Abstract

The mushroom body is a center for associative memory in insect brains. Sparse activity in the ~2,000 mushroom body (MB) intrinsic neurons, the Kenyon cells, is thought to represent the identity of environmental cues such as odors. Distinct dopaminergic neurons innervating the MB convey information about reward or punishment. The convergence of these inputs allows the MB to adaptively assign valence to sensory stimuli, permitting an individual fly to appropriately adjust its behavioral response when it later encounters the same stimulus. MB output neurons (MBONs) are thought to read and translate the activity of Kenyon cells to bias selection of behavioral responses, but little is know about how they accomplish this task. We have described the detailed projection patterns of the 21 types of MBO Ns and 20 types of dopaminergic and other modulatory neurons. Each of the 21 MBON type elaborates segregated dendritic arbors in the MB lobes, forming 15 compartments that collectively tile the lobes. Using intersectional split-GAL4 drivers that allow manipulation of individual cell types, we have begun the process of determining the nature of the information conveyed by MBO Ns. We show that optogenetic activation of MBO Ns can, depending on the specific cell type, either repel or attract untrained flies by biasing directional turning. The effects of different MBO Ns are additive, implying that activities of MBO Ns representing opposing valence are balanced in untrained flies. We propose that the ensemble of the MBO Ns collectively encodes overall valence and changing the balance between MBO Ns by dopamine modulation biases behavioral responses. Based on this initial work, we are performing further anatomical and behavioral analyses with the goal of understanding the circuit principles for a system of memory-based valuation and action selection. I will present examples from our recent unpublished work.