Both first sessions of the second day here at the Janelia Farm Conference “Learning and Memory: A Synthesis of Flies and Honeybees” were about the neural networks underlying learning and memory. At 9am, Andreas Thum kicked the double whammie off with his presentation on larval learning in Drosophila. He told us that fly larve have exactly 21 olfactory receptor neurons and 5 types of intrinsic antennal lobe neurons. 21 uniglomerular projection neurons project to the mushroom-body calyx forming 34 glomeruli. About 200-300 Kenyon cells of embryonic origin make up the mushroom-body. Larval learning relies on the gustatory system to provide either appetitive or aversive stimuli. 90 gustatory receptor neurons project into the subesophageal ganglion. In a so far unknown way, these neurons are connected to the dopaminergic system. Activity in these eurons can substitute aversive stimulation and lead to aversive olfactory learning. 17 dopaminergic neurons on each side of the brain that innervate the mushroom-bodies. Analogous to the dopaminergic neurons, activating the octopaminergic neurons can be used to condition the larvae appetitively. However, ablating all octopaminergic neurons in the brain does not lead to any learning phenotype, suggesting that these neurons are sufficient, but not necessary for appetitive conditioning. There are 3 octopaminergic neurons on each side of the brain that innervate the mushroom-body. Finally, Andreas talked about serotonin. None of the 85 serotonerig in the larval nervous system innervates the mushrom-body and ablating all serotonergic neurons has no effect on larval olfactory learning, neither appetitive nor aversive.

Next up was André Fiala who talked about generalization in Drosophila olfactory learning. He showed that, not unlike such experiments in other animals, differential conditioning leads to less generalization than single odor conditioning. Often generalization happens between chemically similar odorants, but sometimes also between odorants that are chemically vers different. Imaging the neuronal response to odor presentations revealed that even though the different odorants lead to clearly differentiable neuronal responses on the level of olfactory receptor neurons, the (chemically very different) odors that are generalized behaviorally are not very well resolved on the projection neuron level.

The last speaker before the half-time break was Jean-Christophe Sandoz who talked about long term memory in the honeybee. Jean-Christophe studies classical olfactory conditioning of the proboscis extension response (PER). A few pairings of an odor with sucrose leads to a life-long memory trace in the honey bee brain. He first trained the bees with a schedule leading to long-term memory and then dissected their brains and analyzed the volume and the neuronal activity of the glomeruli in their antennal lobes. They found a specific increase in the volume of specific glomeruli and a volume-related increase in the glomerular activity. Moving up the olfactory chain, he next showed us that the volume of the micro-glomeruli in the mushroom-body calyces is increased in the trained but not the control group. Moreover, the microglomerular density in the lip of the calyces increases in a transscription-dependent manner. He next presented imaging results comparing the activity in the glomeruli connected to the mACT and those connected to the lACT. The main difference between these systems appears to be that the m-system codes mainly functional groups, whereas the l-system codes chain length. Finally he turned to the often-neglected lateral horn. They imaged the lateral horn neural activity during odor presentations. They find that also in the lateral horn, different odors lead to activity in different regions for different odors. Interestingly, the two components of the bee alarm pheromone elicit activity in the same regions of the lateral horn.