The third day of the conference was kicked of by a plenary lecture by Leslie Griffith from Brandeis. As the title suggests, Leslie's lab works on courtship conditioning in flies. This is a paradigm where naive males encounter a mated female. Mated females are not receptive and the male learns to associate the rejection with the smell of mated females. Upon further encounters, the male will avoid courting mated females but does not suppress courtship towards virgin females. This is a very well-established, classic learning paradigm in the invertebrate learning world and a lot is known about how the brain does it.
For instance, it is know that only the chemical cues are required for the learning to occur. The anatomical circuits required for the learning are also well mapped out. Leslie told us that training was mediated by a set of neurons in the antennal lobes and that memory takes place one synapse downstream, in the mushroom-bodies.
The olfactory cue leading to suppression, i.e. the active component of the unconditioned stimulus (US) is contained in the hydrocarbon pheromones on the surface of the female. In fact, it's a component that gets transferred from the mating male to the female during copulation - cis Vaccenyl Acetate (cVA). In other word, during copulation, males transfer a compound onto the female which later is the active compound involved in other males learning that this female is actually already mated and doesn't want to mate again. Ponder the evolutionary aspects of this for a while!
cVA is synthesized in the male ejaculatory bulb and is transferred to the female in the ejaculate and when a male courts the mated female later, the female extends its ovipositor and effectively uses cVA as "mace". Interestingly, cVA together with food odors acts in exactly the opposite way: it is attractive for males. Only with a female does it act aversively. How can that be? Flies express two olfactory receptors specific for cVA (Or65a and Or67d). Which of these mediates the two different functions? It turned out that only Or65a mediates cVA-dependent courtship suppression, while Or67d appears to mediate the appetitive effect of cVA. But how does the brain know which of the two receptors to use? This is currently ongoing and one important aspect seems to be GABAergic presynaptic inhibition of Or65a olfactory receptor neurons.