The results show that the OECs from olfactory bulb may induce hyperalgesia when they were transplanted into the hemisected spinal cord in rats. And this phenomenon seemed to be associated with the activation of ERK and the increase of BDNF level, indicated by RT-PCR and Western Blot.
Currently, cell transplantation is one novel therapeutic strategy for the treatment of SCI. As one type of glial cells ensheathed in the olfactory axons, OECs are featured by the function of promoting the axonal regeneration. Several studies emphasized the optimal role of OECs in the treatment of SCI, which may induce function repair for improving locomotor level [8
] and promoting axonal regeneration [14
], in the paraplegic [10
], transected [13
], contused [8
] SCI models and multiple lumbar rhizotomy [16
]. Moreover, the introduction of OECs can remyelinate the demyelinated axons in the lesions of the animal spinal cord [17
]. Unfortunately, however, there is no evidence to reveal how this transplantation affects the sensory function. Therefore, we aim to test whether the treatment of OEC transplantation in rat hSCI model may affect the regulation of sensory function as the motor function.
The innocuous mechanical and thermal stimuli test showed that the OECs treatment group had lower PWL than the untreated group. This result suggests that the OEC transplantation into spinal cord injured rats may induce obvious hyperalgesic responses. For this reason, the analgesic approach should be developed when we consider the OEC transplantation therapy.
To investigate the underlying mechanism of the allodynia following OEC implantation, we determined the expressions of BDNF and ERK. Consequently, BDNF expression in injured spinal cord was greatly increased in OEC-transplanted rats on 21st day and BDNF was located in neurons of the superficial layer in the dorsal horn. It is known that the productions of growth factors (NGF, BDNF, and GDNF) and the expressions of corresponding receptors may contribute to axonal regeneration following OEC transplantation [5
]. BDNF as one type of the neurotrophin family of growth factors can not only support the survival of the existing neurons and promote the growth and differentiation of new neurons and synapses [20
], but also trigger the pain in sciatic nerve separation model [22
] and ligation model [23
]. In our experiments, immunofluorescent staining was performed to observe the expression of BDNF at the injured segment, and the red fluorescence which symbolized “BDNF positive” was found in the superficial layer of dorsal horn. Simultaneously, the upregulation of BDNF expression was also confirmed. These results indicated that OEC transplantation could increase BDNF expression in sensory neurons of the dorsal horn, which may contribute to the observed phenomena like hyperalgesia.
More importantly, we also found the p-ERK (phosphorylation of extracellular signal-regulated kinase) level was increased in the injured spinal cord after OEC transplantation. The potential correlation of the activation of ERK following spinal neuron injury with the pain hypersensitivity and central sensitization was well documented [24
]. Within a minute after a noxious stimulus, many p- ERK-positive neurons were observed, predominantly in laminas I and II of the ipsilateral dorsal horn [26
], and the phosphorylation of ERK occurring in the nociceptive neurons may contribute to persistent inflammatory and neuropathic pains [27
]. In present study, we observed the activation of ERK following OEC transplantation, which may be related to hyperalgesia. In this observation, several events including behavior and molecule have occurred at different time points. Reportedly, the ERK phosphorylation may be a reason for the increase of BDNF [28
]. Therefore, the increase of ERK phosphorylation on the 14th day may induce the increase of BDNF level on the 21st day. As the behavioral changes were generally resulting from the cellular and molecular changes, hyperalgesia was found after 1 month. Together, the phosphorylation of ERK results in the upregulation of BDNF, and eventually gives rise to the behavioral hyperalgesia.
We also found that BDNF was expressed both in OECs and in host neurons. As the neurons with BDNF staining are expressing TrkB simultaneously, probably the BDNF released from OECs could be enrolled into neurons by TrkB receptor. These could underline some linkage by BDNF for the consequent hyperalgesia between transplanted OECs and host neurons.
In summary, these findings may provide novel insight into the hyperalgesia responses following OEC transplantation. In using OEC transplantation for the treatment of SCI, it is useful to consider the inhibition of ERK phosphorylation and BDNF blockage to relieve the nociceptive behaviors.