Initial segment Kv2.2 channels mediate a slow delayed rectifier and maintain high frequency action potential firing in medial nucleus of the trapezoid body neurons

Abstract

The medial nucleus of the trapezoid body (MNTB) is specialized for high frequency firing by expression of Kv3 channels, which minimize action potential (AP) duration, and Kv1 channels, which suppress multiple AP firing, during each calyceal giant EPSC. However, the outward K + current in MNTB neurons is dominated by another unidentified delayed rectifier. It has slow kinetics and a peak conductance of ∼37 nS; it is half-activated at −9.2 ± 2.1 mV and half-inactivated at −35.9 ± 1.5 mV. It is blocked by several non-specific potassium channel antagonists including quinine (100 μm) and high concentrations of extracellular tetraethylammonium (TEA; IC 50 = 11.8 mm), but no specific antagonists were found. These characteristics are similar to recombinant Kv2-mediated currents. Quantitative RT-PCR showed that Kv2.2 mRNA was much more prevalent than Kv2.1 in the MNTB. A Kv2.2 antibody showed specific staining and Western blots confirmed that it recognized a protein ∼110 kDa which was absent in brainstem tissue from a Kv2.2 knockout mouse. Confocal imaging showed that Kv2.2 was highly expressed in axon initial segments of MNTB neurons. In the absence of a specific antagonist, Hodgkin-Huxley modelling of voltage-gated conductances showed that Kv2.2 has a minor role during single APs (due to its slow activation) but assists recovery of voltage-gated sodium channels (Nav) from inactivation by hyperpolarizing interspike potentials during repetitive AP firing. Current-clamp recordings during high frequency firing and characterization of Nav inactivation confirmed this hypothesis. We conclude that Kv2.2-containing channels have a distinctive initial segment location and crucial function in maintaining AP amplitude by regulating the interspike potential during high frequency firing. Potassium currents have multiple and diverse roles in shaping electrical signalling, with different suites of voltage-gated and rectifying non-gated channels setting neuronal membrane potentials, firing threshold, action potential waveform and firing patterns. Identification of the channel family and subunits contributing to these functions in native neurons is complicated by their heterogeneous subunit composition, their particular functional localization to the plasma membrane and by the absence of specific antagonists for some families. For these reasons the full complement of the K + channels and subunits underlying native K + currents are still not known for any identified central neuron. We have focused on studying the potassium currents of a 'simple' neuron This paper has online supplemental material.within the medial nucleus of the trapezoid body (MNTB), which serves as a relay in the binaural circuits involved in sound source localization. These neurons receive the glutamatergic calyx of Held giant synapse, which reliably triggers postsynaptic APs with large, well-timed EPSCs which have a magnitude of around 30 times firing threshold. In vivo recordings show the calyceal input fires spontaneously at frequencies ranging between 0 and 100 Hz ...

Kv2 Channel Regulation of Action Potential Repolarization and Firing Patterns in Superior Cervical Ganglion Neurons and Hippocampal CA1 Pyramidal Neurons

“…Such inactivation of potassium currents is consistent with the substantially broader spikes evoked from Ϫ70 mV (1.02 Ϯ 0.06 ms) compared with spikes evoked from Ϫ80 mV (0.83 Ϯ 0.04 ms, n ϭ 7; p ϭ 0.04). This effect is consistent with previous evidence for a major role of I A in repolarization of action potentials in CA1 pyramidal neurons (Kim et al, 2005;Andrásfalvy et al, 2008) and it is also possible that steady-state inactivation of other channel types such as Kv1 family (D-type) or Kv3 family channels could be involved. These results show that the relative role of Kv2 channels in early action potential repolarization in CA1 pyramidal neurons depends on resting potential, with a significant role in early repolarization for action potentials elicited from Ϫ70 mV.…”

“…We believe the GxTX-resistant current likely contains multiple components, because the speed and degree of inactivation varied considerably from cell to cell. Based on previous analysis of potassium currents in hippocampal and cortical pyramidal neurons, the GxTXresistant current includes components from Kv4 and Kv1 family subunits (Kim et al, 2005;Chen et al, 2006;Guan et al, 2007aGuan et al, , 2007bGuan et al, , 2011Carrasquillo et al, 2012;Kim and Hoffman, 2012). We found that some of the GxTX-resistant current could be blocked by 1 mM TEA, but this block was not selective because 1 mM TEA also clearly partially inhibited the slowly-activating slowly-deactivating GxTX-sensitive component of current.…”

“…Knock-down of Kv2 with antisense or dominant-negative expression has suggested that Kv2 channels are activated only minimally during single action potentials in cortical or CA1 pyramidal neurons (Du et al, 2000;Guan et al, 2013), but can contribute significantly to repolarization of wider action potentials in sympathetic neurons (Malin and Nerbonne, 2002). In both CA1 and sympathetic neurons, loss of Kv2 disrupts maintained repetitive or tonic firing (Du et al, 2000;Malin and Nerbonne, 2002;Guan et al, 2013), an effect seen also with computer modeling of neuron firing in the medial nucleus of the trapezoid body (Johnston et al, , 2010. The function of Kv2 channels is of particular interest because of highly plastic expression (Misonou et al, 2005;Nataraj et al, 2010;Fox et al, 2013) and strong modulation by multiple pathways (Mohapatra et al, 2007;Cerda and Trimmer, 2011;Plant et al, 2011;Steinert et al, 2011).…”

“…Mammalian neurons express multiple types of voltage-gated potassium channels that activate with different voltage dependence and kinetics and together enable a wide range of firing patterns (Hille, 2001;Trimmer and Rhodes, 2004;Vacher et al, 2008;Johnston et al, 2010). Among the most widely expressed potassium channels in neurons of the mammalian brain are Kv2 family channels (Trimmer, 1991;Hwang et al, 1993;Scannevin et al, 1996;Du et al, 1998Du et al, , 2000Guan et al, 2007).…”

Maturation of NaVand KVChannel Topographies in the Auditory Nerve Spike Initiator before and after Developmental Onset of Hearing Function

“…Whereas K V 3 channels turn on and off with the rising and falling phases of the action potential, the slower K V 2 channels may hyperpolarize the interspike potential and thereby expedite recovery of Na V channels from inactivation. Such a mechanism was suggested for MNTB neurons, in which K V 2.2 channels were enriched on one part of the AIS (Johnston et al, 2008). The role of K V 2.2 channels awaits investigation in ANFs, in which they are positioned along internodes (Fig.…”