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
Results
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?
Errors in the Radial-Arm Maze at 6 Months
Enriched
Non Enriched
Overall Mean Errors During the 8 Trials at 6 months
Standard Cage
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Medium Enriched Cage
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Large Enriched Cage
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Significant differences betw een
the non enriched (all groupsshow n here the standard cage
vs. enriched groups (p< 0.05)
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Fig. 1
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Errors
Errors
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Mean Errors
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Standard Cage
Medium Cage
Large Cage
Medium Enriched
Large Enriched
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Standard Cage
Medium Size Cage
Large Size Cage
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Fig. 3
Fig. 2
Figure 1, 2 & 3. At 6 months, there was a main effect for environment, F(4, 28) = 5.40, p < .002, partial eta2 = .44; a main effect of trial, F(7,196) = 8.42, p <
.001, partial eta2 = .23, and a significant environment X trial interaction, F(28, 196) = 1.79, p < .05, partial eta2 = .20. Over the 8 trials, regardless of cage
size, rats in the enriched environments had significantly less errors overall (StandardNE: 6.25+0.53 errors, MediumNE: 6.71+0.62 errors, LargeNE: 4.62+ 0.79
errors, MediumE: 2.77+0.72 errors, LargeE: 3.0+0.89 errors).
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 8”h), a medium non-enriched cage (Medium NE: 25”x 16” x 14.75”h), 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.
Errors in the Radial-Arm Maze at 12 Months
8-Arm Maze Task at 12 mo - Non Enriched
8-Arm Maze Task at 12 mo - Non Enriched
(Short & Long-Term Memory)
(Short & Long-Term Memory)
18
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 (N=34, 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.
18
Standard Cage
Medium Size Cage
Large Size Cage
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Standard Cage
Medium Enriched Cage
Large Enriched Cage
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Overall Mean Errors During the 8 Trials at 12 months
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7
Standard
Medium
Large
Medium Enriched
Large Enriched
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no differences between
the groups (p=1.00)
9
Errors
Errors
12
Mean Errors
6
Significant differences betw een the
non enriched (all groups- show n here
the standard cage vs. enriched
groups (p< .005)
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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.
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Summary of Findings
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Trial
Standard Cage
(19” x 10.5” x 8”h)
Design: 5X8
8-Arm Maze
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8
Trial
Fig. 4
Environment (1 year - test at
6 mo & 12 mo)
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Fig. 6
Fig. 5
Figure 4, 5 & 6. At 12 months, there was a main effect for environment, F(4, 22) =8.87, p < .001, partial eta2 = .60; a main effect of trial, F(7,154) = 12.23, p
< .001, partial eta2 = .36, and a significant environment X trial interaction, F(28, 154) = 3.03, p < .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 4.77+0.41 errors, MediumNE: 4.10+0.41 errors, LargeE: 4.12+0.44 errors, MediumE: 2.35+0.41 errors, LargeE: 1.62+0.5 errors).
Trial ( 1 trial a day for 8 consec days)
Trial 1
Medium Cage
(25” x 16” x 14.75”h)
Trial 2
Large Cage
(40.5” x 18” x 20.5”h)
Time to complete the Radial-Arm Maze at 6 Months
Trial 3
Time to complete the Radial-Arm Maze at 12 Months
Trial 4
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Standard
Medium
Large
Medium Enriched
Large Enriched
Fig. 7
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: 6.22+0.63 min,
MediumNE: 7.72+0.63 min, LargeNE:
5.77+0.69 min, MediumE: 3.37+0.63 min,
LargeE: 4.31+0.78 min).
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Acknowledgements
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Figure 7. At 6 months, a main effect for
environment was shown, rats in the
enriched environments completed the
spatial task faster than their nonenriched counter parts (StandardNE:
7.03+0.89 min, MediumNE: 8.46+1.0 min,
LargeNE: 5.05+1.3 min, MediumE:
3.33+1.2 min, LargeE: 3.96+1.4 min).
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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.
Standard
Medium
Large
Medium Enriched
Large Enriched
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Trial 8
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).
an
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Trial 7
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Trial 6
Minutes
Trial 5
St
Large Enriched
(40.5” x 18” x 20.5”h)
Overall Mean Time to Complete Task
During the 8 Trials at 12 months
Overall Mean Time to Complete Task
During the 8 Trials at 6 months
Minutes
Medium Enriched
(25” x 16” x 14.75”h)
References
Supported by the McNair Scholars Program at
California State University, Bakersfield