Phrenic Motor Neuron Expression of Receptors Known to Elicit Phrenic Motor Facilitation After Daily Acute Intermittent Hypoxia Preconditioning

Abstract

Spinal serotonin receptor activation elicits a form of respiratory motor plasticity known as phrenic motor facilitation (pMF). One specific form of pMF is phrenic long‐term facilitation (pLTF) following acute intermittent hypoxia (AIH) exposure. pMF exhibits metaplasticity, defined as a future change in the capacity to express plasticity based on experience. For example, repetitive AIH preconditioning enhances AIH‐induced pLTF.Distinct cellular mechanisms give rise to pMF, including the Q and S pathways, which are induced by 5HT2A/5HT2B and 5HT7 or adenosine 2A signaling, respectively. Although Q and S pathways each elicit pMF when acting alone, they are mutually inhibitory; thus, pLTF following moderate AIH results predominantly via the Q pathway, and is partially restrained by concurrent S pathway activation. Our working hypothesis is that repetitive AIH preconditioning causes pLTF metaplasticity by: 1) reducing inhibitory interactions between the S and Q pathways, and 2) increasing the relative contribution of the S pathway to pLTF. To begin exploring this global hypothesis, we began by investigating the impact of repetitive AIH on the phrenic motor neuron expression of receptors initiating the Q (5HT2A,5HT2B) versus S (5HT7, A2A) pathways to pMF. Four days after intrapleural injections of Cholera toxin B fragment (CTB) to identify phrenic motor neurons, rats were exposed to normoxia or daily AIH (10, 5 min episodes per day, 5 min intervals; 14 days). The rats were perfused and tissue sections were analyzed for 5HT2A, 5HT2B, 5HT7 and A2A receptors, and a downstream mediator of the S pathway (EPAC2) via immunofluorescence. Following dAIH: 1) 5HT2A, 5HT2B and A2A receptor expression were unchanged; but 2) 5HT7 and EPAC2 expression were increased versus controls. Increased 5HT7 and EPAC2 protein expression may support enhanced contributions of the S pathway to pMF, but do not provide information concerning the possibility that cross‐talk inhibition between the Q and S pathways was reduced. Priming a system with dAIH may increase the capacity for respiratory plasticity, and may be a useful therapeutic intervention to treat disorders that cause respiratory motor deficits, including spinal injury, sleep apnea or motor neuron disease.Support or Funding InformationSupported by: T32 Neuromuscular Plasticity Training Program NICHD: T32‐HD043730, NIH HL69064, and the McKnight Brain Institute. We acknowledge Dr. Mark Brownfield for providing us with the serotonin receptor antibodies.