Elsevier

Epilepsy Research

Volume 32, Issues 1–2, 1 September 1998, Pages 104-113
Epilepsy Research

`Dormant' inhibitory neurons: do they exist and what is their functional impact?

https://doi.org/10.1016/S0920-1211(98)00044-8Get rights and content

Abstract

The concept of dormant interneurons is proving to be hard to define precisely. We argue here that the term is best used as an operational description of interneurons which are not lost from the epileptic brain, but which fail to perform adequately. We present evidence for the existence of functionally dormant interneurons in the tetanus toxin model of chronic epilepsy, and we explore the roles of a partial dormancy (and also of charge-screening) in the acute low magnesium model of epilepsy.

Introduction

The hypothesis of `dormant' inhibitory neurons in epilepsy was developed to explain the disruption of functional inhibition, in the presence of viable inhibitory neurons and of functional GABAA receptors. The idea was proposed by R.S. Sloviter on the basis of his experiments using prolonged stimulation of the perforant path to replicate status epilepticus (Sloviter, 1987Sloviter, 1991). In these experiments, initially performed in the dentate area, he demonstrated a loss of paired pulse inhibition, despite the presence of a normal number of GABA positive basket cells. The pathology that he found consisted of a loss of mossy cells and of somatostatin and neuropeptide-Y containing neurons in the hilus. Sloviter (1987)proposed that the loss of mossy cells would reduce the excitatory drive to the basket cells and other interneurons that would normally inhibit granule cells. In a subsequent paper, Sloviter (1991)demonstrated a similar loss of paired pulse inhibition in the CA1 region in the face of preserved pyramidal cells and interneurons. In the latter case he proposed that the loss of CA3 pyramidal cells reduced the excitatory drive to CA1 inhibitory neurons, diminishing their responsiveness, and thereby weakening paired pulse inhibition.

Bekenstein and Lothman (1993)tested this hypothesis of reduced interneuronal responsiveness in the CA1 region. They used intracellular recordings in brain slices from rats made chronically epileptic by repetitive stimulation some 2–5 months previously. Their evidence for functional disconnection of the interneurons came from comparing the responses to stimulation near to (≲200 μm) and remote from (>1 mm) the impaled CA1 pyramidal cell. In control tissue, stimulation produces EPSPs, fast IPSPs and slow IPSPs, whether the stimulus is close to or remote from the recorded cell. However, in slices from chronically epileptic rats, stimulation of the remote sites failed to elicit IPSPs but still produced a fast EPSP. In contrast, stimulation within a few hundred microns of the impaled cell produced normal looking sequences of fast and slow IPSPs in the presence of drugs that block EPSPs. Bekenstein and Lothman (1993)interpreted this result to show (a) that interneuron-mediated inhibition onto pyramidal cells could be elicited directly when the stimulation site was close to the pyramidal cell, indicating that inhibitory neurons were present and viable; but (b) that these interneurons could not be excited to fire through afferents normally reaching them from more remote parts of the CA1.

Changes resembling dormancy were also found in the kainate-lesion model. Injection of kainic acid into the ventricles leads to acute epileptic seizures and focal neuronal loss, typically including the hippocampal CA3 region. In the longer term, this acute phase can be followed by the development of chronic epileptic seizures (Ben-Ari et al., 1980Lothman et al., 1981Nadler, 1981). During this chronic epileptic phase there is evidence that inhibitory neurons are present and that their synapses onto pyramidal cells can work, but that they are activated more weakly than normal. Nakajima et al. (1991)used paired intracellular recording in the CA1 region to show that the frequency of synaptic coupling between pyramidal cells and interneurons was less than in non-epileptic tissue slices, and that there was no marked increase in excitatory connections between pyramidal cells. In their study, evoked inhibition was weak, but recovered with very strong stimulation that could activate interneurons directly. In another study of the kainate model, IPSPs were reduced in the hyperexcitable parts of slices prepared from chronically epileptic rats, but pharmacologically isolated monosynaptic IPSPs remained normal, again pointing to a functional weakness in the activation of interneurons (Williams et al., 1993).

Here we will describe evidence from a different chronic experimental epilepsy, the intrahippocampal tetanus toxin model, that supports the existence of dormant interneurons. We also describe an acute experimental model to demonstrate the concept of partial disconnection of inhibitory neurons from their excitatory inputs.

Section snippets

Methods

Adult male rats received intrahippocampal injections of <50 ng of tetanus toxin (kind gift of Wellcome Biotech, Beckenham, UK) while under general anaesthesia with a mixture of Hypnorm and Hypnovel (Whittington and Jefferys, 1994). The injection site was 2.8 mm caudal to bregma, 3.5 mm lateral to midline and 3.5 mm below cortical surface. Several days after recovery they developed an epileptic syndrome, which has been described in detail elsewhere (Mellanby et al., 1977, Hawkins and Mellanby,

Results

Rats which had received 50 ng of tetanus toxin injected into the hippocampus developed epileptic seizures that recurred intermittently over a period of up to 10 weeks after the injection. Each of these seizures lasted less than 2 min, possibly explaining the absence of consistent neuropathology in any of the hippocampal regions, either in the pyramidal cell population (Traub et al., 1994) or in the GABAergic interneurons (Najlerahim et al., 1992). We have also found that the size distribution

Acute model: low magnesium epilepsy

It has been argued that a partial loss of excitation to inhibitory neurons will have minimal impact on epileptic discharge because of the massive excitation that the interneurons should receive when pyramidal cells fire synchronously. In our work on the low-[Mg2+]o model of epilepsy, we used selective glutamate receptor antagonists to dissect out the roles of different classes of receptor in the generation of epileptic events. Incubating brain slices in ACSF containing low-[Mg2+]o produces

Discussion

The evidence from the tetanus toxin chronic epilepsy argues for a loss of function of those inhibitory neurons that remain present in the epileptogenic region. The details differ from dormant interneurons as described in chronic models of epilepsy induced by status epilepticus or kainic acid, where gross lesions occur (Nakajima et al., 1991Bekenstein and Lothman, 1993Williams et al., 1993Sloviter, 1994), in that there is no early anatomical loss of neurons in the tetanus toxin model. Recently

Acknowledgements

RDT is a Wellcome Principal Research Fellow. This work was funded by grants from the Wellcome Trust and Human Frontiers.

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