Response Characteristics of an Identified, Sexually Dimorphic Olfactory Glomerulus

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Partitioning of synaptic neuropil into glomeruli is a common feature of primary olfactory centers in most animal species. The functional significance of glomeruli, however, is not yet well understood. The present study is part of our effort to test the hypothesis that each glomerulus is a functional unit dedicated to processing information about a particular odorant or attribute of odor molecules and that the glomerular array constitutes a map of "odor space." We investigated the physiological and morphological features of uniglomerular projection neurons (PNs) associated with an identified glomerulus in each antennal lobe of the female sphinx moth, Manduca sexta. This "lateral large female glomerulus" (latLFG) is sexually dimorphic and therefore may play a female-specific role, such as processing of information about one or more odorants important for orientation of a female to host plants for oviposition. Together with the medial LFG (medLFG), the latLFG resides outside the array of spheroidal ordinary glomeruli, near the entrance of the antennal (olfactory) nerve. Each LFG is innervated by four to five PNs. Using intracellular recording and staining, we examined the responses of latLFG-PNs to odorants that represent major classes of volatiles released by host plants of M. sexta. All latLFG-PNs were excited when the ipsilateral antenna was stimulated with low concentrations of the monoterpenoid linalool. Dose-response analysis showed that neither other monoterpenoids nor representatives of other classes of host plant volatiles were similarly stimulatory to latLFG-PNs. These findings are consistent with the idea that each glomerulus has a characteristic, limited molecular receptive range.