Alterations in auditory midbrain processing is observed in both female and male mouse model of Fragile X Syndrome

Introduction

Sensory processing deficits presenting as auditory hypersensitivity is a common phenotype associated with Fragile X Syndrome (FXS), a leading monogenic cause of intellectual disability and autism spectrum disorder. Auditory hypersensitivity can also be observed in the FMR1- knockout (KO) mice, a well-established mouse model of autism and FXS. FXS is an X-linked disorder that is more prevalent in males compared to females, as a result most auditory and electrophysiology studies are performed in males. Previous in-vivo electrophysiology studies at the inferior colliculus (IC), an essential component of the auditory pathway, in male FMR1- KO mice at post-natal days 14, 21 and 34 (P14, P21 and P34) demonstrated increased neuronal firing, suggesting that the IC could play a role in auditory hypersensitivity. However, very little is known about the role of the central nucleus of the IC (ICc) and auditory hypersensitivity in females.

Methods

Here we investigated auditory processing at P20 and P30, representative of early and late developmental stage of auditory development, respectively, at the central nucleus of the IC (ICc) of both female and male FMR1- KO mice, compared with wildtype (WT) animals.

Results

In-vivo electrophysiology recordings from the ICc neurons of the FMR1- KO mice at both developmental ages demonstrated increased response magnitude measured by spike number to pure tones of varying frequency and amplitude, compared with age- and sex-matched WT animals. In addition, within the FMR1- KO group we also observed significant developmental and sex difference wherein higher response magnitudes were displayed at P20 and in the female mice. Minimum threshold of ICc neuron in the KO mice was significantly decreased at both P20 and P30. The ICc neurons in the KO mice also displayed increased response duration compared to WT animals at both P20 and P30, but significant sex difference was only observed at P30. Our data also indicated that the ICc neurons of both groups displayed weak negative relationship between latency and response magnitude at P20, and at P30 the WT mice showed a stronger relationship only in the female group.

In terms of developmental changes, we observed decreasing neuronal firing only in the KO mice between 20- and 30-day old female and male mice. Reduced response duration was observed in 30-day old mice of both sexes of both genotypes. Regarding minimum threshold, we observed a decline between early and late auditory development only in the male mice. Finally, our results also indicated that in the WT mice the reverse relationship between latency and response magnitude became more pronounced and consistent with age, a developmental trend that was absent in both the female and male FMR1 -KO mice.

Discussion

Overall, our findings demonstrate that auditory processing deficits can also be observed in the female FMR1 -KO mice using in-vivo electrophysiology studies, highlighting the importance of including female subjects in future studies. These results also indicate that auditory hypersensitivity can be observed robustly in younger mice, suggesting that the early development stage could be an ideal target for interventions.