Elsevier

Neuroscience

Volume 83, Issue 4, 12 January 1998, Pages 1047-1062
Neuroscience

Tachykinins may modify spontaneous epileptiform activity in the rat entorhinal cortex in vitro by activating GABAergic inhibition

https://doi.org/10.1016/S0306-4522(97)00469-7Get rights and content

Abstract

The effects of substance P and related tachykinins on intrinsic membrane properties and synaptic responses of neurons in cortical slices were determined. Substance P had no detectable effect on membrane properties of principal neurons in layer II or V of the rat medial entorhinal cortex or on neurons in either layer of the anterior cingulate cortex. Specific agonists at the neurokinin1-receptor were also without effect as were agonists at both neurokinin2- and neurokinin3-receptors. Substance P hyperpolarized a small number of principal neurons. These responses were weak and desensitized with repeated applications. Similar effects were seen with other neurokinin1-receptor agonists. Excitatory synaptic potentials mediated by either α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate- or N-methyl-d-aspartate-receptors in principal neurons of the entorhinal cortex were unaffected by substance P. Responses of entorhinal neurons to iontophoretically applied glutamate and N-methyl-d-aspartate were also unaffected. Inhibitory synaptic potentials mediated by either GABAA- or GABAB-receptors in entorhinal neurons were slightly but consistently enhanced by substance P. Neurons identified as interneurons on the basis of their firing characteristics were consistently depolarized by substance P. These responses also desensitized with repeated applications. Spontaneous epileptiform discharges evoked in entorhinal cortex by perfusion with a GABAA-receptor antagonist (bicuculline), were reduced in frequency and, sometimes, in duration by substance P. This effect was mimicked by other neurokinin1-receptor agonists and blocked by neurokinin1-receptor antagonists. It was also mimicked by neurokinin A but not by a specific neurokinin2-receptor agonist. The reduction in frequency of discharges was also mimicked by a GABAB-receptor agonist, l-baclofen, and blocked by the GABAB-receptor antagonist, CGP55845A. Neurokinin B, and a specific neurokinin3-receptor agonist (senktide), increased the frequency and (sometimes) duration of epileptiform discharges. Substance P could also increase frequency but this usually succeeded or preceded a decrease in frequency. The effect of neurokinin B was reduced by a metabotropic glutamate receptor antagonist.

Substance P appears to have little direct effect on principal neurons of the entorhinal cortex but may hyperpolarize them indirectly by activating interneurons and releasing GABA. This indirect inhibition may be responsible for the ability of substance P to reduce the frequency of epileptiform discharges in the entorhinal cortex and may suggest that neurokinin1-receptor agonists have potential as anticonvulsant drugs.

Section snippets

Experimental procedures

Most experiments were performed on brain slices prepared from male Wistar rats (120–150 g; Bantin & Kingman, Hull, U.K.). Slices containing the EC, hippocampus and temporal cortex were prepared as described previously.[21]In some experiments coronal slices containing anterior cingulate cortex were also examined. Animals were anaesthetized with a mixture of ketamine (120 mg/kg) and xylazine (8 mg/kg) administered i.m., and decapitated. The brain was removed and submerged in chilled (4–5°C)

Membrane effects

The effects of bath application of SP were tested on 115 principle neurons (as opposed to interneurons) in the MEC, 61 in layer V and 54 in layer II. SP had little effect on most neurons in either layer, even at very high concentrations (up to 10 μM). Membrane potential and input resistance (mean±S.E.M.) in layer V neurons just before application of SP (n=51) were −72.7±0.9 mV and 35.8±6.5 MΩ, respectively. During application of the peptide the respective values were −71.8±0.8 mV and 35.2±6.6 MΩ.

Discussion

In summary, we have found that the vast majority of principle neurons in the rat MEC are largely unaffected by SP. This lack of effect was surprising in view of the powerful excitatory actions of SP on cortical neurons seen in vivo,1, 23, 24, 25, 26, 29, 40and considering data showing a high density of binding sites for SP in the EC.6, 9, 27, 31A similar lack of effect in slices of rat anterior cingulate cortex suggested that regional differences are unlikely to be a factor. The present results

Conclusion

Whether our results in slices have any relevance for epileptogenesis or anti-epileptic effects in man can only be the subject of conjecture at present. However, NK1-receptors are phylogenetically preserved in higher species including man[8]and it would be worthwhile, therefore to investigate the use of agonists at these receptors as anticonvulsants.

Acknowledgements

We are grateful to the Wellcome Trust, the Royal Society and Glaxo Group Research for financial support for this study.

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    Present Address: Merck Sharpe and Dohme Research Laboratories, Neuroscience Research Centre, Terlings Park, Harlow, Essex CM20 2QR, U.K.

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