The effects of chronic stress on hippocampal morphology and function: An evaluation of chronic restraint paradigms

Abstract

Chronic restraint stress for 6  h/21 days causes hippocampal CA3 apical dendritic retraction, which parallels spatial memory impairments in male rats. Recent research suggests that chronic immobilization stress for 2  h/10 days induces CA3 dendritic retraction [Vyas, A., Mitra, R., Shankaranarayana Rao, B.S., Chattarji, S., 2002. Chronic stress induces contrasting patterns of dendritic remodeling in hippocampal and amygdaloid neurons. J. Neurosci. 22, 6810–6818.] and questions whether CA3 dendritic retraction and spatial memory deficits can be produced sooner than found following 6  h/21 days of restraint stress. Therefore, this study investigated the effects of four different durations of chronic restraint stress (varied by hours/day and total number of days) and the subsequent effects on hippocampal CA3 morphology and spatial memory in the same male Sprague–Dawley rats. The results showed that only rats exposed to the 6  h/21 days restraint paradigm exhibited CA3 apical dendritic retraction, consistent spatial memory deficits, and decreased body weight gain compared to experimental counterparts and controls. While chronically stressing a rat with wire mesh restraint has a physical component, it acts primarily as a psychological stressor, and these findings support the interpretation that chronic psychological stress produces hippocampal-dependent cognitive deficits that are consistent with hippocampal structural changes. Differences in stress effects observed across different studies may be due to rat strain, type of stressor, and housing conditions; however, the current findings support the use of chronic restraint stress, with wire mesh, for 6  h/21 days as a reliable and efficient method to produce psychological stress and to cause CA3 dendritic retraction and spatial memory deficits in male Sprague–Dawley rats.

Introduction

The hippocampus plays a critical role in spatial memory ability because damage to the hippocampus corresponds with spatial memory impairments in both lab animals (Conrad et al., 1996, Fortin et al., 2002, Kleen et al., 2006, Luine et al., 1996) and humans (Astur et al., 2002, Kessels et al., 2001, King et al., 2002). Chronic stress may affect hippocampal function through such mechanisms as CA3 neuronal remodeling (for review, see Conrad, 2006), suppression of synaptic activity (Kim and Diamond, 2002, Stewart et al., 2005), and altered neurogenesis (for review, see Leuner et al., 2006, Shors, 2006). In male rats, stress-induced alterations in CA3 apical dendritic arborization parallel deficits in hippocampal function, such as spatial memory impairments in the radial arm maze (Luine et al., 1994) and Y-maze (Conrad et al., 1996, Conrad et al., 2003, Wright and Conrad, 2005), and spatial learning deficits in the water maze (Sandi et al., 2003, Venero et al., 2002). Therefore, stress-induced changes in hippocampal CA3 neurons are consistent with deficits in hippocampal function, including spatial memory.

Hippocampal CA3 dendritic retraction, a remodeling of CA3 dendritic properties, is characterized by decreases in branch points, branch length, and synaptic suppression (Sousa et al., 2000). A variety of experimental conditions produce CA3 dendritic retraction, including 21 days of predator stress combined with high-fat diet (Baran et al., 2005), 6 days of activity stress combined with food restriction (Lambert et al., 1998), 1 month of chronic, unpredictable stress (Sousa et al., 2000) and chronic social stress paradigms, including 14 days of social defeat stress (McKittrick et al., 2000), and 11 sessions of social defeat stress (Kole et al., 2004). In rodents, a common stress paradigm used to elicit CA3 dendritic retraction is chronic restraint stress (Conrad et al., 1999, Magarinos and McEwen, 1995, Stewart et al., 2005, Watanabe et al., 1992a, Watanabe et al., 1992b). Experimental preference for this paradigm includes such factors as the reversal of stress-induced dendritic retraction within 4–10 days of restraint termination (no permanent cell death) and its ease and cost-effectiveness for the researcher. In the chronic restraint paradigm, rats are placed in ventilated, but snug wire mesh restraints for 6 h/21 days. This type of restraint involves a physical component (immobilization), but acts primarily as a psychological stressor through awareness of the inability to escape (Glavin et al., 1994, Servatius et al., 2000). In addition to inducing hippocampal dendritic retraction, chronic restraint stress has also been widely used to assess other hippocampal properties including molecular expression and synaptic activity (Donahue et al., 2006, Ejchel-Cohen et al., 2006, Gao et al., 2006, McEwen, 1999, Stewart et al., 2005, Venero et al., 2002), and hippocampal-dependent behaviors such as spatial memory (Bowman et al., 2003, Conrad et al., 1996, Kleen et al., 2006, Luine et al., 1996, McLaughlin et al., 2005, Sandi et al., 2003, Srikumar et al., 2006). Earlier work with chronic restraint has emphasized that 6 h/13 days of restraint does not alter hippocampal dendritic complexity, and males tested under this paradigm actually show a slight enhancement in spatial memory on the radial arm maze (Luine et al., 1996). However, a recent report suggests that CA3 dendritic retraction can be produced with a shorter duration of restraint (2 h/10 days, Vyas et al., 2002). This finding questions whether researchers should adapt to a new chronic stress paradigm.

A concern when comparing across studies is that different rat strains and restraint mechanisms are used. Rat strains can perceive stressors differently, which can subsequently influence hormonal profiles in response to stress. Therefore, the purpose of the current experiment was to compare different durations of chronic stress, using wire mesh restraint, on CA3 dendritic morphology and spatial memory within the same male Sprague–Dawley rats. The comparison of these different restraint paradigms will be useful in determining the most efficient and effective form of restraint stress needed to induce CA3 dendritic retraction and spatial memory impairments. Moreover, this study will measure brain and behavioral outcomes within the same group of rats to directly demonstrate whether changes in CA3 dendritic complexity are consistent with spatial ability.