Exploring 5‐HT2 Receptors as Targets for Treating Epilepsy in Fragile X Syndrome: A Preclinical Study of Fmr1 Knock‐out Mice
Authors: Saraf Tanishka S. , Chen Yiming , Canal Clinton E. 
Published: April 2020

Recent clinical data report that the serotonin 2 receptor (5‐HT2R) agonist, lorcaserin, and the 5‐HT releaser, fenfluramine, abate seizures in children with Dravet and Lennox‐Gastaut syndromes (e.g., Griffin et al., 2017, Brain). Evidence suggests their neuropharmacotherapeutic effects, e.g., anorexigenic, are dependent on 5‐HT2CR activation. At high doses, however, lorcaserin and the metabolite of fenfluramine, norfenfluramine, also activate other 5‐HTRs, including 5‐HT2ARs. The present experiments were designed to evaluate the efficacy and possible 5‐HT2AR and/or 5‐HT2CR mechanism(s) of lorcaserin to treat audiogenic seizures (AGS) in juvenile (P23–P25), Fmr1 knock‐out mice, a genetic model useful for studying fragile X syndrome (FXS). FXS is a monogenetic, neurodevelopmental disorder characterized by cognitive disabilities, severe anxiety, autism, and sensory hypersensitivities. Approximately 30% of individuals with FXS also have epilepsy, and AGS in Fmr1 knock‐out mice closely model seizures in FXS that can be triggered by excessive sensory stimulation. The experimental design includes male and female, wild‐type and Fmr1 knock‐out mice (FVB strain) treated with vehicle (wild‐type and Fmr1 knock‐out mice), 1, 3, or 10 mg/kg lorcaserin (Fmr1 knock‐out mice only), that are exposed 30 min later to a 120‐dB alarm for five consecutive minutes. Separate groups are pre‐treated with M100907 (0.03 mg/kg) or SB242084 (0.3 mg/kg) to discern contributions of 5‐HT2ARs or 5‐HT2CRs, respectively. Scored behaviors include wild‐running and jumping, tonic‐clonic seizures with full recovery, and tonic‐clonic seizures progressing to respiratory arrest—this behavioral sequence is typical. Wild‐running and jumping is considered, here, a pre‐seizure event, whereas AGS are defined as tonic‐clonic seizures. Immediately after testing, vehicle‐treated wild‐type and Fmr1 knock‐out mice are euthanized, brains are extracted, and membranes are collected for saturation binding experiments to determine binding site densities of 5‐HT2ARs (frontal cortex) and 5‐HT2CRs (coronal sections containing striatum and choroid plexus) using [3H]Ketanserin and [3H]Mesulergine, respectively, with appropriate cold ligands to block off‐target binding. AGS prevalence in vehicle‐treated, wild‐type mice was 14% (2 of 14), whereas AGS prevalence was 67% in vehicle‐treated Fmr1 knock‐out mice (10 of 15) with an onset latency of 50 (10) seconds (mean (SEM)). Lorcaserin 1 mg/kg was ineffective at preventing or reducing onset latency of AGS; AGS prevalence was 73% (8 of 11), and onset latency was 40 (3) seconds. Lorcaserin 3 mg/kg decreased AGS prevalence to 46% (6 of 13) and though not statistically significant (P=0.14), decreased lethality caused by seizures by 31%, relative to vehicle; there were no effects on the latency to onset of AGS. Tests of 10 mg/kg lorcaserin are not completed. Preliminary results (N=3–4) from 5‐HT2AR and 5‐HT2CR saturation binding assays suggest decreased expression of both receptors. Results from this project could guide repurposing of 5‐HT2 agonists to treat seizures in FXS.Support or Funding InformationFunded by: FRAXA Fellowship and DOD W81XWH‐17‐1‐0329