Status epilepticus enhances tonic GABA currents and depolarizes GABA reversal potential in dentate fast-spiking basket cells

Abstract

Temporal lobe epilepsy is associated with loss of interneurons and inhibitory dysfunction in the dentate gyrus. While status epilepticus (SE) leads to changes in granule cell inhibition, whether dentate basket cells critical for regulating granule cell feedforward and feedback inhibition express tonic GABA currents (I(GABA)) and undergo changes in inhibition after SE is not known. We find that interneurons immunoreactive for parvalbumin in the hilar-subgranular region express GABAA receptor (GABA(A)R) δ-subunits, which are known to underlie tonic I(GABA). Dentate fast-spiking basket cells (FS-BCs) demonstrate baseline tonic I(GABA) blocked by GABA(A)R antagonists. In morphologically and physiologically identified FS-BCs, tonic I(GABA) is enhanced 1 wk after pilocarpine-induced SE, despite simultaneous reduction in spontaneous inhibitory postsynaptic current (sIPSC) frequency. Amplitude of tonic I(GABA) in control and post-SE FS-BCs is enhanced by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), demonstrating the contribution of GABA(A)R δ-subunits. Whereas FS-BC resting membrane potential is unchanged after SE, perforated-patch recordings from FS-BCs show that the reversal potential for GABA currents (E(GABA)) is depolarized after SE. In model FS-BCs, increasing tonic GABA conductance decreased excitability when E(GABA) was shunting and increased excitability when E(GABA) was depolarizing. Although simulated focal afferent activation evoked seizurelike activity in model dentate networks with FS-BC tonic GABA conductance and shunting E(GABA), excitability of identical networks with depolarizing FS-BC E(GABA) showed lower activity levels. Thus, together, post-SE changes in tonic I(GABA) and E(GABA) maintain homeostasis of FS-BC activity and limit increases in dentate excitability. These findings have implications for normal FS-BC function and can inform studies examining comorbidities and therapeutics following SE.

Functional Reduction in Cannabinoid-Sensitive Heterotypic Inhibition of Dentate Basket Cells in Epilepsy: Impact on Network Rhythms

“…Post hoc biocytin immunostaining and morphological analysis were used to definitively identify AC-INs based on dendrites in the hilus and molecular layer and axonal projections either in the inner molecular layer (HICAP) or spanning the molecular layer (TML) Hosp et al 2014). FS-BCs had axon terminals in the granule cell layer (Hefft and Jonas 2005;Yu et al 2013). Both pre-and postsynaptic cells were identified based on physiological characteristics followed by post hoc confirmation based on either morphological or neurochemical identity or both.…”

Fluid percussion injury device for the precise control of injury parameters

“…1-week post injury, adolescent rats were anesthetized with 1-3% isoflurane, decapitated, brains removed, and cooled to 4 • C in oxygenated sucrose-aCSF (artificial cerebrospinal fluid with 95% O 2 and 5% CO 2 at pH 7.4) (Gupta et al, 2012;Yu et al, 2013). Brains were cut into 400 m horizontal brain slices using a vibratome (Leica, VT1200S), sagittally bisected into two hemispherical components (ipsilateral and contralateral to the injury site) and held at room temperature in oxygenated aCSF.…”

“…Brains were cut into 400 m horizontal brain slices using a vibratome (Leica, VT1200S), sagittally bisected into two hemispherical components (ipsilateral and contralateral to the injury site) and held at room temperature in oxygenated aCSF. Individual slices were transferred to an interfaced recording chamber (BSC2, Automatic Scientific), and perfused with oxygenated recording-aCSF (Gupta et al, 2012;Yu et al, 2013).…”

“…Field recordings were performed in the granule cell layer of the dentate gyrus by using patch pipettes (4 M , KG-33 borosilicate glass capillary with 1.5 mm outer diameter from Harvard Apparatus, pulled using a Sutter P-1000 horizontal micropipette puller) filled with recording-aCSF as in earlier studies (Gupta et al, 2012;Yu et al, 2013;Neuberger et al, 2014). Responses were evoked by constant-current stimulation of the perforant path fibers (4 mA, 50 s) delivered at 0.1 Hz through custom made, parallel bipolar 90-m tungsten stimulating electrode placed in the junction of the dorsal blade and the crest as previously described (Gupta et al, 2012;Yu et al, 2013;Neuberger et al, 2014).…”

“…Responses were evoked by constant-current stimulation of the perforant path fibers (4 mA, 50 s) delivered at 0.1 Hz through custom made, parallel bipolar 90-m tungsten stimulating electrode placed in the junction of the dorsal blade and the crest as previously described (Gupta et al, 2012;Yu et al, 2013;Neuberger et al, 2014). Recordings were obtained from granule cell layer of the dentate gyrus with an AxoPatch 200B amplifier and digitized at 10 kHz with DigiData 1440A (Molecular Devices, Sunnyvale, CA).…”