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 (PaO 2 ~40-50 mmHg) induced phrenic motor plasticity in a 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; FIO 2 =0.09) or 3, 5-min (3x5; FIO 2 =0.105) moderate hypoxic episodes, we tested the hypothesis that diurnal cycle regulates AIH-induced vLTF in unanesthetized male Sprague Dawley rats. Minute ventilation (V̇ E ), tidal volume (V T ), breathing frequency (f R ), and metabolic CO 2 production (V̇ CO 2 ) were assessed via whole-body plethysmography during mid-rest (light) and mid-active (dark) phases. Since V̇ E and V T correlate strongly with V̇ CO 2 across the rest/active cycle, and AIH decreases V̇ CO 2 , we normalized V̇ E and V T to V̇ CO 2 (V̇ E /V̇ CO 2 and V T /V̇ CO 2 ) 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.