1. Responses in brain produced by the activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subtype of ionotropic receptor for L-glutamate are often rapidly desensitizing. AMPA-induced desensitization and its characteristics, and the potentiating effect of cyclothiazide were investigated in vitro by analysing AMPA-induced release of [3H]-noradrenaline from prisms of rat hippocampus. 2. AMPA (1-1000 microM) stimulated the release of [3H]-noradrenaline in a concentration-dependent manner that was both calcium-dependent and tetrodotoxin-sensitive, and attenuated by the AMPA-selective antagonists, NBQX (1 and 10 microM), LY 293558 (1 and 10 microM) and GYKI 52466 (10 and 30 microM). 3. By use of an experimental procedure with consecutive applications of AMPA (100 microM, 28 min apart), the second response was reduced, indicative of receptor desensitization, and was reversed by cyclothiazide in a concentration-dependent manner (1-300 microM). The concentration-response curve for AMPA-induced release of [3H]-noradrenaline was shifted leftwards, but the reversal by cyclothiazide of the desensitized response was partial and failed to reach the maximal response of the first stimulus. 4. Observations made with various schedules of cyclothiazide application indicated that the initial AMPA-evoked response was already partially desensitized (150% potentiation by 100 microM cyclothiazide) and that the desensitization was not likely to be due to a time-dependent diminution and was longlasting (second application of cyclothiazide was ineffective). 5. Co-application of a number of drugs with actions on second messenger systems, in association with the second AMPA stimulus, revealed significant potentiation of the AMPA-induced release of [3H]-noradrenaline: forskolin (10 microM, +78%), Rp-cAMPS (100 microM, +65%), Ro 31-8220 (10 microM, +163%) and thapsigargin (100 pM, + 161%). 6. The AMPA receptor-mediated response regulating the release of [3H]-noradrenaline from rat hippocampal slices was desensitized and cyclothiazide acted to reverse partially the desensitization in a concentration-dependent manner. Since the time-course of desensitization was longer lasting than that noted in previous electrophysiological studies, multiple events may be involved in the down-regulation of AMPA receptor activity including receptor phosphorylation and depletion of intracellular Ca2+ stores.