Assessment of Pain and Spinal Cord levels of Calcitonin Gene Related Peptide and Interleukin-6 in a Model of Neuropathic Pain in Female Rats

Background: Neuropathic pain is a form of chronic pain that arises from functional changes in the pain sensory system after peripheral nerve injury. It is manifested by spontaneous pain, hyperalgesia and allodynia. Calcitonin gene related peptide (CGRP) is 37 amino acid neuropeptide that is widely distributed in the central and peripheral nervous system and is involved in processing of nociceptive information. Evidences from human and animal studies whether estrogen is pro or antinociceptive is inconsistent. Transcutaneous electric nerve stimulation (TENS) is a non pharmacologic therapy for pain used to produce analgesia in inflammatory conditions. Aim of the present work was assessment of pain and spinal cord levels of CGRP and interleukin-6 (IL-6) in a model of neuropathic pain induced in female rats by chronic constriction injury (CCI) of sciatic nerve. The possible modulation of neuropathic pain by estradiol and TENS was also assessed in ovariectomized rats with CCI of sciatic nerve. Material & Methods: The study was carried out on 48 adult female rats divided into six groups (8 rats/each). Group 1: sham operated rats, group II: CCI of sciatic nerve in gonadally intact female rats, group III: ovariectomized (OVX) rats, group IV: OVX+CCI, group V: OVX+CCI/17  estradiol, group VI: OVX+ CCI/TENS. All ovariectomized rats were included in the study 3 weeks after ovariectomy for depletion of circulating estrogen. Exogenous estrogen was given to ovarietomized rats of group V as 6g/kg body weight (b.wt.) per day subcutaneous for 3 weeks starting on the same day of CCI-surgery (day 0). TENS was delivered to OVX+CCI rats (group VI) starting from day 0 and continued for 3 weeks on alternate days under light ether anaesthesia. Pain behavior was assessed by pin prick test. Mechanical hyperalgesia was defined as increased hindpaw withdrawal frequencies percent (PWF%) to mechanical stimuli. The assessment began on the day of CCI surgery (day 0) before the operation (pre-operative, baseline data) and followed up weekly on post-CCI days 8, 14 and 21 (post operative assessment). Ovariectomized rats that were not subjected to CCI (group III) followed the same schedule for assessment of PWF%. At the end of the experiment, rats were sacrificed, lumbar spinal cord was dissected and used for later assessment of mRNA expression of calcitonin gene-related peptide (CGRP) using an Reverse transcriptase –Polymerase chain reaction (RT-PCR) method. Also levels of CGRP and IL-6 in the spinal cord were assessed by Enzyme Immunoassay Kits. Results: The expression of CGRP in the spinal cord tissue, levels of CGRP and IL-6 were significantly higher in lumbar spinal cord after CCI, OVX or OVX+CCI compared with sham operated rats. After estrogen or TENS therapy, a significant decrease of expression of CGRP, levels of CGRP and IL-6 were found compared with untreated (OVX+CCI) group. However, the values of CGRP and IL-6 after treatment were still above control level. Rats in group II (CCT of sciatic nerve in gonadally intact female rats) had significant increase of PWF% on post-CCI days 8, 14 and 21. Group III: OVX rats had significant increase of PWF% on day 0 and post-CCI days 8,14 and 21 versus sham operated rats. Group IV (OVX+CCI) showed significant increase of PWF% on days 8, 14 and 21 post-CCI compared to baseline. These values were significantly higher than those of sham operated rats. PWF% in group IV (OVX+CCI) tended to be higher at all the time points of post-CCI follow up than corresponding values in group II (CCI in gonadally intact females) and the difference was statistically significant on day 14. Group V (OVX+CCI/17  estradiol) showed significant decrease of PWF% starting from day 8 and continued on days 14 and 21 post-CCI versus baseline, but remained significantly higher than sham operated rats. Group VI: OVX+CCI/TENS showed significant decrease of PWF% on post-CCI days 8, 14 and 21 compared with baseline, but remained significantly higher than sham operated controls. No statistically significant differences in PWF% were found between estradiol and TENS treated groups on post CCI days 8, 14 and 21. Conclusion: increased spinal cord levels of CGRP and IL-6 could be involved in the mechanism of mechanical hyperalgesia after CCI surgery of sciatic nerve and ovariectomy. Administration of estradiol or TENS significantly attenuated the development of hyperalgesia in (OVX+CCI) group, possibly by decreasing spinal cord levels of CGRP and IL-6. The findings suggested that TENS can be considered as alternative to hormonal therapy for neuropathic pain in ovariectomized rats. [Gihan Sharara, Hala M. Abou Heif and Abeer El-Emam Deif. Assessment of Pain and Spinal Cord levels of Calcitonin Gene Related Peptide and Interleukin-6 in a Model of Neuropathic Pain in Female Rats. J Am Sci 2012;8(6):145-157]. (ISSN: 1545-1003). http://www.sciencepub. net/american. 18