study examined the effects of trimethyltin (TMT) induced hippocampal lesions
on spatial reference and working memory in two strains of mice via the
Morris water maze (Morris, Garrud, Rawlins, & O’Keefe, 1982).
C57BL/6J and CF1 mice were randomly divided into two groups of each strain.
One group of each strain received a saline injection and the other group
of each strain received 3 mg/kg TMT injected intraperitoneally. All
of the C57BL/6J mice that were injected with TMT died. In the reference
task, both the CF1 TMT and saline mice were significantly impaired compared
to the saline C57BL/6J group. In the working memory task, only the
CF1 saline mice were significantly impaired compared to saline C57BL/6J
mice. The lack of significance found between the CF1 saline and TMT
mice on both tasks may reflect a recovery from the effect of TMT.
These data also show that C57BL/6J mice may have a more accurate spatial
reference and working memory. This may be a result of their well
developed CA3 and CA4 hippocampal regions. This may further implicate
these areas in reference and working memory. Further research may
look at the brain physiology of the C57BL/6J mice in more detail and compare
it to that of the CF1 mice through histological examination.
TMT affect spatial working and reference memory in CF1 and C57/BL/6J mice
as assessed by the Morris water maze? If so, does it have more of
an affect on memory in C57/BL/6J mice, which have a more highly developed
CA3 hippocampal region?
A total of
21 male C57 mice and ten adult male CF1 mice participated in this experiment.
They were kept on a 12-hour light/dark cycle and housed in separate cages
with food and water available to them ad libitum. The 21 C57 mice
were randomly assigned to two groups, one of ten (control) and one of 11
(experimental). The ten CF1 mice were randomly assigned to two groups
of five, one control group and one experimental group. The control
groups received a saline injection, while the experimental groups received
a 3 mg/kg injection of TMT. The mean weight of the mice was 30g at
time of injection. All mice were cared for in a manner consistent
with the ethical guidelines of the American Psychological Association.
a random selection of mice for the control or the TMT group. The
three mg/kg TMT or .25 ml of saline was injected intraperitoneally.
Three mg/kg was used because higher doses are often lethal in mice, while
lower doses are usually not sufficient in lesioning the hippocampus (Earley
et al, 1992). The mice were monitored for a period of seven days
post-TMT so they could recover from the initial effects of the TMT.
water maze (Morris et al, 1982) was used to assess spatial memory.
The maze consists of a submerged platform within a pool of water (Morris,
et al, 1982). It has a diameter and a height of three feet.
The pool was entirely black fiberglass. The wooden platform was also
black and was submerged 1.5 cm below the surface of the water. The
pool was divided into four quadrants with white paint and the platform
was located in the center of quadrant four. The start position for
the reference memory task was in the center of quadrant one, marked by
an X in white paint. For the working memory task, the start position
was an X painted in the center of quadrant three. A large lamp and
a video camera were visible from within the pool and remained in the same
positions throughout the entire experiment. The data were measured
using a stopwatch and a digital video camera that was positioned on top
of a counter adjacent to the pool.
Prior to testing
11 of the C57s and five of the CF1 mice were randomly selected and injected
with 3 mg/kg TMT. The remaining mice of each strain were injected
with .25 ml of saline. After injection, the mice were given one full
week to recover from the initial effects of the drug.
The mice were
placed in the pool and had to locate the platform to escape. They
were unable to see the platform, and therefore had to rely on memory of
distal cues to locate it. Resting on the platform provided the motivation,
and therefore the mice did not need food- or water-deprivation or an extended
For the reference
memory task, the mice were placed in quadrant one at start position one
and allowed to swim freely for 60 seconds or until they found the platform.
If the mouse was unable to find the platform within that time period, they
were placed there. After finding the platform or being placed there,
each mouse rested on it for 30 seconds. There were 22 trials over
a period of 11 consecutive days. Each mouse was tested twice each
day, once in the morning and once in the afternoon, at approximately the
In the working
memory task, each mouse was placed in the pool at start position two in
quadrant three, everything else (e.g., distal cues, water level, position
of the platform, etc.) remained the same as in the reference memory task.
The mice were again allowed to swim freely for 60 seconds and if they were
unable to locate the platform within that time period, they were placed
there. All of the mice rested on the platform for 30 seconds at the
end of each trial. The working memory task was completed in one day,
with three trials. Two consecutive trials were run in the morning,
and the third occurred that afternoon.
The mice were
measured for how long it took to find the platform in each trial of both
the reference and working memory tasks. The mean score for each mouse
across trials for each task was then computed. A score of 60 seconds
was automatically given for that trial if a mouse did not find the platform
within the cut-off period.
purpose of this experiment was to both examine the effects of TMT on spatial
reference and working memory, and to compare CF1 and C57 mice on a spatial
memory task in which time was the dependant variable. After injection
of the TMT all 11 of the C57 mice died. Univariate analyses of variance
were computed across the three remaining groups (CF1 saline, CF1 TMT, C57
For the reference
task, a significant difference between groups was found (F(2,12) = 5.07,
p<.05). A post-hoc comparison was then performed and the CF1 mice,
both control (p=.01) and TMT (p=.036) groups, were found to be significantly
impaired compared to the C57s (see figure 1) with means of 44, 40, and
26 respectively. However, no significant difference was found between
the CF1 control and TMT mice (p=.513).
For the working
memory task, no significant effect was found for TMT (F(2,12) = 3.21, p>.05),
although it was approaching significance. However, a post-hoc comparison
showed that the CF1 saline group (p=.028) was significantly impaired compared
to the C57s (see figure 2) with means of 8 and 33, respectively.
suggest that reference memory is not dependent on the areas of the brain
affected by TMT because the CF1 control and TMT groups were not significantly
different from each other. However, because the C57s were much faster
and more accurate in both acquisition and performance in completing the
reference memory task than both of the CF1 groups, it is very possible
that the differences are a result of the different brain physiology of
the two strains.
the CF1 control and TMT groups were not significantly different from each
other in the working memory task, the areas of the brain destroyed by the
TMT may not be necessary for working memory to function properly.
In fact, the CF1 TMT group did slightly better than the CF1 saline group
in this task, with means of 24 and 33, respectively. The C57s once
again outperformed both the CF1 saline and TMT groups. This suggests
that the different brain physiology of the C57s may be responsible for
more accurate working memory as well.
areas of this study were inconsistent with previous research. This
may be due to possible confounds. First, because there were only
five mice in each group due to the lethality of TMT in the C57s, the sample
may not have been large enough to give an accurate representation of the
mice's abilities. Second, the scoring of the mice was subject to
experimenter reliability, and a more accurate method would entail using
a computer program that would score each mouse in a consistent manner.
Third, as previously mentioned, the TMT mice were injected approximately
six months prior to testing and the effects of the drug may have worn off
by the time it began. Fourth, it is possible that the somewhat inconsistent
testing patterns caused some fatigue. For the working memory portion
of the experiment, two of the three trials were run consecutively and the
third trial was run a few hours later that same day. Finally, the
dose of TMT that was administered to the C57 mice was too high. These
mice may require a much lower dose to produce lesions in the hippocampal
area and affect spatial working memory.
As the many
confounds may have skewed the results, in the future researchers might
attempt to repeat this study using more consistent and reliable techniques.
For instance, a larger sample size should be used so that it can be safely
assumed that the results are representative of the strain of mouse being
tested. Also, testing the mice approximately one to two weeks after
TMT-injection will ensure that the effects of the drug have not worn
off. Furthermore, testing should be done in a consistent manner,
with a set number of trials each day, dispersed evenly throughout to reduce
fatigue effects. Finally, to avoid lethality while pursuing the effects
of TMT on C57 mice, a lower dose should be injected (maybe as low as .5
on the effects of TMT is very important. TMT is found in many large
and vital industries, including plastics, agriculture, paper, and disinfectants
(Wenger et al, 1982). Furthermore, approximately 100 people developed
toxic symptoms in the 1950s in France, after treatment with a preparation
containing triethyltin, an organic tin closely related to trimethyltin
(Wenger et al, 1982). This is what initially spurred interest into the
effects of TMT on rats and mice.
this area is also important for understanding what the regions of the brain
affected by the TMT are responsible for. Although, in this study
TMT did not appear to negatively affect spatial reference or working memory
ability in mice, much previous research has found that TMT does impair
working memory in rats (Alessandri et al, 1994; Bushnell & Angell,
1992; Earley et al, 1992; Nation et al, 1984) and mice (Oullette, 1998).
Previous studies have also found increased aggression, hyperirritability,
tremor, hyperreactivity, and changes in schedule-controlled behavior in
rats as a result of TMT exposure (Earley et al, 1992). This suggests
that the areas of the brain affected by TMT are involved in more than spatial
reference and working memory. Future research should further address
what these areas are responsible for.
results of this study suggest that C57 mice perform better on the Morris
water maze than CF1 mice. Further, despite previous research findings
in the area, these results do not support a link between TMT and spatial-working
memory impairment. Further research with C57 mice and TMT will be
essential to a better understanding of spatial memory in humans and animals.