Olfactory learning skews mushroom body output pathways to steer behavioral choice in Drosophila

Abstract

Learning permits animals to attach meaning and context to sensory stimuli. How this information is coded in neural networks in the brain, and appropriately retrieved and utilized to guide behavior, is poorly understood. In the fruit fly olfactory memories of particular value are represented within sparse populations of odor-activated Kenyon cells (KCs) in the mushroom body ensemble. During learning reinforcing dopaminergic neurons skew the mushroom body network by driving zonally restricted plasticity at synaptic junctions between the KCs and subsets of the overall small collection of mushroom body output neurons. Reactivation of this skewed KC-output neuron network retrieves memory of odor valence and guides appropriate approach or avoidance behavior.

Figure 1

Schematics of reinforcing dopaminergic neurons that innervate the MB. (a) The MB-MP1 [PPL1-γ1pedc] and MB-MV1 [PPL1-γ2α′1] DANs in the protocerebral posterior lateral (PPL) 1 cluster provide negative reinforcement signals. The MB-MV1 neuron projects to the lower stalk and junction region and the MB-MP1 neuron innervates the heel and distal peduncle. In addition, the aversive MB-M3 (PAM-β2β′2a) neuron from the protocerebral anterior medial (PAM) cluster ramifies on the tip of the β lobe. All neurons shown have an identical paired neuron that primarily innervates the contralateral MB lobes. (b) DANs in the PAM cluster mostly provide positive reinforcement signals. PAM DANs representing sugar sweetness (green with yellow outline), nutritious value of sugar (darker green) and water (blue) project to discrete zones of the horizontal β, β′ and γ MB lobes (marked with dotted outlines for γ); sweet taste to β′2am and γ4, sugar nutrient to γ5b (tip) and α1 (and possibly β1 β2, not illustrated), and water to a subregion of γ4 that appears distinct from the sweet-taste DANs. In addition, naïve evaluation of water vapor in thirsty flies requires a DAN that innervates β′2p. Reward type is therefore differentially represented in the DAN population and along the MB lobes. Several of the PAM DANs also have a projection to the contralateral MB. A single γ KC is shown with inset illustrating a model where adjoining segments of the KC arbor contain KC presynaptic terminals that are reinforced by DANs for water, sweet taste, or sugar nutrient value. These presynapses are assumed to wire to MBONs with a very similar zonally restricted anatomy to that of the DANs. Cell body position is not stereotyped and diagrams are not intended to be anatomically accurate. These illustrations are edited from [25].

Figure 2

A piano-playing model for learned valence in the KC-MBON network. (a) The canonical view on higher order processing in the fly brain places the LH (not shown) as instrumental for the expression of innate odor-driven behaviors. Experiments blocking all synaptic output from KCs either by ablation or acute silencing suggested the MBs were dispensable for innate odor-driven behavior, but essential for learned responses. However recent findings demonstrate that blocking the MBONs from the tips of the horizontal MB lobes radically alters naïve and learned odor-driven behaviors [13••, 15•]. In addition, the activity of particular MBONs is now known to favor either avoidance (red arrows) or approach (green arrows) [13••, 46••]. It therefore seems logical that the contribution of these MBONs is integrated and balanced in the naïve fly, leading to an apparent lack of contribution from the MB and neutrality in naïve odor-driven tasks. For simplicity we illustrated the balance as equal numbers of outputs (4 plus and 4 minus = zero, neutrality), but it need only be balanced by the relative weights. (b) Reward learning with sugar depresses the odor-specific KC connections to avoidance MBONs (black arrows). In addition it modulates/enhances approach connections (thicker green arrow). This skewed balance (4 plus and 1 minus) now favors odor-driven approach. (c) Aversive conditioning depresses the odor-specific KC connections to approach MBONs (black arrows). In addition it enhances avoidance connections (thicker red arrow). This skewed balance (1 plus and 4 minus) now favors odor-driven avoidance. Avoidance neurons are glutamatergic whereas approach neurons are cholinergic or GABAergic [13••, 46••].

Figure 3

Model how local feed-forward inhibitory interneurons in the MB could mediate the motivational control of sugar memory retrieval. (a) The MB-MP1 [PPL1-γ1pedc] DANs that innervate the heel and peduncle of the MB provide the inhibitory constraint of satiety on the expression of sugar reward memory [53]. The MB-MP1 presynaptic terminals overlap with the dendrites of the GABAergic MVP2 [MBON-γ1pedc > α/β] (dark blue) [46^••] suggesting that MB-MP1 DANs drive plasticity between KC synapses in these regions and the MVP2 MBONs. In the satiated fly the MB-MP1 DANs are tonically active/on and therefore inhibit odor-drive to MVP2, reducing feed-forward inhibition to MBON junctions, such as M4 [MBON-β2 β′2a, MBON-β′2mp] and M6 [MBON-γ5β′2a] outputs on the horizontal lobe tips that drive avoidance. This situation inhibits the expression of reward memories. (b) In hungry flies the MB-MP1 neurons are inhibited/turned off by the action of Neuropeptide F [53]. This results in increased odor-drive to MVP2 and therefore more feed-forward inhibition (MVP2 neuron now light blue) to MBON avoidance pathways (dashed red arrows). This situation favors expression of conditioned odor approach behavior. Interestingly, only nutrient-dependent sugar memory expression requires the flies to be hungry [32^••] and MVP2 innervates the relevant α1 zone of the MB. Furthermore, water-reinforced memory expression is promoted by thirst and not hunger and the MVP2 neuron does not seem to have an arbor in the γ4 water-reinforcement zone. A similar mechanism could provide state-dependence to visual and tastant memories.