¹Shanghai Jiao Tong University School of Medicine, Shanghai Mental Health Center, Shanghai, China, ²Shanghai Jiao Tong University, School of Psychology, Shanghai, China

Background: Obsessive-compulsive disorder (OCD), a disabling psychiatric disorder characterized by obsessions (commonly anxiety-associated) or compulsions, remains mechanistically elusive, which hinders therapeutic development. Our previous study revealed elevated succinic acid levels in both the serum of OCD patients and in the serum and medial prefrontal cortex (mPFC) of OCD-patient-feces-colonized mice, suggesting its potential role as a pathogenic metabolite. However, the biological mechanisms linking excessive succinate to OCD pathophysiology remains unclear.

Aims & Objectives: This study aims to investigate whether succinate induces compulsive-like behavior in mice and elucidate its neurobiological underpinnings.

Method: Sodium succinate (SA) was administrated to adult male mice through drinking water solution, while control (Ctrl) mice received regular drinking water without SA. Compulsive-like behaviors were assessed using open field test, elevated plus maze, marble burying test, and grooming analysis. Additionally, spontaneous behavior was quantified using a machine learning-powered 3D behavioral analysis platform. Transmission electron microscopy evaluated myelination and postsynaptic density (PSD) ultrastructure in mPFC. Whole-cell patch clamp recordings measured neuronal excitability, synaptic transmission and excitation-inhibition (E/I) balance, complemented by optical-fiber recordings. Using chemogenetic methods, we activated glutamatergic neurons in the mPFC to verify their regulatory functions in succinate-induced compulsive-like behaviors.

Results: Succinate-treated mice exhibited significant compulsive-like behavior profiles compared to Ctrl. The SA group showed reduced myelin sheath thickness and PSD depth in mPFC. The number of acting potential spikes evoked by depolarizing current injection and the amplitude of spontaneous excitatory postsynaptic currents (sEPSC) were significantly lower, while the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) was significantly higher in mPFC pyramidal neurons of SA-treated mice. During exploratory and grooming behaviors, the Ca²⁺ signals of mPFC glutamatergic neurons significantly decreased in SA-treated mice compared to the Ctrl group. Activation of mPFC glutamatergic neurons significantly attenuated succinate-induced compulsive-like behaviors.

Discussion & Conclusions: Succinate induces compulsive-like behaviors accompanied by ultrastructural and functional impairments in mPFC glutamatergic neurons. Hypoactivity and E/I imbalance characterized by inhibitory bias in these neurons can mediate succinate-induced behavioral abnormalities.