Motor neurons essential for normal sciatic function in neonatally nerve-injured rats

Hypoglossal nerve injury with long nerve resection leading to slow motoneuron death

Crush injury to peripheral nerves in adult animals is considered not to trigger retrograde neuronal cell death; however, several studies reported neuronal cell death following severe injuries including nerve transection, resection, or avulsion. However, the rate of neuronal cell death varied among studies. In this study, we evaluated the outcomes of very severe nerve injury by long nerve resection in adult rats. Right hypoglossal (XII) nerve was exposed, and a 9-mm section was resected. At 4, 8, and 12 weeks after the resection, the number of XII neurons were counted in from the rostral to caudal sections. The number of XII neurons in the injured right side was reduced after the XII nerve resection compared with the uninjured left side. The mean rates of surviving neurons at 4, 8, and 12 weeks after the nerve resection were 83.5 %, 73.9 %, and 61.1 %, respectively, which were significantly lower than those of the control. The number of XII neurons after extensive XII nerve resection declined gradually over a relatively long time period, revealing that extensive nerve resection led to slow cell death of the injured neurons.

Effects of repeated nerve injuries at different time intervals on functional recovery and nerve innervation

Effects of repeated nerve injuries on functional recovery and nerve innervation were examined in rodents. Crush injuries of the sciatic nerve were inflicted on adult rats and repeated twice or thrice at different time intervals of 1, 2, 3, and 4 weeks. Motor function was assessed by the static sciatic index at 1, 7, 14, 21, 28, 35, 42, 49, and 56 days after the final crush. The rates of nerve innervation of the tibialis anterior muscle, a main muscle innervated by the common peroneal nerve, were evaluated by the quantification of βIII-tubulin-positive nerve terminals and α-bungarotoxin-positive acetylcholine receptors 21 and 56 days after the final crush of triple nerve injuries at 1-, 2-, 3-, and 4-week intervals. Compared with single nerve crush injury, delayed recovery of motor function was observed in repeated crush injuries. In addition, recoveries in the triple crush groups were slower than those in the double crush groups. The rates of reinnervation were lower in the triple crush groups than in the single crush groups, both at 21 days (single: 59.7%; triple: 54.1%-56.1%) and 56 days (single: 88.8%; triple: 72.5%-83.0%) after the final crush, except in the groups with 1-week (triple: 73.8%) and 2-week (triple: 70.5%) intervals at 21 days after the final crush. We concluded that the recovery of motor function was delayed according to the number of repetitions of crush injuries, and that the rates of nerve innervation were still low in the triple crush groups 8 weeks after the final crush.

Stereological assessment of the total number of hypoglossal neurons after repeated crush injuries to the hypoglossal nerve in adult rats

OBJECTIVE

Retrograde neuronal cell death does not occur in mature motoneurons following the axonal injury of peripheral nerves. However, a previous study suggested that retrograde neuronal cell death does occur in adult rats after the creation of double lesions on the hypoglossal (XII) nerve based on a substantial decrease in the number of XII neurons. Using stereological methods, we examined neuronal apoptosis in XII neurons and the total number of XII neurons following repeated crush injuries to the XII nerve.

METHODS

The right XII nerve of adult rats was crushed three times at one-week intervals with a brain aneurysm clip. At 4 weeks after the final crush, the total numbers of XII neurons on the injured right and uninjured left sides were estimated stereologically.

RESULTS

After repeated crush injuries, no apoptosis was evident in XII neurons as indicated by immunostaining for cleaved caspase-3. Moreover, immunohistochemistry for the vesicular acetylcholine transporter revealed axonal elongation in the tongue 4 weeks after repeated crush injuries. At 4 weeks, the total numbers of XII neurons were 7800 ± 290 on the injured right side and 8000 ± 230 on the uninjured left side, and no significant difference was evident between the injured and uninjured sides.

CONCLUSION

Neuronal cell death does not occur in XII neurons and the total number of XII neurons does not decrease after repeated crush injuries of the XII nerve in adult rats.

Effect of graded nerve pressure injuries on motor function

OBJECT

The purpose of this study was to determine the minimum amount of nerve fibers required to maintain normal motor function after nerve injury in rats.

METHODS

The authors first confirmed that a common peroneal nerve injury caused more aggravating effects on lower limb motor function than tibial nerve injury, as assessed by the static sciatic index (SSI). Thereafter, rats were subjected to varying degrees of crush injury to the common peroneal nerve. At 48 hours after the injury, motor function was assessed using the SSI and slope-walking ability (with slope angles of 30° and 45°). The tibialis anterior muscle, a main muscle innervated by the common peroneal nerve, was removed. Muscle sections were co-labeled with neuronal class III β-tubulin polyclonal antibody to identify the presence of axons and Alexa Fluor 488-conjugated α-bungarotoxin to identify the presence of motor endplates.

RESULTS

The evaluation of neuromuscular innervation showed a correlation between SSI scores and ratios of residual axons (rs = 0.68, p < 0.01), and there was a statistically significant difference between slope-walking ability and ratios of residual axons (p < 0.01). Moreover, the ratios of residual axons in the nerve-crushed rats with normal motor function (SSI above -20) ranged from 36.5% to 88.7%, and those ratios in the success group with slope-walking angles of 30° and 45° ranged from 14.7% to 88.7% and from 39.8% to 88.7%, respectively.

CONCLUSIONS

In this study of rodents, less than half of the motor axons were sufficient to maintain normal motor function of the lower limb.

Correlation between motor function and axonal morphology in neonatally sciatic nerve-injured rats

The present study was conducted to investigate the correlation between motor function and axonal morphology in neonatally sciatic nerve-injured rats. The left sciatic nerve of newborn rats was transected or crushed, and functionality of the sciatic nerve was assessed by the static sciatic index after 8 weeks. After functional assessment, the common peroneal nerves in the control, nerve-transected, and nerve-crushed rats were removed and prepared for morphometric examinations. The cross-sectional area of the nerve, total number of myelinated axons, and size of each myelinated axon were analyzed for each group. The control rats showed normal motor function, whereas the nerve-transected rats showed severe motor dysfunction. The cross-sectional area of the nerve and total number of myelinated axons were reduced after nerve transection. Moreover, the percentage per size class of myelinated axons was almost uniform in the control rats, while the distribution was shifted to the left in the nerve-transected rats. Furthermore, no large myelinated axons were observed in the nerve-transected rats. The nerve-crushed rats showed various gait functions with various distribution patterns of axonal size, and the rats were divided into two groups with and without uninjured residual large axons. The results showed that the importance of regenerated medium-sized axons in cases without large axons and of residual large axons in cases with large axons in motor function. It was revealed that motor function was related closely to axonal size in neonatally nerve-injured rats.

Quantitative analysis of survival of hypoglossal neurons in neonatally nerve-injured rats: Correlation with milk intake

INTRODUCTION

Tongue movement innervated by the hypoglossal (XII) nerve is essential for the survival of neonatal rats. The pups with bilateral XII nerve resection failed to suckle milk and did not survive, and the pups with unilateral XII nerve resection showed disturbed suckling capability and lower survival rates. The present study was performed to investigate the relation between neuronal population and milk intake of developing rats that had received various degrees of crush injuries to the unilateral XII nerve during the neonatal period.

METHODS

The right XII nerve of postnatal day 1 (P1) pups was crushed and milk intake was estimated at 3 days and 6 days after the nerve injury. As nerve injury at the neonatal stage results in death of axotomized neurons, varying degrees of crushing was estimated by the number of survived motor neurons.

RESULTS

In nerve-crushed rats, the populations of XII motor neurons and amounts of milk intake were reduced in a varied manner. Statistically significant positive correlations were observed between increasing XII neuron survival and increasing milk intake at 3 (r=0.62) and 6 (r=0.71) days after the nerve injury.

CONCLUSION

The results indicate that there is a strong relationship between the number of XII motor neurons and the amount of milk intake in neonatally XII nerve-injured rats.

Effects of repeated crush injuries on motor functional recovery of the sciatic nerve

OBJECTIVES

The present study was conducted to examine whether repeated crush injuries have significant effects on motor functional recovery of peripheral nerves.

METHODS

Repeated crush injuries of the sciatic nerve were inflicted on adult rats at 1-week intervals, and functionality of the sciatic nerve was assessed by the static sciatic index each week for 8 weeks after the final injury. To determine the effects of repeated crush injuries on motor functional recovery of the sciatic nerve, tibialis anterior muscle fibers from single and triple crush injuries were examined, and fiber size and fiber reinnervation during the 2- to 4-week period after the final injury were measured.

RESULTS

Compared to single crush injuries, which completely recovered by post-injury week 4, double crush injuries resulted in retarded, but complete recovery by post-injury week 6, whereas triple crush injuries resulted in marked retardation in the regenerative process with incomplete recovery during week 8 of the experimental period. Muscle fiber size for rats with triple crush did not recover to normal range at post-injury week 4, despite its normal size for rats with single crush. The rate of reinnervation increased prominently between post-injury weeks 2 and 3 in both injuries, but the rate with triple crush was lower than that with single crush at post-injury week 3.

DISCUSSION

These results, which contradict those of a previous study that reported early functional recovery, indicate that repeated crush injuries inhibit motor functional recovery of the damaged sciatic nerve, as evidenced by delayed and incomplete regeneration, atrophied muscle fibers, and delayed reinnervation.

Determination of functionally essential neuronal population of the olfactory epithelium for nipple search and subsequent suckling behavior in newborn rats

Neuronal population of the olfactory epithelium required for nipple search and subsequent suckling behavior was examined in newborn rats. After unilateral ablation of the olfactory bulb, different concentrations of ZnSO(4) were contralaterally injected into the nasal cavity to produce varying degrees of neuronal degeneration in the olfactory epithelium. The ZnSO(4)-treatment resulted in two populations of pups. One exhibited suckling while the other did not, and intact olfactory receptor neurons were quantified immunohistochemically using an antibody for olfactory marker protein (OMP), a marker protein for olfactory receptor neurons. The total numbers of the OMP (+) cells in the ZnSO(4)-treated pups with suckling capability ranged between 2457 and 4615, whereas those in the ZnSO(4)-treated pups without suckling capability ranged between 112 and 2398. With the mean value (4969) of the total numbers of the OMP (+) cells of the normal/control pups assumed to represent 100%, the total numbers of the OMP (+) cells accounted for 49-93% in the suckling (+) group and 2-48% in the suckling (-) group. From these findings, we conclude that approximately 50% of neuronal population of the olfactory epithelium is a critical value to distinguish between the two groups with and without suckling capability in the unilateral olfactory system of newborn rats.