Negative Regulation of Opioid Receptor-G Protein-Ca2+ Channel Pathway by the Nootropic Nefiracetam

It has recently been reported that nefiracetam, a nootropic agent, is capable of attenuating the development of morphine dependence and tolerance in mice. The mechanism of this antimorphine action is not clear. The present study was designed to address this issue using Xenopus oocytes expressing δ-opioid receptors, G proteins (G i 3 α or G o 1 α ), and N-type (α 1 B ) Ca 2 + channels. Membrane currents through Ca 2 + channels were recorded from the oocytes under voltage-clamp conditions. The Ca 2 + channel currents were reduced reversibly by 40-60% in the presence of 1 μM leucine-enkephalin (Leu-Enk). The Leu-Enk-induced current inhibition was recovered promptly by nefiracetam (1 μM), while control currents in the absence of Leu-Enk were not influenced by nefiracetam. A binding assay revealed that 3 H-nefiracetam preferentially bound to the membrane fraction of oocytes expressing G i 3 α . When 8-opioid receptors were coexpressed, the binding was significantly increased. However, an additional expression of α 1 B Ca 2 + channels decreased the binding. The results suggest that nefiracetam preferentially binds to G i 3 α associated with 6-opioid receptors, thereby inhibiting the association of G proteins with Ca 2 + channels. In conclusion, nefiracetam negatively regulates the inhibitory pathway of opioid receptor-G protein-Ca 2 + channel.