Mated female European grapevine moths find the scent of grapevine berries irresistible, since it lures them to a feast for their larvae. This attraction of moth to vine, which results in grapes infested with moth larvae, has incurred the wrath of wine producers worldwide. But it has also garnered the interest of scientists in Bordeaux. Unlike winery owners, however, Sylvia Anton and her colleagues see these moths not as pests, but as opportunities to study how odour is coded in insect brains(p. 1147). How is the scent of grapevine berries processed in the moth's primary olfactory centre,the antennal lobe?
To begin answering this question, Anton wanted to identify where plant odour information is processed; she needed a three-dimensional map of the moth antennal lobe. To create this atlas, Anton's doctoral student, Ingwild Masante-Roca, had to identify the antennal lobe's individual glomeruli,spherical structures that receive input from olfactory neurons. Using specialized software to record the outlines of individual glomeruli manually from scanned optical sections of a single moth brain, Masante-Roca painstakingly created an antennal lobe atlas. She also compared the shapes and positions of all glomeruli in each of three brains to identify landmark glomeruli to serve as reference points.
With map in hand, the team could now identify individual glomeruli that house the dendritic branches of antennal lobe projection neurons, which output to higher integration centres in the brain. First, Masante-Roca exposed mated females' antennae to various grapevine berry odour components and recorded the intracellular responses of the projection neurons. Given the tiny size of the grapevine moth brain, the smallest of any lepidopteran species studied thus far, this was not an easy feat: Masante-Roca had to `expose the brain of the insect, remove the muscles in the head and keep the insect alive during this procedure', explains Anton. After the recordings, the team stained the projection neurons, dissected the brain, and referenced the 3D map to trace the glomerular destinations of the projection neurons.
Tracing the paths of output projection neurons revealed a complicated story. Other studies suggest that odour information entering the antennal lobes input in a direct way: receptor neurons that respond to a particular plant odour compound target a few identifiable glomeruli. This study, however,shows that there is no simple match between what goes into the antennal lobe and what comes out. Odour information leaving the antennal lobe is represented in a complex array of projection neurons: projection neurons that respond to a particular odour compound can target completely different glomeruli, while the projection neurons targeting the same glomeruli can respond to a variety of odours. Anton explains that such a pattern must be the product of complex intraglomerular interactions within the antennal lobe.
Noting that a moth's reaction to grapevine berry scent depends on mating status and environmental conditions, Anton wonders how odour coding inside the moths' brains changes under different conditions. With the 3D atlas now available, the team is poised to make such comparisons. What they find may explain how connection patterns between neurons and glomeruli differ between a moth that finds grapevine berry aroma alluring and one that does not.