Effects of developmental PCB exposure on audiogenic and amygdala kindling models of seizure

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that can accumulate in the adipose tissue. PCBs can then be mobilized and transported across the placenta and through breast milk to affect developing offspring. Previously we have observed that developmental exposure to PCBs results in increased susceptibility to audiogenic seizures (AGS) in 12 month old rats that had previously been exposed to a noise induced hearing loss paradigm. Here, studies were conducted to assess the susceptibility to AGS in young adult rats following developmental PCB exposure alone (without loud noise exposure), and to determine if there was an underlying decrease in GABA inhibitory neurotransmission in the inferior colliculus (IC) that could potentially explain this effect. Rats were developmentally exposed to the environmentally relevant ‘Fox River PCB mixture’ at 0 or 6 mg/kg/day through gestation and until weaning. As young adults 1 male and 1 female from each of the exposed litters were tested on a classic AGS paradigm. Another male and female pair from each litter was chosen for western blot analysis of glutamic acid decarboxylase (GAD) protein and GABAAα1 receptor protein expression in the IC, the site in the auditory brainstem where AGS are initiated. There was a significant increase in the number and severity of AGS in rats (both males and females) from the 6 mg/kg PCB dose group compared to control. There also was a significant decrease in GAD65 but no decrease in GAD67 or GABAAα1 protein levels in the IC of PCB exposed animals compared to controls. A study was then conducted to assess whether developmental PCB exposure would result in increased seizure susceptibility outside the auditory brainstem using the amygdala kindling model (a well characterized model of temporal lobe epilepsy in rodents). Two males from each of the litters described above were implanted bilaterally with electrodes in the basolateral amygdala as adults. For each animal a number of seizure indices were measured, which included, afterdischarge (AD) thresholds, primary and total AD duration, sessions to each stage of behavioral seizure, sessions spent in each behavioral stage and generalized seizure thresholds. Although developmental PCB exposure did not affect the AD threshold or the total AD duration, PCB exposure did increase the latency to behavioral manifestations of seizure propagation. PCB exposed animals took significantly longer to reach stage 2 seizures compared to control animals indicating that PCB exposure attenuated focal (amygdaloidal) excitability. Given this unexpected finding, measures of GAD65 and GAD67 protein levels were made in the basolateral amygdala. No significant effect of PCB exposure was seen for either GAD65 or GAD67 in the basolateral amygdala. These series of experiments confirmed that developmental PCB exposure caused AGS to occur in adulthood and that reduced GAD65 in the IC may explain this increased susceptibility. The amygdala kindling study exposed gaps in our knowledge of PCB action outside the auditory system. The delay in kindling may be explained by reduced plasticity due to PCB exposure, but further research is needed to confirm this. Together, these studies have shown that PCB exposure during development has complex actions in neural systems and that more research is needed in order to better understand the potential for negative human health impacts