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© Copyright 2004 The British Pharmacological Society

021P University of Bath
Summer Meeting July 2004

mGluR4a inhibits N and P/Q-type calcium channels to depress evoked glutamate release in the rat entorhinal cortex

Roland. S.G. Jones, D. Ieuan Evans, Göher Ayman and Gavin L. Woodhall, Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD

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Jones RSG
Evans DI
Ayman G
Woodhall GL

We have previously demonstrated that activation of presynaptic group III metabotropic glutamate receptors (mGluR) enhances the spontaneous release of glutamate at synapses in layer V of the entorhinal cortex (EC), evidenced by an increase in spontaneous excitatory postsynaptic current (sEPSC) frequency. This effect was due to activation of the mGluR4a subtype (Evans et al., 2000a). However, EPSCs evoked by concurrent stimulation of afferent inputs (eEPSCs) were concurrently depressed (Evans et al., 2000b). In the current study we have determined whether the effect of mGluR4a activation on eEPSCs is mediated by inhibition of voltage gated Ca2+-channels (VGCC).

EC slices were prepared from the male Wistar rats (50-110g). Whole cell voltage clamp recordings were made from neurones in layer V, visualized using differential interference contrast optics and an infrared video camera. eEPSCs were elicited by electrical stimulation (10-100 mV, 0.02 ms duration) via a bipolar electrode placed on the surface of the slice in layer V lateral to the recording site. A paired Students t-test was used for comparison of eEPSC amplitudes. All error values refer to standard error of the mean. Drugs were applied by bath perfusion.

The specific mGluR4a agonist, ACPT-1 reduced the amplitude of eEPSCs from 29.4±1.2 pA to 18.0±1.4 pA (P<0.05). Specific inhibitors were then used to determine whether inhibition of VGCC may underlie this effect. -conotoxin GVIA (CTx, 400 nM) was used to block N-type channels and agatoxin IVA (AgTX, 200 nM) and Ni2+ (25 µM) to block, P/Q and R-type channels, respectively. CTx reduced the normalized mean fractional eEPSCs to 0.75±0.02 (P<0.01; n=8). A similar effect was seen with AgTx (0.75±0.02; P<0.01; n=8). The Addition of ACPT-1 with either toxin saw further reductions to 0.49±0.02 and 0.52±0.03, respectively (P<0.01in both cases). Ni2+ reduced the fractional eEPSC amplitude to 0.71±0.03 (P<0.01; n=7), but in the presence of ACPT-1 the additional reduction, to 0.26±0.01, was much more pronounced (P<0.01).

Thus, N, P/Q and R-type channels all contribute to glutamate release at these terminals. Inhibition of either P/Q or N-type channels alone did not prevent the depression of the eEPSC by mGluR4a activation, but blockade of R-type channels essentially doubled it. One explanation for these data would be that the effect of ACPT-1 depends on inhibition of both N and P/Q-type, but not R-type channels. Inhibition of either P/Q or N-type channels alone would allow the unblocked N or P/Q-type channels to fully mediate the effects of mGluR activation. In these circumstances, mGluR-insensitive R-type channels would help maintain eEPSC amplitude. With R-type channels blocked, glutamate release would depend only on N and P/Q-type channels, both of which would be inhibited by mGluR activation, strongly reducing the intraterminal Ca2+ transient and hence the eEPSC.

Evans, D I P et al., (2000a). J. Neurophysiol. 83, 2519-2525
Evans, D I P et al., (2000b). J. Physiol. 527P, 100P