A novel, competitive mGlu5 receptor antagonist (LY344545) blocks DHPG-induced potentiation of NMDA responses but not the induction of LTP in rat hippocampal slices

1 We have investigated the pharmacological properties of LY344545, a structurally related epimer of the broad spectrum competitive metabotropic glutamate receptor antagonist, LY341495. We have found that LY344545 also antagonizes competitively nearly all mGlu receptor subtypes, but with a wide spectrum of activity. The order of potency for the human receptor isoforms was mGlu(5a) (IC50 of 5.5 +/- 0.6 mu M) > mGlu(2) = mGlu(3) > mGlu(1 alpha) = mGlu(7) > mGlu(6) = mGlu(8). No significant mGlu(4) receptor antagonist activity was detected at the highest concentration used (100 mu M). 100 mu M LY344545 displaced 50+/-5% of [H-3]-CGP39653 binding, but less than 30% of [H-3]-kainate or [H-3]-AMPA in radioligand binding assays. 2 LY344545 antagonized L-glutamate stimulated Ca2+ release in CHO cells transfected with mGlu receptors in a concentration dependent manner with a 10 fold higher affinity for the rat mGlu(5a) receptor (K-i=2.1+/-0.6 mu M) compared to the rat mGlu1 alpha receptor (K-i=20.5+/-2.1 mu M). 50 mu M (1S, 3R)-ACPD-induced Ca2+ rises in hippocampal CA1 neurones were also antagonized (IC50 = 6.8+/-0.7 mu M). 3 LY344545 antagonized 10 mu M (S)-3,5-DHPG-induced potentiation of NMDA depolarizations in CAI neurones (EC50=10.6+/-1.0 mu M). At higher concentrations (greater than or equal to 100 mu M), LY344545 was an NMDA receptor antagonist. 4 LY344545 also blocked the induction, but not the expression, of LTP at CA3 to CA1 synapses with an IC50 > 300 mu M. This effect is consistent with its weak activity at NMDA receptors. 5 These results demonstrate that the binding of ligands to mGlu receptor subtypes is critically dependent on the spatial orientation of the same molecular substituents within a given chemical pharmacophore. The identification of LY344545 as the first competitive antagonist to show selectivity towards mGlu(5) receptors supports the potential to design more selective and potent competitive antagonists of this receptor. 6 These results further indicate that mGlu receptor-mediated potentiation of NMDA responses is not essential for the induction of LTP.