Outward Currents Contributing to Inspiratory Burst Termination in preBötzinger Complex Neurons of Neonatal Mice Studied in Vitro

Thomas B. Arnold,Christopher A. Del Negro,Adam M. Goodreau,Rebecca A. Krey

doi:10.3389/fncir.2010.00124

Abstract

We studied preBötzinger Complex (preBötC) inspiratory interneurons to determine the cellular mechanisms that influence burst termination in a mammalian central pattern generator. Neonatal mouse slice preparations that retain preBötC neurons generate respiratory motor rhythms in vitro. Inspiratory-related bursts rely on inward currents that flux Na+, thus outward currents coupled to Na+ accumulation are logical candidates for assisting in, or causing, burst termination. We examined Na+/K+ ATPase electrogenic pump current (Ipump), Na+-dependent K+ current (IK–Na), and ATP-dependent K+ current (IK–ATP). The pharmacological blockade of Ipump, IK–Na, or IK–ATP caused pathological depolarization akin to a burst that cannot terminate, which impeded respiratory rhythm generation and reversibly stopped motor output. By simulating inspiratory bursts with current-step commands in synaptically isolated preBötC neurons, we determined that each current generates approximately 3–8 mV of transient post-burst hyperpolarization that decays in 50–1600 ms. Ipump, IK–Na, and – to a lesser extent – IK–ATP contribute to terminating inspiratory bursts in the context of respiratory rhythm generation by responding to activity dependent cues such as Na+ accumulation.

Highlights

Breathing in mammals depends on inspiratory rhythms that originate in the preBötzinger Complex of the ventral medulla (Tan et al, 2008; Bouvier et al, 2010; Gray et al, 2010)
XII output stopped in four preparations after 8.3 ± 6.0 min of continuous exposure, at which time the recorded preBötzinger Complex (preBötC) neuron was quiescent at a depolarized state, without rhythmic drive
7 min 15 min Discussion Post-synaptic mechanisms that contribute to inspiratory burst termination include Ipump, IK–Na, and IK–ATP, outward currents evoked by the accumulation of Na+ or the depletion of ATP due to Na+ pumping

Summary

Introduction

Breathing in mammals depends on inspiratory rhythms that originate in the preBötzinger Complex (preBötC) of the ventral medulla (Tan et al, 2008; Bouvier et al, 2010; Gray et al, 2010). INaP and ICAN flux Na+ during inspiratory bursts, so burst termination may depend on activity-dependent outward currents linked to Na+ accumulation, as well as the related depletion of ATP due to Na+ pumping. Na/K ATPase electrogenic pump current (Ipump) has been linked to burst termination in spinal locomotor and oral-motor central pattern generator (CPG) networks (Ballerini et al, 1997; Del Negro et al, 1999; Darbon et al, 2003) and midbrain dopamine neurons (Johnson et al, 1992). In addition to generating net outward current, Na/K ATPase pumps consume ATP, which evokes ATP-sensitive K+ current (IK–ATP) in respiratory neurons (Pierrefiche et al, 1996; Mironov et al, 1998; Haller et al, 2001), and could influence burst termination. Depending on subunit composition, IK–Na could act in conjunction with both Ipump and IK–ATP to assist in burst termination

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