Immune activation paired with intraoral sucrose conditions oral rejection

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.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Ingestion of bacterial lipopolysaccharide inhibits peripheral taste responses to sucrose in mice

A fundamental role of the taste system is to discriminate between nutritive and toxic foods. However, it is unknown whether bacterial pathogens that might contaminate food and water modulate the transmission of taste input to the brain. We hypothesized that exogenous, bacterially-derived lipopolysaccharide (LPS), modulates neural responses to taste stimuli. Neurophysiological responses from the chorda tympani nerve, which innervates taste cells on the anterior tongue, were unchanged by acute exposure to LPS. Instead, neural responses to sucrose were selectively inhibited in mice that drank LPS during a single overnight period. Decreased sucrose sensitivity appeared 7days after LPS ingestion, in parallel with decreased lingual expression of Tas1r2 and Tas1r3 transcripts, which are translated to T1R2+T1R3 subunits forming the sweet taste receptor. Tas1r2 and Tas1r3 mRNA expression levels and neural responses to sucrose were restored by 14 days after LPS consumption. Ingestion of LPS, rather than contact with taste receptor cells, appears to be necessary to suppress sucrose responses. Furthermore, mice lacking the Toll-like receptor (TLR) 4 for LPS were resistant to neurophysiological changes following LPS consumption. These findings demonstrate that ingestion of LPS during a single period specifically and transiently inhibits neural responses to sucrose. We suggest that LPS drinking initiates TLR4-dependent hormonal signals that downregulate sweet taste receptor genes in taste buds. Delayed inhibition of sweet taste signaling may influence food selection and the complex interplay between gastrointestinal bacteria and obesity.

Grooming Behavior as a Mechanism of Insect Disease Defense

Grooming is a well-recognized, multipurpose, behavior in arthropods and vertebrates. In this paper, we review the literature to highlight the physical function, neurophysiological mechanisms, and role that grooming plays in insect defense against pathogenic infection. The intricate relationships between the physical, neurological and immunological mechanisms of grooming are discussed to illustrate the importance of this behavior when examining the ecology of insect-pathogen interactions.

Weakened [corrected] taste-LPS association during endotoxin tolerance

In naive individuals, the administration of bacterial lipopolysaccharide (LPS) provokes a rapid systemic increase in pro-inflammatory cytokines such as tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta and IL-6, inducing an acute phase response including sickness behavior. Strong associative learning occurs when relevant gustatory/olfactory stimuli precede the activation of the immune system, affecting long-term individual food selection and nutritional strategies. Repeated LPS administration results in the development of an endotoxin tolerance status, characterized by a drastic reduction in the LPS-induced cytokine response. Here we investigated how the postprandial categorization of a relevant taste (0.2% saccharin) changed after administration of a high dose of LPS (0.5 mg/kg i.p.) in LPS-tolerant animals. Determination of the consummatory fluid intake revealed that, in contrast to LPS-naive rats, taste-LPS association did not occur during endotoxin tolerance. Ninety minutes after the single association trial, the plasma responses of TNF-alpha, IL-1beta and IL-6 were completely blunted in LPS-tolerant animals, which also resulted in low LPS-adipsogenic and LPS-anorexic effects. These findings indicate that an identical immune challenge can result in completely different neuro-behavioral consequences depending on the immune history of the individual, thus revealing part of the complex interconnection between the immune and neuro-endocrine systems in regulating food selection and consumption during the infectious process.

Central blockade of IL-1 does not impair taste-LPS associative learning

After saccharin intake is associated with the consequences of peripheral lipopolysaccharide (LPS) administration, rats develop a strong conditioned avoidance behavior against this gustatory stimulus. To investigate the role of central interleukin-1 (IL-1) as a key signal during taste-LPS engram formation, rats were chronically infused with IL-1 receptor antagonist into the lateral ventricle of the brain before, during and after a single association trial. The results indicate that a stable taste-LPS engram can be formed even under the chronic blockade of central IL-1 signaling during engram formation and consolidation. More importantly, our data show that animals which did not experience a fever response during association phase (due to the LPS encounter) were unable to elicit hyperthermia as part of the conditioned response. These data indicate that pairing a relevant taste stimulus with an immune challenge, such as LPS, might result in the formation of multiple engrams, specifically codifying independent information.

The effects of lipopolysaccharide and lithium chloride on the ingestion of a bitter-sweet taste: comparing intake and palatability

Activation of the immune system with lipopolysaccharide (LPS) has been shown to result in decreased consumption of normally preferred substances while at the same time not affecting palatability. The present study examined the effects LPS administration on both intake and palatability of a relatively unpalatable bitter-sweet taste. Bitter is thought to signal a danger cue to an animal representing a potential toxin-containing food. Using a one-bottle consumption test, voluntary intake of a sucrose-quinine (0.15 M sucrose + 0.00015 M quinine; S-Q) solution was assessed in rats on two conditioning days (days 1 and 4) after a systemic injection with LPS, LiCl, or NaCl. On the test day (day 7), rats were given 1h access to the same solution in the absence of any injection. In a separate experiment, rats fitted with intraoral cannulae received similar testing schedules, however, the solution was delivered intraorally, activating only the consummatory responses of the animal. During conditioning, rats received 5 brief (1 min) intraoral infusions of the taste across a 1h period following injections of LPS, LiCl or NaCl. Individual taste reactivity responses were recorded and analyzed. Both LPS and LiCl resulted in decreased consumption of the unpalatable taste relative to controls on the test day, suggesting typical conditioned taste avoidance. When the consummatory responses were examined, LPS-treatment produced an increase in active oral rejection relative to NaCl- and LiCl-treated groups on both conditioning days. The present study demonstrates that although both LPS- and LiCl-treatment result in similar conditioned avoidance using an intake measure, they do not elicit similar patterns of taste reactivity responding to intraoral infusions of the bitter-sweet taste. Furthermore, the present results suggest that immune activation with LPS-treatment results in increased rejection of a mildly aversive stimulus and supports the hypothesis that reorganization of behavioral priorities occurs during bacteria-induced sickness.