The Effects Of Lacosamide On The Synaptic Transmission Of The Spinal Nociceptive Pathway In Rats

Pain is the result of the nociceptor activation. Frequently, the symptom of pain manifests uncomfortable emotional experience. World Health Organization(WHO) defined the pain as the fifth vital signs in human, in addition to temperature, respiration, pulse and blood pressure. Furthermore, it has been pointed out that the pain relief is the fundamental human rights. Pain derived from the nocuous stimulus is usually called ―the physiological pain‖, which is beneficial to the organisms’ survival and evolution, and it acts as the protective role to avoid further lesion or injuries. On the other hand, chronic pain or pathological pain often rises from the injuries of the peripheral and the central nerve system. The clinical manifestation often sustained persistently and chronically even if the injuries healed. The chronic pain as well as the pathological pain do not have the protective role but only exert great influence on the patients’ physical and psychological state, as well as the quality of life.Neuropathic pain is the most common and unmet medical problems. International Association for the Study Pain(IASP) has defined it: pain resulting from the lesion ordisease which affected the somatosensory system. Neuropathic pain has three distinguished features: spontaneous pain, allodynia and hyperalgesia. Nowadays, the prevailing therapies in clinical to treat the neuropathic pain include drug therapy and non-drug therapy. Drug therapy is the fundmental and the most common approach in clinical practice, and can be easily accept by the patients.Currently, in the clinical practice a wide range of drugs have been developed to treat with the neuropathic pain, for example: the selective serotonin re-uptake inhibitor(SSRI), tricyclic anti-depressants(TCAs), narcotic analgesics, non steroid anti-inflammatory drugs(NSAIDS). The unsatisfactory therapeutic effect of the drugs for neuropathic pain limits their widespread usage in clinical practice.Lacosemide, developed by the Union chimique belge(UCB) company of Belgium, is a new NMDA receptor glycine binding-site antagoist, and a novel functionally anti-epileptic drug. It was approved to the markets in European Union, German, Britain and FDA of the US in 2007 and 2008, respectively. Originally, the drug was used for the adjuvant treatment of partial-onset seizures in adults. After years of more researches on the drug, scientists revealed that LCM is sentitive to the slow- and not fast-inactivating of the voltage-gated sodium channels(VGSCs), but insentitive to its fast inactivation state. Also, LCM can interacts with collapsin-response mediator protein 2(CRMP-2).More reaserches focus on the relationship between the sodium channel and the neuropathic pain, because the generation of action potential of the neurons and even the nerve conduction was contributed primarily by the sodium ion, and the sodium channel is the most important channel in regulating the cell membrane excitability of the neurons. Then, researches aslo revealed that: LCM is sensitive to Nav1.3, Nav1.7 and Nav1.8. And as we knew, Nav1.8 is generally believed closely related to the sense of pain, and serves as the important target for the treatment of neuropathic pain in the current pharmacological studies.It was reported that in the clinical practice, LCM can alleviate cancer-pain, osteoarthritis pain, tumor- and chemotherapy-induced cancer pain, painful diabetic neuropathy to a certain extent. In the rats inflammatory neuropathic model and the sciatic nerve injury model, lacosemide can improve the rats’ paw withdrawal mechaical threshold, and it has the inhibiting effects on pain in rats. However, there is no literature reporting the exact mechanism of LCM acts on the synaptic transmission of spinal nociceptive pathway. Spinal dorsal horn is the central-regulation of pain in the level of the spine. The noxious signals are transmitted from periphery nociceptors to the spinal dorsal horn(SDH) neurons. So, it is significant to probe deeply into the exact mechanism of LCM on the SDH neurons, and it will be beneficial to develop LCM into an ideal antinociceptive drug.Experiment 1: The anti-allodynia effects of LCM in the spinal nerve ligation model(SNL model).Objective: To evaluate the anti-allodynia effects of LCM in the spinal nerve ligation model.Methods: Adult Sprague–Dawley rats(180--250g) were chose to make the Spinal nerve ligation(SNL) neuropathic pain model according to the Kim’s method.. Three days after the operation, the von Frey fibers were used to measure the rats’ paw withdrawal mechanical threshold(PWMT). Then,the rats were randomly divided into 3 groups: Control group, High-dose LCM(100mg/kg) and Low-dose LCM(50mg/kg).Drugs were intraperitoneally injected( 50mg/kg, 100mg/kg, respcetively). To determine the anti-allodynia ettects of LCM, PWMT were measured at the time points as follow: 5、15、30、45、60、90、120、150 min after the intraperitoneal injection.Results: Compared with the control group, LCM(100 mg/kg, i.p.) significantly increases the PWMT after SNL(P<0.01; SNK-q). The peak effect appears 40 mins after adminstion of LCM, and lasts at least 70 mins. While the controlateral side,LCM(50mg/kg, i.p.),vehicle group and control group shows no significant diferences(P> 0.05).Experiment 2: The inhibitory effect of lacosemide on the spinal nociceptive pathway in rats.Objective: To investigate the effects of lacosamide(LCM) on the spinal nociceptive Aδ fiber and C fiber mediated synaptic transmission on the spinal slices with a dorsal root attached.Methods: Adult Sprague–Dawley rats(3-5 weeks old, 80--100g) were perfused with ACSF through the heart. Then a parasagittal spinal slice with a dorsal root attached was made. Whole-cell patch-clamp recordings were used to record the SDH neurons.Holding voltage was set at-70 m V. The input resistance was less than 10MΩ. The effects of LCM on the s EPSCs and e EPSCs were evaluated by the model of constant-voltage stimulation.Results:(1)Lacosamide significantly inhibits the monosynaptic peak amplitude of evoked Excitatory Postsynaptic Current(e EPSC) mediate by the Aδ fibers and C fibers in the SDH neurons in a dose-dependent manner.(2)Lacosamide attenuates the frequency of spontaneous Excitatory Postsynaptic Current(s EPSCs) in the SDH neurons, without affecting their amplitude.(P < 0.01; K–S test).Conclusions:1. LCM can inhibit the evoked excitatory synaptic transmission(e EPSCs) between the Aδ and C fibers and the SDH neurons in a dose dependent manner.2. LCM attenuates the frequency of spontaneous excitatory synaptic transmission(s EPSCs)of the SDH neurons without affecting their amplitude,suggesting principally pre-synaptic action.3. Intraperitoneal administration of LCM(100 mg/kg) significantly increases the PWMT after SNL, suggesting its anti-allodynia effect.