Infralimbic projections to the basal forebrain mediate extinction learning
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Fear extinction learning and retrieval are critical for decreasing fear responses to a stimulus that no longer poses a threat. While it is known that the infralimbic region (IL) of the medial prefrontal cortex mediates retrieval of an extinction memory through projections to the basolateral amygdala (BLA), the contribution of the IL to extinction learning is not well-understood. Given the strong projection from the IL to the basal forebrain (BF), a center of attentional processing, we investigated whether this pathway participates in extinction, and compared it to the IL-BLA pathway. Using retrograde tracing, we first demonstrate that projections from the IL to the BF originate from superficial (L2/3) and deep cortical layers (L5), and that they are denser than IL projections to the BLA. Next, combining retrograde tracing with labeling of the immediate early gene cFos, we show increased activity of the L5 IL-BF pathway during extinction learning and increased activity of the L2/3 IL-BLA pathway during extinction retrieval. Our in vitro recordings demonstrate that neurons in the IL-BF pathway become more excitable towards the end of extinction learning, but less excitable during extinction retrieval. Finally, using optogenetics we show that inactivation of the IL-BF pathway impairs extinction learning, leaving retrieval intact. We propose that the IL acts as a switchboard operator during extinction, with increased L5 IL-BF communication during learning and increased L2/3 IL-BLA communication during retrieval. Anxiety and stress-related changes in IL physiology could affect one or multiple lines of communication, impairing different aspects of extinction.
Significance Statement
Extinction of conditioned fear is a widely used behavioral approach to diminish fear, with projections from infralimbic prefrontal cortex to the amygdala known for mediating extinction memory retrieval. However, less is known about infralimbic pathways involved in extinction learning. We use neuroanatomical tracing, behavior, slice recordings, and circuit manipulation to show that infralimbic output to the basal forebrain, an attention processing center, is denser than to the amygdala, and is active during extinction learning rather than retrieval. Neurons in the infralimbic-basal forebrain pathway become more excitable as extinction learning progresses, and then less excitable during extinction retrieval. Moreover, inhibiting this pathway impairs extinction learning. Our findings identify different lines of communication the infralimbic cortex uses for extinction learning and retrieval.
