CEST MRI reveals nicotine-induced alterations in glutamate-associated molecular connectivity in the mouse brain

Introduction Understanding how neurotransmitter systems organize into large-scale networks is essential for elucidating the mechanisms through which drugs, diseases, and behavioral states alter brain function. Existing imaging modalities such as functional MRI (fMRI) and positron emission tomography (PET) provide measures of hemodynamic and metabolic connectivity, but cannot noninvasively map neurotransmitter-associated networks with high spatial resolution. Herein, we introduce a chemical exchange saturation transfer (CEST) MRI-based framework for mapping glutamate-associated molecular connectivity and apply it to characterize nicotine-induced network reorganization in the mouse brain. Methods Male C57BL/6 mice underwent dynamic glutamate-weighted CEST (gluCEST) MRI before and after seven days of nicotine exposure. Regional glutamate-weighted CEST time series were extracted from 51 brain regions, and connectivity was evaluated using within-subject temporal correlation and inter-subject covariance analyses. Results Graph theory analyses identified four baseline glutamate-associated modules involving frontal-sensorimotor, cortico-hippocampal, intra-hippocampal, and cortico-striatal circuits. Nicotine exposure attenuated these baseline networks in analyses performed without global signal regression (GSR) and with conditional GSR, whereas full GSR reduced the apparent magnitude of these effects. Nicotine also reduced nodal strength in the CA1 and insular cortex. In contrast, nicotine selectively strengthened a thalamo-striato-motor circuit involving the motor cortex, mediodorsal and ventral thalamic nuclei, and caudoputamen. This pattern remained evident even under full GSR. Subject-level covariance analysis confirmed widespread nicotine-induced attenuation of glutamate-associated connectivity and revealed a distinct sensory-limbic module involving the lateral geniculate nucleus, amygdala, and piriform cortex that was selectively disrupted following nicotine exposure. Discussion These results demonstrate the feasibility of dynamic gluCEST MRI for mapping glutamate-associated molecular connectivity in vivo and detecting pharmacologically induced network remodeling. This approach provides a noninvasive platform for investigating glutamatergic dysregulation in addiction, neuropsychiatric disorders, and therapeutic response.