Quantitative assessment of phrenic nerve functional recovery mediated by the crossed phrenic reflex at various time intervals after spinal cord injury

Effects of serotonin inhibition on neuronal and astrocyte plasticity in the phrenic nucleus 4 h following C2 spinal cord hemisection

C2 spinal cord hemisection results in synaptic and astroglial changes in the phrenic nucleus which have been associated with the recovery of the ipsilateral hemidiaphragm during expression of the crossed phrenic phenomenon. As part of our ongoing analysis of the neurotransmitters involved, the present study investigated the effects of systemic administration of para-chlorophenylalanine (p-CPA), a serotonin (5-HT) synthesis inhibitor, on plasticity in the rat phrenic nucleus 4 h following C2 hemisection. Hemisected control rats demonstrated typical morphological changes in the ipsilateral phrenic nucleus including: (1) an increased number and length of synaptic active zones and (2) an increased number and length of dendrodendritic membrane appositions. p-CPA treatment 3 days prior to hemisection reduced 5-HT levels and resulted in an attenuation of these changes in the ipsilateral phrenic nucleus 4 h following hemisection compared to hemisected controls. In addition, p-CPA treatment attenuated injury-induced alterations in immunohistochemical staining of glial fibrillary acidic protein (GFAP), although Western blot analysis demonstrated that overall levels of GFAP did not differ significantly between groups. The results suggest that inhibition of 5-HT synthesis by p-CPA attenuates hemisection-induced plasticity in the phrenic nucleus 4 h following an ipsilateral C2 hemisection.

Effects of serotonin on crossed phrenic nerve activity in cervical spinal cord hemisected rats

The present study investigates the effect of 5-hydroxytryptophan (5-HTP), a serotonin precursor, on crossed phrenic nerve activity (CPNA) in rats subjected to a left C2 spinal cord hemisection. Electrophysiological experiments were conducted on anesthetized, vagotomized, paralyzed, and artificially ventilated rats to assess phrenic nerve activity. The left phrenic nerve lost rhythmic activity due to the disruption of the bulbospinal respiratory pathways following spinal cord hemisection. Activity was induced in the left phrenic nerve (CPNA) by temporary asphyxia. 5-HTP administration increased CPNA during asphyxia in the left phrenic nerve in a dose-dependent fashion. Specifically, in a group of eight animals, application of 5-HTP at 0.5, 1.0, and 2.0 mg/kg significantly increased CPNA by 102.2+/-18.5%, 200.8+/-58.1%, and 615.0+/-356.9% compared with predrug control values, respectively. 5-HTP-induced increases in CPNA were reversed by methysergide (2-6 mg/kg, i.v.), a serotonin receptor antagonist. The results suggest that serotonin is involved in the modulation of crossed phrenic nerve activity following spinal cord injury.

Basic fibroblast growth factor increases long-term survival of spinal motor neurons and improves respiratory function after experimental spinal cord injury

Acute focal injection of basic fibroblast growth factor (FGF2) protects ventral horn (VH) neurons from death after experimental contusive spinal cord injury (SCI) at T8. Because these neurons innervate respiratory muscles, we hypothesized that respiratory deficits resulting from SCI would be attenuated by FGF2 treatment. To test this hypothesis we used a head-out plethysmograph system to evaluate respiratory parameters in conscious rats before and at 24 hr and 7, 28, and 35 d after SCI. Two groups of rats (n = 8 per group) received either FGF2 (3 microg) beginning 5 min after injury or vehicle (VEH) solution alone. We found significantly increased respiratory rate and decreased tidal volume at 24 hr and 7 d after SCI in the VEH-treated group. Ventilatory response to breathing 5 or 7% CO(2) was also significantly reduced. Recovery took place over time. Respiration remained normal in the FGF2-treated group. At 35 d after injury, histological analyses were used to compare long-term neuron survival. FGF2 treatment doubled the survival of VH neurons adjacent to the injury site. Because the number of surviving VH neurons rostral to the injury epicenter was significantly correlated to the ventilatory response to CO(2), it is likely that the absence of respiratory deficits in FGF2-treated rats was caused by its neuroprotective effect. Our results demonstrate that FGF2 treatment prevents the respiratory deficits produced by thoracic SCI. Because FGF2 also reduced the loss of preganglionic sympathetic motoneurons after injury, this neurotrophic factor may have broad therapeutic potential for SCI.

The use of single phrenic axon recordings to assess diaphragm recovery after cervical spinal cord injury

Electrophysiological recordings taken from the whole phrenic nerve have been utilized previously to describe the gradual increase in functional recovery of a hemidiaphragm paralyzed by ipsilateral C2 hemisection during the crossed phrenic phenomenon (CPP). Although the increase in activity has been temporally correlated with hemisection-induced morphological alterations of the phrenic nucleus, suggesting an association of the increased activity with the morphological alterations, whole phrenic nerve recordings during the CPP can provide only limited information. The purpose of the present study, therefore, was to use phrenic single-axon recording techniques to better understand the mechanisms underlying the recovery of respiratory activity during the expression of the CPP. Recordings from the whole phrenic nerve on the right side and from small fascicles of the phrenic nerve that show only the activity of single phrenic axons (units) on the left side were made in the neck before left spinal hemisection and during the CPP. The results indicated that there were two types of units firing before and during the CPP: an early- and a late-firing unit based on the time of their firing onset in relation to whole phrenic nerve activity. Ten early units and 25 late units were identified according to the shape of their spikes before hemisection as well as during the CPP. In addition to these units, 20 new units were recruited during CPP activity. These new units were mainly of the late-onset type. The results also indicated that there was a significant increase in the frequency of firing of both early and late units. The results specifically indicate therefore that the increase in respiratory activity recorded previously in the whole phrenic nerve during the CPP is most likely due to: (i) an increase in firing frequency for both early- and late-firing units and (ii) a recruitment of predominantly late-firing units into the CPP response. These results are important in understanding more completely the mechanisms that can facilitate recovery of the diaphragm after spinal cord injury.

Theophylline-induced recovery in a hemidiaphragm paralyzed by hemisection in rats: contribution of adenosine receptors

Previously, we demonstrated that a single intravenous injection of theophylline can induce recovery in a hemidiaphragm paralyzed by cervical (C2) spinal cord hemisection for up to 3 h. The present study contrasts the actions of enprofylline and theophylline on inducing hemidiaphragmatic recovery after cervical spinal cord hemisection. Both drugs are methylxanthines; however, theophylline is an adenosine receptor antagonist while enprofylline is not. To further test the involvement of adenosine receptors, N6 (L-2-phenylisopropyl) adenosine (L-PIA), an analogue of adenosine was used in conjunction with theophylline. Following a left C2 spinal cord hemisection, animals were injected with either enprofylline (2.5-20 mg/kg) or theophylline (15 mg/kg) alone or in combination. Theophylline-injected animals demonstrated robust respiratory-related activity in the previously quiescent left phrenic nerve and hemidiaphragm. No recovery was observed in any of the enprofylline-injected rats. When enprofylline injection was followed later with theophylline, recovery occurred. Prior L-PIA administration blocked theophylline-induced recovery. When given after theophylline, L-PIA attenuated and then blocked the induced activity in both the nerve and hemidiaphragm ipsilateral to spinal cord hemisection. We conclude that adenosine receptor antagonism is implicated in hemidiaphragmatic recovery after hemisection and theophylline may be useful in the treatment of spinal cord injured patients with respiratory deficits.

Quantitative assessment of respiratory function following contusion injury of the cervical spinal cord

In this study, we describe a new method for quantitative assessment of phrenic inspiratory motor activity in two models of cervical spinal cord contusion injury. Anesthetized rats received contusion injury either to the descending bulbospinal respiratory pathway on one side of the spinal cord alone (C2 lateralized contusion) or to both the descending pathway, as well as the phrenic motoneuron pool bilaterally (C4/C5 midline contusion). Following injury, respiratory-associated phrenic nerve motor activity was recorded under standardized and then asphyxic conditions. Phrenic nerve efferent activity was rectified, integrated, and quantitated by determining the mean area under the integrated neurograms. The mean integrated area of the four inspiratory bursts recorded just before turning off the ventilator (to induce asphyxia) was determined and divided by the integrated area under the single largest respiratory burst recorded during asphyxia. This latter value was taken as the maximal inspiratory motor response that the rat was capable of generating during respiratory stress. Thus, a percentage of the maximal inspiratory motor drive was established for breathing in control and injured rats under standardized conditions. The results indicate that noninjured rats use 52 +/- 1.8% of maximal inspiratory motor drive under standardized conditions. In C2-contused rats, the results showed that while the percentage of maximal inspiratory motor drive on the noncontused side was similar to the control (55 +/- 4.1%), it was increased on the contused side (78 +/- 2.6%). In C4/5 lesions, the results indicate that this percentage was increased on both sides (77 +/- 4.4%). The results show the feasibility for performing quantitative evaluation of respiratory dysfunction in an animal model of cervical contusion injury. These findings lend to further development of this model for investigations of neuroplasticity and/or therapeutic interventions directed at ameliorating respiratory compromise following cervical spinal cord trauma.

Glial changes in the phrenic nucleus following superimposed cervical spinal cord hemisection and peripheral chronic phrenicotomy injuries in adult rats

The objective of the present study was to characterize the microglial and astroglial reaction in the phrenic nucleus following either an ipsilateral C2 spinal cord hemisection, a peripheral phrenicotomy, or a combination of the two injuries in the same adult rat. The present study used three different fluorescent markers and a confocal laser image analysis system to study glial cells and phrenic motoneurons at the light microscopic level. Young adult female rats were divided into one combined injury group (left phrenicotomy and left C2 spinal hemisection with periods of 1 to 4 weeks between injuries, N = 12) and three other groups consisting of noninjured animals (N = 3), animals that received C2 hemisection only (N = 3), and animals with phrenicotomy only (survival periods of 2 (N = 3) and 4 (N = 3) weeks after phrenicotomy). Fluorogold was injected into the diaphragm to label phrenic motoneurons in all animals. Microglia and astrocytes were labeled with Texas red and fluorescein, respectively, and were visualized simultaneously along with phrenic motoneurons. The results suggest that the microglial and astrocytic response in the superimposed injury model are similar to the glial reactions characteristically seen in a peripheral axotomy alone model. These reactions include proliferation and migration of microglial cells along the perineuronal surface (peaking at 2 weeks) and the hypertrophy of astrocytes (peaking at 4 weeks). In addition, the increase in astrocytic tissue, which is characteristically seen in response to axotomy alone, is significantly enhanced in the superimposed injury model. Also, there is a large and rapid increase in GFAP-positive astrocytes within 24 hours after hemisection alone. The information gained from the present study will aid in determining, predicting, and eventually manipulating central nervous system responses to multiple injuries with the objective of reestablishing function in the damaged CNS.

Morphological plasticity induced in the phrenic nucleus following cervical cold block of descending respiratory drive

Morphological plasticity occurs in the phrenic nucleus within hours following an ipsilateral C2 spinal cord hemisection. The plasticity has been associated with the unmasking of a latent respiratory pathway (the crossed phrenic pathway) which allows recovery of the hemidiaphragm paralyzed by the hemisection during a reflex known as the crossed phrenic phenomenon. This study tests if the plasticity is induced by the generalized effects of spinal cord trauma or the more specific effect of interrupting the main descending respiratory drive to phrenic motoneurons. Electron microscopic quantitative morphometric analysis of the phrenic nucleus neuropil was carried out on four Sprague-Dawley rats (200-250 g) sacrificed 4 h following unilateral reversible cold block of the descending bulbospinal respiratory drive at the second cervical segment of the spinal cord (C2). The data from four sham-operated control animals were compared with those of the experimental group. The following morphological alterations were documented in cold block animals compared to controls: (1) a significant increase in the number of multiple synapses (i.e., terminals with synaptic active zones contacting two or more postsynaptic profiles in the same plane of section), (2) a significant increase in the number of dendrodendritic appositions, and (3) a significant increase in the length of symmetric and asymmetric synaptic active zones. The above changes are similar to the changes induced in the phrenic nucleus following C2 hemisection. We conclude therefore, that injury to the spinal cord is not a requirement for this type of morphological plasticity in the phrenic nucleus, but rather the induced changes are activity-dependent and are likely caused by the interruption of the descending bulbospinal respiratory drive to the phrenic nucleus.

The superimposed effects of chronic phrenicotomy and cervical spinal cord hemisection on synaptic cytoarchitecture in the rat phrenic nucleus

The present study was carried out to determine the effects of a combined peripheral phrenicotomy and rostral spinal cord hemisection on the synaptic architecture in the ipsilateral rat phrenic nucleus. Young adult female Sprague-Dawley rats were divided into a hemisection-alone and two hemisection-plus-phrenicotomy (HPP) groups. In all animals, DiI, a fluorescent carbocyanine dye was injected into the left hemidiaphragm to retrogradely label the ipsilateral phrenic motoneurons. In the HPP groups, left intrathoracic phrenicotomies were carried out at 2 and 4 weeks prior to sacrificing. Hemisection-alone animals were not subjected to phrenicotomy. In all animals, a left C2 spinal cord hemisection was performed 24 h prior to death. Quantitative morphometric analysis of the phrenic nucleus showed that the number of synapses contacting phrenic profiles is significantly less in the HPP (2 week) group as compared to the hemisection-alone group, but this number returns to a level not significantly different from the hemisection-alone value in the HPP (4 week) group. The results suggest that the transient change in the number of synapses might contribute to the differential expression of the crossed phrenic phenomenon documented in another group of animals subjected to the same surgical procedures. Furthermore, the different stages of glial reaction induced by phrenicotomy/spinal cord hemisection might underlie the change in synaptic number.

Structural changes of anterior horn neurons and their synaptic input caudal to a low thoracic spinal cord hemisection in the adult rat: a light and electron microscopic study

Structural changes in lumbosacral ventral horn neurons and their synaptic input were studied at 3, 10, 21, 42, and 90 days following low thoracic cord hemisection in adult rats by light microscopic examination of synaptophysin immunoreactivity (SYN-IR) and by electron microscopy. There was an ipsilateral transient decrease in SYN-IR at the somal and proximal dendritic surfaces of anterior horn neurons which extended caudally from the site of injury over a postoperative (p.o.) period of 42 days. Concomitantly, at 21 days p.o., perineuronal SYN-IR started to recover in upper lumbar segments. By 90 days p.o., a normal staining pattern of SYN was noted in upper and mid lumbar segments, but the perineuronal SYN-IR was still slightly below normal levels in low lumbar and sacral segments. Electron microscopy revealed ultrastructural changes coincident with the alterations in SYN-IR. At 3 days p.o., phagocytosis of degenerating axon terminals by activated microglial cells was observed at the somal and proximal dendritic surfaces of ventral horn neurons. These changes were most prominent up to two segments caudal to the lesion. At 10 days p.o., advanced stages of bouton phagocytosis were still detectable in all lumbosacral motor nuclei. Additionally, abnormal axon terminals, with a few dispersed synaptic vesicles and accumulations of large mitochondria, appeared at the scalloped somal surfaces of anterior horn neurons. At 21 days p.o., several large lumbosacral motoneurons had developed chromatolysis-like ultrastructural alterations and motoneuronal cell bodies had become partially covered by astrocytic lamellae. At 42 days p.o., there was a transient appearance of polyribosomes in some M-type boutons. In addition, at 42 and 90 days p.o., a few degenerating motoneurons were detected in all lumbosacral segments, but most displayed normal neuronal cell bodies contacted by numerous intact synapses as well as by astrocytic processes. In contrast to these striking alterations of synaptic input at somal and proximal dendritic surfaces of motoneurons, relatively few degenerating boutons were detected in the neuropil of motor nuclei at all the p.o. times studied. We suggest that the preferential disturbance of the predominantly inhibitory axosomatic synapses on ventral horn neurons may be involved in the mechanisms which influence the well-established increase in motoneuronal excitability after spinal cord injury.

Bulbospinal respiratory neurons are a source of double synapses onto phrenic motoneurons following cervical spinal cord hemisection in adult rats

The purpose of this study was to determine if the medullary neurons that provide the primary excitatory drive to phrenic motoneurons (i.e., rostral ventral respiratory group, rVRG) are a source of double synapse formation in the phrenic nucleus after spinal cord hemisection. The axons of rVRG neurons either ipsilateral or contralateral to the hemisection were labeled by injection of a mixture of HRP and WGA-HRP into the rostral ventral respiratory group. Phrenic motoneurons ipsilateral and caudal to the hemisection were labeled by the retrograde transport of HRP. The ultrastructural results indicated that after hemisection, rVRG neurons from both sides of the medulla formed labelled double synapses in the phrenic nucleus.

Distribution of bulbospinal neurons supplying bilateral innervation to the phrenic nucleus in the rat

The location of bulbospinal neurons with axon collaterals in both phrenic nuclei were determined by injecting two different fluorescent tracers into the right and left C4 cervical spinal cord. In contrast to single-labeled neurons that were found throughout the rostrocaudal extent of the medulla, the majority of double-labeled neurons were located in the rostral ventral respiratory group. Only a few double-labeled neurons were found in the ventrolateral nucleus of the solitary tract. The role of this bilateral pathway in synchronizing the activity of the phrenic nucleus is discussed.