Functional ultrasound imaging of recent and remote memory recall in the associative fear neural network in mice.

Emotional learning and memory are affected in numerous psychiatric disorders. At a systems level, however, the underlying neural circuitry is not well defined. Rodent fear conditioning (FC) provides a translational model to study the networks underlying associative memory retrieval. In the current study, functional connectivity among regions related to the cue associative fear network were investigated using functional ultrasound (fUS), a novel imaging technique with great potential for detecting neural activity through cerebral blood flow. Behavioral fear expression and fUS imaging were performed one and thirty-one days after FC to assess recent and remote memory recall. Cue-evoked increases in functional connectivity were detected throughout the amygdala, with the lateral (LA) and central (CeA) amygdalar nuclei emerging as major hubs of connectivity, although CeA connectivity was reduced during remote recall. Hippocampal and sensory cortical regions displayed heightened connectivity with the LA during remote recall, whereas interconnectivity between the primary auditory cortex and temporal association areas was reduced. Subregions of the prefrontal cortex exhibited variable changes, where prelimbic connectivity with the amygdala was refined while specific connections between the infralimbic cortex and amygdalar subregions emerged during remote memory retrieval, a signature of extinction memory. Moreover, freezing behavior positively correlated with functional connectivity between hubs of the associative fear network, suggesting that emotional response intensity reflected the strength of the cue-evoked functional network. Overall, our data provide evidence of the functionality of fUS imaging to investigate the neural dynamics of memory retrieval, applicable in the development of innovative treatments for affective disorders.