Electrical synapses between mushroom body neurons are critical for consolidated memory retrieval in Drosophila

Abstract

Electrical synapses between neurons, also known as gap junctions, are direct cell membrane channels between adjacent neurons. Gap junctions play a role in the synchronization of neuronal network activity; however, their involvement in cognition has not been well characterized. Three-hour olfactory associative memory in Drosophila has two components: consolidated anesthesia-resistant memory (ARM) and labile anesthesia-sensitive memory (ASM). Here, we show that knockdown of the gap junction gene innexin5 (inx5) in mushroom body (MB) neurons disrupted ARM, while leaving ASM intact. Whole-mount brain immunohistochemistry indicated that INX5 protein was preferentially expressed in the somas, calyxes, and lobes regions of the MB neurons. Adult-stage-specific knockdown of inx5 in αβ neurons disrupted ARM, suggesting a specific requirement of INX5 in αβ neurons for ARM formation. Hyperpolarization of αβ neurons during memory retrieval by expressing an engineered halorhodopsin (eNpHR) also disrupted ARM. Administration of the gap junction blocker carbenoxolone (CBX) reduced the proportion of odor responsive αβ neurons to the training odor 3 hours after training. Finally, the α-branch-specific 3-hour ARM-specific memory trace was also diminished with CBX treatment and in inx5 knockdown flies. Altogether, our results suggest INX5 gap junction channels in αβ neurons for ARM retrieval and also provide a more detailed neuronal mechanism for consolidated memory in Drosophila .

Mushroom body subsets encode CREB2-dependent water-reward long-term memory in Drosophila

“…Functional in vivo calcium imaging has provided insights into neuronal responses to specific odors in a living fly brain by allowing the examination of increased or decreased calcium responses to training odor after conditioning [ 19 ]. Several previous studies have shown that different calcium responses to training odor (called memory trace) occurs in the distinct axons of MB neurons at different time windows after odor/shock conditioning [ 20 – 23 ]. We therefore investigated whether the thirsty flies could form a memory trace in MB neurons after odor/water conditioning, and the paired and unpaired training protocols were used for in vivo calcium imaging analysis.…”

“…A previous study showed that the fly forms α-lobe branch-specific aversive LTM trace 24-hour after odor/shock conditioning [ 20 ]. In our recent study, we also observed the α-lobe branch-specific aversive anesthesia-resistant memory (ARM) trace 3-hour after odor/shock conditioning [ 23 ]. Intriguingly, we found that both α- and β- lobes of the surface neurons show increased calcium response to training odor 24-hour after odor/water conditioning, suggesting that the wLTM trace is not specific to the α-lobe branch ( Fig 6 and S5 Fig ).…”

“…All the flies were reared on standard cornmeal food at 25°C and 60% relative humidity on a 12 h:12 h, light-dark cycle. R13F02-GAL4 , R16A06-GAL4 , 5HT1B-GAL4 , C739-GAL4 , VT44966-GAL4 , VT30604-GAL4 , VT57244-GAL4 , tub-GAL80 ts ; UAS-dCREB2b , UAS-mCD8 :: GFP; UAS-mCD8 :: GFP , UAS-shi ts , and UAS-GCaMP6m fly strains have been used as described previously [ 1 , 23 , 35 , 37 , 43 – 45 ]. UAS-ricin CS and MB607B-GAL4 flies were obtained from Ann-Shyn Chiang.…”

“…GFP; UAS-mCD8::GFP, UAS-shi ts , and UAS-GCaMP6m fly strains have been used as described previously [1,23,35,37,[43][44][45]. UAS-ricin CS and MB607B-GAL4 flies were obtained from Ann-Shyn Chiang.…”

The cellular architecture of memory modules inDrosophilasupports stochastic input integration

“…Hence, it is still unclear how dopamine signaling can result in two different modes of synaptic tuning. Recent studies have observed an increased representa- tion of stimuli in KCs which was mediated through electrical synapses [49]. Furthermore, conditioning induced a functional decorrelation of calcium traces in KC lobes [6,15].…”