Involvement of glucocorticoid-mediated Zn²⁺ signaling in attenuation of hippocampal CA1 LTP by acute stress

Abstract

Glucocorticoid–glutamatergic interactions have been proposed as a potential model to explain stress-mediated impairment of cognition. However, it is unknown whether glucocorticoid–zincergic interactions are involved in this impairment. Histochemically reactive zinc (Zn²⁺) is co-released with glutamate from zincergic neurons. In the present study, involvement of synaptic Zn²⁺ in stress-induced attenuation of CA1 LTP was examined in hippocampal slices from young rats after exposure to tail suspension stress for 30 s, which significantly increased serum corticosterone. Stress-induced attenuation of CA1 LTP was ameliorated by administration of clioquinol, a membrane permeable zinc chelator, to rats prior to exposure to stress, implying that the reduction of synaptic Zn²⁺ by clioquinol participates in this amelioration. To pursue the involvement of corticosterone-mediated Zn²⁺ signal in the attenuated CA1 LTP by stress, dynamics of synaptic Zn²⁺ was checked in hippocampal slices exposed to corticosterone. Corticosterone increased extracellular Zn²⁺ levels measured with ZnAF-2 dose-dependently, as well as the intracellular Ca²⁺ levels measured with calcium orange AM, suggesting that corticosterone excites zincergic neurons in the hippocampus and increases Zn²⁺ release from the neuron terminals. Intracellular Zn²⁺ levels measured with ZnAF-2DA were also increased dose-dependently, but not in the coexistence of CaEDTA, a membrane-impermeable zinc chelator, suggesting that intracellular Zn²⁺ levels is increased by the influx of extracellular Zn²⁺. Furthermore, corticosterone-induced attenuation of CA1 LTP was abolished in the coexistence of CaEDTA. The present study suggests that corticosterone-mediated increase in postsynaptic Zn²⁺ signal in the cytosolic compartment is involved in the attenuation of CA1 LTP after exposure to acute stress.

Introduction

Stress disturbs physiological and psychological homeostasis of humans and animals (Lee et al., 2002). Stress activates the hypothalamo-pituitary-adrenocortical (HPA) system, increases glucocorticoid secretion from the adrenal cortex, and affects learning and memory processes. The hippocampus participates in cognitive function and also plays an important role in stress response. Hippocampal neurons are enriched with glucocorticoid receptors, in addition to mineralocorticoid receptors and negatively modulate the HPA system activity (Kim and Yoon, 1998). However, the hippocampus is vulnerable to stress (McEwen, 1999, Garcia, 2001, Kim et al., 2006).

Stress and glucocorticoids have diverse effects on synaptic plasticity such as long-term potentiation (LTP) that is thought to be the cellular mechanism of memory (Wong et al., 2007, Howland and Wang, 2008). The mechanisms of the diverse effects are poorly understood. Glucocorticoid–glutamatergic interactions during information processing are proposed as a potential model to explain many of the diverse effects of glucocorticoids on cognition (Sandi, 2011). Glucocorticoids can affect glutamatergic pathways through the increase in extracellular glutamate. An increase in serum corticosterone level produces a rapid increase in corticosterone level in the hippocampus in parallel with a specific increase in extracellular glutamate level (Venero and Borrell, 1999, Droste et al., 2008). In the hippocampus, corticosterone-induced increase in extracellular glutamate can be exerted through a variety of mechanisms, including glucocorticoid receptor-mediated inhibition of glutamate uptake (Virgin et al., 1991, Yang et al., 2005) and membrane mineralocorticoid receptor- or/and glucocorticoid receptor-mediated increase of presynaptic glutamate release probability (Karst et al., 2005, Olijslagers et al., 2008, Wang and Wang, 2009).

Histochemically reactive (chelatable) zinc (Zn²⁺) is co-released with glutamate from zincergic neurons, a subclass of glutamatergic neurons and serves as a signal factor (Zn²⁺ signal) in both the extracellular and intracellular compartments (Frederickson et al., 2005, Takeda and Tamano, 2009). Thus, it is possible that presynaptic release probability of glutamate and zinc is increased by corticosterone in the hippocampus. However, there is no report on glucocorticoid–zincergic interactions in the hippocampus. It is important to study the involvement of glucocorticoid–zincergic interactions in stress-induced impairment of synaptic plasticity and memory, because corticosterone potentially excites zincergic neurons and releases Zn²⁺ from the neuron terminals (Takeda and Tamano, 2010). This idea is supported by the data on dynamics of synaptic Zn²⁺ in the hippocampus after exposure to stress. Restraint stress increases glutamate and zinc in the extracellular fluid of the hippocampus. In contrast, in the cases of exposure to novelty stress (Takeda et al., 2006, Takeda et al., 2009a) and tail suspension stress for a short period (Takeda et al., 2009b), extracellular zinc is decreased, in spite of the increase in extracellular glutamate. The decrease in extracellular zinc is estimated to be due to the increase in zinc influx into neuronal and glial cells. Serum zinc level is decreased after exposure to acute stress and this decrease may be associated with corticosterone-mediated synthesis of metallothioneins in the tissues, especially in the liver (Cousins, 1986, Cousins et al., 1986, Tamano et al., 2000). It is possible that psychological stress facilitates cellular zinc influx through the action of corticosterone and that physical stress, i.e., restraint stress, induces more increases in corticosterone secretion, which might lead to excess of extracellular zinc.

In the present study, the involvement of synaptic Zn²⁺ in stress-induced attenuation of hippocampal CA1 LTP was examined by using clioquinol (5-chloro-7-iodo-8-hydroxyquinoline), a membrane-permeable zinc chelator. Clioquinol has a relatively weak affinity for zinc (K_d, approximately 1 × 10⁻⁷ M) and can reduce synaptic Zn²⁺ (Cherny et al., 2001). On the basis of the data that clioquinol ameliorated stress-induced attenuation of CA1 LTP, the present study also evaluates corticosterone-mediated dynamics of synaptic Zn²⁺ and its significance in the attenuated CA1 LTP.