MULTITASKING IN THE OLFACTORY SYSTEM - CONTEXT-DEPENDENT RESPONSES TOODORS REVEAL DUAL GABA-REGULATED CODING MECHANISMS IN SINGLE OLFACTORY PROJECTION NEURONS

Studies of olfaction have focused mainly on neural processing of infor mation about the chemistry of odors, but olfactory stimuli have other properties that also affect central responses and thus influence behav ior. In moths, continuous and intermittent stimulation with the same o dor evokes two distinct flight behaviors, but the neural basis of this differential response is unknown. Here we show that certain projectio n neurons (PNs) in the primary olfactory center in the brain give cont ext-dependent responses to a specific odor blend, and these responses are shaped in several ways by a bicuculline-sensitive GABA receptor. P harmacological dissection of PN responses reveals that bicuculline blo cks GABA(A)-type receptors/chloride channels in PNs, and that these re ceptors play a critical role in shaping the responses of these glomeru lar output neurons. The firing patterns of PNs are not odor-specific b ut are strongly modulated by the temporal pattern of the odor stimulus . Brief repetitive odor pulses evoke fast inhibitory potentials, follo wed by discrete bursts of action potentials that are phase-locked to t he pulses. In contrast, the response to a single prolonged stimulus wi th the same odor is a series of slow oscillations underlying irregular firing. Bicuculline disrupts the timing of both types of responses, s uggesting that GABA(A)-like receptors underlie both coding mechanisms. These results suggest that glomerular output neurons could use more t han one coding scheme to represent a single olfactory stimulus. Moreov er, these context-dependent odor responses encode information about bo th the chemical composition and the temporal pattern of the odor signa l. Together with behavioral evidence, these findings suggest that cont ext-dependent odor responses evoke different perceptions in the brain that provide the animal with important information about the spatiotem poral variations that occur in natural odor plumes.