Drosophila mutants lacking octopamine exhibit impairment in aversive olfactory associative learning.

Octopamine is a biogenic amine in invertebrates that is considered a functional homolog of vertebrate norepinephrine, acting as a neurotransmitter, neuromodulator and neurohormone. Octopamine regulates many physiological processes such as metabolism, reproduction and different types of behaviour including learning and memory. Previous studies in insects led to the notion that acquisition of an olfactory memory depends on the octopaminergic system during appetitive (reward-based) learning, but not in the case of aversive (punishment-based) learning. Here, we provide several lines of evidence demonstrating that aversive associative olfactory learning in Drosophila is also dependent on octopamine signalling. Specifically, we used Drosophila Tβh (tyramine-β-hydroxylase) mutants, which lack octopamine and are female sterile, to determine whether octopamine plays a role in aversive learning. We show that Tβh mutant flies exhibit a significant reduction in learning compared to control lines that is independent of either genetic background or the methods used to induce aversive olfactory memory. We also show that the learning deficits observed in Tβh mutants are not due to defects in sensorimotor behaviours. Finally, to unambiguously demonstrate that octopamine synthesis plays a role in aversive olfactory learning, we performed rescue experiments using the Gal4/UAS system. We show that expression of UAS-Tβh in octopamine/tyraminergic neurons using Tdc2-Gal4 in Tβh null mutant flies fully rescued both the aversive learning defects and female sterility observed in Tβh mutants.

Iliadi KG ，Iliadi N ，Boulianne GL 《-》

Concerted Actions of Octopamine and Dopamine Receptors Drive Olfactory Learning.

Aminergic signaling modulates associative learning and memory. Substantial advance has been made in Drosophila on the dopamine receptors and circuits mediating olfactory learning; however, our knowledge of other aminergic modulation lags behind. To address this knowledge gap, we investigated the role of octopamine in olfactory conditioning. Here, we report that octopamine activity through the β-adrenergic-like receptor Octβ1R drives aversive and appetitive learning: Octβ1R in the mushroom body αβ neurons processes aversive learning, whereas Octβ1R in the projection neurons mediates appetitive learning. Our genetic interaction and imaging studies pinpoint cAMP signaling as a key downstream effector for Octβ1R function. The rutabaga-adenylyl cyclase synthesizes cAMP in a Ca2+/calmodulin-dependent manner, serving as a coincidence detector for associative learning and likely representing a downstream target for Octβ1R. Supporting this notion, the double heterozygous rutabaga/+;octβ1r/+ flies perform poorly in both aversive and appetitive conditioning, while individual heterozygous rutabaga/+ and octβ1r/+ flies behave like the wild-type control. Consistently, the mushroom body and projection neurons in the octβ1r brain exhibit blunted responses to octopamine when cAMP levels are monitored through the cAMP sensor. We previously demonstrated the pivotal functions of the D1 receptor dDA1 in aversive and appetitive learning, and the α1 adrenergic-like receptor OAMB in appetitive learning. As expected, octβ1r genetically interacts with dumb (dDA1 mutant) in aversive and appetitive learning, but it interacts with oamb only in appetitive learning. This study uncovers the indispensable contributions of dopamine and octopamine signaling to aversive and appetitive learning. All experiments were performed on mixed sex unless otherwise noted.SIGNIFICANCE STATEMENT Animals make flexible behavioral choices that are constantly shaped by experience. This plasticity is vital for animals to appropriately respond to the cues predicting benefit or harm. In Drosophila, dopamine is known to mediate both reward-based and punishment-based learning while octopamine function is important only for reward. Here, we demonstrate that the octopamine-Octβ1R-cAMP pathway processes both aversive and appetitive learning in distinct neural sites of the olfactory circuit. Furthermore, we show that the octopamine-Octβ1R and dopamine-dDA1 signals together drive both aversive and appetitive learning, whereas the octopamine-Octβ1R and octopamine-OAMB pathways jointly facilitate appetitive, but not aversive, learning. This study identifies the cognate actions of octopamine and dopamine signaling as a key neural mechanism for associative learning.

Sabandal JM ，Sabandal PR ，Kim YC ，Han KA ... -  《-》

An octopamine-mushroom body circuit modulates the formation of anesthesia-resistant memory in Drosophila.

Drosophila olfactory aversive conditioning produces two components of intermediate-term memory: anesthesia-sensitive memory (ASM) and anesthesia-resistant memory (ARM). Recently, the anterior paired lateral (APL) neuron innervating the whole mushroom body (MB) has been shown to modulate ASM via gap-junctional communication in olfactory conditioning. Octopamine (OA), an invertebrate analog of norepinephrine, is involved in appetitive conditioning, but its role in aversive memory remains uncertain. Here, we show that chemical neurotransmission from the APL neuron, after conditioning but before testing, is necessary for aversive ARM formation. The APL neurons are tyramine, Tβh, and OA immunopositive. An adult-stage-specific RNAi knockdown of Tβh in the APL neurons or Octβ2R OA receptors in the MB α'β' Kenyon cells (KCs) impaired ARM. Importantly, an additive ARM deficit occurred when Tβh knockdown in the APL neurons was in the radish mutant flies or in the wild-type flies with inhibited serotonin synthesis. OA released from the APL neurons acts on α'β' KCs via Octβ2R receptor to modulate Drosophila ARM formation. Additive effects suggest that two parallel ARM pathways, serotoninergic DPM-αβ KCs and octopaminergic APL-α'β' KCs, exist in the MB.

Wu CL ，Shih MF ，Lee PT ，Chiang AS ... -  《-》

Place memory formation in Drosophila is independent of proper octopamine signaling.

Sitaraman D ，Zars M ，Zars T 《-》

Characterization of the octopaminergic and tyraminergic neurons in the central brain of Drosophila larvae.

Drosophila larvae are able to evaluate sensory information based on prior experience, similarly to adult flies, other insect species, and vertebrates. Larvae and adult flies can be taught to associate odor stimuli with sugar reward, and prior work has implicated both the octopaminergic and the dopaminergic modulatory systems in reinforcement signaling. Here we use genetics to analyze the anatomy, up to the single-cell level, of the octopaminergic/tyraminergic system in the larval brain and subesophageal ganglion. Genetic ablation of subsets of these neurons allowed us to determine their necessity for appetitive olfactory learning. These experiments reveal that a small subset of about 39 largely morphologically distinguishable octopaminergic/tyraminergic neurons is involved in signaling reward in the Drosophila larval brain. In addition to prior work on larval locomotion, these data functionally separate the octopaminergic/tyraminergic system into two sets of about 40 neurons. Those situated in the thoracic/abdominal ganglion are involved in larval locomotion, whereas the others in the subesophageal ganglion and brain hemispheres mediate reward signaling.

Selcho M ，Pauls D ，Huser A ，Stocker RF ，Thum AS ... -  《-》