Increased activation of presynaptic GluR5 kainate receptors may contribute to generation of slow wave oscillations in entorhinal cortical neurones in vitro

Sophie Chamberlain, Stuart Greenhill, Roland Jones. University of Bath, Bath, United Kingdom.

Slow wave oscillations (SWO) can be induced in rat entorhinal cortex (EC) slices when [Mg++]0 is reduced. One factor involved in the initiation of SWO is increased activation of a kainate receptor (KAr), since SWO are abolished by UBP 302, an antagonist of GluR5-containing KAr (Cunningham et al., 2005). We have been studying the role of GluR5 receptors in the EC, and have found that they can act as presynaptic autoreceptors to enhance excitatory transmission (Chamberlain and Jones, 2007). In this study we have determined whether lowering [Mg++]0 alters the autoreceptor function of the GluR5 KAr.

Spontaneous excitatory postsynaptic currents (sEPSCs) were used as a reporter of presynaptic glutamate release. sEPSCs were recorded using whole cell-patch clamp from layer III neurones in EC slices prepared from male Wistar rats (60-70g). Under the conditions of the experiments, sEPSCs are mediated almost exclusively by activation of AMPA receptors.

In “normal” [Mg++]0 (2 mM), the GluR5-specific agonist, ATPA (500 nM, n=5), decreased the mean (±sem) inter-event interval (IEI) of sEPSCs from 211.3±41.1 to 64.1±9.3 ms, reflecting an increase in frequency of around 300%. The mean amplitude of events was not significantly altered (11.5±0.9 v 13.4±1.5 pA). These data suggest that the GluR5 receptor presynaptically facilitates glutamate release at these synapses. UBP 302 (20 μM, n=10) had no effect on either IEI (256.0±51.7 v 273.9±55.5 ms) or amplitude (15.5±3.4 v 16.1±1.6 pA) of sEPSCs. Thus, GluR5 receptors are unlikely to contribute to postsynaptic excitation but, more importantly, presynaptic GluR5 receptors are not tonically activated by on-going glutamate release. Reduction of [Mg++]0 to 1.25 mM resulted in a decrease in IEI of sEPSCs from 180.4±30.7 to 89.2±16.0 ms (n=5), indicative of a substantial increase baseline release of glutamate Mean amplitude of events was significantly altered (15.6±1.35 v 12.8±1.35 pA). Under these conditions, application of UBP 302 (n=6) now resulted in a significant increase in IEI (decreased frequency) of sEPSCs from 102.4±22.1 to 214.84±39.7 ms, once again without change in amplitude (12.9±1.12 v 13.6±1.7 pA). We repeated the experiments in the presence of the NMDA antagonist, 2-AP5 (30 μM, n=6), since Mg++ also regulates NMDA receptors, but this had no effect on the outcome (not shown).

Thus, the results suggest that a reduction of [Mg++]0, which can induce SWO, increases baseline glutamate release. This probably arises as a result of a reduction in Mg++-induced inhibition of voltage-gated calcium influx into the glutamate terminals. With the higher level of ambient glutamate, a tonic facilitation of release by presynaptic GluR5 receptors is revealed. This will help to maintain a high level of release and may be involved in the initiation of the SWO.

Chamberlain, S.E.L. and Jones, R.S.G. (2007) Proc. BNA. 19, abstr 13.05, p69.
Cunningham, M.O. et al., (2006) P.N.A.S. 103, 5597-5601

We thank the Wellcome Trust, Epilepsy Research UK and the BBSRC for financial support.