Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Behavioral Determination of Odor Preference is Coded by the Oscillation Frequency in a Collective Oscillating Network of a Terrestrial Mollusk
School of Pharmaceutical Sciences, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Correspondence to be sent to Yutaka Kirino, e-mail: kirino{at}mayqueen.f.u-tokyo.ac.jp
Key words: acetylcholine, Limax, odor-aversion conditioning, olfactory system, procerebrum
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Introduction |
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 Top Introduction Odor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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The brain receives sensory inputs, processes them and finally produces outputs in the form of behavior. In olfaction, many excellent studies have clarified the initial step of neural processing. However, the next step, i.e. how the meaning of the odor is added to the recognized quality of the chemical odor, is poorly understood. This is a critical problem in brain science, because the meaning of a stimulus finally determines the behavior of the animal. In this paper, we first review the characteristics of the olfactory CNS of the land slug Limax marginatus and related species, focusing on the synchronized network oscillation of membrane potentials of the neurons and then report our recent study on a neural mechanism for determining the aversive meaning of odors in the mollusk.
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Odor-aversion conditioning in the Limax |
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TopIntroductionOdor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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The land slug Limax has a highly developed ability to discriminate and identify odors and can be conditioned aversively (Sahley et al., 1981
). If a slug is subjected to a
paired presentation of the odor of a favorite food and a noxious stimulus such as
electrical shock to the head, or quinidine to the lips, the slug learns the relationship
between the two stimuli and starts to avoid the previously attractive odor. |
Oscillatory activity of the neural network in the Limax olfactory center |
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TopIntroductionOdor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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The procerebrum (PC) is an olfactory center of the gastropod mollusks (Chase, 1985
). The Limax PC is a highly developed
neural network consisting of 
105 neurons that are classified into two
types, bursting (B) and nonbursting (NB) neurons (Gelperin et al., 1989;
Kleinfeld et al., 1994). B
neurons have projections within the cell body layer of the PC while NB neurons have
projections extending to the neuropil layers (Watanabe et al., 1998). The NB neurons receive
direct input from the afferent fibers from the tentacle ganglion, the primary olfactory
center (Inoue et al., 2000).
The projection pattern of NB neurons is topographically organized, potentially enabling
the mapping of information onto a spatial pattern (Kawahara et al., 1997). Furthermore, we have
recently studied odor-evoked activity in individual NB neurons using perforated patch
recordings (Murakami et al.,
2004) and the result suggest that activity of NB neurons may encode odor
identity.
The PC shows regular oscillatory activity at ~0.7 Hz (Gelperin and Tank, 1990;
Kleinfeld et al., 1994;
Kawahara et al., 1997;
Kimura et al., 1998) with a
slight phase difference along the PC, which produces waves propagating from the apical to
basal region (Kleinfeld et al.,
1994;
Kawahara et al., 1997;
Inoue et al., 1998).
Perforated-patch recording revealed that B neurons have Cl–-dependent
excitatory and K+-dependent inhibitory glutamate receptors (Watanabe et al., 1999). On the other
hand, NB neurons only have the latter ones. Our recent study revealed a spatial
difference in the Cl–-dependent periodic depolarizations in B neurons,
which can account for the unidirectional propagation of waves (Watanabe et al., 2003).
Neuronal interactions producing the oscillatory activity are mediated by various
putative transmitters including glutamate, acetylcholine (Watanabe et al., 2001), serotonin and NO
(Gelperin et al., 1993;
Inoue et al., 2001). The
activity level of the bursting neurons determines the oscillation frequency, which can be
increased by action of acetylcholine on B neurons (Watanabe et al., 2001).
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What is the role of the oscillation in the PC in information processing? |
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TopIntroductionOdor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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One of the major issues with the neural oscillation is what role the oscillation could have in information processing. There are reports that the PC oscillation is involved in odor discrimination (Teyke and Gelperin, 1999
). We also proposed a phase-dependent filtering of olfactory information
(Inoue et al., 2000).
However, one way to answer this question more clearly is to identify the neural pathway
from the PC to behavioral output and to analyze the causal relationship between
oscillatory dynamics in olfactory processing networks and odor-guided behavior. Since
such attempts remain elusive in the mammalian brain, an in vitro nervous system
of the Limax may lead to an answer. An advantage of using Limax is that
molluscan nervous systems retain their function in the isolated brain, which can survive
for a long period of time in physiological saline. However, there have been so far few
studies focusing on the regions outside the PC, which may lead to identification of
pathways from the PC to a motor output. |
In vitro index of odor-guided behavior |
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TopIntroductionOdor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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We identified the odor-evoked motor output pathway (Inoue et al., 2004
). We observed the behavior of
the slug in response to odorants very carefully and found that when the animal approached
an aversive odorant (innately or conditioned), it showed shortening of the mantle, before
turning away from the repellant. In contrast, attractive odorants did not induce mantle
shortening. Therefore, contraction of the mantle muscle can be used as an index of an
aversive response. We identified a motoneuron that projects to the mantle and innervates
the mantle muscle and named it posterior visceral neuron, p-VN. Electrical
stimulation of the superior tentacle nerve or application of innately aversive odor
induced the action potentials in the p-VN. On the other hand, the attractive
odors hardly induced firing of the p-VN. These results indicate that aversive odors selectively increase the number of p-VN spikes, namely, that odor-elicited muscle contraction in vivo can be reproduced in vitro, in terms of the activity of the p-VN.
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Neural correlate of the aversive meaning of the odor |
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TopIntroductionOdor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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The isolated nose–brain preparation was conditioned by simultaneous application of an odor of attractive food to the tentacle and electrical stimulation of the lip nerve. In the test of the learning, the conditioned (now aversive) and unconditioned (attractive) odors were applied to the nose while monitoring the activity of the p-VN and LFP of the PC. Application of the conditioned odor elicited discharges of the motoneuron. However, application of an unconditioned odor did not induce the discharges of the motoneuron. These results were obtained repeatedly, indicating that the isolated brains can acquire the odor memory. Thus an in vitro conditioning system has been established.
Simultaneous recording of the LFP oscillation of the PC lobe and p-VN activity has revealed strong correlation between discharges of the p-VN and the frequency increase in the LFP oscillation, both of which are induced selectively by aversive odors, the frequency increase preceding the p-VN discharge.
We examined the effect of the acetylcholine application, which increases LFP oscillation frequency, on the discharges of the p-VN. Selective application of acetylcholine to the PC greatly increases the LFP oscillation frequency, but has little effect to the motoneuronal discharges. However, application of odor, not only aversive but also attractive, could elicit discharges of the p-VN while the PC oscillation frequency was increased by ACh application. This result suggests that the increase in the oscillation frequency in the PC does not activate the p-VN directly, but has a modulatory effect on the odor recognition, which results in the discharges of the p-VN and the subsequent contraction of the mantle muscle.
The conclusion of the present study is that the oscillation frequency in the olfactory CNS is a regulatory element for determining the aversive meaning of odors. In other words, the present study revealed how the meaning of the odor is added to the recognized quality of the chemical odor and how it leads to behavior.
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Acknowledgements |
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TopIntroductionOdor-aversion conditioning in...Oscillatory activity of the...What is the role...In vitro index of...Neural correlate of the...AcknowledgementsReferences |
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This work was supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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