Serotonin receptor 7 stimulation rescues fear generalization in a male mouse model of post-traumatic stress disorder

Introduction: Post-traumatic stress disorder (PTSD) is a chronically impairing mental disorder occurring after exposure to traumatic events. Invalidating symptoms range from intrusive memories, persistent avoidance of trauma-related stimuli and hypervigilance [1] and are thought to arise from maladaptive memory processes that can include a generalization of fear to neutral stimuli [2]. To date, resolutive interventions are not available, and patients display a high degree of resistance or relapse to currently available treatments [3]. Among the numerous neurotransmitter pathways reported to be altered in patients with PTSD, the serotonin system has been proposed to play a key role in dealing with stressful situations. In particular, the serotonin receptor 7 (5-HT7R) is clearly involved in endocrine and behavioural responses undertaken to cope with stress, as well as in brain structural changes underneath learning and memory processes [4], but its role in PTSD has not been investigated yet. Aim: we sought to evaluate the potential involvement of the 5-HT7R in PTSD-like symptomatology, along with the therapeutic effects of its pharmacological stimulation, in transgenic mice carrying a hypofunctional form of the epigenetic factor methyl-CpG binding protein 2 (MeCP2-308), that are particularly vulnerable to the negative outcomes of trauma exposure [5]. Methods: to this aim, MeCP2-308 male mice and wild-type (wt) littermate controls were systemically treated for 7 days (0.25 mg/kg/day) with the brain-penetrant and selective 5-HT7R agonist LP-211, or vehicle (1%DMSO in saline), starting 24 hours after the traumatic experience (2 x 0.8 mA unescapable footshocks). Mice of both genotypes not receiving footshocks and treated with vehicle were included, as controls. Throughout the treatment schedule, mice were tested for cued and contextual fear memory recall (1st and 7th days of treatment) and avoidance of trauma-related stimuli (6th day of treatment), to assess treatment effects on PTSD-like behavioural alterations. Statistical significance was ascertained by ANOVA, and Tukey’s test was used for post-hoc comparisons, setting α at 0.05. Results: Trauma-exposed MeCP2-308 mice performed significantly more freezing when exposed to a novel, non-threatening context compared to non-shocked controls of both genotypes (p<0.001), suggesting that they generalized fear to neutral stimuli. Consistently, despite the absence of proper avoidance of trauma reminders, in the avoidance task traumatized MeCP2-308 mice exhibited increased fear regardless of the presence of trauma-related stimuli, compared to non-shocked controls of both genotypes (p<0.001). The treatment with LP-211 normalized both freezing responses (p<0.05 and p<0.01 for MeCP2-308 shocked+vehicle vs MeCP2-308 shocked+LP-211). Trauma-exposed MeCP2-308 mice also displayed an improper freezing response when re-exposed to the non-predicting cue (p<0.01 for MeCP2-308 shocked+vehicle vs wt shocked+vehicle), which was reduced after 7 days of treatment with LP-211 (p<0.001 for MeCP2-308 shocked+LP-211 at the 1st vs the 7th days of treatment). Conclusions: Present data provide novel evidence for the potential involvement of 5-HT7R in PTSD-related alterations of memory processes, and suggest that 5-HT7R agonism might prove beneficial for counteracting fear generalization, a hallmark of aberrant traumatic memories. Further studies are needed to clarify the mechanisms involved, which might contribute to a deeper understanding of the neurobiology of PTSD. References [1] Miao, X.-R., Chen, Q.-B., Wei, K., Tao, K.-M., & Lu, Z.-J. (2018). Posttraumatic stress disorder: from diagnosis to prevention. Military Medical Research, 5(1), 1–7. [2] Liberzon, I., & Abelson, J. L. (2016). Context Processing and the Neurobiology of Post-Traumatic Stress Disorder. Neuron, 92(1), 14–30. [3] Richter-Levin, G., Stork, O., & Schmidt, M. V. (2019). Animal models of PTSD: a challenge to be met. Molecular Psychiatry, 24(8), 1135–1156. [4] Ciranna, L., & Catania, M. V. (2014). 5-HT7 receptors as modulators of neuronal excitability, synaptic transmission and plasticity: Physiological role and possible implications in autism spectrum disorders. Frontiers in Cellular Neuroscience, 8(AUG), 1–17. [5] Cosentino, L., Vigli, D., Medici, V., Flor, H., Lucarelli, M., Fuso, A., & De Filippis, B. (2019). Methyl-CpG binding protein 2 functional alterations provide vulnerability to develop behavioral and molecular features of post-traumatic stress disorder in male mice. Neuropharmacology, 160(June), 107664.