Adaptation of Respiratory-Related Brain Regions to Long-Term Hypercapnia: Focus on Neuropeptides in the RTN

Abstract

Long-term hypercapnia is associated with respiratory conditions including obstructive sleep apnea, chronic obstructive pulmonary disease and obesity hypoventilation syndrome. Animal studies have demonstrated an initial (within hours) increase in ventilatory drive followed by a decrease in this response over the long-term (daysweeks) in response hypercapnia. Little is known about whether changes in the central respiratory chemoreflex are involved. Here we investigated whether central respiratory chemoreceptor neurons of the retrotrapezoid nucleus (RTN), which project to the respiratory pattern generator within the ventral respiratory column (VRC) have a role in the mechanism of neuroplasticity associated with long-term hypercapnia. Adult male C57BL/6 mice (n = 5/group) were used. Our aims were (1) to determine if galanin, neuromedin B and gastrin-releasing peptide gene expression is altered in the RTN after long-term hypercapnia. This was achieved using qPCR to measure mRNA expression changes of neuropeptides in the RTN after short-term hypercapnia (6 or 8 h, 5 or 8% CO 2 ) or long-term hypercapnia exposure (10 day, 5 or 8% CO 2 ), (2) in the mouse brainstem, to determine the distribution of preprogalanin in chemoreceptors, and the cooccurrence of the galanin receptor 1 (GalR1:Gi-coupled receptor) with inhibitory GlyT2 ventral respiratory column neurons using in situ hybridization (ISH) to better characterize galaninergic RTN-VRC circuitry, (3) to investigate whether long-term hypercapnia causes changes to recruitment (detected by cFos immunohistochemistry) of respiratory related neural populations including the RTN neurons and their galaninergic subset, in vivo. Collectively, we found that hypercapnia decreases neuropeptide expression in the RTN in the short-term and has the opposite effect over the long-term. Following long term hypercapnia, the number of RTN galanin neurons remains unchanged, and their responsiveness to acute chemoreflex is sustained; in contrast, we identified multiple respiratory related sites that exhibit blunted chemoreflex activation. GalR1 was distributed in 11% of preBötC and 30% of BötC glycinergic neurons. Our working hypothesis is that during long-term hypercapnia, galanin co-release from RTN neurons may counterbalance glutamatergic inputs to respiratory centers to downscale energetically wasteful hyperventilation, thereby having a role in neuroplasticity by contributing to a decrease in ventilation, through the inhibitory effects of galanin.

Whole-brain analysis of CO2chemosensitive regions and identification of the retrotrapezoid and medullary raphé nuclei in the common marmoset (Callithrix jacchus)

“…Therefore, to identify CO 2activated Phox2b-expressing neurons (Kumar et al 2015;Stornetta et al 2006), we used immunostaining of both Phox2b and c-fos in marmosets exposed to 6% CO 2 and mapped the Phox2b + c-fos + cells ventral to the facial nucleus, investigating the putative location of the RTN. Similar to other regions in the ventral brainstem, there was a basal level of c-fos activity within the RTN region in the control animals (animals not expose to acute hypercapnia) which could be related to the involvement of Phox2b + neurons in modulation of breathing activity (Dereli et al 2019;Fu et al 2019;Fu et al 2017;Tian et al 2021). However, the number of Phox2b + c-fos + cells in parafacial region were increased by 10 folds in marmosets exposed to 6% CO 2 (Figure 2).…”

Correlation between Galanin and its receptor with the serum electrolytes in Long-COVID patients

“…Increased Galanin levels were linked to moderate OSAS [72]. Galanin co-release from retrotrapezoid nucleus neurons may counterbalance glutamatergic inputs to respiratory centers to downscale energetically wasteful hyperventilation, contributing to neuroplasticity by decreasing ventilation [73]. GALR1 controls galanergic signaling by acting as a dimer outside a cell and moving inside when triggered by galanin [74].…”

Ventilatory neural drive in chronically hypercapnic patients with COPD: effects of sleep and nocturnal noninvasive ventilation

“…Neuroplasticity of regions of the brain responsible for ventilatory sensing and neural drive may play a role in the altered responses seen in chronic hypercapnic patients, although this hypothesis has thus far only been explored in animal studies. Within the retrotrapezoid nucleus (RTN; a region containing many chemosensory cells relevant to respiration), expression of several neuropeptides decreases with short-term hypercapnia, but increases with chronically elevated P aCO 2 [ 70 ]. One of these is galanin, which inhibits ventilatory signals including the acute chemosensory response to hypercapnia and hypoxia when injected into the Bötzinger and pre-Bötzinger complexes of rats [ 71 ].…”

“…One of these is galanin, which inhibits ventilatory signals including the acute chemosensory response to hypercapnia and hypoxia when injected into the Bötzinger and pre-Bötzinger complexes of rats [ 71 ]. This offers a potential mechanism to explain findings of blunted acute chemosensitivity in hypercapnic patients, although the limitations of applying animal physiology to human physiological function must be considered [ 70 ]. However, the neuropeptide neuromedin B, which is an excitatory neurotransmitter potentially implicated in increased minute ventilation, is also expressed in increasing quantities in rat RTN neurons as hypercapnia progresses, which may explain the sustained increase in ventilation during chronic hypercapnia [ 70 ].…”

“…Additionally, COPD patients frequently experience arousal from sleep, which can be made even worse by coexistent OSA [ 104 ]. These arousals have been associated with increased neural drive, which could complicate our understanding of ventilatory drive during sleep and changes in blood gasses [ 26 , 70 ]. However, the details of COPD–OSA overlap and arousals are beyond the scope of this article, as we aim to predominantly review the physiological underpinnings of hypercapnic COPD in isolation.…”