The Effect Of NMDA Receptor Antagonist On Intrathecal Morphine Induced Analgesia And Pruritus And Its Mechanisms

IntroductionPruritus is the most common complication of intrathecal morphine. It can have a significant negative impact on the efficacy of analgesia by morphine in some severe cases. However, the exact molecular mechanism of morphine induced pruritus is unclear, and the treatment remains a challenge. Some itch specific neuron in spinal cord express μ-opioid receptor(MOR) isoform MOR1D, which mediates intrathecal morphine induced pruritus. Studies have shown that MORs and N-methyl-D-aspartate receptors(NMDARs) associate in many regions of the central nervous system. NMDAR activation negatively regulates the analgesic capacity of morphine. And considering the increased activity of NMDAR could participate in the relaying of spinal itch sensation, we proposed that NMDAR antagonist should be a potent drug for the treatment of intrathecal morphine induced pruritus. In this study, we build the mice model of intrathecal morphine induced pruritus and analgesia. On that basis, we tested the effect of NMDAR antagonist on intrathecal morphine induced analgesia and pruritus, with pERK as a marker for itch neuron activation.MethodMale C57BL/6 mice were randomly assigned into naive, control(saline) group and different doses of morphine groups to create the mice model of intrathecal morphine induced pruritus and analgesia. I.t. injections to the lumbar region of conscious mice were performed. The number of scratching responses was counted for 30min post-injection to evaluate pruritus. Warm water tail immersion assay was conducted before and until 120min post-injection at 30min intervals. Percent of maximal possible potential effect(%MPE) and area under curve(AUC) were calculated based on tail flicking latency to evaluate analgesic efficacy. The effects of ketamine or ifenprodil coinjection with 0.5μg morphine were evaluated. And lumbar spinal cord tissue were collected at 5min post-injection to identify the expression pERK by Western blot.Results Compared with the control group, morphine 0.5μg i.t. injection elicited obvious scratching response within 30mins post-injection. After i.t. morphine, scratching increased dramatically within 5min, and decreased quickly after 10min. In contrast, analgesia maintained for 90min.In MO+KET group, ketamine lμg coinjected with morphine 0.5μg partially reduced the number of scratch within 30min post-injection. As the dose increased, ketamine 3μg coinjection completely abolished the scratching induced by i.t. morphine 0.5μg (4.38±4.60, vs.39.13±18.98, respectively, n=8, p<0.001). In MO+IFEN group, scratching was significantly attenuated by both ifenprodil 0.1μg and ifenprodil 0.5μg coinjection, compared with i.t. morphine 0.5μg+DMSO(13.00±5.61,1.63±1.60, vs. 39.25±17.99, respectively, n=8, p<0.001).The analgesic efficacy of i.t. morphine was not affected by ketamine coadministration. However, both ifenprodil 0.1μg and ifenprodil 0.5μg coinjected with morphine intrathecally respectively increased %MPE at each time point. And there was a significant difference in AUC, compared with i.t. morphine 0.5μg group(187.6±34.99, 192.2± 44.05,141.7±35.73, respectively, n=8, p<0.05).Compared with control group, i.t. morphine 0.5μg significantly increased the expression of pERK1/2 in lumbar dorsal horn, which was decreased by ketamine or ifenprodil coadministration with morphine.Conclusion1. Intrathecal morphine could induce pruritus in mice.2. NMDAR antagonist, both ketamine and ifenprodil coadministration with morphine intrathecally could alleviate morphine induced pruritus in mice model. NR2B selective antagonist ifenprodil had the potential to increase the analgesic effect of intrathecal morphine.3. Intrathecal morphine could increase the expression of pERK in lumbar spinal dorsal horn. Both ketamine and ifenprodil could alleviate morphine induced pruritus, and significantly decrease the expression of pERK as well.