Different kenyon cell populations drive learned approach and avoidance in Drosophila

Abstract

In Drosophila, anatomically discrete dopamine neurons that innervate distinct zones of the mushroom body (MB) assign opposing valence to odors during olfactory learning. Subsets of MB neurons have temporally unique roles in memory processing, but valence-related organization has not been demonstrated. We functionally subdivided the αβ neurons, revealing a value-specific role for the ∼160 αβ core (αβc) neurons. Blocking neurotransmission from αβ surface (αβs) neurons revealed a requirement during retrieval of aversive and appetitive memory, whereas blocking αβc only impaired appetitive memory. The αβc were also required to express memory in a differential aversive paradigm demonstrating a role in relative valuation and approach behavior. Strikingly, both reinforcing dopamine neurons and efferent pathways differentially innervate αβc and αβs in the MB lobes. We propose that conditioned approach requires pooling synaptic outputs from across the αβ ensemble but only from the αβs for conditioned aversion.

Figure 1

Anatomically Distinct Subsets of MB αβ Neurons (A) Model of the left fly MB outlining the different subsets of intrinsic αβ KCs within the lobes. (B–E) Projection views of confocal stacks at the level of the left MB lobes from c739-αβ_scp (B), 0770-αβ_s (C), NP7175-αβ_c (D), and c708a-αβ_p (E) flies driving mCD8::GFP (green). In all panels, the overall MB is labeled with rCD2::RFP (magenta). The inset shows a horizontal cross-section through the vertical collateral at the level of the dashed line in (A). Scale bar represents 20 μm. See also Figure S1.

Figure 2

Functional Subdivision of αβ Neurons in 3 hr Memory Retrieval Flies were trained at the permissive 23°C and αβ subsets blocked by shifting the flies to restrictive 33°C 30 min before and during testing 3 hr memory (schematic). (A) αβ_c and αβ_s neurons are required for retrieval of 3 hr appetitive memory. Blocking transmission from c739, 0770, and NP7175 neurons during testing impaired appetitive memory (all p < 0.001), whereas blocking c708a neurons had no effect (p = 0.10). (B) αβ_c neurons are not required for retrieval of aversive memory. Blocking transmission from c739 and 0770 neurons during testing impaired aversive memory (both p < 0.001), whereas blocking NP7175 or c708a neurons had no effect (p > 0.9 and p > 0.5). Odors used in (A) and (B) are 4-methylcyclohexanol (MCH) and 3-octanol (OCT). (C and D) Repeat of experiments in (A) and (B) using isoamyl acetate (IAA) and ethyl butyrate (EB) as odors. The requirement for output from the αβ_s and αβ_c neurons for the retrieval of 3 hr appetitive memory was reproduced, whereas αβ_p neuron output remained dispensable. (C) Blocking transmission from c739, 0770, and NP7175 neurons during testing impaired appetitive memory (all p < 0.001), whereas blocking c708a neurons had no effect (p > 0.5). (D) Blocking transmission from c739 and 0770 neurons during testing impaired aversive memory (p < 0.01 and p < 0.001), whereas blocking NP7175 or c708a neurons had no effect (p > 0.4 and p > 0.2). An asterisk denotes significant difference between marked group and the relevant genetic controls (all p < 0.01, ANOVA). The wild-type group corresponds to pooled data from independent experiments and all data are represented as the mean ± SEM. See also Figures S1, S2, S4, and S7 and Table S1.

Figure 3

αβ_c Neurons Only Contribute to Appetitive Memory Expression (A–C) Projection views of confocal stacks at the level of the left MB lobes from αβ subset GAL4 lines driving mCD8::GFP (green). The inset shows a cross-section through the vertical collateral at the level of the dashed line in Figure 1A. The overall MB is labeled with rCD2::RFP (magenta). Scale bar represents 20 μm. (A) NP5286 αβ_s neurons. (B) ChaGAL80 inhibits GAL4 in c739 labeled αβ_s neurons and leaves expression in αβ_c neurons. (C) NP6024 labels αβ_c neurons and inner αβ_s neurons. (D) Illustration of a cross-section of the α lobe neurons labeled by NP5286-αβ_s, c739;ChaGAL80-αβ_c, and NP6024-αβ_sc GAL4 lines. (E) Flies were trained at the permissive 23°C and αβ subsets were blocked by shifting the flies to restrictive 33°C 30 min before and during testing 3 hr memory (schematic). The αβ_s and αβ_c neurons are required for 3 hr appetitive memory retrieval. Blocking transmission from NP5286, c739;ChaGAL80, or NP6024 neurons during testing impaired appetitive memory (p < 0.01). (F) The αβ_s but not the αβ_c neurons are required for 3 hr aversive memory retrieval. Blocking transmission from NP5286 or NP6024 neurons during testing impaired aversive memory (both p < 0.05), whereas suppressing αβ_s expression in c739 reversed the 3 hr aversive memory retrieval phenotype. Blocking c739;ChaGAL80 neuron output did not impair aversive memory retrieval (p = 0.9). An asterisk denotes significant difference between marked group and the relevant genetic controls (all p < 0.05, ANOVA). Odors used are OCT and MCH. Data are represented as mean ± SEM. See also Figure S3.

Figure 4

Odors Evoke Responses in All αβ Neuron Subsets (A) The four odors used in conditioning evoke a robust increase in GCaMP5 fluorescence in αβ_s and αβ_c neurons and a decrease in αβ_p neurons of naive flies. Time courses of odor-evoked GCaMP5 responses (ΔF/F) collected at the level of the tip of the MB α-lobe represented by the panels shown in (B). Responses from individual flies are shown as light traces and the average responses from all flies in bold traces. The αβ_c and αβ_s KCs were activated by all odors, whereas αβ_p KCs were inhibited by all odors used in this study. n = 4–5. (B) Anatomical segregation and distribution of odor-evoked responses. Pseudocolored activity maps of odor responses overlaid on grayscale images of baseline fluorescence. The four odors used elicit different patterns of activation in each KC subgroup. Scale bar represents 5 μm. See also Figure S5.

Figure 5

Output from αβ_c Neurons Is Required for Relative Aversive Choice (A) Schematic of the absolute and relative aversive training paradigm. (B–D) Relative but not absolute aversive conditioning requires output from rewarding DA neurons during acquisition. (B) Blocking transmission from 0104 neurons during training impaired 30 min Y₆₀ versus Z₃₀ relative choice memory (p < 0.001). (C) Blocking 0104 DA neurons during training did not disrupt 30 min X₀ versus Y₆₀ absolute choice memory (p > 0.05). (D) No differences were apparent when flies were trained and tested for relative Y₆₀ versus Z₃₀ choice memory at permissive 23°C (p > 0.9). (E–H) Relative but not absolute choice memory retrieval requires output from αβ_c neurons. (E) Blocking output from NP7175 αβ_c, c739;ChaGAL80 αβ_c, or 0770 αβ_s neurons during retrieval impaired 30 min Y₆₀ versus Z₃₀ relative choice memory (p < 0.001). (F) No statistical differences were apparent when flies were trained and tested for relative Y₆₀ versus Z₃₀ choice memory at permissive 23°C (p > 0.05). (G) Blocking transmission from 0770 αβ_s (p < 0.01) but not NP7175 αβ_c or c739;ChaGAL80 αβ_c (both p > 0.5) impaired X₀ versus Y₆₀ absolute choice memory. (H) No statistical differences were evident between 0770 flies trained and tested for absolute X₀ versus Y₆₀ choice memory at permissive 23°C (p > 0.05). An asterisk denotes significant difference between marked group and the relevant controls (all p < 0.05, ANOVA). Odors used are OCT, MCH, and IAA. Data are represented as mean ± SEM.

Figure 6

Reinforcing DA Neurons and MB Efferent Neurons Differentially Innervate αβ Layers (A–D) Rewarding DA input neurons from the PAM cluster ramify throughout the β lobe. (A) Frontal projection view of a confocal stack of the brain from a fly expressing mCD8::GFP (black) driven by 0279 GAL4. The MB is labeled with rCD2::RFP (red). Scale bar represents 50 μm. The 0279 labeled neurons ramify throughout the β lobe (inset, magnified sagittal section through the β lobe at the level of the dashed black line in A; scale bar represents 10 μm). (B) uas-dTrpA1-mediated activation of 0279 neurons contingent with odor presentation (2 min at 33°C, red) forms robust appetitive olfactory memory (ANOVA between the relevant control groups, p < 0.001, denoted by an asterisk). Odors used are OCT and MCH. Data are represented as mean ± SEM. (C) 0279 neurons (GFP/green) colocalize with tyrosine hydroxylase immunoreactivity (TH-ir, magenta), suggesting they are dopaminergic. (D) 0279 DA neurons are presynaptic to the ipsilateral β₁ and contralateral β₂ lobe (DSyd1::GFP, green). DenMark (magenta) labels the postsynaptic compartment in the superior lateral protocerebrum. Scale bar represents 20 μm. (E and F) Aversive DA input neurons mostly innervate the αβ surface. (E) Single frontal section (0.5 μm) of the left MB (labeled with rCD2::RFP, magenta) at the level of the heel/peduncle transition (dashed box in inset) showing innervation of the aversive reinforcing and appetitive motivational control MB-MP1 neurons, labeled by c061;MBGAL80-driven mCD8::GFP (green). MB-MP1 neurons do not innervate the region of the distal peduncle that is occupied by αβ_c KCs. Scale bar represents 10 μm. (F) Single sagittal section (0.5 μm) through the left MB (labeled with rCD2::RFP, magenta) at the level of the horizontal lobe tips (dashed line in inset) showing MB-M3 neurons labeled by NP5272-driven mCD8::GFP (green). Aversive reinforcing MB-M3 neurons only innervate the surface and not the core of the β lobe tip (white dashed circles). Scale bar represents 10 μm. (G) Single horizontal section (0.5 μm) of the left MB (labeled with rCD2::RFP, magenta) through the vertical lobe tips (at the level of the dashed line in inset) showing MB-V3-nonselective output neurons labeled by 12-244-driven mCD8::GFP (green). MB-V3 ramifies across the αβ_c and αβ_s layers in the α lobe tip. (H) Single horizontal section (0.5 μm) of the left MB (labeled with rCD2::RFP, magenta) through the vertical lobe stalk (at the level of the dashed line in inset) showing MB-V2α-aversive output neurons labeled by NP2492-driven mCD8::GFP (green). MB-V2α more densely innervates the α surface than α core. Scale bar represents 10 μm. (E–H) Each inset shows the expression pattern of the respective GAL4 line (GFP/green) within the left MB lobes, labeled with rCD2::RFP (magenta). Scale bar represents 20 μm. See also Figure S6.

Figure 7

Model Illustrating the Differential Role of αβ Surface and Core Neurons in Conditioned Approach and Aversion (A) During appetitive conditioning, rewarding dopaminergic MB-M8 neurons (green) from the PAM cluster reinforces odor-activated synapses of αβ_s (blue) and αβ_c (black) neurons in the β₁ and β₂ zones of the horizontal β lobe. (B) During testing, appetitive memory is retrieved at least in part through MB-V3 efferent neurons (dark red) that pool inputs from across the αβ ensemble in the α₃ region of the vertical α lobe tip region and drive approach behavior through a putative premotor area. Expression of conditioned approach is additionally gated in a hunger state-dependent manner by the MB-MP1 DA neurons (orange; Krashes et al., 2009). (C) During aversive training, MB-MP1 and MB-M3 DA neurons (green) reinforce odor-activated synapses in the αβ_s region of the peduncle and only αβ_s neurons in the β₂ region of the horizontal β lobe tip. (D) During testing, aversive memory is retrieved at least in part through MB-V3 neurons and the MB-V2α efferent neurons (both dark red) that collect inputs from the αβ_s neurons in the tip and α₂ region of the vertical α-stalk and drive avoidance behavior through the putative premotor area.