Antidepressant-like effects of cinnamamide derivative M2 via D₂ receptors in the mouse medial prefrontal cortex

Major depressive disorder is a global mental illness associated with severe mortality and disability. The dopaminergic system is involved in both the etiology and therapeutics of depression. Distinct functions of dopamine D₁ and D₂ receptor subtypes have attracted considerable research interest, and their roles in the pathogenesis of depression and interaction with antidepressants need to be comprehensively elucidated. Herein, we investigated the antidepressant effects of a candidate antidepressant from a cinnamamide derivative, M2, and examined underlying neural mechanisms. We observed that a single dose of M2 (30 mg/kg, ip) produced rapid antidepressant-like effects in mice subjected to the forced swim and tail suspension tests. Using whole-cell recordings in mouse coronal brain slices, we found that application of M2 (10-150 μM) concentration-dependently increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) of the pyramidal neurons in the medial prefrontal cortex (mPFC). Furthermore, M2-induced enhancement of sEPSC frequency was abolished by sulpiride (10 µM), a dopamine D2 receptor antagonist, but not by the dopamine receptor D₁ antagonist, SCH23390 (10 μM). In addition, M2 administration significantly increased expression levels of synaptogenesis-related proteins, including p-mTOR and p-TrkB, in the mPFC at 30 min, and increased postsynaptic protein PSD-95 at 24 h. Our results demonstrated that M2 produces rapid antidepressant actions through a novel mechanism via dopamine D₂ receptor-mediated enhancement of mPFC neurotransmission.