Si-Ni-San alleviates depressive-like behaviors in adolescent male mice by restoring DRN-NAc serotonergic circuit function and 5-HT4R-dependent synaptic plasticity

Background: Early-life stress (ELS) is a key risk factor for adolescent depression. Si-Ni-San (SNS), a classic traditional Chinese medicine formula, has shown antidepressant potential, yet its effects on the dorsal raphe nucleus (DRN)-nucleus accumbens (NAc) serotonergic circuit remain unclear.

Purpose: This study aimed to investigate whether SNS alleviates adolescent depression by restoring DRN-NAc serotonergic circuit function and to identify the serotonin receptor mediating its synaptic effects in the NAc.

Study design: Firstly, the antidepressant efficacy of SNS was evaluated in a mouse model of ELS. Subsequently, its underlying mechanism was explored through integrated neurophysiological, molecular, and pharmacological analyses.

Methods: Depressive- and anxiety-like behaviors were assessed using behavioral tests (sucrose preference, tail suspension, forced swim, open field, and elevated plus maze). In vivo electrophysiolog was employed to monitor DRN neuronal activity. Chemogenetic manipulation was employed to regulate the DRN-NAc serotonergic circuit, while 5-HT4R function was assessed through pharmacological intervention and viral knockdown. Synaptic and molecular mechanisms were examined using Western blotting, qPCR, ELISA, and immunofluorescence.

Results: SNS alleviated depressive-like behaviors, enhanced neural activity and low-frequency oscillations in the DRN, and restored 5-hydroxytryptamine (5-HT) levels in the NAc. Mechanistically, SNS upregulated Tryptophan Hydroxylase 2 (TPH2) while downregulating indoleamine 2,3-dioxygenase 1 (IDO1), thus promoting 5-HT synthesis. Critically, the antidepressant effects of SNS were blocked by either chemogenetic inhibition of the DRN-NAc serotonergic circuit or pharmacological blockade of 5-HT4R in the NAc. Meanwhile, the knockdown of 5-HT4R abolished the ameliorative effects of SNS on depressive-like behaviors and associated synaptic remodeling, including the upregulation of brain-derived neurotrophic factor, postsynaptic density protein 95, and mushroom spine density.

Conclusion: These results demonstrate that SNS alleviates depressive-like behaviors in adolescent male mice by restoring DRN-NAc serotonergic circuit function, enhancing 5-HT bioavailability, and promoting 5-HT4R-dependent synaptic plasticity in the NAc, revealing a circuit- and receptor-specific therapeutic mechanism.

Adolescent depression; Dorsal raphe nucleus; Nucleus accumbens; Si-Ni-San; Spine plasticity.