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The pharmacological specificity of N‐methyl‐ d ‐aspartate receptors in rat cerebral cortex: correspondence between radioligand binding and electrophysiological measurements
Author(s) -
Grimwood Sarah,
Foster Alan C.,
Kemp John A.
Publication year - 1991
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1991.tb09799.x
Subject(s) - radioligand , electrophysiology , neuroscience , receptor , cerebral cortex , radioligand assay , chemistry , pharmacology , biology , biochemistry
1 The pharmacological specificity of N‐methyl‐ d ‐aspartate (NMDA) receptors has been investigated in the rat cerebral cortex by use of radioligand binding and electrophysiological techniques. 2 A comparison was made between a functional assay (NMDA‐induced depolarizations in a rat cortical slice preparation) and NMDA‐sensitive l ‐[ 3 H]‐glutamate binding in the same brain region and species, to provide accurate affinity values for agonists and antagonists at the NMDA recognition site. 3 In a preparation of crude postsynaptic densities (PSD) from rat cortex, l ‐[ 3 H]‐glutamate bound with high affinity to an NMDA‐sensitive population of sites with K D (geometric mean (–s.e.mean, +s.e.mean) = 120 (114, 126) n m , B max (mean ± s.e.mean) = 11.4 ± 0.8 pmol mg −1 protein and Hill coefficient (mean ± s.e.mean) = 1.2 ± 0.17 ( n = 3). 4 There was a good agreement between the relative affinities in radioligand binding and electrophysiological assays for the receptor agonists NMDA, N‐methyl‐ l ‐aspartate, quinolinate and trans ‐2,3‐piperidine dicarboxylate, which are poor substrates of acidic amino acid transport systems. However, agonists which are good substrates for high affinity uptake systems ( l ‐ and d ‐glutamate, l ‐ and d ‐aspartate, d ‐aspartate‐β‐hydroxamate and l ‐glutamate‐γ‐hydroxamate) were much weaker in the electrophysiological experiments. 5 Schild analysis of the antagonism of NMDA responses in the rat cortical slice by dl ‐3‐(2‐carboxypiperazin‐4‐yl)propyl‐1‐phosphonate, d ‐ and dl ‐2‐amino‐5‐phosphonovalerate, d ‐ and dl ‐2‐amino‐ 7‐phosphonoheptanoate, d ‐β‐aspartylaminomethylphosphonate, d ‐γ‐glutamylglycine and d ‐α‐aminoadipate ( d ‐AA) indicated a competitive interaction with respective pA 2 values of 6.17, 5.62, 5.24, 5.28, 5.20, 5.00, 4.43 and 3.97. 6 In the radioligand binding experiments the same antagonists inhibited only the NMDA‐sensitive component of l ‐[ 3 H]‐glutamate binding. IC 50 values showed a good correlation with the pA 2 values (correlation coefficient = 0.96), with the exception of d ‐AA which was more potent than anticipated in the binding experiments (IC 50 = 9.8 μ m ). 7 These results confirm that NMDA‐sensitive l ‐[ 3 H]‐glutamate binding sites represent the NMDA recognition site of the NMDA receptor and provide affinity values for both agonists and antagonists in the rat cerebral cortex, agreeing well with previous estimates in this and other tissues.