Protein synthesis inhibition and extinction:

1
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 Lynchburg, VA
24503
Results All rats met acquisition and extinction
criteria. CHX rats showed amnesia for
extinction, while VEH rats did not. More CHX rats
(n7) dug on the test than VEH rats (n2)
?24.337, p0.037. 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 t1.123, pgt0.05 test 2 t2.333,
pgt0.05). Any VEH rats that dug (n2) showed
perfect preference scores for the correct odor
(i.e., 1.0) (see Figure 2).
Introduction 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.
Figure 1
Shaping (Day 1) One cup of unscented sand (two
trials)
Figure 2
Shaping (Day 2) Two cups of unscented sand (two
trials)
Shaping (Day 3) Two cups of unscented sand (six
trials)

• Discussion
• 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.

Training (Day 4) Trained on one odor
discrimination (at least six trials with 75
correct total)
Method Subjects Ninety-day old, male Long-Evans
rats (N19) 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 12-hour
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). 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
(n10) or a vehicle (VEH) injection (n9).
Twenty-four hours later, rats received two test
trials.
Extinction (Day 5) Extinction trials for the odor
discrimination (at least two trials and no
digging for 30 consecutive seconds)
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.
Testing (Day 6) Two test trials for odor
discrimination