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

Abstract

Rhythmicity is critical for the generation of rhythmic behaviors and higher brain functions. This review discusses common mechanisms of rhythm generation, including the role of synaptic inhibition and excitation, with a focus on the mammalian respiratory network. This network generates three phases of breathing and is highly integrated with brain regions associated with numerous non-ventilatory behaviors. We hypothesize that during evolution multiple rhythmogenic microcircuits were recruited to accommodate the generation of each breathing phase. While these microcircuits relied primarily on excitatory mechanisms, synaptic inhibition became increasingly important to coordinate the different microcircuits and to integrate breathing into a rich behavioral repertoire that links breathing to sensory processing, arousal, and emotions as well as learning and memory.

Figure 1

Microcircuits in mammalian respiratory rhythm generation. (a) Isolated preBötC neurons have various activity patterns including bursting ‘pacemaker’, tonic firing, and silent that are largely determined by conductance characteristics including persistent sodium (INaP) and non-specific cation (ICAN) currents (Adapted with permission from [96]). (b) The microcircuit constituting the preBötC consists of glia and both excitatory and inhibitory neurons that primarily fire (>80%) in phase with the inspiratory phase of breathing (Adapted with permission from [71]). (c and d) In vivo optogenetic manipulations of respiratory microcircuits coupled to the preBötC. (c) The parafacial lateral region (pF_L; blue) is a conditional oscillator that generates active expiration visualized in abdominal (Abd) activity; excitation of this microcircuit elicits an Abd_EMG burst and inhibits diaphragm activity (Dia_EMG) (Adapted with permission from [100]). (d) The post-inspiratory complex (PiCo; purple) generates postinspiration visualized in the vagus nerve (X N); stimulation of this microcircuit elicits a vagal nerve burst and delays the onset of inspiratory activity observed in hypoglossal motor output (XII N). BötC, Bötzinger complex; VRG, ventral respiratory group. (e) The contribution of each microcircuit to the generation of the respiratory rhythm is dynamically regulated and integrated with brain regions controlling distinct respiratory-related and non-respiratory behaviors.

Figure 2

The anatomy of an inspiratory population burst. Recurrent synaptic excitation among sparsely connected neurons begins to increase action potential rates and build network excitability. As presynaptic Ca⁺² summates, vesicle release probability increases, strengthening synaptic transmission. Burst generating Ca⁺² and voltage gated conductances become increasingly active as neurons in the network depolarize, leading to non-linear amplification of action potential rates and network synchrony. The high rate of action potentials generated during the population burst depletes the ready-releasable pool of synaptic vesicles, which reduces synaptic transmission, leading to the loss of synchronization, termination of the burst, and a refractory period for inspiratory activity.

Figure 3

Vertebrate rhythmogenic respiratory microcircuits. (a) Dorsal representation of respiratory rhythm generation in lamprey, which seems to be localized to a single microcircuit located in the pons — the para trigeminal respiratory group (pTRG). The pTRG utilizes excitatory mechanisms for rhythm generation and may be homologous to the mammalian preBötC. Respiratory activity in this primitive vertebrate generates gill movements, which can be observed in vagus nerve (X N) motor output (bottom trace) (Adapted with permission from [25^•]). (b) Dorsal representation of respiratory microcircuits located near cranial nerve nuclei in bullfrogs. It is thought that three distinct respiratory oscillators generate the buccal (orange), lung priming (purple), and lung powerstroke (blue) rhythms. These respiratory activities can be differentially observed in cranial nerve activity, as shown here in facial (VII N), vagus (X N) and hypoglossal (XII N) nerve motor output (Adapted with permission from [14^••]). (c) Sagittal representation of the three identified excitatory rhythmogenic respiratory microcircuits in the mouse. The ‘triple-oscillator’ hypothesis: three anatomically distinct coupled excitatory microcircuits generate the three phases of the mammalian breathing rhythm — the preBötC, PiCo, and pFL, generate inspiration (I), postinspiration (PI), and active expiration (AE), respectively. These breathing phases are observed in motor output from respiratory-related nerves (Abd, abdominal; Ph, phrenic; cVN, cervical vagus nerve), which is precisely coordinated to produce a breath (Adapted with permission from [101]). BötC, Bötzinger complex, NA, nucleus ambiguus; LRt, lateral reticular nucleus.