Developmental exposure to PCBs, MeHg, or both: long-term effects on auditory function

Abstract

Background: Developmental exposure to polychlorinated biphenyls (PCBs) or methylmercury (MeHg) can result in a variety of neurotoxic effects, including long-term auditory deficits. However, little is known about the effects of combined exposure to PCBs and MeHg on auditory function.

Objective: We developmentally exposed rats to PCBs and/or MeHg and assessed auditory function in adulthood to determine the effects of exposure to these contaminants individually and in combination.

Methods: We exposed female Long-Evans rats to 1 or 3 mg/kg PCB in corn oil, 1.5 or 4.5 ppm MeHg in drinking water, or combined exposure to 1 mg/kg PCB + 1.5 ppm MeHg or 3 mg/kg PCB + 4.5 ppm MeHg. Controls received corn oil vehicle and unadulterated water. Dosing began 28 days before breeding and continued until weaning at postnatal day (PND) 21. Auditory function of the offspring was assessed at approximately PND 200 by measuring distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs).

Results: Groups exposed to PCBs alone had attenuated DPOAE amplitudes, elevated DPOAE thresholds, and elevated ABR thresholds compared with controls. Groups exposed to MeHg alone did not differ from controls. Unexpectedly, the effects of PCB exposure appeared to be attenuated by coexposure to MeHg.

Conclusion: Developmental exposure to PCBs can result in permanent hearing deficits, and the changes in DPOAE amplitudes and thresholds suggest a cochlear site of action. Coexposure to MeHg appeared to attenuate the PCB-related deficits, but the mechanism for this unexpected interaction remains to be determined.

Keywords: ABR; DPOAE; MeHg; PCB; auditory; development.

Figure 1

An ABR waveform generated from a control rat in response to a 6-kHz tone at 65 dB, indicating peaks 1–4.

Figure 2

Group mean DPOAE amplitudes of control rats compared with exposed rats. Exposure to PCBs only (A), MeHg only (B), or PCBs + MeHg (C). *Main effect across all frequencies is significantly different from controls (p < 0.05).

Figure 3

Group mean DPOAE thresholds of control rats compared with exposed rats. Exposure to PCBs only (A, D), MeHg only (B, E), or PCBs + MeHg (C, F) for male rats and female rats. Letters indicate a significant difference (p < 0.05) between controls and 3 mg/kg PCB (a), 1 mg/kg PCB (b), 3 mg/kg PCB + 4.5 ppm MeHg (c), and 1 mg/kg PCB + 1.5 ppm MeHg (d).

Figure 4

Group mean ABR thresholds of control rats compared with exposed rats. Exposure to PCBs only (A), MeHg only (B), or PCBs + MeHg (C). Letters indicate a significant difference (p < 0.05) between controls and 3 mg/kg PCB (a), 1 mg/kg PCB (b), 4.5 ppm MeHg (c), 1 mg/kg PCB + 1.5 ppm MeHg (d), and 3 mg/kg PCB + 4.5 ppm MeHg (e).

Figure 5

Group mean ABR peak 1 amplitude across all frequencies. *Significant difference (p < 0.05).