| Pain is a kind of protective warning signal when the body confronts nocuousstimulus or disease. The long-lasting chronic pain, however, loses its warning effect, andexerts to the patients debilitating suffering. Neuropathic pain is the most common, moststubborn chronic pain that is lack of effective treatment. International Association for theStudy of Pain (IASP) defined neuropathic pain as pain arising as a direct consequence ofa lesion or disease affecting the somatosensory system. The mechanism of neuropathicpain development is still poorly understood. At present, the main measures of treatmentinvolve drug therapy and non-drug therapy, the latter includes surgical treatment,interventional therapy and so on. But all the treatment measures mentioned above havesignificant disadvantage. The serious side-effect and unsatisfactory effect are the mostcommon obstacles for the drug treatment. Besides, the surgical treatment is quite difficultand the medical cost is so expensive that hamper its widespread use. Therefore,developing ideal analgesic is urgent needed. The cannabinoid has been used for thousands of years to deal with pain. At present,two kind of endocannabinoids, AEA (anandamine) and2-AG (2-arachidonoyl glycerol),has been found to participate in a variety of physiological activity, including the immuneregulation, cardiovascular system, energy metabolism, pain, synaptic plasticity andneural protection. Previous studies have demonstrated that2-AG can exert its analgesiceffects on a variety of animal models, such as acute pain, inflammatory pain andneuropathic pain models, but there are no literature reported the effect of2-AG on SGneurons (substantia gelatinosa neurons in the dorsal horn of spinal cord) which constitutethe′Spinal center of pain modulationμ. For the purpose of further understanding theanalgesic mechanism of endocannabinoid, the present study examined the effect of2-AGon synaptic transmission from primary afferent fibers to SG neurons.Experiment one: The analgesic effect of2-AG in neuropathic painmodel.Objective: To observe the analgesic effect of intrathecal injection of2-AG inneuropathic pain model.Methods:200~250g adult SpragueíDawley rats were used in the behavioralexperiments. The paw withdrawal mechanical threshold (PWMT) and thermal withdrawlatency (TWL) were performed before spinal nerve ligation (SNL). The rats with normalbase threshold were chose to make SNL neuropathic pain model and conduct intrathecalcatheterization. Three days after SNL, the PWMT and TWL were tested again whenneuropathic pain developed fully. Rats were divided into two groups randomly:experimental group with20μg2-AG intrathecal injection, control group with the samevolume of normal saline intrathecal injection. The value of PWMT and TWL weremeasured at the time points as follow:5,10,15,20,25,30,40,50,60min afterintrathecal injection, respectively, to observe the analgesic effect of exogenous2íAG.Results: Compared with the control, exogenous2-AG significantly increased the paw withdrawal mechanical threshold (P <0.001) and prolonged thermal withdraw latency (P<0.001) for at most60min, while vehicle injection had no significant effect.(P>0.05,one-way AVOVA).Experiment2: The effect of exogenous2-AG on the excitatory synaptictransmission of SG neurons in spinal dorsal horn.Objective: To observe the effect of2-AG on excitatory postsynaptic currents (eEPSCs)induced by dorsal root stimulation in spinal cord slice.Methods: Adult SpragueíDawley rats (4-6weeks old) were used to make spinal cordslices. Whole-cell, voltage-clamp recordings were made from SG neurons in spinaldorsal horn. To observe the effect of2-AG on synaptic transmission and excitability ofSG neurons receiving different types of primary afferent fibers, the spontaneousexcitatory postsynaptic currents (sEPSCs) and dorsal root evoked eEPSCs were recorded.Results:(1) Exogenous2-AG significantly inhibited the peak amplitude ofmonosynaptic eEPSCs mediated by A and C afferent fibers, however, potentialized thepeak amplitude of monosynaptic eEPSCs mediated by Aβ fibers. Either the inhibitory orenhanced effect can be blocked by specific CB1receptor antagonist-AM251.(2)2-AGsignificantly decreased the frequency of sEPSCs in SG neurons, while had no significanteffect to its amplitude (P <0.01; K-S test).Summary:1. Single intrathecal injection of2-AG attenuates the neuropathic pain induced by nerveinjury. Its maximum analgesic effect on mechanical allodynia appeared at20min afterdrug injection, and the similar effect on thermal hyperalgesia appeared at10min afterdrug injection. Both the anti-allodynia effect and anti-hyperalgesia effect lasted for atmost1h.2.2-AG plays a different role on synaptic transmission in SG neurons depending on the different types of afferent fibers. It significantly inhibits the synaptic transmissionmediated by nociceptive fibers (A and C fiber), but enhances the synaptic transmissionmediated by innocuous mechanoreceptive fibers (Aβ fiber) in a dose-dependent manner.Either inhibitory or enhanced effect mentioned above was mediated by CB1receptors.3.2-AG inhibits the frequency of sEPSCs of SG neurons and has no effect on itsamplitude, indicating that2-AG activates the presynaptic CB1receptors, which isconsistent with the viewpoint that CB1receptor is located on the presynaptic membrane.Conclusion:Spinal2-AG may inhibit the release of excitatory transmitter glutamate andattenuate allodynia and hyperalgesia through the activation of CB1receptors on primarynociceptive fibers. The potentiated effect of2-AG on A fibers needs furtherinvestigation. |
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