Daryl P. Fields

Spinal metaplasticity in respiratory motor control

A hallmark feature of the neural system controlling breathing is its ability to exhibit plasticity. Less appreciated is the ability to exhibit metaplasticity, a change in the capacity to express plasticity (i.e., “plastic plasticity”). Recent advances in our understanding of cellular mechanisms giving rise to respiratory motor plasticity lay the groundwork for (ongoing) investigations of metaplasticity. This detailed understanding of respiratory metaplasticity will be essential as we harness metaplasticity to restore breathing capacity in clinical disorders that compromise breathing, such as cervical spinal injury, motor neuron disease and other neuromuscular diseases. In this brief review, we discuss key examples of metaplasticity in respiratory motor control, and our current understanding of mechanisms giving rise to spinal plasticity and metaplasticity in phrenic motor output; particularly after pre-conditioning with intermittent hypoxia. Progress in this area has led to the realization that similar mechanisms are operative in other spinal motor networks, including those governing limb movement. Further, these mechanisms can be harnessed to restore respiratory and non-respiratory motor function after spinal injury.

Phrenic Long-Term Facilitation Requires PKCθ Activity within Phrenic Motor Neurons

Acute intermittent hypoxia (AIH) induces a form of spinal motor plasticity known as phrenic long-term facilitation (pLTF); pLTF is a prolonged increase in phrenic motor output after AIH has ended. In anesthetized rats, we demonstrate that pLTF requires activity of the novel PKC isoform, PKCθ, and that the relevant PKCθ is within phrenic motor neurons. Whereas spinal PKCθ inhibitors block pLTF, inhibitors targeting other PKC isoforms do not. PKCθ is highly expressed in phrenic motor neurons, and PKCθ knockdown with intrapleural siRNAs abolishes pLTF. Intrapleural siRNAs targeting PKCζ, an atypical PKC isoform expressed in phrenic motor neurons that underlies a distinct form of phrenic motor plasticity, does not affect pLTF. Thus, PKCθ plays a critical role in spinal AIH-induced respiratory motor plasticity, and the relevant PKCθ is localized within phrenic motor neurons. Intrapleural siRNA delivery has considerable potential as a therapeutic tool to selectively manipulate plasticity in vital respiratory motor neurons.

Phrenic motor neuron TrkB expression is necessary for acute intermittent hypoxia-induced phrenic long-term facilitation

ABSTRACT Phrenic long‐term facilitation (pLTF) is a form of hypoxia‐induced spinal respiratory motor plasticity that requires new synthesis of brain derived neurotrophic factor (BDNF) and activation of its high‐affinity receptor, tropomyosin receptor kinase B (TrkB). Since the cellular location of relevant TrkB receptors is not known, we utilized intrapleural siRNA injections to selectively knock down TrkB receptor protein within phrenic motor neurons. TrkB receptors within phrenic motor neurons are necessary for BDNF‐dependent acute intermittent hypoxia‐induced pLTF, demonstrating that phrenic motor neurons are a critical site of respiratory motor plasticity. HighlightsAcute intermittent hypoxia‐induced respiratory motor plasticity requires BDNF signaling.Phrenic motor neurons express the high affinity BDNF receptor TrkB.Intrapleural siRNA targeting TrkB receptors decreased TrkB receptor expression selectively on phrenic motor neurons.Intrapleural siRNA targeting TrkB receptors abolished BDNF dependent, hypoxia‐induced respiratory motor plasticity.Phrenic motor neurons are a critical site for spinal respiratory motor plasticity.

Mammalian target of rapamycin is required for phrenic long-term facilitation following severe but not moderate acute intermittent hypoxia.

Phrenic long-term facilitation (pLTF) is a persistent increase in phrenic nerve activity after acute intermittent hypoxia (AIH). Distinct cell-signaling cascades give rise to pLTF depending on the severity of hypoxemia within hypoxic episodes. Moderate AIH (mAIH; three 5-min episodes, PaO2 ∼35-55 mmHG) elicits pLTF by a serotonin (5-HT)-dependent mechanism that requires new synthesis of brain-derived neurotrophic factor (BDNF), activation of its high-affinity receptor (TrkB), and ERK MAPK signaling. In contrast, severe AIH (sAIH; three 5-min episodes, PaO2 ∼25-30 mmHG) elicits pLTF by an adenosine-dependent mechanism that requires new TrkB synthesis and Akt signaling. Although both mechanisms require spinal protein synthesis, the newly synthesized proteins are distinct, as are the neurochemicals inducing plasticity (serotonin vs. adenosine). In many forms of neuroplasticity, new protein synthesis requires translational regulation via mammalian target of rapamycin (mTOR) signaling. Since Akt regulates mTOR activity, we hypothesized that mTOR activity is necessary for sAIH- but not mAIH-induced pLTF. Phrenic nerve activity in anesthetized, paralyzed, and ventilated rats was recorded before, during, and 60 min after mAIH or sAIH. Rats were pretreated with intrathecal injections of 20% DMSO (vehicle controls) or rapamycin (0.1 mM, 12 μl), a selective mTOR complex 1 inhibitor. Consistent with our hypothesis, rapamycin blocked sAIH- but not mAIH-induced pLTF. Thus spinal mTOR activity is required for adenosine-dependent (sAIH) but not serotonin-dependent (mAIH) pLTF, suggesting that distinct mechanisms regulate new protein synthesis in these forms of spinal neuroplasticity.

Divergent cAMP signaling differentially regulates serotonin-induced spinal motor plasticity

&NA; Spinal metabotropic serotonin receptors encode transient experiences into long‐lasting changes in motor behavior (i.e. motor plasticity). While interactions between serotonin receptor subtypes are known to regulate plasticity, the significance of molecular divergence in downstream G protein coupled receptor signaling is not well understood. Here we tested the hypothesis that distinct cAMP dependent signaling pathways differentially regulate serotonin‐induced phrenic motor facilitation (pMF); a well‐studied model of spinal motor plasticity. Specifically, we studied the capacity of cAMP‐dependent protein kinase A (PKA) and exchange protein activated by cAMP (EPAC) to regulate 5‐HT2A receptor‐induced pMF within adult male rats. Although spinal PKA, EPAC and 5‐HT2A each elicit pMF when activated alone, concurrent PKA and 5‐HT2A activation interact via mutual inhibition thereby blocking pMF expression. Conversely, concurrent EPAC and 5‐HT2A activation enhance pMF expression reflecting additive contributions from both mechanisms. Thus, we demonstrate that distinct downstream cAMP signaling pathways enable differential regulation of 5‐HT2A‐induced pMF. Conditional activation of independent signaling mechanisms may explain experience amendable changes in plasticity expression (i.e. metaplasticity), an emerging concept thought to enable flexible motor control within the adult central nervous system. HighlightscAMP initiates distinct PKA and EPAC signaling cascades in phrenic motor neurons.PKA and EPAC differentially regulate serotonin‐induced spinal, phrenic motor plasticity.Signaling divergence downstream from spinal 5‐HT7 receptors confers flexibile regulation of serotonin‐induced motor plasticity.

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

ABSTRACT For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output—recognizable as a breath—from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity‐induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea. HighlightsWe review evidence for homeostatic plasticity in the neural control of breathing.Spinal mechanisms elicit iPMF in response to decreased phrenic synaptic inputs.iPMF magnitude is proportional to the magnitude of activity deprivation.Homeostatic plasticity may play a role in ventilatory control disorders.

Cross-talk inhibition between 5-HT2B and 5-HT7 receptors in phrenic motor facilitation via NADPH oxidase and PKA

Intermittent spinal serotonin receptor activation elicits phrenic motor facilitation (pMF), a form of spinal respiratory motor plasticity. Episodic activation of either serotonin type 2 (5-HT2) or type 7 (5-HT7) receptors elicits pMF, although they do so via distinct cellular mechanisms known as the Q (5-HT2) and S (5-HT7) pathways to pMF. When coactivated, these pathways interact via mutual cross-talk inhibition. Although we have a rudimentary understanding of mechanisms mediating cross-talk interactions between spinal 5-HT2 subtype A (5-HT2A) and 5-HT7 receptor activation, we do not know if similar interactions exist between 5-HT2 subtype B (5-HT2B) and 5-HT7 receptors. We confirmed that either spinal 5-HT2B or 5-HT7 receptor activation alone elicits pMF and tested the hypotheses that 1) concurrent activation of both receptors suppresses pMF due to cross-talk inhibition; 2) 5-HT7 receptor inhibition of 5-HT2B receptor-induced pMF requires protein kinase A (PKA) activity; and 3) 5-HT2B receptor inhibition of 5-HT7 receptor-induced pMF requires NADPH oxidase (NOX) activity. Selective 5-HT2B and 5-HT7 receptor agonists were administered intrathecally at C4 (3 injections, 5-min intervals) to anesthetized, paralyzed, and ventilated rats. Whereas integrated phrenic nerve burst amplitude increased after selective spinal 5-HT2B or 5-HT7 receptor activation alone (i.e., pMF), pMF was no longer observed with concurrent 5-HT2B and 5-HT7 receptor agonist administration. With concurrent receptor activation, pMF was rescued by inhibiting either NOX or PKA activity, demonstrating their roles in cross-talk inhibition between these pathways to pMF. This report demonstrates cross-talk inhibition between 5-HT2B- and 5-HT7 receptor-induced pMF and that NOX and PKA activity are necessary for that cross-talk inhibition.

Cancer cachexia impairs neural respiratory drive in hypoxia but not hypercapnia: Cancer cachexia impairs neural respiratory drive in hypoxia

Cancer cachexia is an insidious process characterized by muscle atrophy with associated motor deficits, including diaphragm weakness and respiratory insufficiency. Although neuropathology contributes to muscle wasting and motor deficits in many clinical disorders, neural involvement in cachexia‐linked respiratory insufficiency has not been explored.

The multifunctional and multi-system influence of Ghrelin in the treatment of diabetic and spinal cord injury induced Neuropathy.

Ghrelin (Gh), a small peptide, which was originally discovered as a gastrointestinal (GI) tropic hormone, has shown to have a presence and function within multiple tissue systems. Recently, Gh has shown to exhibit anti-inflammatory and regenerative abilities in response to both chemical and mechanical stressors within neural tissues. By continuing to elucidate the potential applications of Gh on pathological neural states, the viability of this peptide hormone for therapeutic uses can be explored for future clinical application.