Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCB012

Poster Communications

Hypoxia inducible factor 1α in nucleus tractus solitarii glutamatergic neurons is necessary for ventilatory acclimatization to hypoxia

E. A. Moya1, A. Go1, Z. Fu1, R. Johnson2, F. L. Powell1

1. Physiology, University of California San Diego, San Diego, California, United States. 2. Department of Physiology, Development and Neurosciencie, University of Cambridge, Cambridge, United Kingdom.

Chronic sustained hypoxia (CH) produces ventilatory acclimatization to hypoxia (VAH), which is a time-dependent increase of resting ventilation and hypoxic ventilatory response (HVR). VAH involves increased O2-sensitivity in carotid bodies and neural plasticity in medullary respiratory centers, including the nucleus tractus solitarii (NTS) at the primary synapse of carotid body chemoreceptors. We hypothesized that VAH requires gene expression induced by hypoxia inducible factor 1-α (HIF-1α) in respiratory neurons in the NTS. To test this, we anesthetized (ketamine/xylazine 40/3 mg/Kg i.p.) transgenic mice (loxP sites flanking HIF-1α gene, 1) and micro-injected adeno-associated virus expressing green fluorescent protein and Cre-recombinase in neurons (Vector Biolabs AAV2-Cre-GFP) in the NTS to delete HIF-1α gene before mice were acclimatized CH (PiO2=70 Torr). Transgenic mice were microinjected in the NTS with AAV expressing only GFP for CH controls. After 7 days of CH, we measured hypoxic and hypercapnic ventilatory responses using whole body plethysmography and metabolic rates (VO2 and VCO2) during acute normoxia and hypoxia (21% and 10% O2). We also tested for HIF-1a deletion (GFP-Cre expression) in glutamaterigic and GABAergic neurons in the NTS using immunohistochemistry (Vglut2 and GAD67, respectively) in another group of CH mice. Finally, we used cFos immunoreactivity as a marker for neural activity in the NTS during acute hypoxia (10% O2 2-3 hrs). HIF-1α deletion in the NTS significantly (p<0.05 multivariate ANOVA, Fisher's test, n=11-14) decreased the HVR in CH mice by decreasing VI during 10% O2 breathing (3320±250 in GFP-Cre vs. 4349±278 ml/(min kg) in GFP mice); there was no significant difference in VI breathing 21% O2 or 7% CO2. Metabolic rate was not different with HIF-1a deletion. However, we measured ventilation normalized for decreased metabolism in acute hypoxia (VI/ VCO2), and HIF-1a deletion significantly decreased VAH during 10% O2 breathing (109.9±7.7 in GFP vs. 86.0±5.6 ml/(min kg) in GFP-Cre mice,). Acute hypoxia increased cFOS immunoreactivity in GFP-Cre positive neurons in the NTS and GFP-Cre positive neurons in the NTS of CH mice co-localized with Vglut2 but not GAD67-positive neurons. The results support HIF-1a-inducing glutamatergic plasticity that is necessary for VAH (2) but experiments are necessary to determine why adeno-associated virus preferentially infects glutamatergic vs. GABAergic neurons in the NTS during CH.

Where applicable, experiments conform with Society ethical requirements