Serotonin 2A and 2B receptor-induced phrenic motor facilitation: differential requirement for spinal NADPH oxidase activity

Abstract

Acute intermittent hypoxia (AIH) facilitates phrenic motor output by a mechanism that requires spinal serotonin (type 2) receptor activation, NADPH oxidase activity and formation of reactive oxygen species (ROS). Episodic spinal serotonin (5-HT) receptor activation alone, without changes in oxygenation, is sufficient to elicit NADPH oxidase-dependent phrenic motor facilitation (pMF). Here we investigated: 1) whether serotonin 2A and/or 2B (5-HT2a/b) receptors are expressed in identified phrenic motor neurons, and 2) which receptor subtype is capable of eliciting NADPH-oxidase-dependent pMF. In anesthetized, artificially ventilated adult rats, episodic C4 intrathecal injections (3 × 6µl injections, 5 min intervals) of a 5-HT2a (DOI) or 5-HT2b (BW723C86) receptor agonist elicited progressive and sustained increases in integrated phrenic nerve burst amplitude (i.e. pMF), an effect lasting at least 90 minutes post-injection for both receptor subtypes. 5-HT2a and 5-HT2b receptor agonist-induced pMF were both blocked by selective antagonists (ketanserin and SB206553, respectively), but not by antagonists to the other receptor subtype. Single injections of either agonist failed to elicit pMF, demonstrating a need for episodic receptor activation. Phrenic motor neurons retrogradely labeled with cholera toxin B fragment expressed both 5-HT2a and 5-HT2b receptors. Pre-treatment with NADPH oxidase inhibitors (apocynin and DPI) blocked 5-HT2b, but not 5-HT2a-induced pMF. Thus, multiple spinal type 2 serotonin receptors elicit pMF, but they act via distinct mechanisms that differ in their requirement for NADPH oxidase activity.

Spinal 5-HT7 receptors induce phrenic motor facilitation via EPAC-mTORC1 signaling

“…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 and Mitchell 2011;MacFarlane et al 2009MacFarlane et al , 2011. Indeed, multiple serotonin receptor subtypes elicit pMF via mechanistically distinct signaling cascades named for the G proteins most often coupled with the initiating metabotropic serotonin receptor (Dale-Nagle et al 2010).…”

“…Indeed, multiple serotonin receptor subtypes elicit pMF via mechanistically distinct signaling cascades named for the G proteins most often coupled with the initiating metabotropic serotonin receptor (Dale-Nagle et al 2010). Specifically, G s proteincoupled serotonin type 7 (5-HT 7 ) receptors give rise to pMF through the "S pathway" (Dale-Nagle et al 2010;Hoffman and Mitchell 2011), whereas G q protein-coupled 5-HT 2 receptors induce the "Q pathway" to pMF (Dale-Nagle et al 2010;MacFarlane et al 2011).…”

“…One manifestation of cross-talk inhibition is the bell-shaped dose-response curve of pMF in response to intermittent intrathecal serotonin injections (MacFarlane and Mitchell 2009). Although low serotonin doses elicit pMF by activating spinal 5-HT 2 receptors MacFarlane et al 2011;MacFarlane and Mitchell 2009), high doses do not unless spinal 5-HT 7 receptors are inhibited. Thus, when activated alone, 5-HT 2 (MacFarlane et al 2011) and 5-HT 7 (Hoffman and Mitchell 2011) receptors each give rise to distinct forms of pMF.…”

“…Although low serotonin doses elicit pMF by activating spinal 5-HT 2 receptors MacFarlane et al 2011;MacFarlane and Mitchell 2009), high doses do not unless spinal 5-HT 7 receptors are inhibited. Thus, when activated alone, 5-HT 2 (MacFarlane et al 2011) and 5-HT 7 (Hoffman and Mitchell 2011) receptors each give rise to distinct forms of pMF. When activated concurrently, they effectively cancel each other because of balanced cross-talk inhibition (Devinney et al 2013;Hoffman and Mitchell 2013;MacFarlane and Mitchell 2009).…”

The Serotonergic System and Neuroplasticity are Implicated in the Effect of Phytochemicals on Mood and Cognitive Disorders

“…Third, 5-HT receptor subtypes interact with components of the oxidative process. For example, NADPH oxidase is necessary for the activation of 5-HT 2 receptors (MacFarlane et al, 2011). NADPH oxidase is recognized for its dual-edge roles in health and disease.…”