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The Journal of Neuroscience, August 1, 1998, 18(15):5999-6008
Multitasking in the Olfactory System: Context-Dependent Responses
to Odors Reveal Dual GABA-Regulated Coding Mechanisms in Single
Olfactory Projection Neurons
Thomas A.
Christensen,
Brian R.
Waldrop, and
John G.
Hildebrand
Arizona Research Laboratories, Division of Neurobiology, University
of Arizona, Tucson, Arizona 85721
Studies of olfaction have focused mainly on neural processing of
information about the chemistry of odors, but olfactory stimuli have
other properties that also affect central responses and thus influence
behavior. In moths, continuous and intermittent stimulation with the
same odor evokes two distinct flight behaviors, but the neural basis of
this differential response is unknown. Here we show that certain
projection neurons (PNs) in the primary olfactory center in the brain
give context-dependent responses to a specific odor blend, and these
responses are shaped in several ways by a bicuculline-sensitive GABA
receptor. Pharmacological dissection of PN responses reveals that
bicuculline blocks GABAA-type receptors/chloride channels
in PNs, and that these receptors play a critical role in shaping the
responses of these glomerular output neurons. The firing patterns of
PNs are not odor-specific but are strongly modulated by the temporal
pattern of the odor stimulus. Brief repetitive odor pulses evoke fast
inhibitory potentials, followed by discrete bursts of action potentials
that are phase-locked to the pulses. In contrast, the response to a
single prolonged stimulus with the same odor is a series of slow
oscillations underlying irregular firing. Bicuculline disrupts the
timing of both types of responses, suggesting that
GABAA-like receptors underlie both coding mechanisms. These
results suggest that glomerular output neurons could use more than one
coding scheme to represent a single olfactory stimulus. Moreover, these
context-dependent odor responses encode information about both the
chemical composition and the temporal pattern of the odor signal.
Together with behavioral evidence, these findings suggest that
context-dependent odor responses evoke different perceptions in the
brain that provide the animal with important information about the
spatiotemporal variations that occur in natural odor plumes.
Key words:
information coding; intermittency; GABA receptors; oscillations; olfaction; insect; moth; Manduca sexta
Copyright © 1998 Society for Neuroscience 0270-6474/98/18155999-10$05.00/0
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