Chapter 8 - Synaptically activated burst-generating conductances may underlie a group-pacemaker mechanism for respiratory rhythm generation in mammals

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Abstract

Breathing, chewing, and walking are critical life-sustaining behaviors in mammals that consist essentially of simple rhythmic movements. Breathing movements in particular involve the diaphragm, thorax, and airways but emanate from a network in the lower brain stem. This network can be studied in reduced preparations in vitro and using simplified mathematical models that make testable predictions. An iterative approach that employs both in vitro and in silico models argues against canonical mechanisms for respiratory rhythm in neonatal rodents that involve reciprocal inhibition and pacemaker properties. We present an alternative model in which emergent network properties play a rhythmogenic role. Specifically, we show evidence that synaptically activated burst-generating conductances—which are only available in the context of network activity—engender robust periodic bursts in respiratory neurons. Because the cellular burst-generating mechanism is linked to network synaptic drive we dub this type of system a group pacemaker.

Introduction

Understanding the neural origins of behaviors like walking, running, swimming, chewing, suckling, and breathing will be tenable via the application of a broad spectrum of techniques from biomechanics to molecular genetics. These cross-disciplinary tools will enable us to cohere data from many levels of analysis spanning intact behaving organisms to intrinsic membrane properties and biochemical signaling pathways studied in vitro.

Here we examine breathing behavior in mammals: rhythmic movements of the diaphragm, thorax, and airways to produce ventilation. We seek cellular- and synaptic-level mechanisms that produce the respiratory rhythm. We limit our focus to a specialized region of the lower brain stem (preBötzinger Complex; preBötC), which is essential for breathing in awake intact adult rodents, and is necessary and sufficient for inspiratory motor rhythms in vitro (Feldman and Del Negro, 2006, Rekling and Feldman, 1998, Smith et al., 1991).

To focus on the neural origins of the essential underlying inspiratory rhythm, we necessarily set aside other, equally important, issues related to the neural control of breathing, including—but not limited to—the developmental genetics of preBötC circuits, integrated functions of the preBötC within the larger respiratory network of the lower medulla, formation of an appropriate spatiotemporal motor pattern for ventilatory movements, regulation of the rhythm via neuromodulation, sensorimotor integration, for example, mechanosensitive feedback from the lungs, as well as peripheral and central chemosensation related to oxygen, carbon dioxide, and pH.

Our analysis of rhythm generation in the preBötC addresses the following two linked questions: (1) Which neurons? (2) How? The preBötC is a functionally and anatomically defined site in the lower brain stem containing several thousand neurons in rodents, and we are interested in understanding which ones are rhythmogenic. Here, we consider intrinsic membrane and anatomical properties that may underlie rhythmogenicity.

Several models—conceptual as well as explicitly mathematical—outline two general paradigms for the mechanism underlying respiratory rhythmogenesis: reciprocal inhibition and/or pacemaker neurons. Here we evaluate some straightforward predictions of these models and conclude that the respiratory rhythm cannot be adequately explained using these paradigms. Instead we present an alternative model: group-pacemaker hypothesis (Feldman and Del Negro, 2006, Rekling and Feldman, 1998, Rekling et al., 1996a, Rubin et al., 2009a). In this model, recurrent synaptic excitation boosts and propagates activity like a conventional network oscillator (Grillner, 2006), yet constituent neurons generate spike bursts with an underlying plateau-like depolarization during the active phase, which is behavior typically associated with intrinsic pacemaker properties (Coombes and Bressloff, 2005).

Section snippets

Rhythmic motor behaviors studied in vitro

Central pattern generator (CPG) networks produce neural rhythms for motor behaviors without need of sensory feedback or commands from higher brain centers (Grillner, 2006, Marder, 2001). Invertebrate model systems provide valuable insights into the structure and function of CPGs because these systems comprise a limited number of constituent neurons (~100) that can be identified and, more importantly, selectively recorded in the context of behavior in vitro. Detailed analyses of CPG neurons and

Early models featured a role for chloride-mediated synaptic inhibition

Prior to 1989, the dominant models of respiratory rhythm generation incorporated a critical and obligatory role for synaptic inhibition (Bradley et al., 1975, Feldman and Cowan, 1975). The general structure of these models posited (a minimum of) three interconnected populations of respiratory neurons including inspiratory and expiratory timing circuits, as well as well as a ramp-generator circuit for inspiratory bursts. The interaction of these populations was thought to generate two or three

PreBötC neurons with early-inspiratory activity and robust bursts may be rhythmogenic

If pacemaker properties after synaptic isolation are not necessarily a reliable and accurate phenotype for putative respiratory rhythm-generators, then one should examine how respiratory neurons behave and, what intrinsic properties they utilize in the context of the functioning network, to elucidate the mechanisms of rhythmogenesis. This type of analysis is possible using slice preparations that retain the preBötC in vitro (Fig. 1b and c). An important pioneering work in that regard was the

A new paradigm for respiratory rhythmogenesis based on emergent network properties

In the absence of obligatory pacemaker neurons, one alternative explanation for rhythm generation is based on recurrent synaptic excitation. This concept remained purely hypothetical without the identification of specific mechanisms through which it could be implemented. However, we now recognize that ICAN activation is linked to mGluRs and AMPA receptor-mediated recruitment of Ca2+ channels, which is a viable mechanism for the normally latent ICAN to be evoked synaptically in the lead-up to

Conclusions

CPGs in vertebrates incorporate thousands of highly interconnected neurons, each of which represents a complex dynamical system with many degrees of freedom (Grillner, 2006, Kozlov et al., 2009). To understand CPGs, we must necessarily focus on the essential rhythmogenic modules. Reciprocal inhibition—where active synaptic inhibition acts as a barrier that must be overcome for phase switching to occur—or specialized pacemaker neurons—which provide a rhythmic template for network activity—are

Acknowledgments

The work was supported by NIH HL-40959, NIH NINDS R21, NS070056-01, NIH-NHLBS R01 HL104127-01, and the Undergraduate Biological Sciences Education and Research Training Grant to The College of William & Mary by the Howard Hughes Memorial Institute.

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    Present address: Neurobiologie & Développement, Institut de Neurobiologie Alfred Fessard, Centre National de la Recherche Scientifique, Gif sur Yvette cedex, France

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