Neurobiology Select

Understanding how neural circuits in the brain respond to external cues resulting in behavioral changes is an important aspect of neuroscience research. A valuable model in this endeavor is the neural circuitry governing courtship behavior in the fruit fly Drosophila melanogaster. New work provides detailed insights into how factors such as sex pheromones direct the intricate courtship behavior of the fly. When male flies are exposed to a female that has already mated, their courtship response to all females is suppressed. Interestingly, this general inhibition of courtship does not occur upon exposure to virgin females. Ejima et al. (2007) uncovered the basis of this aversion phenomenon. First, they showed that the general suppression of courtship in male flies can be attributed to chemosensory molecules because hydrocarbons extracted from the cuticle of mated females, but not virgin females, caused general suppression of courtship activity in males. Analysis of the cuticular hydrocarbon composition revealed one difference: the known male-specific pheromone 11-cis-vaccenyl acetate (cVA) was present in the profile of mated females but not virgin females. cVA is a major component of male cuticular hydrocarbons and is not synthesized in females but is thought to be transferred to them during copulation. Although the olfactory receptor Or67d has been implicated in the response to cVA, Eijima et al. show that the aversive effect of this pheromone is mediated by olfactory neurons expressing the receptor Or65a. The Or65a olfactory receptor neurons are present in the trichoid sensilla (special sensory organs that are located on the fly antenna). The authors propose that this response probably evolved to dissuade males from mating with females already carrying the sperm of another fly, thus protecting the reproductive investment of the first male. Taking an electrophysiological approach, van der Goes van Naters and Carlson (2007) examined the specificity of olfactory receptors expressed in the trichoid sensilla of fly antennae. There are three subtypes of trichoid sensilla, T1, T2, and T3, that contain olfactory neurons expressing one, two, or three different types of olfactory receptor, respectively. The authors first determined that these neu-rons respond to fly odors but not food odors. They then established an experimental system to detect response to compounds at close range given that flies are in intimate contact during courtship. The authors prepared cuticular extracts from male flies as an odor source and found that the three types of trichoid sensilla responded with different sensitivity depending on …