Figure 3.

Feed-forward, feed-back and feed-across networks in the mushroom body. (a) Feed forward inhibition regulates state dependent expression of food memory retrieval. Hunger state controls odor-driven behavior by relaying hunger-dependent dNPF signaling (purple) through the PPL1-MP1 DANs (red) and the GABAergic MVP2 neuron (blue). The MVP2 neuron feeds forward inhibition to the M4/M6 group of glutamatergic MBONs (orange). In a satiated fly, approach behavior is counterbalanced by the avoidance-promoting M4/M6 MBONs, which are not inhibited by MVP2. In a hungry fly, the MVP2 neuron is active and inhibits the M4/M6 MBONs, reducing avoidance and facilitating approach behavior. (b) A closed feedback loop involving the MBON from the α1 compartment, the αβ Kenyon cells and the DANs innervating the α1 compartment has been proposed to stabilize memory after learning. Blocking any of these neurons immediately after sugar reward learning cripples behavioral approach measured 24 h later [50^•], although a functional connection between the glutamatergic α1 MBON and α1 DANs has not been demonstrated. (c) A feedback and feed-across recurrency is crucial for memory reconsolidation. Experiencing training-related cues can switch stable memories back into a labile state and reconsolidation is then required for the memory to return to a persistent condition [9^••]. The reconsolidation process requires the activity of the cholinergic MBON-γ2α′1 which in turn recruits two different sets of dopaminergic neurons: the PPL1-γ2α′1 DANs, which are required during memory retrieval, and a group of PAM-DANs which innervate different compartments and are required in the period following retrieval. These cholinergic MBON-DAN connections are excitatory [9^••].