How to study the neuronal mechanisms underlying chemosensory coding and learning in herbivorous insects

H. Mustaparta

Department of Biology, Neuroscience Unit, Norwegian University of Science and technology, Trondheim, Norway.

We are searching for the neuronal mechanisms underlying the behavior of herbivorous insects, heliothine moths, in their interactions with plants. Host location and selection is to a large extent based on olfaction and taste. Olfactory learning also plays an important role in this process. How the information is detected by the receptor neurons and processed in the neuronal network in the brain leading to behavioral responses, is studied by the use of chemical analyses, electrophysiological recordings, morphological tracing of the neuronal pathways and behavioral experiments.

Separation of naturally produced plant volatiles by two-column gas chromatography linked to electrophysiological recordings form single olfactory receptor neurons is suitable for identifying the active compounds. This is followed by chemical identification of these compounds using linked gas chromatography with mass-spectrometry and verification by retesting authentic materials on the sensory neurons. The identi.ed compounds are considered as biologically relevant odorants for the insect species. They are then used to study how the olfactory information is processed in the brain, by physiological and morphological characterization of inter neurons, using intracellular recordings combined with fluorescent staining. This is followed by confocal laser scanning microscopy and 3-D reconstruction of the neurons and the innervated brain structures in the software AMIRA. Taste receptor neurons and their projections are studied by extracellular recordings combined with fluorescent staining to trace the taste pathway in the central nervous system. The connection between the olfactory and taste pathways being the pathways of the conditioned and unconditioned information, respectively, in appetitive learning is searched for by the use of intracellular recordings and staining. By placing the various identi.ed neurons of each preparation in an averaged brain model the neuronal network can eventually be described, like in the honeybee brain (Krofczik et al 2004). Behavioral experiments, carried out to elucidate the importance of the chemical information and learning ability, include conditioning of the proboscis extension response, observation of walking and flight behavior as well as egg laying performance.

Paper presented at Measuring Behavior 2005 , 5^th International Conference on Methods and Techniques in Behavioral Research, 30 August - 2 September 2005, Wageningen, The Netherlands.

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