Reduction of Iba1-expressing microglial process density in the hippocampus following electroconvulsive shock

Abstract

Recent studies place emphasis on the modulations of immune system in various psychiatric disorders and/or treatments. The aim of this study was to investigate the implications of immune-related glial cells in a rapid-acting treatment for depression, namely, electroconvulsive therapy (ECT). Specifically, the effects of electroconvulsive shock (ECS; animal model of ECT) on microglia were morphologically determined in the mouse hippocampus by using ionized calcium-binding adaptor molecule 1 (Iba1) immunocytochemistry. For comparison, S100β-positive astrocytes, another type of glial cells, were also tested. After 24 hours of acute ECS administration, a meshwork of Iba1-positive microglial processes was largely diminished, although the change was transient. In mice that received chronic ECS administration, the decline of Iba1-positive microglial process meshwork continued even 1 month after the last shock. Morphometric image analysis revealed the significant reduction of Iba1-positive microglial process density following ECS administration. On the other hand, neither acute nor chronic ECS administration made alterations in the patterns of expression of S100β immunoreactivity. No significant changes were detected in the cell surface area of S100β-positive astrocytes following ECS administration. The optical disector analysis demonstrated that ECS did not affect the numerical densities of Iba1-positive microglia and S100β-positive astrocytes in the hippocampus. These results provide some key to understand the potential role of microglia and astrocytes in the antidepressant action of ECT.

Introduction

Microglia represent a distinct type of resident immune-related cells in the brain, which are the first to respond to any changes in brain homeostasis (for review, see Aloisi, 2001). Over the last few decades, a number of studies have been made to show their morphological and functional changes in response to detrimental stimuli (Hailer et al., 1996; for review, see Schwartz et al., 2006). For instance, the microglial process dynamics is regulated by ATP via purinergic receptor at early stages of response to local CNS injury (Haynes et al., 2006). On the other hand, the ramified microglia that were once considered to be at resting state are now drawing major attention as busy and vigilant housekeepers. Recent studies using in vivo two-photon imaging (Davalos et al., 2005, Nimmerjahn et al., 2005) have demonstrated that microglia with a morphology of so called resting cells in non-pathological brains are never quiescent but highly active and continuously checking the local microenvironment with motile processes (for review, see Hanisch and Kettenmann, 2007).

In recent years, increasing attention has been paid to the relationship between neuroimmune system and psychiatric illness (for review, see Schiepers et al., 2005). Especially, the possible immune activation accompanied by elevated production of proinflammatory cytokines has been repeatedly reported in patients with major depression (Lanquillon et al., 2000). Proinflammatory cytokines compromise a heterogenous group of messenger molecules that are released from immune-related cells, such as microglia, astrocytes, and lymphocytes. It should also be noted that microglial activation has been demonstrated in postmortem brains of depressed patients (Bayer et al., 1999, Steiner et al., 2006).

These findings led us to the question whether microglial activity might be regulated by the therapies used in psychiatric practice. Here we focused on a rapid acting psychiatric treatment, electroconvulsive therapy (ECT). Historically, ECT has been mainly used to treat depressive disorders since the late 1930s. Although the risk of relapse should not be ignored, the antidepressant effect is reliable and can persist more than 1 year. Even now, ECT is an indispensable choice for the severest form of major depressive episodes (The UK-ECT, 2003). On the other hand, it may be worth mentioning that many critics have raised concerns about safety of ECT. The controversy continues so far (Sterling, 2000, Fink and Taylor, 2007), although there is an increasing body of evidence that ECT has no detrimental effects (Devanand et al., 1994, Zachrisson et al., 2000, Dwork et al., 2004). In contrast to such big debate on its safety, however, little is known about the therapeutic mechanisms of ECT.

The goal of our study is to elucidate the implications of immune-related glial cells in ECT. To this end, we identified microglia by using ionized calcium-binding adapter molecule 1 (Iba1; Imai et al., 1996) immunocytochemistry and tested possible changes in their morphology and numerical density following electroconvulsive shock (ECS; animal model of ECT) in the hippocampus, which is one of the major targets of modern psychiatric research (for review, see Duman et al., 1997, Henn et al., 2004). For comparison, we examined the effects of ECS on astrocytes, which are also associated with immune system in the brain. As an immunocytochemical marker for astrocytes, we used S100β (Rickmann and Wolff, 1995).