Persistent Hyperactivity of Hippocampal Dentate Interneurons After a Silent Period in the Rat Pilocarpine Model of Epilepsy.

Xiaochen Wang,Xinyu Song,J Victor Nadler,Lin Wu,Ren-Zhi Zhan

doi:10.3389/fncel.2016.00094

Abstract

Profile of GABAergic interneuron activity after pilocarpine-induced status epilepticus (SE) was examined in the rat hippocampal dentate gyrus by analyzing immediate early gene expression and recording spontaneous firing at near resting membrane potential (REM). SE for exact 2 h or more than 2 h was induced in the male Sprague-Dawley rats by an intraperitoneal injection of pilocarpine. Expression of immediate early genes (IEGs) was examined at 1 h, 1 week, 2 weeks or more than 10 weeks after SE. For animals to be examined at 1 h after SE, SE lasted for exact 2 h was terminated by an intraperitoneal injection of diazepam. Spontaneous firing at near the REM was recorded in interneurons located along the border between the granule cell layer and the hilus more than 10 weeks after SE. Results showed that both c-fos and activity-regulated cytoskeleton associated protein (Arc) in hilar GABAergic interneurons were up-regulated after SE in a biphasic manner; they were increased at 1 h and more than 2 weeks, but not at 1 week after SE. Ten weeks after SE, nearly 60% of hilar GABAergic cells expressed c-fos. With the exception of calretinin (CR)-positive cells, percentages of hilar neuronal nitric oxide synthase (nNOS)-, neuropeptide Y (NPY)-, parvalbumin (PV)-, and somatostatin (SOM)-positive cells with c-fos expression are significantly higher than those of controls more than 10 weeks after SE. Without the REM to be more depolarizing and changed threshold potential level in SE-induced rats, cell-attached recording revealed that nearly 90% of hilar interneurons fired spontaneously at near the REM while only 22% of the same cell population did so in the controls. In conclusion, pilocarpine-induced SE eventually leads to a state in which surviving dentate GABAergic interneurons become hyperactive with a subtype-dependent manner; this implies that a fragile balance between excitation and inhibition exists in the dentate gyrus and in addition, the activity-dependent up-regulation of IEGs may underlie plastic changes seen in some types of GABAergic cells in the pilocarpine model of epilepsy.

Highlights

In temporal lobe epilepsy, multiple histopathological changes such as loss of neurons, gliosis, synaptic reorganization, granule cell dispersion, and altered neurogenesis are present in the hippocampal dentate gyrus (Majores et al, 2007; Goldberg and Coulter, 2013)
Through assessing expression of activity-regulated cytoskeletonassociated protein (Arc) and c-fos and recording spontaneous firing at near resting membrane potential (REM) of hippocampal dentate interneurons in a rat pilocarpine model of epilepsy, we revealed that after a short latent period, surviving hilar interneurons after pilocarpine-induced status epilepticus (SE) become persistently hyperactive in a subtype-dependent manner
Despite the fact that both c-fos and Arc are widely used as molecular markers of neuronal activation (Kawashima et al, 2014; Yang et al, 2014), it remains to be known if expressions of these two markers are parallel at different stages after SE

Summary

Introduction

Multiple histopathological changes such as loss of neurons, gliosis, synaptic reorganization, granule cell dispersion, and altered neurogenesis are present in the hippocampal dentate gyrus (Majores et al, 2007; Goldberg and Coulter, 2013). Because of these changes, the dentate gyrus has been studied extensively for its roles in the development of epilepsy and propagation of epileptic waves. The promise of GABAergic progenitor transplantation and activation of GABAergic interneurons through optogenetics in reducing frequency and severity of seizures as well as in improving abnormal behaviors (Calcagnotto et al, 2010; Hunt et al, 2013; Henderson et al, 2014; Southwell et al, 2014; Tyson and Anderson, 2014) that associate with epilepsy raise a question on if specific type(s) of GABAergic interneurons should be transplanted for achieving optimal outcomes

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