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Baertsch NA, Marciante AB, Mitchell GS, Baker TL. Inactivity-induced phrenic motor facilitation requires PKCζ activity within phrenic motor neurons. J Neurophysiol 2024; 131:1188-1199. [PMID: 38691529 DOI: 10.1152/jn.00138.2024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2024] [Revised: 04/29/2024] [Accepted: 04/30/2024] [Indexed: 05/03/2024] Open
Abstract
Prolonged inhibition of respiratory neural activity elicits a long-lasting increase in phrenic nerve amplitude once respiratory neural activity is restored. Such long-lasting facilitation represents a form of respiratory motor plasticity known as inactivity-induced phrenic motor facilitation (iPMF). Although facilitation also occurs in inspiratory intercostal nerve activity after diminished respiratory neural activity (iIMF), it is of shorter duration. Atypical PKC activity in the cervical spinal cord is necessary for iPMF and iIMF, but the site and specific isoform of the relevant atypical PKC are unknown. Here, we used RNA interference to test the hypothesis that the zeta atypical PKC isoform (PKCζ) within phrenic motor neurons is necessary for iPMF but PKCζ within intercostal motor neurons is unnecessary for transient iIMF. Intrapleural injections of siRNAs targeting PKCζ (siPKCζ) to knock down PKCζ mRNA within phrenic and intercostal motor neurons were made in rats. Control rats received a nontargeting siRNA (NTsi) or an active siRNA pool targeting a novel PKC isoform, PKCθ (siPKCθ), which is required for other forms of respiratory motor plasticity. Phrenic nerve burst amplitude and external intercostal (T2) electromyographic (EMG) activity were measured in anesthetized and mechanically ventilated rats exposed to 30 min of respiratory neural inactivity (i.e., neural apnea) created by modest hypocapnia (20 min) or a similar recording duration without neural apnea (time control). Phrenic burst amplitude was increased in rats treated with NTsi (68 ± 10% baseline) and siPKCθ (57 ± 8% baseline) 60 min after neural apnea vs. time control rats (-3 ± 3% baseline), demonstrating iPMF. In contrast, intrapleural siPKCζ virtually abolished iPMF (5 ± 4% baseline). iIMF was transient in all groups exposed to neural apnea; however, intrapleural siPKCζ attenuated iIMF 5 min after neural apnea (50 ± 21% baseline) vs. NTsi (97 ± 22% baseline) and siPKCθ (103 ± 20% baseline). Neural inactivity elevated the phrenic, but not intercostal, responses to hypercapnia, an effect that was blocked by siPKCζ. We conclude that PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient iIMF.NEW & NOTEWORTHY We report important new findings concerning the mechanisms regulating a form of spinal neuroplasticity elicited by prolonged inhibition of respiratory neural activity, inactivity-induced phrenic motor facilitation (iPMF). We demonstrate that the atypical PKC isoform PKCζ within phrenic motor neurons is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for, transient inspiratory intercostal facilitation. Our findings are novel and advance our understanding of mechanisms contributing to phrenic motor plasticity.
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Affiliation(s)
- Nathan A Baertsch
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin, United States
| | - Alexandria B Marciante
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, Florida, United States
| | - Gordon S Mitchell
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida, Gainesville, Florida, United States
| | - Tracy L Baker
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin, United States
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Marciante AB, Tadjalli A, Burrowes KA, Oberto JR, Luca EK, Seven YB, Nikodemova M, Watters JJ, Baker TL, Mitchell GS. Microglia regulate motor neuron plasticity via reciprocal fractalkine/adenosine signaling. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.05.07.592939. [PMID: 38765982 PMCID: PMC11100694 DOI: 10.1101/2024.05.07.592939] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2024]
Abstract
Microglia are innate CNS immune cells that play key roles in supporting key CNS functions including brain plasticity. We now report a previously unknown role for microglia in regulating neuroplasticity within spinal phrenic motor neurons, the neurons driving diaphragm contractions and breathing. We demonstrate that microglia regulate phrenic long-term facilitation (pLTF), a form of respiratory memory lasting hours after repetitive exposures to brief periods of low oxygen (acute intermittent hypoxia; AIH) via neuronal/microglial fractalkine signaling. AIH-induced pLTF is regulated by the balance between competing intracellular signaling cascades initiated by serotonin vs adenosine, respectively. Although brainstem raphe neurons release the relevant serotonin, the cellular source of adenosine is unknown. We tested a model in which hypoxia initiates fractalkine signaling between phrenic motor neurons and nearby microglia that triggers extracellular adenosine accumulation. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF; in contrast, severe AIH drives pLTF by a unique, adenosine-dominant mechanism. Phrenic motor neuron fractalkine knockdown, cervical spinal fractalkine receptor inhibition on nearby microglia, and microglial depletion enhance serotonin-dominant pLTF with moderate AIH but suppress adenosine-dominant pLTF with severe AIH. Thus, microglia play novel functions in the healthy spinal cord, regulating hypoxia-induced neuroplasticity within the motor neurons responsible for breathing.
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Affiliation(s)
- Alexandria B. Marciante
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
| | - Arash Tadjalli
- Current Address: Nova Southeastern University, College of Allopathic Medicine (NSU MD), Department of Medical Education, 3200 South University Drive, Fort Lauderdale, FL 33328-2018
| | - Kayla A. Burrowes
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
| | - Jose R. Oberto
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
| | - Edward K. Luca
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
| | - Yasin B. Seven
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
| | - Maria Nikodemova
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
| | - Jyoti J. Watters
- Current Address: Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706
| | - Tracy L. Baker
- Current Address: Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706
| | - Gordon S. Mitchell
- Breathing Research and Therapeutics Center, Department of Physical Therapy and McKnight Brain Institute, University of Florida; Gainesville, FL, USA 32610
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Anand SK, Lavadi RS, Johnston BR, Chalif JI, Scanlon JM, Wang W, Agarwal N, Hamilton DK, Fields DP, Van't Land CW. Time of day does not impact spinal serotonin levels in humans. Synapse 2024; 78:e22291. [PMID: 38733105 DOI: 10.1002/syn.22291] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2024] [Revised: 04/10/2024] [Accepted: 04/20/2024] [Indexed: 05/13/2024]
Abstract
Spinal serotonin enables neuro-motor recovery (i.e., plasticity) in patients with debilitating paralysis. While there exists time of day fluctuations in serotonin-dependent spinal plasticity, it is unknown, in humans, whether this is due to dynamic changes in spinal serotonin levels or downstream signaling processes. The primary objective of this study was to determine if time of day variations in spinal serotonin levels exists in humans. To assess this, intrathecal drains were placed in seven adults with cerebrospinal fluid (CSF) collected at diurnal (05:00 to 07:00) and nocturnal (17:00 to 19:00) intervals. High performance liquid chromatography with mass spectrometry was used to quantify CSF serotonin levels with comparisons being made using univariate analysis. From the 7 adult patients, 21 distinct CSF samples were collected: 9 during the diurnal interval and 12 during nocturnal. Diurnal CSF samples demonstrated an average serotonin level of 216.6 ± $ \pm $ 67.7 nM. Nocturnal CSF samples demonstrated an average serotonin level of 206.7 ± $ \pm $ 75.8 nM. There was no significant difference between diurnal and nocturnal CSF serotonin levels (p = .762). Within this small cohort of spine healthy adults, there were no differences in diurnal versus nocturnal spinal serotonin levels. These observations exclude spinal serotonin levels as the etiology for time of day fluctuations in serotonin-dependent spinal plasticity expression.
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Affiliation(s)
- Sharath Kumar Anand
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Raj S Lavadi
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Benjamin R Johnston
- Department of Neurological Surgery, Brigham and Women's Hospital, Boston, Massachusetts, USA
| | - Joshua I Chalif
- Department of Neurological Surgery, Brigham and Women's Hospital, Boston, Massachusetts, USA
| | - James M Scanlon
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Weiwen Wang
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Nitin Agarwal
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - David K Hamilton
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
| | - Daryl P Fields
- Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
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Bhagavan H, Burgraff NJ, Ramirez JM. Temporal Dynamics and Therapeutic Implications of Phrenic Long-Term Facilitation in Respiratory Control. FUNCTION 2023; 5:zqad068. [PMID: 38143850 PMCID: PMC10742368 DOI: 10.1093/function/zqad068] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2023] [Accepted: 11/23/2023] [Indexed: 12/26/2023] Open
Affiliation(s)
- Hemalatha Bhagavan
- Center for Integrative Brain Research, Seattle Children’s Research Institute, Seattle, WA 98101, USA
| | - Nicholas J Burgraff
- Center for Integrative Brain Research, Seattle Children’s Research Institute, Seattle, WA 98101, USA
| | - Jan-Marino Ramirez
- Center for Integrative Brain Research, Seattle Children’s Research Institute, Seattle, WA 98101, USA
- Department of Pediatrics, University of Washington, Seattle, WA 98105, USA
- Department of Neurological Surgery, University of Washington, Seattle, WA 98104, USA
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