Abstract
Developmental exposure to polychlorinated biphenyls (PCBs), environmental toxicants found throughout the world, results in neurodevelopmental delays and/or deficits. Previous mechanistic studies have demonstrated that PCBs elicit a broad spectrum of biochemical responses that include slow, graded increases in intracellular Ca2+. Acute exposure of cultures of newborn rodent cortical neurons to the commercial PCB mixture Aroclor 1254 [A1254; 1–20 μM (0.3–6 ppm)], induced recurring oscillations of intracellular Ca2+ concentration (individual Ca2+ amplitudes of 200–600 nM). This oscillatory activity was absent in control (0.5 mM Mg2+-containing) solution. Ca2+ oscillations induced by a 1-h exposure to A1254 were concentration dependent, as measured by cell recruitment (proportion of responding cells) as well as by Ca2+ oscillation frequency and amplitude. Extracellular Ca2+ entry via L-type voltage-sensitive Ca2+ channels (VSCCs) was required to elicit the Ca2+ oscillations because oscillations induced by A1254 were blocked in Ca2+-deficient solution or by addition of 1 μM nifedipine. Tetrodotoxin also blocked the Ca2+oscillations, suggesting that synaptic activity may activate VSCCs. To examine this further, the role of postsynaptic receptors that indirectly activate L-type VSCCs was examined. At 4 to 5 days in vitro, when GABA exerts a depolarizing action and activates L-type channels, addition of bicuculline blocked Ca2+oscillations induced by A1254. After longer maintenance of the cells in vitro (7 days), A1254-induced Ca2+ oscillations were selectively blocked by a combination ofN-methyl-d-aspartate and non-N-methyl-d-aspartate receptor antagonists (d-2-amino-5-phosphonopentanoic acid and 2,3-dihydroxy-6,7-dinitroquinoxaline, respectively). These novel findings show the induction of network activity in an in vitro model by A1254 via activation of excitatory GABAergic and/or glutamatergic synaptic activity, depending on the stage of maturation.
Footnotes
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Send reprint requests to: Jon R. Inglefield, Neurotoxicology Division, MD-74B, National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. E-mail:inglefield.jon{at}epamail.epa.gov
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↵1 This research was funded, reviewed, and approved by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency. Approval does not signify that the contents necessarily reflect the views and policies of the agency nor does mention of trade names or commercial products constitute endorsement or recommendation for use. Portions of this work were presented at the Annual Meeting of the Society of Neuroscience and have been published (Inglefield and Shafer, 1999).
- Abbreviations:
- PCB
- polychlorinated biphenyl
- VSCC
- voltage-sensitive Ca2+ channel
- GABA
- γ-aminobutyric acid
- A1254
- Aroclor 1254
- DIV
- days in vitro
- DMSO
- dimethyl sulfoxide
- NMDA
- N-methyl-d-aspartate
- d-AP5
- d-2-amino-5-phosphonopentanoic acid
- BIC
- bicuculline
- DNQX
- 2,3-dihydroxy-6,7-dinitroquinoxaline
- fura-2/AM
- fura-2/acetoxymethyl ester
- NR
- no response
- Received February 2, 2000.
- Accepted June 15, 2000.
- U.S. Government
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