Olfactory Protocerebral Pathways Processing Sex Pheromone and Plant Odor Information in the Male Moth Agrotis segetum HONG LEI, SYLVIA ANTON,AND BILL S. HANSSON * Department of Ecology, Lund University, Lund SE-22362, Sweden ABSTRACT We investigated protocerebral processing of behaviorally relevant signals in the turnip moth, Agrotis segetum. Single neurons were studied both physiologically and morphologically using intracellular recording techniques. In moth pheromone communication systems, the presence of the complete, female-produced pheromone blend is necessary for male attraction. We predicted that more protocerebral neurons, compared with AL, would display blend interactions. However, only a few protocerebral neurons responded differently to the blend than could be deduced from the response to single components. The majority of the pheromone-sensitive protocerebral neurons identified in this study responded to the major pheromone component. In coding time,most AL neurons can follow a 5-Hz odor stimulus, whereas most protocerebral neurons failed at higher frequencies than 1 Hz. The majority of neurons that responded to the odorants tested innervated one or both of the protocerebral lateral accessory lobes. If only one ofthese was innervated,then the innervation always displayed a varicose appearance, suggesting a presynaptic function. Thus, information seems to be transferred from other protocerebral areas to the lateral accessory lobes. Into these, descending neurons sent smooth, postsynaptic branches. A majority of the neurons inner- vating the superior medial protocerebrum were found to display single-component specificity. Few additional correlations between odor specificity and structural characteristics were apparent. J. Comp. Neurol. 432:356 –370, 2001. © 2001 Wiley-Liss,Inc. Indexing terms: neuroethology; insect; olfaction; protocerebrum Olfactory cues are used by both vertebrates and inver- tebrates in many behavioralcontexts relevant to repro- duction and survival (for review, see Hildebrand and Shepherd,1997).Due to a well-studied behavioral back- ground, identified key stimuli, and a relatively accessible olfactory neural network,insects have provided a favor- able system for studies ofolfactory function.In moths, studies of the primary olfactory center, the antennal lobe (AL), have produced large amounts of data concerning, e.g., neural circuits, response specificity, odor coding, and mechanisms of integration (for reviews, see Christensen and Hildebrand, 1987; Hansson, 1995, Hildebrand, 1996; Hansson and Christensen, 1999).However,when follow- ing the neural projections from the AL to higher centers, the olfactory processing pathways become very complex and are still unknown to a large extent. Communication via female-produced sex pheromones plays a vital role in moths. The female usually emits a blend of components,each forming a vitalcue for male attraction. An incomplete synthetic blend attracts signif- icantly fewer males than a complete blend,and single components often lack activity. The representation of the blend gestalt in the moth nervous system has been a major issue in research into insect olfaction (Hansson and Chris- tensen, 1999). Antennal lobe neurons have been shown to display so-called blend specificity, i.e., responding to the full blend in a way that cannot be predicted from the response to single components (see, e.g., Wu et al., 1996). Grant sponsor: Swedish Natural Science Research Council. *Correspondence to: Professor Bill S. Hansson, Department of Ecology, Lund University, Lund SE-22362, Sweden. E-mail: bill.hansson@ekol.lu.se Received 8 September 2000; Revised 20 November 2000; Accepted 10 January 2001 THE JOURNAL OF COMPARATIVE NEUROLOGY 432:356–370 (2001) ©2001 WILEY-LISS, INC.