Background: 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).

Materials and Methods: 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.

Results: 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.

Conclusion : 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.

Background: The underlying central mechanisms for the development and maintenance of neuropathic pain are unknown. The current study aimed to evaluate the long-term potentiation (LTP) changes in spinal dorsal horn wide dynamic range (WDR) neurons following a peripheral neuropathy model.

Materials and Methods: This study was conducted on 26 male Wistar rats. The spinal nerve ligation (SNL) model was performed to induce neuropathy. After surgery, thermal hyperalgesia and mechanical allodynia were evaluated one day before neuropathy, and then on days 2, 5, 7, 14, 21 and 28 after neuropathy. Single-unit recording was used to study the changes of LTP. The changes of LTP and Aδ- and C-fiber evoked responses by high-frequency stimulation (100 Hz and current intensity six times that of the threshold for activation of C- fibers) of sciatic nerve in spinal WDR synapses were studied on day 14 after surgery up to 2 hours.

Results: Neuropathy was induced thermal hyperalgesia and mechanical allodynia on day 2 and persisted for 28 days after neuropathy. Electrophysiological recording revealed that HFS induced LTP either in the Aδ- or in the C-fibers in both sham and neuropathy groups up to 2 hr on day 14 after neuropathy. Neuropathy also significantly decreased the threshold of these fibers.

Conclusion: LTP-induced HFS in spinal WDR neurons can be one of the underlying central mechanisms in the maintenance of neuropathic pain.