Modulation of kainate--activated currents by diazoxide and cyclothiazide analogues (IDRA) in cerebellar granule neurons.

1. Patch-clamp technique was used in primary cultures of cerebellar granule neurons to study the modulation of the cyclothiazide analogue(IDRA21) and of the diazoxide derivative (IDRA 5) on KA-evoked currents. 2. The dose-response of kainic acid (KA) reveals an EC50=90 mu M and an Hill coefficient of 1.3. IDRA 21 and cyclothiazide potentiate KA-evoked current in a dose-dependent way, being cyclothiazide more potent but less efficacious than IDRA 21. Conversely IDRA 5 acts as a negative modulator of KA evoked -current. 3. Application of IDRA 21 and cyclothiazide results in a current potentiation of 125+/-18 % and 80 +/- 12 % respectively, while IDRA 5 decreases KA-current (-21+/-5 %). Coapplication of cyclothiazide and IDRA 21 produces a potentiation of 110+/- 17 %, suggesting a competition of the two drugs for the same site. 4. In the same experimental model we studied the ability of IDRA compounds of promoting toxicity through AMPA-receptor activation. Under basal conditions AMPA treatment (50 mu M for 1 hour) results in a negligible excitotoxicity. 5. In contrast similar treatment with AMPA + IDRA 21 (1 mM) or + IDRA 5 (1 mM) or + cyclothiazide (100 mu M) induces citotoxicity. The neurotoxic damage induced by IDRA 21 and cyclothiazide is blocked by GYKI 53655 (50 mu M) and by NBQX (10 mu M). Interestingly GYKI and NBQX are ineffective in reducing IDRA 5 toxicity.