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

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.

Robust network oscillations during mammalian respiratory rhythm generation driven by synaptic dynamics

“…Finally, the second part of the SD, characterized by the entry of neurons in a refractory period to let synapses recover to equilibrium, prevents neurons from firing back immediately. This behavior is in agreement with previous studies (34,48). The next cycle starts after the neuronal refractory periods end and when noise-driven spontaneous spiking occurs at several neurons.…”

“…The two-pool model where vesicles can either be in the RRP or RP compartments resolves this difficulty: the arrival of a presynaptic AP triggers fusion of vesicles from the RRP, and the postsynaptic current is proportional to the amount of fused vesicles. Bursting termination in the preBötC is still unknown and might be due to several different processes such as synaptic depression, voltage-dependent/ion-activated outward currents (48), or, as postulated in previous models, due to the deactivation of inward currents (8), or a combination. In our model, burst termination is based on short-term SD, where the ready-to-fuse vesicles are lacking.…”

Rhythmic Bursting in the Pre-Bötzinger Complex

“…This part of our study was motivated by several recent experimental and modeling studies (Crowder et al, 2007; Dunmyre et al, 2011; Krey et al, 2010; Pace and Del Negro, 2008; Pace et al, 2007; Toporikova and Butera, 2011) and focused on simulation of an I CAN -dependent bursting mechanism (see model details and descriptions in Jasinski et al, 2013). In the two models considered, I CAN was activated by intracellular Ca 2+ accumulation ([Ca 2+ ] in ) whose accumulation was provided by the inositol triphosphate (IP 3 )-dependent Ca 2+ release from intracellular stores.This process in our models critically depended on Ca 2+ influx through voltage-gated calcium current ( I Ca ) which provided an initial [Ca 2+ ] in accumulation, which then induced a nonlinear positive feedback mechanism known as Ca 2+ -induced Ca 2+ release (CICR).…”

“…Several proposals have been made concerning other potential burst-terminating mechanisms, including mechanisms based on (a) slowly activating voltage-dependent potassium current (e.g., Butera et al, 1999a, Model 2) or Ca 2+ -activated potassium current (suggesting [Ca 2+ ] in accumulation during bursts via high voltage-activated calcium currents, e.g., Bevan and Wilson, 1999; El Manira et al, 1994; Ryczko et al, 2010), (b) Na + -activated potassium currents (e.g., Krey et al, 2010; Wallen et al, 2007; Yuan et al, 2003), and (c) activation of the Na + /K + electrogenic pump (e.g., Ballerini et al, 1997; Darbon et al, 2003; Del Negro et al, 2009; Krey et al, 2010). The two latter mechanisms suggest an important role of [Na + ] in accumulation during bursts.…”

“…Furthermore, rhythmic activity in the Cd 2+ -sensitive bursters could be blocked by flufenamic acid (FFA), a pharmacological blocker of the calcium-activated nonspecific cation current ( I CAN ) (Del Negro et al, 2005), suggesting that both I Ca and I CAN are involved in bursting generated in the pre-BötC. Further studies of the possible role of I CAN and the metabotropic mechanisms involved in its activation (Beltran-Parrazal et al, 2012; Ben-Mabrouk et al, 2012; Krey et al, 2010; Pace and Del Negro, 2008; Pace et al, 2007; Rubin et al, 2009) produced inconsistent results. As a result, the involvement and specific roles of these and other possible sodium and calcium mechanisms in the bursting activity observed in the pre-BötC remain unresolved and require further investigation.…”