Tramadol and propentofylline coadministration exerted synergistic effects on rat spinal nerve ligation-induced neuropathic pain

Abstract

Neuropathic pain is an intractable clinical problem. Drug treatments such as tramadol have been reported to effectively decrease neuropathic pain by inhibiting the activity of nociceptive neurons. It has also been reported that modulating glial activation could also prevent or reverse neuropathic pain via the administration of a glial modulator or inhibitor, such as propentofylline. Thus far, there has been no clinical strategy incorporating both neuronal and glial participation for treating neuropathic pain. Therefore, the present research study was designed to assess whether coadministration of tramadol and propentofylline, as neuronal and glial activation inhibitors, respectively, would exert a synergistic effect on the reduction of rat spinal nerve ligation (SNL)-induced neuropathic pain. Rats underwent SNL surgery to induce neuropathic pain. Pain behavioral tests were conducted to ascertain the effect of drugs on SNL-induced mechanical allodynia with von-Frey hairs. Proinflammatory factor interleukin-1β (IL-1β) expression was also detected by Real-time RT-PCR. Intrathecal tramadol and propentofylline administered alone relieved SNL-induced mechanical allodynia in a dose-dependent manner. Tramadol and propentofylline coadministration exerted a more potent effect in a synergistic and dose dependent manner than the intrathecal administration of either drug alone. Real-time RT-PCR demonstrated IL-1β up-expression in the ipsilateral spinal dorsal horn after the lesion, which was significantly decreased by tramadol and propentofylline coadministration. Inhibiting proinflammatory factor IL-1β contributed to the synergistic effects of tramadol and propentofylline coadministration on rat peripheral nerve injury-induced neuropathic pain. Thus, our study provided a rationale for utilizing a novel strategy for treating neuropathic pain by blocking the proinflammatory factor related pathways in the central nervous system.

Figure 1. Effect of intrathecal tramadol on spinal nerve ligation (SNL)-induced neuropathic pain.

Graph A shows a dose-dependent effect of intrathecal tramadol on SNL-induced mechanical allodynia. Intrathecal tramadol (10 or 30 µg/rat) significantly elevated the pain threshold, whereas, intrathecal tramadol 3 µg/rat didn't show apparent effect. Graph B means the percentage of anti-allodynia of the maximum possible effect. The Y-axis is the percentage of the antiallodynia after drug administration. % Antiallodynia = 100−100×(baseline of SNL-Drug – post SNL-Drug)/(baseline of SNL-Saline – post SNL-Saline). *P<0.05, compared with that of SNL-Saline. ^# P<0.05, compared with that of SNL-T 10 µg group. 6 rats in each group. POD: post operative day, T: tramadol.

Figure 2. Effect of intrathecal propentofylline on SNL-induced neuropathic pain.

Graph A shows a dose-dependent effect of intrathecal propentofylline on SNL-induced neuropathic pain. Intrathecal propentofylline (0.5 or 2.5 µg/rat) markedly raised the pain threshold, whereas, intrathecal propentofylline 0.1 µg/rat didn't show obvious effect on SNL-induced allodynia. Graph B means the percentage of anti-allodynia of the maximum possible effect. The Y-axis is the percentage of the antiallodynia after drug administration. % Antiallodynia = 100−100×(baseline of SNL-Drug – post SNL-Drug)/(baseline of SNL-Saline – post SNL-Saline). *P<0.05, compared with that of SNL-Saline. ^# P<0.05, compared with that of SNL-P 0.5 µg group. 6 rats in each group. P: propentofylline.

Figure 3. Effect of tramadol and propentofylline intrathecal coadministration on SNL-induced neuropathic pain.

Graph A shows a dose-dependent effect of drugs coadministration on SNL-induced mechanical allodynia. Coadministration of 7.2 µg/rat (6.7 µg/rat tramadol and 0.5 µg/rat propentofylline) remarkably reversed the mechanical allodynia. Combination of 3.6 µg/rat (3.35 µg/rat tramadol and 0.25 µg/rat propentofylline) also effectively elevated the pain threshold. Moreover, intrathecal 1.8 µg/rat (1.68 µg/rat tramadol and 0.12 µg/rat propentofylline) could still relief the mechanical allodynia. Graph B means the percentage of anti-allodynia of the maximum possible effect. The Y-axis is the percentage of the antiallodynia after drug administration. % Antiallodynia = 100−100×(baseline of SNL-Drug – post SNL-Drug)/(baseline of SNL-Saline – post SNL-Saline). *P<0.05, compared with that of SNL-Saline. ^# P<0.05, compared with that of SNL-C 3.6 µg group. 6 rats in each group. C: coadministration.

Figure 4. Isobologram of drugs combination shows the synergistic effect of intrathecal tramadol and propentofylline coadministration on SNL-induced neuropathic pain.

Graph A, B means the ED50 of intrathecal propentofylline or tramadol respectively. The oblique line between A and B is the theoretic additive effect line of tramadol and propentofylline coadministration. Graph C, in the middle of the line, is the theoretical ED50 of drugs combination, which is calculated from the individual drug ED50. Graph D, far below the line, is the experimental ED50 of drugs combination, which is actually observed after drugs coadministration. The experimental ED50 point lies far below the additive line, suggesting a significant synergistic effect of drugs coadministration. *P<0.05, compared with that of theoretical ED50.

Figure 5. Effects of drugs application on SNL-induced IL-1β expression.

Different drugs were injected to detect the effects on SNL-induced IL-1β expression. *P<0.05. 8 rats in each group. C: coadministration.

Figure 6. Effects of drugs on motor performance of rats in the rotarod test.

After a baseline response had been obtained, tramadol 30 µg, propentofylline 2.5 µg and combination 7.2 µg were administered intrathecally and rotarod test was performed once a day for 8 d. Compared with that of the baseline response, there was no statistical differences could be detected from rotarod test after drugs intrathecal administration. 6 rats in each group.