Somatosensory-evoked response induces extensive diffusivity and kurtosis changes associated with neural activity in rodents.

Neural tissue microstructure is dynamic during brain activity, presenting changesin cellular morphology and membrane permeability. The sensitivity of diffusionMRI (dMRI) to restrictions and hindrances in the form of cell membranes orsubcellular structures enables the exploration of brain activity under a newparadigm, offering a more direct functional contrast than itsblood-oxygenation-level-dependent (BOLD) counterpart. The current work aims atprobing Mean Diffusivity (MD) and Mean Kurtosis (MK) changes and theirtime-dependence signature across various regions in the rat brain duringsomatosensory processing and integration, upon unilateral forepaw stimulation.We report adecreasein MD in the contralateral primarysomatosensory cortex, forelimb region (S1FL), previously ascribed to cellularswelling and increased tortuosity in the extracellular space, paralleled by apositive BOLD response. For the first time, we also report a paireddecreasein MK during stimulation in S1FL, suggestingincreased membrane permeability. This observation was further supported by thereduction in exchange time estimated from the kurtosis time-dependence analyses.Conversely, the secondary somatosensory cortex and subcortical areas, formerlyreported as responsive to sensory stimulation in rodents (thalamus, striatum,hippocampal subfields), displayed a marked MD and MKincrease,paralleled by a weak-to-absent BOLD response. Overall, MD and MK uncoveredfunctional-induced changes with higher sensitivity than BOLD. Although the exactorigin of the MD and MK increase is yet to be unraveled, the potential of dMRIto provide complementary functional insights, even below the BOLD detectionthreshold, has been showcased.