Behavioral consequences of serotonin deficiency in TPH2 knock-in mice: Implications for depression, anxiety, and cognitive dysfunction

BackgroundSerotonin (5-hydroxytryptamine, 5-HT) is essential to mood regulation, and its dysfunction has been strongly connected to depression and anxiety disorders. The key enzyme for serotonin production is tryptophan hydroxylase 2 (TPH2); genetic abnormalities influencing TPH2 activity have been connected to mood disorders.ObjectiveOur study aims to understand the consequences of human TPH2 mutation-TPH2 R439H in mice, especially depression and anxiety-like behaviors.MethodsCompared to wild-type, we employed 6-month-old knock-in mice, heterozygotes (one allele) and homozygotes (two alleles) expressing TPH2 R439H analogous to the human R441H TPH2 mutation. We examined behavioral differences between TPH2-KI and WT mice, heterozygotes and homozygotes, and males and females. Tail suspension and forced swim assessed depression-like behavior; open field and light-dark tests assessed anxiety and exploration; the Morris water maze tested memory and spatial learning; rotarod assessed balance and motor coordination; and the novel object test assessed recognition memory.ResultsOur findings demonstrate that TPH2 KI mice exhibited increased depression-like behavior in the forced swim and tail suspension tests; increased avoidance in the light-dark and open field tests revealed anxiety-like phenotypes. Furthermore, serotonin deficit decreased locomotion and coordination in the rotarod. In the novel object, recognition memory was impaired, but spatial learning and memory in the Morris water were unaffected. Homozygotes displayed more severe phenotypes than heterozygotes, indicating a gene dosage-dependent effect.ConclusionsOur findings extend prior behavioral characterizations of TPH2 KI mice by providing an integrated profile at 6-month age point, revealing domain-specific effects across mood, anxiety, locomotion, and recognition memory.