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Mitchell GS, Baker TL. Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity. HANDBOOK OF CLINICAL NEUROLOGY 2022; 188:409-432. [PMID: 35965036 DOI: 10.1016/b978-0-323-91534-2.00016-3] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.
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Affiliation(s)
- Gordon S Mitchell
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, FL, United States.
| | - Tracy L Baker
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI, United States
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2
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Braegelmann KM, Meza A, Agbeh AE, Fields DP, Baker TL. Retinoic acid receptor alpha activation is necessary and sufficient for plasticity induced by recurrent central apnea. J Appl Physiol (1985) 2021; 130:836-845. [PMID: 33411644 DOI: 10.1152/japplphysiol.00287.2020] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Reductions in respiratory-related synaptic inputs to inspiratory motor neurons initiate a form of plasticity that proportionally enhances inspiratory motor output, even in the absence of changing blood gases. This form of plasticity is known as inactivity-induced inspiratory motor facilitation (iMF). iMF triggered by brief, recurrent reductions in respiratory neural activity requires local retinoic acid (RA) synthesis, but receptor subtypes activated by RA are unknown. To test the hypothesis that retinoic acid receptor alpha (RARα) is necessary for iMF, RAR subtype-specific inhibitors were delivered intrathecally above the phrenic motor pool in urethane-anesthetized, ventilated rats before 5, ∼1 min central apneas (without hypoxia; separated by 5 min) while monitoring phrenic inspiratory output. Pretreatment with a spinal RARα inhibitor impaired the capacity for recurrent central apnea to trigger long-lasting increases in phrenic inspiratory output, but plasticity was expressed in rats pretreated with an RARβ/γ inhibitor. Intrathecal RA application in the absence of reduced respiratory neural activity elicited an increase in phrenic inspiratory output, which was prevented by pretreatment with an RARα inhibitor. These data indicate that spinal RARα activation is necessary for iMF triggered by recurrent reductions in respiratory neural activity, and that RARα activation in/near the phrenic motor pool in the absence of respiratory neural activity deprivation is sufficient to elicit phrenic inspiratory motor facilitation. Understanding cellular cascades underlying plasticity induced by reductions in respiratory neural activity may define avenues for pharmacological intervention in disorders in which endogenous compensatory mechanisms that defend ongoing inspiratory motor output are impaired.NEW & NOTEWORTHY Local mechanisms near phrenic motor neurons respond to reductions in respiratory-related synaptic inputs by triggering a chemoreflex-independent, proportional enhancement in inspiratory output, a form of plasticity called inactivity-induced inspiratory motor facilitation (iMF). Here, we show that activation of spinal retinoic acid receptor alpha (RARα) is necessary to trigger phrenic iMF, and that spinal RARα activation in the absence of respiratory neural activity deprivation is sufficient to elicit phrenic inspiratory facilitation.
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Affiliation(s)
- Kendra M Braegelmann
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Armand Meza
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Abiye E Agbeh
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Daryl P Fields
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
| | - Tracy L Baker
- Department of Comparative Biosciences, University of Wisconsin-Madison, Madison, Wisconsin
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Perim RR, El-Chami M, Gonzalez-Rothi EJ, Mitchell GS. Baseline Arterial CO 2 Pressure Regulates Acute Intermittent Hypoxia-Induced Phrenic Long-Term Facilitation in Rats. Front Physiol 2021; 12:573385. [PMID: 33716760 PMCID: PMC7943620 DOI: 10.3389/fphys.2021.573385] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2020] [Accepted: 02/02/2021] [Indexed: 01/25/2023] Open
Abstract
Moderate acute intermittent hypoxia (mAIH) elicits a progressive increase in phrenic motor output lasting hours post-mAIH, a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). mAIH-induced pLTF is initiated by activation of spinally-projecting raphe serotonergic neurons during hypoxia and subsequent serotonin release near phrenic motor neurons. Since raphe serotonergic neurons are also sensitive to pH and CO2, the prevailing arterial CO2 pressure (PaCO2) may modulate their activity (and serotonin release) during hypoxic episodes. Thus, we hypothesized that changes in background PaCO2 directly influence the magnitude of mAIH-induced pLTF. mAIH-induced pLTF was evaluated in anesthetized, vagotomized, paralyzed and ventilated rats, with end-tidal CO2 (i.e., a PaCO2 surrogate) maintained at: (1) ≤39 mmHg (hypocapnia); (2) ∼41 mmHg (normocapnia); or (3) ≥48 mmHg (hypercapnia) throughout experimental protocols. Although baseline phrenic nerve activity tended to be lower in hypocapnia, short-term hypoxic phrenic response, i.e., burst amplitude (Δ = 5.1 ± 1.1 μV) and frequency responses (Δ = 21 ± 4 bpm), was greater than in normocapnic (Δ = 3.6 ± 0.6 μV and 8 ± 4, respectively) or hypercapnic rats (Δ = 2.0 ± 0.6 μV and −2 ± 2, respectively), followed by a progressive increase in phrenic burst amplitude (i.e., pLTF) for at least 60 min post mAIH. pLTF in the hypocapnic group (Δ = 4.9 ± 0.6 μV) was significantly greater than in normocapnic (Δ = 2.8 ± 0.7 μV) or hypercapnic rats (Δ = 1.7 ± 0.4 μV). In contrast, although hypercapnic rats also exhibited significant pLTF, it was attenuated versus hypocapnic rats. When pLTF was expressed as percent change from maximal chemoreflex stimulation, all pairwise comparisons were found to be statistically significant (p < 0.05). We conclude that elevated PaCO2 undermines mAIH-induced pLTF in anesthetized rats. These findings contrast with well-documented effects of PaCO2 on ventilatory LTF in awake humans.
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Affiliation(s)
- Raphael R Perim
- Department of Physical Therapy, McKnight Brain Institute, Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, FL, United States
| | - Mohamed El-Chami
- Department of Physical Therapy, McKnight Brain Institute, Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, FL, United States
| | - Elisa J Gonzalez-Rothi
- Department of Physical Therapy, McKnight Brain Institute, Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, FL, United States
| | - Gordon S Mitchell
- Department of Physical Therapy, McKnight Brain Institute, Center for Respiratory Research and Rehabilitation, University of Florida, Gainesville, FL, United States
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Beyeler SA, Hodges MR, Huxtable AG. Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity. Respir Physiol Neurobiol 2020; 274:103357. [PMID: 31899353 DOI: 10.1016/j.resp.2019.103357] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2019] [Revised: 11/17/2019] [Accepted: 12/02/2019] [Indexed: 10/25/2022]
Abstract
The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.
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Affiliation(s)
- Sarah A Beyeler
- Department of Human Physiology, University of Oregon, Eugene, OR, 97403, United States
| | - Matthew R Hodges
- Department of Physiology, Medical College of Wisconsin, Milwaukee, WI 53226, United States
| | - Adrianne G Huxtable
- Department of Human Physiology, University of Oregon, Eugene, OR, 97403, United States.
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Mateika JH. A reminder that experimentally induced intermittent hypoxia is an incomplete model of obstructive sleep apnea and its outcome measures. J Appl Physiol (1985) 2019; 127:1620-1621. [PMID: 31600097 DOI: 10.1152/japplphysiol.00549.2019] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Affiliation(s)
- Jason H Mateika
- Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan.,John D. Dingell Veterans Affairs Medical Center, Detroit, Michigan.,Department of Internal Medicine, Wayne State University School of Medicine, Detroit, Michigan
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Mateika JH, Panza GS, Alex R, Castillo K. Pushing and pulling with no end in sight! The role of cross-talk between different forms of respiratory plasticity in modifying sleep apnoea. J Physiol 2019; 597:3789-3790. [PMID: 31271221 DOI: 10.1113/jp278230] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Affiliation(s)
- Jason H Mateika
- John D. Dingell Veterans Affairs Medical Center, Detroit, MI, 48201, USA.,Department of Physiology, Wayne State University School of Medicine, Detroit, MI, 48201, USA.,Department of Internal Medicine, Wayne State University School of Medicine, Detroit, MI, 48201, USA
| | - Gino S Panza
- John D. Dingell Veterans Affairs Medical Center, Detroit, MI, 48201, USA.,Department of Physiology, Wayne State University School of Medicine, Detroit, MI, 48201, USA
| | - Raichel Alex
- John D. Dingell Veterans Affairs Medical Center, Detroit, MI, 48201, USA.,Department of Physiology, Wayne State University School of Medicine, Detroit, MI, 48201, USA
| | - Kenia Castillo
- John D. Dingell Veterans Affairs Medical Center, Detroit, MI, 48201, USA.,Department of Physiology, Wayne State University School of Medicine, Detroit, MI, 48201, USA
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