Allosteric modulation of [3H]-CGP39653 binding through the glycine site of the NMDA receptor: further studies in rat and human brain
Abstract
Binding of D,L-(E)-2-amino-4-[(3)H]-propyl-5-phosphono-3-pentenoic acid ([(3)H]-CGP39653), a selective antagonist at the glutamate site of the NMDA receptor, is modulated by glycine in rat brain tissue. We have further investigated this phenomenon in rodent and human brain by means of receptor binding and quantitative autoradiography techniques. In rat cerebral cortical membranes the glycine antagonist 3-[2-(Phenylaminocarbonyl)ethenyl]-4,6-dichloro-indole-2-carboxylic acid sodium salt (GV150526A) did not change basal [(3)H]-CGP39653 binding, but competitively reversed the high affinity component of [(3)H]-CGP39653 binding inhibition by glycine, with a pK(B) value of 8.38, in line with its affinity for the glycine site (pK(i)=8.49 vs. [(3)H]-glycine). Glycine (10 microM) significantly decreased [(3)H]-CGP39653 affinity for the NMDA receptor (with no change in the B(max)), whereas enhanced L-glutamate affinity (P<0.05, paired-samples Student's t-test). In rat brain sections the addition of GV150526A (30 microM) to the incubation medium increased [(3)H]-CGP39653 binding to 208% of control (average between areas), indicating the presence of endogenous glycine. The enhancement presented significant regional differences (P<0.05, two...Continue Reading
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