Journal of applied physiology | 2021
Retinoic acid receptor alpha activation is necessary and sufficient for plasticity induced by recurrent central apnea.
Abstract
Reductions in respiratory-related synaptic inputs to inspiratory motor neurons initiate a form of plasticity that proportionally enhances inspiratory motor output, even in the absence of changing blood gases. This form of plasticity is known as inactivity-induced inspiratory motor facilitation (iMF). iMF triggered by brief, recurrent reductions in respiratory neural activity requires local retinoic acid (RA) synthesis, but receptor subtypes activated by RA are unknown. To test the hypothesis that retinoic acid receptor alpha (RARa) is necessary for iMF, RAR subtype-specific inhibitors were delivered intrathecally above the phrenic motor pool in urethane-anesthetized, ventilated rats prior to 5, ~1min central apneas (without hypoxia; separated by 5 min) while monitoring phrenic inspiratory output. Pre-treatment with a spinal RARa inhibitor impaired the capacity for recurrent central apnea to trigger long-lasting increases in phrenic inspiratory output, but plasticity was expressed in rats pre-treated with an RAR / inhibitor. Intrathecal RA application in the absence of reduced respiratory neural activity elicited an increase in phrenic inspiratory output, which was prevented by pre-treatment with an RARa inhibitor. These data indicate that spinal RARa activation is necessary for iMF triggered by recurrent reductions in respiratory neural activity, and that RARa activation in/near the phrenic motor pool in the absence of respiratory neural activity deprivation is sufficient to elicit phrenic inspiratory motor facilitation. Understanding cellular cascades underlying plasticity induced by reductions in respiratory neural activity may define avenues for pharmacological intervention in disorders in which endogenous compensatory mechanisms that defend on-going inspiratory motor output are impaired.