Neurochemical Plasticity of Serotonin Receptors on Phrenic Motor Neurons: Effects of Cervical Spinal Injury and Intermittent Hypoxia

Abstract

Serotonin is critical for multiple forms of spinal respiratory motor plasticity, including acute intermittent hypoxia (AIH) induced phrenic long‐term facilitation. Relevant serotonin receptor subtypes include 5HT2A, 5HT2B and 5HT7 receptors. Cervical spinal cord injury (cSCI) disrupts serotonergic projections to spinal respiratory motor neurons, transiently abolishing serotonin‐dependent, AIH‐induced phrenic motor plasticity. However, serotonergic innervation recovers with time post‐injury (>8 wks), restoring the capacity for AIH‐induced phrenic motor plasticity. Repetitive “low dose” AIH exposure is emerging as a promising strategy to restore breathing ability following cSCI; conversely, “high dose” chronic intermittent hypoxia (CIH) simulating that experienced during sleep apnea undermines these functional benefits and triggers pathology. Despite the powerful influences of cSCI, repetitive AIH and CIH on breathing ability, we know little concerning the potential of these conditions to change the expression of serotonin receptors (neurochemical plasticity) in phrenic motor neurons. Thus, we tested the hypothesis that cSCI, repetitive AIH and CIH exert differential effects on serotonin receptor expression in phrenic motor neurons. 5HT2A, 5HT2B and 5HT7 receptor expression were determined in male Sprague Dawley rats with and without C2 spinal hemisection (C2Hx; 12 wks post‐injury) that were exposed to 28 days of: 1) normoxia; 2) daily AIH (10, 5‐min 10.5% O2 episodes with 5‐min intervals per day; 1.5 hrs/day); 3) moderate CIH (CIH 5/5: 5‐min 10.5% O2 episodes, 5‐min intervals; 8 hrs/day); and 4) high dose CIH (CIH 2/2: 2‐min 10.5% O2 episodes, 2 min intervals; 8 hrs/day). Rats were then perfused, and cervical spinal cords sectioned (40mm). C3 to C5 sections were processed for 5HT7 receptors and Cholera toxin B subunit (CtB, injected 14 days before C2Hx). Using a custom MATLAB algorithm, optical density in CtB‐positive phrenic motor neurons was quantified. Statistical analysis was performed using MANOVA (SAS JMP). C2Hx had marginal effects on 5HT7 receptor expression (p=0.07). Intermittent hypoxia exerted protocol‐specific effects on 5HT7 receptor expression (CIH 2/2 > CIH 5/5 > daily AIH; p<0.001), and these effects were most prominent after C2Hx (injury/treatment interaction; p<0.011). 5HT2A and 5HT2B quantification are ongoing. The injury‐specific effects of intermittent hypoxia on 5HT7 receptor expression are consistent with the notion that the injured spinal cord has unique properties. Phrenic motor neuron serotonin receptor plasticity may be significant as we work to develop new therapeutic strategies to improve breathing after spinal cord injury.Support or Funding InformationSupported by: NIH T32 HD043730 (LLA) OT2OD023854 (SPARC), NIH K12 HD055929 (EGR), McKnight Brain Institute.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.