A whole-brain analysis of functional connectivity and immediate early gene expression revealed functional network shifts after operant learning

Previous studies of operant learning have addressed neuronal activities and network changes in specific brain areas, such as the striatum, sensorimotor cortex, prefrontal/orbitofrontal cortices, and hippocampus. However, how changes in the whole-brain network are caused by cellular-level changes remains unclear. We combine resting-state functional magnetic resonance imaging (rsfMRI) and whole-brain immunohistochemical analysis of early growth response 1 (EGR1), a marker of neural plasticity, to elucidate the spatiotemporal functional network changes and underlying cellular processes during operant learning. We used an 11.7 Tesla scanner and whole-brain immunohistochemical analysis of EGR1 in mice during the early and late stages of operant learning. In the operant training, mice received a reward when they pressed the left and right buttons alternately and were punished with a bright light when they made a mistake. Control mice spent the same time and received the same amount of reward in the same operant box. A group of mice (n = 22) underwent the first rsfMRI before behavioral sessions, the second after 3 days of sessions (early stage), and the third after 21 days of sessions (late stage). Another group of mice (n = 40) was subjected to histological analysis 15 min after the early or late stages of behavioral sessions. After the early stage of training, functional connectivity was increased between the limbic areas and thalamus or auditory cortex, and the correlations of the number of EGR1-immunopositive cells between the limbic area and auditory cortex were also increased. After the late stage of training, the increases in functional connectivity and correlations of EGR1-immunopositive cells primarily occurred between the motor cortex, somatosensory cortex, and striatum. The subcortical networks centered around the limbic areas that emerged in the early stage have been implicated in rewards, pleasures, and fears. The connectivity between the motor cortex, somatosensory cortex, and striatum that consolidated in the late stage have been implicated in motor learning. Our multimodal longitudinal study successfully revealed the temporal shifts of brain regions involved in behavioral learning together with the underlying cellular-level plasticity between these regions for the first time. Our study represents a first step toward establishing a new experimental paradigm that combines rsfMRI and immunohistochemistry for linking macroscopic and microscopic mechanisms of learning.