Polyamine effects upon N-methyl- D-aspartate receptor functioning: differential alteration by glutamate and glycine site antagonists

Abstract

Polyamines such as spermidine potentiate activation of the N-methyl- D-aspartate (NMDA)-type excitatory amino acid receptor. The goal of the present study was to investigate interactions between the putative polyamine binding site and previously described sites for glutamate and glycine. Binding of the high-potency PCP receptor ligand [ 3H]MK-801 to well-washed rat brain membranes was used as an in vitro probe of NMDA receptor activation. Spermidine concentration-response studies were performed in the absence and presence of both glutamate and glycine, with and without D-(−)-2-amino-5-phosphonovaleric acid ( D(−)AP-5) or 7-chlorokynurenic acid (7Cl-KYN). Incubation in the presence of spermidine alone induced a 20.4-fold increase in [ 3H]MK-801 binding with an EC 50 value of 13.3 μM. The mean concentration of spermidine which induced maximal stimulation of binding was 130 μM ( n = 10,S.E.M.= 24.66,range= 25–250 μM). Glutamate (10 μM) decreased the EC 50 value for spermidine-induced stimulation of [ 3H]MK-801 binding to 3.4 μM. Glycine (10 μM) did not significantly alter either maximum spermidine-induced [ 3H]MK-801 binding or the EC 50 value for spermidine-induced stimulation of [ 3H]MK-801 binding. Incubation in the presence of the specific glutamate antagonist D(−)AP-5 attenuated [ 3H]MK-801 binding in a glutamate-reversible fashion. The competitive glycine antagonist 7Cl-KYN decreased maximum spermidine-induced [ 3H]MK-801 binding in a glycine-reversible fashion. In addition, 7Cl-KYN increased the EC 50 value for spermidine-induced stimulation of [ 3H]MK-801 binding while D(−)AP-5 was without effect. These findings suggest that glutamate and glycine regulate the polyamine binding site differentially. PCP-like agents induce a psychotomimetic state closely resembling schizophrenia by inhibiting NMDA receptor-mediated neurotransmission. The ability of polyamines to modulate NMDA receptor functioning suggests a potential site for pharmacological intervention.