the duration of application at constant agonist concentration and to concentration at constant application duration.

2. I_delay could last for several minutes and was accompanied by a conductance increase, which closely paralleled current amplitude. Depression of the kainate-induced current response at receptor level with CNQX or at ionic level with Na⁺-free solution eliminated I_delay. However, when applied during I_delay neither CNQX nor Na⁺-free solution had any effect on I_delay. Li⁺ effected the same response as Na⁺ in mediating kainate-induced I_delay.

3. GABA-activated Cl^- current, which was associated with the same amount of inwardly directed charge flow at the same potential as that induced by kainate, did not trigger a long-lasting delayed current.

4. I_delay depended on the existence of extracellular K⁺ and its amplitude increased with the increase in K⁺ concentration. Neither applying Cl^-- or Ca²⁺-free solutions nor increasing intracellular Ca²⁺ buffering speed and capacity altered I_delay. Exposure to the specific K_Ca channel blockers apamin and charybdotoxin also failed to alter I_delay. However, I_delay could be blocked by Cs⁺, Ba²⁺ and high concentrations of 4-aminopyridine (4-AP) and TEA.

5. Inside-out excised patch-clamp recordings revealed that low density or highly clustered Na⁺-activated K⁺ channels were expressed in the cell bodies of cultured embryonic rat hippocampal neurones. These could be the elementary channels underlying I_delay.

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