Abstract
ABSTRACTThe catecholaminergic (CA) system has been implicated in many facets of breathing control and offers an important target to better comprehend the underlying etiologies of both developmental and adult respiratory pathophysiologies. Here, we used a noninvasive DREADD-based pharmacogenetic approach to acutely perturb Tg(Th-Cre)FI172Gsat (Th-Cre)-defined neurons in awake and unrestrained mice in an attempt to characterize CA function in breathing. We report that clozapine-N-oxide (CNO)-DREADD-mediated inhibition of Th-Cre-defined neurons results in blunted ventilatory responses under respiratory challenge. Under a hypercapnic challenge (5% CO2/21% O2/74% N2), perturbation of Th-Cre neurons results in reduced fR, and . Under a hypoxic challenge (10% O2/90% N2), we saw reduced fR, and , in addition to instability in both interbreath interval and tidal volume, resulting in a Cheyne-Stokes-like respiratory pattern. These findings demonstrate the necessity of Th-Cre-defined neurons for the hypercapnic and hypoxic ventilatory responses and breathing stability during hypoxia. However, given the expanded non-CA expression domains of the Tg(Th-Cre)FI172Gsat mouse line found in the brainstem, full phenotypic effect cannot be assigned solely to CA neurons. Nonetheless, this work identifies a key respiratory population that may lead to further insights into the circuitry that maintains respiratory stability in the face of homeostatic challenges.
Highlights
The catecholaminergic (CA) system, including the dopaminergic, noradrenergic and adrenergic systems, has been implicated in respiratory homeostasis (Li et al, 2008; Viemari, 2008)
Our results show that hM4Di-mediated inhibition of tyrosine hydroxylase (Th)-Credefined neurons results in reduced hypercapnic and hypoxic reflexes as well as temperature deficits, identifying a population of cells that are critical for maintaining respiratory and metabolic homeostasis
We observed a reduction in periodic instability [Fig. 2F, −40.5% reduction in interbreath interval (IBI) coefficient of variation (CV), P=0.017]
Summary
The catecholaminergic (CA) system, including the dopaminergic, noradrenergic and adrenergic systems, has been implicated in respiratory homeostasis (Li et al, 2008; Viemari, 2008). We aimed to characterize the CA system in respiratory function in adult mice through the use of acute and targeted pharmacogenetic neuronal inhibition. DREADD-mediated neuronal manipulations combined with recombinase based targeting strategies have provided an approach for functional circuit mapping that enables acute noninvasive perturbation of targeted populations while measuring respiratory output in conscious and unrestrained adult mice (Hennessy et al, 2017; Brust et al, 2014; Ray et al, 2012, 2011). To examine the potential aspects of respiratory physiology served by the CA system, we utilized the Cre-responsive RC::PDi mouse in combination with the B6.FVB(Cg)-Tg(Th-Cre)FI172Gsat (hereafter Th-Cre) driver to express the hM4D receptor in CA and other neurons defined by this frequently used Th-Cre driver. Using whole-body plethysmography, we assessed respiratory and metabolic function under baseline (21% O2/79% N2), hypercapnic (5% CO2/21% O2/ 74% N2) and hypoxic (10% O2/90% N2) conditions after CNODREADD-mediated perturbation of targeted cells, examining several aspects of respiratory and metabolic homeostasis, including rate (fR), tidal volume (VT), minute ventilation ðV_ EÞ, oxygen consumption ðV_ O2 Þ and waveform patterning
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