The ability of acromelic acid A to inhibit [3H]-kainic acid and [3H]-(RS)-alpha-amino-3-hydroxy-5-methyloxazole-4-propionic acid ([3H]-AMPA) binding to rat brain synaptic plasma membranes was investigated by equilibrium radioligand binding assay. 2. Kinetic analysis of [3H]-kainic acid binding demonstrated the existence of two kainate binding sites in this tissue preparation and yielded equilibrium dissociation constants for [3H]-kainic acid of KD = 0.4 nM and KD = 20.8 nM. 3. Kainic acid and domoic acid both appeared to displace [3H]-kainic acid from a single binding site with equilibrium binding constants of KD = 19.4 nM and Ki = 14.5 nM respectively. Acromelic acid A exhibited a biphasic inhibition of [3H]-kainic acid binding to synaptic membranes with binding affinities of Ki = 15.1 nM and Ki = 1.49 microM. 4. In the absence of chaotropic ions, the order of potency of inhibition of [3H]-AMPA binding was acromelic acid A (Ki = 26 nM) greater than AMPA (KD = 184 nM) greater than domoic acid (Ki = 499 nM). 5. The inclusion of 100 mM thiocynanate ion in the [3H]-AMPA binding assay resulted in a change in the order of potency to: AMPA (KD = 160 nM) greater than acromelic acid A (Ki = 289 nM) greater than domoic acid (Ki = 9.02 microM). 6. These results show that acromelic acid A distinguishes two kainate binding sites in rat brain synaptic plasma membranes and in addition, that in the absence of chaotropic ions, acromelic acid A is the most potent displacer of [3H]-AMPA binding yet described.
