Sensory Processing of Environmental -CO2 Information in the Insect Brain

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The carbon dioxide (CO2) produced by the respiration of most organisms is a fundamental constituent of their odor. Many insects can sense the concentration of CO2 in the air around them, and they are thought to use that information in vital tasks such as locating food sources (e.g., appropriate plants) and possibly identifying desirable sites for oviposition (egg laying). Whereas considerable research has focused on how the sensory information about CO2 is acquired, little is known about how that information is processed in the central nervous system (CNS). In order to address this issue, this project will take advantage of an experimentally favorable and extensively studied model plant-eating insect, the moth Manduca sexta, using methods already developed and in practice in this laboratory. Pilot studies that led to this project suggested that a particular group of sensory receptor cells, situated in a sensory organ (the labial-palp pit organ, LPO) recessed in a deep invagination on each labial palp (a mouthpart) of the adult moth, are specialized to detect and quantitatively assess CO2 in the air around that organ. Anatomical evidence shows that although the LPO receptor cells are located in that mouthpart, they send their axons to the antennal lobe (AL), the primary-olfactory center resembling the vertebrate olfactory bulb, in the insect's brain. Furthermore, our pilot work clearly shows that some of the central neurons in the AL receive and process synaptic inputs, and thus information about CO2, from the LPO receptor cells. Thus, the project centers on recording of the electrical signals (coded sensory information about CO2) generated by LPO receptor cells and the responding neurons in the AL that receive inputs from the LPO sensory cells. In addition, the morphology of those AL neurons is studied by means of intracellular staining in order to reveal the types and patterns of branching of neurons contributing to processing of CO2 information. This research is the first study of central processing of environmental-CO2 information, and it promises to add significantly to our understanding of CNS olfactory mechanisms in insects. CO2, temperature, and humidity are environmental variables of importance to insects, and the sensory systems for those three kinds of stimuli share certain physiological properties. Therefore it may be that the processing of CO2 information has aspects in common with the processing of information about temperature and/or humidity, such that general principles of information processing in the brain could emerge from the proposed project. These studies are expected to lead to understanding of how information about environmental CO2 is first processed in the AL of the insect brain. Manduca sexta is an excellent experimental animal for this research because it: (a) possesses a highly developed CO2-detecting organ; (b) is large and hence ideal for neurophysiological studies, easily reared in the laboratory, and favorable for experimentation at the molecular, physiological, and organismal levels; (c) has yielded a wealth of information about the neurobiology of insect olfaction and the neuroethology of odor-guided behavior, much of it through previous research done in this laboratory; and (d) is an agricultural pest which, although not very important economically, exhibits behavior and sensory mechanisms that are similar to those of important, herbivorous pest insects. Thus, the findings from this project, complementing other research under way in this laboratory, will benefit ongoing basic research aimed at understanding how nervous systems analyze, recognize, and respond to odors, and should also yield insights that will be useful in the agricultural arena for designing new strategies for protection of crop plants from insect predation. Finally, in addition to its scientific impact and potential benefit to agriculture, this project will contribute to the research training of a postdoctoral associate (the key investigator) and also involve one or more undergraduate students in aspects of the studies.