Influences of quality of maternal care and environmental enrichment on associative memory function in rats with early life lead exposure

Abstract

Introduction: Children in low socioeconomic status (SES) communities are at higher risk of exposure to lead (Pb) and potentially more severe adverse outcomes from Pb exposures. While the factors encompassing SES are complex, low SES households often have less enriching home environments and parent-child interactions. This study investigated the extent to which environmental/behavioral factors (quality of maternal care and richness of the postnatal environment) may modify adverse effects from Pb exposure.

Methods: Long-Evans female rats were randomly assigned to Control (no Pb), Early Postnatal (EPN: birth through weaning), or Perinatal (PERI: 14 days pre-mating through weaning) Pb exposure groups. From postnatal days (PNDs) 2-9, maternal care behaviors were observed, and dams were classified as low or high maternal care based on amounts of licking/grooming and arched back nursing. At weaning, pups were randomly assigned to enriched or non-enriched environments. At PND 55, animals began trace fear conditioning and associative memory was tested on days 1, 2, and 10 postconditioning.

Results: Control offspring showed no significant effects of maternal care or enrichment on task performance. Females with EPN-Pb exposure and males with PERI-Pb exposure living in the non-enriched environment and having an LMC mother had significant memory impairments at days 2 and 10 that were not observed in comparably housed animals with HMC mothers. Enriched animals had no deficits, regardless of maternal care status.

Conclusion: These results show the potential for modulatory influences of maternal care and housing environment on protecting against or reversing at least one aspect of Pb-induced cognitive/behavioral dysfunction.

Keywords: environmental enrichment; lead exposure; maternal care; memory.

FIGURE 1

During the conditioning phase of the trace fear conditioning test, all groups of animals quickly learned the tone‐shock association and performed similarly over the six tone‐shock pairing trials, as demonstrated by the lack of significant differences in freezing responses across the various test groups. EPN, early postnatal Pb exposure; HMC, high maternal care; LMC, low maternal care; PERI, perinatal Pb exposure.

FIGURE 2

Effects of quality of maternal care and environmental enrichment on associative memory function. A decrease in percent freezing during the trace period from day 1 to day 10 postconditioning is indicative of an associative memory impairment. In males, only PERI LMC non‐enriched animals (N = 10) had a significant impairment compared to similar Control non‐Pb‐exposed animals (N = 10). Within the PERI LMC group, there were significant differences in memory performance at day 2 and day 10 postconditioning between enriched (N = 8) and non‐enriched animals (N = 10). In non‐enriched PERI males, there were also significant differences in memory performance at day 2 and day 10 postconditioning between LMC (N = 10) and HMC (N = 6) groups. In females, only EPN LMC non‐enriched animals (N = 9) had a significant impairment compared to similar Control non‐Pb‐exposed animals (N = 8). Within the EPN LMC group, there were significant differences in memory performance at day 2 and day 10 postconditioning between enriched (N = 9) and non‐enriched (N = 9) animals. In non‐enriched EPN females, there was also a significant difference in memory performance at day 10 postconditioning between LMC (N = 9) and HMC (N = 11) groups. *p < .05; **p < .01; ***p < .001.