Olfaction in Mosquitoes -Neuroanatomy and Electrophysiology of the Olfactory System

Ghaninia, M. 2007. Olfaction in Mosquitoes: Neuroanatomy and Electrophysiology of the Olfactory System. Doctoral dissertation. ISSN 1652-6880, ISBN 978-91-576-7392-3 Female mosquitoes are vectors of diseases, affecting both livestock and humans. The host-seeking and identification behaviors of mosquitoes are mediated mainly by olfactory cues. The peripheral olfactory organs of mosquitoes which perceive olfactory cues are the antennae and maxillary palps. These appendages bear numerous hair shaped structures, sensilla, in which olfactory receptor neurons (ORNs) are housed. The ORNs detect and discriminate various odorant molecules and send information regarding odor quality, quantity and spatio-temporal patterns to the central olfactory system in the brain for further analysis. The first goal of this study was to investigate the neuroanatomy of the mosquito central olfactory system. Using different staining techniques, the neuronal architecture of the deutocerebrum as well as 3D reconstructions of antennal lobe (AL) glomeruli were depicted for both sexes of the Afrcian malaria mosquito, Anopheles gambiae and the yellow fever mosquito, Aedes aegypti. To study how mosquitoes detect olfactory cues, single sensillum recordings (SSRs) were performed, which allowed me to investigate electrophysiological properties of individual ORNs housed in four morphological types of the most abundant olfactory sensilla, s. trichodea. I was able to identify 11 functional types which their ORNs displayed distinct responses to a set of compounds. As part of this study, axons of functionally defined ORNs were traced by neurobiotin to indicate which glomeruli they targeted. This resulted in a functional map of AL glomeruli. The map indicated that different functional types of ORNs converged onto different spatially fixed glomeruli. My next step was to identify novel biologically active compounds for the ORNs using gas chromatography coupled SSRs (GC-SSRs). Headspace odors from different human body parts, i.e. armpit, feet and trunk regions as well as from a plant used as a mosquito repellent (Nepeta faassenii) were collected, extracted and eventually injected onto the GC-column. I found that some of the extract components elicited responses in previously defined ORNs as well as in ORNs of the intermediate sensilla. Some of the compounds, which were subsequently identified by using GC-mass spectrometry (GC-MS) were heptanal, octanal, nonanal and decanal.