Negative Regulation of Opioid Receptor-G Protein-Ca super(2+) 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 delta -opioid receptors, G proteins (G sub(i3 alpha ) or G sub(o1 alpha )), and N-type ( alpha sub(1B)) Ca super(2+) channels. Membrane currents through Ca super(2+) channels were recorded from the oocytes under voltage-clamp conditions. The Ca super(2+) channel currents were reduced reversibly by 40-60% in the presence of 1 mu M leucine-enkephalin (Leu- Enk). The Leu-Enk-induced current inhibition was recovered promptly by nefiracetam (1 mu M), while control currents in the absence of Leu-Enk were not influenced by nefiracetam. A binding assay revealed that super(3)H-nefiracetam preferentially bound to the membrane fraction of oocytes expressing G sub(i3 alpha ). When delta -opioid receptors were coexpressed, the binding was significantly increased. However, an additional expression of alpha sub(1B) Ca super(2+) channels decreased the binding. The results suggest that nefiracetam preferentially binds to G sub(i3 alpha ) associated with delta -opioid receptors, thereby inhibiting the association of G proteins with Ca super(2+) channels. In conclusion, nefiracetam negatively regulates the inhibitory pathway of opioid receptor-G protein-Ca super(2+) channel.