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Protein synthesis inhibition and extinction:
Does cycloheximide produce amnesia for extinction of an odor discrimination in rats?
Alexandra Knoppel, Katherine Janson and Gretchen Hanson Gotthard
Randolph College
Much research has shown that the administration of protein synthesis
inhibitors blocks the formation of new fear memories (e.g., Nader, Schafe, &
LeDoux, 2000). In fact, most of the research in this area has focused on the
acquisition of fear responses or used tasks that required animals to respond
under aversive conditions (e.g., Morris water maze; Meiri & Rosenblum, 1998).
Additionally, most studies examining the effects of blocked protein synthesis
have examined acquisition of a completely new response, rather than
extinction of an already-established response. A small number of studies have
begun to examine the effects of protein synthesis inhibition on extinction
(Lattal & Abel, 2001; Suzuki, et al., 2004); however, the results have been mixed
and have been conducted with aversive tasks only (e.g., fear conditioning and
the water maze).
The present studied used an appetitive odor discrimination digging task
(Bunsey & Eichenbaum, 1996) to examine the effects of a protein synthesis
inhibitor (cycloheximide - CHX) on extinction in rats. Considering extinction is
similar to acquisition in that it also involves new learning, it was hypothesized
that protein synthesis inhibition would block extinction and produce continued
high levels of responding during testing.
Results
Lynchburg, VA 24503
Figure 1
Shaping (Day 1)
One cup of unscented
sand (two trials)
All rats met acquisition and extinction criteria. CHX rats showed amnesia for
extinction, while VEH rats did not. More CHX rats (n=7) dug on the test than
VEH rats (n=2); χ2=4.337, p=0.037.
Additionally, CHX rats showed amnesia not only for extinction, but for original
training as well. CHX rats had preference index scores that did not differ from
chance (test 1: t=1.123, p>0.05; test 2: t=2.333, p>0.05). Any VEH rats that
dug (n=2) showed perfect preference scores for the correct odor (i.e., 1.0) (see
Figure 2).
Figure 2
Shaping (Day 2)
Two cups of unscented
sand (two trials)
Shaping (Day 3)
Two cups of unscented
sand (six trials)
Preference Index
Introduction
Preference Scores for Rats that Dug
1
0.5
0
Test Trial 1
Test Trial 2
Cycloheximide Vehicle
Training (Day 4)
Method
Trained on one odor
discrimination (at least six
trials with 75% correct
total)
Subjects
Ninety-day old, male Long-Evans rats (N=19) were reduced to and maintained
at 85% of their free-feeding weights one week prior to and during
experimentation. Water was available ad libitum. Rats were maintained on a 12hour light/dark cycle.
Apparatus
All shaping, training, and testing took place in the rats’ home cages. Plastic
Nalgene® cups (125 ml size) were used for the odor discriminations and were
mounted using Velcro® onto rectangular Plexiglas® bases. Odor
discriminations were created by mixing play sand (148.5 grams) with different
dried spices (i.e., 1.5 grams of cocoa or cinnamon).
•Rats learned to dig in the correct odor (acquisition) and to stop digging in the
correct odor (extinction).
•Administration of a protein synthesis inhibitor (CHX) during extinction
blocked memory for extinction and for original training.
•Results suggest that protein synthesis may be critical for original learning and
subsequent extinction learning in an appetitive odor discrimination task.
Extinction (Day 5)
Extinction trials for the odor
discrimination (at least two trials and
no digging for 30 consecutive
seconds)
Procedure
Rats were shaped to dig in unscented cups of sand for three days prior to
training (10 shaping trials total – see Figure 1 for shaping, training, and testing
procedure). Rats received one day of training during which they learned one
odor discrimination to 75% correct. One day following training, rats received at
least two extinction trials for the odor discrimination with a 1 mg/kg CHX
injection (n=10) or a vehicle (VEH) injection (n=9). Twenty-four hours later,
rats received two test trials.
Discussion
Testing (Day 6)
Two test trials for odor discrimination
References
Bunsey, M. & Eichenbaum, H. (1996). Conservation of hippocampal memory function
in rats and humans. Nature, 379, 255-257.
Lattal, K. M. & Abel, T. (2001). Different requirements for protein synthesis in
acquisition and extinction of spatial preferences and context-evoked fear. The Journal of
Neuroscience, 21(15), 5773-5780.
Meiri, N. & Rosenblum, K. (1998). Lateral ventricle injection of the protein synthesis
inhibitor anisomycin impairs long-term memory in a spatial memory task. Brain Research,
789, 48-55.
Nader, K., Schafe, G. E., & LeDoux, J. E. (2000). Fear memories require protein
synthesis in the amygdala for reconsolidation after retrieval. Nature, 406, 722-726.
Suzuki, A., Josselyn, S. A., Frankland, P. W., Masushige, S., Silva, A. J., & Kida, S.
(2004). Memory reconsolidation and extinction have distinct temporal and biochemical
signatures. The Journal of Neuroscience, 24(20), 4787-4795.