of 9 PRINCIPLES OF NEUROBIOLOGY CHAPTER 6 : OLFACTION , TASTE , AUDITION , AND SOMATOSENSATION

Although considerable effort has been directed toward developing an understanding of vertebrate olfaction (see Sections 6.1–6.10), the complexity of the vertebrate olfactory system presents many challenges. This complexity is in part due to the large number of processing channels in this system: the mouse genome encodes more than a thousand odorant receptors, and the mouse olfactory bulb has more than two thousand glomeruli (see Sections 6.4 and 6.8). The olfactory systems of insects are numerically simpler: the Drosophila genome encodes fewer than sixty olfactory receptors, and the insect analog of the olfactory bulb, the antennal lobe, has approximately fifty glomeruli. Insect and vertebrate olfactory systems are similarly organized (for instance, olfactory receptor neurons expressing a single olfactory receptor type all project to the same glomeruli in the olfactory bulb or antennal lobe in vertebrates and insects, respectively) and use similar strategies to process and represent olfactory information (see Section 6.13). These similarities, along with the numerical simplicity of insect olfactory systems compared to vertebrate olfactory systems, have made insects fruitful model organisms for understanding olfaction throughout the animal kingdom. More broadly, studies of insect olfaction have provided insight into the more general issues of how neural circuits process and represent complex sensory information. Until the 2000s, our understanding of insect olfactory system function was based principally on neurophysiological studies in locusts, moths, and bees. Because of their larger body sizes and the correspondingly larger sizes of their neurons, these insect species were more amenable to electrophysiological study than smaller species such as the fruit fly Drosophila, which has such small neuronal cell bodies that studies using patch clamp techniques (see Section 13.21 and Box 13–2) were traditionally difficult or impossible to perform. However, the genetic tools available in Drosophila (see Section 13.2) and the stereotyped identities of its neurons made it the preferred organism for developmental, anatomical, and functional imaging studies of the olfactory system and motivated the development of methods to record intracellularly from Drosophila neurons. The first patch clamp recordings from neurons in the Drosophila olfactory system were obtained by Rachel Wilson and her colleagues in the early 2000s. Since then, Wilson and her colleagues have published a series of exemplary papers examining the mechanisms and principles of information processing in the early Drosophila olfactory system. Remarkable work from numerous research groups, including Wilson’s, on the development, organization, and function of the Drosophila olfactory system, has shed light on fundamental principles and mechanisms of sensory processing. The present paper explores how olfactory information is transformed by the Drosophila antennal lobe and how this transformation may facilitate reliable and accurate representation of odors.