The mechanism of kainic acid receptor-induced inhibition of I-sAHP in CA1 pyramidal neurones

The slow afterhyperpolarisation current (IsAHP) in CA1 pyramidal neurones is reduced in kainic acid-lesioned hippocampus and this effect leads to an increase in cell excitability (Ashwood et al. 1986). We investigated the mechanisms of kainic acid-induced inhibition using whole-cell patch-clamp experiments.Hippocampal slices were obtained from humanely killed 14- to 21-day-old rats. The recordings were made from CA1 pyramidal cells using KMeSO4-containing pipettes. Voltage steps (60 mV for 80 ms) were applied every 20 s to generate IsAHP.Bath application of kainic acid produced concentration-dependent inhibition of IsAHP that reached a plateau of 34 % at 100-200 nM (IC50 = 15 nM; n = 7). The inhibition was not accompanied by an inward current and persisted in the presence of 1 µM TTX + 5 mM TEA (n = 8), suggesting postsynaptic localisation of the receptors. The AMPA/KA blocker CNQX (20 µM; n = 6) abolished the action of kainic acid, whereas a selective antagonist of AMPA receptors, GYKI52466 (100 µM), was ineffective (n = 5). The effect of kainate could be reproduced by domoic acid (200 nM; n = 7), but not by the potent agonist of AMPA receptors (S)-5-fluorowillardiine (300 nM; n = 7). The GluR5 kainate receptor agonist ATPA (2 µM; n = 5) did not produce any inhibition. The results are consistent with an action of kainate mediated by GluR6-containing kainate receptors, and imply that a metabotropic function may be involved as reported for presynaptic kainate receptors (Rodriguez-Moreno & Lerma, 1998). This was tested by pre-incubation of the slices with the protein kinase C inhibitor calphostin C (1 µM); this treatment prevented kainate-induced inhibition (n = 10). Subsequent application of noradrenaline (10 µM; n = 4) blocked IsAHP, demonstrating that PKA-dependent inhibition remained intact. In current-clamp experiments, kainic acid (200 nM) did not reduce either the width or the amplitude of calcium spikes recorded following a brief depolarising step (n = 8). This suggests that kainate inhibition of IsAHP was not due to a reduction in Ca2+ influx.These data indicate that GluR6 kainate receptors on CA1 pyramidal cells not only mediate ionic currents (Bureau et al. 1999) but have metabotropic actions leading to increased cell excitability.This work is supported by The Wellcome Trust.