(M1330-04-27) Activating 5-HT 1A Receptors as a Treatment for Fragile X Syndrome: A Preclinical Study in Fmr1 Knockout Mice

Purpose: There are no FDA-approved medications for Fragile X syndrome (FXS), a neurodevelopmental disorder caused by an epigenetic mutation of FMR1 that reduces or eliminates expression of the RNA-binding protein FMRP. FXS is characterized by neural circuit dysfunction including neuronal hyperexcitability, that manifests as intellectual disability, seizures, anxiety, sensory hypersensitivities, autism spectrum disorder, and other neuropsychiatric symptoms, as well as neurophysiological and neurobiological abnormalities which are recapitulated in Fmr1 knockout (KO) mice. Serotonin 1A receptors (5-HT_1ARs) are inhibitory, and thus, activating them may correct the overarching neuronal hyperexcitability seen in FXS. Additionally, the gene encoding 5-HT_1AR is differentially expressed in FXS, providing a rationale for investigating 5-HT_1ARs as pharmacotherapeutic targets.

Methods: We used a combinatorial approach—including receptor pharmacology, electroencephalography (EEG), and behavioral experiments—to test the hypothesis that selectively activating 5-HT_1ARs is pharmacotherapeutic for FXS. At the receptor level, we examined whole-brain 5-HT_1AR expression using saturation binding and autoradiography with agonist ([³H]5-CT) and antagonist ([³H]WAY-100635) radioligands. At the circuit level, we recorded resting-state EEG from above the left somatosensory cortex of awake, freely-moving mice at baseline (without treatment) and after treatment with vehicle and FPT, a non-selective 5-HT_1AR agonist. At the behavioral level, we tested in vivo function by evaluating 5-HT_1AR-mediated behaviors such as flat body posture, head weaving, and hind limb abduction using the selective 5-HT_1AR agonist, 8-OH-DPAT (1 and 2 mg/kg). We evaluated the therapeutic efficacy of activating 5-HT_1ARs by measuring antiepileptic effects of another selective 5-HT_1AR agonist, NLX-112 (0.25-2.5 mg/kg) in the audiogenic seizure assay.

Results: At the receptor level, we did not observe any differences in agonist- or antagonist-bound 5-HT_1AR expression (B_MAX from whole brains and autoradiography from select neural systems) or [³H]5-CT affinity at 5-HT_1ARs (K_D) between juvenile Fmr1 KO and control mice. At the circuit level, adult Fmr1 KO mice showed significantly elevated relative gamma (30-100 Hz) power compared to control mice. FPT treatment significantly modulated EEG activity in adult mice. At the behavioral level, we observed significantly lower in vivo 5-HT_1AR-mediated responses in juvenile Fmr1 KO mice, relative to control mice. NLX-112 significantly and dose-dependently prevented audiogenic seizures in juvenile Fmr1 KO mice, an effect blocked by the selective 5-HT_1AR antagonist, WAY-100635 (0.1 mg/kg).

Conclusion: Collectively, these results suggest 5-HT_1AR as a potential target for the treatment of FXS and that activating 5-HT_1ARs may be pharmacotherapeutic for FXS.

Acknowledgements:

Funding: FRAXA Research Foundation, NINDS R15NS118352