Seizures occur in ~12% of individuals with autism spectrum disorder (ASD) and in ~25% of individuals with
fragile X syndrome (FXS)—mutations in FMR1 that cause FXS are also the most common, known genetic risk
factor for ASD. Seizure susceptibility in ASD and FXS is higher in children than in adults, inferring a critical
neurodevelopmental window. Juvenile Fmr1 knock-out mice, useful for the study of FXS and ASD, exhibit
robust sensory-evoked, audiogenic seizures, a phenotype that models sensory hypersensitivity and seizures in
individuals with FXS. New studies show that individuals with FXS and Fmr1 knock-out mice also have
perturbations in electroencephalogram (EEG) activity at rest, e.g., increased gamma power in the cortex, which
we reproduced and report here as preliminary data. Thus, etiologically-valid, Fmr1 knock-out mice exhibit two
phenotypes with translational relevance to individuals with FXS. Numerous studies have reported alterations in
the serotonin (5-hydroxytryptamine, 5-HT) system in FXS and ASD, yet knowledge regarding the impact of
specific 5-HT receptors (5-HTRs) on FXS phenotypes are conspicuously lacking. We now report data showing
that audiogenic seizures in juvenile Fmr1 knock-out mice are entirely prevented by a novel 5-HTR modulator
with 5-HT1AR and 5-HT7R partial agonist activity. Herein, we propose definitive in vivo behavioral
pharmacology and EEG experiments together with ex vivo receptor pharmacology experiments to
mechanistically probe 5-HT1ARs and 5-HT7Rs as targets that can prevent audiogenic seizures (Aim 1) and
correct EEG abnormalities (Aim 2) in juvenile Fmr1 knock-out mice. We hypothesize in Aim 1 that selective 5-HT1AR activation or selective 5-HT7R inactivation will attenuate audiogenic seizures and combined 5-HT1AR
activation/5-HT7R inactivation will prevent audiogenic seizures in juvenile Fmr1 knock-out mice. We extend the
test of this hypothesis in Aim 2, testing this pharmacodynamic effect to correct EEG phenotypes, e.g. to
correct abnormally high gamma band power in the auditory cortex and somatosensory cortex of juvenile Fmr1
knock-out mice. 5-HT1ARs and 5-HT7Rs are densely expressed in the hippocampus, a neural system with a
low seizure threshold that is altered in Fmr1 knock-out mice and in individuals with FXS. In Aim 3, selective
radioligands will be used for saturation binding experiments to evaluate 5-HT1AR and 5-HT7R expression in the
hippocampus, and [35S]GTPγS assays will be conducted to determine the function of 5-HT1ARs and 5-HT7Rs in
the hippocampus of juvenile Fmr1 knock-out mice compared to wild-type mice. 5-HT1ARs and 5-HT7Rs couple
to regulate adenylate cyclase activity in opposing ways, i.e., 5-HT1ARs stimulate Gαi, whereas 5-HT7Rs
stimulate Gαs signaling. Since cAMP is known to be altered in FXS and ASD, outcomes from this project will
provide knowledge to build a critical infrastructure regarding the putative impact of 5-HTR regulation of cAMP
on translationally-valid FXS and ASD phenotypes. Results will also provide important information regarding
whether 5-HT1ARs and/or 5-HT7Rs are viable pharmacotherapeutic targets for FXS or ASD.
