Decreased spinal synaptic inputs to phrenic motor neurons elicit localized inactivity-induced phrenic motor facilitation

Abstract

Phrenic motor neurons receive rhythmic synaptic inputs throughout life. Since even brief disruption in phrenic neural activity is detrimental to life, on-going neural activity may play a key role in shaping phrenic motor output. To test the hypothesis that spinal mechanisms sense and respond to reduced phrenic activity, anesthetized, ventilated rats received micro-injections of procaine in the C2 ventrolateral funiculus (VLF) to transiently (~30 min) block axon conduction in bulbospinal axons from medullary respiratory neurons that innervate one phrenic motor pool; during procaine injections, contralateral phrenic neural activity was maintained. Once axon conduction resumed, a prolonged increase in phrenic burst amplitude was observed in the ipsilateral phrenic nerve, demonstrating inactivity-induced phrenic motor facilitation (iPMF). Inhibition of tumor necrosis factor alpha (TNFα) and atypical PKC (aPKC) activity in spinal segments containing the phrenic motor nucleus impaired ipsilateral iPMF, suggesting a key role for spinal TNFα and aPKC in iPMF following unilateral axon conduction block. A small phrenic burst amplitude facilitation was also observed contralateral to axon conduction block, indicating crossed spinal phrenic motor facilitation (csPMF). csPMF was independent of spinal TNFα and aPKC. Ipsilateral iPMF and csPMF following unilateral withdrawal of phrenic synaptic inputs were associated with proportional increases in phrenic responses to chemoreceptor stimulation (hypercapnia), suggesting iPMF and csPMF increase phrenic dynamic range. These data suggest that local, spinal mechanisms sense and respond to reduced synaptic inputs to phrenic motor neurons. We hypothesize that iPMF and csPMF may represent compensatory mechanisms that assure adequate motor output is maintained in a physiological system in which prolonged inactivity ends life.

Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control?

“…Of note, both TNFα and retinoic acid are known mediators of homeostatic synaptic plasticity in the hippocampus and cortex (Beattie et al, 2002, Stellwagen and Malenka, 2006, Aoto et al, 2008, Maghsoodi et al, 2008, Steinmetz and Turrigiano, 2010, Chen et al, 2014). Our evidence suggests that both TNFα and retinoic acid ultimately converge on the atypical protein kinase C isozyme, protein kinase C zeta (PKCζ; Strey et al, 2012, Baertsch and Baker-Herman, 2015), which stabilizes early, transient increases in phrenic burst amplitude post-activity deprivation into a long-lasting iPMF (Strey et al, 2012, Streeter and Baker-Herman, 2014a, Baertsch and Baker-Herman, 2015). Collectively, these data suggest that while different mechanisms “sense” prolonged versus intermittent reductions in respiratory neural activity, they ultimately result in the activation of PKCζ, likely within phrenic motor neurons (Guenther et al, 2010, Strey et al, 2012), to induce long-lasting increases in phrenic inspiratory output by an unknown mechanism.…”

“…For example, when cortical or hippocampal neurons are forced to fire outside their normal range (either more or less) for an extended period of time, mechanisms of plasticity are induced that alter cellular properties in the right direction to restore normal firing rates (Turrigiano et al, 1998, Turrigiano, 2012, Tatavarty et al, 2013). Recently, we discovered that activity in respiratory motor neurons might also be subject to such homeostatic regulation: when synaptic inputs to respiratory motor neurons are reduced (in the absence of changing blood gases), local mechanisms of plasticity are induced that counteract this reduction and increase respiratory motor output (Streeter and Baker-Herman, 2014a). …”

“…Importantly, since arterial PO 2 was constant throughout the protocol in all treatments, and in two out of three treatments, arterial PCO 2 was constant, a role for fluctuations in blood gases in eliciting this form of plasticity could be ruled out. We termed this form of plasticity inactivity-induced phrenic motor facilitation (iPMF; Mahamed et al, 2011), although this term is a bit of a misnomer since complete suppression of respiratory neural activity is not necessary to induce iPMF (Streeter and Baker-Herman, 2014a; Figure 1). Later studies revealed that plasticity following reduced respiratory neural activity is also expressed in hypoglossal motor output (inactivity-induced hypoglossal motor facilitation, iHMF; Baker-Herman and Strey, 2011) and inspiratory intercostal EMG activity (inactivity-induced intercostal motor facilitation, iIMF; Strey et al, 2013).…”

“…To better understand the role for local mechanisms in sensing and responding to reductions in respiratory neural activity, we developed a novel model whereby synaptic inputs to one phrenic motor pool were reduced while respiratory neural activity was maintained at normal levels in the brainstem and contralateral phrenic nerve (Streeter and Baker-Herman, 2014a). In anesthetized, mechanically ventilated rats (in which blood gases were held constant), we microinjected procaine into the C2 ventrolateral funiculus on one side of the spinal cord to reversibly block axon conduction in descending bulbospinal tracts providing respiratory-related synaptic inputs to ipsilateral phrenic motor neurons.…”

“…Following activity deprivation, ipsilateral phrenic motor output was significantly increased relative to pre-activity deprivation levels, indicating that mechanisms local to the phrenic motor pool sensed and responded to a reduction in synaptic inputs to elicit iPMF. Since ipsilateral phrenic motor output was only reduced, not eliminated, during the period of activity deprivation, we sought to understand the relationship between the amount of activity deprivation and the magnitude of the resulting iPMF; here, we present additional analysis of data presented in Streeter and Baker-Herman, 2014a (Figure 2). Linear regression analysis was used to determine the relationship between the average reduction in phrenic burst amplitude during axon conduction block and the resulting iPMF.…”