L-687,414, a low efficacy NMDA receptor glycine site partial agonist in vitro, does not prevent hippocampal LTP in vivo at plasma levels known to be neuroprotective

Abstract

1. N-methyl-D-aspartic acid (NMDA) receptors are known to play a key role in the induction phase of long-term potentiation (LTP) at certain hippocampal synapses and to represent some component of spatial learning in animals. The ability of NMDA receptor antagonists (or gene knockout) to impair LTP has led to the suggestion that the therapeutic use of such antagonists may impair cognitive function in humans. The present study compares the effects on LTP of NMDA receptor ion channel block by MK-801 and glycine-site antagonism by 3R(+)cis-4-methyl-pyrrollid-2-one (L-687,414).

2. In vitro experiments using rat cortical slices revealed L-687,414 to be ∼3.6 fold more potent than its parent analogue, R(+)HA-966 at antagonizing NMDA-evoked population depolarizations (apparent K_bs: 15 μM and 55 μM, respectively).

3. Whole-cell voltage-clamp experiments using rat cultured cortical neurones revealed L-687,414 to shift to the right the concentration-response relationship for NMDA-evoked inward current responses (pK_b=6.2±0.12). L-687,414 affinity for the glycine site on the NMDA receptor complex was also determined from concentration-inhibition curves, pKi=6.1±0.09. In the latter experiments, L-687,414 and R(+)HA-966 were unable to completely abolish inward current responses suggesting each compound to be a low efficacy partial agonist (estimated intrinsic activity=∼10 and 20% of glycine, respectively).

4. L-687,414 and MK-801 were compared for their effects on NMDA receptor-dependent LTP in the dentate gyrus of anaethestized rats following high frequency stimulation of the medial perforant path (mPP) afferents. Control rats, administered saline (0.4 ml kg⁻¹ followed by 0.0298 ml min⁻¹), showed a robust augmentation of the population e.p.s.p. risetime (LTP) recorded in the dentate hilus following tetanic stimulation of the mPP. LTP was effectively abolished in a separate group of rats treated with an MK-801 dosing regimen (0.12 mg kg⁻¹ i.v. followed by 1.8 μg kg⁻¹ h⁻¹) known to produce maximal neuroprotection in a rat stroke model but, by contrast, remained largely intact in a third group of animals given a similarly neuroprotective L-687,414 treatment (28 mg kg⁻¹ i.v. followed by 28 mg kg⁻¹ h⁻¹).

5. These experiments suggest that a low level of intrinsic activity at the glycine site may be sufficient to support NMDA receptor-dependent LTP but in circumstances where there is likely to be an excessive NMDA receptor activation the agonism associated with a low efficacy partial agonist, such as L-687,414, is dominated by the antagonist properties. Thus, an NMDA receptor partial agonist profile may offer a therapeutic advantage over neutral antagonists by permitting an acceptable level of `normal' synaptic transmission whilst curtailing excessive receptor activation.

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