Comparison of behavioural responses of two neuropathic pain models in male rats

Neuropathic pain syndromes are changes resulted from damage to neuronal pathways which is characterized by spontaneous burning pain with accompanying allodynia and hyperalgesia. The mechanisms underlying neuropathic pain are poorly understood. The present study explores behavioral characteristics of the neuropathic pain models chronic constriction injury (CCI) and spared nerve injury (SNI). Experiments were performed on four groups (n= 8) of male Sprague-Dawley rats (230-280 g). Anesthesia was initially induced with sodium pentobarbital (i.p.) at a dose of 50 mg/kg. The CCI model was made by loose ligation of the sciatic nerve. Also a lesion of two of three terminal branches of the sciatic nerve leads to a SNI model. The animals were tested for behavioral responses cold-and mechano-allodynia and heat-and mechano-hyperalgesia. The cold and mechanical stimulations in the cold- and mechano-allodynia phenomena were applied through acetone and von Frey filament respectively. Pin-prick and radiant heat were applied as thermal and mechanical stimulations in the heat- and mechano-hyperalgesia respectively. Behavioral tests were conducted on the animals prior to surgery (the day 0), and 3, 7, 14, 21 and 28 days post-operation. Our results indicate that, in comparison to the controlled group, the rats in both SNI and CCI groups reveal an obvious difference in behavioral responses. Although the SNI, compared to the CCI group were more sensitive to mechano-allodynia shortly after surgery (p<0.5) however, both groups share a similar pattern of behavior. In the heat-hyperalgesia testing, again, the animals in the CCI and SNI groups behaved differently than those in the controlled group, but no variation was evident among the test groups, themselves. These findings clearly show that the two neuropathic models produce abnormal pain-related disorders in the rats. A major feature of the SNI model was the very marked hypersensitivity to normally innocuous mechanical stimuli.