Sammendrag
We are studying chemosensory coding in Heliothis virescens, a devastating agricultural pest insect. The goal in the present study is to understand how the primary olfactory center, the antennal lobe, process plant odor information and how the information is further mediated to higher olfactory centers in the brain. The neural network of the antennal lobe is formed by synaptic connections in the glomeruli between the receptor neuron terminals, local interneurons and projection neurons. Also terminals of modulatory neurons, GABAergic as well as peptidergic innervate the glomeruli in H. virescens (Berg et al. 2007 Cell Tiss Res 327:385-398, 2009 Cell Tiss Res 335:593-605). By intracellular recordings we characterize antennal lobe interneurons physiologically by testing for sensitivity to antennal stimulation with biologically relevant plant odorants. Receptor neurons, classified by the use of gas chromatography linked to single cell recordings, are narrowly tuned to one primary odorant and respond weaker to a few structurally related odorants (Røstelien et al. 2005 Chem Senses 30:443-461). The minimal overlap of the molecular receptive ranges of the different receptor neuron types indicates that the plant odor information is mediated to the antennal lobe by a labeled-line mechanism. In the present study the test protocol includes primary plant odorants and mixtures as well as pheromone components. The antennal lobe neurons are stained with fluorescent dyes for visualization in confocal laser scanning microscope followed by 3-dimensional reconstruction. The neurons are morphologically classified according to their innervations of glomeruli (uni- or multi-glomerular) and in which of the four antenno-cerebral tracts the axons project to the mushroom bodies (involved in olfactory learning) and the premotoric area in lateral protocerebrum (Rø et al. 2007 J Comp Neurol 500:658-675). The neurons identified in individual brains are integrated into the standard H. virescens brain atlas (Kvello et al. submitted). We here present physiologically and morphologically classified antennal lobe neurons integrated in the standard brain atlas. Most neurons described by this method belong to uniglomerular projection neurons which responded with excitation or inhibition to several of the primary odorants and blends. The results suggest interglomerular excitation as well as inhibition as input to the projection neurons in this moth species. The project was supported by the Norwegian Research Council (nr. 170510).
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