Learning Blocked by Dark-flashes in Mushroom Body-less Flies

Theodor-Boveri-Institut für Biowissenschaften (Biozentrum), Lehrstuhl für Genetik, Am Hubland, 97074 Würzburg, Germany

Mushroom bodies (MBs) in flies and bees are involved in odour discrimination learning (Heisenberg et al., 1985; deBelle and Heisenberg, 1994; Hammer and Menzel, 1995). In Drosophila this conclusion is drawn from two lines of evidence:

1. Flies with genetic or ontogenetic lesions in the MBs are impaired in this property.
   
2. Several genes of the cAMP signalling pathway that were shown to be required for learning and memory, are preferentially expressed in the MBs.

Until now, it is not clear how the MBs contribute to this behavioral plasticity.

Surprisingly, in visual pattern discrimination learning, flies without MBs learn as well as WT-flies. This is shown with tethered flying Drosophila in a so-called flight simulator, in which the angular velocity of a cylindrical panorama surrounding the fly is made to be controlled by the fly’s yaw torque around its vertical body axis. This enables the fly to stabilize the rotational movements of the panorama (i.e. to fly straight) and to adjust certain flight directions with respect to particular visual landmarks.

Flies can be trained operantly or classically. In operant conditioning the fly learns to control the appearance of a reinforcer (e.g. beam of infrared light) by its choice of flight direction with respect to the angular positions of visual patterns at the arena wall. This paradigm is considered operant since no learning is observed if in so-called "replay experiments" the whole sequence of pattern motion during the training session is precisely recorded and afterwards played back to the same (or another) fly in open loop (i.e. the fly has no control of the stimulus). The classical conditioning procedure uses the same conditioned response and a similar conditioned stimulus as the operant one. In the training procedure the flight simulator mode is interrupted and two stable pattern orientations are presented and are interchanged every 3 s. One of them is combined with the reinforcer. In both procedures learning is tested in the same way: The apparatus is switched to the flight simulator mode and the fly's pattern preference is recorded without reinforcement.

In the classical procedure, a subtle modification of the reinforcement has a dramatic effect on MB-less flies. A brief dark period (220 ms) every 3.2 s during or just after the pattern displacement, which does not interfere with the learning performance of the control flies completely blocks learning in animals with ontogenetically ablated MBs. This role of the MBs in bridging short dark periods in pattern discrimination learning is investigated. Is the spatio-temporal continuity between the two pattern orientations required for learning to occur in the MB-less flies or is the flies' learning performance disrupted by a dark period at any time? Do dark periods in MB-less flies impair learning also with operant conditioning? So far, the "dark flash effect" has been demonstrated only in flies with ontogenetic lesions. Do MB mutants show it as well? Moreover, we will also try to block the MBs by combining GAL4-lines expressing the GAL4 in the MBs, with a UASGal4-tetanus toxin transgene.

References:

deBelle SJ and Heisenberg M (1994): Associative Odor Learning in Drosophila Abolished by Chemical Ablation of Mushroom Bodies. Science Vol.263:692-695

Hammer M and Menzel R (1995): Learning and Memory in the Honeybee. J Neurosci 15(3):1617-1630

Heisenberg M, Borst A, Wagner S, and Byers D (1985): Drosophila mushroom body mutants are deficient in olfactory learning. J. Neurogenet. 2:1-30