Somatosensory map expansion and altered processing of tactile inputs in a mouse model of fragile X syndrome

- In a mouse model of Fragile X syndrome, we investigated the processing of tactile stimuli in the somatosensory system.
- Fmr1 KO mice have deficits in the encoding of the tactile stimulation frequency.
- They also display an increase in the size of the tactile receptive fields in the somatosensory cortex.
- In a tactile dependent behavioral task, the KO animals reduced the whisker touch-time.
- The cortical deficits could explain the observed behavioral phenotype, suggestive of tactile hypersensitivity.

Fragile X syndrome (FXS) is a common inherited form of intellectual disability caused by the absence or reduction of the fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. In humans, one symptom of FXS is hypersensitivity to sensory stimuli, including touch. We used a mouse model of FXS (Fmr1 KO) to study sensory processing of tactile information conveyed via the whisker system. In vivo electrophysiological recordings in somatosensory barrel cortex showed layer-specific broadening of the receptive fields at the level of layer 2/3 but not layer 4, in response to whisker stimulation. Furthermore, the encoding of tactile stimuli at different frequencies was severely affected in layer 2/3. The behavioral effect of this broadening of the receptive fields was tested in the gap-crossing task, a whisker-dependent behavioral paradigm. In this task the Fmr1 KO mice showed differences in the number of whisker contacts with platforms, decrease in the whisker sampling duration and reduction in the whisker touch-time while performing the task. We propose that the increased excitability in the somatosensory barrel cortex upon whisker stimulation may contribute to changes in the whisking strategy as well as to other observed behavioral phenotypes related to tactile processing in Fmr1 KO mice.