A cholinergic eligibility trace facilitates amygdala plasticity in flavour avoidance learning
When an animal consumes a new food and consequently feels ill, it rapidly and robustly learns to avoid this food in the future, a form of learning termed conditioned flavour avoidance (CFA). Postingestive malaise often occurs long after novel food consumption, necessitating a neural mechanism that can facilitate plasticity between temporally distant events. Neuromodulators, acting through G-protein-coupled receptors (GPCRs) that can influence neuronal excitability on extended timescales, may facilitate this process. The projection of parabrachial (PB) Calca neurons to the central amygdala (CeA) is critical for formation of CFA. Here, we demonstrate that these neurons overlap with a PB population that releases acetylcholine (ACh) in the CeA. ACh is released in CeA during consumption of a novel solution and subsequent visceral malaise, consistent with a role in CFA acquisition. Two-photon calcium imaging in brain slices reveals that ACh widely activates CeA neurons and enhances glutamatergic responsivity on a timescale consistent with CFA learning. CRISPR-Cas9-mediated genetic knockdown and optogenetics demonstrate that ACh from PB facilitates CFA behavior. Large-scale neuronal recordings in the CeA along with our CRISPR approach reveal that loss of ACh signaling to CeA blocks key signatures of CFA-associated plasticity. Together, these data point to the cholinergic input from PB to central amygdala as a critical neuromodulatory signal that links activity over long timespans to facilitate associative learning in CFA.