Phox2b-Expressing Neurons of the Parafacial Region Regulate Breathing Rate, Inspiration, and Expiration in Conscious Rats

Abstract

The retrotrapezoid nucleus contains Phox2b-expressing glutamatergic neurons (RTN-Phox2b neurons) that regulate breathing in a CO2-dependent manner. Here we use channelrhodopsin-based optogenetics to explore how these neurons control breathing in conscious and anesthetized adult rats. Respiratory entrainment (pacing) of breathing frequency (fR) was produced over 57% (anesthetized) and 28% (conscious) of the natural frequency range by burst activation of RTN-Phox2b neurons (3–8 × 0.5–20 ms pulses at 20 Hz). In conscious rats, pacing under normocapnic conditions increased tidal volume (VT) and each inspiration was preceded by phase 2 (active) expiration, denoting abdominal muscle contraction. During long-term pacing, VT returned to pre-stimulation levels suggesting that central chemoreceptors such as RTN-Phox2b neurons regulate VT partly independently of their effect on fR. Randomly applied light-trains reset the respiratory rhythm and shortened the expiratory phase (TE) when the stimulus coincided with late-inspiration or early-expiration. Importantly, continuous (20 Hz) photostimulation of the RTN-Phox2b neurons and a saturating CO2 concentration produced similar effects on breathing that were much larger than those elicited by phasic RTN stimulation. In sum, consistent with their anatomical projections, RTN-Phox2b neurons regulate lung ventilation by controlling breathing frequency, inspiration and active expiration. Adult RTN-Phox2b neurons can entrain the respiratory rhythm if their discharge is artificially synchronized but continuous activation of these neurons is much more effective at increasing lung ventilation. These results suggest that RTN-Phox2b neurons are no longer rhythmogenic in adulthood and that their average discharge rate may be far more important than their discharge pattern in driving lung ventilation.

State-Dependent Modulation of Breathing in Urethane-Anesthetized Rats

“…ABD EMG activity was also potentiated during CO 2 stimulation, consistent with previous reports (Iizuka and Fregosi, 2007;Marina et al, 2010;Abbott et al, 2011).…”

“…Abdominal muscles are not generally active at rest but are frequently activated during quiet sleep in healthy humans, in patients with obstructive sleep apnea, and in snorers (Skatrud and Dempsey, 1985;Skatrud et al, 1988). In rats, abdominal expiratory-related activity is induced or potentiated by chemosensory stimulation (Iizuka and Fregosi, 2007;Marina et al, 2010;Abbott et al, 2011), exercise, and selective stimulation of the para-facial respiratory group (Pagliardini et al, 2011) [the proposed expiratory rhythm generator (Mellen et al, 2003;Janczewski and Feldman, 2006)]. Thus, it is possible that ABD EMG recruitment during REM-like periods was caused by increased respiratory drive.…”

Midline section of the medulla abolishes inspiratory activity and desynchronizes pre-inspiratory neuron rhythm on both sides of the medulla in newborn rats

“…Indeed, studies in adult cat in vivo preparation (Bainton and Kirkwood 1979;Sears 1977) demonstrated that rhythmic discharges of expiratory bulbospinal units in the medulla were observed to change into a tonic firing pattern. Active expiration was induced effectively by unilateral optogenetic stimulation of the pFRG and a neighboring retrotrapezoid nucleus (Abbott et al 2011;Pagliardini et al 2011), but interactions between pFRGs on the stimulated and contralateral side were not investigated.…”

Interação entre o grupamento parafacial/núcleo retrotrapezóide e a região de kölliker-fuse na modulação do padrão respiratório.

“…Apesar de várias evidências mostrando o envolvimento do pFRG/RTN na inspiração, mais recentemente estudos têm procurado associar o pFRG/RTN também à atividade expiratória (ABBOTT et al, 2011;FELDMAN et al, 2011).…”

“…A musculatura da laringe esta envolvida em diversas funções fisiológicas como o funcionamento adequado das cordas vocais, na regulação da abertura da glote, no controle da resistência das vias aéreas e em reflexos protetores das vias aéreas (HORNER, 2013 (TAKAKURA et al, 2006;, mas uma região integradora de respostas autônomas (MOREIRA et al, 2007a;TAKAKURA et al, 2011; TAKAKURA; MOREIRA, 2011) e respiratórias (ABBOTT et al, 2011(ABBOTT et al, , 2013MOREIRA et al, 2007b;presentes resultados).…”