Repeated injury to the sciatic nerve in immature rats causes motoneuron death and impairs muscle recovery

Nerve Tracing in Juvenile Rats: A Feasible Model for the Study of Brachial Plexus Birth Palsy and Cocontractions?

Brachial plexus birth injuries cause diminished motor function in the upper extremity. The most common sequel is internal rotation contracture. A number of these patients also suffer from cocontractions, preventing the use of an otherwise good passive range of motion in the shoulder. One theory behind the co-contracture problem is that injured nerve fibers grow into distal support tissue not corresponding to the proximal support tissue, resulting in reinnervation of the wrong muscle groups. To further elucidate this hypothesis, we used rat neonates to investigate a possible model for the study of cocontractions in brachial plexus birth injuries. Five-day-old rats were subjected to a crush injury to the C5-C6 roots. After a healing period of 4 weeks, the infraspinatus muscle was injected with Fluoro-Gold. A week later, the animals were perfused and spinal cords harvested and sectioned. Differences in the uptake of Fluoro-Gold and NeuN positive cells of between sides of the spinal cord were recorded. We found a larger amount of Fluoro-Gold positive cells on the uninjured side, while the injured side had positive cells dispersed over a longer area in the craniocaudal direction. Our findings indicate that the method can be used to trace Fluoro-Gold from muscle through a neuroma. Our results also indicate that a neuroma in continuity somewhat prevents the correct connection from being established between the motor neuron pool in the spinal cord and target muscle and that some neurons succumb to a crushing injury. We also present future research ideas.

Normal Development and Pathology of Motoneurons: Anatomy, Electrophysiological Properties, Firing Patterns and Circuit Connectivity

This chapter will provide an introduction into motoneuron anatomy, electrophysiological properties, firing patterns focusing on development and also describing several pathological conditions that affect mononeurons. It starts with a historical retrospective describing the early landmark work into motoneurons. The next section lays out the various types of motoneurons (alpha, beta, and gamma) and their subclasses (fast-twitch fatigable, fast-twitch fatigue-resistant, and slow-twitch fatigue resistant), highlighting the functional relevance of this classification scheme. The third section describes the development of motoneurons' passive and active electrophysiological properties. This section also defines the major terms one uses in describing how a neuron functions electrophysiologically. The electrophysiological aspects of a neuron is critical to understanding how it behaves within a circuit and contributes to behavior since the firing of an action potential is how neurons communicate with each other and with muscles. The electrophysiological changes of motoneurons over development underlies how their function changes over the lifetime of an organism. After describing the properties of individual motoneurons, the chapter then turns to revealing how motoneurons interact within complex neural circuits, with other motoneurons as well as sensory neurons, and how these circuits change over development. Finally, this chapter ends with highlighting some recent advances made in motoneuron pathology, focusing on spinal muscular atrophy, amyotrophic lateral sclerosis, and axotomy.

Effects of repeated transection and coaptation of peripheral nerves on axonal regeneration and motoneuron survival

BACKGROUND AND PURPOSE

Salvage procedures for facial reanimation can involve a second neurorrhaphy operation. It remains unclear whether reuse of the original donor nerve in the salvage procedure remains likely to produce successful outcome. This study aimed to investigate the effect of repeated transection and coaptation of a nerve on axonal regrowth and motoneuron survival.

MATERIALS AND METHODS

The sciatic nerves of Sprague Dawley rats were transected and microsutured once (the one-time group) or repeatedly at eight-week intervals (the repeated group), and the animals remained alive for eight weeks before sacrifice. The gastrocnemius muscle was weighed, and muscle fiber diameter was measured with hematoxylin-eosin staining. Axonal count of the distal nerve stump was calculated by toluidine blue staining. Myelin thickness and axonal diameter were analyzed by transmission electronic microscopy. Finally, motoneurons were retrogradely traced to the spinal cord using Fluoro-Gold.

RESULTS

Repeated coaptation of nerves resulted in significant decreases of the wet weight ratio of gastrocnemius and muscle fiber diameter. The axonal counts and myelin thicknesses of the distal stumps were comparable between the groups, whereas axonal diameter was significantly smaller after repeated injury. Additionally, retrograde tracing demonstrated significantly less motoneurons in the L4-L6 spinal segments of the repeatedly injured animals than that of the one-time group.

CONCLUSIONS

Compared with one-time nerve injury, repetitive transection and coaptation of nerves resulted in compromised axonal regeneration, motoneuron survival, and target muscle recovery. It is possible that the final functional outcome could also be compromised, and the patients should be counseled accordingly.

Differences in c-jun and nNOS expression levels in motoneurons following different kinds of axonal injury in adult rats

In the peripheral nervous system (PNS), root avulsion causes motoneuron degeneration, but the majority of motoneurons can survive axotomy. In order to study the mechanism of motoneuron degeneration, we compared the expression patterns of c-jun and neuronal nitric oxide synthase (nNOS), the well-known molecular players in PNS regeneration and degeneration, among adult rats having undergone axotomy (Ax), avulsion (Av), or pre-axotomy plus secondary avulsion (Ax + Av) of the brachial plexus. Our results showed that the highest and longest-lasting c-jun activation occurred in Ax, which was much stronger than those in Av and Ax + Av. The time course and intensity of c-jun expression in Ax + Av were similar to those in Av except on day 1, while the pre-axotomy condition resulted in a transient up-regulation of c-jun to a level comparable to that in Ax. Axotomy alone did not induce nNOS expression in motoneurons. Pre-axotomy left-shifted the time course of nNOS induction in Ax + Av compared to that in Av. Motoneuron loss was not evident in Ax, while it was 70% in Av and more than 85% in Ax + Av at 8 weeks postinjury. The survival of motoneurons was positively correlated with c-jun induction, but not with nNOS expression in motoneurons. Moreover, c-jun induction was negatively correlated with nNOS induction in injured motoneurons. Our results indicate that functional crosstalk between c-jun and nNOS might play an important role in avulsion-induced motoneuron degeneration, while c-jun might act as a prerequisite survival factor and nNOS might act as a predictor for the onset of motoneuron degeneration.

Effect of precocious locomotor activity on the development of motoneurones and motor units of slow and fast muscles in rat

We have investigated the effect of precociously increasing locomotor activity during early postnatal development by daily treatment with the monoaminergic precursor L-DOPA on the survival of motoneurones supplying the slow soleus (SOL) muscle and the fast, tibialis anterior (TA) and extensor digitorum longus (EDL) muscles as well as the contractile and histochemical properties of these muscles. L-DOPA treatment resulted in a significant loss of motoneurones to the slow SOL muscle, but not to the fast TA and EDL muscles. Moreover, motoneurones to fast muscles also die as when exposed to increased activity in early life, if their axons are repeatedly injured. The loss of normal soleus motoneurones was accompanied by an increase in force of the remaining motor units and sprouting of the surviving axons suggesting a remodelling of motor unit organisation. The time to peak contraction of both SOL and EDL muscles from L-DOPA treated rats was prolonged at 8 weeks of age. At 4 weeks the soleus muscles of the L-DOPA treated animal developed more tension than the saline treated one. This difference between the two groups did not persist and by 8 weeks of age the muscle weight and tetanic tension from either group were not significantly different from control animals. The present study shows that early transient, precocious locomotor activity induced by L-DOPA is damaging to normal soleus but not to normal EDL/TA motoneurones.

Regeneration into Protected and Chronically Denervated Peripheral Nerve Stumps

OBJECTIVE

Delayed repair of peripheral nerve injuries often results in poor motor functional recovery. This may be a result of the deterioration or loss of endoneurial pathways in the distal nerve stump before motor axons can regenerate into the stump.

METHODS

Using the rat femoral nerve, we protected distal endoneurial pathways of the saphenous nerve with either cross-suture of the quadriceps motor nerve (Group A) or resuture of the saphenous nerve (Group B) to compare later motor regeneration into the "protected" saphenous nerve pathway to chronic denervation and "unprotected" saphenous nerve (Group C). A total of 60 rats, 20 per group, were operated on. After this protection (or lack thereof) for 8 weeks, the motor branch of the femoral nerve was cut and sutured to the distal saphenous nerve to allow motor regeneration into protected and unprotected saphenous nerve stumps. The quantitative assessment of axonal regeneration was performed after 6 weeks by use of nerve sampling for axon counts and retrogradely labeled motor neuron counts.

RESULTS

Significantly more myelinated axons innervated the motor (A) than the sensory (B) and no-protection (C) groups. There were significantly more retrogradely labeled femoral motor neurons in Group A than in the unprotected group (C).

CONCLUSION

We conclude that even 2 months of denervation of the distal nerve pathway is deleterious to regeneration and that protection of the pathway improves subsequent reinnervation and regeneration. Moreover, if the desired regeneration is motor, protection of the distal nerve pathway by a motor nerve conditions is better than a sensory nerve.

Magnesium administration provokes motor unit survival, after sciatic nerve injury in neonatal rats

BACKGROUND

We examined the time course of the functional alterations in two types of muscles following sciatic nerve crush in neonatal rats and the neuroprotective effect of Mg2+.

METHODS

The nerve crush was performed on the 2nd postnatal day. MgSO4*7H2O was administered daily for two weeks. Animals were examined for the contractile properties and for the number of motor units of extensor digitorum longus and soleus muscles at three postnatal stages and adulthood. Four experimental groups were included in this study: i) controls, ii) axotomized rats, iii) magnesium treated controls and iv) axotomized and Mg2+-treated rats.

RESULTS

Axotomy resulted in 20% MU survival in EDL and 50% in soleus. In contrast, magnesium treatment resulted in a significant motor unit survival (40% survival in EDL and 80% in soleus). The neuroprotective effects of Mg2+ were evident immediately after the Mg2+-treatment. Immature EDL and soleus muscles were slow and fatigueable. Soleus gradually became fatigue resistant, whereas, after axotomy, soleus remained fatigueable up to adulthood. EDL gradually became fastcontracting. Tetanic contraction in axotomized EDL was just 3,3% of the control side, compared to 15,2% in Mg2+-treated adult rats. The same parameter for axotomized soleus was 12% compared to 97% in Mg2+-treated adult rats.

CONCLUSIONS

These results demonstrate that motoneuron death occurs mostly within two weeks of axotomy. Magnesium administration rescues motoneurons and increases the number of motor units surviving into adulthood. Fast and slow muscles respond differently to axotomy and to subsequent Mg2+ treatment in vivo.