Received for publication June 2, 2004.
Revised September 17, 2004.
Accepted for publication September 23, 2004.

Divergent effects of the purinoceptor antagonists suramin and PPNDS on AMPA receptors

Abstract

Suramin is a large naphthyl-polysulfonate compound that inhibits a vast array of receptors and enzymes, and it has also been reported to block currents mediated by glutamate receptors. This study shows that suramin and several structurally related compounds (NF023, NF279, NF449) reduce binding of [³H]AMPA and [³H]fluorowillardiine to rat brain membranes and homomeric GluR1-4 receptors with IC₅₀ values in the range of 5-180 µM. Inhibition often was less than complete at saturating drug concentrations and thus appears to be non-competitive in nature. PPNDS is a potent antagonist of purinoceptors that shares some structural elements with suramin yet is smaller than the latter. PPNDS also had potent effects on AMPA receptors (EC₅₀ 4 µM) but of a kind not seen with the other compounds in that it increased binding affinity for radioagonists several-fold. In addition, PPNDS slowed association and dissociation rates more than ten times. In physiological experiments with GluR2 receptors, PPNDS at 50 µM reduced the peak current by 30-50% but had only small effects on other waveform aspects like desensitization and steady-state currents. This pattern of effects differentiates PPNDS from other compounds like thiocyanate and up-modulators which increase agonist binding by enhancing desensitization or slowing deactivation, respectively. Receptor model simulations indicate that most effects can be accounted for by assuming that PPNDS slows agonist binding/unbinding and stabilizes the bound-closed state of the receptor. By extension, suramin is proposed to stabilize the unbound state and thereby to reduce affinity for agonists. These drugs thus act through a novel type of non-competitive antagonism.

Key words: Glutamate, Func. analysis receptor/ion channel mutants, Receptor binding studies