Role of Synaptic Inhibition in the Coupling of the Respiratory Rhythms that Underlie Eupnea and Sigh Behaviors

Abstract

The preBötzinger Complex (preBötC) gives rise to two types of breathing behavior under normal physiological conditions: eupnea and sighing. Here, we examine the neural mechanisms that couple their underlying rhythms. We measured breathing in awake intact adult mice and recorded inspiratory rhythms from the preBötC in neonatal mouse brainstem slice preparations. We show previously undocumented variability in the temporal relationship between sigh breaths or bursts and their preceding eupneic breaths or inspiratory bursts. Investigating the synaptic mechanisms for this variability in vitro, we further show that pharmacological blockade of chloride-mediated synaptic inhibition strengthens inspiratory-to-sigh temporal coupling. These findings contrast with previous literature, which suggested glycinergic inhibition linked sigh bursts to their preceding inspiratory bursts with minimal time intervals. Furthermore, we verify that pharmacological disinhibition did not alter the duration of the prolonged interval that follows a sigh burst prior to resumption of the inspiratory rhythm. These results demonstrate that synaptic inhibition does not enhance coupling between sighs and preceding inspiratory events or contribute to post-sigh apneas. Instead, we conclude that excitatory synaptic mechanisms coordinate inspiratory (eupnea) and sigh rhythms. 4 SIGNIFICANCE STATEMENT Normal breathing consists of eupnea and sigh breaths, which differ in their magnitude, frequency, and function. Both breath types emerge from a brainstem microcircuit that coordinates their timing. Here, we advance understanding of these rhythms by assessing the nature and strength of their coordination, and by showing that synaptic inhibition does not constrain their temporal coupling in contrast to conventional understanding. This study elucidates synaptic mechanisms linking oscillations of different amplitude and frequency within one core oscillator microcircuit.

Evaluating the Burstlet Theory of Inspiratory Rhythm and Pattern Generation

“…A sigh burst in the preBötC field recording was distinguished from an inspiratory burst if it met these three criteria: the area of the putative sigh burst exceeded the mean area of all inspiratory bursts by one SD; the cycle period of the putative sigh bursts measured 1-4 min and not outside this range; and the putative sigh burst was followed by a prolonged inter-event interval Ͼ1.3 times the average inspiratory cycle time for six consecutive cycles preceding a putative sigh burst (Lieske et al, 2000;Ruangkittisakul et al, 2008;Borrus et al, 2019).…”

An open-source tool for automated analysis of breathing behaviors in common marmosets and rodents

“…Since incidences of sighs, apneas, and sniffing could contribute to the irregularity of respiration, we measured the frequencies of these essential features of breathing behavior. Sighs can be generated within the inspiratory rhythm-generating circuits of the preBötzinger complex (preBötC) ( Sheikhbahaei et al, 2018 ; Li et al, 2016 ; Lieske et al, 2000 ; Borrus et al, 2020 ; Toporikova et al, 2015 ; Vlemincx et al, 2013 ), and may be modulated by excitatory signals from central chemocenters ( Sheikhbahaei et al, 2018 ; Sheikhbahaei et al, 2017 ; Souza et al, 2018 ; Souza et al, 2019 ; Li et al, 2016 ). In female adult marmosets, sigh frequencies were not different when compared to those in male animals during the baseline in room air (11 ± 1 vs. 12 ± 2 hr –1 in male) ( Figure 5—figure supplement 1 ).…”

Sigh breathing rhythm depends on intracellular calcium oscillations in a population of inspiratory rhythmogenic preBötzinger complex neurons in mice

“…1E, right). We analyzed inspiratory frequency via all events , i.e., both standalone inspiratory bursts and sigh bursts, which typically build off an inspiratory burst with very short latency (≲ 1 s ) (7, 23). Control inspiratory (event) frequency of 0.25 ± 0.03 Hz remained unaffected by 10 nM NMB (0.25 ± 0.04 Hz) and 30 nM NMB (0.24 ± 0.05 Hz) (Fig.…”