The study examined the effects of chronic low-level lead exposure on spatial and object recognition memory in juvenile mice. Mice were exposed to one of three levels of lead (0 ppm, 30 ppm, or 430 ppm) from birth to postnatal day 28 and then tested on an object-in-place visual recognition task. The results showed that low-level lead exposure did not impair memory but decreased activity levels, while higher exposure increased activity when mice were cognitively challenged. This suggests attention and orientation may be affected by chronic low-level lead exposure.

1. ABSTRACT: METHODS:
DISCUSSION:
The results did not support the hypothesis that chronic low-level lead disrupts spatial and object recognition memory
however they suggested that chronic exposure to low-levels of lead reduces activity whereas exposure to higher levels of
lead increases activity of juvenile mice when they are challenged cognitively. Similarly, a recent study suggested decreased
activity in chronic low-level lead exposed mice that were challenged with an exploratory task4. Thus attentional and/or
orientation processes may be disrupted as a result of chronic low-level lead exposure. Future studies should include
behavioral tasks that examine specifically exploratory activity with different behavioral tasks in order to examine under
which circumstances and why there are differences between groups with regard to exploratory activity. These findings
suggest that the C57BL/6J mouse model is valuable for understanding deficits in children exposed to chronic low-level lead.
RESULTS:
Introduction: Lead is an environmental toxin that is dangerous for the
developing brain. Empirical studies have provided convincing evidence that
children exposed to chronic low-levels of lead have diminished memory and
attention1,2,5,6,7. The brain mechanisms underlying these deficits are unknown.
Animal models that examine behavioral tasks that may be sensitive to the
effects of chronic low-level lead exposure in young animals are needed
because they can suggest possible brain areas and/or pathways affected. The
purpose of the present study was to examine the effect that chronic low-level
lead exposure may have on the spatial and object recognition memory of young
mice. It was hypothesized that chronic low-level lead exposure would diminish
spatial and object recognition memory.
Methods: Fifty-two C57BL/6J mice were exposed to one of three levels of lead:
0 ppm (control), 30 ppm (low-dose), and 430 ppm (high-dose). Lead was
administered via dam´s drinking water from birth until PND 28. On this day mice
were tested behaviorally on the object-in-place visual recognition paradigm3
(see Figure 1). Data were collected via a camera mounted over the arena and
analyzed using the SMART software system (Harvard PanLab). The outcome
variables were time spent and rearing time in the zones where the objects were
located, and total rearing time. Data were analyzed using StatView 5.0.1
statistical software. For spatial and object recognition memory discrimination
ratios were calculated by dividing the time exploring the displaced objects
(DOs) or novel object (NO) by the sum of the exploration time of all objects
within each session of interest (sessions 4, 5, 6, and 7). Two-way Repeated
measures ANOVA models were used to test for differences between groups
with regard to behavior. The between-groups factors were lead exposure group
and sex and the within-groups factor was session.
Results:
Spatial Memory Challenge (Sessions 4-5). For time in zones, a significant
main effect for sessions (S) 4-5 (S4 M = 0.43 ± 0.3; S5 M = 0.59 ± 0.3, p < .01)
with no interaction was found. For rearing time in zones, a significant main
effect for sessions 4-5 (S4 M = 0.43 ± 0.5; S5 M = 0.60 ± 0.4, p < .05) with no
interaction was found. For total rearing time, a significant main effect for
sessions 4-5 (S4 M = 34.31 ± 4.38; S5 M = 45.53 ± 5.92, p < .05) and a
significant interaction (session*group) was found (p < .05). Post hoc Bonferroni
tests revealed that the high-dose group had increased total rearing from
session 4 to 5 (S4 M = 31 ± 6.18; S5 M = 78.72 ± 10.59) as compared to the
low-dose (S4 M = 47.81 ± 13.14; S5 M = 43.25 ± 8.03) and control groups (S4
M = 25.56 ± 4.34; S5 M = 16.89 ± 3.05), p < .01. On the other hand, the low-
dose group had decreased total rearing from session 4 to 5 as compared to
controls, p < .01.
Object Recognition Memory Challenge (Sessions 6-7). For time in zones, a
significant main effect for session 6-7 (S6 M = 0.10 ± 0.2; S7 M = 0.43 ± 0.3, p
< .01) with no interaction was found. For rearing time in zones, a significant
main effect for session 6-7 (S6 M = 0.14 ± 0.3; S7 M = 0.51 ± 0.4, p < .01) with
no interaction was found. For total time rearing, a significant main effect for
session (S6 M = 34.31 ± 4.38; S7 M = 45.53 ± 5.92, p < .05) and a significant
interaction (session*group) was found (p < .05). Post hoc Bonferroni tests
revealed that the high-dose group had increased total rearing from session 6 to
7 (S6 M = 100.06 ± 16.31; S7 M = 147.78 ± 12.12) as compared to the low-
dose (S6 M = 49.13 ± 11.71; S7 M = 45.63 ± 11.86) and control groups (S6 M =
11.61 ± 2.21; S7 M = 24.83 ± 5.37), p < .01. The low-dose group did not differ
from controls with regard to total time rearing.
Conclusion: Chronic low-level lead exposure may not impair spatial and object
recognition memory but may result in decreased activity at lower levels of
exposure and increased activity at higher levels of exposure when animals are
challenged cognitively. This suggests that attention and/or spatial orientation
may be compromised as a result of chronic low-level lead exposure. Future
studies should examine whether and how exploratory activity changes
in chronic low-level lead exposed animals.
A Shortened Version of the Object-in-Place Visual Recognition Paradigm Detects
Changes in Activity in Chronic Low-Level Lead Exposed Juvenile Mice
Mayra Gisel Flores-Montoya1, Juan Alvarez2, Christina Sobin1,2,3,4
Acknowledgments: This research was made possible by grants from the National Institute of
Child Health and Human Development (NICHD), National Institutes of Health, (R21HD060120);
the National Center for Research Resources, a component of the National Institutes of Health
(5G12RR008124); the Border Biomedical Research Center (BBRC), University of Texas at El
Paso; the Center for Clinical and Translational Science, The Rockefeller University, New York,
New York; the Paso del Norte Health Foundation, El Paso, Texas; and from the University
Research Institute, University of Texas, El Paso. The funding agencies had no role in the
design, implementation, data analysis, or manuscript preparation for this study.
1. Department of Psychology, University of Texas, El Paso,
Laboratory of Developmental Neurocognition
2. Department of Public Health Sciences, College of Health Sciences,
University of Texas, El Paso
3. Border Biomedical Research Center, Toxicology and Neuroscience Cores
University of Texas, El Paso
4. Laboratory of Neuroendocrinology,
The Rockefeller University, New York, New York
Animals
A total of 52 mice were tested behaviorally
0 ppm group: n = 18 , ( 8 females; 10 males)
30 ppm group: n = 16, ( 10 females; 6 males)
430 ppm group: n = 18, ( 10 females; 8 males)
Object-in-Place Visual Recognition Paradigm
The task measures memory for a familiar vs. a novel object and memory for a familiar vs. a novel location3. (The
pathways governing these types of memories are segregated in the dentate gyrus.) A round Plexiglas arena with
an external cue (38 x 30 cm cardboard painted with vertical black and white stripes) placed on the northwest of
the arena was used. The boundaries of 9 zones (8 different locations within the arena in which objects were
placed) and the perimeter of the arena were defined using the SMART system. Behavior was recorded by a video
camera placed on top of the arena and was automatically later quantified using the SMART system. Each animal
completed 7 trials. In Trial 1, the animal was placed in the empty Plexiglas arena. This was the habituation trial. In
Trials 2, 3, and 4 the animal was exposed to 5 objects. These were the exposure/learning trials. Trials 5 and 7
were the testing trials. In Trials 5 and 6 two of the five familiar objects were switched to new locations (one familiar
location now called and one novel location). In Trial 7 a novel object in novel location was introduced. Each
session lasted 4 min with an inter-trial-interval of 2 min.
University of Texas at El Paso
REFERENCES:
Spatial Memory Challenge
Fig 2. Time in zones (sessions 4-5)
Fig 4. Rearing time in zones (sessions 4-5) Fig 5. Rearing time in zones (sessions 6-7)
Object Recognition Memory Challenge
Fig 3. Time in zones (sessions 6-7)
Discriminationratio
Session 4 Session 5
Repeated measures effect = p < .01
Discriminationratio
Session 6 Session 7
Repeated measures effect = p < .01
Session 4 Session 5 Session 6 Session 7
Repeated measures effect = p < .05
Repeated measures effect = p < .01
Fig 6. Total rearing time (sessions 4-5)
Session 4 Session 5
Repeated measures effect = p < .05
Session*Group effect = p < .05
Totalrearingtime
Repeated measures effect = p < .05
Session*Group effect = p < .05
Fig 7. Total rearing time (sessions 6-7)
Totalrearingtime
Discriminationratio
Session 6 Session 7
Discriminationratio
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