Regulation of epileptiform activity by two distinct subtypes of extrasynaptic GABAA receptors

Abstract

BackgroundGABAergic deficit is one of the major mechanisms underlying epileptic seizures. Previous studies have mainly focused on alterations of synaptic GABAergic inhibition during epileptogenesis. Recent work suggested that tonic inhibition may also play a role in regulating epileptogenesis, but the underlying mechanism is not well understood.ResultsWe employed molecular and pharmacological tools to investigate the role of tonic inhibition during epileptogenesis both in vitro and in vivo. We overexpressed two distinct subtypes of extrasynaptic GABAA receptors, α5β3γ2 and α6β3δ receptors, in cultured hippocampal neurons. We demonstrated that overexpression of both α5β3γ2 and α6β3δ receptors enhanced tonic inhibition and reduced epileptiform activity in vitro. We then showed that injection of THIP (5 μM), a selective agonist for extrasynaptic GABAA receptors at low concentration, into rat brain also suppressed epileptiform burst activity and behavioral seizures in vivo. Mechanistically, we discovered that low concentration of THIP had no effect on GABAergic synaptic transmission and did not affect the basal level of action potentials, but significantly inhibited high frequency neuronal activity induced by epileptogenic agents.ConclusionsOur studies suggest that extrasynaptic GABAA receptors play an important role in controlling hyperexcitatory activity, such as that during epileptogenesis, but a less prominent role in modulating a low level of basal activity. We propose that tonic inhibition may play a greater role under pathological conditions than in physiological conditions in terms of modulating neural network activity.

Activation of extrasynaptic GABAA receptors inhibits cyclothiazide-induced epileptiform activity in hippocampal CA1 neurons

“…Extrasynaptic GABA A Rs have been suggested to exert a regulatory function during epileptogenesis and could be novel antiepileptic drug targets [13] . Hence, we went on to …”

“…Extrasynaptic GABA A Rs are not sensitive to most of the benzodiazepines; however, they are highly sensitive to 4,5,6,7-tetra-hydroisoxazolo [5,4-c] pyridine-3-ol (THIP) [5,9] . The role of synaptic GABAR-mediated phasic inhibition in epileptogenesis has been well investigated [10,11] , yet recent research efforts have also revealed that extrasynaptic GABA A R-mediated tonic inhibition plays an equivalent or even more critical role in the regulation of epilepsy [12,13] . Moreover, clinical studies have shown that there is a significant down-regulation of δ-subunitcontaining GABA A Rs in brain samples from patients with temporal lobe epilepsy [14] , and that mutation of the δ-subunit is one of the pathogenic mechanisms of epilepsy [15] .…”

“…Furthermore, the injection of the selective extrasynaptic GABA A R agonist THIP inhibits both epileptiform burst activity in anesthetized rats and seizure behavior in freelymoving rats [12,13] .…”

Homeostatic Competition between Phasic and Tonic Inhibition

“…significantly increased the tonic current in cultured hippocampal neurons (41). Interestingly, we found that after overexpressing ␣5␤3␥2 receptors, the frequency of mIPSCs was reduced significantly compared with non-transfected controls (Fig.…”

“…The overexpression of the ␣5 subunit may displace some ␣1 subunit and, hence, decrease synaptic GABAergic inhibition. Our recent work demonstrated that overexpression of both the ␣6␤3␦ and ␣5␤3␥2 receptors significantly inhibited the formation of epileptiform activity in hippocampal neurons (41). This work suggests that synaptic GABA inhibition is actually reduced after enhancing tonic inhibition.…”