Mechanisms of enhanced phrenic long-term facilitation in SOD1G93a rats

Abstract

Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease, causing muscle paralysis and death from respiratory failure. Effective means to preserve/restore ventilation are necessary to increase the quality and duration of life in ALS patients. At disease end-stage in a rat ALS model (SOD1G93A), acute intermittent hypoxia (AIH) restores phrenic nerve activity to normal levels via enhanced phrenic long-term facilitation (pLTF). Mechanisms enhancing pLTF in end-stage SOD1G93A rats are not known. Moderate AIH-induced pLTF is normally elicited via cellular mechanisms that require the following: Gq-protein-coupled 5-HT2 receptor activation, new BDNF synthesis, and MEK/ERK signaling (the Q pathway). In contrast, severe AIH elicits pLTF via a distinct mechanism that requires the following: Gs-protein-coupled adenosine 2A receptor activation, new TrkB synthesis, and PI3K/Akt signaling (the S pathway). In end-stage male SOD1G93A rats and wild-type littermates, we investigated relative Q versus S pathway contributions to enhanced pLTF via intrathecal (C4) delivery of small interfering RNAs targeting BDNF or TrkB mRNA, and MEK/ERK (U0126) or PI3 kinase/Akt (PI828) inhibitors. In anesthetized, paralyzed and ventilated rats, moderate AIH-induced pLTF was abolished by siBDNF and UO126, but not siTrkB or PI828, demonstrating that enhanced pLTF occurs via the Q pathway. Although phrenic motor neuron numbers were decreased in end-stage SOD1G93A rats (~30% survival; p ~ 0.001), BDNF and phosphorylated ERK expression were increased in spared phrenic motor neurons (p ~ 0.05), consistent with increased Q-pathway contributions to pLTF. Our results increase understanding of respiratory plasticity and its potential to preserve/restore breathing capacity in ALS.