Cassidy T. Burke

Summary: When an animal feels ill after consumption of a novel food, it rapidly learns to avoid this food in the future, a form of learning termed conditioned flavour avoidance (CFA). Postingestive malaise often occurs long after 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) promotes formation of CFA. Here, we demonstrate that these neurons overlap with a PB population that releases the neuromodulator acetylcholine (ACh) in the CeA during both consumption of a novel solution and subsequent visceral malaise, consistent with a role in CFA acquisition. 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 acquisition. Loss of ACh signalling to CeA blocks key signatures of CFA-associated plasticity. Together, these data reveal that cholinergic input from PB to central amygdala is a critical neuromodulatory signal that links activity over long timespans to facilitate associative learning.

The ventral pallidum (VP) was defined as a basal ganglia nucleus with dense input from ventral striatum. To further investigate its regional identity, we conducted a cross-species transcriptional characterization of VP cell types. We performed single-nucleus RNA sequencing of VP tissue from mice and rats and identified 16 conserved VP neuronal subclasses, including 14 γ-aminobutyric acid–releasing subclasses from three developmental classes. Combining our sequencing data with a spatial atlas revealed that all VP subclasses extended beyond the traditional borders in VP. Integrating our VP data with published striatal, hypothalamic, and extended amygdalar sequencing confirmed that cell types are shared among these regions. Given the role of VP in feeding, we also assessed the transcriptional impact of high-fat diet, which altered expression of genes involved in oxidative phosphorylation and inhibitory signaling. Overall, our results demonstrate that VP is not a transcriptionally discrete nucleus; rather, VP contains cell types with diverse expression patterns overlapping with regions beyond the basal ganglia.