Zinc differentially acts on components of long-term potentiation at hippocampal CA1 synapses

Abstract

Long-term potentiation (LTP) at hippocampal CA1 synapses consists of N-methyl-d-aspartate (NMDA) receptor-dependent and NMDA receptor-independent forms. The action of divalent heavy metals, which are NMDA receptor antagonists, was examined focusing on the evidence that CA1 LTP induced by a 100-Hz tetanus for 1 s is abolished in the presence of 2-amino-5-phosphonovalerate (APV), a NMDA receptor antagonist. Only ZnCl₂ (5 μM) of heavy metals tested potentiated CA1 LTP. CA1 LTP induced by repeated 100-Hz tetanus (1 s, 6 times, 10 min interval), which reached a plateau in magnitude, was abolished in the presence of 50 μM APV. In this case, CA1 LTP after the first tetanus was potentiated in the presence of 5 μM ZnCl₂, whereas CA1 LTP after the last tetanus was not potentiated. These results indicate that the magnitude of NMDA receptor-dependent CA1 LTP can be positively shifted with 5 μM ZnCl₂ in the range of the maximum magnitude. CA1 LTP induced by a 200-Hz tetanus for 1 s was not potentiated in the presence of 5 μM ZnCl₂ and was partially inhibited in the presence of APV. Furthermore, CA1 LTP induced by a 200-Hz tetanus for 1 s in the presence of APV was not potentiated in the presence of 5 μM ZnCl₂, indicating that NMDA receptor-independent CA1 LTP is not potentiated with 5 μM ZnCl₂. The present study suggests that zinc differentially acts on CA1 LTP components.

Introduction

The hippocampus plays an important role in learning, memory and recognition of novelty. The most widely accepted mechanisms of memory formation are synaptic plasticity (Bliss and Lomo, 1973, Bliss and Collingridge, 1993). The mechanisms of plasticity at the Schaffer collateral/commissural synapses in the hippocampal CA1 region have been extensively studied in the brain (Bliss and Collingridge, 1993, Malenka and Nicoll, 1999, Luscher et al., 2000). It is well established that long-term potentiation (LTP) at this pathway involves the synaptic activation of N-methyl-d-aspartate (NMDA) receptors (Nicoll and Malenka, 1999, Malenka and Bear, 2004). This activation plays a key role for the increase in postsynaptic Ca²⁺ concentration.

The activation of NMDA receptors, which are heterogeneous with multiple subclasses (Mayer and Armstrong, 2004), is blocked by divalent heavy metals such as zinc (IC₅₀ for the low-affinity site, approximately 20 μM at − 40 mV) and copper (IC₅₀, 0.27 μM) (Vlachova et al., 1996, Paoletti et al., 2009). Zinc and copper are trace metals that play essential roles in the brain function as well as brain development (Prohaska, 1987, Mathie et al., 2006). They are present at high levels in the brain, especially in the hippocampus (Hanig and Aprison, 1967, Donaldson et al., 1974, Tarohda et al., 2004). A major portion of both metals serves as key components in many proteins and co-factors for the activity of many enzymes that are critical for brain function (Vallee and Falchuk, 1993). Both ions can also function as signaling molecules; zinc and copper, which are histochemically reactive as revealed by Timm's sulfide-silver staining method, are concentrated in synaptic vesicles, especially in some glutamatergic neurons (Frederickson, 1989).

It is estimated that the basal concentrations of zinc and copper in the brain extracellular space are < 0.5 μM and 0.2–1.7 μM, respectively (Weiss et al., 2000, Mathie et al., 2006). Because they are co-released with neurotransmitters during neuronal excitation (Vogt et al., 2000, Hopt et al., 2003), it is possible that the extracellular concentrations of zinc and copper in the brain are changed spatially and temporally. However, the changes in their concentrations are poorly understood. The extracellular concentration of zinc in the hippocampus, which is stained at high densities by Timm's method, is estimated to be less than 1 μM, judging from the data on in vivo microdialysis experiments (Takeda et al., 2003). Although the extracellular concentration of zinc reached during LTP induction is a matter of debate, the data that zinc (5 μM) multi-functionally modulates LTP induction in the hippocampus imply that it is very low micromolar (Takeda et al., 2008, Takeda et al., 2009a).

It is likely that other heavy metals such as manganese exist in the synaptic vesicles (Takeda 2003). Manganese can be released into the extracellular space by neuronal excitation (Takeda et al., 1998), although extracellular manganese concentration after tetanic stimulation is estimated to be much less than zinc and copper. Cadmium is believed to be unnecessary for brain function. When cadmium is transported into brain extracellular space after exposure to cadmium, it is taken up into neurons and also released into the extracellular space by the excitation (Minami et al., 2001). Manganese and cadmium can serve as NMDA receptor blockers (IC₅₀, approximately 36 μM and 48 μM for manganese and cadmium, respectively) (Mayer et al., 1989, Guilarte and Chen, 2007) and voltage-dependent calcium channel (VDCC) blockers (Kostyuk, 1986, Castelli et al., 2003). However, the action of manganese and cadmium in synaptic neurotransmission remains to be clarified (Minami et al., 2001, Takeda, 2003).

On the basis of the evidence that ZnCl₂ (5 μM) potentiates NMDA receptor-dependent CA1 LTP (Takeda et al., 2009a), the effect of zinc, copper, manganese and cadmium on NMDA receptor-dependent CA1 LTP was compared in the present study. The potentiation of NMDA receptor-dependent CA1 LTP was zinc-specific in heavy metals tested. CA1 LTP is also induced in NMDA receptor-independent pathways (Harris and Cotman, 1986, Grover and Teyler, 1990, Cavuş and Teyler, 1996). Thus, the effect of zinc on NMDA receptor-independent forms was examined.