Sodium and calcium current-mediated pacemaker neurons and respiratory rhythm generation

Department of Applied Science, College of William and Mary, Williamsburg, Virginia, United States
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 02/2005; 25(2):446-53. DOI: 10.1523/JNEUROSCI.2237-04.2005
Source: PubMed


The breathing motor pattern in mammals originates in brainstem networks. Whether pacemaker neurons play an obligatory role remains a key unanswered question. We performed whole-cell recordings in the preBotzinger Complex in slice preparations from neonatal rodents and tested for pacemaker activity. We observed persistent Na+ current (I(NaP))-mediated bursting in approximately 5% of inspiratory neurons in postnatal day 0 (P0)-P5 and in P8-P10 slices. I(NaP)-mediated bursting was voltage dependent and blocked by 20 mum riluzole (RIL). We found Ca2+ current (I(Ca))-dependent bursting in 7.5% of inspiratory neurons in P8-P10 slices, but in P0-P5 slices these cells were exceedingly rare (0.6%). This bursting was voltage independent and blocked by 100 microm Cd2+ or flufenamic acid (FFA) (10-200 microm), which suggests that a Ca2+-activated inward cationic current (I(CAN)) underlies burst generation. These data substantiate our observation that P0-P5 slices exposed to RIL contain few (if any) pacemaker neurons, yet maintain respiratory rhythm. We also show that 20 nm TTX or coapplication of 20 microm RIL + FFA (100-200 microm) stops the respiratory rhythm, but that adding 2 mum substance P restarts it. We conclude that I(NaP) and I(CAN) enhance neuronal excitability and promote rhythmogenesis, even if their magnitude is insufficient to support bursting-pacemaker activity in individual neurons. When I(NaP) and I(CAN) are removed pharmacologically, the rhythm can be maintained by boosting neural excitability, which is inconsistent with a pacemaker-essential mechanism of respiratory rhythmogenesis by the preBotzinger complex.

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Available from: Consuelo Morgado-Valle, Apr 13, 2015
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    • "The inspiratory phase of the respiratory cycle in vitro results from preBötC neurons firing a synchronous burst of action potentials (APs) on top of a 10-to 20-mV, 0.3-to 0.8-s depolarization dubbed inspiratory drive potential, which results mainly from AMPA receptor (AMPAR)-mediated postsynaptic currents (Funk et al. 1993; Greer et al. 1991; Morgado-Valle and Feldman 2007). Although preBötC neurons express pacemaking-promoting currents such as persistent Na ϩ current (I NaP ) and Ca 2ϩ -activated nonselective cation current (I CAN ) (Del Negro et al. 2002; Pena and Ramirez 2004), Ͻ10% are intrinsic pacemaker neurons (Del Negro et al. 2005) and the vast majority are nonpacemaker neurons requiring excitatory synaptic input to burst rhythmically. The existence of multiple oscillatory regimes and the state dependence of pacemaker neurons in the preBötC suggest that further studies are needed to understand their role in respiratory rhythm generation (Rybak et al. 2014). "
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    ABSTRACT: The preBötC underlies inspiratory rhythm generation. As a result of network interactions, preBötC neurons burst synchronously to produce rhythmic premotor inspiratory activity. Each inspiratory burst consists of action potentials (AP) on top of a 10-20 mV synchronous depolarization lasting 0.3-0.8 s known as inspiratory drive potential. The mechanisms underlying the initiation and termination of the inspiratory burst are unclear, and the role of Ca(2+) is a matter of intense debate. In order to investigate the role of extracellular Ca(2+) in inspiratory burst initiation and termination, we substituted the extracellular Ca(2+) by Sr(2+). We found for the first time an ionic manipulation that significantly interferes with burst termination. In a rhythmically active slice, we current-clamped preBötC neurons (Vm≅-60 mV) while recording integrated hypoglossal nerve (∫XIIn) activity as motor output. Substitution of extracellular Ca(2+) by either 1.5 or 2.5 mM Sr(2+) significantly prolonged the duration of inspiratory bursts from 653.4±30.7 ms in control conditions to 981.6±78.5 ms in 1.5 mM Sr(2+) and 2048.2±448.5 ms in 2.5 mM Sr(2+), with a concomitant increase in decay time and area. Substitution of extracellular Ca(2+) by Sr(2+) is a well-established method to desynchronize neurotransmitter release. Our findings suggest that the increase in inspiratory burst duration is determined by a presynaptic mechanism involving desynchronization of glutamate release within the network. Copyright © 2014, Journal of Neurophysiology.
    Full-text · Article · Nov 2014 · Journal of Neurophysiology
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    • "Respiratory neurons with intrinsic bursting properties were identified and incorporated into mechanistic computational models for respiratory rhythm generation in neonatal rodents. Experimental evidence suggested that intrinsically bursting respiratory neurons were not necessary for rhythm generation (Del Negro et al., 2005). Since experimental methods do not currently exist for specifically inactivating the specific ionic currents underlying intrinsic bursting properties in respiratory neurons, attention was redirected toward alternative mechanisms underlying rhythmogenesis. "
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    ABSTRACT: It is not known whether respiratory neurons with intrinsic bursting properties exist within ectothermic vertebrate respiratory control systems. Thus, isolated adult turtle brainstems spontaneously producing respiratory motor output were used to identify and classify respiratory neurons based on their firing pattern relative to hypoglossal (XII) nerve activity. Most respiratory neurons (183/212) had peak activity during the expiratory phase, while inspiratory, post-inspiratory, and novel pre-expiratory neurons were less common. During synaptic blockade conditions, ∼10% of respiratory neurons fired bursts of action potentials, with post-inspiratory cells (6/9) having the highest percentage of intrinsic burst properties. Most intrinsically bursting respiratory neurons were clustered at the level of the vagus (X) nerve root. Synaptic inhibition blockade caused seizure-like activity throughout the turtle brainstem, which shows that the turtle respiratory control system is not transformed into a network driven by intrinsically bursting respiratory neurons. We hypothesize that intrinsically bursting respiratory neurons are evolutionarily conserved and represent a potential rhythmogenic mechanism contributing to respiration in adult turtles.
    Full-text · Article · Nov 2014 · Respiratory Physiology & Neurobiology
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    • "heterogeneous group of glutamatertgic neurons expressing, constantly or transiently, DbX1 (Bouvier et al., 2010; Gray et al., 2010; Picardo et al., 2013), Robo3 (Bouvier et al., 2010), NK1R (Gray et al., 1999, 2010; Guyenet et al., 2002), and somatostatin (Gray et al., 1999, 2010; Llona and Eugenín, 2005; Stornetta et al., 2003), constitute an essential component of the preBötC. Several attempts have been made to relate the presence of such molecular markers with morphological and/or electrophysiological features of the preBötC neurons (Bouvier et al., 2008, 2010; Hayes and Del Negro, 2007; Koizumi et al., 2008, 2013; Morgado-Valle et al., 2010; Pagliardini et al., 2005; Picardo et al., 2013). Here, to obtain more information regarding the functional–anatomical characteristics of the neuronal elements that constitute the respiratory network contained in the preBötC, we have characterized the morphology of cells identified as respiratory neurons. "
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    ABSTRACT: Although the pre-Bötzinger complex (preBötC) was defined as the inspiratory rhythm generator long ago, the functional-anatomical characterization of its neuronal components is still being achieved. Recent advances have identified the expression of molecular markers in the preBötC neurons that, however, are not exclusive to specific respiratory neuron subtypes and have not always been related to specific cell morphologies. Here, we evaluated the morphology and the axonal projections of electrophysiologically defined respiratory neurons in the preBötC using whole-cell recordings and intracellular biocytin labeling. We found that respiratory pacemaker neurons are larger than expiratory neurons and that inspiratory neurons are smaller than pacemaker and expiratory neurons. Other morphological features such as somata shapes or dendritic branching patterns were not found to be significantly different among the preBötC neurons sampled. We also found that both pacemaker and inspiratory nonpacemaker neurons, but not expiratory neurons, show extensive axonal projections to the contralateral preBötC and show signs of electrical coupling. Overall, our data suggest that there are morphological differences between subtypes of preBötC respiratory neurons. It will be important to take such differences in consideration since morphological differences would influence synaptic responses and action potential propagation.
    Full-text · Article · Apr 2014 · Progress in brain research
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