EFFECTS OF HEPARIN ON THE PROPERTIES OF SOLUBILIZED AND RECONSTITUTEDRAT-BRAIN AMPA RECEPTORS

Heparin was found to bind to lpha-amino-3-hydroxy-5-methylisoxazole-4- propionic acid (AMPA) receptors and to alter their functional properti es. AMPA receptors solubilized in 0.4% Triton X-100 bound to a heparin -agarose column and were eluted by 0.4 M NaCl. Soluble heparin inhibit ed 10 nM [H-3]AMPA binding to detergent-solubilized receptors by 75% ( IC50 = 10 mu g/ml), but had little effect on binding to membrane-assoc iated receptors. The inhibition of [H-3]AMPA binding to detergent-solu bilized receptors was not observed when binding was measured in the pr esence of 0.4 M NaCI, and no effect of heparin was observed on binding of the AMPA receptor antagonist [H-3] 6-cyano-7-nitroquinoxaline-2,3- dione (CNQX). Scatchard analyses of [H-3]AMPA binding to solubilized r eceptors revealed that the inhibition induced by heparin was caused by a decrease in the apparent affinity of a portion of the total binding sites. Studies on AMPA receptors reconstituted in artificial lipid bi layers indicated that 10 mu g/ml heparin enhanced cooperativity betwee n channels and prolonged the lifetime of the open channel, but did not affect the amplitude of single channel currents. Thus, heparin may be added to the list of compounds known to modulate AMPA receptor functi on. These data also raise the possibility that heparin-containing prot eoglycans, which are known to be concentrated at synaptic junctions, m ight be able to bind AMPA receptors and influence their functional cha racteristics.