Elsevier

Neuroscience

Volume 117, Issue 2, 21 March 2003, Pages 249-264
Neuroscience

Research paper
Dual effect of Zn2+ on multiple types of voltage-dependent Ca2+ currents in rat palaeocortical neurons

https://doi.org/10.1016/S0306-4522(02)00865-5Get rights and content

Abstract

The effects of Zn2+ were evaluated on high-voltage-activated Ca2+ currents expressed by pyramidal neurons acutely dissociated from rat piriform cortex. Whole-cell, patch-clamp experiments were carried out using Ba2+ (5 mM) as the charge carrier. Zn2+ blocked total high-voltage-activated Ba2+ currents with an IC50 of approximately 21 μM. In addition, after application of non-saturating Zn2+ concentrations, residual currents activated with substantially slower kinetics than control Ba2+ currents. Both of the above-mentioned effects of Zn2+ were also observed in high-voltage-activated currents recorded in the presence of nearly-physiological concentrations of extracellular Ca2+ (1 and 2 mM) rather than Ba2+. Under the latter conditions, 30 μM Zn2+ inhibited high-voltage-activated currents somewhat less than observed in extracellular Ba2+ (∼47% and ∼41%, respectively, vs. ∼59%), but slowed Ca2+-current activation to very similar degrees. All of the pharmacological components in which Ba2+ currents could be dissected (L-, N-, P/Q-, and R-type) were inhibited by Zn2+, the percentage of current blocked by 30 μM Zn2+ ranging from 34 to 57%. Moreover, the activation kinetics of all pharmacological Ba2+ current components were slowed by Zn2+. Hence, the lower activation speed observed in residual Ba2+ currents after Zn2+ block is due to a true slowing of macroscopic Ca2+-current activation kinetics and not to the preferential inhibition of a fast-activating current component. The inhibitory effect of Zn2+ on Ba2+ current amplitude was voltage-independent over the whole voltage range explored (–60 to +30 mV), hence the Zn2+-dependent decrease of Ba2+ current activation speed is not the consequence of a voltage- and time-dependent relief from block. Zn2+ also caused a slight, but significant, reduction of Ba2+ current deactivation speed upon repolarization, which is further evidence against a depolarization-dependent unblocking mechanism. Finally, the slowing effect of Zn2+ on Ca2+-channel activation kinetics was found to result in a significant, extra reduction of Ba2+ current amplitude when action-potential-like waveforms, rather than step pulses, were used as depolarizing stimuli. We conclude that Zn2+ exerts a dual action on multiple types of voltage-gated Ca2+ channels, causing a blocking effect and altering the speed at which channels are delivered to conducting states, with mechanism(s) that could be distinct.

Section snippets

Cell preparation

Young (P12–P22) Wistar rats of either sex were decapitated, according to a procedure approved by the University of Pavia Ethical Committee and compliant with the national laws on animal research. The brain was quickly extracted under hypothermic conditions, the two hemispheres were separated, and each was cut with a McIlwain tissue chopper into 350-μm thick slices, the plane of which was normal to the main axis of the lateral olfactory tract. Layer II of anterior piriform cortex was carefully

Results

Voltage-dependent IBas and ICas were recorded in 145 pyramidal neurons from rat PC layer II. Currents were routinely elicited by delivering 50-ms depolarizing steps starting from 2-s prepulses at –60 mV, which allowed for recording of HVA currents in isolation (see Magistretti and de Curtis, 1998). Unless otherwise explicitly stated, 5 mM Ba2+ was routinely used as the charge carrier.

Discussion

The present study shows that zinc ions can exert a relatively potent blocking action on multiple subtypes of HVA Ca2+ currents in mammalian paleocortical neurons. Although most group VIIA-IIB metal divalent cations (including Mn2+, Co2+, Ni2+, and Cd2+) are known to interact with voltage-gated Ca2+ channels in a number of cell systems, thus exerting, on the corresponding currents, more or less potent inhibitory effects that have been characterized in detail Lansman et al 1986, Swandulla and

Conclusions

In conclusion, we have demonstrated that Zn2+ modulates multiple types of neuronal voltage-gated Ca2+ channels at concentrations that are likely to be of physiological relevance in the CNS. This modulation could modify presynaptic processes, but also postsynaptic excitability (directly, via the action on VDCC, or indirectly, by affecting Ca2+-dependent conductances) and/or Ca2+-dependent cell functions. The verification of these possibilities deserves further investigations.

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