Roles of adenosine receptors in the regulation of kainic acid-induced neurotoxic responses in mice

Abstract

Kainic acid (KA) is a well-known excitatory and neurotoxic substance. In ICR mice, morphological damage of hippocampus induced by KA administered intracerebroventricularly (i.c.v.) was markedly concentrated on the hippocampal CA3 pyramidal neurons. In the present study, the possible role of adenosine receptors in hippocampal cell death induced by KA (0.1 μg) administered i.c.v. was examined. It has been shown that 3,7-dimethyl-1-propargylxanthine (DMPX; A2 adenosine receptors antagonist, 20 μg) reduced KA-induced CA3 pyramidal cell death. KA dramatically increased the phosphorylated extracellular signal-regulated kinase (p-ERK) immunoreactivities (IR) in dentate gyrus (DG) and mossy fibers. In addition, c-Jun, c-Fos, Fos-related antigen 1 (Fra-1) and Fos-related antigen 2 (Fra-2) protein levels were increased in hippocampal area in KA-injected mice. DMPX attenuated KA-induced p-ERK, c-Jun, Fra-1 and Fra-2 IR. However, 1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine (PACPX; A1 adenosine receptor antagonist, 20 μg) did not affect KA-induced p-ERK, c-Jun, Fra-1 and Fra-2 IR. KA also increased the complement receptor type 3 (OX-42) IR in CA3 region of hippocampus. DMPX, but not PACPX, blocked KA-induced OX-42 IR. Our results suggest that p-ERK and c-Jun may function as important regulators responsible for the hippocampal cell death induced by KA administered i.c.v. in mice. Activated microglia, which was detected by OX-42 IR, may be related to phagocytosis of degenerated neuronal elements by KA excitotoxicity. Furthermore, it is implicated that A2, but not A1, adenosine receptors appear to be involved in hippocampal CA3 pyramidal cell death induced by KA administered i.c.v. in mice.

Introduction

Kainic acid (KA), the analog of the excitatory amino acid l-glutamate, is a neuroexcitatory and neurotoxic substance. KA, upon binding to non-N-methyl-d-aspartate (non-NMDA) glutamate receptors, causes depolarization of neurons followed by severe status epilepticus, neurodegeneration, plasticity, memory loss and neuronal damage in the mammalian central nervous system, especially the hippocampal formation. [1], [3], [12], [14], [17], [27], [29], [41], [44], [51].

The phosphorylated extracellular signal-regulated kinase (p-ERK) is increased by systemic administration of KA in the rat hippocampus [15], [26]. Seizures induce the sprouting of mossy fibers in the CA3 region of rat hippocampus [4]. We have also demonstrated p-ERK immunoreactivity (IR) is increased in the mouse hippocampus by KA administered intracerebroventricularly (i.c.v.) [23]. The activation of ERK protein has been reported to be important in determining whether a cell survives or undergoes apoptosis [49], and also in activating transcription factors [46].

KA leads to the induction of several types of proto-oncogene products, such as Jun and Fos proteins, which serve as the third messengers in the hippocampus [17], [47]. Administration of KA at convulsant dose also induces the expression of c-Fos, Fos-related antigen 1 (Fra-1), Fos-related antigen 2 (Fra-2) and Jun in rat and mouse hippocampus [18], [22], [30], [35], [38], [50]. The increased expression of c-Fos and c-Jun induced by KA might be a maker in seizure activity, excitotoxicity or the activation of target genes [19], [42], [48].

The activation of microglia can be detected using an activated microglial marker, the complement receptor type 3 (OX-42) IR. It is known that KA applied to CA3 region causes selective pyramidal cell death and activates microglial cells [7], [32].

It has been reported that adenosine release is evoked by KA and adenosine is a powerful anticonvulsant substance [5], [13]. An increase of hippocampal adenosine release and metabolism is associated with seizure activity and the increased adenosine may attenuate seizure activity [5], [11], [34]. Adenosine modulates neuronal activity and neurotransmitter release through interacting with their receptors. Adenosine blocks synaptic transmission by attenuating neurotransmitter release. For example, adenosine enhances the inhibitory action of tetrodotoxin on nerve conduction [40]. Ochiishi et al. [31] have investigated the possible changes of adenosine A1 receptors expression in kainate-treated rats and found that A1 receptors are distributed in the CA2 and CA3 field in the normal rat hippocampus. The mRNA expression of adenosine A2a receptors is distributed in the hippocampus CA1, CA3, dentate gyrus (DG) and cerebral cortex, mainly localized in the pyramidal and granular cells; the same hippocampal regions shows adenosine A1 receptor mRNA expression [10]. GABA release is depressed by A2a receptor activation in the hippocampus [9]. It has been accepted that adenosine shows neuroprotective and neurotoxic roles in the CNS. A1 receptors are known to exert the protective action against hippocampal neuronal death induced by KA epileptic preconditioning or sublethal global ischemia [37]. However, the role of A2 receptors in the regulation of cell death induced by KA administered intracerebroventriculary (i.c.v.) in mice has not been characterized. Furthermore, the cellular mechanism of A2 receptors involved in KA-induced neurotoxicity has not yet been well elucidated. Thus, the effects of adenosine receptor antagonists on KA-induced hippocampal cell death and expression of several signal molecules such as p-ERK, c-Jun, c-Fos, Fra-1, Fra-2 and OX-42 were examined in the present study.