A2A and 5‐HT Receptors are Differentially Required for Respiratory Plasticity Over the Course of Motor Neuron Loss in Intrapleurally CTB‐SAP Treated Rats

Abstract

Loss of respiratory (e.g., phrenic) motor neurons leads to ventilatory failure in patients with neuromuscular diseases, but there are currently no treatments to maintain ventilation or to significantly and consistently preserve/restore ventilatory capacity in these patients. Genetic models have been used to study the impact of motor neuron loss; however, the rate and amount of motor neuron death cannot be controlled in these models, and degeneration is not limited to only respiratory motor neurons making it difficult to study treatment modalities to improve ventilation. Thus, we have developed a novel model of controlled respiratory motor neuron death using intrapleural injection of cholera toxin B conjugated to saporin (CTB‐SAP), in which we find that eupneic ventilation is maintained despite deficits in maximal ventilatory capacity. Our previous results demonstrated that respiratory plasticity is not only observed 7 days (d) after CTB‐SAP delivery, but that it is actually enhanced compared to control treatments. At 28d, however, respiratory plasticity is comparable to control levels, suggesting that the underlying mechanisms of plasticity at both time points may differ. Further, we suggest that neuroplasticity mechanisms contribute to maintain ventilation in the face of respiratory motor neuron loss. Although multiple cellular pathways give rise to respiratory plasticity, this study will focus on serotonin (5‐HT) and adenosine 2A (A2A) receptors based on our preliminary receptor expression data. Our preliminary data indicate that A2A receptor expression is increased in 7d, not 28d, CTB‐SAP treated rats, whereas 5‐HT receptor expression decreases at 7d and recovers to control levels in 28d CTB‐SAP treated rats. Thus, we hypothesize that distinct mechanisms (A2A receptor‐dependent at 7d, 5‐HT receptor‐dependent at 28d) enable respiratory plasticity and maintain eupneic ventilation in CTB‐SAP‐induced neuropathology. Using anesthetized, paralyzed and ventilated CTB‐SAP treated adult Sprague Dawley rats, we studied acute intermittent hypoxia (AIH; 3, 5 min bouts of 10.5% O2) induced phrenic long‐term facilitation (pLTF) following intrathecal C4 delivery of either the A2A receptor antagonist (10 μM MSX‐3) or the 5‐HT receptor antagonist (20 mM methysergide) to rats that received bilateral, intrapleural injections of: 1) CTB‐SAP (25μg), or 2) un‐conjugated CTB and SAP (control) in rats treated 7d or 28d prior. Our preliminary results indicate that pLTF is abolished after MSX‐3 at 7d and methysergide at 28d vs. controls and vehicle treated CTB‐SAP rats. These observations suggest that A2A receptors are necessary for respiratory plasticity early (7d), but 5‐HT receptors are required late (28d). Studies are underway to assess whether these receptors are required to maintain eupneic ventilation using whole‐body plethysmography. Results from this project will increase our understanding of how respiratory plasticity is initiated and how eupneic ventilation is maintained over the course of neuropathology, and will have broad implications for potential therapies to preserve/restore ventilatory function in disorders in which respiratory motor neuron death is associated.Support or Funding InformationNIH R00 HL119606 and SCIDRPThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.