Spatial alternation deficits following developmental exposure to Aroclor 1254 and/or methylmercury in rats

Polychlorinated biphenyls (PCBs) and methylmercury (MeHg) are ubiquitous environmental contaminants that alter cognitive function in both humans and animals. Because PCBs and MeHg often occur together in the environment, it is important to understand whether these two contaminants have the potential to interact, causing additive or greater than additive effects. The current study examined the combined effects of gestational and lactational exposure to Aroclor 1254 (A1254), a commercial PCB mixture, and MeHg on a series of spatial alternation tasks including cued spatial alternation (CA), non-cued spatial alternation (NCA), and delayed spatial alternation (DSA) in rats using standard two-lever operant testing chambers. Pregnant Long-Evans rats received either 6 mg/kg A1254 pipetted onto a Keebler Vanilla Wafer cookie (PCB-only group), 0.5 ppm. MeHg dissolved in the drinking water (MeHg-only group), 6 mg/kg A1254 + 0.5 ppm. MeHg (PCB + MeHg group), or corn oil vehicle and normal tap water (control group) beginning 28 days prior to mating and continuing through postnatal day 16. One male and one female from each litter began testing on spatial alternation at approximately 110 days of age. Animals were reinforced for pressing the lever opposite that pressed on the previous trial. In general, animals exposed to A1254 and/or MeHg were impaired relative to control rats on the NCA and DSA tasks. Significant reductions in NCA performance were observed in the MeHg-only and PCB + MeHg groups, while significant reductions in DSA performance were observed in the PCB-only and MeHg-only groups. The PCB + MeHg group showed a similar magnitude reduction in performance on DSA, but this difference was not statistically significant due to increased variability in that group. The reductions in DSA performance were observed across most of the delays, indicating that memory impairments were not likely the cause of the deficit. Instead, the DSA deficits following exposure to A1254 and/or MeHg are indicative of either an associative or attentional impairment. The results from the current study indicate that combined exposure to PCBs and MeHg does not exacerbate the PCB- or MeHg-induced impairments on spatial alternation tasks.