Vol. 295, Issue 1, 105-113, October 2000

Polychlorinated Biphenyl-Stimulation of Ca²⁺ Oscillations in Developing Neocortical Cells: A Role for Excitatory Transmitters and L-Type Voltage-Sensitive Ca²⁺ Channels¹

Jon R. Inglefield and Timothy J. Shafer

Neurotoxicology Division, National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina

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 of N-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.