Developmental PCB Exposure Disrupts Synaptic Transmission and Connectivity in the Rat Auditory Cortex, Independent of Its Effects on Peripheral Hearing Threshold

Abstract

Polychlorinated biphenyls (PCBs) are enduring environmental toxicants and exposure is associated with neurodevelopmental deficits. The auditory system appears particularly sensitive, as previous work has shown that developmental PCB exposure causes both hearing loss and gross disruptions in the organization of the rat auditory cortex. However, the mechanisms underlying PCB-induced changes are not known, nor is it known whether the central effects of PCBs are a consequence of peripheral hearing loss. Here, we study changes in both peripheral and central auditory function in rats with developmental PCB exposure using a combination of optical and electrophysiological approaches. Female rats were exposed to an environmental PCB mixture in utero and until weaning. At adulthood, auditory brainstem responses (ABRs) were measured, and synaptic currents were recorded in slices from auditory cortex layer 2/3 neurons. Spontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSCs) were more frequent in PCB-exposed rats compared with controls and the normal relationship between IPSC parameters and peripheral hearing was eliminated in PCB-exposed rats. No changes in spontaneous EPSCs were found. Conversely, when synaptic currents were evoked by laser photostimulation of caged-glutamate, PCB exposure did not affect evoked inhibitory transmission, but increased the total excitatory charge, the number and distance of sites that evoke a significant response. Together, these findings indicate that early developmental exposure to PCBs causes long-lasting changes in both inhibitory and excitatory neurotransmission in the auditory cortex that are independent of peripheral hearing changes, suggesting the effects are because of the direct impact of PCBs on the developing auditory cortex.

Keywords: PCB; auditory cortex; laser photostimulation; patch-clamp; toxin; uncaging.

Figure 1.

Experimental design and summary timeline of PCB treatment. Rows indicate significant experimental timepoints: beginning of dosing (day 0), pairing with male (day 28), parturition (approximately day 56), and weaning (approximately day 77).

Figure 2.

Comparison of ABR thresholds. A, Comparison of ABR thresholds in response to noise between control (blue) and PCB (red) treatments. Boxplots indicate median (horizontal bar), 25th and 75th percentiles (box), range of non-outlier points (vertical whiskers), and outliers (crosses). Black asterisks indicate significant comparisons; *p < 0.05, ***p < 0.001. B, Comparison of thresholds to 4-, 8-, 16-, and 32-kHz tones.

Figure 3.

Image of auditory cortex slice and example voltage-clamp recording. A, Example image of recording electrode placement in a coronal slice containing auditory cortex. Recording pipette walls are highlighted in yellow lines. B, Example of membrane current recorded in voltage clamp with holding potential of −65 mV and bath application of 20 μm GABAzine.

Figure 4.

Comparison of spontaneous and miniature synaptic currents. A, Comparison of frequency of synaptic currents between control (blue) and PCB-treated (red) neurons (sample sizes indicate numbers of neurons). Boxplots indicate median (horizontal bar), 25th and 75th percentiles (box), range of non-outlier points (vertical whiskers), and outliers (crosses). Black asterisks indicate significant comparisons, *p < 0.05. B, Relationship of ABR threshold and sIPSC frequency for control (blue points) and PCB-exposed (red points) groups. Dashed lines indicate robust linear regression fits.

Figure 5.

Demonstration of LSPS mapping of input charge. A, Example image demonstrating positions of stimulation grid and recording electrodes in a coronal slice containing auditory cortex. Cyan points mark the sites of the 32 × 32 photostimulation grid. Recording pipette walls are highlighted in yellow lines. In the example, current recordings were simultaneously collected from two neurons. B, An example photostimulation-evoked current response from the cell positioned on the bottom right. Holding potential was −65 mV. Timing of the laser pulse (1 ms in duration) is indicated by the red arrowhead. A pronounced negative peak begins shortly after the laser onset. C, Map of input charge from current responses to photostimulation at all sites of the 32 × 32 stimulation grid. For recordings of excitatory responses, measured charge is inverted to positive values, and represented by color. D, Distribution of all IPSC latencies across all neurons in this study. The black arrow corresponds to the latency used to distinguish between direct and synaptic events in this study.

Figure 6.

Group averaged maps of photostimulation-evoked synaptic strength. Photostimulation-evoked input maps of charge (A) and amplitude (B) aligned to the recorded cell body. Measured charge at each site is averaged across all cells from each treatment group and is represented in color. EPSC charge maps are presented in the top plots, and IPSC charge maps are presented in the bottom plots. Black vertical scale bar marks 200 μm.

Figure 7.

Comparison of photostimulation-evoked currents between treatments. Comparison of total excitatory (A) and inhibitory (B) input charge, input area, and input distance, between control (blue) and PCB-exposed (red) treated neurons. Boxplots indicate median (horizontal bar), 25th and 75th percentiles (box), range of non-outlier points (vertical whiskers), and outliers (crosses). Black asterisks indicate significant comparisons; *p < 0.05. C, Ratio of excitatory to inhibitory charge between control and PCB-exposed groups.

Figure 8.

Spatial profile of synaptic input. Distance profile of input charge and connection probability for control (blue) and PCB-exposed (red) treatment groups. Response measures are binned by input distance in 80-μm bins, and interpolation between bin means are marked by the solid lines. The shaded areas indicate 1 standard error bounds around the means.