Peripheral nerve regeneration is a complex biological process involving interactions among multiple cells, neurotrophic factors and extracellular matrice proteins . This report led us to believe that a systemic examination of DRG neuron responses to vit B12 is advantageous to better understand the role of vitamin B12 in peripheral nerve regeneration.
The present study evaluated the protective effects of vit B12on the number of DRG cells recovery after inducing SNI in the rats.
In a serious trauma like SNI, a short period of localized ischemia is followed by evident increase in the pressure of endoneural fluid and defect of the normal capillary blood flow in the endoneurium . Subsequently, Wallerian degeneration may arise distal to the lesion [14, 17]. It has been shown that sciatic nerve crush leads to histological changes in the DRG cells number . Loss of the neurons occurs and the nerve cells become less after peripheral nerve injury in the DRG [15, 18]. This finding is in accordance with our results in which we showed the number of the cells were decreased.
Histological changes, such as decrease in the number of DRG cells after SNI, have been reported by other researchers [34–36]. Our results are in accordance with these findings, in which we demonstrated that the total number of L5-DRG cells after SNI were decreased.
Vit B12is also a good scavenger of the reactive oxygen species and is suggested to be a good neuroprotectant. It can pass through the blood brain barrier, which is an evidence of amplification of its neuroprotectant potential in neurodegenerative disorders such as Parkinson's and Alzheimer's diseases . In addition, it has been reported that vit B12 has protective effects after spinal cord injuries. The previous studies have stressed the anti-apoptotic and anti-necrotic effects of the vit B12 on the neurons . These reports may explain the beneficial effects of vit B12 in the present study after SNI. These properties including anti-inflammatory, antioxidant, anti-apoptotic and anti-necrotic might be effective in the present study.
In previous reports on the effects of vit B12 on neurons in vivo, high levels of vit B12 improved nerve conduction and regeneration in streptozotocin-diabetic rats , and experimental acrylamide neuropathy . We used a rat SNI model in the present study to evaluate the effects of vit B12
in vivo. However, we used continuous administration of doses of vit B12 (0.5 and 1 mg/kg/day) higher than that used in previous reports (500 μg/kg/day). Our morphological and histological evaluation offers a possible explanation of this phenomenon. Vit B12increased the regeneration of axons.
We consider that repair discrepancy is due to the differences in the doses of vit B12and time of administration. In our study, vit B12had not remarkable affect on neurons at low-dose concentrations, especially with low-dose administration or short time, because the concentration of vit B12declines immediately  and the absorption of vit B12from the ileum and entire intestine is regulated by limitation of binding to gastric intrinsic factor and passive diffusion [40, 41]. From the results of our study, we consider that it is necessary to devise new clinical methods, such as high-dose systemic or long time, to deliver high doses of vit B12 to target organs to treat nervous disorders more effectively. Thus, the high-dose vit B12 treatment together with long time might have the potential to treat not only peripheral nerve injury but also central nervous injury.
The results of our study indicated that the experimental groups, under treatment with vit B12, were prevented from reducing the number of neurons in DRG (Table. 2). Moreover, the DRG neurons decrease of left side of the operated experimental group compared to non-operated right side of the same group was slightly, especially it reached to the lowest degree (6336± 142) in 1mg/kg dosage in 42-day, and the result was significant compared to the other groups (p<0/05). On the other hand, the left neurons of 0/5mg/kg group in 21-day shows an increase of 7779± 123 compared to the same ganglion of the control group.
Also, according to Table 3, the neuronal mean increase shows a significant rise in association with the increase of vit B12 dosage and time that such variation in 0/5 and1 mg/kg group reached to 7023± 164 to 7095± 171respectively (Table 3). In addition, the decrease in the neurons death percentage of the right side of the non-crushing sciatic nerves compared to the same side of the control group was 20/32, 20/27, 20/05 and 19/51 percent (Table 3 and Figure 1 ).
On the other hand, the regeneration and healing in Dorsal Spinal Ganglion will be improved by increase of administration time and vit B12 dose, indicating that such vitamin was able to progress recovery process of peripheral nerves damage in experimental rats.
In parallel with our research several studies have been conducted. Yamatsu evaluated the effect of vitaminB12 on the nerve repair after a crush injury. Vit B12 demonstrated significant increase in the regeneration and improvement on SNI tests suggesting an inhibitory effect on the nerve degeneration and facilitating the regeneration after an injury . Okada et al. evaluated the effectiveness of vit B12 on nerve regeneration and found that it demonstrated the greatest improvement on nerve regeneration . Yamazaki also demonstrated that vit B12 improves the neuromuscular junction by decreasing nerve degeneration and improving nerve regeneration in the motor nerve terminals in a gracile axonal dystrophy mice model .
In addition to, the results of this study confirm the results of one-way ANOVA and represent that the neurons construction and repairing started by time passage. Because the average number of neurons in under-prescription groups by vit B12 in 0.5mg/Kg and 1mg/Kg doses increased in 42-day experimental groups from 7435± 200 to7779± 123, and this difference was obvious and significant between the control group and 42-day experimental group in 1mg/kg dosage (p<0/05). In agreement with our study, Yagihushi et al., 1976 suggested that a constant 16-week cures by vit B12 (500 microgram/kg) had a completely repairing effect on the injured peripheral nerves in experimental diabetic neuropathy of rats. In another study, Taniguchi et al., 1987 indicated that vit B12 cured neuropathy in uromicina patients. They advised that vitamin B12 could be used as a cure in such forms of neuropathy . In addition, another study by, Hasegawa et al. (1978) studies showed that the vitamin B complex facilitated functional recovery from nerve injury faster than its components, and showed that B1 and B12 had significant facilitating effects on the functional recovery as well .
In this study, we showed that the vitamin B12 was the most effective in promoting neuronal survival in DRG neurons. These results suggest that the metabolic pathway of vit B12, is associated with neuronal survival, but further understanding of this role and development of an effective delivery system for vitamin B12 may enable us to treat several nervous disorders and to obtain new insights into nerve regeneration.