Chronic 5-HT7R activation drives depressive phenotypes and synaptic dysfunction.

Selective serotonin reuptake inhibitors (SSRIs) are commonly used to treat depression, but their chronic use is associated with side effects and residual symptoms of depression. Both effects induced by SSRIs are mediated by serotonin receptor-dependent signaling pathways, yet the molecular mechanisms underlying these effects remain unclear. Here, we investigated the impact of chronic and acute activation of the 5-HT7 receptor (5-HT7R) using the selective agonist AGH-194 in male mice. Behavioral assessment revealed that chronic AGH-194 administration induced depressive-like effects in the novelty suppressed feeding test (NSFT), female urine sniffing test (FUST), and novel object location test (NOLT). After acute injection, depressive-like effects were observed only in NSFT. At the molecular level, AGH-194 administration activated matrix metalloproteinase 9 (MMP-9) through a 5-HT7R-Gαs signaling-dependent mechanism. Acute treatment induced transient activation, while chronic treatment led to prolonged enzymatic activity, accompanied by a reduction in the expression of the GluA1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) in the hippocampus. At the cellular level, acute but not chronic AGH-194 treatment induced a shift toward more juvenile dendritic spine morphology in the CA1 and dentate gyrus (DG) regions of the hippocampus, along with an increase in dendritic spine density in DG. Electrophysiological recordings demonstrated that acute AGH-194 administration enhanced hippocampal excitability by increasing population spike amplitude in CA1. Chronic AGH-194 treatment further modulated short-term plasticity, increasing both population spike and extracellular field potential paired-pulse ratios (PS-PPR and EPSP-PPR) in CA1, while also enhancing the maximum EPSP slope amplitude. These findings provide novel evidence that chronic 5-HT7R activation can induce depressive-like behaviors in male mice, potentially through sustained MMP-9 activation and alterations in synaptic plasticity. Understanding the molecular and electrophysiological consequences of selective 5-HT7R stimulation may provide insights into receptor-specific mechanisms that could contribute to SSRI-induced side effects, thereby contributing to the development of improved antidepressant strategies.