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

Intrahippocampal transplantation of transgenic neural precursor cells overexpressing interleukin-1 receptor antagonist blocks chronic isolation-induced impairment in memory and neurogenesis

The proinflammatory cytokine interleukin-1 (IL-1) within the brain is critically involved in mediating the memory impairment induced by acute inflammatory challenges and psychological stress. However, the role of IL-1 in memory impairment and suppressed neurogenesis induced by chronic stress exposure has not been investigated before now. We report here that mice that were isolated for 4 weeks displayed a significant elevation in hippocampal IL-1beta levels concomitantly with body weight loss, specific impairment in hippocampal-dependent memory, and decreased hippocampal neurogenesis. To examine the causal role of IL-1 in these effects, we developed a novel approach for long-term delivery of IL-1 receptor antagonist (IL-1ra) into the brain, using transplantation of neural precursor cells (NPCs), obtained from neonatal mice with transgenic overexpression of IL-1ra (IL-1raTG) under the glial fibrillary acidic protein promoter. Four weeks following intrahippocampal transplantation of IL-1raTG NPCs labeled with PKH-26, the transplanted cells were incorporated within the dentate gyrus and expressed mainly astrocytic markers. IL-1ra levels were markedly elevated in the hippocampus, but not in other brain regions, by 10 days and for at least 4 weeks post-transplantation. Transplantation of IL-1raTG NPCs completely rescued the chronic isolation-induced body weight loss, memory impairment, and suppressed hippocampal neurogenesis, compared with isolated mice transplanted with WT cells or sham operated. The transplantation had no effect in group-housed mice. These findings elucidate the role of IL-1 in the pathophysiology of chronic isolation and suggest that transplantation of IL-1raTG NPCs may provide a useful therapeutic procedure for IL-1-mediated memory disturbances in chronic inflammatory and neurological conditions.

Early-life infection leads to altered BDNF and IL-1beta mRNA expression in rat hippocampus following learning in adulthood

Neonatal bacterial infection in rats leads to profound hippocampal-dependent memory impairments following a peripheral immune challenge in adulthood. Here, we determined whether neonatal infection plus an immune challenge in adult rats is associated with impaired induction of brain-derived neurotrophic factor (BDNF) within the hippocampus (CA1, CA3, and dentate gyrus) following fear conditioning. BDNF is well characterized for its critical role in learning and memory. Rats injected on postnatal day 4 with PBS (vehicle) or Escherichia coli received as adults either no conditioning or a single 2min trial of fear conditioning. Half of the rats in the conditioned group then received a peripheral injection of 25mug/kg lipopolysaccharide (LPS) and all were sacrificed 1 or 4h later. Basal (unconditioned) BDNF mRNA did not differ between groups. However, following conditioning, neonatal infection with E. coli led to decreased BDNF mRNA induction in all regions compared to PBS-treated rats. This decrease in E. coli-treated rats was accompanied by a large increase in IL-1beta mRNA in CA1. Taken together, these data indicate that early infection strongly influences the induction of IL-1beta and BDNF within distinct regions of the hippocampus, which likely contribute to observed memory impairments in adulthood.

Prostaglandins are necessary and sufficient to induce contextual fear learning impairments after interleukin-1 beta injections into the dorsal hippocampus

The intra-hippocampal administration of interleukin-1beta (IL-1beta) as well as the induction of elevated but physiological levels of IL-1beta within the hippocampus interferes with the formation of long-term memory. There is evidence suggesting that the induction of prostaglandin (PG) formation by IL-1beta is involved in impairments in working and spatial memory following IL-1beta. The present experiments extend these findings by showing that PGs are responsible for memory deficits in contextual fear conditioning that occur following IL-1beta injection into the dorsal hippocampus of Sprague-Dawley rats. Cyclooxygenase (COX) inhibition blocked the disruption in contextual fear conditioning produced by IL-1beta and COX inhibition alone also disrupted contextual memory, suggesting an inverted U-shaped relationship between PG levels and memory. In addition to demonstrating the necessity of PGs in IL-1beta-mediated memory deficits, we also show that PGs injected directly into the dorsal hippocampus are sufficient to impair context memory and significantly reduce post-conditioning levels of BDNF within the hippocampus, suggesting a possible mechanism for the memory-impairing effects of PGs.

A dual role for interleukin-1 in hippocampal-dependent memory processes

Ample research demonstrates that pathophysiological levels of the pro-inflammatory cytokine interleukin-1 (IL-1) produces detrimental effects on memory functioning. However, recent evidence suggests that IL-1 may be required for the normal physiological regulation of hippocampal-dependent memory. To substantiate the physiological role of IL-1 in learning and memory we examined the induction of IL-1 gene expression following a learning experience, and the effects of IL-1 signaling blockade, by either genetic or pharmacological manipulations, on memory functioning. We show that IL-1 gene expression is induced in the hippocampus 24h following fear-conditioning in wild type mice, but not in two mouse strains with impaired IL-1 signaling. Moreover, we report that mice with transgenic over-expression of IL-1 receptor antagonist restricted to the CNS (IL-1raTG) display impaired hippocampal-dependent and intact hippocampal-independent memory in the water maze and fear-conditioning paradigms. We further demonstrate that continuous administration of IL-1ra via osmotic minipumps during prenatal development disrupt memory performance in adult mice, suggesting that IL-1 plays a critical role not only in the formation of hippocampal-dependent memory but also in normal hippocampal development. Finally, we tested the dual role of IL-1 in memory by intracerebroventricular (ICV) administration of different doses of IL-1beta and IL-1ra following learning, providing the first systematic evidence that the involvement of IL-1 in hippocampal-dependent memory follows an inverted U-shaped pattern, i.e., a slight increase in brain IL-1 levels can improve memory, whereas any deviation from the physiological range, either by excess elevation in IL-1 levels or by blockade of IL-1 signaling, results in impaired memory.

Age increases vulnerability to bacterial endotoxin-induced behavioral decrements

Peripheral lipopolysaccharide (LPS) or proinflammatory cytokines produce alterations in learning, memory, and other behaviors. Additionally, research has demonstrated that factors such as dose, route of administration, species, strain, gender, and age are important modulatory factors in the effects of endotoxin exposure. Previous research from our laboratory and others indicate that LPS-induced behavioral deficits are greater in older subjects. The current study examined avoidance learning in a negatively reinforced operant procedure (i.e., two-way active avoidance conditioning) following single or repeated intraperitoneal LPS injections in 2- and 12-month-old male C57BL/6J mice. LPS-treated subjects show impaired acquisition of the task regardless of the age of the subject, as these animals performed significantly fewer avoidance responses than controls. However, the effects of LPS administration were more pronounced in the 12-month-old animals, particularly for the subjects given repeated LPS injections. These results support the hypothesis that endotoxin exposure is capable of altering performance in this task in a way that may reflect deficits in learning, and provide evidence that increased age may exacerbate these deleterious behavioral effects.

Alleviation of the effects of endotoxin exposure on behavior and hippocampal IL-1beta by a selective non-peptide antagonist of corticotropin-releasing factor receptors

Previous research has shown that lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) administration produces learning/memory deficits in a variety of paradigms. In our laboratory, we have consistently observed LPS-induced behavioral alterations in a two-way active avoidance conditioning paradigm. Following LPS administration, one factor that affects cytokine production is corticotropin-releasing factor (CRF). CRF has well known anti-inflammatory effects, via stimulation of ACTH and corticosterone release. However, CRF acting directly on immune cells or within the CNS may potentiate proinflammatory effects. The current experiments explored the potential of antalarmin, a CRF-R1 non-peptide antagonist, to diminish or negate deficits observed with LPS administration. On the first day of testing, four-month-old male C57BL/6J mice received an intraperitoneal (i.p.) injection of antalarmin, followed 90min later by a second i.p. injection of LPS 4h prior to two-way active avoidance conditioning testing. As hypothesized, LPS administration altered performance. However, pretreatment with antalarmin attenuated the adverse effects of LPS administration. Moreover, evidence indicates that antalarmin attenuated hippocampal, but not peripheral, cytokine release. The behavioral results cannot be explained by alterations in the HPA axis, as antalarmin did not affect the LPS-induced rise in corticosterone. The current research contributes preliminary evidence that CRF may be an important factor in the development of LPS-induced behavioral effects, and that blocking the activity of CRF may be sufficient to alleviate some of the effects of endotoxin exposure, possibly due to diminished LPS-induced IL-1beta release in the dorsal hippocampus.

Interleukin-1beta and caspase-1: players in the regulation of age-related cognitive dysfunction

Scientific research on the unprecedented and growing number of older adults in the United States and other industrialized countries has focused much attention on the health consequences of aging. Over the last few decades, inflammation in the brain and its implication in the progression of aging and age-related cognitive dysfunction has been an area of increasing importance to neuroscientists and is now considered as one of the most interesting and promising topics for aging research. One of the critical aspects of inflammatory processes is that the activation of one upstream inflammatory molecule initiates a cascade of self-sustaining inflammatory events which leads to the activation of a number of different downstream functions. Recently, a great deal of attention has been given to the interplay between inflammatory and apoptotic processes and the regulation of these processes by the caspases. The caspase family of proteases can be divided into proapoptotic and pro-inflammatory members. The present review summarizes recent observations of the interactions between the inflammatory cytokine interleuldn-1 (IL-1) beta and the inflammatory/apoptotic caspase-1 and their involvement in age-related impairments in cognition. A comprehensive understanding of these mechanisms could potentially lead to the development of preventive or protective therapies that reduce or inhibit the cognitive decline associated with aging and age-related neurodegenerative disease.

Spatial learning and memory in HIV-1 transgenic rats

HIV-1 infection of the central nervous system impairs neural, cognitive, and behavioral functioning in patients despite antiretroviral therapy. However, studying mechanisms underlying HIV-1-related neurological and cognitive dysfunction has been limited without an adequate animal model. A novel, noninfectious HIV-1 transgenic (HIV-1Tg) rat model was recently created that expresses an HIV-1 provirus with a deletion of functional gag and pol genes. This HIV-1Tg rat reportedly develops clinical manifestations of human HIV disease and thus appears to mimic the persistent infection that results from the presence of HIV viral proteins in the host. We evaluated the HIV-1Tg rat model using the Morris water maze, a popular paradigm for testing learning and memory deficits in rodents. Because of congenital cataracts in HIV-1Tg rats, however, the traditional use of visual navigational cues in this paradigm were precluded. We first designed a modified Morris water maze and demonstrated that neurologically intact rats can effectively learn the water maze in the absence of visual cues and in the presence of non-visual navigation cues. We then tested HIV-1Tg rats in this modified Morris water maze. These HIV-1Tg rats showed a deficit in learning how to swim to the location of the hidden platform but did not show a deficit in their memory of the general location of the hidden platform. These results suggest that the noninfectious HIV-1Tg rat can be a valid model for the behavioral studies of HIV-related neurological dysfunction.

Differential effects of neonatal handling on early life infection-induced alterations in cognition in adulthood

We have previously demonstrated that bacterial infection (Escherichia coli) in neonatal rats is associated with impaired memory in a fear-conditioning task in adulthood. This impairment, however, is only observed if a peripheral immune challenge (lipopolysaccharide; LPS) is administered around the time of learning. We used a brief separation/handling paradigm to determine if the adult memory impairment associated with neonatal-infection could be prevented. Naturally occurring variations in maternal care promote striking variations in offspring cognitive development, and handling paradigms are used to manipulate the quality and quantity of maternal care. Rats were injected on post natal (P) day 4 with E. coli or PBS, and half from each group were handled for 15 min/day from P4 to 20. All rats were then tested in adulthood. Neonatal handling of rats infected as neonates prevented the increase in microglial cell marker reactivity within the hippocampus, and the exaggerated brain IL-1beta production to LPS normally produced by the infection. Thus, these neural processes were now comparable to levels of non-infected PBS controls. Furthermore, handling completely prevented LPS-induced memory impairment in a context-fear task in adult rats infected as neonates. Finally, neonatal handling dramatically improved spatial learning and memory and decreased anxiety in rats treated early with PBS, but had no beneficial effect on these measures in rats infected as neonates. Taken together, these data suggest that maternal care may profoundly influence neuroinflammatory processes in adulthood, and that infection may also prevent maternal care influences on cognition later in life.

Regulation of food intake by inflammatory cytokines in the brain

A number of inflammatory cytokines are synthesized and released after activation of the immune system. In addition to other biological effects, these cytokines can potently inhibit food intake. Cytokine-mediated inhibition of food intake is of particular importance because excessive production of peripheral inflammatory cytokines is often associated with the cachexia-anorexia syndrome seen in some chronic diseases. The weight loss in cachexia is associated with an increase in morbidity and mortality. Understanding how cytokines regulate food intake may be crucial in enhancing quality of life and facilitating recovery in patients exhibiting cachexia. This review describes the main inflammatory cytokines that influence food intake and explores how peripheral cytokines communicate with hypothalamic nuclei to influence feeding.

Interleukin-6 facilitates lipopolysaccharide-induced disruption in working memory and expression of other proinflammatory cytokines in hippocampal neuronal cell layers

Proinflammatory cytokines inhibit learning and memory but the significance of interleukin-6 (IL-6) in acute cognitive deficits induced by the peripheral innate immune system is not known. To examine the functional role of IL-6 in hippocampus-mediated cognitive impairments associated with peripheral infections, C57BL6/J (IL-6(+/+)) and IL-6 knock-out (IL-6(-/-)) mice were trained in a matching-to-place version of the water maze. After an acquisition phase, IL-6(+/+) mice injected intraperitoneally with lipopolysaccharide (LPS) exhibited deficits in working memory. However, IL-6(-/-) mice were refractory to the LPS-induced impairment in working memory. To determine the mechanism by which IL-6 deficiency conferred protection from disruption in working memory, plasma IL-1beta and tumor necrosis factor alpha (TNFalpha), c-Fos immunoreactivity in the nucleus of the solitary tract (NTS), and steady-state levels of IL-1beta and TNFalpha mRNA in neuronal layers of the hippocampus were determined in IL-6(+/+) and IL-6(-/-) mice after injection of LPS. Plasma IL-1beta and TNFalpha and c-Fos immunoreactivity in the NTS were increased similarly in IL-6(+/+) and IL-6(-/-) mice after LPS, indicating high circulating levels of IL-1beta and TNFalpha and activation of vagal afferent pathways were not sufficient to disrupt working memory in the absence of IL-6. However, the LPS-induced upregulation of IL-1beta and TNFalpha mRNA that was evident in hippocampal tissue of IL-6(+/+) mice was greatly attenuated or entirely absent in IL-6(-/-) mice. Collectively, these data suggest that humoral and neural immune-to-brain communication pathways are intact in IL-6-deficient mice but that, in the absence of IL-6, the central cytokine compartment is hyporesponsive.

Interleukin-1β induces anorexia but not spatial learning and memory deficits in the rat

Sickness behaviors are a set of adaptive responses to infection that include lethargy, anorexia, and, of direct relevance to this work, learning and memory impairments. The proinflammatory cytokine, interleukin-1 beta (IL-1beta) has been proposed as the primary peripheral mediator of these sickness behaviors, though few studies have investigated the effects of peripheral IL-1beta on learning and memory. We used three different versions of the Morris water task (Morris water task), a spatial learning and memory task, to separately assess the effects of peripheral IL-1beta on acquisition, consolidation, and retention of spatial location information. Using a dose that induced anorexia, assessed as a significant reduction in body weight, we observed no performance impairments in the IL-1beta-treated rats across the different versions of the task, suggesting that peripheral IL-1beta alone is insufficient to induce spatial learning and memory impairments in the rat. The observed dissociation of anorexia and cognitive dysfunction suggests that, either spatial learning and memory are not principal components of the sickness response, or cognitive dysfunction requires different or additional peripheral mediator(s).

Peripheral infection and aging interact to impair hippocampal memory consolidation

We report that a peripheral injection of Escherichia coli produces both anterograde and retrograde amnesia in 24 month old, but not 3 month old rats for memories that depend on the hippocampus, that is, memory of context, contextual fear, and place learning. The anterograde effect was restricted to measures of long-term memory. Short-term memory was not affected, nor did E. coli produce amnesia for auditory-cue fear conditioning. There were no age related effects on memory in vehicle-treated rats. In addition to these age-related cognitive effects of E.coli, we report that it produced a marked increased in IL-1beta levels in the hippocampus, but not in parietal cortex or serum. These findings support the hypothesis that age is a vulnerability factor that increases the likelihood that an immune challenge will produce a cognitive impairment. It is possible that this cognitive vulnerability is mediated by age-related changes in the glial environment that results in an exaggerated brain pro-inflammatory response to infection.

Neonatal infection-induced memory impairment after lipopolysaccharide in adulthood is prevented via caspase-1 inhibition

We have reported that neonatal infection leads to memory impairment after an immune challenge in adulthood. Here we explored whether events occurring as a result of early infection alter the response to a subsequent immune challenge in adult rats, which may then impair memory. In experiment 1, peripheral infection with Escherichia coli on postnatal day 4 increased cytokines and corticosterone in the periphery, and cytokine and microglial cell marker gene expression in the hippocampus of neonate pups. Next, rats treated neonatally with E. coli or PBS were injected in adulthood with lipopolysaccharide (LPS) or saline and killed 1-24 h later. Microglial cell marker mRNA was elevated in hippocampus in saline controls infected as neonates. Furthermore, LPS induced a greater increase in glial cell marker mRNA in hippocampus of neonatally infected rats, and this increase remained elevated at 24 h versus controls. After LPS, neonatally infected rats exhibited faster increases in interleukin-1beta (IL-1beta) within the hippocampus and cortex and a prolonged response within the cortex. There were no group differences in peripheral cytokines or corticosterone. In experiment 2, rats treated neonatally with E. coli or PBS received as adults either saline or a centrally administered caspase-1 inhibitor, which specifically prevents the synthesis of IL-1beta, 1 h before a learning event and subsequent LPS challenge. Caspase-1 inhibition completely prevented LPS-induced memory impairment in neonatally infected rats. These data implicate IL-1beta in the set of immune/inflammatory events that occur in the brain as a result of neonatal infection, which likely contribute to cognitive alterations in adulthood.

Amygdala regulation of immediate-early gene expression in the hippocampus induced by contextual fear conditioning

The basolateral nuclei of the amygdala (BLA) are thought to modulate memory storage in other brain regions (McGaugh, 2004). We reported that BLA modulates the memory for both an explored context and for contextual fear conditioning. Both of these memories depend on the hippocampus. Here, we examined the hypothesis that the BLA exerts its modulatory effect by regulating the expression of immediate-early genes (IEGs) in the hippocampus. The main findings of these experiments were: (1) Arc activity-regulated cytoskeletal protein (Arc), an immediate-early gene (also termed Arg 3.1) and c-fos mRNA are induced in the hippocampus after a context exposure, or context plus shock experience, but not after an immediate shock; and (2) BLA inactivation with muscimol attenuated the increase in Arc and c-fos mRNA in the hippocampus associated with contextual fear conditioning but did not influence Arc mRNA associated with context exploration. These results support the hypothesis that the amygdala modulates contextual fear memory by regulating expression of IEGs in the hippocampus.

Differential involvement of the hippocampus, anterior cingulate cortex, and basolateral amygdala in memory for context and footshock

Extensive evidence from contextual fear conditioning experiments suggests that the hippocampus is involved in processing memory for contextual information. Evidence also suggests that the rostral anterior cingulate cortex (rACC) may be selectively involved in memory for nociceptive stimulation. In contrast, many findings indicate that the basolateral amygdala (BLA) is more broadly involved in modulating the consolidation of different kinds of information. To investigate further the differential involvement of these brain regions in memory consolidation, the present experiments used a modified inhibitory avoidance training procedure that took place on 2 sequential days to separate context training from footshock training. Male Sprague-Dawley rats were implanted with unilateral cannulae aimed at the (i) hippocampus, (ii) rACC, or (iii) BLA, and given infusions of the muscarinic cholinergic agonist oxotremorine (OXO) immediately after either context training (day 1) or footshock training in that context (day 2). OXO enhanced retention when infused into the hippocampus after context, but not footshock, training. Conversely, OXO infusions enhanced memory when administered into the rACC immediately after footshock, but not context, training. Lastly, intra-BLA OXO infusions enhanced retention when administered after either context or footshock training. These findings are consistent with evidence that the hippocampus and rACC play selective roles in memory for specific components of training experiences. Additionally, they provide further evidence that the BLA is more liberally involved in modulating memory consolidation for various aspects of emotionally arousing experiences.

Improvement of memory for context by inhibition of caspase-1 in aged rats

Impaired learning and memory is a common pathologic feature associated with numerous neurologic disorders. There is strong evidence that central inflammation contributes significantly to the progression of several neurodegenerative diseases as well as to the ageing process. For example, in aged rats an increase in interleukin-1beta (IL-1beta) is implicated in the decline of synaptic plasticity in the hippocampus and impaired performance on cognitive tasks such as contextual fear conditioning. IL-1beta is a proinflammatory cytokine initially synthesized in an inactive precursor form that is cleaved by caspase-1 to generate the biologically mature form. In the present study, cleavage of IL-1beta was chronically inhibited using a specific caspase-1 inhibitor (Ac-YVAD-CMK; 10 pmol) in both aged (22 month) and young (4 month) rats. Both groups received Ac-YVAD-CMK for 28 days intracerebroventricularly through a brain infusion cannula connected to an osmotic minipump. On day 20 the animals were trained in contextual fear conditioning, and memory for context was tested on day 22. Chronic infusion of a specific caspase-1 inhibitor in aged rats ameliorated age-related increases in hippocampal IL-1beta and improved memory for context.

Effects of post-training hippocampal injections of midazolam on fear conditioning

Benzodiazepines have been useful tools for investigating mechanisms underlying learning and memory. The present set of experiments investigates the role of hippocampal GABA(A)/benzodiazepine receptors in memory consolidation using Pavlovian fear conditioning. Rats were prepared with cannulae aimed at the dorsal hippocampus and trained with a series of white noise-shock pairings. In the first experiment, animals received intrahippocampal infusion of midazolam or vehicle immediately or 3 h after training. Then, 24 h later, freezing to the training context and the white noise were measured independently. Results show infusion of midazolam immediately, but not 3 h, after training selectively attenuates contextual fear conditioning. In the second experiment, animals received intrahippocampal infusions of an antisense oligodeoxynucleotide (ODN) targeting the alpha5 subunit of the GABA(A) receptor or a missense control for several days prior to training and testing. Immediately after training, animals received an infusion of either midazolam or vehicle. Western blots conducted after testing showed a significant decrease in alpha5-containing GABA(A) receptor protein. This reduction did not alter the effectiveness of midazolam immediately after training at impairing context fear memory. Therefore, alpha5-containing GABA(A) receptors may not contribute to the effects of midazolam on context fear conditioning when given immediately post-training.

Context pre-exposure obscures amygdala modulation of contextual-fear conditioning

We report that post-training inactivation of basolateral amygdala region (BLA) with muscimol impaired memory for contextual-fear conditioning (as measured by freezing) and intra-BLA norepinephrine enhanced this memory. However, pre-exposure to the context eliminated both of these effects. These findings provide a likely explanation of why an earler study failed to observe that the BLA modulates contextual fear conditioning-they pre-exposed their rats to the context. These results also suggest that the amygdala modulates the storage of the context fear memory and may do so by influencing the storage of the representation of the context in which the shock occurred.

A behavioural characterization of neonatal infection-facilitated memory impairment in adult rats

We have reported that exposure to bacteria (Escherichia coli) during the neonatal period in rats is associated with impaired memory for a novel context in adulthood. However, impairment is only observed if a peripheral immune challenge (bacterial lipopolysaccharide (LPS)) is administered immediately following context exposure. The goal of the current study was to more fully characterize this phenomenon. In Experiment 1, memory impairment as a result of neonatal infection and subsequent LPS challenge was observed in juvenile rats, indicating that the changes induced by infection occur early on and are then manifest throughout the lifespan. In Experiment 2, infection in juvenile rats did not lead to LPS-induced memory impairment in adulthood, suggesting there is a critical period for early infection-induced alterations. In Experiments 3 and 4, memory for a novel context was impaired in neonatally infected rats, a task that is dependent on the hippocampus, whereas cued memory for a tone, which does not depend on the hippocampus, was not impaired. Furthermore, long-term, but not short-term contextual memory was impaired in adult rats infected as neonates following an LPS challenge either 24 h before or immediately after conditioning. Finally, in Experiment 5, no neonatal group differences were observed in corticosterone or open field behaviour, suggesting that decreased freezing to a conditioned context reflects impaired memory, and not simply hyperactivity or altered stress reactivity. Taken together, we have demonstrated that neonatal infection results in robust hippocampal-dependent memory impairment following an immune challenge in adulthood using a number of conditioning paradigms.

Bidirectional communication between the brain and the immune system: implications for physiological sleep and disorders with disrupted sleep

This review describes mechanisms of immune-to-brain and brain-to-immune signaling involved in mediating physiological sleep and altered sleep with disease. The central nervous system (CNS) modulates immune function by signaling target cells of the immune system through autonomic and neuroendocrine pathways. Neurotransmitters and hormones produced and released by these pathways interact with immune cells to alter immune functions, including cytokine production. Cytokines produced by cells of the immune and nervous systems regulate sleep. Cytokines released by immune cells, particularly interleukin-1beta and tumor necrosis factor-alpha, signal neuroendocrine, autonomic, limbic and cortical areas of the CNS to affect neural activity and modify behaviors (including sleep), hormone release and autonomic function. In this manner, immune cells function as a sense organ, informing the CNS of peripheral events related to infection and injury. Equally important, homeostatic mechanisms, involving all levels of the neuroaxis, are needed, not only to turn off the immune response after a pathogen is cleared or tissue repair is completed, but also to restore and regulate natural diurnal fluctuations in cytokine production and sleep. The immune system's ability to affect behavior has important implications for understanding normal and pathological sleep. Sleep disorders are commonly associated with chronic inflammatory diseases and chronic age- or stress-related disorders. The best studied are rheumatoid arthritis, fibromyalgia and chronic fatigue syndromes. This article reviews our current understanding of neuroimmune interactions in normal sleep and sleep deprivation, and the influence of these interactions on selected disorders characterized by pathological sleep.

Catecholamines mediate stress-induced increases in peripheral and central inflammatory cytokines

Proinflammatory cytokines act at receptors in the CNS to alter physiological and behavioral responses. Exposure to stressors increases both peripheral and central proinflammatory cytokines, yet the mechanism(s) of induction remain unknown. Experiments here examined the role of catecholamines in the in vivo induction of proinflammatory cytokines following tailshock stress. Rats were pretreated i.p. with 2.0 mg/kg prazosin (alpha1-adrenoceptor antagonist), 10.0 mg/kg propranolol (beta-adrenoceptor antagonist), or 5.0 mg/kg labetalol (alpha1- and beta-adrenoceptor antagonist) 30 min prior to tailshock exposure and plasma interleukin-1beta (IL-1beta) and IL-6, along with tissue interleukin-1beta from the hypothalamus, hippocampus, and pituitary were measured immediately following stressor termination. Prazosin attenuated stress-induced plasma IL-1beta and IL-6, but had no effect on tissue IL-1beta levels, while propranolol attenuated plasma IL-6 and blocked tissue IL-1beta elevation, and labetalol, which cannot cross the blood-brain barrier, attenuated plasma IL-1beta and IL-6, blocked pituitary IL-1beta, but had no effect on central tissue IL-1beta levels. Furthermore, administration of 50.0 mg/kg N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride, a neurotoxin that lesions neural projections from the locus coeruleus, prevented stress-induced elevation in hippocampal IL-1beta, a region highly innervated by the locus coeruleus, but had no effect on hypothalamic IL-1beta, a region that receives few locus coeruleus projections. Finally, i.p. injection of 5.0 mg/kg isoproterenol (beta-adrenoceptor agonist) was sufficient to induce circulating IL-1 and IL-6, and tissue IL-1beta. These data suggest catecholamines play an important role in the induction of stress-induced proinflammatory cytokines and that beta-adrenoceptors are critical for tissue IL-1beta induction, while both alpha- and beta-adrenoceptors contribute to the induction of plasma cytokines.

Systemic administration of lipopolysaccharide and interleukin-1β have different effects on memory consolidation

Sickness behaviour is an adaptive response to infection that includes lethargy, anorexia and of direct relevance to this work, learning and memory impairments. It has been suggested that proinflammatory cytokines may disrupt learning and memory by interfering with memory consolidation [C.R. Pugh, K. Kumagawa, M. Fleshner, L.R. Watkins, S.F. Maier, J.R. Rudy, Selective effects of peripheral lipopolysaccharide administration on contextual and auditory-cue fear conditioning, Brain Behav. Immun. 12 (1998) 212-229]. We tested whether systemic interleukin-1beta is sufficient to induce impairments in memory consolidation by comparing the effects of post-learning administration of interleukin-1beta with, the potent endotoxin, lipopolysaccharide; and saline, on retention of conditioned fear of a context. We administered an acute intraperitoneal injection of lipopolysaccharide, interleukin-1beta or saline immediately following a single conditioning episode in which rats received two tone-shock pairings. Two days following the learning episode, animals were tested for strength of conditioned responding to both the context and tone. Lipopolysaccharide-injected animals, but not interleukin-1beta-injected animals, exhibited less conditioned fear of context compared to saline-treated controls. All groups showed similar conditioned fear of tone. Our results suggest that systemic interleukin-1beta is not sufficient to disrupt memory consolidation, but rather, the synergistic actions of the proinflammatory cytokines released by lipopolysaccharide are required to disrupt memory consolidation.

Peripheral lipopolysaccharide administration impairs two-way active avoidance conditioning in C57BL/6J mice

Peripheral administration of lipopolysaccharide (LPS) or interleukin-1 (IL-1) may lead to alterations of CNS function and behavioral changes designated "sickness behavior." Further, some experiments show evidence of LPS- and cytokine-mediated alterations in learning and memory. The current series of experiments examined the effects of a single or repeated intraperitoneal LPS injections, at a number of doses and time points before or after test sessions, on behavior in a two-way active avoidance conditioning paradigm. Subjects were able to avoid the mild shock stimulus, escape it, or fail to respond to it. Subjects treated with LPS at many, but not all, of the time points sampled showed impaired learning, by exhibiting significantly fewer avoidance responses than controls. Furthermore, an LPS-induced increase in non-cued inter-trial interval crossings was observed during the later days of testing, suggesting that a greater percentage of their avoidance responses was not conditioned and their behavior was less efficient. Taken together, the results suggest that LPS-treated animals showed a diminished association between conditioned stimulus (CS) and unconditioned stimulus (US). These results support the theory that peripheral immune stimuli may induce deleterious effects on learning, and extend the work to a negatively reinforced operant procedure.

Neonatal infection induces memory impairments following an immune challenge in adulthood

Exposure to infectious agents during early postnatal life often alters glucocorticoid responses to stress and immune outcomes in adulthood. The authors examined whether neonatal infection results in memory impairments in adult animals. Rats infected with Escherichia coli (E. coli) as neonates displayed impaired memory for a recently explored context in adulthood. This impairment, however, was only observed in rats that received a peripheral immune challenge (lipopolysaccharide; LPS) immediately following context exposure. Adult rats treated neonatally with E. coli also had decreased hippocampal astrocytes compared with phosphate-buffered saline-treated rats, but displayed increased astrocyte reactivity in the hippocampus and decreased brain interleukin-1beta following lipopolysaccharide. Infection during development appears to alter glia within the hippocampus, which may contribute to altered cytokine responses and memory impairment.

The temporal dynamics of retention of a context memory: something is missing

We use a variation of contextual fear conditioning, called the context pre-exposure facilitation effect (CPFE) to study the rat's memory for context. In this paradigm, the rat is pre-exposed to a conditioning context and later returned to that context, where it is immediately shocked. The memory context is revealed by the fact that pre-exposure to the conditioning context, but not to a different context, greatly enhances conditioned fear produced by immediate shock. We report that rat's retention of the context memory is a nonmonotonic U-shaped function of the interval separating pre-exposure and immediate shock. Retention performance decays rapidly so that within 2 min of pre-exposure there is no evidence that the rat was pre-exposed to the context. Within a few hours, however, a strong CPFE was observed that persisted for at least 28 d. Two hypotheses are discussed: (1) the descending arm of the U represents a retrieval failure, and (2) the U-shaped function represents two discontinuous memory processes initiated in parallel-short-term synaptic changes that are rapidly initiated, but also decay rapidly, and long-term synaptic processes that take time to generate but can endure for days.

Competitive interactions between endogenous LTD and LTP in the hippocampus underlie the storage of emotional memories and stress-induced amnesia

This speculative review serves two purposes. First, it as an extension of the ideas we developed in a previous review (Diamond et al., Hippocampus, 2004;14:281-291), and second, it is a rebuttal to Abraham's (Hippocampus, 2004;14:675-676) critique of that review. We had speculated on the functional significance of the finding that post-training LTP induction produces retrograde amnesia. We noted the similarities between the findings that strong tetanizing stimulation can produce LTP and retrograde amnesia, and that a strong emotional experience can produce a long-lasting memory and retrograde amnesia, as well. The commonalities between LTP induction and emotional learning provided the basis of our hypothesis that an emotional experience generates endogenous LTD/depotentiation, which reverses synaptic plasticity formed during previous learning experiences, and endogenous LTP, which underlies the storage of new information. Abraham raised several concerns with our review, including the criticism that our speculation "falters because there is no evidence that stress causes LTD or depotentiation," and that research on stress and hippocampus has "failed to report any LTP-like changes." Abraham's points are well-taken because stress, in isolation, does not appear to generate long-lasting changes in baseline measures of hippocampal excitability. Here, within the context of a reply to Abraham's critique, we have provided a review of the literature on the influence of stress, novelty, fear conditioning, and the retrieval of emotional memories on cognitive and physiological measures of hippocampal functioning. An emphasis of this review is our hypothesis that endogenous forms of depotentiation, LTD and LTP are generated only when arousing experiences occur in conjunction with memory-related activation of the hippocampus and amygdala. We conclude with speculation that interactions among the different forms of endogenous plasticity underlie a form of competition by synapses and memories for access to retrieval resources.

The Ventral Hippocampus Supports a Memory Representation of Context and Contextual Fear Conditioning: Implications for a Unitary Function of the Hippocampus

The authors report that either inactivating the ventral hippocampus (VH) with muscimol prior to context preexposure or injecting anisomycin into the VH after preexposure significantly impaired rats' memory for context. Injecting anisomycin into the VH prior to contextual fear conditioning also greatly reduced long-term memory (48-hr retention test) but had no effect on short-term memory (1-hr retention test) for contextual fear. Together with other results, these data suggest that the memory for a novel context is distributed throughout the longitudinal extent of the hippocampus and that this representation helps to support contextual fear conditioning.

BDNF mRNA expression in rat hippocampus following contextual learning is blocked by intrahippocampal IL-1β administration

The present study examined the modulating effects of an intrahippocampal injection of interleukin-1beta (IL-1beta) on brain-derived neurotrophic factor (BDNF) mRNA expression 0.5, 2, 4, and 6 h following contextual fear conditioning, a task known to increase BDNF mRNA, in rats. Contextual fear conditioning produced a time-dependent increase in BDNF mRNA that varied by region of hippocampus. IL-1beta blocked or reduced these increases in BDNF mRNA in the CA1, CA2, and dentate gyrus regions of the hippocampus, but had no effect in cortical regions. These data support the idea that IL-1beta-produced memory deficits may be mediated via BDNF mRNA reductions in hippocampus.

Interleukin-6: a cytokine to forget

It is known that proinflammatory cytokines such as interleukin-6 (IL-6) are expressed in the central nervous system (CNS) during disease conditions and affect several brain functions including memory and learning. In contrast to these effects observed during pathological conditions, here we describe a physiological function of IL-6 in the "healthy" brain in synaptic plasticity and memory consolidation. During long-term potentiation (LTP) in vitro and in freely moving rats, IL-6 gene expression in the hippocampus was substantially increased. This increase was long lasting, specific to potentiation, and was prevented by inhibition of N-methyl-D-aspartate receptors with (+/-)-2-amino-5-phosphonopentanoic acid (AP-5). Blockade of endogenous IL-6 by application of a neutralizing anti-IL-6 antibody 90 min after tetanus caused a remarkable prolongation of LTP. Consistently, blockade of endogenous IL-6, 90 min after hippocampus-dependent spatial alternation learning resulted in a significant improvement of long-term memory. In view of the suggested role of LTP in memory formation, these data implicate IL-6 in the mechanisms controlling the kinetics and amount of information storage.

Proinflammatory cytokines differentially affect leverpress avoidance acquisition in rats

Recent evidence indicates that the pro-inflammatory cytokines (PICs) can affect learning and memory processes. To examine the effect of the PICs on leverpress escape/avoidance conditioning, we injected male Sprague-Dawley rats with IL-1beta, IL-6 (both 3.0 microg/kg, i.p.), TNFalpha (6.0 microg/kg, i.p.) or vehicle, 24h before a single 4-h session of leverpress escape/avoidance conditioning. The TNFalpha-treated animals made more avoidance responses and fewer escape responses than controls during the last hour of the session. Further, both TNFalpha- and IL-1beta-treated animals had a higher percent avoidance than controls during the 4th hour of the session. None of the cytokines had an effect on the number of leverpresses during safety, a putative measure of anxiety. Results are discussed in terms of the differential central effects of the pro-inflammatory cytokines and the possible relationship to avoidance conditioning.

Involvement of arachidonic acid cascade in working memory impairment induced by interleukin-1 beta

The aim of the present study is to clarify the possible relevance of the arachidonic acid cascade to working memory in rats, by using a three-panel runway apparatus. Interleukin-1 beta, injected bilaterally into the dorsal hippocampus at a dose of 100 ng/side, significantly impaired working memory, and this impairment was attenuated by pretreatment with 10 mg/kg (s.c.) of diclofenac, a cyclooxygenase inhibitor. Working memory was also impaired in rats administered a bilateral intrahippocampal injection of prostaglandin E2, in a dose-dependent manner at 0.01-1 microg/side. Furthermore, an injection of 100 ng/side of interleukin-1 beta significantly increased production of prostaglandin E2 (580 +/- 32 pg to 1142 +/- 101 pg/100 mg wet tissue) in the hippocampus. Taken together, these findings suggest that the activation of the arachidonic acid cascade was causative of the working memory impairment induced by interleukin-1 beta.

Interleukin-1beta mediates the memory impairment associated with a delayed type hypersensitivity response to bacillus Calmette-Guérin in the rat hippocampus

Interleukin-1beta (IL-1beta) plays a major role in the initiation and exacerbation of brain inflammation, and its action is limited by the natural antagonist of IL-1 receptors, IL-1Ra. The aim of the present study was to test the hypothesis that IL-1beta mediates the functional consequences of inflammation during the course of delayed-type hypersensitivity response to bacillus Calmette-Guérin (BCG) in the hippocampus of Lewis rats. Animals were primed with an injection of BCG in the right hippocampus and challenged 4 weeks later with BCG administered subcutaneously. Concentrations of IL-1beta and IL-1Ra were measured by ELISA in the BCG injected hippocampus and compared to those measured in the contralateral hippocampus during the first 2 weeks post-challenge. IL-1beta levels increased in response to BCG challenge and peaked 12 days after challenge. The same variations appeared in the contralateral hippocampus but to a lesser extent. Hippocampal IL-1Ra levels increased in response to intrahippocampal injection of BCG. They further increased at days 6 and 9 post-challenge and decreased from day 12 back to baseline values on day 16. The increase in IL-1beta levels and the decline in IL-1Ra levels were associated with an impairment in spatial memory in a Y-maze on day 16 post-challenge, that was abrogated by chronic administration of IL-1Ra via a subcutaneously implanted osmotic minipump geared to deliver 7 mg IL-1Ra/day. These results show that overexpression of IL-1beta in the brain during the course of a chronic inflammation has deleterious consequences on cognitive processes, that are reversed by blockade of IL-1 receptors.

The role of the dorsal hippocampus in the acquisition and retrieval of context memory representations

It is argued that the hippocampus contributes to contextual fear conditioning by supporting the acquisition of a conjunctive memory representation of context, which associates with shock. This function was examined by studying the context pre-exposure facilitation effect (CPFE). A rat that is shocked immediately after being placed into a context subsequently displays almost no fear of that context. However, if it is pre-exposed to the context the day before immediate shock, it displays significant freezing to that context. By using 5-aminomethyl-3-hydroxysoxazole to temporarily inactivate the dorsal hippocampus (DH) at three different phases of the procedure, which produces the CPFE, we show that the hippocampus is necessary for the following: (1) acquisition of the context memory, (2) retrieval of this memory at the time of immediate shock, and (3) retrieval of the context-shock memory at the time of testing. In contrast, inactivating the DH before a standard contextual shock experience had no effect on contextual fear conditioning. These results support the view that two processes can support contextual fear conditioning: (1) conditioning to the conjunctive representation, which depends on the hippocampus, and (2) conditioning to the features that make up the context, which does not.

Brain-derived neurotrophic factor mRNA downregulation produced by social isolation is blocked by intrahippocampal interleukin-1 receptor antagonist

Manipulations that increase the expression of the pro-inflammatory cytokine interleukin-1beta (IL-1beta) in the hippocampus (e.g. peripheral administration of lipopolysaccharide, i.c.v. glycoprotein 120, social isolation) as well as the intrahippocampal injection of IL-1beta following a learning experience, dramatically impair the memory of that experience if the formation of the memory requires the hippocampus. Here we employed social isolation to further study this phenomenon, as well as its relation to brain-derived neurotrophic factor (BDNF). BDNF was studied because of its well-documented role in the formation of hippocampally based memory. A 6 h period of social isolation immediately after contextual fear conditioning impaired memory for context fear measured 48 h later, and decreased BDNF mRNA in the dentate gyrus and the CA3 region of the hippocampus assessed immediately after the isolation. Moreover, an intrahippocampal injection of the IL-1 receptor antagonist prior to the isolation period prevented both the BDNF downregulation and the memory impairments produced by the isolation. These data suggest that hippocampal-dependent memory impairments induced by elevated levels of brain IL-1beta may occur via an IL-1beta-induced downregulation in hippocampal BDNF.

The amygdala modulates hippocampus-dependent context memory formation and stores cue-shock associations

Preexposing rats to the context facilitates subsequent contextual fear conditioning. This effect depends on the hippocampus (J. W. Rudy, R. M. Barrientos, & R. C. O'Reilly, 2002). The authors report that inactivating the basolateral region of the amygdala (BLA) by injecting muscimol, a GABAA agonist, before or after preexposure reduced this effect. In contrast, bilateral injections of anisomycin, a protein synthesis inhibitor, into BLA did not impair the consolidation of the context memory. However, when injected after fear conditioning, anisomycin impaired consolidation of both contextual and auditory-cue fear conditioning. Results are consistent with 2 ideas about the amygdala's contribution to memory: (a) It modulates memory formation in other regions of the brain, and (b) it is a storage site for cue-shock associations.

Peripheral and central proinflammatory cytokine response to a severe acute stressor

The role of proinflammatory cytokines in the response to acute stressor exposure has received recent attention. Exposure to a single session of inescapable shock (IS) induces peripheral and central proinflammatory cytokines. Other stressors also increase expression of proinflammatory cytokine mRNA and/or protein in various tissues. However, the induction of central and peripheral proinflammatory cytokines by stressors remains controversial and the pattern of cytokine induction is not consistent across stressors. The present experiments sought to examine the pattern of the proinflammatory cytokine response to a stressor known to cause elevations of IL-1beta protein. mRNA expression for three proinflammatory cytokines, IL-1beta, TNF-alpha and IL-6, and IL-1beta protein was examined after IS. IS increases IL-1beta mRNA and/or protein in a variety of tissues, including hypothalamus, hippocampus, pituitary and spleen. Furthermore, IS concomitantly alters IL-1beta mRNA and protein in hypothalamus and spleen, while the IL-1beta mRNA increase in pituitary lags behind the increase of IL-1beta protein. Interestingly, IL-1beta mRNA is elevated in hippocampus 4 h after IS, but an increase of IL-1beta protein in hippocampus is not detected. Expression of TNF-alpha and IL-6 mRNA do not increase in response to IS. Indeed, TNF-alpha mRNA expression decreases in cortex, pituitary and liver immediately after IS. These findings suggest that alterations of proinflammatory cytokine expression by stressors, and IS in particular, are region- and cytokine-specific.

Understanding contextual fear conditioning: insights from a two-process model

Contextual fear conditioning is an important behavioral paradigm for studying the neurobiology of learning and memory and the mnemonic function of the hippocampus. We suggest that research in this domain can profit by a better theoretical understanding of the processes that contribute to this phenomenon. To facilitate this understanding, we describe a theory which assumes that physical elements of a conditioning context represented in the brain as either (a) a set of independent features or (b) features bound into a conjunctive representation by the hippocampus which supports pattern completion. Conditioning produced by shocking a rat in a particular context, in principle, can be produced by strengthening connections between the feature representations and/or the conjunctive representation and basolateral region of the amygdala. We illustrate how this theory clarifies some of the complexities associated with the existing literature and how it can be used to guide future empirical work. We also argue that the mechanisms (conjunctive representations and pattern completion) that mediate the contribution the hippocampus makes to contextual fear conditioning are the same ones that enable the hippocampus to support declarative memory in humans.

Time of day differences in IL1β and TNFα mRNA levels in specific regions of the rat brain

Interleukin-1beta (IL1beta) and tumor necrosis factor-alpha (TNFalpha) are involved in several physiological functions regulated by brain. The current studies were performed to determine whether a diurnal rhythm of IL1beta and TNFalpha mRNAs, determined by real time RT-PCR, existed in specific brain regions linked to the functions of these cytokines. Rats were sacrificed 2 h after light onset (AM) and 1 h prior to dark onset (PM). IL1beta mRNA levels in the AM were significantly higher than those obtained from rats sacrificed in the PM in the prefrontal cortex, parietal cortex, amygdala/piriform cortex, ventral hippocampus, hypothalamus, nucleus tractus solitarius, and nucleus accumbens, but not in the dorsal hippocampus. Time-of-day differences in TNFalpha mRNA levels were observed in all these brain regions. These results support the hypothesis that TNFalpha and IL1beta have physiological roles within the brain.

Immune-to-central nervous system communication and its role in modulating pain and cognition: Implications for cancer and cancer treatment

This paper reviews the nature of communication from the immune system to the brain and some implications of this communication for phenomena that are not ordinarily considered to be modulated by immune function. Pro-inflammatory cytokines released by activated immune cells signal the brain by both blood-borne and neural routes, leading to alterations in neural activity. The cascade of altered neural activity includes the induction of pro-inflammatory cytokines within the brain and spinal cord. The cytokines in the brain, specifically in the hippocampus, interfere with the consolidation of memory, while the cytokines within the spinal cord exaggerate pain. Activation of this immune-to-central nervous system pathway, with the consequent production of cytokines within the central nervous system, may be involved in the mediation of a number of phenomena that occur during cancer and cancer treatment.