Modulation of hippocampal synaptic transmission and functional evaluation of the kynurenine pathway member xanthurenic acid

CS Copeland1, SA Neale1,2, VN Uebele3, FJ Thomson3, TE Salt1. 1Institute of Ophthalmology, UCL, London, EC1V 9EL, UK, 2Neurexpert Ltd, London, EC1V 2NX, UK, 3Merck Research Laboratories, West Point, PA, 19486-0004, USA

Xanthurenic acid (XA), an endogenous kynurenine, is known to be an inhibitor of vesicular glutamate transport (VGLUT) and has also been suggested to be an agonist of Group II mGlu receptors (Fazio et al., 2011; Thompson et al., 2005). Changes to these systems have been implicated in the pathophysiology of schizophrenia and other psychiatric disorders, however little is known of how XA affects synaptic transmission. We thought it important to understand how XA may modulate synaptic function and have investigated the effects of XA on synaptic transmission at two glutamatergic pathways in the hippocampus and have additionally evaluated the ability of XA to displace mGlu2/3 antagonist binding and to inhibit the forskolin-induced formation of cAMP.

Field excitatory postsynaptic potentials (fEPSPs) were recorded from either the dentate gyrus (DG) or CA1 region of mouse (C57black/6j) hippocampal slices in vitro (Kew et al., 2002). Addition of XA to the bathing medium (1mM-10mM) resulted in a dose-related reduction of fEPSP amplitudes in both the DG (up to 52±9% reduction, n=4; P < 0.05) and CA1 (18±5% reduction, n=5; P < 0.05) regions. In the DG, the mGlu2/3 agonist LY354740 (30nM-1µM) also reduced fEPSPs (up to 74±3% reduction, n=18; P < 0.001). The mGlu2/3 antagonist LY341495 (300nM) reversed the effect of LY354740 (P<0.05) but not the effect of XA. The effects of LY354740 were associated with a change in paired-pulse ratio, whereas no change in paired-pulse ratio was seen with XA. Congo red (1mM), another VGLUT inhibitor (Thompson et al., 2005), also depressed the fEPSP amplitude by 61±2% (n=3; P<0.05). In binding experiments in membranes with human mGlu2 receptors, XA had no effect on specific binding of 1nM [3H]LY341495. XA (75-300µM) was able to inhibit the forskolin-stimulated cAMP formation of CHO-K1 cells expressing human mGlu2 receptors (EC50=67±3.6uM) but this effect was not affected by LY341495 (300nM), whereas the effect of LY354740 (0.3-3µM) (EC50=1.9±0.3nM) was antagonized by LY341495 in this assay (P<0.05). Addition of the positive allosteric modulator (PAM) of mGlu2, LY487379 (300nM), had no significant effect on the EC50 for XA, but did reduce the EC50 for LY354740 to 0.7±0.4nM (P<0.05).

We conclude that XA can modulate synaptic transmission via a mechanism that appears to involve VGLUT inhibition and/or modulation of cAMP rather than activation of mGlu2/3 receptors, and that this could be important in the pathophysiology of nervous system disorders including schizophrenia.

References

Fazio F, Molinaro G, Iacovelli L, Bernabucci M, Mauro G, Di Menna L, et al. (2011). Xanthurenic Acid as a Novel Putative Agonist of Group II Metabotropic Glutamate Receptors. Current Neuropharmacology 9(Suppl 1): 19.

Kew JNC, Pflimlin M-C, Kemp JA, Mutel V (2002). Differential regulation of synaptic transmission by mGlu2 and mGlu3 at the perforant path inputs to the dentate gyrus and CA1 revealed in mGlu2 -/- mice. Neuropharmacology 43: 215-221.

Thompson CM, Davis E, Carrigan CN, Cox HD, Bridges RJ, Gerdes JM (2005). Inhibitors of the Glutamate Vesicular Transporter (VGLUT). Current Medicinal Chemistry 12: 2041-2056.