Memory-Relevant Mushroom Body Output Synapses Are Cholinergic

Abstract

Memories are stored in the fan-out fan-in neural architectures of the mammalian cerebellum and hippocampus and the insect mushroom bodies. However, whereas key plasticity occurs at glutamatergic synapses in mammals, the neurochemistry of the memory-storing mushroom body Kenyon cell output synapses is unknown. Here we demonstrate a role for acetylcholine (ACh) in Drosophila. Kenyon cells express the ACh-processing proteins ChAT and VAChT, and reducing their expression impairs learned olfactory-driven behavior. Local ACh application, or direct Kenyon cell activation, evokes activity in mushroom body output neurons (MBONs). MBON activation depends on VAChT expression in Kenyon cells and is blocked by ACh receptor antagonism. Furthermore, reducing nicotinic ACh receptor subunit expression in MBONs compromises odor-evoked activation and redirects odor-driven behavior. Lastly, peptidergic corelease enhances ACh-evoked responses in MBONs, suggesting an interaction between the fast- and slow-acting transmitters. Therefore, olfactory memories in Drosophila are likely stored as plasticity of cholinergic synapses.

Figure 1

Kenyon Cells Express ChAT and VAChT, and Compromised ChAT or VAChT Expression Impairs MB Function (A) MBs label with an antibody to ChAT (upper row) and an antibody to VAChT (bottom row). Pseudocolored images of single confocal sections at the level of the MB γ lobe. OK107-GAL4-driven UAS-ChAT^RNAi in KCs reduces anti-ChAT label in the MB. UAS-VAChT^RNAi reduces anti-ChAT and anti-VAChT immunoreactivity in MB. See Figure S1 for additional data. Scale bar, 20 μm. (B) Quantification of (A). Anti-ChAT label is significantly lower in MBs in UAS-ChAT^RNAi;OK107-GAL4 and UAS-VAChT^RNAi;OK107-GAL4 flies as compared to all genetic controls. Anti-VAChT signal is significantly lower in MBs in UAS-VAChT^RNAi;OK107-GAL4 flies (n = 3–9, asterisks denote p < 0.05; one-way ANOVA, Tukey’s HSD post hoc test). Similar results are evident in the α, α′, β, and β′ lobes (data not shown). (C) Aversive olfactory memory expression requires ACh function in KCs. Performance of UAS-ChAT^RNAi;OK107-GAL4 and UAS-VAChT^RNAi;OK107-GAL4 flies is significantly different from that of heterozygous controls (n = 6, asterisks denote p < 0.01; one-way ANOVA, Tukey’s HSD post hoc test). See Figures S2A–S2C for additional experiments. Error bars in (B) and (C) represent the standard error of the mean (SEM).

Figure 2

ACh Evokes Calcium Transients in M4/6 Neuron Dendrites (A) Schematic of experimental setup. A micropipette connected to a pressure ejection system is placed near the dendrites of M4/6 MBONs in the tip of the horizontal MB lobe. M4/6 neural activity is monitored using R21D02-GAL4 (enhancer fragment from the Dα6 locus)-driven UAS-GCaMP. Brain shown is costained with anti-Bruchpilot, a general neuropil marker (Wagh et al., 2006). (B) Bright-field image of sample explant brain and micropipette. Region of interest (ROI) from which calcium traces were extracted is marked with a red dashed circle. (C) Pseudocolor fluorescence image of the same area as in (B). (D) Direct application of 1 mM of each candidate KC transmitter to M4/6 dendrites reveals activation by ACh only. n = 5 brains for each transmitter with ten trials per brain). Each colored trace represents an individual brain with the solid line representing the mean, and the shade representing the SEM. (E) Dose-response curve of ACh responses (n = 5 brains per concentration). The line is a sigmoidal fit to the data. Data in (A)–(E) were acquired from R21D02-GAL4; UAS-GCaMP5 brains. (F) Application of 10 μM nicotine elicits strong calcium responses in M4/6 dendrites. n = 3 brains; each colored line represents an individual brain. Solid line is the mean of five trials and shade the SEM. (G) Calcium responses to increasing nicotine concentrations. No change is registered after applying muscarine. n = 3 brains each condition, mean of three trials. Data in (F) and (G) were acquired from R21D02-GAL4;UAS-GCaMP6f brains. Scale bar in (A)–(C), 20 μm. See Figure S3 for additional experiments. Error bars in (E) and (G) represent SEM.

Figure 3

ACh Evokes Responses in Multiple Classes of MBONs (A–F) Release of ACh onto the dendrites of other MBONs evokes calcium responses. Each panel shows the anatomy of the relevant MBON and below the corresponding GCaMP6f measured physiological responses evoked by 1 mM ACh. (A–C) Glutamatergic MBONs on the horizontal MB lobe. (A) MBON-β′2mp (M4β′)/R39A05-GAL4, (B) MBON-β2β′2a (M4β)/R56F01-GAL4, and (C) MBON-γ5β′2a (M6)/R66C08-GAL4. (D and E) Cholinergic MBONs on the vertical MB lobe. (D) MBON-α′3ap (V2α′), MBON-α2sc (V2α)/MB549C, (E) MBON-α′3ap (V2α′), MBON-α′3m (V2α′)/R24H08-GAL4. (F) GABAergic MBON in the heel region, MBON-γ1pedc>αβ (MVP2)/MB112C. Each colored line represents the mean of ten trials per individual brain, and the shade represents the respective SEM (three brains per genotype). Scale bar, 20 μm. See Figure S4 for additional examples.

Figure 4

Optogenetic Activation of KCs Evokes ACh-Dependent Calcium Responses in M4/6 Neurons (A–F) Optogenetic stimulation of KCs triggers M4/6 calcium responses that are blocked by mecamylamine. 247-LexA-driven lexAop-CsChrimson was used to activate KCs, and M4/6 neuron calcium transients were monitored with R21D02-GAL4 driven UAS-GCaMP6f. (A) Samples of two-photon images of M4/6 presynaptic arbors collected on the first frame after LED stimulation, before and after mecamylamine application. Dotted white circle represents ROI from which ΔF/F₀ is calculated. (B and C) A total of 200 ms of red LED light (indicated by red vertical lines) triggers strong calcium responses in M4/6 neuron axons that are abolished 5 min after applying 250 μM mecamylamine. (B) Sample trace and (C) quantification, n = 5 brains, asterisk denotes p < 0.05, paired samples t test. (D) KC-evoked M4/6 neuron responses remain stable after application of vehicle. (E and F) (E) Sample trace and (F) quantification, n = 5 brains, p > 0.05, paired samples t test. See Figure S5 for washout experiments and others using 100 μM mecamylamine or additional nicotinic antagonists. (G–J) KC-expressed UAS-VAChT^RNAi reduces KC-triggered M4 calcium responses. (G) Illustration of experimental setup. R13F02-GAL4-driven UAS-CsChrimson was used to optogenetically activate KCs while M4 calcium transients were monitored with R15B01-LexA driven lexAop-GCaMP6f. (H) Samples of two-photon images of M4 presynaptic arbors collected before and after LED stimulation in control brains (upper panel) or those coexpressing UAS-VAChT^RNAi in KCs (lower panel). Dotted white circles represent ROI from which ΔF/F₀ is calculated. (I and J) A total of 200 ms of red light evokes calcium responses in presynaptic arbors of M4 neurons that have a significantly reduced peak when VAChT^RNAi is coexpressed in KCs. (I) Sample trace and (J) quantification, n = 4–6 brains, asterisk denotes p < 0.05, Mann-Whitney U-test. Data are normalized to controls. Scale bars, 5 μM. Error bars represent SEM.

Figure 5

Reducing nAChR Subunit Expression in M4/6 MBONs Impairs Physiological and Behavioral Odor-Evoked Responses (A) nAChR subunit expression is required for MCH- and OCT-evoked calcium transients in M4/6 neuron axons. Mean traces of activity evoked by 5 s of odor presentation, with SEM as shade (n = 9–16 from 5–10 animals). Head-fixed live flies carrying R21D02-GAL4-driven UAS-GCaMP6f show robust calcium responses to MCH and OCT odors that are impaired by expressing nAChR-directed RNAi. (B) Quantification of peak calcium responses in (A). MCH-evoked responses are significantly reduced in flies expressing UAS-Dα1^RNAi, UAS-Dα4^RNAi, UAS-Dα5^RNAi, and UAS-Dα6^RNAi (n = 9–16, asterisk denotes p < 0.05, one-way ANOVA with Dunnett’s multiple comparisons test). OCT-evoked responses are significantly reduced in flies expressing UAS-Dα4^RNAi and UAS-Dα6^RNAi (n = 9–16, asterisk denotes p < 0.05, one-way ANOVA with Dunnett’s multiple comparisons test). (C) nAChR RNAi in M4/6 neurons phenocopies the behavioral consequence of neural blockade. Flies were given 2 min in a T maze to choose between a tube perfused with 1:1,000 MCH and another with clean air. VT1211-GAL4;UAS-Dα1^RNAi, VT1211-GAL4;UAS-Dα4^RNAi, VT1211-GAL4;UAS-Dα5^RNAi, and VT1211-GAL4;UAS-Dα6^RNAi exhibit a significant preference toward MCH when compared to their two respective genetic control groups (n = 18–38, asterisk denotes p < 0.001, one-way ANOVA with Tukey’s multiple comparisons test). Error bars in (B) and (C) represent SEM.

Figure 6

sNPF Increases ACh-Evoked Calcium Transients in M4/6 Neurons A total of 100 μM of ACh was applied through a micropipette onto the M4/6 dendrites, with either 100 μM sNPF or vehicle. Calcium transients were measured from M4/6 dendrites of R21D02-GAL4; UAS-GCaMP6f explant brains. (A and B) Mean calcium traces of ten trials from 24 brains per condition. (C) Overlaid mean traces from (A) ACh plus sNPF and (B) ACh plus vehicle. Shade represents SEM. (D) Mean peak calcium responses are significantly higher when ACh is coapplied with 100 μM sNPF than with vehicle (t[39.09] = 2.12, p < 0.05, unpaired t test with Welch’s correction). Error bars represent SEM.