Doctor of Philosophy
Pharmacology & Toxicology
Billy R. Martin
Polychlorinated biphenyls (PCBs) are industrial compounds whose ubiquitous environmental contamination has been known since the late 1960's. Incidents of human intoxication have been reported, most notably, the "Yusho" incident that affected over 1600 people in Japan in 1968. A broad spectrum of adverse effects are produced by PCBs, including some neurological symptoms. While examination of the toxic effects of PCBs has received much attention, there has been little work on the neurotoxicity of PCBs. The intent of the experiments presented in this thesis is to provide further information on the central neurotoxicity of PCBs in mice.
A commercial mixture of PCBS (Aroclor 1254) was used in these experiments. The first set of experiments was initiated to evaluate the behavioral effects of orally administered Aroclor 1254 in mice. A dose of 500 mg/kg depressed spontaneous motor activity at times of 15 min to 3 hr after gavage, with the peak effect observed at 45 min. A dose-response curve generated at 45 min revealed statistically significant effects only at the highest dose, 500 mg/kg. Aroclor 1254 had no effect on the other behavioral measures evaluated - performance on the rotor rod and inverted screen and effect on pentylenetetrazol-induced convulsions. These results suggest that Aroclor 1254 may be exerting a CNS depressant effect as evidenced by a suppression of spontaneous activity in the absence of impaired motor coordination.
In order to examine further the suggested CNS depressant effect, the interaction of Aroclor 1254 with a known CNS depressant, pentobarbital, was investigated. Pretreatment with a single dose of Aroclor 1254 (500 mg/kg) significantly enhanced pentobarbital-induced sleep time at pretreatment times of 0, 0.75, 2, 4 and 8 hr. A dose-response relationship was demonstrated with doses of 5 to 500 mg/kg of Aroclor‘ 1254 given at the pretreatment time of peak effect, 2 hr. By contrast, subchronic (14 day) administration of Aroclor 1254 (30 or 100 mg/kg) reduced pentobarbital induced sleep time in a dose-dependent fashion when pentobarbital was given 45 min after the last dose of Aroclor 1254 with a further reduction when given 24 hr after Aroclor 1254. Results of studies of the disposition of 14C-pentobarbital following Aroclor 1254-pretreatment suggested that acute Aroclor 1254-pretreatment inhibits pentobarbital metabolism while subchronic PCB-pretreatment enhances pentobarbital metabolism as evidenced by tissue levels of pentobarbital and metabolites. Therefore, it appears that the observed increase in pentobarbital-induced sleep time is not a result of altered CNS sensitivity or combined depressant effects of the two compounds, but rather is due to Aroclor 1254-induced alterations in the pharmacokinetics of pentobarbital.
The next series of experiments was aimed at evaluating the effects of Aroclor 1254 on neurochemistry. Aroclor 1254 produced concentration-dependent alterations in the following parameters measured in vitro: inhibition of neurotransmitter and precursor uptake, enhancement of neurotransmitter release, enhancement of synaptosomal 45Ca++ uptake, stimulation of ATP-supported mitochondrial 45Ca++ uptake, inhibition of synaptosomal Na+ /K+ - and Mg++- ATPases, and inhibition of mitochondrial Mg++ -ATPases. Effective concentrations of Aroclor 1254 in these in vitro systems ranged from 10-6 to 10-4 M. Acute (up to 500 mg/kg) and subchronic (90 day) administration of Aroclor 1254 had no effect on' uptake of neurotransmitters or precursors by synaptosomes isolated from Aroclor 1254-treated mice. Nor was there any effect on ATPase activities in synaptosomes or mitochondria isolated from mice exposed to acute or subchronic (14 day) Aroclor 1254 regimens.
This disparity between the results of in vitro and in vivo exposure to Aroclor 1254 prompted studies of the disposition of 14C-PCBs. A time course of radioactivity in plasma and brain following a 500 mg/kg dose of 14C-PCBS (a mixture of PCBs with 50% Cl by weight and uniformly labeled with 14C)compared favorably with that of the behavioral activity of Aroclor 1254. No differences in brain regional distribution of 14C-PCBs was observed. Most importantly, levels of radioactivity in synaptosomes and mitochondria of sub fractionated brains of mice that had been exposed to a behaviorally active dose of 14C-PCBS (500 mg/kg) were comparable to tissue levels in isolated synaptosomes and mitochondria that had been incubated with concentrations of 14C-PCBs (10-5 to 10-4M) that altered neurochemistry in vitro. Thus, effective concentrations of 14C-PCBS are apparently achieved in subcellular organelles of brain following oral administration of a behaviorally active dose. The lack of observable neurochemical effects in Aroclor 1254-treated mice may be due to compensatory mechanisms that maintain homeostasis in the intact brain.
In summary, Aroclor 1254 has been shown to produce a depression of spontaneous motor activity in mice, changes in pentobarbital-induced sleep time (an effect that is probably a manifestation of changes in the pharmacokinetic behavior of pentobarbital rather than a direct effect of Aroclor 1254 on brain), and alterations in a number of neurochemical events in vitro. These results may help to expand the limited data base on the neurotoxicity of PCBs.
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