Molecular Mechanisms of Exercise-induced Hippocampal Neurogenesis and Antidepressant Effects

According to a World Health Organization report, there are approximately 280 million depressed people worldwide (1) . Depression is a common mental illness to us all, and the socioeconomic loss caused by depression is very large. The National Health Promotion Movement in the 21st Century (Health Japan 21) launched by the Japanese government emphasizes the importance of the prevention and early treatment of depression from the perspective of mental health. Currently, antidepressants, including selective serotonin reuptake inhibitors (SSRIs), are used for the pharmacological treatment of depression. However, a significant proportion of depressed patients do not achieve remission, and the therapeutic effect is not sufficient (2) . The large number of patients with treatment-resist-ant depression who do not respond to existing antidepressants has become a major social problem, and there is an urgent medical need to develop novel and effective therapeutic agents for depression (3) .

It is well known that exercise is beneficial for the prevention and improvement of cardiovascular diseases, diabetes, osteoporosis, and so on. Recent studies have revealed that exercise also has beneficial effects on the brain (4), (5) . The effects of exercise on the brain have been reported at the experimental animal and human levels, including the promotion of neurogenesis in the hippocampus (6), (7) , prevention and improvement of depression (antidepressant effects) (8), (9) , and enhancement of learning ability (10), (11) . Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter that works in the central nervous system to regulate various physiological functions, such as thermoregulation, feeding behavior, sleep-wakefulness, emotion, and memory, and its involvement in depression and hippocampal neurogenesis has been reported (12), (13) . Recently, it was reported that exercise increases the release of serotonin in the hippocampus and that serotonin plays an important role in the increase in hippocampal neurogenesis and antidepressant effects induced by exercise (14) . However, the detailed mechanism of how exercise-induced serotonin increase in the hippocampus promotes hippocampal neurogenesis and produces antidepressant effects has not been clarified (15) . The serotonin receptor consists of seven subfamilies (5-HT1 to 5-HT7 receptors) (16) . Most of them are G protein-coupled receptors, but the 5-HT3 receptor is the only ionotropic receptor (17) . The 5-HT3 receptor is expressed in the limbic regions of the brain, including the hippocampus, amygdala, and prefrontal cortex (18), (19) , and has been reported to be involved in emotion and memory (20), (21), (22) , but its possible role in hippocampal neurogenesis and depression has not been clarified. Therefore, we sought to investigate the possible relationship of 5-HT3 receptors with exercise-induced increase in hippocampal neurogenesis and antidepressant effects using 5-HT3 receptor knockout (Htr3a −/− ) mice.

- Animals: Wild-type mice and 5-HT3 receptor knockout (Htr3a −/−) mice were used to assess the role of 5-HT3 receptors in exercise-induced neurogenesis and antidepressant effects. - Exercise paradigm: Mice were housed in an exercise environment equipped with a running wheel for 3 weeks; total locomotor activity (wheel revolutions) was recorded and did not differ between genotypes. - Neurogenesis assays: Dividing cells were labeled with bromodeoxyuridine (BrdU). Immunohistochemistry quantified BrdU/NeuN double-labeled mature granule cells and BrdU/DCX double-labeled neural progenitor cells in the subgranular zone of the hippocampal dentate gyrus. - Behavioral tests: Depressive-like behavior was assessed via the forced swimming test and tail suspension test; immobility time served as the primary outcome. - Pharmacology: A 5-HT3 receptor agonist (SR 57227A) was administered to assess acute antidepressant-like effects and neurogenesis. An SSRI (fluoxetine) was administered across doses (including 20 mg/kg) to compare mechanisms; combination treatments (fluoxetine plus SR 57227A) were also evaluated. - Expression mapping: 5-HT3 receptor-EGFP reporter mice were used to map receptor expression in the hippocampus; immunohistochemistry and in situ hybridization identified co-expression with IGF-1 in neurons of the subgranular zone. - IGF-1 measurements: In vivo microdialysis quantified IGF-1 levels in extracellular hippocampal fluid following SR 57227A or fluoxetine; serum IGF-1 levels were also measured. - Pathway manipulation: An IGF-1 receptor antagonist (AG 1024) was administered intrahippocampally to determine whether IGF-1 signaling mediates SR 57227A-induced increases in neurogenesis. - Disease model confirmation: A lipopolysaccharide-induced depression model in mice was used to confirm that a 5-HT3 receptor agonist promotes hippocampal neurogenesis and improves depressive-like behavior.

- Exercise increased BrdU/NeuN double-labeled mature granule cells in the hippocampal dentate gyrus of wild-type mice but not in Htr3a −/− mice after 3 weeks, indicating 5-HT3 receptor dependence for exercise-induced neurogenesis. - Exercise reduced immobility time in forced swim and tail suspension tests in wild-type but not in Htr3a −/− mice, demonstrating that 5-HT3 receptors are essential for exercise-induced antidepressant effects. - The 5-HT3 receptor agonist SR 57227A decreased immobility time (antidepressant-like effect) in wild-type but not in Htr3a −/− mice. - Fluoxetine produced dose-dependent antidepressant effects in both wild-type and Htr3a −/− mice, suggesting 5-HT3 receptors are not required for SSRI effects; combining fluoxetine (20 mg/kg) with SR 57227A (5 mg/kg) enhanced antidepressant effects beyond fluoxetine alone. - SR 57227A increased BrdU/DCX double-labeled neural progenitor cells in the dentate gyrus after 3 days in wild-type but not in Htr3a −/− mice; fluoxetine required 3 weeks of treatment to increase progenitor cells in both genotypes. - 5-HT3 receptors are highly expressed in neurons in the subgranular zone of the dentate gyrus; these neurons co-express IGF-1. - SR 57227A increased IGF-1 levels in extracellular hippocampal fluid without changing serum IGF-1; fluoxetine did not alter hippocampal IGF-1 levels. SR 57227A failed to increase hippocampal IGF-1 in Htr3a −/− mice. - The increase in neural progenitor cells induced by SR 57227A was blocked by intrahippocampal administration of an IGF-1 receptor antagonist (AG 1024), implicating IGF-1 signaling in 5-HT3-mediated neurogenesis and antidepressant effects. - In a lipopolysaccharide-induced depression model, a 5-HT3 receptor agonist increased hippocampal neurogenesis and improved depressive-like behavior. - Reported statistical significance levels included p < 0.05, p < 0.01, and p < 0.001 where indicated in figures.

The study addresses how exercise-induced increases in hippocampal serotonin translate into enhanced neurogenesis and antidepressant effects. Using genetic and pharmacological tools, the authors demonstrate that 5-HT3 receptors are essential mediators of exercise-induced hippocampal neurogenesis and behavioral antidepressant effects. Mechanistically, 5-HT3 receptors are enriched on neurons in the dentate gyrus subgranular zone that produce IGF-1. Agonist stimulation of 5-HT3 receptors rapidly elevates hippocampal extracellular IGF-1 and increases neural progenitor cell numbers via IGF-1 receptor signaling; blocking IGF-1 receptors prevents this neurogenic effect. These findings delineate a 5-HT3 receptor–IGF-1 pathway that is distinct from the mechanism of SSRIs, which require chronic treatment to enhance neurogenesis and do not involve changes in hippocampal IGF-1. The data suggest therapeutic complementarity: 5-HT3 receptor agonists can produce rapid neurogenic and antidepressant-like effects and augment SSRI efficacy. This mechanism provides a rationale for developing novel antidepressants targeting 5-HT3 receptors and IGF-1 signaling, potentially benefiting patients who are unresponsive to current SSRIs.

The paper identifies 5-HT3 receptors as essential for exercise-induced hippocampal neurogenesis and antidepressant effects and elucidates a novel mechanism wherein 5-HT3 receptor activation stimulates IGF-1 release in the hippocampus, enhancing neurogenesis via IGF-1 signaling and producing antidepressant effects. This pathway is mechanistically distinct from SSRI action and can synergize with SSRIs. The findings support development of 5-HT3 receptor–targeted therapeutics for depression, including treatment-resistant cases, and encourage further research to translate this mechanism into clinical interventions.