Long-Term Enriched Environment But Not Cage Size Improves Memory in Adulthood and

1
Long-Term Enriched Environment But Not Cage Size
Improves Memory in Adulthood and Late Adulthood
in Male Rats I.C. Sumaya, J.H. Calderon, V.
Rodriguez, C.M. Franz A.A. Amaya Department of
Psychology, Behavioral Neuroscience Laboratory,
California State University, Bakersfield, CA .
Background
Environments
It has been well established that enriched
environments positively impact rodent learning
and memory. Rats placed in enriched environments
have shown improvement in performance in a
variety of memory tasks including the Morris
Water Maze (Harburger et al., 2007 ), the novel
object recognition task (Bruel-Jungerman et al.,
2005), and the radial arm maze task (Hoffmann et
l., 2009). One of the proposed underlying
mechanisms of action for improvement of memory is
that of neurogenesis in the hippocampus, a region
important for learning and memory in both rodents
(Kempermann et al., 1997 Veena et al., 2009
Nilsson et al., 1999) and humans (Suthana et al.,
2009 Goodrich-Hunsacker et al., 2009). Although
many studies have investigated the effects of
neurogenesis on learning and memory in rodents,
much of the data reported have been collected
during short-term enriched housing (2 weeks to 2
months) and in younger populations at one time
point. The aim of our experiment was to
investigate the effects of long-term enriched
housing on spatial learning (8 arm-maze task) at
two developmental time points, adulthood (6
months) and later adulthood (12 months) in rats
that were born, raised, and matured in enriched
environments. Additionally, we were interested
in the importance of cage size within th e
context of the enriched environments. That is to
say, does cage size matter?
Results
Errors in the Radial-Arm Maze at 6 Months
Fig. 3
Fig. 1
Fig. 2
Method
Figure 1, 2 3. At 6 months, there was a main
effect for environment, F(4, 28) 5.40, p lt
.002, partial eta2 .44 a main effect of trial,
F(7,196) 8.42, p lt .001, partial eta2 .23,
and a significant environment X trial
interaction, F(28, 196) 1.79, p lt .05, partial
eta2 .20. Over the 8 trials, regardless of cage
size, rats in the enriched environments had
significantly less errors overall (StandardNE
6.250.53 errors, MediumNE 6.710.62 errors,
LargeNE 4.62 0.79 errors, MediumE 2.770.72
errors, LargeE 3.00.89 errors).
Summary of Findings
Animals Five pairs of Sprague-Dawley (Charles
River, Wilmington, MA) male and female (2 months
old) rats were randomly assigned to one of five
environments for breeding. All pairs were kept in
a 12L/12D cycle with lights on at 0600, fed
standard rat chow ad libitum and fresh water
daily. The five environments included a standard
non-enriched cage (Standard NE 19 x 10.5 x
8h), a medium non-enriched cage (Medium NE 25x
16 x 14.75h), a large non-enriched cage (Large
NE 40.5" x 18 x 20.5 h), a medium enriched cage
(Medium E), and a large enriched cage (Large E).
Enriched environments included running wheels,
ledges, toys and edible rat shacks. Breeding
Male rats were placed in a cage with clean
bedding for 24 hours prior to placing the female
in the cage. The pairs were kept together for 7
days after which the males were removed from the
cages. All females were impregnated and gave
birth approximately 21 days later. Once the dams
gave birth, and the pups were weaned, the pups
were separated by gender, the mothers were
removed from the environment with the offspring
remaining in the environment they were born in
(N34, only report male data here). At 6 months
the rats were tested in the radial 8-arm maze
task (1 trial/day for 8 days) with perseveration
errors and time to completion measured. Six
months later, the rats were again tested.
Over the 8 trials at 6 months, regardless of cage
size, rats in the enriched environments had
significantly less errors overall and completed
the spatial task faster than their non-enriched
counterparts. The same pattern occurred at 12
months of age. Regardless of cage size, rats in
the enriched environments continued to have
significantly less errors overall. Additionally,
the rats in the enriched environments also
completed the task quicker at both 6 months and
12 months of age. These data show that enriched
environments continue to positively impact
spatial learning and memory as measured in the
radial arm maze into late adulthood. Our
unanticipated results were that the size of the
environment is not an important contributing
factor in the positive impact on memory.
Errors in the Radial-Arm Maze at 12 Months
Fig. 6
Fig. 4
Fig. 5
References
8-Arm Maze
Design 5X8
Figure 4, 5 6. At 12 months, there was a main
effect for environment, F(4, 22) 8.87, p lt .001,
partial eta2 .60 a main effect of trial,
F(7,154) 12.23, p lt .001, partial eta2 .36,
and a significant environment X trial
interaction, F(28, 154) 3.03, p lt .001, partial
eta2 .36. At 12 months of age the rats showed
similar patterns of learning over the 8 trials.
Regardless of cage size, rats in the enriched
environments continued to have significantly less
errors overall (StandardNE 4.770.41 errors,
MediumNE 4.100.41 errors, LargeE 4.120.44
errors, MediumE 2.350.41 errors, LargeE
1.620.5 errors).
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Time to complete the Radial-Arm Maze at 12 Months
Time to complete the Radial-Arm Maze at 6 Months
Figure 8. At 12 months, a main effect for
environment was shown, rats in the enriched
environments completed the spatial task faster
than their non-enriched counter parts
(StandardNE 6.220.63 min, MediumNE 7.720.63
min, LargeNE 5.770.69 min, MediumE 3.370.63
min, LargeE 4.310.78 min).
Figure 7. At 6 months, a main effect for
environment was shown, rats in the enriched
environments completed the spatial task faster
than their non-enriched counter parts
(StandardNE 7.030.89 min, MediumNE 8.461.0
min, LargeNE 5.051.3 min, MediumE 3.331.2
min, LargeE 3.961.4 min).
Radial 8-Arm Maze The maze was made of Plexiglas
(arms 61cm L, x 23cm W x 23cm H) whereby the
arms were baited with fruitloops (Kelloggs,
Cincinnati, Ohio). A day prior to testing all
rats were placed in the maze for a 2-hour
acclimation period. The arms were baited with
fuitloops (Kelloggs). Data were recorded for
number of errors in the 8 arm maze, and the time
animals took to complete task (time data not
shown here).
Statistics A 5X8 Mixed ANOVA was used with
environment as the between subjects factor and
Trial as the within groups factor. Post hoc
tests were performed with the Bonferroni
correction. Alpha was at 0.05.
Acknowledgements
Supported by the McNair Scholars Program at
California State University, Bakersfield
Fig. 7
Fig. 8