Fate of the supraspinal collaterals of cord-projection neurons following upper spinal axonal injury

The efficacy of end-to-end and end-to-side nerve repair (neurorrhaphy) in the rat brachial plexus

Proximal nerve injury often requires nerve transfer to restore function. Here we evaluated the efficacy of end-to-end and end-to-side neurorrhaphy of rat musculocutaneous nerve, the recipient, to ulnar nerve, the donor. The donor was transected for end-to-end, while an epineurial window was exposed for end-to-side neurorrhaphy. Retrograde tracing showed that 70% donor motor and sensory neurons grew into the recipient 3 months following end-to-end neurorrhaphy compared to 40-50% at 6 months following end-to-side neurorrhaphy. In end-to-end neurorrhaphy, regenerating axons appeared as thick fibers which regained diameters comparable to those of controls in 3-4 months. However, end-to-side neurorrhaphy induced slow sprouting fibers of mostly thin collaterals that barely approached control diameters by 6 months. The motor end plates regained their control density at 4 months following end-to-end but remained low 6 months following end-to-side neurorrhaphy. The short-latency compound muscle action potential, typical of that of control, was readily restored following end-to-end neurorrhaphy. End-to-side neurorrhaphy had low amplitude and wide-ranging latency at 4 months and failed to regain control sizes by 6 months. Grooming test recovered successfully at 3 and 6 months following end-to-end and end-to-side neurorrhaphy, respectively, suggesting that powerful muscle was not required. In short, both neurorrhaphies resulted in functional recovery but end-to-end neurorrhaphy was quicker and better, albeit at the expense of donor function. End-to-side neurorrhaphy supplemented with factors to overcome the slow collateral sprouting and weak motor recovery may warrant further exploration.

Spinal Axonal Injury Induces Brief Downregulation of Ionotropic Glutamate Receptors and No Stripping of Synapses in Cord-Projection Central Neurons

Spinal cord injury often damages the axons of cord-projecting central neurons. To determine whether their excitatory inputs are altered following axonal injury, we used rat rubrospinal neurons as a model and examined their excitatory input following upper cervical axotomy. Anterograde tracing showed that the primary afferents from the cerebellum terminated in a pattern similar to that of control animals. Ultrastructurally, neurons in the injured nucleus were contacted by excitatory synapses of normal appearance, with no sign of glial stripping. Since cerebellar fibers are glutamatergic, we examined the expression of ionotropic receptor subunits GluR1-4 and NR1 for AMPA and NMDA receptors, respectively, in control and injured neurons using immunolabeling methods. In control neurons, GluR2 appeared to be low as compared to GluR1, GluR3, and GluR4, while NR1 labeling was intense. Following unilateral tractotomy, the levels of expression of each subunit in axotomized neurons appeared to be normal, with the exception that they were lower than those of control neurons of the nonlesioned side at 2-6 days postinjury. These findings suggest that axotomized neurons are only temporarily protected from excitotoxicity. This is in sharp contrast to the responses of central neurons that innervate peripheral targets, in which both synaptic stripping and reduction of their ionotropic glutamate receptor subunits persist following axotomy. The absence of an injury-induced trimming of afferents and stripping of synapses and the lack of a persistent downregulation of postsynaptic receptors might enable injured cord-projection neurons to continue to control their supraspinal targets during most of their postinjury survival. Although this may support neurons by providing trophic influences, it nevertheless may subject them to excitotoxicity and ultimately lead to their degenerative fate.

Spinal axonal injury transiently elevates the level of metabotropic glutamate receptor 5, but not 1, in cord-projection central neurons

In investigating the effect of spinal injury on cord-projection central neurons, we found that rat rubrospinal neurons retained glutamatergic afferents and, in general, ionotropic glutamate receptor expression following spinal axotomy. Since glutamate also acts on second-messenger-coupled metabotropic receptors, the expression of group I metabotropic glutamate receptors, mGluR1 and mGluR5, was examined following similar treatment. mGluR1 expression began to decline in the perikarya 2 days postlesion and a day later in the neuropil. The decline slowed down by the fifth day and recovered in both the perikarya and neuropil 1 week postlesion. However, expression in both the perikarya and neuropil declined again and persisted up to 2 years postlesion. Similarly, the mGluR5 displayed an early transient decrease and returned to normal levels by 7 days post-lesion. However, rather than progressing to a secondary decline, the expression of mGluR5 increased to levels dramatically higher than those of control nuclei at 2-4 weeks postlesion, subsiding again by 8 weeks, and remaining low up to 2 years postinjury. Although mGluR5 has been shown to save cultured neurons from excitotoxic cell death, its elevated expression in the present model corresponds in time to an increased input/output relationship and excitability of the injured neurons as well as a period of maximal somatic shrinkage and cell loss. In addition to the cell bodies and dendrites, axon-like profiles also contain mGluR1. Their decrease following rubrospinal axotomy suggests that axonal injury may also compromise the presynaptic regulation of afferent activities onto injured cord-projection central neurons.

Respiratory motor recovery after unilateral spinal cord injury: eliminating crossed phrenic activity decreases tidal volume and increases contralateral respiratory motor output

By 2 months after unilateral cervical spinal cord injury (SCI), respiratory motor output resumes in the previously quiescent phrenic nerve. This activity is derived from bulbospinal pathways that cross the spinal midline caudal to the lesion (crossed phrenic pathways). To determine whether crossed phrenic pathways contribute to tidal volume in spinally injured rats, spontaneous breathing was measured in anesthetized C2 hemisected rats at 2 months after injury with an intact ipsilateral phrenic nerve, or with ipsilateral phrenicotomy performed at the time of the SCI (i.e., crossed phrenic pathways rendered ineffective) (dual injury). Ipsilateral phrenicotomy did not alter the rapid shallow eupneic breathing pattern in C2 injured rats. However, the ability to generate large inspiratory volumes after either vagotomy or during augmented breaths was impaired if crossed phrenic activity was abolished. We also investigated whether compensatory plasticity in contralateral motoneurons would be affected by eliminating crossed phrenic activity. Thus, contralateral phrenic motor output was recorded in anesthetized, vagotomized, and mechanically ventilated rats with dual injury during chemoreceptor stimulation. Hypercapnia, hypoxia, and asphyxia increased contralateral phrenic burst amplitude in the dual injury group more than in rats with SCI alone. Dual injury rats also had elevated baseline burst frequency. Together, these results demonstrate a functional role of crossed phrenic activity after SCI. Moreover, by preventing ipsilateral phrenic motor recovery in rats with unilateral SCI, segmental and supraspinal changes could be induced in contralateral respiratory motor output beyond that seen with SCI alone.

Close axonal injury of rubrospinal neurons induced transient perineuronal astrocytic and microglial reaction that coincided with their massive degeneration

To learn more about the pathophysiology of axonal injury and the significance of axon collaterals on the survival of axotomized cord-projection central neurons, we studied the survival rate, surrounding astrocytic and microglial reactions, and bouton coverage on rat rubrospinal cell bodies following their axonal lesion at the brain stem and upper cervical level. The brain stem lesion disconnected most rubrospinal neurons from all their targets, while the upper cervical lesion spared their supraspinal collaterals. Much higher cell loss accompanied by robust astrocytic and microglial reaction was found following brain stem than upper cervical lesion starting 4 days postaxotomy. The reaction of astrocytes had subsided while microglial reaction remained relatively robust by 10 weeks postaxotomy when the cell loss had slowed down. Ultrastructural observation revealed that reactive astrocytes covered 40%, an increase from the 20% of control, of brain stem-axotomized rubrospinal cell body surface at 4 days and 2 weeks and returned to normal levels by 10 weeks postlesion. An increase of apposition by axons and dendrites and a moderate decrease of round and flattened vesicle-containing bouton contacts at 4 days and 2 weeks and returning to normal levels at 10 weeks postaxotomy accompanied this. It appears that although axotomy induced robust astrocytic reaction around cord-projection central neurons, this, unlike their periphery-projection counterparts, failed to effectively strip their somatic synapses. In effect, this might in part determine neuronal fate following axonal injury.

Fate of the soma and dendrites of cord-projection central neurons after proximal and distal spinal axotomy: an intracellular dye injection study

We used rat rubrospinal neurons as a model to study the soma-dendritic morphology of cord-projection neurons following spinal axonal injury. We examined lumbar-projection neurons following both upper cervical and lower thoracic axotomy to find out whether changes were dependent on the proximity of the lesion to the cell body. Axotomized neurons were marked with retrograde tracer and studied 4 and 8 weeks later with intracellular dye injection technique. Axotomy resulted in prominent shrinkage of their soma and relatively minor reduction of their dendritic spreads. The degree of soma shrinkage depended on both the duration of survival and the proximity of lesion. In addition, dendritic modification peaked 4 weeks following proximal lesion, which was also achieved 8 weeks following distal axotomy. Tractotomy at upper cervical and lower thoracic levels also allowed us to compare the effect of distal axotomy on cervical and lumbar-projection neurons. Results show that although cervical-projection neurons responded more quickly than lumbar-projecting ones, they however showed a similar degree of alteration in both their soma and dendrites 8 weeks following distal axotomy. In summary, cord-projection neurons survived 8 weeks following either upper cervical or lower thoracic axotomy with relatively intact dendritic features. Taken together, our data thus far suggest that cord-projection central neurons continue to integrate inputs and control supraspinal targets following spinal axotomy. The minor dendritic shrinkage within two months of spinal axotomy rejuvenates hopes for functional recovery if regeneration of their spinal axons can be achieved at least within this time frame.

Altered respiratory motor drive after spinal cord injury: supraspinal and bilateral effects of a unilateral lesion

Because some bulbospinal respiratory premotor neurons have bilateral projections to the phrenic nuclei, we investigated whether changes in contralateral phrenic motoneuron function would occur after unilateral axotomy via C(2) hemisection. Phrenic neurograms were recorded under baseline conditions and during hypercapnic and hypoxic challenge in C(2) hemisected, normal, and sham-operated rats at 1 and 2 months after injury. The rats were anesthetized, vagotomized, and mechanically ventilated. No group differences were seen in contralateral neurograms at 1 month after injury. At 2 months, however, there was a statistically significant decrease in respiratory rate (RR) at normocapnia, an elevated RR during hypoxia, and an attenuated increase in phrenic neurogram amplitude during hypercapnia in the C(2)-hemisected animals. To test whether C(2) hemisection had induced a supraspinal change in respiratory motor drive, we recorded ipsilateral and contralateral hypoglossal neurograms during hypercapnia. As with the phrenic motor function data, no change in hypoglossal output was evident until 2 months had elapsed when hypoglossal amplitudes were significantly decreased bilaterally. Last, the influence of serotonin-containing neurons on the injury-induced change in phrenic motoneuron function was examined in rats treated with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Pretreatment with 5,7-dihydroxytryptamine prevented the effects of C(2) hemisection on contralateral phrenic neurogram amplitude and normalized the change in RR during hypoxia. The results of this study show novel neuroplastic changes in segmental and brainstem respiratory motor output after C(2) hemisection that coincided with the spontaneous recovery of some ipsilateral phrenic function. Some of these effects may be modulated by serotonin-containing neurons.