Full Length ArticleDevelopmental exposure to PCBs alters the activation of the auditory cortex in response to GABAA antagonism
Introduction
Polychlorinated biphenyls (PCBs) are persistent environmental contaminants that were used as dielectric fluids in transformers and capacitors until the 1970s, when they were banned. Exposure of humans to this toxicant is thought to arise mainly from the ingestion of contaminated fish and seafood (Crinnion, 2011). However, additional sources of PCB exposure include caulking materials and fluorescent light ballasts (Anezaki and Nakano, 2015, Hu and Hornbuckle, 2010), and recent work has also revealed that PCBs are inadvertently produced as by-products during the synthesis of paint pigments (Anezaki et al., 2015). PCBs are stable and lipophilic compounds, characteristics which lead to accumulation in adipose tissue, mobilization of PCBs into breast milk, transport across the placental barrier, and resulting exposure to both the fetus and infant (DeKoning and Karmaus, 2000, Jacobson et al., 1984). During brain development, numerous processes must occur in a time dependent and coordinated manner. As a result, this represents a particularly vulnerable period during which exposure to PCBs can induce long-lasting effects on the brain and behavior.
Impairments in auditory function are a consistent finding in human and rodent models examining effects of developmental PCBs (Jusko et al., 2014, Trnovec et al., 2010, Powers et al., 2006, Powers et al., 2009, Poon et al., 2011). Previously, our lab has reported that auditory brainstem response (ABR) thresholds were elevated across all frequencies tested in PCB-exposed rats, indicative of dysfunction in the cochlea or auditory nerve (Powers et al., 2006). Also, developmental PCB exposure in rats decreased amplitudes and increased thresholds for distortion product otoacoustic emissions (DPOAEs), which provide a measure of integrity of the outer hair cells of the cochlea (Powers et al., 2006, Powers et al., 2009, Poon et al., 2011). These studies demonstrate that peripheral structures of auditory system are vulnerable to developmental PCB exposure.
Previous work from our lab has also demonstrated that PCB exposure during early development leads to an increase in audiogenic seizure incidence, a decrease in the latency to onset of audiogenic seizures, and an increase in the severity of seizures in adult male and female rats (Poon et al., 2015). Importantly, these effects were observed in mature adult rats many months after PCB exposure ended, suggesting that PCB exposure altered developmental processes, leading to a permanent increase in susceptibility to audiogenic seizures. These findings were recently replicated in a second study (Bandara et al., 2016), in which it was also demonstrated that GAD67 (glutamic acid decarboxylase), an enzyme that converts glutamate in gamma-aminobutyric acid (GABA), was decreased in the inferior colliculus (IC) in adulthood after developmental exposure to PCBs. Given that the IC is important for the initiation and propagation of audiogenic seizures (N’Gouemo and Faingold, 1996, Ross and Coleman, 2000), this suggests that levels of inhibitory neurotransmission in the IC are permanently decreased after developmental exposure to PCBs, a potential mechanism for the increase in audiogenic seizures seen in these animals.
Although the auditory cortex (AC) has not been implicated directly in the initiation and propagation of audiogenic seizures, this structure has ascending and descending connections to other auditory structures, such as the IC, and plays a modulatory role in many aspects of the response properties of neurons from other auditory and thalamic structures, such as sensitivity to sound frequency in the IC (Bajo and King, 2013, Suga, 2008, Stebbings et al., 2014). In addition, cells within the AC from fragile X knockout mice demonstrate a hyper-responsiveness to sound, and this animal model also displays an increased incidence of audiogenic seizures (Rotschafer and Razak, 2013, Rotschafer and Razak, 2014).
Interestingly, one previous study has reported changes in the AC of adult rats after developmental exposure to PCBs. Exposure to PCB 95, a specific penta-chlorinated congener, led to large changes in the functioning and organization of the AC (Kenet et al., 2007), including alterations in the tonotopic and topographic gradients and receptive field characteristics of neurons in the primary AC. Additionally, the normal plasticity that occurs in response to noise exposure in vivo was altered after developmental PCB exposure. Lastly, the best frequencies to elicit tone-evoked inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs) in the AC were correlated in control animals, while they were not correlated in PCB-exposed rats (Kenet et al., 2007). These findings suggest that the AC is susceptible to perturbations by developmental PCB exposure. However, due to recent findings demonstrating increased audiogenic seizures and changes within the AC, more work is needed to better understand the long-lasting functional changes in inhibition and excitation in the AC after developmental exposure to PCBs.
In the current study, we adapted a flavoprotein autofluorescence technique previously used in mice (Llano et al., 2012, Stebbings et al., 2016) to measure neural activation in response to electrical stimulation of thalamocortical afferents from the medial geniculate body of the thalamus (MGB) to the AC in brain slices from adult rats developmentally exposed to an environmentally relevant mixture of PCBs. In addition to measuring changes under control artificial cerebral spinal fluid (aCSF) bath conditions, GABAA antagonists and N-methyl-d-aspartate (NMDA) antagonists were added to the bath to determine whether the AC activation would differ between slices from oil and PCB-exposed rats in response to blockade of inhibitory or excitatory neurotransmitters, respectively.
Section snippets
Animals
Female and male Long-Evans rats were purchased from Harlan (Indianapolis, IN) as adults and were individually housed in standard polycarbonate cages with woodchip bedding. All rats (including breeders and offspring) were given chow (Harlan Teklad rodent diet 8604) and water ad libitum and were kept on a 12/12-h light cycle (lights on at 0700 h) throughout the entire duration of the study. The rats were maintained in facilities accredited by the Association for the Assessment and Accreditation of
Activation of the auditory cortex
Shown in Fig. 2, under control aCSF conditions, there were no significant main effects of PCB treatment and no PCB treatment × stimulation amplitude interactions in either the upper (Fig. 2A) or lower layers (Fig. 2B) of the AC.
However, analysis of the ratio of SR-95531/aCSF activation revealed significant main effects of treatment (p < 0.01) and stimulation amplitude (p < 0.001), and a significant treatment × stimulation amplitude interaction (p < 0.001) in the upper layers of the AC (Fig. 2C). Post-hoc
Discussion
The current study examined changes in the activation of the AC in adult female rats that were exposed to either oil vehicle or 6 mg/kg PCBs during early development. Although no differences in flavoprotein activation were seen in response to electrical stimulation of thalamocortical afferents during the aCSF bath condition or AP5/aCSF bath condition, slices from PCB-exposed rats had an increased response, most notably at the low amplitudes of stimulation, to blockade of GABAA receptors via
Conflict of interest
None.
Funding
This work was supported by a grant from the National Institute of Environmental Health Sciences (NIEHS) (R01 ES015687 to S.L.S) and by a Beckman Institute Postdoctoral Fellowship (R. N. S.).
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