A genetic screen for olfactory habituation mutations in Drosophila: analysis of novel foraging alleles and an underlying neural circuit.

Habituation is a form of non-associative learning that enables animals to reduce their reaction to repeated harmless stimuli. When exposed to ethanol vapor, Drosophila show an olfactory-mediated startle response characterized by a transient increase in locomotor activity. Upon repeated exposures, this olfactory startle attenuates with the characteristics of habituation. Here we describe the results of a genetic screen to identify olfactory startle habituation (OSH) mutants. One mutation is a transcript specific allele of foraging (for) encoding a cGMP-dependent kinase. We show this allele of for reduces expression of a for-T1 isoform expressed in the head and functions normally to inhibit OSH. We localize for-T1 function to a limited set of neurons that include olfactory receptor neurons (ORNs) and the mushroom body (MB). Overexpression of for-T1 in ORNs inhibits OSH, an effect also seen upon synaptic silencing of the ORNs; for-T1 may therefore function in ORNs to decrease synaptic release upon repeated exposure to ethanol vapor. Overall, this work contributes to our understanding of the genes and neurons underlying olfactory habituation in Drosophila.

A model study on the circuit mechanism underlying decision-making in Drosophila.

Previous elegant experiments in a flight simulator showed that conditioned Drosophila is able to make a clear-cut decision to avoid potential danger. When confronted with conflicting visual cues, the relative saliency of two competing cues is found to be a sensory ruler for flies to judge which cue should be used for decision-making. Further genetic manipulations and immunohistological analysis revealed that the dopamine system and mushroom bodies are indispensable for such a clear-cut or nonlinear decision. The neural circuit mechanism, however, is far from being clear. In this paper, we adopt a computational modeling approach to investigate how different brain areas and the dopamine system work together to drive a fly to make a decision. By developing a systems-level neural network, a two-pathway circuit is proposed. Besides a direct pathway from a feature binding area to the motor center, another connects two areas via the mushroom body, a target of dopamine release. A raised dopamine level is hypothesized to be induced by complex choice tasks and to enhance lateral inhibition and steepen the units' response gain in the mushroom body. Simulations show that training helps to assign values to formerly neutral features. For a circuit model with a blocked mushroom body, the direct pathway passes all alternatives to the motor center without changing original values, giving rise to a simple choice characterized by a linear choice curve. With respect to an intact circuit, enhanced lateral inhibition dependent on dopamine critically promotes competition between alternatives, turning the linear- into nonlinear choice behavior. Results account well for experimental data, supporting the reasonableness of model working hypotheses. Several testable predictions are made for future studies.

Partial unilateral lesions of the mushroom bodies affect olfactory learning in honeybees Apis mellifera L.

The mushroom bodies (MBs) are central structures in the insect brain that have been associated with olfactory learning and memory. Here we used hydroxyurea (HU) to treat honeybee larvae and induce partial MB ablations at the adult stage. We studied olfactory learning in honeybees with unilateral loss of the median calyces of their MBs and compared their ability to solve different forms of olfactory discrimination. When odorants were delivered in a side-specific manner, ablated bees could not solve either discrimination of the unambiguous problem (Paradigm 1: A+, B- on one antenna, C+, D- on the other; A+B-/C+D-) whereas they could solve at least one of both discriminations of the ambiguous problem (Paradigm 2: A+B-/A-B+), namely that proposed to their intact brain side. Non-ablated bees could learn side-specific discriminations on both brain sides. When odorants were delivered simultaneously to both antennae (Paradigm 3: A+B-C+D-), HU-ablated bees learned slower than HU-normal bees. Thus, in all three paradigms, the unilateral loss of a median calyx affected olfactory learning. We propose that the MBs are required for solving elemental olfactory tasks whose complexity is increased by the number of stimuli involved and that MB ablations could have an effect on the inhibition of information exchange between brain hemispheres.

Centrophobism/thigmotaxis, a new role for the mushroom bodies in Drosophila.

Insects, like vertebrates, exhibit spatially complex locomotor activity patterns when foraging or navigating. Open field studies recently showed that Drosophila avoids central zones and stays at the periphery, an effect that can be interpreted as centrophobism and/or thigmotaxis. In this study, we further characterized this phenomenon and studied the responsible underlying neural mechanisms. The implication of the Drosophila mushroom bodies (MBs) in olfactory learning and memory processes is well documented. In an open field situation in which fly locomotor activity is recorded by video tracking, we show that center avoidance is greatly diminished in flies with hydroxyurea-ablated MBs, suggesting a new role for these structures. Furthermore, the temperature-sensitive allele of the dynamin gene shibire was expressed in various enhancer-trap P[GAL4] lines, disrupting synaptic transmission in different MB lobes. Specifically blocking the gamma lobes alters centrophobism/thigmotaxis while blocking the alpha/beta lobes does not, suggesting a functional specialization of MB lobes. Drosophila may serve as a new model system for elucidating the genetic and neural bases of such complex phenomena as centrophobism/thigmotaxis.

Habituation of an odorant-induced startle response in Drosophila.

Habituation is a fundamental form of behavioral plasticity that permits organisms to ignore inconsequential stimuli. Here we describe the habituation of a locomotor response to ethanol and other odorants in Drosophila, measured by an automated high-throughput locomotor tracking system. Flies exhibit an immediate and transient startle response upon exposure to a novel odor. Surgical removal of the antennae, the fly's major olfactory organs, abolishes this startle response. With repeated discrete exposures to ethanol vapor, the startle response habituates. Habituation is reversible by a mechanical stimulus and is not due to the accumulation of ethanol in the organism, nor to non-specific mechanisms. Ablation or inactivation of the mushroom bodies, central brain structures involved in olfactory and courtship conditioning, results in decreased olfactory habituation. In addition, olfactory habituation to ethanol generalizes to odorants that activate separate olfactory receptors. Finally, habituation is impaired in rutabaga, an adenylyl cyclase mutant isolated based on a defect in olfactory associative learning. These data demonstrate that olfactory habituation operates, at least in part, through central mechanisms. This novel model of olfactory habituation in freely moving Drosophila provides a scalable method for studying the molecular and neural bases of this simple and ubiquitous form of learning.