knock-out rats exhibit abundant spike-wave discharges in EEG, exacerbated with valproate treatment.

OBJECTIVE

To elucidate the functional role of gamma-aminobutyric acid (GABA)-ergic inhibition in suppressing epileptic brain activities such as spike-wave discharge (SWD), we recorded electroencephalogram (EEG) in knockout rats for (), which encodes one of the two GABA-synthesizing enzymes in mammals. We also examined how anti-epileptic drug valproate (VPA) acts on the SWDs present in rats and affects GABA synthesis in the reticular thalamic nucleus (RTN), which is known to play an essential role in suppressing SWD.

METHODS

Chronic EEG recordings were performed in freely moving control rats and homozygous knockout (-/-) rats. Buzzer tones (82 dB) were delivered to the rats during EEG monitoring to test whether acoustic stimulation could interrupt ongoing SWDs. VPA was administered orally to the rats, and the change in the number of SWDs was examined. The distribution of GABA in the RTN was examined immunohistochemically.

RESULTS

SWDs were abundant in EEG from (-/-) rats as young as 2 months old. Although SWDs were universally detected in older rats irrespective of their genotype, SWD symptom was most severe in (-/-) rats. Acoustic stimulation readily interrupted ongoing SWDs irrespective of the genotype, whereas SWDs were more resistant to interruption in (-/-) rats. VPA treatment alleviated SWD symptoms in control rats, however, counterintuitively exacerbated the symptoms in (-/-) rats. The immunohistochemistry results indicated that GABA immunoreactivity was significantly reduced in the somata of RTN neurons in (-/-) rats but not in their axons targeting the thalamus. VPA treatment greatly increased GABA immunoreactivity in the RTN neurons of (-/-) rats, which is likely due to the intact GAD2, another GAD isozyme, in these neurons.

DISCUSSION

Our results revealed two opposing roles of GABA in SWD generation: suppression and enhancement of SWD. To account for these contradictory roles, we propose a model in which GABA produced by GAD1 in the RTN neuronal somata is released extrasynaptically and mediates intra-RTN inhibition.