Sex and rearing condition modify the effects of perinatal lead exposure on learning and memory

Abstract

Developmental lead (Pb) exposure is associated with cognitive impairments in humans and rodents alike. In particular, impaired spatial learning and memory, as assessed using the Morris water maze (MWM), has been noted in developmentally Pb-exposed rats. Although sex and rearing environment can influence MWM performance in normal animals, the interactions of sex and rearing environment on the impact of developmental Pb exposure on hippocampal-dependent processes has not been well characterized. The present study examined the effects of perinatal exposure (i.e., gestation through weaning) to different levels of Pb (250, 750 and 1500 ppm Pb acetate in food) in males and females raised in a non-enriched environment (standard cage with 3 animals and no toys) or an enriched environment (large cage containing a variety of toys that were changed twice weekly). Testing in the MWM began at postnatal day 55. Behavioral outcomes were influenced by sex and rearing environment, with complex interactions with Pb exposure. In non-Pb exposed control animals, beneficial effects of environmental enrichment on spatial learning and memory were observed in males and females, with greater effects in females. Pb exposure in females mitigated at least some of the benefits of enrichment on learning, particularly at the lowest and highest exposure levels. In males, enrichment conferred a modest learning advantage and for the most part, Pb exposure did not affect this. However, in males with the highest Pb exposure, enrichment did help to overcome detrimental effects of Pb on learning. In females, any potential benefit to reference memory contributed by enrichment was muted by exposure to Pb and for the most part, this was not reproduced in males. Thus, there are complex interactions between sex, environment, and Pb exposure on spatial learning and memory. Environmental manipulation is a potential risk modifier of developmental Pb exposure and interacts with other factors including sex and amount of Pb exposure to affect the functional influences of Pb on the brain.

Fig. 1

Blood lead levels during the perinatal exposure period. (A). Maternal blood lead levels. Prior to lead exposure all animals had equivalent lead levels. After 10 days of lead exposure and prior to being paired with a male for breeding, there was a dose-dependent increase in blood lead levels. Blood lead levels still showed a dose-dependent increase immediately post-partum and immediately post-weaning. (B). Male blood lead levels from birth to weaning. (C) Female blood lead levels from birth to weaning. Statistical comparisons are indicated on the graphs, with significance levels as follows: * p<0.05, ** p<0.01, *** p<0.001, and **** p<0.0001.

Fig. 2

Water maze task acquisition data for male rats. Escape Latency: All groups of animals acquired a spatial map by the fourth day of training as defined by an escape latency of less than 20 seconds (learning criterion is shown by the dotted line in each graph). Enriched animals acquired a spatial map faster than non-enriched animals and virtually all enriched groups reached criterion by the completion of the second day of training. Non-enriched groups required 3 days of training to reach criterion. By day 3, all lead-exposed non-enriched animals had escape latencies similar to matched enriched animals, with the exception of 1,500 ppm lead-exposed non-enriched animals. Path Efficiency: All groups of animals regardless of treatment showed greater path efficiency (a measure of the efficiency of the path taken by the animal from the release point to the location of the hidden platform) over time. All lead-exposed and non-enriched animals performed worse than matched enriched animals at all time points. Group Sizes: Enriched Animals - Control (n=18), 250ppm Pb-exposure (n=10), 750ppm Pb-exposure (n=10), and 1500ppm Pb-exposure (n=11); Non-Enriched Animals: Control (n=11), 250ppm Pb-exposure (n=10), 750ppm Pb-exposure (n=13), and 1500ppm Pb-exposure (n=12). All data are shown as group mean ± S.E.M. Statistical differences are between enriched and non-enriched groups within a given level of lead exposure. * p<0.05, ** p<0.01, *** p<0.001, and **** p<0.0001.

Fig. 3

Water maze task acquisition data for female rats. Escape Latency: All females acquired a spatial map by the 7^th day of training as defined by an escape latency of less than 20 seconds, with most groups of animals at or below criterion by the 5^th day of training (learning criterion is shown by the dotted line in each graph). In all groups, except those exposed to 1,500 ppm Pb, enriched animals acquired the task faster than non-enriched animals. The non-enriched 250 ppm lead–exposed group was the only group that did not reach learning criterion by day 7. Path Efficiency: All groups of animals showed improved path efficiency (a measure of the efficiency of the path taken by the animal from the release point to the location of the hidden platform) over time. There was little difference in the performance of non-enriched and enriched animals in the 250 ppm and 1,500 ppm exposure groups with some residual benefits from enrichment in the 750 ppm exposure group. All data are shown as group mean ± S.E.M. Statistical differences are between enriched and non-enriched groups within a given exposure level. Group Sizes: Enriched Animals - Control (n=20), 250ppm Pb-exposure (n=16), 750ppm Pb-exposure (n=10), and 1500ppm Pb-exposure (n=20); Non-Enriched Animals: Control (n=17), 250ppm Pb-exposure (n=5), 750ppm Pb-exposure (n=12), and 1500ppm Pb-exposure (n=11). * p<0.05, ** p<0.01, *** p<0.001 and **** p<0.0001.

Fig. 4

Water maze probe trial performance of male rats. (A) Percent of time spent in the Northeast (NE) quadrant during the first 30 seconds of the probe test. Only the non-enriched animals exposed to 750 ppm lead performed significantly differently than matched enriched animals. (B) Percent of time spent in the NE (platform) annulus during the first 30 seconds of the probe test. There was no significant effect of lead exposure or rearing environment on this measure. (C) Path efficiency of first entry into the NE (platform) annulus during the first 30s of the probe test. Non-enriched animals exposed to either 250 ppm or 1,500 ppm lead (but not 750 ppm lead) were impaired on this measure compared to matched enriched animals. (D) Swim speed within the first 30s of the probe test. Statistically significant but not functionally meaningful differences were observed within enriched environment at 1500ppm Pb exposure. Group Sizes: Enriched Animals - Control (n=18), 250ppm Pb-exposure (n=10), 750ppm Pb-exposure (n=10), and 1500ppm Pb-exposure (n=11); Non-Enriched Animals: Control (n=11), 250ppm Pb-exposure (n=10), 750ppm Pb-exposure (n=13), and 1500ppm Pb-exposure (n=12). All data are shown as group mean ± S.E.M. Statistical differences between enriched and non-enriched groups within a given exposure level are defined as follows: ♦ p<0.05, ♦♦ p<0.01. Statistical differences within environment are defined using linking bars: ** p<0.01.

Fig. 5

Water maze probe trial performance of female rats. (A) Percent of time spent in the Northeast (NE) quadrant during the first 30 seconds of the probe test. There was no significant effects within lead exposure or rearing environment on this measure. (B) Percent of time spent in the NE annulus during the first 30 seconds of the probe test. Non-enriched 750 ppm-exposed and control animals were significantly impaired on this measure compared to matched enriched animals. (C) Path efficiency of first entry into the NE annulus during the first 30s of the probe test. Non-enriched animals exposed to either 250 ppm or 750 ppm lead (but not 1,500 ppm lead) were impaired on this measure compared to matched enriched animals. (D) Swim speed within the first 30s of the probe test was statistically slower in the 250 ppm, non-enriched group compared to matched enriched animals. Group Sizes: Enriched Animals - Control (n=20), 250ppm Pb-exposure (n=16), 750ppm Pb-exposure (n=10), and 1500ppm Pb-exposure (n=20); Non-Enriched Animals: Control (n=17), 250ppm Pb-exposure (n=5), 750ppm Pb-exposure (n=12), and 1500ppm Pb-exposure (n=11). All data are shown as group mean ± S.E.M. Statistical differences between enriched and non-enriched groups within a given exposure level are defined as follows: ♦ p<0.05, ♦♦ p<0.01.