Lipopolysaccharide inhibits the simultaneous establishment of LiCl-induced anticipatory nausea and intravascularly conditioned taste avoidance in the rat

Lipopolysaccharide (LPS) attenuates the primary conditioning of lithium chloride (LiCl)-induced context aversion but not the secondary conditioning of context aversion or taste avoidance

A first-order association can be formed between toxin-induced nausea and a context, as well as nausea and a taste cue. However, comparatively little is understood about second-order associations. The present study examined if the bacterial endotoxin, LPS, could impair the first- and second-order conditioning of context aversion (anticipatory nausea paradigm) and subsequent conditioned taste avoidance (two-bottle task). Adult male Long Evans rats were treated with LiCl (127 mg/kg, intraperitoneal [i.p.]) or vehicle control (NaCl) and then exposed to a distinct context for 4 first-order conditioning trials. LPS (200 μg/kg, i.p.) or NaCl were administered 24 h after each trial. Seventy-two h after the final first-order conditioning trial, rats underwent 2 second-order conditioning trials where they were treated with 2% saccharin (i.p.) and then exposed to the same context. Twenty-four h after the final second-order conditioning trial, rats were tested in a two-bottle task (2 trials), where they were given a choice between water and a palatable 0.2% saccharin solution. LiCl-treated rats demonstrated a context aversion by the 3rd conditioning trial in the anticipatory nausea paradigm. Rats previously exposed to LiCl also displayed a conditioned taste avoidance of saccharin within the two-bottle task. LPS attenuated first-order context aversion but did not alter either second-order context aversion or conditioned taste avoidance in the two-bottle task. This study demonstrated that a secondary association formed within an aversive context could result in a conditioned taste avoidance. Further, LPS may be able to attenuate primary conditioning, but not secondary conditioning.

Immune activation attenuates memory acquisition and consolidation of conditioned disgust (anticipatory nausea) in rats

Anticipatory nausea is a classically conditioned response to cues (e.g. contexts) that have been previously paired with a nauseating stimulus, such as chemotherapy in humans. In rodents, anticipatory nausea can be modeled by pairing a novel context with lithium chloride (LiCl), which leads to conditioned disgust behaviours (such as gaping) when exposed to the context alone. Growing evidence suggests that selective immune activation attenuates various forms of learning and memory. The present study investigated the effects of the endotoxin lipopolysaccharide (LPS) on LiCl-induced anticipatory nausea across critical stages of associative memory including acquisition, consolidation, and extinction. Adult male Long Evans rats were subject to intraperitoneal (i.p.) LiCl (127 mg/kg) or vehicle control (NaCl) paired with a 30 min conditioning trial in a distinct context for a total of 4 trials. To study acquisition, rats were administered either LPS or NaCl (200 μg/kg, i.p.) 90 mins before the conditioning trials. To study consolidation, different rats were administered either LPS or NaCl (200 μg/kg, i.p.) immediately after the conditioning trials. These trials were followed by 4 drug-free extinction trials within the same context. LPS significantly reduced conditioned gaping behaviours by the 4th conditioning trial and on the 1st drug-free extinction trial when administered 90 mins before or immediately after the conditioning trials. LPS had no significant effect on extinction. The present study provides strong evidence for the attenuating effects of LPS exposure on the acquisition and consolidation of LiCl-induced anticipatory nausea.

Infection, Learning, and Memory: Focus on Immune Activation and Aversive Conditioning

Here we review the effects of immune activation primarily via lipopolysaccharide (LPS), a cell wall component of Gram-negative bacteria, on hippocampal and non-hippocampal-dependent learning and memory. Rodent studies have found that LPS alters both the acquisition and consolidation of aversive learning and memory, such as those evoking evolutionarily adaptive responses like fear and disgust. The inhibitory effects of LPS on the acquisition and consolidation of contextual fear memory are discussed. LPS-induced alterations in the acquisition of taste and place-related conditioned disgust memory within bottle preference tasks and taste reactivity tests (taste-related), in addition to conditioned context avoidance tasks and the anticipatory nausea paradigm (place-related), are highlighted. Further, conditioned disgust memory consolidation may also be influenced by LPS-induced effects. Growing evidence suggests a central role of immune activation, especially pro-inflammatory cytokine activity, in eliciting the effects described here. Understanding how infection-induced immune activation alters learning and memory is increasingly important as bacterial and viral infections are found to present a risk of learning and memory impairment.

LiCl-induced sickness modulates rat gustatory cortical responses

Gustatory cortex (GC), a structure deeply involved in the making of consumption decisions, presumably performs this function by integrating information about taste, experiences, and internal states related to the animal's health, such as illness. Here, we investigated this assertion, examining whether illness is represented in GC activity, and how this representation impacts taste responses and behavior. We recorded GC single-neuron activity and local field potentials (LFPs) from healthy rats and rats made ill (via LiCl injection). We show (consistent with the extant literature) that the onset of illness-related behaviors arises contemporaneously with alterations in 7 to 12 Hz LFP power at approximately 12 min following injection. This process was accompanied by reductions in single-neuron taste response magnitudes and discriminability, and with enhancements in palatability-relatedness-a result reflecting the collapse of responses toward a simple "good-bad" code visible in the entire sample, but focused on a specific subset of GC neurons. Overall, our data show that a state (illness) that profoundly reduces consumption changes basic properties of the sensory cortical response to tastes, in a manner that can easily explain illness' impact on consumption.

Memory deficits in males and females long after subchronic immune challenge

Memory impairments and cognitive decline persist long after recovery from major illness or injury, and correlate with increased risk of later dementia. Here we developed a subchronic peripheral immune challenge model to examine delayed and persistent memory impairments in females and in males. We show that intermittent injections of either lipopolysaccharides or Poly I:C cause memory decline in both sexes that are evident eight weeks after the immune challenge. Importantly, we observed sex-specific patterns of deficits. Females showed impairments in object recognition one week after challenge that persisted for at least eight weeks. In contrast, males had intact memory one week after the immune challenge but exhibited broad impairments in memory tasks including object recognition, and both context and tone fear conditioning several months later. The differential patterns of memory deficits in males and in females were observed without sustained microglial activation or changes in blood-brain barrier permeability. Together, these data suggest that transient neuroimmune activity results in differential vulnerabilities of females and males to memory decline after immune challenge. This model will be an important tool for determining the mechanisms in both sexes that contribute to memory impairments that develop over the weeks and months after recovery from illness. Future studies using this model will provide new insights into the role of chronic inflammation in the pathogenesis of long-lasting memory decline and dementias.

Neuroimmune Activation Drives Multiple Brain States

Neuroimmune signaling is increasingly identified as a critical component of neuronal processes underlying memory, emotion and cognition. The interactions of microglia and astrocytes with neurons and synapses, and the individual cytokines and immune signaling molecules that mediate these interactions are a current focus of much research. Here, we discuss neuroimmune activation as a mechanism triggering different states that modulate cognitive and affective processes to allow for appropriate behavior during and after illness or injury. We propose that these states lie on a continuum from a naïve homeostatic baseline state in the absence of stimulation, to acute neuroimmune activity and chronic activation. Importantly, consequences of illness or injury including cognitive deficits and mood impairments can persist long after resolution of immune signaling. This suggests that neuroimmune activation also results in an enduring shift in the homeostatic baseline state with long lasting consequences for neural function and behavior. Such different states can be identified in a multidimensional way, using patterns of cytokine and glial activation, behavioral and cognitive changes, and epigenetic signatures. Identifying distinct neuroimmune states and their consequences for neural function will provide a framework for predicting vulnerability to disorders of memory, cognition and emotion both during and long after recovery from illness.

Oxytocin, social factors, and the expression of conditioned disgust (anticipatory nausea) in male rats

Disgust has been proposed to have evolved as a means to rid the body and mouth of noxious substances and toxins, as well as to motivate and facilitate avoidance of contact with disease-causing organisms and infectious materials. Nonemetic species, such as the rat, show distinctive facial expressions, including the gaping reaction, indicative of nausea-based disgust. These conditioned disgust responses can be used to model anticipatory nausea in humans, which is a learned response observed following chemotherapy treatment. As social factors play a role in the modulation and expression of conditioned disgust responses in rats, and the nonapeptide, oxytocin (OT), is involved in the modulation of social behavior, the present study examined the effects of an OT antagonist, L-368 899, on the development and expression of socially mediated conditioned disgust in male rats. When administered 10 min before testing in a distinct context (different from the original conditioning context), L-368 899 (5 mg/kg) significantly decreased gaping behavior in rats that were conditioned with a social partner. LiCl-treated rats administered L-368 899 before testing also showed decreased social initiations toward their social partner. These findings suggest that OT may play a role in the modulation and expression of socially mediated conditioned disgust in rats.

Impairment of lithium chloride-induced conditioned gaping responses (anticipatory nausea) following immune system stimulation with lipopolysaccharide (LPS) occurs in both LPS tolerant and LPS non-tolerant rats

Anticipatory nausea is a classically conditioned response to a context that has been previously paired with toxin-induced nausea and/or vomiting. When injected with a nausea-inducing drug, such as lithium chloride (LiCl), rats will show a distinctive conditioned gaping response that has been suggested to be an index of nausea. Previous studies have found that immune system activation with an endotoxin, such as lipopolysaccharide (LPS), attenuates LiCl-induced conditioned gaping in rats. The present study examined the acquisition of LiCl-induced conditioned gaping in rats that were either LPS tolerant or LPS non-tolerant, as little is known about the effects of endotoxin tolerance on learning and memory. Male Long-Evan rats were given four systemic injections of LPS (200 μg/kg) or isotonic saline (NaCl) to induce LPS tolerance, indexed with 24 h changes in body weight following treatment. The animals were then given 4 acquisition trials in a LiCl-induced conditioned gaping paradigm. On conditioning days animals were treated with LPS (200 μg/kg) or saline followed 90 min later by injection of LiCl (127 mg/kg) or saline and then placed in a distinctive context for 30 min and their behavior video-recorded. On a drug free test day all animals were again placed in the distinctive context for 10 min and behavior was video-recorded. Gaping responses were scored for all acquisition days and the test day. Spleen and body weights were also obtained for all rats at the end of the experiment. Gaping responses were attenuated in rats treated with LPS in both the LPS tolerant and LPS non-tolerant groups. There were significant negative correlations between spleen weight as well as spleen/body weight ratios, and levels of conditioned gaping responses in LiCl treated rats, but not control rats. These results show that LPS interferes with learning/memory in the anticipatory nausea paradigm in rats that are both LPS tolerant and LPS non-tolerant.