Abstracts

Rationale: The unique ecological niche and social organization of naked mole-rats (Heterocephalus Glaber) make them a useful model to study how environmental, hormonal, and social factors relate to epilepsy. We have found that naked mole-rat hippocampal slices exhibit spontaneous epileptiform burst discharges in area CA3 with a morphology and frequency similar to those shown previously in chronic pilocarpine-treated rats. The purpose of the present study was to test whether abnormal hippocampal circuitry may help explain this pathophysiology. Methods: Naked mole-rats were deeply anesthetized with CO2 and decapitated. Horizontal 300μm hippocampal slices were maintained in an interface chamber with humidified 95% CO2/5% O2 at 32±1°C. Slices were equilibrated in a sucrose-based artificial cerebrospinal fluid (ACSF) and perfused with a saline-based ACSF. Recording began more than 60 minutes after perfusion of saline-based solution. Slices were first recorded in the CA3 pyramidal cell layer using a glass extracellular electrode. A stimulating electrode was placed in the subgranular zone of the lower blade of the dentate gyrus to activate the mossy fiber pathway. Stimuli used were always less than half of the maximal stimulus used to evoke an othodromic population spike in the CA3 PCL (100-300 μA, 100msec). Slices with biocytin iontophoretically injected at the site of stimulation were incubated overnight in streptavidin conjugated quantum dots (705 nm) and imaged on a confocal microscope. Results: When the stimulating electrode was placed in the subgranular zone and the recording electrode placed in the CA3b-c region, each of 9 slices from 4 animals demonstrated an early, negative-going potential that exceeded the amplitude of the typical mossy fiber-evoked population spike that followed.This putative antidromic signal was absent when the stimulating electrode was placed at an equal distance in the oriens-lacunosum moleculare axis, ruling out current spread artifacts. The morphology of the negative-going potential was similar when antidromic responses were elicited from the fimbria or Schaffer Collateral pathway. Bath application of the AMPA/kainate antagonist CNQX (10 μM, n=2) abolished the orthodromic population spike but not the putative antidromic spike. In slices where biocytin was injected at the mossy fiber stimulation site (n=3), clearly labeled pyramidal somata could be detected near the recording site. Conclusions: This study demonstrates that there is an excitatory feedback pathway from the CA3 pyramidal cells to the dentate gyrus in the naked mole-rat. This pathway may play a role in the epileptiform burst discharges which are common in area CA3 of the naked mole-rat, and are very similar to those we have previously characterized in chronic pilocarpine-treated rats. In fact, a recent study (Zhang et al. 2012, J Neurosci. 32(4)1183-96) demonstrated a similar pathway in chronic pilocarpine-treated rats. Although the termination zones and functional consequences of these projections remain to be determined, this pathway presents an exciting new potential mechanism for hippocampal pathophysiology