Major depressive disorder (MDD), bipolar depression (BP), and schizophrenia (SCZ) represent significant global health burdens, yet our understanding of their pathophysiology remains incomplete. This necessitates the development of novel diagnostic and therapeutic strategies. Research efforts are increasingly focused on identifying individual constructs of behavior and brain function, rather than solely relying on traditional disease models. This study takes a combinatorial approach to explore the role of Signal transducer and activator of transcription 3 (STAT3), a transcription factor activated by Janus kinase (JAK) in response to diverse stimuli, in behavioral phenotypes relevant to psychiatric disorders. The rationale for focusing on STAT3 stems from several observations: its role in cellular immune processes implicated in psychopathology; its activation by various upstream regulators relevant to neural function (growth factors, hormones, endocannabinoids); its influence on synaptic plasticity and epigenetic processes; and its emerging links to serotonergic neurotransmission. Prior research suggests a role for STAT3 in psychopathology and relevant animal models, indicating its potential as a regulatory lever for negative emotional behavior. This study hypothesizes that STAT3 signaling within the brain's serotonergic system plays a crucial role in regulating behavioral traits related to mental illness. The researchers used a serotonergic cell-specific STAT3 knockout mouse model and viral-mediated STAT3 knockdown in the dorsal raphe (DR) to investigate this hypothesis, complemented by in vivo electrophysiological recordings and transcriptomic analysis of the DR.

The introduction extensively cites previous research supporting the involvement of STAT3 in neural function and its association with psychopathology. The authors reference studies highlighting STAT3's role in immune responses, synaptic plasticity, epigenetic regulation, and its interaction with the serotonergic system. This literature review establishes the context for the current study and underscores the rationale for investigating STAT3's role in behavioral regulation.

The study used conditional STAT3 knockout mice (*SertCre/+; Stat3fl/fl*) and littermate controls (*SertCre/+; Stat3+/+*) bred from founder pairs of B6.129S1-*Stat3<tm1Xyfu>/J* and B6.129(Cg)-*Slc6a4<tm1(cre)Xz>/Cnrm*. *Sert++; Stat3fl/fl* mice were used for viral knockdown experiments. Behavioral testing included the sucrose preference test (SPT), novelty-suppressed feeding (NSF), forced swim test (FST), open field test (OFT), light-dark box (LDB), elevated plus maze (EPM), rotarod (RR), amphetamine sensitization, and conditioned place preference (CPP). In vivo electrophysiological recordings of DR 5-hydroxytryptamine (5-HT) neurons were performed using an Axoclamp-2B amplifier and Digidata-1440 interface. Immunohistochemistry and quantification of fluorescence were used to assess STAT3 expression in different cell types within the DR. RNA sequencing (RNA-Seq) and quantitative real-time PCR (qRT-PCR) were employed for transcriptomic analysis. Viral knockdown of STAT3 in the midbrain DR was achieved through the infusion of AAV-Cre or AAV-GFP. Statistical analysis involved two-tailed, two-sample Student's t-tests (with Welch correction where appropriate) and repeated-measures ANOVA. Sample sizes were determined based on previous studies and all procedures adhered to ethical guidelines.

Conditional STAT3 knockout mice showed a significant reduction in STAT3 expression within serotonergic neurons of the dorsal raphe (DR). These mice exhibited reduced negative behavioral reactivity, as evidenced by decreased immobility in the forced swim test and reduced anxiety-like behavior in the light-dark box and elevated plus maze. They also showed a blunted response to amphetamine sensitization. In vivo electrophysiological recordings revealed altered firing rates of DR 5-HT neurons in the knockout mice. Transcriptomic analysis identified alterations in gene networks associated with neuropsychiatric disorders in the DR of knockout mice. Viral-mediated knockdown of STAT3 in the adult DR of wild-type mice phenocopied the behavioral alterations observed in the knockout mice, demonstrating that the effects are not solely due to developmental factors. The results indicate a specific role for STAT3 in the DR in regulating behavioral reactivity.

The findings support the hypothesis that STAT3 signaling within the DR serotonergic system plays a significant role in the regulation of behavioral traits relevant to psychopathology. The reduced negative behavioral reactivity and blunted amphetamine sensitization observed in both the knockout and viral knockdown mice point towards a direct involvement of DR STAT3 in modulating emotional responses and reward processing. The altered neuronal firing rates and transcriptional changes further support this conclusion, highlighting the molecular mechanisms underlying the observed behavioral effects. These findings provide a mechanistic link between upstream activators of STAT3, serotonergic neurotransmission, and psychopathology, suggesting that targeting DR STAT3 signaling could offer a novel therapeutic avenue for neuropsychiatric disorders.

This study provides strong evidence for the role of dorsal raphe STAT3 as a molecular gate regulating behavioral reactivity and associated gene networks implicated in psychopathology. The consistent findings across both genetic knockout and viral knockdown models highlight the importance of this specific brain region and signaling pathway. Future research could focus on identifying the specific downstream targets of STAT3 involved in these behavioral effects, and exploring the potential therapeutic implications of targeting STAT3 signaling for the treatment of neuropsychiatric disorders.

The study primarily focused on mice, which may limit the generalizability of the findings to humans. Further research is needed to validate these findings in other animal models and ultimately in humans. The specific downstream molecular pathways influenced by STAT3 in the DR require further investigation. The study primarily focused on the effects of STAT3 reduction; investigation of STAT3 overactivation is also warranted.