|Year : 2013 | Volume
| Issue : 1 | Page : 34-38
Comparing the efficacy of carbamazepine, gabapentin and lamotrigine in chronic constriction injury model of neuropathic pain in rats
Bharti Chogtu1, Kurady Laxminarayana Bairy1, Penuganchiprolu Himabindu2, Supurna Dhar3
1 Department of Pharmacology, Kasturba Medical College, Manipal, Karnataka, India
2 Melaka Manipal Medical College, Manipal, Karnataka, India
3 International Center, Manipal University, Manipal, Karnataka, India
|Date of Submission||31-May-2012|
|Date of Acceptance||22-Jun-2012|
|Date of Web Publication||6-Feb-2013|
Department of Pharmacology, Kasturba Medical College, Manipal University, Manipal, Karnataka-576 104
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Context: Anti-epileptics are used in the treatment of neuropathic pain. Newer anti- epileptics with improved safety profile are being used in neuropathic pain. The animal models of neuropathic pain have helped to find newer drugs for this condition. As the efficacy of these drugs is modest, there is always a need to compare the efficacy of newer anti-epileptics with conventional drugs in this condition. Aims: The aim of this study was to compare the efficacy of anti-epileptics in neuropathic pain induced by chronic constriction injury. Settings and Design: It was a randomised, prospective study. Materials and Methods: The rats were divided into four groups. The first three groups were administered each of carbamazepine, gabapentin and lamotrigine respectively and the fourth group acted as control. Assessment was started on day five of surgery by hot plate method and Randall Selitto test. Statistical Analysis Used: Statistical analysis was done by one way analysis of variance followed by posthoc test. Results: In mechanical allodynia rats treated with lamotrigine and gabapentin (P = 0.001) showed a significant increase in mechanical threshold whereas in thermal hyperalgesia, it was the carbamazepine (P = 0.027) which was superior to other two drugs. Conclusions: Lamotrigine and gabapentin are efficacious in mechanical allodynia whereas carbamazepine is superior in thermal hyperalgesia. This shows that effect of drugs vary in different behavioral tests. So, the choice of treatment in different types of neuropathic pain can vary.
Keywords: Anti-epileptics, chronic constriction injury, neuropathic pain
|How to cite this article:|
Chogtu B, Bairy KL, Himabindu P, Dhar S. Comparing the efficacy of carbamazepine, gabapentin and lamotrigine in chronic constriction injury model of neuropathic pain in rats. Int J Nutr Pharmacol Neurol Dis 2013;3:34-8
|How to cite this URL:|
Chogtu B, Bairy KL, Himabindu P, Dhar S. Comparing the efficacy of carbamazepine, gabapentin and lamotrigine in chronic constriction injury model of neuropathic pain in rats. Int J Nutr Pharmacol Neurol Dis [serial online] 2013 [cited 2019 Jun 15];3:34-8. Available from: http://www.ijnpnd.com/text.asp?2013/3/1/34/106987
| Introduction|| |
Neuropathic pain is defined as pain initiated or caused by primary lesion or dysfunction in the nervous system.  It leads to chronic and disabling condition and frequently presents as continuous pain, allodynia and hyperalgesia. Considerable research is going on in the development of therapeutic approaches. Tricyclic antidepressants and anticonvulsants are the mainstay of treatment of clinical therapy for neuropathic pain.  Currently available drugs offer modest efficacy only for neuropathic pain.  Newer anti-epileptics have better tolerability and improved pharmacokinetics rather than improved efficacy in neuropathic pain.  So there is a need to compare the efficacy of anti-epileptics in neuropathic pain.
Several animal models of neuropathic pain have been developed. Chronic constriction injury,  partial sciatic nerve ligation,  spinal nerve ligation  are the three commonly used models. These share partial injury of sciatic nerve leading to alteration in hind limb withdrawal thresholds to sensory stimuli. In this study we compared the efficacy of three anti-epileptic drugs in neuropathic pain using chronic constriction injury model.
| Materials and Methods|| |
Adult male Wistar rats weighing between 150- 200 g were used. Animals were acclimatized to the laboratory environment for 5-7 days before entering in the study. They were allowed free access to water and were maintained on standard rat diet under laboratory conditions. 12-hour light/dark cycle was maintained. All procedures were carried with approval of Institutional Animal Ethics Committee (IAEC).
Carbamazepine (100 mg/kg),  gabapentin (60 mg/ kg),  lamotrigine (40 mg/kg)  were used in the doses as per the previous study. Carbamazepine and lamotrigine were dissolved in 2% gum acacia and gabapentin in distilled water. All the drugs were administered by oral gavage.
Induction of pain
Pain was induced by chronic constriction injury (CCI) which was performed as originally described by Bennett and Xie.  Animals were anaesthetized with ketamine and xylaline (85 mg/kg and 13 mg/kg)  which was administered by intra-peritoneal route. Surgery was performed, skin and musculature of left lateral leg was incised to expose the sciatic nerve at the middle of thigh by blunt dissection through biceps femoris. The nerve was loosely ligated with four simple interrupted 4-0 chromic gut sutures. After hemostasis was checked, overlying muscles and skin was closed routinely. After the surgery animals were housed in plastic cages with soft bedding. Rat food pellets and water was available ad libitum.
Postoperative tests were started on day 5 of surgery. At this time most of the rats developed distinct neuropathic behavior such as allodynia and hyperalgesia. We set an arbitrary response criterion for demonstrating tactile allodynia on the operated side at <10 g. Only animals that passed this screening test were used for drug testing. About 15% of animals operated were excluded on these grounds. Animals screened were divided into control and three test groups. The drugs were given once a day daily from 5 th to 14 th day. Mechanical allodynia was assessed on day 8 th , 10 th , 12 th and 14 th . Thermal hyperalgesia was assessed on 5 th , 7 th , 11 th and 13 th day of surgery.
Thermal hyperalgesia was assessed using a hot plate. The radiant heat was applied to the plantar surface of hind paw and withdrawal latency time recorded. The thermal nociceptive threshold was defined as the time required for eliciting either a hind paw lick or a jump. The hind paw withdrawal latency to this stimulus was tested three times at intervals of not less than 3 minutes and a mean calculated. The cut off time was taken as 30 seconds to avoid risk of thermal injury to the skin.
Mechanical hyperalgesia was assessed by Randall Selitto test.  Analgesymeter (UGO-Basile apparatus) was used to assess the pressure pain threshold. The observer was blind to group treatment. The rat's left hind paw was placed in pressure applicator and steadily increased pressure was applied to the dorsal surface of paw until the rat withdrew. The force (in grams) needed to produce paw withdrawal was regarded as pain threshold.
All results were expressed in mean ± SEM. Statistical analysis among different groups was done by univariate method by one way analysis of variance followed by Scheffe's post hoc test. For all analyses P value less than 0.05 was considered statistically significant.
| Results|| |
Most of the animals included in the analysis of study exhibited altered sensory threshold from 5 th day following CCI.
Gabapentin (P = 0.001) and lamotrigine (P = 0.001) groups showed a significant increase in mechanical threshold as compared to control. However, the rats treated with carbamazepine did not show a significant difference (P = 0.10) as compared to control on assessing mechanical allodynia [Figure 1].
|Figure 1: Pain threshold in animals treated with lamotrigine, gabapentin, and carbamazepine. Values are in mean ± standard error (n = 6) lam = lamotrigine, gab = gabapentin, cbz = carbamazepine|
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On comparing the three groups by ANOVA, it was shown that there was no statistical significant difference between the three groups (P = 0.379).
Carbamazepine showed a significant increase in hot plate latency as compared to control (P = 0.027). However, gabapentin (P = 0.46) and lamotrigine (P = 0.52) did not show a significant increase in hot plate latency as compared to control.
On comparing the three groups by ANOVA, a statistical difference was obtained (P = 0.001). Scheffe's post-hoc test revealed a statistical difference between carbamazepine and gabapentin (P = 0.002) [Figure 2] a, carbamazepine and lamotrigine (P = 0.005) [Figure 2] b. However, no statistical difference was observed between gabapentin and lamotrigine (P = 0.71) treated animals.
|Figure 2: (a) Withdrawal latency in rats treated with carbamazepine and gabapentin in hot plate method. Values are in mean ± standard error. (n = 6). Cbz = carbamazepine, Gab = gabapentin. (b). Withdrawal latency in rats treated with carbamazepine and lamotrigine in hot plate method. Values are in mean ± standard error. (n = 6). Cbz = carbamazepine, lam = lamotrigine|
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| Discussion|| |
Chronic constriction injury simulates the symptoms of chronic nerve compression. In this model, four ligatures of chromic gut (4-0) are loosely tied around the sciatic nerve at the middle of the thigh. The chromic gut used induces a local inflammatory reaction and subsequent edema that increases the compression of axons.  The nerve inflammation induced by the chromic gut ligature is accompanied by an immune reaction  including invasion of immune cells  and production of immune modulators.  Chronic constriction injury in rats simulates chronic nerve compression in clinical situations like nerve entrapment neuropathy or spinal root irritation by lumbar disk herniation. Topical preparations are also being tried in clinical settings of neuropathic pain. Capsaicin topically is being tried and showing promising results in patients with diabetic neuropathic pain. 
In this study 5 days after CCI, rats showed a relatively higher degree of allodynia and hyperalgesia against mechanical and thermal stimuli. The role of anticonvulsant drugs in the treatment of neuropathic pain is evolving and has been clearly demonstrated with gabapentin and carbamazepine.
Carbamazepine, the first anticonvulsant studied in clinical trials, probably alleviates pain by decreasing conductance in Na + channels and inhibiting ectopic discharges. Though results from clinical trials have been positive in the treatment of trigeminal neuralgia, painful diabetic neuropathy and post herpetic neuralgia,  carbamazepine did not affect mechanical hyperlgesia or tactile allodynia induced by partial sciatic nerve ligation in rat following oral administration.  Similarly in our study carbamazepine group did not shows any effect on mechanical allodynia whereas it was superior to control group in thermal hyperalgesia.
Gabapentin acts on the α2δ subunit of voltage-dependent calcium channels,  which may be associated with reduction in transmitter release and neuronal activity. Gabapentin reduces the release of excitatory amino acids in the spinal cord following the inflammatory stimulation of the periphery.  It inhibits ectopic discharges of the sciatic nerve in a partial sciatic nerve ligation model.  Studies also suggest that gabapentin besides opening potassium channels may partially activate the NO- cyclic GMP-PKG spinal pathway in Chung model of neuropathy.  These findings taken together suggest that gabapentin acts on the primary afferents to suppress pain. The spinal nerve ligation induces the up-regulation of the α2δ subunit in the dorsal root ganglion, which is associated with the anti-allodynic effect of gabapentin.  As per previous reports, gabapentin did not produce any anti allodynic effect in neuropathic pain induced by CCI in rats.  However, in our study gabapentin produced anti allodynic effect but did not increase the withdrawal latency in hot plate method. The effect shown by gabapentin is similar to our earlier study in paclitaxel induced neuropathic pain where gabapentin was effective in mechanical allodynia but did not show a significant response in hot plate withdrawal latency. 
Lamotrigine acts by stabilizing the slow inactivated conformation of a subtype of Na + channel  and modulates Ca ++ and K + currents.  Single and repeated injections of lamotrigine were devoid of effects on von Frey filament induced hypersensitivity in trigeminal neuropathic pain.  Lamotrigine produced a slight inhibition of tactile allodynia in rat only at the highest dose i.e. 100 mg/kg per orally.  Intra- thecal lamotrigine reverses tactile allodynia but not thermal hyperalgesia when given after induction of inflammation.  These results are in concordance with our study in which lamotrigine did not increase the latency in hot plate method. It was superior to control group in mechanical allodynia as well. This shows differential effect on thermal hyperalgesia and mechanical allodynia. The differential effect may be due to difference in potency of drugs in activating the NMDA and non NMDA receptors as previously reported. 
Gabapentin did not show a significant effect on thermal hyperalgesia as compared to control in this study. Earlier studies also show that gabapentin is poor in thermal hyperalgesia and this is most probably due to different modes of nerve damage in these models.  On systemic administration, gabapentin and lamotrigine were not effective in inhibiting responses to hot plate method.  Gabapentin had no effect on transient nociceptive signaling but inhibits sensitized signaling associated with allodynia. This is supported by the fact that gabapentin has no effect on afferent nerve fiber activity but inhibits ectopic discharge with peripheral nerve injury. 
To summarize, anticonvulsants are effective in different types of neuropathic pain. In case of neuropathic pain due to CCI, carbamazepine was superior as an analgesic in thermal hyperalgesia whereas gabapentin and lamotrigine showed antiallodynic effect. Different effects on mechanical and thermal hyperalgesia are seen in this study which may be due to different pathogenesis underlying the two behavioral tests.
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[Figure 1], [Figure 2]