Network synchronization and synchrony propagation: emergent elements of inspiration

Abstract

While now well established that the preBötzinger Complex (preBötC) 1 is the kernel of breathing rhythmogenesis in mammals, the underlying mechanisms remain a mystery 2 . Critically, two long-favored hypotheses for rhythmogenesis, i.e., that the rhythm is generated by pacemaker neurons or by simple circuits dependent on inhibition including post-inhibitory rebound, are not supported by experimental tests 2-8 . Here, we explore an alternative (though non-exclusive) hypothesis, that the rhythm is an emergent property of the preBötC microcircuit 2,6,9,10 . We assessed preBötC network dynamics by recording synaptic inputs to preBötC neurons under respiratory rhythmic and non-rhythmic conditions in in vitro slices from neonatal mouse. Our analyses uncover a dynamic reorganization of preBötC network activity correlated with and, we hypothesize, essential to, rhythmicity. In each cycle under rhythmic conditions, an inspiratory burst (I-burst) emerges as (presumptive) preBötC rhythmogenic neurons transition away from aperiodic uncorrelated population spike activity to become increasingly synchronized during preinspiration; this burst activity subsides and the cycle repeats. Strikingly, in a slice in nonrhythmic conditions, antagonizing GABA A receptors can initiate this periodic synchronization and consequent rhythm, while simultaneously inducing a high conductance (HC) state in preBötC nonrhythmogenic output neurons. This co-emergence of input synchrony and HC state in these output neurons unveils a novel network mechanism for generation of the rhythm that emerges in preBötC and that ultimately propagates to inspiratory motoneurons, effecting inspiratory muscle contraction and consequential breathing movements. In the light of divergent projections of preBötC output neurons through mono-and oligo-synaptic connections 11 , synchronized oscillations in inspiratory motoneurons 12,13 and entrainment of limbic neurons by respiratory corollary discharge 14 , we hypothesize that the evolution and propagation of preBötC synchrony plays a critical role in the extraordinary reliability and robustness of networks underlying respiratory motor output as well as in the use of breathing rhythms for binding of activity with and across higher brain regions.

Opioids modulate an emergent rhythmogenic process to depress breathing

“…We put forth a hypothesis that respiratory rhythmogenesis is driven not by strong bursts, but by an emergent process that manifests as preBötC burstlets (Kam et al, 2013a). In contrast to pacemaker-driven networks (Phillips et al, 2019), where rhythm depends on the kinetics of conductances, such as I h or persistent inward currents (Marder and Calabrese, 1996), in this model, the period is determined by the gradual synchronization of spontaneously active, recurrently connected excitatory preBötC neurons (Ashhad and Feldman, 2019; Feldman and Kam, 2015). Each cycle consists of the generation, propagation, and integration of spontaneous activity within the preBötC and culminates in the generation of a burstlet (Ashhad and Feldman, 2019; Feldman and Kam, 2015).…”

“…Because preBötC rhythmic activity does not rely on intrinsic ‘pacemaker’ conductances or inhibitory synaptic transmission and can consist solely of burstlets that do not produce motoneuronal output, we hypothesize that inspiratory rhythmogenesis is an emergent process, distinct from burst generation, that manifests as burstlets (Feldman and Kam, 2015; Kam et al, 2013a). In this model, burstlets occur when spontaneous firing among preBötC neurons reaches a threshold for synchrony, and the time required for preBötC neurons to assemble and achieve this synchrony (percolation) in each cycle is an essential determinant of the period of the rhythm (Ashhad and Feldman, 2019; Feldman and Kam, 2015; Kam et al, 2013a). A strong prediction of this hypothesis is that manipulations that slow breathing act specifically on this emergent rhythmogenic mechanism.…”

Evaluating the Burstlet Theory of Inspiratory Rhythm and Pattern Generation

“…It may have to do with a quorum: a certain number of rhythmogenic interneurons must be active (i.e., spiking) to trigger an irreversible cascade that ostensibly activates all (or nearly all) preBötC neurons. Or, it may have to do with synchrony: a certain number of rhythmogenic interneurons must be spiking in sync to trigger that cascade (Ashhad and Feldman, 2019). The former focuses on mass action of constituent interneurons wherein temporal precision is inconsequential.…”

Evaluating the burstlet theory of inspiratory rhythm and pattern generation

“…The second scenario focuses on phasic precision rather than mass action. Our present data cannot distinguish which mechanism is at work but Ashhad and Feldman argue for the preeminent importance of synchrony in burst generation (Ashhad and Feldman 2019). Given that burstlet amplitude is voltage-dependent and the number of constituent neurons participating in burstlets may vary (~100s) at any given level of excitability, the quorum model 390 seems less feasible.…”

“…5B). preBötC field recordings and extracellular unit recordings were shown previously (Ashhad and Feldman 2019;Kam, Worrell, Janczewski, et al 2013), but these are the first intracellular recordings to 265…”