Fields DP, Springborn SR, Mitchell GS. Spinal 5-HT 7 receptors induce phrenic motor facilitation via EPAC-mTORC1 signaling. J Neurophysiol 114: -2022. First published August 12, 2015 doi:10.1152/jn.00374.2015.-Spinal serotonin type 7 (5-HT 7 ) receptors elicit complex effects on motor activity. Whereas 5-HT 7 receptor activation gives rise to long-lasting phrenic motor facilitation (pMF), it also constrains 5-HT 2 receptor-induced pMF via "cross-talk inhibition." We hypothesized that divergent cAMP-dependent signaling pathways give rise to these distinct 5-HT 7 receptor actions. Specifically, we hypothesized that protein kinase A (PKA) mediates cross-talk inhibition of 5-HT 2 receptor-induced pMF whereas 5-HT 7 receptor-induced pMF results from exchange protein activated by cAMP (EPAC) signaling. Anesthetized, paralyzed, and ventilated rats receiving intrathecal (C 4 ) 5-HT 7 receptor agonist (AS-19) injections expressed pMF for Ͼ90 min, an effect abolished by pretreatment with a selective EPAC inhibitor (ESI-05) but not a selective PKA inhibitor (KT-5720). Furthermore, intrathecal injections of a selective EPAC activator (8-pCPT-2=-Me-cAMP) were sufficient to elicit pMF. Finally, spinal mammalian target of rapamycin complex-1 (mTORC1) inhibition via intrathecal rapamycin abolished 5-HT 7 receptor-and EPAC-induced pMF, demonstrating that spinal 5-HT 7 receptors elicit pMF by an EPAC-mTORC1 signaling pathway. Thus 5-HT 7 receptors elicit and constrain spinal phrenic motor plasticity via distinct signaling mechanisms that diverge at cAMP (EPAC vs. PKA). Selective manipulation of these molecules may enable refined regulation of serotonin-dependent spinal motor plasticity for therapeutic advantage. motor neuron; phrenic nerve; spinal cord; respiratory plasticity; neuroplasticity; 5-HT 7 ; receptor; exchange protein activated by cAMP; protein kinase A; rapamycin; mTOR SEROTONIN PLAYS A KEY ROLE in important forms of sensorymotor plasticity, including sensitization of the gill withdrawal reflex in Aplysia (reviewed in Kandel 2012). For example, episodic serotonin presentations enhance sensory motor synaptic transmission, giving rise to the gill withdrawal reflex (Brunelli et al. 1976). This well-studied form of plasticity in an invertebrate model system relies on multiple serotonin receptor subtypes, each activating unique kinase signaling cascades (Barbas et al. 2003).In ways similar to sensory motor facilitation in Aplysia, episodic serotonin receptor activation is necessary and sufficient for important forms of spinal respiratory motor plasticity, such as long-lasting (Ͼ90 min) phrenic motor facilitation (pMF) following acute intermittent hypoxia (AIH; reviewed by Mahamed and Mitchell 2007;Dale-Nagle et al. 2010;Devinney et al. 2013) or direct injections of serotonin or serotonin receptor agonists into the cervical spinal cord of rats (Hoffman

Daily acute intermittent hypoxia (dAIH) elicits respiratory plasticity, enhancing respiratory motor output and restoring breathing capacity after incomplete cervical spinal injuries (cSCI). We hypothesized that dAIH-induced functional recovery of breathing capacity would occur after both acute (2 weeks) and chronic (8 weeks) cSCI, but through distinct cellular mechanisms. Specifically, we hypothesized that dAIH-induced breathing recovery would occur through serotonin-independent mechanisms 2wks post C2 cervical hemisection (C2Hs), versus serotonin-dependent mechanisms 8wks post C2Hs. In two independent studies, dAIH or sham (normoxia) was initiated 1 week (Study 1) or 7 weeks (Study 2) post-C2Hs to test our hypothesis. Rats were pre-treated with intra-peritoneal vehicle or methysergide, a broad-spectrum serotonin receptor antagonist, to determine the role of serotonin signaling in dAIH-induced functional recovery. Our data support the hypothesis that dAIH-induced recovery of breathing capacity transitions from a serotonin-independent mechanism with acute C2Hs to a serotonin-dependent mechanism with chronic C2Hs. An understanding of shifting mechanisms giving rise to dAIH-induced respiratory motor plasticity is vital for clinical translation of dAIH as a therapeutic modality.