Astrocytes mediate the post‐hypoxic persistent respiratory augmentation

Abstract

Acute hypoxia increases ventilation. After brief hypoxic exposure, switchback of inspired gas to room air is accompanied by short-term potentiation of breathing, i.e., ventilation remains elevated above the baseline level. Short-term potentiation of breathing has been thought to play a stabilizing role in control of breathing, e.g., prevention of periodic breathing. However, the mechanism of this potentiation is unclear. We aimed to test the hypothesis that astrocytes mediate this potentiation, i.e., post-hypoxic persistent respiratory augmentation (PHRA), and performed a series of in vitro and in vivo experiments. In vitro, isolated brainstem-spinal cord preparations of neonatal rats were superfused with artificial cerebrospinal fluid (aCSF) without or with arundic acid (500 µM), an inhibitory modulator of astrocytic function. Both groups were sequentially superfused with oxygenated (95% O2, 5% CO2) and hypoxic (95% N2, 5% CO2) aCSF for 5 min, and again with oxygenated aCSF for 17 min. Hypoxia universally increased the respiratory burst rate. Without arundic acid, the burst rate transiently decreased with reoxygenation, and then recovered to and remained at the pre-hypoxic level until the end of the post-hypoxic observation. Contrarily, the burst rate steadily decreased throughout the post-hypoxic phase with arundic acid, declining below the pre-hypoxic level. In vivo, the hypoxic ventilatory response was investigated by whole-body plethysmography in unanesthetized adult mice. The animals breathed room air, hypoxic gas (7% O2, 93% N2) for 2 min, and again room air for 10 min before and after i.p. administration of low (100 mg/kg) and high (300 mg/kg) doses of arundic acid. Minute ventilation in the post-hypoxic recovery was maintained above the pre-hypoxic level without arundic acid. However, it was suppressed with arundic acid. Notably with high dose arundic acid, it decreased below the pre-hypoxic level. Finally, we investigated the significance of the astrocytic TRPA1 channel, which is a ventilatory hypoxic sensor, in the PHRA using astrocyte-specific Trpa1 knockout (asTrpa1-/-) and floxed Trpa1 (Trpa1f/f) mice. Although the PHRA was observed in both Trpa1f/f and as Trpa1-/- mice, it was weaker in asTrpa1-/- as compared to Trpa1f/f mice. This observation indicates that the astrocytic TRPA1 channel plays a role in the PHRA but this by itself is not sufficient, suggesting the existence of additional key factors. These in vitro and in vivo experimental findings collectively demonstrate that astrocytes mediate the PHRA partially through the TRPA1 channel.