Reorganization of the septohippocampal cholinergic fiber system in experimental epilepsy

J Comp Neurol. 2017 Aug 15;525(12):2690-2705. doi: 10.1002/cne.24235. Epub 2017 May 22.

Abstract

The septohippocampal cholinergic neurotransmission has long been implicated in seizures, but little is known about the structural features of this projection system in epileptic brain. We evaluated the effects of experimental epilepsy on the areal density of cholinergic terminals (fiber varicosities) in the dentate gyrus. For this purpose, we used two distinct post-status epilepticus rat models, in which epilepsy was induced with injections of either kainic acid or pilocarpine. To visualize the cholinergic fibers, we used brain sections immunostained for the vesicular acetylcholine transporter. It was found that the density of cholinergic fiber varicosities was higher in epileptic rats versus control rats in the inner and outer zones of the dentate molecular layer, but it was reduced in the dentate hilus. We further evaluated the effects of kainate treatment on the total number, density, and soma volume of septal cholinergic cells, which were visualized in brain sections stained for either vesicular acetylcholine transporter or choline acetyltransferase (ChAT). Both the number of septal cells with cholinergic phenotype and their density were increased in epileptic rats when compared to control rats. The septal cells stained for vesicular acetylcholine transporter, but not for ChAT, have enlarged perikarya in epileptic rats. These results revealed previously unknown details of structural reorganization of the septohippocampal cholinergic system in experimental epilepsy, involving fiber sprouting into the dentate molecular layer and a parallel fiber retraction from the dentate hilus. We hypothesize that epilepsy-related neuroplasticity of septohippocampal cholinergic neurons is capable of increasing neuronal excitability of the dentate gyrus.

Keywords: RRID:AB_11214110; RRID:AB_2079751; fiber sprouting; hippocampal formation; medial septum and diagonal band of Broca; stereology; temporal lobe epilepsy.

MeSH terms

  • Analysis of Variance
  • Animals
  • Choline O-Acetyltransferase / metabolism
  • Cholinergic Fibers / metabolism
  • Cholinergic Fibers / pathology*
  • Disease Models, Animal
  • Electroencephalography
  • Epilepsy / chemically induced
  • Epilepsy / pathology*
  • Epilepsy / physiopathology*
  • Gene Expression Regulation / drug effects
  • Hippocampus / pathology*
  • Kainic Acid / toxicity
  • Male
  • Neurons / metabolism
  • Neurons / pathology
  • Rats
  • Rats, Wistar
  • Septum of Brain / metabolism
  • Septum of Brain / pathology*
  • Vesicular Acetylcholine Transport Proteins / metabolism

Substances

  • Slc18a3 protein, rat
  • Vesicular Acetylcholine Transport Proteins
  • Choline O-Acetyltransferase
  • Kainic Acid