Hallucinogenic 5-HT_2A receptor (5-HT_2AR) agonists-induced head-twitch response (HTR) is regulated by G_s signaling pathway. Formation of heterodimers between 5-HT_2AR and metabotropic glutamate mGlu2 receptor (mGluR2) is essential for the hallucinogenic 5-HT_2AR agonist-induced HTR. In order to investigate the effects of mGluR2 agonists and inverse agonists on hallucinogenic 5-HT_2AR agonists DOM-induced HTR, C57BL/6 mice were pretreated with mGluR2 agonists (LY379268, LY354740, LY404039) or the inverse agonist LY341495, and the HTR was manually counted after administering DOM immediately. IP-One (IP1) HTRF assay and cAMP assay were performed to evaluate the effect of LY341495 or LY354740 on DOM-induced G_q and G_s activation in Human Embryonic Kidney-293 (HEK-293) T-type cells co-expressing 5-HT_2AR and mGluR2. The results showed that DOM-induced HTR in mice was dose-dependently inhibited by LY379268, LY354740, and LY404039, while it was dose-dependently enhanced by LY341495. Moreover, LY341495 reversed the inhibitory effect of LY354740 on DOM-induced HTR. In HEK-293T cells co-expressing 5-HT_2AR and mGluR2, DOM-induced cAMP level was decreased by LY354740 and increased by LY341495, but DOM-induced IP1 level was not regulated by LY354740 or LY341495. The regulation of DOM-induced HTR by mGluR2 agonists and inverse agonists is closely related to 5-HT_2AR-mediated G_s signaling pathway. In HEK-293T cells co-expressing 5-HT_2AR and mGluR2 A677S/A681P/A685G mutant (mGluR2 3?A mutant), DOM-induced cAMP level was not regulated by LY354740, but was significantly enhanced by LY341495. The 5-HT_2AR/mGluR2 heterodimers is critical for DOM-induced HTR and cAMP level, both of which are inhibited by mGluR2 agonists and enhanced by mGluR2 inverse agonists.