Diurnal cycle and acute intermittent hypoxia protocol interact to regulate ventilatory long-term facilitation in unanesthetized rats

Acute intermittent hypoxia (AIH) elicits plasticity in multiple respiratory motor pools and is a promising therapeutic approach to restore breathing ability in individuals with neuromuscular injury/disease. In anesthetized rats, diurnal cycle affects the magnitude and mechanism of moderate AIH (arterial partial pressure of oxygen [Formula: see text] ∼40-50 mmHg) induced phrenic motor plasticity in an AIH protocol-specific manner. However, diurnal cycle and AIH protocol effects on ventilatory long-term facilitation (vLTF) in unanesthetized rats have not been reported. Measurements of vLTF assess the collective physiological impact of plasticity in all respiratory motor pools and control for the effects of anesthesia, paralysis, and vagotomy common in phrenic LTF studies. Using 2 AIH protocols consisting of 15, 1-min (15x1; [Formula: see text] = 0.09) or 3, 5-min (3x5; [Formula: see text] = 0.105) moderate hypoxic episodes, we tested the hypothesis that diurnal cycle regulates AIH-induced vLTF in unanesthetized male Sprague-Dawley rats. Minute ventilation (V̇e), tidal volume (V_T), breathing frequency (f_R), and metabolic CO₂ production (V̇co₂) were assessed via whole-body plethysmography during mid-rest (light) and mid-active (dark) phases. Since V̇e and V_T correlate strongly with V̇co₂ across the rest/active cycle, and AIH decreases V̇co₂, we normalized V̇e and V_T to V̇co₂ (V̇e/V̇co₂ and V_T/V̇co₂) to account for changes in ventilation linked to metabolism. We found that 15x1 AIH elicits greater vLTF in mid-rest, whereas 3x5 AIH is more effective in mid-active phase, indicating that diurnal cycle regulates respiratory motor plasticity in an AIH protocol-specific manner in unanesthetized rats. Diurnal cycle is an important consideration as we translate knowledge based on nocturnal rodents to diurnal humans with neuromuscular injury/disease.<b>NEW & NOTEWORTHY</b> Although acute intermittent hypoxia (AIH) elicits phrenic motor plasticity in a manner influenced by diurnal cycle and the specific AIH protocol used, it is unknown how diurnal cycle and AIH protocol regulate AIH-induced ventilatory long-term facilitation (vLTF) in unanesthetized rodents. We report that shorter hypoxic episodes elicit greater vLTF in mid-rest phase, whereas longer episodes are favorable in mid-active phase. Thus, diurnal cycle and AIH protocol must be considered in studies of respiratory motor plasticity.