Glial source of nitric oxide in epileptogenesis: A target for disease modification in epilepsy

Abstract

Epileptogenesis is the process of developing an epileptic condition and/or its progression once it is established. The molecules that initiate, promote, and propagate remarkable changes in the brain during epileptogenesis are emerging as targets for prevention/treatment of epilepsy. Epileptogenesis is a continuous process that follows immediately after status epilepticus (SE) in animal models of acquired temporal lobe epilepsy (TLE). Both SE and epileptogenesis are potential therapeutic targets for the discovery of anticonvulsants and antiepileptogenic or disease-modifying agents. For translational studies, SE targets are appropriate for screening anticonvulsive drugs prior to their advancement as therapeutic agents, while targets of epileptogenesis are relevant for identification and development of therapeutic agents that can either prevent or modify the disease or its onset. The acute seizure models do not reveal antiepileptogenic properties of anticonvulsive drugs. This review highlights the important components of epileptogenesis and the long-term impact of intervening one of these components, nitric oxide (NO), in rat and mouse kainate models of TLE. NO is a putative pleotropic gaseous neurotransmitter and an important contributor of nitro-oxidative stress that coexists with neuroinflammation and epileptogenesis. The long-term impact of inhibiting the glial source of NO during early epileptogenesis in the rat model of TLE is reviewed. The importance of sex as a biological variable in disease modification strategies in epilepsy is also briefly discussed.

Keywords: antiepileptogenic; kainate; neuroinflammation; nitro-oxidative stress; status epilepticus; temporal lobe epilepsy.

Figure 1

Comparison of the spontaneous CS frequencies between the kainate models of rat, C57BL/6J and crossbred wildtype mice. The seizures were quantified from three months of continuous video-EEG recordings. The behavioral spontaneous CS were verified against EEG pattern and the power spectrum as described previously for the rat and mouse kainate model of TLE (Puttachary et al., 2015b and 2016b). The CS were progressive in the rat, while they decreased over time in the mouse models. Mann-Whitney test, *p<0.05, n=6–8. CS, convulsive seizures.

Figure 2

SHD and RLD methods of kainate administration in C57BL/6J mice and their impact on SE, and diazepam treatment on spiking activity (A–D). In the severe group, diazepam treatment was effective on behavioral seizures, but not on the electrographic seizures (A, B). In the mild group, it was effective on both behavioral and electrographic seizures (C, D). Each vertical bar within the box represents spike train that could contain epileptiform spikes, spike clusters (<12s) and/or a seizure (>12s) at a given time point. The spike trains could be continuous for several minutes or intermittent. Upward red arrow represents the onset of first convulsive seizure, i.e. stage ≥3 and grey arrow represents the time when diazepam was administered after 2 hours of the onset of first CS. SHD, single high dose; RLD, repeated low dose; SE. status epilepticus.

Figure 3

Examples of 30 min EEG traces, after the DZP treatment, from the mouse (A) and rat (B) that had severe SE after administering the kainate. The DZP administration did not control epileptiform spiking in the animals that had severe SE.

Figure 4

The ion chromatogram showing the relative abundance of kainate in the hippocampus. The LC-MS analysis confirmed that the kainate was present in the hippocampus at higher levels at 4h, but persisted even at 24h post-administration. n=4.

Figure 5

A) The common features of epileptogenesis that occur soon after SE is illustrated. B) An example of epileptiform spikes on EEG from an epileptic brain is compared with a normal brain. C) IHC images from the hippocampus (i to vi, and ix to x) and the dentate gyrus (vii, viii). The astrocytes [green in (i) and (ii)] and microglia [red in (iii) and (iv), and green in (ix) and (x)] become reactive (ii, iv, x) in an epileptic brain. Reactive gliosis causes production of proinflammatory cytokines, chemokines, and ROS/RNS to induce neurodegeneration [FJB+NeuN (vi)]. Red labelled cells are NeuN positive in the panel (v), (vi), (ix), and (x). Yellow in (v) and (vi) represents FJB positive cells. Epileptic brain showed increased production of neuroblasts (pink labelled cells) in the subgranular zone of the dentate gyrus (white arrows in viii) in contrast to the control brain (vii). Further details on these parameters and the quantified data can be found in Puttachary et al., 2016a and . Scale bar, all 100 μm.