Abstracts

Rationale: In the central nervous system, principal neurons are primarily regulated by local inhibitory interneurons through GABAergic transmission onto GABAA receptors (GABAARs). Two forms of GABAergic inhibition exist: Phasic inhibition provides a transient hyperpolarizing current through low-affinity GABAARs at the synapse, while tonic inhibition refers to a sustained conductance mediated by high-affinity extrasynaptic GABAARs, often those with a subunit. Although the role of GABAergic interneurons in constraining principal neuron excitability has been studied extensively, much less is known about how these interneurons are regulated themselves. We hypothesized that tonic GABAergic inhibition in interneurons plays a key role in regulating their activity in physiological and pathological conditions, and examined its impact on network plasticity and seizure susceptibility.Methods: To investigate the role of tonic inhibition in regulating interneurons, we generated a novel mouse model in which the GABAAR subunit was selectively removed from GABAergic interneurons (Gabrd/Gad mice). We examined network plasticity in Gabrd/Gad mice using electrophysiology. Hippocampal field recordings were performed in coronal slices in an interface chamber perfused with oxygenated aCSF at 32-34 C. Field excitatory postsynaptic potentials (fEPSPs) were measured from the stratum radiatum of CA1 in response to stimulation of Schaffer collaterals at 50% of maximum intensity. Long-term potentiation (LTP) was induced by theta burst stimulation. The slopes of fEPSPs an hour after induction were compared to baseline. We examined seizure susceptibility in Gabrd/Gad mice using the kainic acid model of temporal lobe epilepsy. EEGs were recorded for 2 h following an intraperitoneal injection of kainic acid (20 mg/kg). Epileptiform activity (including ictal events and other abnormal periods of rhythmic spiking) was defined as high amplitude activity (2-3 times the standard deviation of the baseline) lasting longer than 5 s in duration, with consistent changes in power and frequency as analyzed with fast Fourier transform.Results: Loss of tonic GABAergic inhibition in interneurons led to protection from seizures induced with kainic acid. Both heterozygous and homozygous Gabrd/Gad mice exhibited less epileptiform activity than littermate controls during the 2 h following kainic acid administration. Episodes of epileptiform activity in Gabrd/Gad mice were shorter and less intense than those in control mice. However, the disinhibition of interneurons also induced a deficit in LTP in slices from Gabrd/Gad mice.Conclusions: These data demonstrate the importance of tonic GABAergic inhibition in interneurons in physiological and pathological states. Targeting extrasynaptic GABAA R subtypes to influence tonic inhibition can be a potential therapeutic strategy for diseases such as epilepsy, but care must be taken as similar GABAA Rs exist on both principal neurons and interneurons.