Screening And Evaluation Of Selective NMDA Receptor NR2B Antagonists

Opioids have long been used for the treatment of moderate to severe pain. Despite their strong antinociceptive effects, the use of opioids in the treatment of pain is restricted by their physical and psychological dependence. In addition, opioids induce drug addiction among people. The pathophysiological mechanism of opioid dependence is related to the adaptation of many neurotransmitters and their receptors after chronic opioid treatment. Many studies demonstrated that glutamate-NMDA receptor system was an important modulatory system that has effect on the pharmacological actions of opioids.Glutamate is a major excitatory neurotransmitter in mammal central nervous system. NMDA receptor is an important ionotropic glutamate receptor, which is a ligand-gated positive ion channel composed by one NR1 subunit and at least one NR2 subunit. There are 4 genes encoding the NR2 subunit, including NR2A, NR2B, NR2C and NR2D. The property of NMDA receptor depended on the NR2 subunit which formed the heteromeric. NMDA receptor, especially NR2B subunit, plays important role in the pathophysiological process of drug dependence. Therefore, the main purpose of this investigation is to screen and evaluate novel selective NR2B antagonist that has inhibitory effect on opioid dependence, hopefully we can find potential new drugs for the prevention and therapy of opioid dependence and relapse.4-substituted piperdines were chosen as the lead structure. A series of novel compounds were designed and synthesized in this study. Because activation of NMDA receptors results in calcium influx which evokes membrane current. Xenopus oocytes expressing NMDA receptors (NR1A/NR2B) were established in this test. Two-electrode voltage clamp experiment was used to screen these compounds which had inhibitory effect on membrane current evoked by NMDA receptor channel in Xenopus oocytes. In addition, many studies revealed that this kind of antagonist had peripheral analgesic activity. Mice acetic acid writhing test was also used to screen these compounds. Y-IP5 blocked membrane current evoked by NMDA receptor channel significantly. Y-IP9 and Y-IP10 had relatively strong analgesic activity. Based on the results of preliminary screening, the structure-activity relationship was analyzed as follow:1) When the piperdine ring of lead structure was replaced by piperazine ring, the analgesic activity was retained,but the ability to block membrane current was attenuated. 2) When the piperazine ring or piperdine ring was replaced by bulky amino groups such as memantine, both the analgesic activity and the ability to block membrane current were attenuated significantly. 3) When the linker between 1-site of piperazine ring and aryl was ethylenecarbonyl, the analgesic activity and the ability to block membrane current were strong. 4) When the aryl group was 2-benzoxazolone-6-yl or 2-benzothioazolone-yl, the analgesic activity and the ability to block membrane current were retained or enhanced.Among these compounds, Y-IP9 and Y-IP10 had relatively strong analgesic activity. Y-IP5 blocked membrane current significantly. The pharmacological properties of these three compounds were investigated thereafter.Y-IP9 and Y-IP10 showed potent antinociceptive effects with ED50 of 3.31 mg·kg-1 and 3.80 mg·kg-1 respectively in the mice acetic acid writhing test. In rat diabetic neuropathy pain model these two compounds also showed antinociceptive effects. Y-IP9 and Y-IP10 bound opioid receptors with low affinity, which implicated their analgesic action were not related to activation of opioid receptor. Both Y-IP9 and Y-IP10 inhibited the development of morphine-induced conditioned place preference in mice. Y-IP10 also inhibited the expression of physical dependence in morphine-dependent mice. In addition, both Y-IP9 and Y-IP10 inhibited locomotor activity in mice. Y-IP5 blocked membrane current evoked by NMDA receptor channel with the inhibition rate of 84.4%. In chronic morphine tolerance model, Y-IP5 inhibited the development of morphine-induced tolerance. Y-IP5 didn't influence locomotor activity in mice, but inhibited acute morphine-induced hyperactivity. Y-IP5 inhibited the development of morphine-induced physical dependence. Furthermore, Y-IP5 inhibited the development of morphine-induced behavioral sensitization and conditioned place preference in mice.In conclusion, Y-IP5 was found to have inhibitory effect on tolerance to and dependence on opioid, it could be taken as a potential leading compound in prevention and therapy of opioid dependence. Further investigation still needs to do to evaluate the possible effect of Y-IP5 on opioid dependence.