Lipopolysaccharide induces memory-processing deficits in day-old chicks

Abstract

Recent evidence has demonstrated that immune activation can result in cognitive deficits due to the actions of the proinflammatory cytokines. These series of studies examined the effects of peripheral administration of lipopolysaccharide (LPS) on the memory processes of day-old chicks trained on a single-trial passive-avoidance task. LPS impaired performance in a dose- and time-dependent manner. Maximal impairment was produced by a dose of 2.5-mg/kg LPS administered 60 min prior to training. Retention tests revealed that deficits in memory processing appeared between 10 and 20 min posttraining. These results demonstrate an inhibitory effect of LPS on memory processing at the transition point from short-term memory to intermediate-term memory.

Introduction

To combat infection and injury, organisms undergo a range of physiological and behavioural changes including fever, decreased food and water intake, decreased social exploration, and increased slow wave sleep (Hart, 1988). These responses are collectively known as sickness behaviour (Kent et al., 1992a). Anecdotal and experimental evidence suggests that colds and influenza also produce cognitive deficits in humans (Smith, 1992). While the mechanisms underlying the cognitive effects of sickness are as yet unknown, they are presumed to be due to the central actions of the proinflammatory cytokines.

Lipopolysaccharide (LPS) is the active fragment of gram-negative bacteria. When injected systemically or centrally, LPS mimics the effect of live bacteria and induces sickness behaviour Kent et al., 1992b, Kluger, 1991, Layé et al., 1994. This effect is attributed to the cascade of cytokine synthesis and release from macrophages and other related cell types that is triggered by LPS. The primary proinflammatory cytokines induced by LPS are interleukin (IL)-1β, IL-6, and tumour necrosis factor α (TNF-α).

LPS has been used to induce sickness behaviour in several animal models to assess the link between immune activation and cognition. Intraperitoneal injection of LPS impairs contextual, but not auditory-cue, fear conditioning in rats (Pugh et al., 1998). The selectivity of this impairment is indicative of disruptions in hippocampal processing, as hippocampal lesions have previously been shown to produce the same pattern of results (Phillips and LeDoux, 1994). Further, pretreatment with IL-1 receptor antagonist (IL-1ra) prevented the LPS-induced impairment in contextual fear conditioning. Thus, the authors suggest that IL-1 acting on the hippocampus plays a role in the LPS-induced impairment. It has also been reported that direct injection of LPS into rat hippocampus impairs spatial learning and memory in two other tasks, the Morris water maze (MWM) and the Y-maze when tested between 10 and 17 days later (Yamada et al., 1999). Interestingly, nonspatial long-term memory (LTM) as assessed by a passive-avoidance test was not affected by hippocampal injection of LPS. In two similar studies, bilateral hippocampal infusion of LPS or IL-6 impaired the acquisition and retention of an active avoidance task in the rat when tested 10–21 days later Ma and Zhu, 1997a, Ma and Zhu, 1997b. Avoidance learning is also thought to involve the hippocampus (Lipp et al., 1984).

IL-1β administered intraperitoneally to mice (Gibertini et al., 1995) and into the cerebral ventricles in rats (Oitzl et al., 1993) also produces spatial learning and memory deficits on the MWM. Mice infected with Legionella pneumophila demonstrate spatial learning deficits on the MWM, which were reversed by treatment with anti-IL-1β antibodies, suggesting that they are due to the presence of circulating IL-1β (Gibertini et al., 1995). In a follow-up study, it was reported that IL-1β only impaired MWM performance when the starting position of the animals was varied, but not when they entered at a fixed position (Gibertini, 1996). Learning under the random-start protocol is thought to be hippocampally dependent, while that in the fixed-start protocol is not (McNamara and Skelton, 1993). Thus, the most parsimonious explanation of these results is that hippocampal functioning is diminished in sick animals, disturbing their ability to learn complex relations in their environment, but not simpler motor procedures.

The current study investigated the effects of peripheral immune activation induced by LPS administration on the memory processes of day-old chicks using a single-trial passive-avoidance task. LPS induces hyperthermia and sickness behaviour, including fever, reduced food intake, and increased somnolence in the chicken when administered either peripherally or centrally (Johnson et al., 1993). A large amount of previous experimentation conducted with day-old chicks has provided valuable information regarding biochemical stages underlying memory formation following learning on the passive-avoidance task Andrew, 1991, Gibbs and Ng, 1977, Rose, 2000.

The Gibbs and Ng Gibbs and Ng, 1977, Ng and Gibbs, 1991 model of memory formation consists of three sequentially dependent stages. Short-term memory (STM) is thought to last for 5–10 min following learning, and its formation is attributed to neuronal hyperpolarization arising from an activity-induced increase in potassium conductance. A second stage, intermediate-term memory (ITM), lasts from 20 to 50 min postlearning. The formation of ITM is attributed to neuronal hyperpolarization induced by the electrogenic sodium/potassium (Na⁺/K⁺ ATPase) pump. Sodium pump blockers, such as the cardiac glycoside ouabain, inhibit the formation of ITM and induce retention deficits apparent 15 min postlearning. This stage consists of two distinct phases, ITM(A) and ITM(B), which possess distinct temporal parameters and are susceptible to inhibition by different compounds. ITM(A) is energy-dependent and is susceptible to blockade by the ATP synthesis inhibitor 2,4-dinitrophenol (DNP), whereas ITM(B) is not susceptible to DNP blockade. It is believed that the neuronal events that trigger the transition from ITM(A) to ITM(B) give rise to cellular activities that culminate in LTM. This final stage is defined as retention beyond 60 min postlearning, and is thought to be dependent on protein synthesis. These temporal parameters are consistent with behavioural observations demonstrating that retention of the task consists of three distinct stages of high levels of retention separated by two points of transient retention deficit, one at 15 min and the other at 55 min, which presumably occur as one stage develops into the next.

Three separate experiments were conducted in the present report. The first determined the optimal dose at which LPS induced memory deficits, the second ascertained the time required for LPS to impair memory, and the third determined the stage in memory formation when deficits become apparent.