Differential effects of sound repetition rate on auditory cortex development and behavior in fragile X syndrome mouse model

Fragile X syndrome (FXS) is a leading genetic form of autism and intellectual disability that is associated with a loss-of-function mutation in the Fragile X messenger ribonucleoprotein 1 (Fmr1) gene. The Fmr1 knockout (KO) mouse model displays many aspects of FXS-related phenotypes and is used to study FXS pathophysiology. Sensory manipulations, such as sound exposure, are considered as a non-invasive approach to alleviate FXS phenotypes. However, it is unclear what specific sound attributes may have beneficial effects.

In this study, we examined the effects of sound repetition rate on auditory cortex development and FXS-associated behaviors in a mouse model of FXS. KO and wild-type (WT) male littermates were exposed to 14 kHz pure tone trains with 1 Hz or 5 Hz repetition rates during postnatal day (P)9-P21 developmental period. We analyzed the effects of developmental sound exposure on PV cell development, cortical activity and exploratory behaviors in sound-exposed WT and KO mice. We found that parvalbumin (PV) cell density was lower in the auditory cortex (AuC) of KO compared to WT mice raised in sound-attenuated environment, but was increased following the exposure to both 1 Hz and 5 Hz sound trains. However, PV protein levels were upregulated only in AuC of 5 Hz rate exposed KO mice. Interestingly, analysis of baseline cortical activity using electroencephalography (EEG) recordings showed that sound attenuation or exposure to sound trains with 5 Hz, but not 1 Hz, repetition rates corrected enhanced resting state gamma power in AuC of KO mice to WT levels. In addition, sound attenuation and exposure to 5 Hz showed some beneficial effects on the synchronization to frequency-modulated chirp in the frontal cortex (FC) of both WT and KO mice. Analysis of event-related potentials (ERP) in response to broadband sound showed increased ongoing responses and decreased habituation to noise stimuli in the AuC and FC of naive KO mice. While sound-attenuation and exposure to 5 Hz showed no significant effects on the power of onset and ongoing responses, exposure to 1 Hz further enhanced ongoing responses and decreased habituation to sound in both WT and KO mice. Finally, developmental exposure to sound trains with 5 Hz, but not 1 Hz, repetition rates normalized exploratory behaviors and improved social novelty preference but not hyperactivity in KO mice.

Summarizing, our results show that developmental exposure of mice to sound trains with 5 Hz, but not 1 Hz, repetition rate had beneficial effects on PV cell development, overall cortical activity and behaviors in KO mice. While sound attenuation alone normalized some EEG phenotypes, it did not improve PV development or behaviors. These findings may have a significant impact on developing new approaches to alleviate FXS phenotypes and open possibilities for a combination of sound exposure with drug treatment which may offer highly novel therapeutic approaches.