Time of day and pattern of acute intermittent hypoxia regulate ventilatory long-term facilitation in unanesthetized rats

Plasticity is a hallmark feature of the neural system controlling breathing. Acute intermittent hypoxia (AIH) elicits plasticity in multiple respiratory motor pools (e.g., phrenic, intercostal, XII) and has emerged as a promising therapeutic approach to restore lost respiratory (and non-respiratory) function in people with clinical disorders that compromise movement, such as spinal cord injury. In anesthetized rats, time of day affects both the magnitude and mechanism of moderate AIH (PaO2 ~40-50 mmHg) induced phrenic motor plasticity, depending on the specific AIH protocol studied. However, time of day and AIH protocol effects on ventilatory long-term facilitation (LTF) in unanesthetized rats have not been studied. Thus, two AIH protocols consisting of 15, 1-min episodes (15x1; FIO2 = 0.09) or 3, 5-min episodes (3x5; FIO2 = 0.105), each eliciting a SpO2 of ~70%, were applied to test the hypothesis that time of day regulates AIH-induced ventilatory LTF in male Sprague-Dawley rats (3-5 months old). Using whole-body plethysmography, minute ventilation (V·E) and metabolic CO2 production (V·CO2) were assessed during the mid-rest (i.e. light) and mid-active (i.e. dark) phases, and V·E was expressed as the air convection requirement (i.e. V·E/V·CO2). In the rest phase, 15x1 AIH (n = 9) elicited significantly greater ventilatory LTF (ΔV·E/V·CO2) vs 3x5 AIH (n = 8) when averaged 60-90 minutes post-AIH (p = 0.0080). In the active phase, both 15x1 (n = 7) and 3x5 (n = 8) AIH protocols elicited similar ventilatory LTF (p = 0.7185). While the 15x1 protocol produced similar ΔV·E/V·CO2 in both phases (p = 0.3019), the 3x5 protocol elicited significantly greater ventilatory LTF in the active vs rest phase (p = 0.0457). Our results demonstrate that time of day regulates ventilatory LTF in unanesthetized rats, but that these effects depend on the pattern of the AIH protocol (i.e., hypoxic episode number/duration). Mechanisms whereby time of day affects ventilatory LTF are not yet known. These findings may help optimize hypoxic protocols in future studies using AIH to restore breathing function in cases of neuromuscular injury or disease. NIH F32HL176044 (Jones); R01HL148030 & R01HL147554 (Mitchell) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.