Iberiotoxin-sensitive and -insensitive BK currents in Purkinje neuron somata

Abstract

Purkinje cells have specialized intrinsic ionic conductances that generate high-frequency action potentials. Disruptions of their Ca or Ca-activated K (KCa) currents correlate with altered firing patterns in vitro and impaired motor behavior in vivo. To examine the properties of somatic KCa currents, we recorded voltage-clamped KCa currents in Purkinje cell bodies isolated from postnatal day 17–21 mouse cerebellum. Currents were evoked by endogenous Ca influx with approximately physiological Ca buffering. Purkinje somata expressed voltage-activated, Cd-sensitive KCa currents with iberiotoxin (IBTX)-sensitive (>100 nS) and IBTX-insensitive (>75 nS) components. IBTX-sensitive currents activated and partially inactivated within milliseconds. Rapid, incomplete macroscopic inactivation was also evident during 50- or 100-Hz trains of 1-ms depolarizations. In contrast, IBTX-insensitive currents activated more slowly and did not inactivate. These currents were insensitive to the small- and intermediate-conductance KCa channel blockers apamin, scyllatoxin, UCL1684, bicuculline methiodide, and TRAM-34, but were largely blocked by 1 mM tetraethylammonium. The underlying channels had single-channel conductances of ∼150 pS, suggesting that the currents are carried by IBTX-resistant (β4-containing) large-conductance KCa (BK) channels. IBTX-insensitive currents were nevertheless increased by small-conductance KCa channel agonists EBIO, chlorzoxazone, and CyPPA. During trains of brief depolarizations, IBTX-insensitive currents flowed during interstep intervals, and the accumulation of interstep outward current was enhanced by EBIO. In current clamp, EBIO slowed spiking, especially during depolarizing current injections. The two components of BK current in Purkinje somata likely contribute differently to spike repolarization and firing rate. Moreover, augmentation of BK current may partially underlie the action of EBIO and chlorzoxazone to alleviate disrupted Purkinje cell firing associated with genetic ataxias.

BK Channels in the Central Nervous System

“…BK channels showing the characteristic properties of β4-containing channels have been observed in a variety of neuronal populations, including neurohypophysial terminals, dentate gyrus granule cells, cerebellar Purkinje cells, and trigeminal caudal nuclei (TCN) terminals (Benton, Lewis, Bant, & Raman, 2013; Bielefeldt, Rotter, & Jackson, 1992; Brenner et al, 2005; Deng et al, 2013; Dopico, Widmer, Wang, Lemos, & Treistman, 1999; Samengo, Curro, Barrese, Taglialatela, & Martire, 2014; Shruti et al, 2012). It is somewhat puzzling that a large fraction of CNS BK channels is sensitive to iberiotoxin despite the prominent expression of β4 in this tissue.…”

Targeting potassium channels to treat cerebellar ataxia

“…The EGTA concentration was chosen based on previous studies which indicate that 0.5-1.0 mmol/L EGTA maintains Purkinje neuron calcium-activated potassium channel function similar to endogenous calcium buffering. [1][2][3][26][27][28][29][30] In order to block potassium channels in some dendritic excitability experiments, pipettes were filled with an internal recording solution containing the following (in mmol/L): 140 CsCl, 2 MgCl 2 , 1 CaCl 2 , 10 EGTA, 10 HEPES, 4 Na 2 ATP, pH 7.3, osmolarity 287 mOsm. experiments.…”

Knockout of the BK β4-subunit promotes a functional coupling of BK channels and ryanodine receptors that mediate a fAHP-induced increase in excitability

“…This results in so-called frequency-dependent spike broadening that is seen in CA1 (Shao et al 1999) and lateral amygdala neurons (Faber and Sah 2003). Neuronal ␤ 4 -subunits also modulate BK channel function by slowing channel opening and closing, which reduces BK channel contribution to AP repolarization in dentate gyrus (DG) granule neurons (Brenner et al 2005) and CA3 neurons (Deng et al 2013), but may sustain BK channel activity during interspike times of cerebellar Purkinje neurons (Benton et al 2013).…”