Synaptic hyperexcitability of deep layer neocortical cells in a genetic model of absence seizures

Abstract

We used sharp-electrode, intracellular recordings in an in vitro brain slice preparation to study the excitability of neocortical neurons located in the deep layers (>900 mm from the pia) of epileptic (180-210-days old) Wistar Albino Glaxo/Rijswijk (WAG/Rij) and age-matched, non-epileptic control (NEC) rats. Wistar Albino Glaxo/ Rijswijk rats represent a genetic model of absence seizures associated with generalized spike and wave (SW) discharges in vivo. When filled with neurobiotin, these neurons had a typical pyramidal shape with extensive apical and basal dendritic trees; moreover, WAG/Rij and NEC cells had similar fundamental electrophysiological and repetitive firing properties. Sequences of excitatory postsynaptic potentials (EPSPs) and hyperpolarizing inhibitory postsynaptic potentials (IPSPs) were induced in both the strains by electrical stimuli delivered to the underlying white matter or within the neocortex; however, in 24 of 55 regularly firing WAG/Rij cells but only in 2 of 25 NEC neurons, we identified a late EPSP that (1) led to action potential discharge and (2) was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (20 mM; n = 8/8 WAG/Rij cells). Finally, we found that the fast and slow components of the stimulus-induced IPSPs recorded during the application of glutamatergic receptor antagonists had similar reversal potentials in the two strains, while the peak conductance of the fast IPSP was significantly reduced in WAG/Rij cells. These findings document an increase in synaptic excitability that is mediated by NMDA receptors, in epileptic WAG/Rij rat neurons located in neocortical deep layers. We propose that this mechanism may be instrumental for initiating and maintaining generalized SW discharges in vivo.

Reduced GABAB receptor subunit expression and paired-pulse depression in a genetic model of absence seizures

“…Experimental evidence obtained to date from in vitro slice studies indicates that epileptic WAG/Rij neocortical neurons present with: (i) marked reduction in I h (a cation current that is known to modulate neuron excitability and rhythmicity) (Strauss et al, 2004), (ii) decreased function of GABA A receptor-mediated post-synaptic inhibition (Luhmann et al, 1995) and (iii) increased NMDA receptor-mediated events leading to prolonged depolarizing responses and to action potential discharge (Luhmann et al, 1995;D'Arcangelo et al, 2002;D'Antuono et al, 2006). The latter mechanism has also been identified in vivo both in WAG/Rij (Peeters et al, 1989) and GAERS (Pumain et al, 1992) animals.…”

“…The alterations in GABA B receptor subunit expression and localization identified here in WAG/Rij neocortical tissue are likely to contribute to synaptic hyperexcitability (Luhmann et al, 1995;D'Antuono et al, 2006) and thus to the ability of somatosensory networks of these animals to initiate SW dicharges in vivo (Meeren et al, 2002). Although the exact functional meaning of changes in subunit composition of GABA B receptors in the epileptic WAG/Rij rat neocortex remains to be established, our findings indicate that paired-pulse depression of excitatory and inhibitory responses are markedly reduced in the neocortex of epileptic animals as compared with NECs.…”

“…Indeed, recent evidence points to the perioral area of the somatosensory cortex as the site of initiation of generalized SW activity in both WAG/Rij and GAERS epileptic animals (Meeren et al, 2002;Manning et al, 2004). Several in vivo and in vitro studies have shown that neocortical hyperexcitability in these genetic models is caused by changes in intrinsic (Strauss et al, 2004;Klein et al, 2004) and synaptic mechanisms (Peeters et al, 1989;Pumain et al, 1992;Luhmann et al, 1995;D'Arcangelo et al, 2002;Pinault, 2003;D'Antuono et al, 2006).…”

Upholding WAG/Rij rats as a model of absence epileptogenesis: Hidden mechanisms and a new theory on seizure development

“…Altered GABA A receptor function has been described in the cerebral cortex and thalamus of WAG/Rij rats and may contribute to abnormal brain states of absence epilepsy (D'Antuono et al, 2006;Luhmann et al, 1995). In WAG/Rij rats, GABAergic inhibition is reduced in upper-layer frontal cortical neurons (Luhmann et al, 1995), and GABA A receptor-mediated fast hyperpolarizing inhibitory postsynaptic potentials (IPSPs) show decreased peak conductances in deeper-layer neurons (D'Antuono et al, 2006). WAG/Rij rats in comparison to Wistar rats, exhibited a significant reduction in the efficiency of intracortical GABAergic inhibition concomitant with hyperexcitability (Luhmann et al, 1995) and interestingly, many cortical areas are lacking parvalbumin-containing interneurons.…”

“…In agreement with this, it has recently been demonstrated that the removal of only both foci completely abolishes SWDs (Scicchitano et al, 2015) while fMRI (functional magnetic resonance imaging) studies have confirmed the presence of bilateral activated regions in the somatosensory cortex . Based on this background, we can then suppose that these focal epileptic areas may be genetically predetermined to be hyperexcitable (D'Antuono et al, 2006;Kole et al, 2007;Luttjohann et al, 2011;Strauss et al, 2004) from a certain developmental age onwards. These areas might then trigger neuroadaptive changes into the cortico-thalamo-cortical network, which will then become responsive to the hyperexcitable stimulus generating seizures.…”