Download CH11L1

Olfactory based learning and memory in the Fruit Fly Drosophila
girl
boy
What’s so great about Drosophila?
•Small: you can have large populations
of flies in a small area
•Reproduce rapidly and in great quantity
•Easy to maintain in culture
•Small genome
Seymore Benzer: father of fly genetics and one of the
pioneers of the molecular revolution
Established a method of mutating DNA
•Apply a mutagen which breaks up the DNA
•DNA reassembles with errors
•Most errors are lethal
•Rarely a mutation is not but it will affect the animal in
some nominal to major way.
•Those which live can be established as highly inbred
(homogenous) colonies and studied.
Assumptions of the approach
•Genes -> proteins -> structural and functional basis of
behavior basis of behavior
•Mutating genes one by one should cause specific
defects in behavior
•Identification of gene reveals molecules necessary for
that behavior
Limitations of the approach:
•What if the mutation disrupts its proteins function not only as an adult but
during development and/or in systems other that the one being studied?
•Effect of interest can be compounded with other effects and/or
secondary effects.
•Solution: mutations can now be triggered in adults at will.
•Non affected genes/proteins can compensate: Functional protein isoforms
can compensate for mutated proteins.
Odor-shock learning in the fruit fly (the early days)
•Groups of flies are placed in a start tube.
•Start tube shuttles them to the grid and rest
tubes.
•Grid tubes contain an odor (A or B)
•Grid tubes contain shock grids
•Flourescent light used to attract flies into
and out of tubes.
•One odor is paired with shock
•Between training trials flies are shuttled to
rest tube.
What kind of conditioning paradigm is this?
Differential conditioning yields a measure of associative processes.
CS+
Comparative differences in this
measure as a function of mutations
provides an assay of learning and
memory.
CSCS+
CS-
Key learning mutants:
•Dunce
•Turnip
•Cabbage
•Rutabaga
•DCO
•Amnesiac
Different learning and memory mutants show subtly different deficits
Normal learning in amnesiac flies
Abnormal retention
Tim Tully and the new and improved odor-shock learning method
•No light: Flies are put into training chamber and odor is
shuttled in and out.
•More Pavlovian: The paradigm is imposed on the flies (they
cannot escape odor-shock pairing by not going into the grid
tubes.
•Flies can’t avoid the test (forced choice)
Results: up to 90 percent as compared to 33 percent change in behavior
Old results
Inter-stimulus interval is important
Best learning occurs when CS
and US are overlapped
Comparison of learning and retention across different mutants reveals multiple phases
Acquisition in wildtype
Index of learning
Index of STM
Index of LTM
Summary of mutation effects
In all cases mutants had learned something and lost it but:
•Dunce: poor learning no STM LTM
•Rutabaga/turnip: moderate learning little STM little LTM
•Amnesiac: good learning little STM LTM
Based on what we know of Aplysia, what can we predict about the molecules
involved in these mutants?
Genetic lesioning: Specific ablation of a single gene/protein
amnesiac
Amnesiac: disrupted PACAP-like
protein which acts like serotonin
leaving the second messenger
pathway active
Dunce: disrupted PDE increases ambient
cAMP levels: possibly blocks cAMP
pathway by masking learning-dependent
changes in cAMP levels
Rutabaga: disrupts the adenylyl cyclase
binding site for calmodulin: blocks
coincidence detection (of CS and US)
DCO: Disrupts the PKA catalytic subunit
thus blocks the phosphoralation of the K
channels (no spike broadening)
Other potential mutants that fit the cAMP model:
•Turnip: gene regulates PKC
•Ddc: gene regulates 5HT and Dopamine
•Shaker: A potassium channel gene
So where are these genes expressed in the fruit fly brain?
Antibody staining indicates that the mushroom bodies contain the molecular
components that are key for learning:
•Adenylyl cyclase
•PKA
•cAMP phosphodiesterase
Genetic dissection of MB function
• MB deranged: normal MB as larvae
but deformed MBs as adults
• Learning is fine in larvae but
abolished in adults (similar results in
bees).
Proposed CS-US and behavior pathways
Martin Heisenberg