| Summary: | 碩士 === 臺北醫學大學 === 醫學研究所 === 95 === Dextromethorphan (DM), an antitussive agent, shares the same chemical structure as morphine. But as a D-form isoform, it did not have any effect on opioid receptor. For decades, DM was found to be a non-competitive antagonist on the N-methyl-D-aspartate (NMDA) receptor, one subtype receptor of glutamate receptor, by blocking the NMDA receptor-coupled ion channel. Previous investigation, the binding site on the NMDA receptor by using [3H]-DM ligand binding assay did not well-define the pharmacological feature of DM binding. Therefore, we further elucidate this issue by using characterized [3H]-DM ligand binding assay in membrane prepared from dissected rat cortical tissues and as examined the effect of DM on the NMDA-induced intracellular 45Ca2+ accumulation in rat primary cortical cell culture. The binding of [3H]-DM in the cortical membrane preparation had two distinct binding kinetics, namely a high affinity site with Kd of 53 ± 14 nM, and a low affinity site with Kd of 5.9 ± 1.1 μM. Addition of NMDA or D-AP5, a competitive antagonist on the NMDA binding site of the NMDA receptor, concentration-dependently inhibited the binding of [3H]-DM. On the contrary, addition of glycine or ACBC, a competitive antagonist on the glycine binding site of the NMDA receptor, did not have significant effect. The inhibitory effect of NMDA or D-AP5 on the [3H]-DM is mainly by decreasing the affinity of high affinity site without affecting the low affinity binding. However, DM did not influence the binding of [3H]-glutamate or [3H]-CGP39653. DM concentration-dependently inhibited the NMDA-induced intracellular 45Ca2+ accumulation in rat primary cortical cell culture with a IC50 of 4.8 ± 0.9μM. This inhibitory effect of DM is likely a non-compeptive manner since addition of DM did not affect the potency of NMDA but decreased the maximal efficacy of NMDA. These result indicated that DM have two distinct binding site on the NMDA receptor in which the ligand binding to the NMDA binding site on the NMDA receptor can competitive inhibit the high affinity binding of DM on the NMDA receptor-coupled channel, but the binding of DM did not affect the ligand binding of NMDA binding site. Such particular finding not only improves our knowledge of DM binding site on the brain but also adding another complexity regarding the multiple ligand binding sites on the NMDA receptor.
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