Memory for context is impaired by a post context exposure injection of interleukin-1 beta into dorsal hippocampus

Abstract

Prior research has revealed that treatments that elevate the level of the pro-inflammatory cytokine IL-1β in the brain, if given after training, impair contextual but not auditory-cue fear conditioning. The present experiments add to these finding by showing that, (a) IL-1β exerts its effect on contextual fear conditioning by impairing consolidation processes that support the storage of the memory representation of the context; (b) the dorsal hippocampus is a critical site for the effect of IL-1β; (c) the effect of IL-1β cannot be attributed to its effect on glucocorticoid levels; and (d) IL-1β injected into dorsal hippocampus either, immediately, 3, or 24 h, but not 48 h, after training produces this impairment. At this time the mechanisms responsible for this impairment are not understood, but may involve late-phase protein synthesis processes associated with LTP, because later consolidation processes are being disrupted.

Introduction

There has been recent interest in the influence that pro-inflammatory cytokines in the brain have on learning and memory [29]. This interest derives in part from the finding that conditions such as Alzheimer's disease and AIDS-related dementia complex are associated with elevated brain levels of the pro-inflammatory cytokines interleukin-1 beta (IL-1β), intereleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α) [10], [16]. Furthermore, a variety of conditions in which circulating peripheral levels of pro-inflammatory cytokines are elevated are also associated with cognitive and memory disturbances. These include autoimmune diseases, post-surgical periods, infection, chemotherapy, and depression [39], [40], [41]. Moreover, peripheral cytokine or endotoxin administration to humans produces pronounced cognitive and memory impairments [28], [34].

These facts are noted because peripheral cytokines signal the brain via both blood-borne and neural routes [5], [19], [33] and lead to cytokine production within the brain parenchyma by glial cells and neurons [5], [21], [33]. It is especially important to note that IL-1β is prominently increased in the hippocampus following the peripheral induction of cytokines [2], [21]. Thus, conditions which are associated with elevated peripheral cytokines are likely to also involve increased levels of cytokines in the hippocampus.

The possibility that these products might be functionally significant for memory is further encouraged by the fact that IL-1β interferes with hippocampal long-term potentiation (LTP) [4], [42], and hippocampal LTP has been viewed as an essential element in some forms of memory [20].

Consequently, several investigators have explored the possibility that peripheral immune activation might interfere selectively with performance on learning and memory of tasks that depend on processes supported by the hippocampus. Thus, it has been reported that infection with Legionella pneumophilia selectively interferes with the spatial version of the Morris water maze task [9], as does the intracerebroventricular (ICV) administration of IL-1β itself [23].

More recently, we have used the fear conditioning task to study the impact of immune activation on hippocampal-dependent memory processes (see [29] for a review). In the classic fear conditioning paradigm, rats exposed to tone-footshock pairings in a novel context later display defensive fear responses when re-exposed to either the conditioning context (contextual fear conditioning) or to the tone that preceded shock (auditory-cue fear conditioning). Importantly, contextual fear conditioning depends on the integrity of the hippocampus, whereas auditory-cue fear conditioning does not [14], [26]. We have found that several post-conditioning treatments that increase IL-1β in the hippocampus selectively impair contextual but not auditory cue fear conditioning. These treatments include peripheral injections of lipopolysaccharide (LPS, a constituent of the cell wall of Gram negative bacteria), isolation-induced stress, and ICV injection of gp120 and IL-1β. It is also important to note that the effects of these treatments were blocked in rats that were treated with the IL-1 receptor antagonist (IL-1ra) [29], [30], [31], [32].

These data suggest that elevated levels of brain IL-1β interfere with hippocampal-dependent memory processes [29]. However, a number of other issues remain unanswered. The first of these issues concerns which of the processes assumed to contribute to contextual fear conditioning is impaired by elevated brain IL-1β. Several theorists have proposed that there are at least two processes that contribute to contextual fear conditioning. First, the organism must form and store a conjunctive representation of the context which ‘binds together’ features of the context into a unified neural ensemble. Second, the organism must associate this representation with the unconditioned stimulus, the shock. In this framework, the hippocampus is assumed to be critical in the formation and storage of contextual representations but not in its association with shock [8], [22], [36], [45].

The implication of this view is that the impaired contextual fear conditioning we have previously reported [29] was due to IL-1β's effect on the processes that form and store the memory of the conjunctive representation of context, and not on the associative processes linking the contextual representation to shock. One purpose of the present experiments was to test this hypothesis more directly by studying the so-called immediate-shock effect [6]. If a rat is placed in the conditioning context for about 1 min before the onset of the shock, it will subsequently display fear of that context. However, if it is shocked immediately upon being placed in the chamber, it will subsequently show almost no fear. Fanselow's important observation was that pre-exposing the rat to the context prior to the shock session significantly facilitated the contextual fear conditioning produced by immediate shock (see also [13]). We will call this outcome the context-pre-exposure facilitation effect.

If the hypothesis that IL-1β impairs the hippocampal-dependent processes that support the storage of the contextual representation is correct, then an injection into the brain following context pre-exposure should prevent the context-pre-exposure facilitation effect.

A second issue concerns the hypothesis that IL-1β's impact on memory is due to its interference with processes supported by the hippocampus. Although our previous data are consistent with this idea, the evidence is indirect because our treatments were administered either peripherally or ICV [29], [30], [31], [32]. Thus, in the studies that follow, IL-1β is injected directly into dorsal hippocampus.

A third issue concerns the possibility that the effects of IL-1β on memory might be mediated indirectly by its influence on the adrenal glucocorticoid response. Both peripheral immune activation and ICV IL-1β administration produce corticosterone (CORT) increases [3], [37], and high levels of CORT can interfere with hippocampal-dependent learning and memory processes [43]. It has been shown that an acute intra-hippocampal administration of 10 ng/0.5 μl per side IL-1β produces significant CORT increases [24]. However, it is not known whether the dose to be used in the present experiments (1 ng/0.5 μl per side IL-1β) produces CORT increases, and so this possibility is investigated here. If this dose of intra-hippocampal IL-1β does not significantly increase CORT levels above levels produced by a vehicle injection, then CORT action cannot account for any effects of intra-hippocampal IL-1β on memory in the present paradigm.

Finally, all prior experiments have administered agents that elevate IL-1β immediately after the termination of training, and so the timecourse of interference with memory produced by brain IL-1β is not known. Thus, the interval of time between context pre-exposure and intra-hippocampal IL-1β administration is manipulated here.