Nickel modulates the electrical activity of cultured cortical neurons through a specific effect on N-methyl-d-aspartate receptor channels

Abstract

Nickel (Ni2+) is a toxic metal that affects the function of several ionic channels. In the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor (NR), it causes activity enhancement of the channels containing the NR2B subunit and voltage-independent inhibition of those containing NR2A. Thus, it may represent a functional marker for the identification of NR native channel subunits. We investigated the effect of Ni2+ on spontaneous NR currents in cortical neurons, dissociated from 18-day rat embryos and maintained in culture for up to ∼40 days. In whole-cell voltage-clamp at −60 mV, in a Mg2+-free bath containing the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) (10 μM), spontaneous currents were blocked by 10 μM D(–)-2-Amino-5-phosphonopentanoic acid (APV) (10 μM), and by NR2B antagonists, ifenprodil (10 μM) or Ro25-6981 (Ro25, 1 μM), indicating that they are due to NRs containing predominantly the NR2B subunit. In the presence of Ni2+ (30 μM) the amplitude and the frequency of spontaneous currents were increased and the decay time decreased. A higher dose (300 μM) blocked all electrical activity. In current-clamp, Ni2+ (30 μM) caused a ∼5 mV reversible depolarization. The effect of Ni2+, as well as that of NR2B antagonists, was almost independent of days in vitro (DIV) in the range from 18 to 33 DIV. The electrical activity of the neuronal networks measured by microelectrode arrays (MEAs) was also affected by Ni2+, which caused a decrease in firing rate, but an increase in burst duration, while Ro25 (1–10 μM) caused a decrease in both firing rate and burst duration. Finally, reverse transcription polymerase chain reaction (RT-PCR) revealed a predominant expression of NR2B, with no modification during DIV. These results demonstrate that, in these cultured cells, the NR spontaneous current is almost entirely due by NR2B-containing receptors and that Ni2+ affects the electrical activity through a specific effect on NR channels.