Decoupling of GABA and Glutamine-Glutamate Dynamics and their role in tactile perception: An fMRS Study

Tactile processing is fundamental for our daily lives. In particular, adaptation, the mechanism by which neural (and behavioural) responses change due to repeated stimulation, is key in adjusting our responses to the environment and is often affected in neurodevelopmental conditions such as autism and ADHD. While GABA and glutamate--the main inhibitory and excitatory neurotransmitters-- are known to be fundamental for encoding sensory input, we know little regarding the dynamic responses of the GABA and glutamatergic systems during tactile processing. Here, we examine how GABA and glutamine+glutamate (Glx) in vivo dynamics change during repetitive tactile stimulation and how these changes relate to tactile perception in a healthy population, using functional magnetic resonance spectroscopy (fMRS). Our study showed that repetitive tactile stimulation induced a decoupling between GABA and Glx during the first stimulation blocks as suggested by a negative correlation between GABA and Glx, which changed from a positive correlation at baseline. Subsequently, a multivariate time series analysis showed a predictive temporal relationship between Glx and GABA, showing that changes in these metabolites are temporally linked with an estimated lag of 6 seconds informing on a complex metabolite response function. The absence of gross metabolite change suggests that Glx and GABA adjust in relation to each other in response to repeated tactile stimulation. Furthermore, individual differences in the changed GABA and Glx levels correlated with perceptual measures of touch. Together, our study highlights the complex relationship between GABA and glutamate in tactile processing and demonstrates that experience-dependence plasticity induces a decoupling between these key metabolites. Further study into their dynamic interplay may be key to understanding adaptation as meso-levels in the brain and how these mechanisms differ in neurodevelopmental and neurological conditions.