COMBINATORIAL ODOR DISCRIMINATION IN THE BRAIN - ATTRACTIVE AND ANTAGONIST ODOR BLENDS ARE REPRESENTED IN DISTINCT COMBINATIONS OF UNIQUELYIDENTIFABLE GLOMERULI

The rules governing the central discrimination of odors are complex an d poorly understood, but a growing body of evidence supports the hypot hesis that olfactory glomeruli may represent functionally distinct cod ing modules in the brain. Testing this hypothesis requires that both t he functional characteristics and the spatial position of the glomerul us under study be uniquely identifiable. To address these questions, w e examined a specialized array of glomeruli (the macroglomerular compl ex; MGC) in the antennal lobe of male moths that receives input from o lfactory receptor cells tuned specifically to female-released odorants that either promote upwind flight (conspecific sex pheromones) or inh ibit it (interspecific antagonists). By using a three-dimensional reco nstruction method based on high-resolution laser-scanning confocal mic roscopy, we generated precise spatial maps of the MGC glomeruli in two related noctuid species with similar pheromone chemistry, Heliothis v irescens and Helicoverpa tea. To determine the breadth of tuning of in dividual MGC glomeruli in processing information about these social si gnals, we used intracellular recording and staining methods to examine the responses of projection (output) neurons that innervate MGC glome ruli and that each project an axon to higher integrative centers. In b oth species, a close correspondence was found between the odor specifi city of the projection neurons and the glomerulus (or glomeruli) suppl ied by them. The binary blend of pheromone components for each species was represented by neural activity in only two distinct glomeruli in both H. virescens and H. tea. Odorants that antagonize upwind flight w hen they are added to the respective pheromonal blends evoked excitato ry activity in output neurons restricted to a third glomerulus in the MGCs of both species. In summary, these results suggest that the selec tive activation of different combinations of functionally distinct MGC glomeruli is a general means for discriminating these specific attrac tant and antagonist chemical signals in the brain. (C) 1998 Wiley-Liss , Inc.