Effects of SSRIs on the spatial transcriptome of dorsal raphe serotonin neurons

The serotonin system is the main therapeutic target for selective serotonin reuptake inhibitors (SSRIs) in treating depression, yet the mechanism of action of SSRIs remains incompletely understood. To investigate the molecular and transcriptional effects of SSRI administration on serotonin neurons, we performed spatial transcriptomics, a spatially resolved RNA-sequencing method in intact brain tissue. Mouse brain sections containing the dorsal raphe nucleus and adjacent midbrain structures were analyzed, revealing six distinct serotonergic subpopulations with unique molecular signatures and spatial distributions. Both acute and chronic fluoxetine treatment induced a large number of changes in gene expression in the dorsal raphe nucleus. Notably, Htr1a expression increased following acute treatment but decreased after chronic administration, supporting previous findings on serotonin transporter blockade effects on 5HT1a autoreceptors. Gene enrichment and network analysis identified key pathways modulated by SSRI administration, including Ras, MAPK and cAMP signalling pathways as well as pathways involved in axonal guidance. Additionally, we observed treatment-dependent opposing transcriptional changes in neuropeptides, particularly thyrotropin-releasing hormone (Trh) and Prodynorphin (Pdyn), with distinct spatial localization within the dorsal raphe nucleus. Collectively, our transcriptomic and in situ hybridization analyses reveal spatial and cell-type-specific heterogeneity in SSRI action within the dorsal raphe nucleus, providing new insights into the molecular basis of SSRI treatment effects.