Christof Angst

[3H]CGP 39653: a new N-methyl-D-aspartate antagonist radioligand with low nanomolar affinity in rat brain

CGP 39653 (D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid) was initially discovered to inhibit the binding of [3H]L-glutamate and [3H]3-[+/-)2-carboxypiperazin-4-yl)-propyl-1- phosphonic acid [( 3H]CPP) with Ki values of 230 and 5 nM, respectively. The radiolabeled compound [3H]CGP 39653 binds to rat frontal cortical membranes in a saturable and reversible manner. Analysis of saturation experiments revealed that the ligand labels one binding site with a Kd value of 6 nM. Competition experiments indicated that the order of potency of a number of competitive excitatory amino acid agonist and antagonist compounds was similar to that found previously for other N-methyl-D-aspartate (NMDA) receptor ligands. In contrast to these competitive inhibitors, which produced steep inhibition curves, glycine inhibited binding in a complex manner. When the functional activity of the unlabeled compound was explored, CGP 39653 blocked NMDA-evoked depolarizations in the rat cortical wedge in vitro and inhibited L-glutamate stimulated [3H]N(1-[2-thienyl]cyclohexyl)3,4-piperidine [( 3H]TCP) binding in cortical membranes. These results suggest that [3H]CGP 39653 selectively binds to the NMDA receptor as an antagonist with high affinity and is currently the ligand of choice for labeling the NMDA receptor.

CGP 37849 and CGP 39551: novel and potent competitive N-methyl-D-aspartate receptor antagonists with oral activity.

1 The pharmacological properties of CGP 37849 (dl‐(E)‐2‐amino‐4‐methyl‐5‐phosphono‐3‐pentenoic acid; 4‐methyl‐APPA) and its carboxyethylester, CGP 39551, novel unsaturated analogues of the N‐methyl‐d‐aspartate (NMDA) receptor antagonist, 2‐amino‐5‐phosphonopentanoate (AP5), were evaluated in rodent brain in vitro and in vivo. 2 Radioligand binding experiments demonstrated that CGP 37849 potently (Ki 220 nm) and competitively inhibited NMDA‐sensitive l‐[3H]‐glutamate binding to postsynaptic density (PSD) fractions from rat brain. It inhibited the binding of the selective NMDA receptor antagonist, [3H]‐(±)‐3‐(2‐carboxypiperazin‐4‐yl)propyl‐1‐phosphonate (CPP), with a Ki of 35 nm, and was 4, 5 and 7 fold more potent than the antagonists ((±)‐cis‐4‐phosphonomethylpiperidine‐2‐carboxylic acid) (CGS 19755), CPP and d‐AP5, respectively. Inhibitory activity was associated exclusively with the trans configuration of the APPA molecule and with the d‐stereoisomer. CGP 39551 showed weaker activity at NMDA receptor recognition sites and both compounds were weak or inactive at 18 other receptor binding sites. 3 CGP 37849 and CGP 39551 were inactive as inhibitors of l‐[3H]‐glutamate uptake into rat brain synaptosomes and had no effect on the release of endogenous glutamate from rat hippocampal slices evoked by electrical field stimulation. 4 In the hippocampal slice in vitro, CGP 37849 selectively and reversibly antagonized NMDA‐evoked increases in CA1 pyramidal cell firing rate. In slices bathed in medium containing low Mg2+ levels, concentrations of CGP 37849 up to 10 μm suppressed burst firing evoked in CA1 neurones by stimulation of Schaffer collateral‐commissural fibres without affecting the magnitude of the initial population spike; CGP 39551 exerted the same effect but was weaker. In vivo, oral administration to rats of either CGP 37849 or CGP 39551 selectively blocked firing in hippocampal neurones induced by ionophoretically‐applied NMDA, without affecting the responses to quisqualate or kainate. 5 CGP 37849 and CGP 39551 suppressed maximal electroshock‐induced seizures in mice with ED50s of 21 and 4 mg kg−1 p.o., respectively. 6 CGP 37849 and CGP 39551 are potent and competitive NMDA receptor antagonists which show significant central effects following oral administration to animals. As such, they may find value as tools to elucidate the roles of NMDA receptors in brain function, and potentially as therapeutic agents for the treatment of neurological disorders such as epilepsy and ischaemic brain damage in man.

Differentiation of glycine antagonist sites of N-methyl-D-aspartate receptor subtypes: Preferential interaction of CGP 61594 with NR1/2B receptors

The binding site for the co-agonist glycine onN-methyl-d-aspartate (NMDA) receptors has been mapped to the NR1 subunit whereas binding of the principal agonist glutamate is mediated by the NR2 subunits. Using the novel glycine site antagonist and photoaffinity label CGP 61594, distinct contributions of the NR2 subunit variants to the glycine antagonist binding domains of NMDA receptor subtypes are demonstrated. High affinity sites for CGP 61594 were exclusively displayed by NR1/2B receptors, as shown by their co-distribution with the NR2B subunit, by subunit-selective immunoprecipitation and by functional analysis of NR1/2B receptors expressed in Xenopus oocytes (inhibitory potency, IC50 = 45 ± 11 nm). Other NMDA receptor subtypes are clearly distinguished by reduced inhibitory potencies for CGP 61594, being low for NR1/2A and NR1/2D receptors (IC50 = 430 ± 105 nm and 340 ± 61 nm, respectively) and intermediate for NR1/2C receptors (IC50 = 164 ± 27 nm). Glycine antagonist sites with low and intermediate affinity for [3H]CGP 61594 were detected also in situ by radioligand binding in brain areas predominantly expressing the NR2A and NR2C subunits, respectively. Thus, [3H]CGP 61594 is the first antagonist radioligand that reliably distinguishes the glycine site of NMDA receptor subtypes. [3H]CGP 61594 is a promising tool to identify the NR2 subunit domains that contribute to differential glycine antagonist sites of NMDA receptor subtypes.

[3H]CGP 61594, the first photoaffinity ligand for the glycine site of NMDA receptors

Activation of NMDA receptors requires the presence of glycine as a coagonist which binds to a site that is allosterically linked to the glutamate binding site. To identify the protein constituents of the glycine binding site in situ the photoaffinity label [3H]CGP 61594 was synthesized. In reversible binding assays using crude rat brain membranes, [3H]CGP 61594 labeled with high affinity (K(D) = 23 nM) the glycine site of the NMDA receptor. This was evident from the Scatchard analysis, the displacing potencies of various glycine site ligands and the allosteric modulation of [3H]CGP 61594 binding by ligands of the glutamate and polyamine sites. Electrophysiological experiments in a neocortical slice preparation identified CGP 61594 as a glycine antagonist. Upon UV-irradiation, a protein band of 115 kDa was specifically photolabeled by [3H]CGP 61594 in brain membrane preparations. The photolabeled protein was identified as the NR1 subunit of the NMDA receptor by NR1 subunit-specific immunoaffinity chromatography. Thus, [3H]CGP 61594 is the first photoaffinity label for the glycine site of NMDA receptors. It will serve as a tool for the identification of structural elements that are involved in the formation of the glycine binding domain of NMDA receptors in situ and will thereby complement the mutational analysis of recombinant receptors.

Photoaffinity labeling of the NMDA receptor

The structure of NMDA receptors in situ has been probed with the novel photoaffinity ligand 125I-CGP 55802A. By covalently linking the radioactive high-affinity photolabel to NMDA receptors in bovine brain we have identified a protein of 175 kDa associated with the binding site for NMDA receptor agonists and competitive antagonists. Based on its molecular size the photolabeled protein is likely to correspond to the NR2A and/or NR2B subunit. The photoaffinity ligand will permit the assessment of regulatory changes in NMDA receptor subunit expression.