Synaptic Depression Influences Inspiratory–Expiratory Phase Transition in Dbx1 Interneurons of the preBötzinger Complex in Neonatal Mice

Abstract

The brainstem preBötzinger complex (preBötC) generates the rhythm underlying inspiratory breathing movements and its core interneurons are derived from Dbx1-expressing precursors. Recurrent synaptic excitation is required to initiate inspiratory bursts, but whether excitatory synaptic mechanisms also contribute to inspiratory-expiratory phase transition is unknown. Here, we examined the role of short-term synaptic depression using a rhythmically active neonatal mouse brainstem slice preparation. We show that afferent axonal projections to Dbx1 preBötC neurons undergo activity-dependent depression and we identify a refractory period (ϳ2 s) after endogenous inspiratory bursts that precludes light-evoked bursts in channelrhodopsin-expressing Dbx1 preBötC neurons. We demonstrate that the duration of the refractory period-but neither the cycle period nor the magnitude of endogenous inspiratory bursts-is sensitive to changes in extracellular Ca 2ϩ . Further, we show that postsynaptic factors are unlikely to explain the refractory period or its modulation by Ca 2ϩ . Our findings are consistent with the hypothesis that short-term synaptic depression in Dbx1 preBötC neurons influences the inspiratory-expiratory phase transition during respiratory rhythmogenesis.

Defining preBötzinger Complex Rhythm- and Pattern-Generating Neural Microcircuits In Vivo

“…In Dbx1-ChR2 mice, bilateral preBötC SPP during expiration excited all (recorded) preBötC preinspiratory neurons. Although in vivo extracellular recording cannot distinguish between neurons directly expressing ChR2 and those postsynaptic to ChR2-expressing neurons, these photoresponsive preinspiratory preBötC neurons in vivo likely correspond to photoresponsive preinspiratory preBötC Dbx1 + neurons that were directly excited in vitro (Kottick and Del Negro, 2015). Additionally, in Dbx1-ChR2 mice, preBötC photostimulation had excitatory effects on inspiratory burst timing as: i) bilateral preBötC SPP during mid- or late-expiration generated ectopic bursts that were phase-shifted inspiratory bursts and ii) bilateral preBötC LPP increased f .…”

Microcircuits in respiratory rhythm generation: commonalities with other rhythm generating networks and evolutionary perspectives

“…The synaptic interactions between glutamatergic Dbx1-derived neurons dynamically regulate both burst frequency and burst termination [76 •• ]. The volley of action potentials generated during synchronization of Dbx1 neurons depletes the ready-releasable pool of synaptic vesicles (i.e., pre-synaptic depression) resulting in a ‘refractory period’ for activating the subsequent Dbx1 burst (Figure 2) [34 • ]. Thus, the degree of network synchronization likely influences the magnitude of pre-synaptic depression.…”

“…Mechanisms commonly found in rhythm generating networks include reciprocal inhibition [26 • ,27,28], rhythmic pacemaker properties [11 • ,29–33] and recurrent excitatory network mechanisms [10,34 • ,35,36 • ]. However, their roles within a given network vary and are often different than originally hypothesized.…”

Functional roles of short-term synaptic plasticity with an emphasis on inhibition

“…Slice recordings of preBötC neurons reveal synaptic facilitation and depression within the timescale predicted by the model. The role of synaptic depression in producing the refractory period is also supported by experiments on specific preBötC neurons [59]*, which have been shown to be necessary for inspiratory rhythm generation [60]. …”