Modulation of spontaneous and miniature excitatory synaptic transmission at central synapses by group II metabotropic glutamate receptors G. Ayman, R. S. G. Jones, G. L. Woodhall. Dept. of Physiology, School of Medical Sciences, University of Bristol, University Walk, Bristol, BS8 1TD.

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Previous studies of the effects of group III metabotropic glutamate receptors (mGluRs) on glutamatergic transmission of the entorhinal cortex (EC) have shown that their activation increases the frequency of spontaneous excitatory postsynaptic currents (sEPSCs), but concurrently decreases the amplitude of stimulus-evoked EPSCs (Evans et al., 2000a,b) in layer V. Group II mGluRs (mGlu₂ and ₃) have similar presynaptic locations to group III (mGluR_4,6,7 and ₈) and common intracellular signaling pathways. We are currently investigating whether differential modulation of glutamate-mediated transmission is a fundamental aspect of mGluR function. Here, we describe the effect of group II mGluRs on sEPSCs and activity-independent miniature (m)EPSCs at glutamate synapses in layer V of the EC.

Brain slices comprising EC and hippocampus were prepared from male Wistar rats (45-60g) anaesthetised with ketamine (120 mg/kg i.m.) and xylazine (8 mg/kg i.m.) prior to decapitation. Slices were transferred to a submersion recording chamber continuously perfused with oxygenated (95% O₂: 5% CO₂) aCSF containing the NMDA receptor antagonist, MK-801 (10 µM). Whole-cell voltage clamp recordings of s/mEPSCs were made from neurones voltage clamped at -60 mV using standard patch-clamp techniques. Drugs used were: 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 5 µM) and 2S-2-amino-2-(1S,2S-2-carboxycyclo-pro-1-yl)-3-(xanth-9yl)propanoic acid (LY341495; 20 nM). Data were analysed using Minianalysis software (Synaptosoft, USA). Statistical analysis was performed using the nonparametric Kolmogorov-Smirnoff (KS) test. below refer to mean ± s.e.m.

The mean inter-event interval (IEI) of sEPSCs was 411.1 ± 16.6 ms (n=8 neurones). Perfusion with the group II mGluR agonist, DCG-IV decreased IEI to 333.2 ± 15.2 ms, (P=0.001). Subsequent perfusion with the selective group II antagonist, LY341495, increased the IEI above that seen in control conditions (800.5 ± 54.2 ms, P=0.001). Neither drug had any effect on mean sEPSC amplitude (control 11.2 ± 0.2 pA v 11.7 ± 0.1 pA in DCG-IV, and 11.0 ± 0.2 pA on addition of LY341495). Interestingly, whilst mean amplitude was unchanged by addition of LY341495 to drug-naïve slices, amplitude distribution was skewed towards larger sEPSCs at the expense of smaller events (P<0.001, KS, n=7). For example, the number of events in a 50-75 pA amplitude bin increased to 203% of control, and those in a 0-25 pA bin decreased to 89.3%. Linear regression analysis confirmed that in LY341495, the number of sEPSCs in successive amplitude bins (expressed as % of control) increased monotonically with increasing amplitude size (r²=0.95).

We tested the hypothesis that the shift toward larger amplitude sEPSCs in LY341495 was due to an alteration in action-potential dependent glutamate release by isolating mEPSCs using tetrodotoxin (1 µM). When LY341495 was perfused alone, neither mEPSC amplitude nor IEI was significantly altered (control IEI 1262 ± 54 ms v IEI 1222 ± 52 ms in LY341495; control amplitude 12.43 ± 0.2 pA v 12.44 ± 0.2 pA, P>0.05 in each case, n=6). These data suggest that group II mGluRs may differentially modulate spontaneous and miniature glutamate release at terminals in layer V of the EC.

Evans D. I. P. et al., (2000a) J. Neurophysiol. 83: 2519-25.
Evans D. I. P. et al., (2000b) J. Physiol. 527: 100P.