The effects of enhanced sensory perception on
learning and memory retention in Drosophila
By
Ann Cooper1, Changsoo Kim2,
and R.L. Cooper1
1Depratment
of Biology, University of Kentucky, Lexington, USA;
Research Center, School of Biological Sciences, Chonnam
National University, Gwangju, South Korea;
2Hormone
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INTRODUCTION
Injuries frequently occurring in animals in wild-life elicit painful sensation so that they are
prevented from being further touched; this is beneficial for curing of damaged tissues. Painful sensation
is initiated by pain sensors/channels located on specialized sensory neurons called nociceptors in the
peripheral nervous system, and the pain signals are relayed through spinal cord to pain centers in the
brain where it is interpretated as unpleasant pain; coordinated avoidance behaviors are then followed.
Studies mostly from mammals revealed that there are anti-nociceptive pathways in addition to
nociceptive pathways which allow modulating the amount of pain received in the central pain center.
For example, endogenous morphine induced excites morphinergic neural circuits, leading to reduction
of pain so that animals feel better.
Chemical irritants such as capsaicin, the pungent ingredient of chilly peppers, and
isothiocynate, an active component of wasabi, activate the responsible receptors TRPV1 (transient
receptor potential vanilloid type 1) and TRPA1, respectively, in mammals. They are present in sensory
nociceptors as primary sensory receptors for certain type of nociceptive stimuli. The capsaicin receptor
TRPV1 responds to heat (>42°C) and inflammation soups as well as capsaicin, mediating inflammatory
and injury-induced pain in mammals. In Drosophila, the pain channel, painless TRPA1, a homolog of
mammalian TRPA1 channel, is discovered through direct mutant screening in which mutations in
painless gene caused larvae to show reduced response to noxious heat (Tracey et al., 2003), and it also
mediates heat and wasabi response in adult flies.
As for the mammalian pain sensory neurons with free nerve endings Drosophila also possess
sensory nociceptive neurons with free nerve endings, which are multi-dendritic (MD) neurons with its
prominent multiple dendrites. Inactivation of these neurons by expressing tetanus toxin reduced larval
and adult response to a heated probe (Tracey et al., 2003), suggesting that these neurons mediate
nociceptive sensation. MD neurons are scattered all over the body and give rise to abundant dendrites
underneath the skin at the peripheral nervous system.
Here we expressed the TRPV1 channel in MD neurons to generate the transgenic flies that
exhibit elevated nociceptive response to stimuli (low temperature or capsaicin) that does not cause
nociceptive behaviors in wild-type flies. Furthermore, using the flies we developed a semi-quantitative
method for assessing nociception elicited through a simple feeding regimen based on the pain leading
to starvation induced death. The death rate quantitatively reflects the amount of nociception the flies
experienced. With this system, a slight fluctuation of the pain pathway will affect the death rate, and
commonly used analgesic drugs reduced the death rate.
We chose capsaicin and the capsaicin/heat receptor TRPV1 to selectively activate MD neurons
since channels responding to capsaicin like TRPV1 is not present in Drosophila and capsaicin does not
induce nociceptive behaviors in wild-type flies
The experiments to conduct are to examine the differences in behavior upon
placing wild type and extra sensitive larva on to agar that has been coated with capsaicin.
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MATERIALS AND METHODS
Experimentation are conducted at the temperature the animals were raised to
eliminate temperature changes. To test for the effects of capsaicin (1nM) on the larvae,
the chemical was mixed in agar plates. Third instar larva of the wild type and extra
sensitive strains are placed on the agar dish. Their behaviors are observed under the
dissecting microscope.
REFERENCES
Tracey, W. D., Jr., Wilson, R. I., Laurent, G. & Benzer, S. (2003) Cell 113, 261-73.
Rollmann, S.M., et al. Pleiotropic Effects of Drosophila neuralized on Complex
Behaviors and Brain Structure. Genetics. 2008;179:1327-1336.
Rubin et al., Comparative Genomics of the Eukaryotes Science 24 March 2000; 287:
2204 – 2215
Figure 1: Life cycle of the Drosophila melanogaster
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Figure 2: On the left is the capsaicin sensitive fly that is marked with the curled wing.
On the right is the background strain of the capsaicin sensitive fly without the curled
wing.
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