Examining the effects of lipopolysaccharide and cholecystokinin on water ingestion: comparing intake and palatability

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.

Sickness Behavior Score Is Associated with Neuroinflammation and Late Behavioral Changes in Polymicrobial Sepsis Animal Model

The use of reliable scores is a constant development in critical illness. According to Sepsis-3 consensus, the use of Sequential Organ Failure Assessment (SOFA) score of 2 or more is associated with a higher mortality of sepsis patients. In experimental research, due murine animal model limitations, the use of a score systems can be an alternative to assess sepsis severity. In this work, we suggest a sickness behavior score (SBS) that uses physiological variables to assess sepsis severity and mortality. Animals were evaluated daily by the presence of six indicators of sickness behavior: temperature alteration, preference of water/sucrose, liquid intake, food intake, body weight, and movimentation. Male adult Wistar rats were evaluated daily after sepsis induction by cecal ligation and puncture (CLP) or laparotomy only (sham) for determination of SBS. Oxidative stress, IL-6, and HPA axis markers (corticosterone and adrenal gland weight) were evaluated 24 h after CLP to determine the correlation with the acute SBS and neuroinflammation. Also, BDNF and four cognitive behavioral tests were correlated with the chronic SBS, i.e., sum of 8 days after surgery. In result, septic rats presented higher SBS than sham animals. Sepsis severity markers were associated with acute and chronic SBS. Also, SBS was negative correlated with the cognitive tests. In conclusion, SBS shows to be reliable score to predict sepsis severity and mortality. The use of score system provides the analysis of global sickness behavior, beyond evaluation of each parameter individually.

Genetic and Pharmacologic Manipulation of TLR4 Has Minimal Impact on Ethanol Consumption in Rodents

Toll-like receptor 4 (TLR4) is a critical component of innate immune signaling and has been implicated in alcohol responses in preclinical and clinical models. Members of the Integrative Neuroscience Initiative on Alcoholism (INIA-Neuroimmune) consortium tested the hypothesis that TLR4 mediates excessive ethanol drinking using the following models: (1) Tlr4 knock-out (KO) rats, (2) selective knockdown of Tlr4 mRNA in mouse nucleus accumbens (NAc), and (3) injection of the TLR4 antagonist (+)-naloxone in mice. Lipopolysaccharide (LPS) decreased food/water intake and body weight in ethanol-naive and ethanol-trained wild-type (WT), but not Tlr4 KO rats. There were no consistent genotypic differences in two-bottle choice chronic ethanol intake or operant self-administration in rats before or after dependence. In mice, (+)-naloxone did not decrease drinking-in-the-dark and only modestly inhibited dependence-driven consumption at the highest dose. Tlr4 knockdown in mouse NAc did not decrease drinking in the two-bottle choice continuous or intermittent access tests. However, the latency to ethanol-induced loss of righting reflex increased and the duration decreased in KO versus WT rats. In rat central amygdala neurons, deletion of Tlr4 altered GABA_A receptor function, but not GABA release. Although there were no genotype differences in acute ethanol effects before or after chronic intermittent ethanol exposure, genotype differences were observed after LPS exposure. Using different species and sexes, different methods to inhibit TLR4 signaling, and different ethanol consumption tests, our comprehensive studies indicate that TLR4 may play a role in ethanol-induced sedation and GABA_A receptor function, but does not regulate excessive drinking directly and would not be an effective therapeutic target.

SIGNIFICANCE STATEMENT

Toll-like receptor 4 (TLR4) is a key mediator of innate immune signaling and has been implicated in alcohol responses in animal models and human alcoholics. Members of the Integrative Neuroscience Initiative on Alcoholism (INIA-Neuroimmune) consortium participated in the first comprehensive study across multiple laboratories to test the hypothesis that TLR4 regulates excessive alcohol consumption in different species and different models of chronic, dependence-driven, and binge-like drinking. Although TLR4 was not a critical determinant of excessive drinking, it was important in the acute sedative effects of alcohol. Current research efforts are directed at determining which neuroimmune pathways mediate excessive alcohol drinking and these findings will help to prioritize relevant pathways and potential therapeutic targets.

Participation of α2 -adrenoceptors in sodium appetite inhibition during sickness behaviour following administration of lipopolysaccharide

Sickness behaviour, a syndrome characterized by a general reduction in animal activity, is part of the active-phase response to fight infection. Lipopolysaccharide (LPS), an effective endotoxin to model sickness behaviour, reduces thirst and sodium excretion, and increases neurohypophysial secretion. Here we review the effects of LPS on thirst and sodium appetite. Altered renal function and hydromineral fluid intake in response to LPS occur in the context of behavioural reorganization, which manifests itself as part of the syndrome. Recent data show that, in addition to its classical effect on thirst, non-septic doses of LPS injected intraperitoneally produce a preferential inhibition of intracellular thirst versus extracellular thirst. Moreover, LPS also reduced hypertonic NaCl intake in sodium-depleted rats that entered a sodium appetite test. Antagonism of α2 -adrenoceptors abolished the effect of LPS on sodium appetite. LPS and cytokine transduction potentially recruit brain noradrenaline and α2 -adrenoceptors to control sodium appetite and sickness behaviour.

Lipopolysaccharide reduces incentive motivation while boosting preference for high reward in mice

Inflammation has been implicated in the development of various psychiatric disorders, including depression. However, the neurobehavioral mechanism involved in this relationship remains elusive. This gap in knowledge may best be filled by evaluating elementary neurobehavioral units affected by inflammation rather than behavioral changes in conventional animal tests of depression. To this end, the current study used a concurrent choice paradigm to evaluate inflammation-induced motivational changes. Male C57BL/6J mice (n=27) were food restricted to between 85 and 90% of their free-feeding weight and were trained to perform a concurrent choice task where they nose-poked for grain rewards on a fixed ratio (FR) 1 schedule (low effort/low reward) and chocolate-flavored rewards on a FR-10 schedule (high effort/high reward). A counterbalanced-within subjects design was used. A single intraperitoneal injection of 0.33 mg/kg lipopolysaccharide (LPS) was used to induce peripheral inflammation. Twenty-four hours after LPS administration, mice showed a reduction in the total number of nose pokes. A proportionally greater reduction in nose pokes was observed for grain, resulting in an increase in percent chocolate pellets earned. These behavioral changes cannot be explained by reduced appetite as feeding before the test led to a similar increase in percent chocolate pellets earned but without any decrease in responding. These results indicate that inflammation modulates incentive motivation by affecting willingness to exert effort for reward and not by reducing sensitivity to reward.

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

This study examined the effects of the bacterial endotoxin, lipopolysaccharide (LPS), on the establishment of anticipatory nausea and conditioned taste avoidance in a simultaneous conditioning paradigm using an intravascular/intraperitoneal saccharin taste. 83 naïve adult male Long-Evans rats were injected (intraperitoneal) with either 200 μg/kg LPS or 0.9% saline (NaCl), 90 min prior to ip treatment with either 64 mg/kg LiCl, 64 mg/kg LiCl+2.0% saccharin, 0.9% NaCl, or 0.9% NaCl+2.0% saccharin, and immediately placed into a distinctive context for 30 min (repeated over 4 conditioning days, spaced 72 h apart). 72 h following the final conditioning day, each animal was re-exposed to the context on a drug-free test day where orofacial responding was recorded. The next day, animals received a 24 h 2-bottle preference test with a choice between water and a palatable 0.2% saccharin solution. Results showed that LPS exposure, prior to LiCl or LiCl+Saccharin treatment, inhibited the establishment of anticipatory nausea, as evidenced by significantly lower conditioned gaping frequencies relative to animals pre-treated with NaCl followed by LiCl or LiCl+Saccharin. LPS pre-treatment also inhibited the formation of LiCl-induced taste avoidance, as evidence by significantly higher saccharin preferences in Group LPS-LiCl+Saccharin relative to Group NaCl-LiCl+Saccharin. The results of the current study provide additional evidence for the deleterious effects of LPS on learning and memory in aversive conditioning.

Inhibition of LiCl-induced conditioning of anticipatory nausea in rats following immune system stimulation: comparing the immunogens lipopolysaccharide, muramyl dipeptide, and polyinosinic: polycytidylic acid

The effects of the bacterial endotoxins, lipopolysaccharide (LPS) and muramyl dipeptide (MDP; Experiment 1), and the viral mimetic, polyinosinic: polycytidylic acid (poly I:C; Experiment 2), on the acquisition of "conditioned gaping" behavior in the rodent model of LiCl-induced anticipatory nausea were examined. Experimentally naïve adult male Long-Evans rats were injected (intraperitoneal, i.p.) with either 200 μg/kg LPS, 1.6 mg/kg MDP, or 0.9% saline (Experiment 1), or 4.0 mg/kg poly I:C or 0.9% saline (Experiment 2), 90 min prior to treatment with 127 mg/kg LiCl or saline control and immediately placed into a distinctive context for 30 min (repeated over 4 conditioning days, spaced 72 h apart). On a drug-free test day (72 h following conditioning day 4), each animal was re-exposed to the context for 10 min, and orofacial and aversive behavioral responses were video recorded and analyzed. The results showed that pre-treatment with LPS, MDP (Experiment 1), or poly I:C (Experiment 2) prior to LiCl+context conditioning significantly impaired the establishment of conditioned gaping behavior, thus blocking the acquisition of anticipatory nausea. Results varied in regards to peripheral acute-phase response sickness behaviors, with significantly reduced weight loss in LPS-treated animals, less robust weight loss in poly I:C-treated animals, and no significant reductions in body weight in MDP-treated animals. The learning impairments observed in the current study suggest that endotoxin treatment with bacterial and viral endotoxin may have stronger central effects on learning and memory behavior, relative to peripheral effects on body weight and other sickness-related responses.

Analgesia accompanying food consumption requires ingestion of hedonic foods

Animals eat rather than react to moderate pain. Here, we examined the behavioral, hedonic, and neural requirements for ingestion analgesia in ad libitum fed rats. Noxious heat-evoked withdrawals were similarly suppressed during self-initiated chocolate eating and ingestion of intraorally infused water, sucrose, or saccharin, demonstrating that ingestion analgesia does not require feeding motivation, self-initiated food procurement, sucrose, or calories. Rather, food hedonics is important because neither salt ingestion nor quinine rejection elicited analgesia. During quinine-induced nausea and lipopolysaccharide (LPS)-induced illness, conditions when chocolate eating was presumably less pleasurable, analgesia accompanying chocolate consumption was attenuated, yet analgesia during water ingestion was preserved in LPS-injected rats who showed enhanced palatability for water within this context. The dependence of ingestion analgesia on the positive hedonics of an ingestate was confirmed in rats with a conditioned taste aversion to sucrose: after paired exposure to sucrose and LPS, rats no longer showed analgesia during sucrose ingestion but continued to show analgesia during chocolate consumption. Eating pauses tended to occur less often and for shorter durations in the presence of ingestion analgesia than in its absence. Therefore, we propose that ingestion analgesia functions to defend eating from ending. Muscimol inactivation of the medullary raphe magnus blocked the analgesia normally observed during water ingestion, showing the involvement of brainstem endogenous pain inhibitory mechanisms in ingestion analgesia. Brainstem-mediated defense of the consumption of palatable foods may explain, at least in part, why overeating tasty foods is so irresistible even in the face of opposing cognitive and motivational forces.

Lipopolysaccharide dose dependently impairs rapid toxin (LiCl)-induced gustatory conditioning: a taste reactivity examination of the conditioned taste aversion

There is much debate on how immune activation affects cognitive processing. Research has shown that stimulation of the immune system can significantly impair, have no adverse effects, or enhance learning and memory processes in animals. The present experiment evaluated the effects of the bacterial endotoxin, lipopolysaccharide (LPS) on the acquisition of a rapidly acquired conditioned taste aversion using a toxin-containing food. Male Long Evans rats were fitted with intraoral cannulae and habituated to the taste reactivity procedure. Rats received two conditioning days, 72 h apart, in which they were injected systemically with LPS (200, 100, or 50 microg/kg) or NaCl (0.9% vehicle) and 90 min later placed in the taste reactivity test chamber. Rats were given 5 brief (1 min) intraoral infusions of either a LiCl-adulterated sucrose solution (0.15M LiCl+0.3M sucrose) or NaCl-sucrose solution (0.15M NaCl+0.3M sucrose) across a 1h period. On the test day (72 h after the last conditioning trial), rats were given a 2 min intraoral infusion of the respective taste in a drug-free state. Individual taste reactivity responses were recorded and analyzed. Results demonstrate that rats treated with LPS dose-dependently increased ingestive responding to the LiCl-sucrose flavor while at the same time showing reduced rejection response frequency on the two conditioning days. LPS treatment did not alter taste reactivity responding to the NaCl-sucrose solution. On the test day, the LPS groups again displayed a dose dependent increase in ingestive responses and a decrease in rejection responses to the LiCl-sucrose taste. The present results suggest that LPS-induced immune system activation, significantly impairs the rapid acquisition of a conditioned taste aversion.

Neonatal immune system activation with lipopolysaccharide enhances behavioural sensitization to the dopamine agonist, quinpirole, in adult female but not male rats

Administration of the bacterial cell wall component, lipopolysaccharide (LPS), stimulates the immune and endocrine systems inducing an acute phase of sickness and stress responses in adult and neonatal rats. Neonatal LPS exposure has been shown to alter a variety of behavioural and physiological processes in the adult animal. Early developmental stress, such as maternal separation, causes similar acute as well as long-term behavioural changes in adults, including altered sensitivity to drugs of abuse. Moreover, results of studies have shown evidence of a direct link between immune activation and sensitivity to dopamine-based drugs of abuse. The current study examined the effects of neonatal LPS treatment on subsequent locomotor sensitization to the dopamine (D(2)/D(3)) agonist, quinpirole, in adult rats as an index of drug sensitivity. Male and female Long-Evans rats were treated systemically with either LPS (50microg/kg) or saline (0.9%) on postnatal days 3 and 5. Locomotor sensitization was then examined in the adult rats (postnatal day 70). Animals were injected with quinpirole (0.5mg/kg, s.c.) or saline every other day for a total of 10 injections and locomotor activity was assessed for 60min immediately following injections 1, 2, 4, 6, and 10. Animals also received a 'challenge' injection of 0.5mg/kg quinpirole 28 days after injection 10, to assess persistence of behavioural sensitization. Locomotor activity progressively increased with repeated administration of quinpirole, indicating locomotor sensitization in all of the drug-treated groups. There was an overall sex difference, with females showing significantly greater sensitization than males. Moreover, neonatal LPS treatment potentiated both the level and the rate of development of locomotor sensitization to quinpirole administration in females, but not in males. Thus, the current study revealed that neonatal exposure to bacterial infection increases dopamine (D(2)/D(3)) agonist sensitivity in a sex-specific manner. These findings have important implications for the sexually dimorphic development of addictions to both natural and artificial rewards.

Lack of angiotensin II conversion to angiotensin III increases water but not alcohol consumption in aminopeptidase A-deficient mice

Elevated central concentrations of the vasopressor octapeptide angiotensin (Ang) II increase the water intake in mammals. Recently, we showed that central AngII is also crucial in alcohol-consuming behavior. Since the heptapeptide AngIII, an AngII metabolite, is discussed to mediate AngII-related effects, we investigated water and alcohol consumption in mice, genetically deficient in aminopeptidase A (APA), a peptidase responsible for AngII conversion to AngIII. Sixteen male APA-deficient mice and their age matched wild-type controls were monitored on their water intake under basal conditions and total fluid and alcohol intake before and after social stress in a two-bottle free-choice paradigm. Alterations were connected to the regulation in activity of Ang-related peptidases (APA, ACE; ACE2) in brain regions involved in alcohol intake and peripheral organs. In comparison to their wild-type controls, APA-deficient mice drank significantly more water but not more alcohol at all investigated time points. A reduction in water intake, as observed in wild-type animals after social stress, did not occur in knockout mice. However, the reduction in alcohol consumption after social stress was significantly reduced in both strains. Alcohol consumption upregulated all three peptidases in the kidney, but not in lung. Notable, renal ACE2 activity was significantly higher in APA-deficient mice under basal condition. While the inhibition of AngII metabolism to AngIII does not influence the alcohol intake, water consumption in mice deficient for APA was significantly elevated. These differences induced by an altered AngII/AngIII ratio oppose the hypothesis that central AngII and AngIII act in a congruent pattern.

Lipopolysaccharide and interleukin-1beta decrease sucrose intake but do not affect expression of place preference in rats

Immune system activation has been shown to induce decreased interest in pleasurable stimuli. Studies of this phenomenon have assessed the effect of cytokines or lipopolysaccharide (LPS) on behavior maintained by primary reinforcers, stimuli, such as palatable solutions, that effectively reinforce behavior without prior training. The studies reported in this paper replicated findings of immune system activation decreasing intake of a palatable solution and assessed the effects of immune activation on behavior maintained by a conditioned reinforcer, a stimulus paired with a the palatable solution. Using a conditioned place preference procedure, the effects of LPS and interleukin-1beta (IL-1beta) on sucrose intake (primary reinforcer) and preference for a sucrose-paired environment (conditioned reinforcer) were tested. LPS and IL-1beta decreased sucrose intake but had no effect on the expression of a sucrose-induced place preference, indicating a differential effect of immune system activation on appetitive behaviors maintained by primary and conditioned reinforcers. Finally, it was shown that a sucrose-induced place preference is sensitive to the motivational state of the subjects at the time of testing; a sucrose-induced place preference was demonstrated if rats were tested when water deprived but not if tested after free access to water.

Influence of the estrous cycle on tolerance development to LPS-induced sickness behaviors in rats

The relations between the estrous cycle, inflammatory responses and the development of tolerance to endotoxin were examined. Female Long-Evans rats were injected intraperitoneally with lipopolysaccharide (LPS; 200 microg/kg) or saline vehicle at 08:00h on either diestrus (D) or proestrus (P). Ninety-five minutes after injection locomotor activity was assessed in an automated non-novel open-field for 20 min. To assess tolerance development to LPS, rats were re-injected at the next identical stage (i.e. 4 days later; groups: DD, PP) or at the alternate stage (i.e. 6 days later; groups: DP, PD) of the estrous cycle and locomotor activity was again assessed. On Test Day 1 all groups injected with LPS exhibited similar significant activity decrements, regardless of the stage of the estrous cycle. However, on Test Day 2 rats which received both injections of LPS during proestrus (PP) showed no signs of tolerance development, whereas rats in all other groups were tolerant to LPS. In a follow up study, the time between injections was extended to 8 days. Still the animals injected both times at proestrus showed no signs of tolerance to LPS after the second injection. Thus, the stages of the estrous cycle both at the time of initial exposure and of re-exposure appear critical in the formation of behavioral tolerance to LPS in rats.

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.

The taste of sickness: Lipopolysaccharide-induced finickiness in rats

Decrease in food intake is one of the most documented non-specific symptoms of inflammatory processes. However, attention has been mainly focused on quantitative analysis. The present paper reports studies undertaken to test the possible contribution of changes in taste processes in inflammatory-induced alteration of feeding behavior. In a first experiment, the effects of lipopolysaccharide-induced sickness were assessed on preference for saccharin and aversion for quinine in rats using the two-bottle test paradigm. In a second experiment, effect of lipopolysaccharide (LPS) on the behavioral reactivity to palatable, unpalatable and mixed solutions was analyzed using the taste-reactivity paradigm. Our results show that LPS decreased total fluid intake but did not change taste responses to unpalatable or palatable substances. However, LPS increased aversive reactions and decreased hedonic responses to mixed taste. These LPS-induced changes are interpreted as an increase in finickiness and are discussed in regard to their potential role in the adaptation of individuals to sickness.

Comparing immune activation (lipopolysaccharide) and toxin (lithium chloride)-induced gustatory conditioning: lipopolysaccharide produces conditioned taste avoidance but not aversion

Feeding and drinking typically involve both appetitive and consummatory behaviors. Appetitive behaviors include those behaviors produced by an animal prior to the actual consumption, such as approach movements, whereas consummatory behaviors (such as licking and chewing) are involved in the actual consumption of food. The present research compared the gustatory conditioning effects of bacterial lipopolysaccharide (LPS) and lithium chloride (LiCl) in two different paradigms, conditioned taste avoidance and conditioned taste aversion which differentially affect the appetitive and consummatory components of feeding. Male rats were implanted with intraoral cannulae and habituated to a water deprivation schedule and afterwards received two conditioning days (Days 1 and 4). Each conditioning day consisted of 1 h access to a novel sucrose solution (0.3 M) immediately followed by a systemic injection of LPS (200 microg/kg), LiCl (0.15 M, 3 meq) or NaCl vehicle. Conditioned taste aversion was assessed using the taste reactivity test on Day 7, where orofacial and somatic responses were videotaped and analyzed during 3 brief (1 min) exposures to the sucrose solution. Conditioned taste avoidance was assessed on Days 8 and 9 using a two-bottle preference test (sucrose versus water). Animals conditioned with LiCl displayed typical aversive-like responses in the taste reactivity paradigm evidenced by significant reductions in positive ingestive responses (P<0.05) and an increase in active aversive responses (P<0.05) relative to controls. Furthermore, LiCl treatment resulted in conditioned avoidance of sucrose in the two-bottle preference test characterized by a decreased sucrose preference (P<0.05). Conditioning with LPS produced a reduced sucrose preference (P<0.05) relative to controls, comparable to the avoidance seen in LiCl-treated rats. In contrast, conditioning with LPS resulted in similar positive ingestive responses to intraorally infused sucrose as seen in controls. The present results demonstrate that LPS treatment produces conditioned avoidance but not aversion and suggest that LPS can selectively condition the appetitive aspects of feeding whereas the consummatory behaviors remain unaffected.

Effects of lipopolysaccharide treatment on feeding of goldfish: role of appetite-regulating peptides

The gram-negative bacteria-derived endotoxin lipopolysaccharide (LPS) is known to play an important role in immune and neurological manifestations during bacterial infections. In mammals, peripheral or brain administration of LPS induces anorexia and is thought to exert its effects through activation of pro-inflammatory cytokines. In this study, we investigated the effects of peripheral (intraperitoneal, IP) and central (intracerebroventricular, ICV) injections of LPS on food intake of goldfish. Fish treated IP with 10, 25, 50, 100 or 250 ng/g LPS or ICV with 1, 10 and 100 ng/g LPS showed a significant dose-dependent decrease in food intake, compared to the saline-treated fish. We also examined the brain mRNA expression of several hypothalamic appetite-related neuropeptides in response to the administration of LPS. IP injections of LPS (100 ng/g) induced a decrease in NPY expression and an increase in CCK, CRF and CART expression. These results indicate that LPS is a potent anorexigenic factor in goldfish and that this endotoxin induces a reduction in appetite, at least in part, by influencing gene expression of appetite-related neuropeptides.

Sex differences in the effects of muramyl dipeptide and lipopolysaccharide on locomotor activity and the development of behavioral tolerance in rats

Administration of bacterial agents, such as muramyl dipeptide (MDP) or lipopolysaccharide (LPS), induces a number of illness symptoms including decreased locomotor activity and weight loss. This study provides a detailed multivariate assessment of the effects of repeated exposures of various doses of MDP and LPS, alone and in combination, on various aspects of locomotion in male and female rats. Animals were given a single intraperitoneal injection of either MDP (0.8 or 1.6 mg/kg), LPS (100 or 200 microg/kg), a combination of MDP and LPS (0.8 mg/kg and 100 microg/kg, respectively), or vehicle on Days 1, 4, and 7. Two hours after each injection, locomotor activity was recorded for 30 min in an automated open-field. Both doses of LPS and the high dose of MDP produced significant decrements in locomotor activity in male and female rats, with tolerance becoming evident over repeated administrations, although LPS decreased activity more robustly than MDP. Sex differences were evident in the combined effects of MDP and LPS. Together, MDP and LPS reduced male activity levels in an additive manner but significantly potentiated both horizontal and vertical activity decrements in females. In addition, the rate of behavioral tolerance development to repeated bacterial injections was significantly higher in females than in males. These findings provide evidence for sex differences in the actions of MDP and LPS on various aspects of locomotor activity and in the development of behavioral tolerance to infection.