Writing memories with light-addressable reinforcement circuitry

Abstract

Dopaminergic neurons are thought to drive learning by signaling changes in the expectations of salient events, such as rewards or punishments. Olfactory conditioning in Drosophila requires direct dopamine action on intrinsic mushroom body neurons, the likely storage sites of olfactory memories. Neither the cellular sources of the conditioning dopamine nor its precise postsynaptic targets are known. By optically controlling genetically circumscribed subsets of dopaminergic neurons in the behaving fly, we have mapped the origin of aversive reinforcement signals to the PPL1 cluster of 12 dopaminergic cells. PPL1 projections target restricted domains in the vertical lobes and heel of the mushroom body. Artificially evoked activity in a small number of identifiable cells thus suffices for programming behaviorally meaningful memories. The delineation of core reinforcement circuitry is an essential first step in dissecting the neural mechanisms that compute and represent valuations, store associations, and guide actions.

Figure 1. Pavlovian Olfactory Conditioning

(A and B) Positions in a behavioral chamber (horizontal dimension) as a function of time (vertical dimension) of 20 Canton-S flies choosing between MCH (blue) and OCT (orange). The traces are sorted by untrained preference. Bar graphs on the right indicate population averages of decisions in favor of the left and right chamber halves before and after conditioning (pre and post), in the presence of odors (colored bars) or air (white and gray bars). (A) Mock conditioning without electric shock preserves individual pre-training preferences. (B) Pairing the presentation of MCH with electric shock causes conditioned avoidance of MCH.

Figure 2. Performance of Individually Trained Flies

(A) Percentage of time spent in OCT before and after mock-conditioning (left) and Pavlovian training against MCH (right). **, p < 0.0001; permutation test. The graphs summarize data from Figures 1A and 1B; black lines connect data points corresponding to the same individual. (B) Canton-S flies trained against MCH (column b) or OCT (column d) avoid the shock-associated odor, but trained rutabaga²⁰⁸⁰ flies (column c) perform indistinguishably from mock-conditioned Canton-S animals (column a). p < 0.0001; Kruskal-Wallis ANOVA; **, significantly different from mock-conditioned controls in post-hoc comparison (n = 20 flies per condition; means ± SEM). (C) Persistence of memory (n = 20 flies per timepoint; means ± SEM).

Figure 3. Action-Contingent Olfactory Conditioning

(A) Positions in a behavioral chamber (horizontal dimension) as a function of time (vertical dimension) of 20 Canton-S flies choosing between MCH (blue) and OCT (orange). The traces are sorted by untrained preference. During four 1-min training periods, entries into MCH are punished by electric shock. Bar graphs on the right indicate population averages of decisions in favor of the left and right chamber halves before and after conditioning (pre and post), in the presence of odors (colored bars) or air (white and gray bars). The MAT-file used to generate this figure can be downloaded for further analysis (Data S1). (B) Locomotion traces at an expanded scale of 10 individuals (see corresponding numbers in A) during four epochs of action-contingent conditioning. A red tick mark to the left of a trace indicates the delivery of one electric shock. Animals receive 2–17 shocks during training; the selected individuals represent the minimum (trace 0) and 9 deciles (traces 1–9) in the frequency distribution of shock consumption. Fast learners (traces 0–4) tend to consume reinforcement only during the first two training epochs, whereas slow learners (traces 5–9) are reinforced throughout. (C) Effective conditioning requires a functional rutabaga gene product (column b) and contingency between olfactory choice behavior and electric shock; learning does not occur when this contingency is broken by randomizing reinforcement (column c). p < 0.0001; Kruskal-Wallis ANOVA; **, significantly different from Canton-S animals in post-hoc comparison (n = 20 flies per condition; means ± SEM). (D) Comparison of the performance of Canton-S flies after 2 and 4 min of Pavlovian and action-contingent training, normalized to the number of electric shocks consumed. p < 0.0001; Kruskal-Wallis ANOVA; **, significantly different from 2 min of Pavlovian conditioning in post-hoc comparison (n = 20 flies per condition; means ± SEM).

Figure 4. Optical Implantation of Memory

(A) Examples of conditioned odor avoidance in TH-GAL4:UAS-P2X₂ flies after genetically targeted photostimulation of dopaminergic neurons. Positions in a behavioral chamber (horizontal dimension) as a function of time (vertical dimension) of 10 flies choosing between MCH (blue) and OCT (orange). The traces are sorted by untrained preference. During four 1-min training periods, entries into MCH activate 10-ms laser pulses. Laser pulses are repeated at 0.2 Hz while the fly remains in the reinforcement zone. Note the conditioned avoidance of MCH (blue) after training. (B) Bar graphs indicate population averages (n = 68 flies) of decisions in favor of the left and right chamber halves before and after conditioning (pre and post), in the presence of odors (colored bars) or air (white and gray bars).

Figure 5. Sources of Dopaminergic Reinforcement Signals

(A) Action-contingent photoactivation of P2X₂ in dopaminergic neurons under TH-GAL4 control produces conditioned odor avoidance (columns a and b). Optically reinforced flies achieve the same level of performance as animals trained conventionally via electric shock (horizontal shaded band; mean ± SEM). Effective conditioning requires a functional rutabaga gene product (column c) and contingency between olfactory choice behavior and optically evoked dopamine release; learning does not occur when this contingency is broken by randomizing reinforcement (column d). Activation of P2X₂ in dopaminergic neurons under HL9-GAL4 control (column e) or ATP uncaging in flies lacking P2X₂ expression (columns f–h) are equally ineffective. p < 0.0001; Kruskal-Wallis ANOVA; **, significantly different from electric shock conditioning in post-hoc comparison (n = 20–68 flies per condition; means ± SEM). (B) Temperature-induced expression of Kir2.1 in dopaminergic neurons under TH-GAL4 control (dark gray columns), but not under HL9-GAL4 control (medium gray columns), blocks action-contingent conditioning (column b). p = 0.0062; Kruskal-Wallis ANOVA; **, significantly different from permissive temperature in post-hoc comparison (n = 19–58 flies per condition; means ± SEM). (C) Temperature-induced expression of Kir2.1 in dopaminergic neurons, under either TH-GAL4 control (dark gray columns) or HL9-GAL4 control (medium gray columns), inhibits locomotion (columns b and d). p < 0.0001; Kruskal-Wallis ANOVA; **, significantly different from permissive temperature in post-hoc comparison (n = 82–120 flies per condition; means ± SEM).

Figure 6. Anatomy of Two Functionally Distinct Sets of Dopaminergic Neurons

(A and B) TH-GAL4 (top row) and HL9-GAL4 (bottom row) mark distinct but partially overlapping clusters of dopaminergic neurons. (C to F) Maximum intensity projections of confocal sections reveal 7 paired neuronal clusters expressing tyrosine hydroxylase in the central brain (red pie charts in A and B, cell numbers in parentheses; see Table S1 for statistics); the fractions of neurons co-expressing mCD8-GFP in the two GAL4 lines are indicated in green. Neuropil was stained with nc82 antibodies (blue), dopaminergic neurons with antibodies against tyrosine hydroxylase (red), and mCD8-GFPexpressing neurons with antibodies against mCD8 (green). Scale bar, 50 µm. (G and H) Maximum intensity projections of confocal sections through the mushroom body. KCs express the mb247-DsRed transgene; dopaminergic projections are labeled by mCD8-GFP. Scale bar, 10 µm.

Figure 7. Projections of Dopaminergic Neurons

Three-dimensional reconstructions of PA-GFP-labeled dopaminergic arborizations (green) and mb247-DsRed expressing KCs (gray). The expression of PAGFP is controlled by TH-GAL4 (A, C to F) or HL9-GAL4 (B). Only PPL1 neurons (A, captured by TH-GAL4) and PAM neurons (B, captured by HL9-GAL4) innervate the mushroom body lobes. The two cell clusters target the vertical and horizontal lobes, respectively. In each panel, a right-handed coordinate system near the left mushroom body indicates the anterior (A), lateral (L), and dorsal (D) directions.