GABAA receptor RDL inhibits Drosophila olfactory associative learning

Central synaptic mechanisms underlie short-term olfactory habituation in Drosophila larvae

Naive Drosophila larvae show vigorous chemotaxis toward many odorants including ethyl acetate (EA). Chemotaxis toward EA is substantially reduced after a 5-min pre-exposure to the odorant and recovers with a half-time of ∼20 min. An analogous behavioral decrement can be induced without odorant-receptor activation through channelrhodopsin-based, direct photoexcitation of odorant sensory neurons (OSNs). The neural mechanism of short-term habituation (STH) requires the (1) rutabaga adenylate cyclase; (2) transmitter release from predominantly GABAergic local interneurons (LNs); (3) GABA-A receptor function in projection neurons (PNs) that receive excitatory inputs from OSNs; and (4) NMDA-receptor function in PNs. These features of STH cannot be explained by simple sensory adaptation and, instead, point to plasticity of olfactory synapses in the antennal lobe as the underlying mechanism. Our observations suggest a model in which NMDAR-dependent depression of the OSN-PN synapse and/or NMDAR-dependent facilitation of inhibitory transmission from LNs to PNs contributes substantially to short-term habituation.

Mutation of the Drosophila vesicular GABA transporter disrupts visual figure detection

The role of gamma amino butyric acid (GABA) release and inhibitory neurotransmission in regulating most behaviors remains unclear. The vesicular GABA transporter (VGAT) is required for the storage of GABA in synaptic vesicles and provides a potentially useful probe for inhibitory circuits. However, specific pharmacologic agents for VGAT are not available, and VGAT knockout mice are embryonically lethal, thus precluding behavioral studies. We have identified the Drosophila ortholog of the vesicular GABA transporter gene (which we refer to as dVGAT), immunocytologically mapped dVGAT protein expression in the larva and adult and characterized a dVGAT(minos) mutant allele. dVGAT is embryonically lethal and we do not detect residual dVGAT expression, suggesting that it is either a strong hypomorph or a null. To investigate the function of VGAT and GABA signaling in adult visual flight behavior, we have selectively rescued the dVGAT mutant during development. We show that reduced GABA release does not compromise the active optomotor control of wide-field pattern motion. Conversely, reduced dVGAT expression disrupts normal object tracking and figure-ground discrimination. These results demonstrate that visual behaviors are segregated by the level of GABA signaling in flies, and more generally establish dVGAT as a model to study the contribution of GABA release to other complex behaviors.

A distinct set of Drosophila brain neurons required for neurofibromatosis type 1-dependent learning and memory

Nonspecific cognitive impairments are one of the many manifestations of neurofibromatosis type 1 (NF1). A learning phenotype is also present in Drosophila melanogaster that lack a functional neurofibromin gene (nf1). Multiple studies have indicated that Nf1-dependent learning in Drosophila involves the cAMP pathway, including the demonstration of a genetic interaction between Nf1 and the rutabaga-encoded adenylyl cyclase (Rut-AC). Olfactory classical conditioning experiments have previously demonstrated a requirement for Rut-AC activity and downstream cAMP pathway signaling in neurons of the mushroom bodies. However, Nf1 expression in adult mushroom body neurons has not been observed. Here, we address this discrepancy by demonstrating (1) that Rut-AC is required for the acquisition and stability of olfactory memories, whereas Nf1 is only required for acquisition, (2) that expression of nf1 RNA can be detected in the cell bodies of mushroom body neurons, and (3) that expression of an nf1 transgene only in the alpha/beta subset of mushroom body neurons is sufficient to restore both protein synthesis-independent and protein synthesis-dependent memory. Our observations indicate that memory-related functions of Rut-AC are both Nf1-dependent and -independent, that Nf1 mediates the formation of two distinct memory components within a single neuron population, and that our understanding of Nf1 function in memory processes may be dissected from its role in other brain functions by specifically studying the alpha/beta mushroom body neurons.

Seizure sensitivity is ameliorated by targeted expression of K+-Cl- cotransporter function in the mushroom body of the Drosophila brain

The kcc(DHS1) allele of kazachoc (kcc) was identified as a seizure-enhancer mutation exacerbating the bang-sensitive (BS) paralytic behavioral phenotypes of several seizure-sensitive Drosophila mutants. On their own, young kcc(DHS1) flies also display seizure-like behavior and demonstrate a reduced threshold for seizures induced by electroconvulsive shock. The product of kcc shows substantial homology to KCC2, the mammalian neuronal K(+)-Cl(-) cotransporter. The kcc(DHS1) allele is a hypomorph, and its seizure-like phenotype reflects reduced expression of the kcc gene. We report here that kcc functions as a K(+)-Cl(-) cotransporter when expressed heterologously in Xenopus laevis oocytes: under hypotonic conditions that induce oocyte swelling, oocytes that express Drosophila kcc display robust ion transport activity observed as a Cl(-)-dependent uptake of the K(+) congener (86)Rb(+). Ectopic, spatially restricted expression of a UAS-kcc(+) transgene was used to determine where cotransporter function is required in order to rescue the kcc(DHS1) BS paralytic phenotype. Interestingly, phenotypic rescue is largely accounted for by targeted, circumscribed expression in the mushroom bodies (MBs) and the ellipsoid body (EB) of the central complex. Intriguingly, we observed that MB induction of kcc(+) functioned as a general seizure suppressor in Drosophila. Drosophila MBs have generated considerable interest especially for their role as the neural substrate for olfactory learning and memory; they have not been previously implicated in seizure susceptibility. We show that kcc(DHS1) seizure sensitivity in MB neurons acts via a weakening of chemical synaptic inhibition by GABAergic transmission and suggest that this is due to disruption of intracellular Cl(-) gradients in MB neurons.

Glutamatergic and GABAergic effects of fipronil on olfactory learning and memory in the honeybee

We investigated here the role of transmissions mediated by GABA and glutamate-gated chloride channels (GluCls) in olfactory learning and memory in honeybees, both of these channels being a target for fipronil. To do so, we combined olfactory conditioning with injections of either the GABA- and glutamate-interfering fipronil alone, or in combination with the blocker of glutamate transporter L-trans-Pyrrolidine-2,4-Dicarboxylicacid (L-trans-PDC), or the GABA analog Trans-4-Aminocrotonic Acid (TACA). Our results show that a low dose of fipronil (0.1 ng/bee) impaired olfactory memory, while a higher dose (0.5 ng/bee) had no effect. The detrimental effect induced by the low dose of fipronil was rescued by the coinjection of L-trans-PDC but was rather increased by the coinjection of TACA. Moreover, using whole-cell patch-clamp recordings, we observed that L-trans-PDC reduced glutamate-induced chloride currents in antennal lobe cells. We interpret these results as reflecting the involvement of both GluCl and GABA receptors in the impairment of olfactory memory induced by fipronil.

Adenosine-to-inosine genetic recoding is required in the adult stage nervous system for coordinated behavior in Drosophila

Adenosine deaminases acting on RNA (ADARs) catalyze the deamination of adenosine to inosine in double-stranded RNA templates, a process known as RNA editing. In Drosophila, multiple ADAR isoforms are generated from a single locus (dAdar) via post-transcriptional modifications. Collectively, these isoforms act to edit a wide range of transcripts involved in neuronal signaling, as well as the precursors of endogenous small interfering RNAs. The phenotypic consequences of a loss of dADAR activity have been well characterized and consist of profound behavioral defects manifested at the adult stage, including extreme uncoordination, seizures, and temperature-sensitive paralysis. However, the spatio-temporal requirements of adenosine to inosine editing for correct behavior are unclear. Using transgenic RNA interference, we show that network-wide editing in the nervous system is required for normal adult locomotion. Regulated restoration of editing activity demonstrates that the neuronal requirement of dADAR activity has a significant adult stage component. Furthermore we show that in relation to behavior there are no observable genetic interactions between dAdar and several loci encoding RNA interference components, suggesting that editing of neuronal transcripts is the key mode of ADAR activity for normal behavior in Drosophila.

Ion channels to inactivate neurons in Drosophila

Ion channels are the determinants of excitability; therefore, manipulation of their levels and properties provides an opportunity for the investigator to modulate neuronal and circuit function. There are a number of ways to suppress electrical activity in Drosophila neurons, for instance, over-expression of potassium channels (i.e. Shaker Kv1, Shaw Kv3, Kir2.1 and DORK) that are open at resting membrane potential. This will result in increased potassium efflux and membrane hyperpolarisation setting resting membrane potential below the threshold required to fire action potentials. Alternatively over-expression of other channels, pumps or co-transporters that result in a hyperpolarised membrane potential will also prevent firing. Lastly, neurons can be inactivated by, disrupting or reducing the level of functional voltage-gated sodium (Nav1 paralytic) or calcium (Cav2 cacophony) channels that mediate the depolarisation phase of action potentials. Similarly, strategies involving the opposite channel manipulation should allow net depolarisation and hyperexcitation in a given neuron. These changes in ion channel expression can be brought about by the versatile transgenic (i.e. Gal4/UAS based) systems available in Drosophila allowing fine temporal and spatial control of (channel) transgene expression. These systems are making it possible to electrically inactivate (or hyperexcite) any neuron or neural circuit in the fly brain, and much like an exquisite lesion experiment, potentially elucidate whatever interesting behaviour or phenotype each network mediates. These techniques are now being used in Drosophila to reprogram electrical activity of well-defined circuits and bring about robust and easily quantifiable changes in behaviour, allowing different models and hypotheses to be rapidly tested.

Splice-variant- and stage-specific RNA editing of the Drosophila GABA receptor modulates agonist potency

The molecular diversity of many gene products functioning in the nervous system is enhanced by alternative splicing and adenosine-to-inosine editing of pre-mRNA. Using RDL, a Drosophila melanogaster GABA-gated ion channel, we examined the functional impact of RNA editing at several sites along with alternative splicing of more than one exon. We show that alternative splicing and RNA editing have a combined influence on the potency of the neurotransmitter GABA, and the editing isoforms detected in vivo span the entire functional range of potencies seen for all possible edit variants expressed in Xenopus laevis oocytes. The extent of RNA editing is developmentally regulated and can also be linked to the choice of alternative exons. These results provide insights into how the rich diversity of signaling necessary for complex brain function can be achieved by relatively few genes.

Gamma-aminobutyric acid (GABA)-mediated neural connections in the Drosophila antennal lobe

Inhibitory synaptic connections mediated by gamma-aminobutyric acid (GABA) play important roles in the neural computation of the brain. To obtain a detailed overview of the neural connections mediated by GABA signals, we analyzed the distribution of the cells that produce and receive GABA in the Drosophila adult brain. Relatively small numbers of the cells, which form clusters in several areas of the brain, express the GABA synthesis enzyme Gad1. On the other hand, many cells scattered across the brain express ionotropic GABA(A) receptor subunits (Lcch3 and Rdl) and metabotropic GABA(B) receptor subtypes (GABA-B-R1, -2, and -3). To analyze the expression of these genes in distinct identified cell types, we focused on the antennal lobe, where GABAergic neurons play important roles in odor coding. By combining fluorescent in situ hybridization and immunolabeling against GFP expressed with cell-type-specific GAL4 driver strains, we quantified the percentage of the cells that produce or receive GABA for each cell type. GABA was synthesized in the middle antennocerebral tract (mACT) projection neurons and two types of local neurons. Among them, mACT neurons had few presynaptic sites in the antennal lobe, making the local neurons essentially the sole provider of GABA signals there. On the other hand, not only these local neurons but also all types of projection neurons expressed both ionotropic and metabotropic GABA receptors. Thus, even though inhibitory signals are released from only a few, specific types of local neurons, the signals are read by most of the neurons in the antennal lobe neural circuitry.

A dual role for the adaptor protein DRK in Drosophila olfactory learning and memory

Participation of RAS, RAF, and mitogen-activated protein kinase (MAPK) in learning and memory has been demonstrated in a number of studies, but the molecular events requisite for cascade activation and regulation have not been explored. We demonstrate that the adapter protein DRK (downstream of receptor kinase) which is essential for signaling to RAS in developmental contexts, is preferentially distributed in the adult mushroom bodies, centers for olfactory learning and memory. We demonstrate that drk mutant heterozygotes exhibit deficits in olfactory learning and memory, apparent under limited training conditions, but are not impaired in sensory responses requisite for the association of the stimuli, or brain neuroanatomy. Furthermore, we demonstrate that the protein is required acutely within mushroom body neurons to mediate efficient learning, a process that requires RAF activation. Importantly, 90 min memory remained impaired, even after differential training yielding equivalent learning in animals with compromised DRK levels and controls and did not require RAF. Sustained MAPK activation is compromised in drk mutants and surprisingly is negatively regulated by constitutive RAF activity. The data establish a role for DRK in Drosophila behavioral neuroplasticity and suggest a dual role for the protein, first in RAF activation-dependent learning and additionally in RAF-inhibition dependent sustained MAPK activation essential for memory formation or stability.

The GABA(A) receptor RDL acts in peptidergic PDF neurons to promote sleep in Drosophila

Sleep is regulated by a circadian clock that times sleep and wake to specific times of day and a homeostat that drives sleep as a function of prior wakefulness. To analyze the role of the circadian clock, we have used the fruit fly Drosophila. Flies display the core behavioral features of sleep, including relative immobility, elevated arousal thresholds, and homeostatic regulation. We assessed sleep-wake modulation by a core set of circadian pacemaker neurons that express the neuropeptide PDF. We find that disruption of PDF function increases sleep during the late night in light:dark and the first subjective day of constant darkness. Flies deploy genetic and neurotransmitter pathways to regulate sleep that are similar to those of their mammalian counterparts, including GABA. We find that RNA interference-mediated knockdown of the GABA(A) receptor gene, Resistant to dieldrin (Rdl), in PDF neurons reduces sleep, consistent with a role for GABA in inhibiting PDF neuron function. Patch-clamp electrophysiology reveals GABA-activated picrotoxin-sensitive chloride currents on PDF+ neurons. In addition, RDL is detectable most strongly on the large subset of PDF+ pacemaker neurons. These results suggest that GABAergic inhibition of arousal-promoting PDF neurons is an important mode of sleep-wake regulation in vivo.

The GABAA receptor RDL suppresses the conditioned stimulus pathway for olfactory learning

Assigning a gene's function to specific pathways used for classical conditioning, such as conditioned stimulus (CS) and unconditioned stimulus (US) pathway, is important for understanding the fundamental molecular and cellular mechanisms underlying memory formation. Prior studies have shown that the GABA receptor RDL inhibits aversive olfactory learning via its role in the Drosophila mushroom bodies (MBs). Here, we describe the results of further behavioral tests to further define the pathway involvement of RDL. The expression level of Rdl in the MBs influenced both appetitive and aversive olfactory learning, suggesting that it functions by suppressing a common pathway used for both forms of olfactory learning. Rdl knock down failed to enhance learning in animals carrying mutations in genes of the cAMP signaling pathway, such as rutabaga and NF1, suggesting that RDL works up stream of these functions in CS/US integration. Finally, knocking down Rdl or over expressing the dopamine receptor dDA1 in the MBs enhanced olfactory learning, but no significant additional enhancement was detected with both manipulations. The combined data suggest that RDL suppresses olfactory learning via CS pathway involvement.

The mushroom body of adult Drosophila characterized by GAL4 drivers

The mushroom body is required for a variety of behaviors of Drosophila melanogaster. Different types of intrinsic and extrinsic mushroom body neurons might underlie its functional diversity. There have been many GAL4 driver lines identified that prominently label the mushroom body intrinsic neurons, which are known as "Kenyon cells." Under one constant experimental condition, we analyzed and compared the the expression patterns of 25 GAL4 drivers labeling the mushroom body. As an internet resource, we established a digital catalog indexing representative confocal data of them. Further more, we counted the number of GAL4-positive Kenyon cells in each line. We found that approximately 2,000 Kenyon cells can be genetically labeled in total. Three major Kenyon cell subtypes, the gamma, alpha'/beta', and alpha/beta neurons, respectively, contribute to 33, 18, and 49% of 2,000 Kenyon cells. Taken together, this study lays groundwork for functional dissection of the mushroom body.

The GABAergic anterior paired lateral neuron suppresses and is suppressed by olfactory learning

GABAergic neurotransmitter systems are important for many cognitive processes, including learning and memory. We identified a single neuron in each hemisphere of the Drosophila brain, the anterior paired lateral (APL) neuron, as a GABAergic neuron that broadly innervated the mushroom bodies. Reducing GABA synthesis in the APL neuron enhanced olfactory learning, suggesting that the APL neuron suppressed learning by releasing the inhibitory neurotransmitter GABA. Functional optical-imaging experiments revealed that the APL neuron responded to both odor and electric-shock stimuli that was presented to the fly with increases of intracellular calcium and released neurotransmitter. Notably, a memory trace formed in the APL neuron by pairing odor with electric shock. This trace was detected as a reduced calcium response in the APL neuron after conditioning specifically to the trained odor. These results demonstrate a mutual suppression between the GABAergic APL neuron and olfactory learning, and emphasize the functional neuroplasticity of the GABAergic system as a result of learning.

Neural representations of airflow in Drosophila mushroom body

The Drosophila mushroom body (MB) is a higher olfactory center where olfactory and other sensory information are thought to be associated. However, how MB neurons of Drosophila respond to sensory stimuli other than odor is not known. Here, we characterized the responses of MB neurons to a change in airflow, a stimulus associated with odor perception. In vivo calcium imaging from MB neurons revealed surprisingly strong and dynamic responses to an airflow stimulus. This response was dependent on the movement of the 3^rd antennal segment, suggesting that Johnston's organ may be detecting the airflow. The calyx, the input region of the MB, responded homogeneously to airflow on. However, in the output lobes of the MB, different types of MB neurons responded with different patterns of activity to airflow on and off. Furthermore, detailed spatial analysis of the responses revealed that even within a lobe that is composed of a single type of MB neuron, there are subdivisions that respond differently to airflow on and off. These subdivisions within a single lobe were organized in a stereotypic manner across flies. For the first time, we show that changes in airflow affect MB neurons significantly and these effects are spatially organized into divisions smaller than previously defined MB neuron types.

Heisenberg's roadmap guides our journey to the small cognitive world of Drosophila

Professor Martin Heisenberg is one of the pioneers in the exploration of neuroethology. With his inspiration and earnest help, we employed the fruitfly as a model system to investigate the underlying neural mechanism of cognitive behaviors. Here, we recalled the help from Martin in the early years and introduced some findings from our lab about visual cognition behaviors in Drosophila, such as decision making, selective attention, and experience-dependent visual pattern recognition. From the results so far, the circuit composed of mushroom bodies, central complex, and dopaminergic neurons may play an essential role in these behaviors.

PDF cells are a GABA-responsive wake-promoting component of the Drosophila sleep circuit

Daily sleep cycles in humans are driven by a complex circuit within which GABAergic sleep-promoting neurons oppose arousal. Drosophila sleep has recently been shown to be controlled by GABA, which acts on unknown cells expressing the Rdl GABAA receptor. We identify here the relevant Rdl-containing cells as PDF-expressing small and large ventral lateral neurons (LNvs) of the circadian clock. LNv activity regulates total sleep as well as the rate of sleep onset; both large and small LNvs are part of the sleep circuit. Flies mutant for pdf or its receptor are hypersomnolent, and PDF acts on the LNvs themselves to control sleep. These features of the Drosophila sleep circuit, GABAergic control of onset and maintenance as well as peptidergic control of arousal, support the idea that features of sleep-circuit architecture as well as the mechanisms governing the behavioral transitions between sleep and wake are conserved between mammals and insects.

Inhibitory neurotransmission and olfactory memory in honeybees

In insects, gamma-aminobutyric acid (GABA) and glutamate mediate fast inhibitory neurotransmission through ligand-gated chloride channel receptors. Both GABA and glutamate have been identified in the olfactory circuit of the honeybee. Here we investigated the role of inhibitory transmission mediated by GABA and glutamate-gated chloride channels (GluCls) in olfactory learning and memory in honeybees. We combined olfactory conditioning with injection of ivermectin, an agonist of GluCl receptors. We also injected a blocker of glutamate transporters (L-trans-PDC) or a GABA analog (TACA). We measured acquisition and retention 1, 24 and 48 h after the last acquisition trial. A low dose of ivermectin (0.01 ng/bee) impaired long-term olfactory memory (48 h) while a higher dose (0.05 ng/bee) had no effect. Double injections of ivermectin and L-trans-PDC or TACA had different effects on memory retention, depending on the doses and agents combined. When the low dose of ivermectin was injected after Ringer, long-term memory was again impaired (48 h). Such an effect was rescued by injection of both TACA and L-trans-PDC. A combination of the higher dose of ivermectin and TACA decreased retention at 48 h. We interpret these results as reflecting the involvement of both GluCl and GABA receptors in the impairment of olfactory long-term memory induced by ivermectin. These results illustrate the diversity of inhibitory transmission and its implication in long-term olfactory memory in honeybees.