Interaction of domoic acid and several derivatives with kainic acid and AMPA binding sites in rat brain

Abstract

We have determined the inhibitory potencies of domoic acid and a series of derivatives of domoic acid at kainic acid and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) binding sites in rat forebrain membranes. These derivatives of domoic acid differed in the configuration, stereochemistry, and degree of saturation of the side chain attached to C-4 of the prolyl ring. The binding data were analyzed in terms of one or two classes of sites as appropriate. Domoic acid and kainic acid displayed similar inhibition constant at [ 3H]kainic acid sites (IC 50 = 5 and 7 nM, espectively). At both kainic acid and AMPA binding sites, all of the compounds tested were less potent than domoic acid itself. At high affinity [ 3H]kainic acid sites, the derivatives could be categorized into two groups; those with nanomolar affinity and those with micromolar affinity. All members of the former group possessed a side chain with the first double bond intact and in the Z (cis) configuration. The more distal atoms present in the extended side chain of domoic acid did not appear to contribute to the high affinity interaction with the kainic acid receptor. Although all the compounds tested were weaker inhibitors of [ 3H]AMPA binding compared to [ 3H]kainic acid binding, there was a high correlation between the rank order of potency of the seven domoic acid derivatives at [ 3H]kainic acid and at [ 3H]AMPA binding sites. The inhibition data for kainic acid at [ 3H]AMPA binding sites were described adequately in terms of a 1-site model, whereas the data for domoic acid required two classes of sites. The IC 50 at the higher affinity sites for domoic acid was 9 nM. The IC 50 at the lower affinity sites for domoic acid (2.4 μM) was similar to the IC 50 observed for kainic acid (5.6 μM). Although the fraction of the specific binding contributed by the high affinity sites for domoic acid was small (12%), the existence of a high affinity interaction between domoic acid and [ 3H]AMPA binding sites in mammalian brain may explain the reported higher neuroexcitatory and neurotoxic potency of domoic acid relative to kainic acid.