Phrenic motoneuron discharge patterns following chronic cervical spinal cord injury
Introduction
The injured spinal cord is, in principle, a “new spinal cord” in which neural networks and control mechanisms affecting virtually every functional domain are altered (Dimitrijevic, 1998, Edgerton et al., 2001). Accordingly, respiratory control mechanisms that have been firmly established in the spinal-intact condition are likely to be significantly altered after chronic spinal cord injury (SCI) (Sandhu et al., 2009). A basic understanding of how SCI and spontaneous neuroplastic processes impact respiratory motor control is important for developing and optimizing spinal rehabilitation and repair approaches.
Much of our current understanding of respiratory motor plasticity following SCI derives from experiments which have utilized a lateral cervical (C2) hemisection model (C2Hx) (Goshgarian, 2003, Goshgarian, 2009, Lane et al., 2008a, Sandhu et al., 2009, Zimmer et al., 2007). The C2Hx model has been used to carefully describe both the phrenic neurogram (i.e., compound action potentials) and diaphragm electromyogram (EMG) recovery profiles after incomplete cervical SCI (reviewed in Goshgarian, 2003, Goshgarian, 2009, Sandhu et al., 2009). However, it is unknown whether the gradual and functionally incomplete recovery process after chronic cervical SCI (Dougherty et al., 2012) coincides with fundamental changes in phrenic motoneuron (PhMN) recruitment profiles and discharge patterns (Lee and Fuller, 2011). In the spinal intact condition, respiratory-related PhMN bursting and recruitment are determined by both intrinsic motoneuron properties (i.e., resistance, rheobase (Berger, 1979, Dick et al., 1987, Webber and Pleschka, 1976)) and descending (i.e., bulbospinal) and possibly propriospinal synaptic inputs (Monteau et al., 1985). There is a debate as to whether PhMN recruitment patterns during breathing are driven exclusively by membrane properties (i.e., the “size principle” of motoneuron recruitment, (Henneman, 1957)), or if there is also some degree of selective synaptic input to cells recruited early vs. late in the inspiratory effort (reviewed in Lee and Fuller, 2011). In either case, since injuries to the cervical spinal cord are likely to alter both synaptic inputs (Goshgarian et al., 1989) and intrinsic motoneuron properties (Mantilla and Sieck, 2009, Sperry and Goshgarian, 1993), fundamental changes in how the central nervous system regulates PhMNs are likely after cervical SCI.
Within the cervical spinal cord, neuroplastic changes occurring after C2Hx have been postulated to facilitate PhMN recruitment (Goshgarian, 2009, Tai et al., 1997a, Tai et al., 1997b) and lead to partial functional recovery. For example, increases in cervical spinal glutamatergic (Alilain and Goshgarian, 2008, Mantilla et al., 2012) and/or serotonergic receptor expression (Basura et al., 2001, Fuller et al., 2005, Mantilla et al., 2012) could both serve to increase PhMN excitability. A similar functional impact could result from reductions in PhMN soma dimensions (Mantilla and Sieck, 2009), changes in dendrodentritic appositions, or increased number of synaptic active zones (Goshgarian, 2003, Goshgarian et al., 1989, Sperry and Goshgarian, 1993). The primary purpose of the present work was to determine how and whether PhMN discharge patterns and recruitment profiles are altered in a model of chronic, incomplete cervical SCI. Our overall hypothesis was that PhMNs would show a time-dependent increase in inspiratory burst frequency that paralleled the gradual recovery in phrenic motor output which follows C2Hx.
Section snippets
Animals
Male Sprague–Dawley rats were purchased from Harlan Inc. (Indianapolis, IN, USA). Rats were randomly assigned to the following groups: control, uninjured (N = 18, 388 ± 9 g, age 121 ± 4 days) or C2Hx injury. Rats with C2Hx were studied at 2 wks (N = 8, 327 ± 6 g, 108 ± 1 days), 4 wks (N = 8, 367 ± 7 g, 118 ± 2 days) or 8 wks (N = 7, 380 ± 15 g, 145 ± 3 days) post-injury. All experimental procedures were approved by the Institutional Animal Care and Use Committee at the University of Florida.
Spinal cord injury
The C2Hx injury was induced at ~ 3
Heart rate and blood pressure
Mean arterial blood pressure (MAP) was similar between control and C2Hx rats (Table 1). Furthermore, MAP declined in all groups when PETCO2 was reduced (P < 0.01, Table 1). Heart rate (HR) was similar between experimental groups, but some modest differences were noted (Table 1). While uninjured control rats showed an increase in HR as MAP decreased (P < 0.01), HR did not change with declining MAP in rats studied at 2 wks post-injury (Table 1). However, at 4 wks post-injury, the HR responses to
Discussion
The present results represent the first detailed analyses of PhMN firing patterns following chronic high cervical SCI. Using a C2Hx model, we observed that the distribution of PhMN bursting shifted from cells that predominantly initiated bursting early in the inspiratory effort (i.e., Early-I) to cells with a Late-I phenotype. Reductions in PhMN discharge duration and burst frequency were also prominent after C2Hx. In addition, a unique PhMN discharge pattern emerged after C2Hx in which tonic
Impact of C2Hx on PhMN discharge patterns
Respiratory-related PhMN burst patterns have been extensively studied in a variety of species in the spinal-intact condition (see reference Lee and Fuller, 2011 for a detailed review). Collectively, the literature indicates that inspiratory PhMN discharge patterns are not homogenous, but rather bursting can be separated into two groups based on the recruitment profile during inspiration. Thus, as in the current study (e.g., Fig. 1, Fig. 2), PhMN can be phenotypically classified as Early-I or
Mechanisms influencing PhMN activity following chronic C2Hx injury
The mechanisms underlying phrenic motor recovery following incomplete cervical SCI can be broadly considered either as “spinal” or “supraspinal” (Golder and Mitchell, 2005, Golder et al., 2001, Zimmer and Goshgarian, 2007). It must be emphasized, however, that spinal and supraspinal plasticity following SCI are not mutually exclusive. Indeed, the literature has documented neuroplastic changes in both the cervical spinal cord (Alilain and Goshgarian, 2008, Fuller et al., 2003, Mantilla et al.,
Tonic PhMN activity after C2Hx
The most unique aspect of PhMN activity after C2Hx was the appearance of tonic burst patterns. Tonic bursting occurred in some PhMNs throughout the respiratory cycle, and in other cells it was observed during periods of inspiratory apnea. Tonic PhMN bursting could reflect the loss of inhibitory synaptic inputs and/or the appearance of novel tonic excitatory inputs, or changes in intrinsic membrane properties. In the spinal intact condition, there is little evidence to support the prevalence of
Conclusion
A unilateral cervical SCI which removes descending synaptic inputs to the ipsilateral phrenic motor pool (i.e., C2Hx) results in alterations in the recruitment patterns and burst profiles of PhMNs. The delay in PhMN burst onset and reductions in burst frequency are likely to reflect the persistent reduction in bulbospinal excitatory synaptic inputs to the phrenic pool, although spinal neuroplasticity, including formation of de novo intraspinal circuits, may play a role. We hypothesize that the
Acknowledgments
Support for this work was provided by grants from the National Institutes of Health (NIH): 1R01NS080180-01A1 (DDF). KZL was supported by the Paralyzed Veterans of America Research Foundation (#2691), the National Science Council (NSC) NSC100-2320-B-110-003-MY2, the National Health Research Institutes (NHRI-EX102-10223NC) and the NSYSU-KMU Joint research Project (2013-I006).
References (70)
- et al.
Glutamate receptor plasticity and activity-regulated cytoskeletal associated protein regulation in the phrenic motor nucleus may mediate spontaneous recovery of the hemidiaphragm following chronic cervical spinal cord injury
Exp. Neurol.
(2008) - et al.
Distribution of serotonin 2A and 2C receptor mRNA expression in the cervical ventral horn and phrenic motoneurons following spinal cord hemisection
Exp. Neurol.
(2001) - et al.
Respiratory interneurons in the C5 segment of the spinal cord of the cat
Brain Res.
(1990) - et al.
The use of single phrenic axon recordings to assess diaphragm recovery after cervical spinal cord injury
Exp. Neurol.
(1999) - et al.
The role of propriospinal interneurons in recovery from spinal cord injury
Neuropharmacology
(2011) - et al.
Modest spontaneous recovery of ventilation following chronic high cervical hemisection in rats
Exp. Neurol.
(2008) - et al.
Graded unilateral cervical spinal cord injury and respiratory motor recovery
Respir. Physiol. Neurobiol.
(2009) The role of cervical afferent nerve fiber inhibition of the crossed phrenic phenomenon
Exp. Neurol.
(1981)The crossed phrenic phenomenon and recovery of function following spinal cord injury
Respir. Physiol. Neurobiol.
(2009)Plasticity of interneuronal networks of the functionally isolated human spinal cord
Brain Res. Rev.
(2008)
Spinal respiratory motoneurons and interneurons
Respir. Physiol. Neurobiol.
Respiratory neuroplasticity and cervical spinal cord injury: translational perspectives
Trends Neurosci.
Spinal circuitry and respiratory recovery following spinal cord injury
Respir. Physiol. Neurobiol.
Respiratory function following bilateral mid-cervical contusion injury in the adult rat
Exp. Neurol.
Neural control of phrenic motoneuron discharge
Respir. Physiol. Neurobiol.
Phrenic motoneuron expression of serotonergic and glutamatergic receptors following upper cervical spinal cord injury
Exp. Neurol.
Neuromuscular adaptations to respiratory muscle inactivity
Respir. Physiol. Neurobiol.
Phrenic motor unit recruitment during ventilatory and non-ventilatory behaviors
Respir. Physiol. Neurobiol.
Central determination of recruitment order: intracellular study of phrenic motoneurons
Neurosci. Lett.
Respiratory muscle plasticity
Respir. Physiol. Neurobiol.
Respiratory recovery following high cervical hemisection
Respir. Physiol. Neurobiol.
Role of neurotrophins in recovery of phrenic motor function following spinal cord injury
Respir. Physiol. Neurobiol.
Ultrastructural changes in the rat phrenic nucleus developing within 2 h after cervical spinal cord hemisection
Exp. Neurol.
Comparison of phrenic motoneuron activity in eupnea and apneusis
Respir. Physiol.
Distribution of calcium channel Ca(V)1.3 immunoreactivity in the rat spinal cord and brain stem
Neuroscience
Spinal cord injury in neonates alters respiratory motor output via supraspinal mechanisms
Exp. Neurol.
Light-induced rescue of breathing after spinal cord injury
J. Neurosci.
The injured spinal cord spontaneously forms a new intraspinal circuit in adult rats
Nat. Neurosci.
Synaptic inhibition of cat phrenic motoneurons by internal intercostal nerve stimulation
J. Neurophysiol.
Phrenic motoneurons in the cat: subpopulations and nature of respiratory drive potentials
J. Neurophysiol.
Antagonists with negative intrinsic activity at delta opioid receptors coupled to GTP-binding proteins
Proc. Natl. Acad. Sci. U. S. A.
Transformation of nonfunctional spinal circuits into functional states after the loss of brain input
Nat. Neurosci.
Recovery of supraspinal control of stepping via indirect propriospinal relay connections after spinal cord injury
Nat. Med.
Correlation of recruitment order with axonal conduction velocity for supraspinally driven diaphragmatic motor units
J. Neurophysiol.
Motor control in human spinal cord injury
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