Context makes the difference: Temporally Resolved Dopaminergic Teaching Signals Shape Associative Memory in Drosophila Larvae
Animals can adapt their behavioral responses to environmental cues by learning from experience. This ability relies on the formation and recall of memories that are shaped by beneficial or detrimental consequences and regulated by the dopaminergic system, which is highly conserved across insect species. In the Drosophila melanogaster larva, eight of total [~]120 dopaminergic neurons (DANs) innervate the mushroom body (MB), a key center for associative memory. This subset of DANs can be anatomically grouped into two clusters of four cells: the primary protocerebral anterior medial (pPAM) cluster, associated with reward signaling, and the dorsolateral 1 (DL1) cluster, associated with punishment. Such a functional dichotomy is observed in larval and adult Drosophila and reflects a fundamental organizational principle of reinforcement learning across invertebrate and even vertebrate species. Aversive reinforcement through high-salt exposure is encoded within the DL1 cluster in a combinatorial and heterogeneous manner, critically involving two neurons, DAN-f1 and DAN-g1. Using temporally precise optogenetic activation and inhibition during olfactory conditioning, we show that these neurons can modulate memory strength and valence. Their effects are most often consistent and predictable, enabling accurate computational modeling of DAN-driven teaching signals. By manipulating the intrinsic physiology of DAN-f1 and DAN-g1 and altering the valence of gustatory input, we are beginning to understand at the single-cell level how dopaminergic activity is systematically adjusted to control memory formation.