Senik, Mohd Harizal
(2020)
The effect of pharmacologically-induced epileptiform-like activity on inter-hemispheric hippocampal functional connectivity in the anaesthetised rats.
PhD thesis, University of Nottingham.
Abstract
Medial temporal lobe epilepsy (mTLE) is one of the most common epilepsies in the human population, yet it is difficult to control. Neuronal activity in a seizure reflects increased firing of individual neurones and alterations in the connectivity of neural networks.
In vivo electrophysiological recordings of neuronal ensemble single-unit and local field potential (LFP) activity were performed in both left and right hippocampi of isoflurane-anaesthetised rats with the aim of establishing a model of acute epileptiform-like activity induced by intrahippocampal kainic acid (KA) administration. Secondly, the main objective of this study was to examine the effects of the anticonvulsants valproate (VPA, 20 mg/kg administered intravenously), WIN 55,212-2 (WIN, 1.2 mg/kg, administered intravenously) and flurazepam (FLU, 2 mg/kg, administered intravenously) on acute epileptiform-like activity induced by KA. The effects of the selected drugs were evaluated according to normalised mean firing rate; normalised mean synchrony index, cross-correlations (mean K-index) and LFP frequency bands distribution.
We evaluated two concentrations of KA (1 mM and 10 mM administered locally) to establish the acute mTLE rat model. Epileptiform-like activity was induced by both KA 1 mM and 10 mM in the ipsilateral hippocampus. Although there was no direct administration of KA to the contralateral hippocampus (only injected in the ipsilateral hippocampus), the neuronal hyper-excitability effect was also propagated to the contralateral hippocampus. The inter-hemispheric hippocampal connectivity and the connectivity between neurones in the hippocampus (intra-hippocampal) during epileptiform activity in the mTLE model were evaluated according to four parameters studied: normalised mean firing rate, normalised mean synchrony index, cross-correlation (K-index) and LFP frequency band distribution. Our study has shown that administering intrahippocampal KA 1 mM is effective to induce epileptiform-like activity through significant increases in normalised mean firing rate in ipsilateral-CA1 and contralateral-CA3 hippocampus (p<0.05 and p<0.0001 when compared to basal, respectively). Increased normalised mean synchrony index in ipsilateral-CA1 (p<0.001) and increased normalised mean power in the LFP frequency band of delta (1-4 Hz) and theta (4-8 Hz) in the ipsilateral and contralateral hippocampus were found in both CA1 and CA3, respectively. Therefore, we chose to administer KA 1 mM locally for our mTLE model.
Based on our findings, 20 mg/kg of VPA administered intravenously 20 minutes before KA prevented the neuronal hyper-excitability induced by KA in the ipsilateral-CA1 hippocampus, but there were no changes in the ipsilateral-CA3 and contralateral-CA1 hippocampus when compared to basal. However, 20 mg/kg VPA reduced the neuronal hyper-excitability in contralateral-CA3 by 55% compared to the experiment without VPA (mTLE model). VPA administered 20 min after KA administration was observed to attenuate the neuronal hyper-excitability induced by KA, since there was no neuronal hyper-excitability in any brain regions. These results indicated that VPA prevents KA inducing further neuronal hyper-excitability. Due to these effects, we proposed that VPA acts to prevent, reduce, and attenuate epileptiform-like activity. A dose of 1.2 mg/kg WIN administered before KA prevented neuronal hyper-excitability induced by KA, while the administration of 20 mg/kg WIN 20 min after KA showed that WIN attenuated the firing induced by KA. A 1.2 mg/kg intravenous administration of FLU - lower than the dosage used in the literature (30-60 mg/kg) - attenuated epileptiform-like activity induced by KA.
In conclusion, the data presented in this thesis have addressed the aims and hypothesis of this study by providing further evidence supporting the use of VPA, WIN and FLU in controlling hyper-excitable neurones in the hippocampi. This thesis generates three main contributions; the mTLE model for anaesthetised rats is proposed; VPA administered after KA is the most effective treatment to attenuate the effect of KA than WIN and FLU; WIN and FLU are proposed as potential anti-epileptic drugs (AEDs) in mTLE model. Our study has highlighted the significance of using an electrophysiological approach in an animal model of mTLE using anaesthetised rats, and functional connectivity in the hippocampi, which are lacking in the present literature.
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