John G. HILDEBRAND
ARL Division of Neurobiology, University of Arizona, 611 Gould-Simpson Building, PO Box 210077, Tucson AZ 85721-0077
ln moths and other insects, olfactory information in the environment is detected by antennal receptor cells, the axons of which project into the ipsilateral antennal lobe (AL) in the deutocerebrum of the brain. In the experimentally favorable giant sphinx moth Manduca sexta, the AL comprises a distinctive neuropil with a characteristic array of glomenuli bordered by 3 groups of neuronalsomata. Each antennal olfactory-receptor axon projects to a single glomerulus in the AL neuropil and provides excitatory synaptic input to neurites of AL neurons innervating that glomerulus. In particular, axons of male receptor cells tuned to 2 key components of the female's sex pheromone project to 2 of the 3 identified male-specific glomeruli of the sexually dimorphic macroglomerular complex (MGC). Three main classes of central neurons participate in synaptic processing of olfactory information in the AL glomeruli: local interneurons (LNs), projection neurons (PNs), and centrifugal neurons. LNs typically have multiglomerular arborizations confined to the AL and receive primary-afferent, excitatory synaptic input. PNs may be uni- or multiglomerular and have axons that project in characteristic patterns to olfactory foci in the protocerebrum. Much has been learned about the physiology and functions of AL LNs and PNs, as well as their synaptic interactions, by means of intracellular recording and staining methods. This presentation will focus on the analysis and representation of the sex-pheromonal mixture and its temporal intermittence by the neuronal circuitry of the MGC. The importance of inhibitory synaptic interactions, apparently mediated by GABAergic LNs, in the generation of characteristic PN responses that encode information about the pheromone will be emphasized. In addition, a prominent example of a centrifugal neuron -- a neuromodulatory serotonergic neuron -- in the AL will be introduced.
This work on the male-specific olfactory subsystem has provided important insights about olfactory mechanisms in insects and about the neuroethology of sex-pheromonal communication. A major challenge now is to understand how other behaviorally significant odors, such as those emitted by hosts (food sources, oviposition sites, etc.), are analyzed in the apparently sexually isomorphic olfactory subsystem. This presentation will conclude with a brief discussion of work-in-progress on the detection and central processing of host-plant odors in Manduca.
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