Competitive inhibition of NMDA-mediated responses by guanine nucleotides in brain synaptic membranes treated with Triton X-100

Abstract

The effect of guanine nucleotides on physiological responses mediated by the N- methyl- d-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors was examined by using NMDA-sensitive [ 3H] l-glutamic acid (Glu) binding as well as [ 3H](+)-5-methyl-10,11-dihydro -5H- dibenzo[a,d] cyclohepten-5,10- imine (MK-801) binding in rat brain synaptic membranes treated with a low concentration of Triton X-100. The NMDA-sensitive [ 3H]Glu binding was significantly inhibited by the addition of some guanine nucleotides such as GTP, GDP, 5′-guanylylimidodiphosphate and guanosine 5′- O-(3-thiotriphosphate), but not by other nucleotides or nucleosides such as guanosine, cyclic GMP, adenosine, AMP, ADP, ATP, CTP, ITP and UTP. Inclusion of GTP not only attenuated the ability of NMDA to displace [ 3H]Glu binding in a concentration-dependent manner, but also lowered the affinity of the binding sites for [ 3H]Glu without altering their densities. The inhibitory potency of an antagonist highly selective to the NMDA receptors (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate on [ 3H]Glu binding also deteriorated with GTP at concentrations above 10 μM. Addition of Glu induced a concentration-dependent potentiation of [ 3H]MK-801 binding through an activation of the NMDA-sensitive receptors, and the potency of Glu to potentiate the binding was markedly reduced by the afore-mentioned positive guanine nucleotides in a competitive manner. In contrast, GTP at 0.1 mM non-competitively weakened the stimulatory property of glycine to additionally enhance the binding found in the presence of Glu alone. These results suggest that some guanine nucleotides may have a relatively high affinity for NMDA recognition sites within the NMDA receptor complex in the brain.