Elsevier

Neuroscience

Volume 95, Issue 2, December 1999, Pages 343-351
Neuroscience

Reciprocal modulation of glutamate and GABA release may underlie the anticonvulsant effect of phenytoin

https://doi.org/10.1016/S0306-4522(99)00468-6Get rights and content

Abstract

Although conventional wisdom suggests that the effectiveness of phenytoin as an anticonvulsant is due to blockade of Na+-channels this is unlikely to be it's sole mechanism of action. In the present paper we examined the effects of phenytoin on evoked and spontaneous transmission at excitatory (glutamate) and inhibitory (GABA) synapses, in the rat entorhinal cortex in vitro. Evoked excitatory postsynaptic potentials at glutamate synapses exhibited frequency-dependent enhancement, and phenytoin reduced this enhancement without altering responses evoked at low frequency. In whole-cell patch-clamp recordings the frequency of excitatory postsynaptic currents resulting from the spontaneous release of glutamate was reduced by phenytoin, with no change in amplitude, rise time or decay time. Similar effects were seen on miniature excitatory postsynaptic currents, recorded in the presence of tetrodotoxin. Evoked inhibitory postsynaptic potentials at GABA synapses displayed a frequency-dependent decrease in amplitude. Phenytoin caused a reduction in this decrement without affecting the responses evoked at low frequency. The frequency of spontaneous GABA-mediated inhibitory postsynaptic currents, recorded in whole-cell patch mode, was increased by phenytoin, and this was accompanied by the appearance of much larger amplitude events. The effect of phenytoin on the frequency of inhibitory postsynaptic currents persisted in the presence of tetrodotoxin, but the change in amplitude distribution largely disappeared.

These results demonstrate for the first time that phenytoin can cause a simultaneous reduction in synaptic excitation and an increase in inhibition in cortical networks. The shift in balance in favour of inhibition could be a major factor in the anticonvulsant action of phenytoin.

Section snippets

Slice preparation

Experiments were performed on slices containing EC and hippocampus prepared from male Wistar rats (n=47, 120–150 g). All experiments were performed in accordance with the U.K. Animals (Scientific Procedures) Act 1986, European Communities Council Directive 1986 (86/609/EEC) and the University of Bristol ethical review document. All efforts were made to minimise the number of animals utilized in these experiments and to eliminate any suffering. Animals were decapitated under anaesthesia induced

Results

The current studies have all been conducted on neurons in the rat EC. We have previously suggested21 that the deeper layers IV/V may be more susceptible to epileptogenesis than the superficial layer (II), and provided information as to why this may be so.2., 18., 50. In the present study, we studied neurons in both deep and superficial layers but could detect no difference in the effects of phenytoin in the different layers.

Glutamate transmission

A number of studies have suggested that low-frequency responses at glutamate synapses, or responses to exogenous activation of AMPA and NMDA receptors, may be reduced by phenytoin.3., 14., 23., 48. In agreement with the data of others24., 25., 26., 30., 32. we found little effect of phenytoin on either AMPA or NMDA-mediated responses evoked at low frequency, indicating that phenytoin is unlikely to interact directly with the postsynaptic receptors in the EC. However, the frequency-dependent

Conclusion

The present results show that phenytoin effectively increases synaptic inhibition and at the same time decreases synaptic excitation. These reciprocal effects on both background and evoked inhibition and excitation are highly desirable actions required in an effective anticonvulsant.

Acknowledgements

We thank the Wellcome Trust, the MRC and the Taberner Trust for financial support, Dr John Dempster for the Strathclyde Software, and Novartis for the gift of CGP55845A.

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