Interleukin-1 receptor antagonist does not reverse lipopolysaccharide-induced inhibition of water intake in rat

The gut-brain axis and sodium appetite: Can inflammation-related signaling influence the control of sodium intake?

Sodium is the main cation present in the extracellular fluid. Sodium and water content in the body are responsible for volume and osmotic homeostasis through mechanisms involving sodium and water excretion and intake. When body sodium content decreases below the homeostatic threshold, a condition termed sodium deficiency, highly motivated sodium seeking, and intake occurs. This is termed sodium appetite. Classically, sodium and water intakes are controlled by a number of neuroendocrine mechanisms that include signaling molecules from the renin-angiotensin-aldosterone system acting in the central nervous system (CNS). However, recent findings have shown that sodium and water intakes can also be influenced by inflammatory agents and mediators acting in the CNS. For instance, central infusion of IL-1β or TNF-α can directly affect sodium and water consumption in animal models. Some dietary conditions, such as high salt intake, have been shown to change the intestinal microbiome composition, stimulating the immune branch of the gut-brain axis through the production of inflammatory cytokines, such as IL-17, which can stimulate the brain immune system. In this review, we address the latest findings supporting the hypothesis that immune signaling in the brain could produce a reduction in thirst and sodium appetite and, therefore, contribute to sodium intake control.

Interleukin-1α in the ventral hippocampus increases stress vulnerability and inflammation-related processes

Mechanisms of stress vulnerability remain elusive. Previous research demonstrated that inflammation-related processes in the brain play a role in stress vulnerability. Our previous research showed that inflammatory processes in the ventral hippocampus (vHPC) induced a stress vulnerable phenotype. To further understand neuroinflammatory processes in the vHPC in stressed rats, we determined that protein levels of the pro-inflammatory cytokine interleukin-1-α (IL-1α), but not interleukin-1β (IL-1β), were increased in the vHPC of rats vulnerable to the effects of repeated social defeat compared to rats resilient to its effects. Injections of IL-1α into the vHPC increased stress vulnerability as characterized by increases in passive coping during defeat and subsequent decreased social interactions. Conversely, injections of recombinant IL-1 receptor antagonist (IL1-RA) increased latencies to social defeat and decreased anxiety-like behaviors during social interaction, suggesting an reduction in stress vulnerability. Protein analyses revealed increased FosB expression in the vHPC of IL-1α-injected rats, and increased HPA activation following a social encounter. Further analysis of vHPC of IL1-α-injected rats showed increased density of microglia, increased expression of the pro-inflammatory cytokine HMGB1, and increases in a marker for vascular remodeling. Taken together, these data show increasing IL-1α during stress exposure is sufficient to produce a stress vulnerable phenotype potentially by increasing inflammation-related processes in the vHPC.LAY SUMMARYOur previous research demonstrated that inflammation-related processes in the brain play a role in inducing vulnerability to the effects of repeated social stress in rats. Here we demonstrate that a pro-inflammatory cytokine interleukin-1-α (IL-1α) induces inflammatory processes in the vHPC and behavioral vulnerability in stressed rats, whereas blocking IL receptors produces the opposite effects on behavioral vulnerability. Together, these results identify a substrate in the vHPC that produces vulnerability to stress by increasing inflammation-related processes in the vHPC.

Inflammatory Factors Mediate Vulnerability to a Social Stress-Induced Depressive-like Phenotype in Passive Coping Rats

BACKGROUND

Coping strategy impacts susceptibility to psychosocial stress. The locus coeruleus (LC) and dorsal raphe (DR) are monoamine nuclei implicated in stress-related disorders. Our goal was to identify genes in these nuclei that distinguish active and passive coping strategies in response to social stress.

METHODS

Rats were exposed to repeated resident-intruder stress and coping strategy determined. Gene and protein expression in the LC and DR were determined by polymerase chain reaction array and enzyme-linked immunosorbent assay and compared between active and passive stress-coping and unstressed rats. The effect of daily interleukin (IL)-1 receptor antagonist before stress on anhedonia was also determined.

RESULTS

Rats exhibited passive or active coping strategies based on a short latency (SL) or longer latency (LL) to assume a defeat posture, respectively. Stress differentially regulated 19 and 26 genes in the LC and DR of SL and LL rats, respectively, many of which encoded for inflammatory factors. Notably, Il-1β was increased in SL and decreased in LL rats in both the LC and DR. Protein changes were generally consistent with a proinflammatory response to stress in SL rats selectively. Stress produced anhedonia selectively in SL rats and this was prevented by IL-1 receptor antagonist, consistent with a role for IL-1β in stress vulnerability.

CONCLUSIONS

This study highlighted distinctions in gene expression related to coping strategy in response to social stress. Passive coping was associated with a bias toward proinflammatory processes, particularly IL-1β, whereas active coping and resistance to stress-related pathology was associated with suppression of inflammatory processes.

Blockade of central kappa-opioid receptors inhibits the antidipsogenic effect of interleukin-1beta

The objective of the present study was to investigate the role of brain kappa-opioid receptors (KOR) in the antidipsogenic effect promoted by third ventricle injections of interleukin-1beta (IL-1beta). Wistar male rats were submitted to three different, thirst-inducing, physiological conditions: dehydration induced by water deprivation, hyperosmolarity induced by salt-load and hypovolemia induced by polyethylene glycol subcutaneous injection. Third ventricle injections of IL-1beta significantly inhibited the increase in water intake observed in those situations. The pharmacological blockade of central KOR by the selective KOR antagonist nor-binaltorphimine (BNI) at different doses significantly inhibited the antidipsogenic effect induced by the central administration of IL-1beta in all conditions tested: dehydration, hypovolemia and hyperosmolarity. The central administration of IL-1beta failed to induce any locomotor deficit, as verified in an open field test. Stimulation of the central interleukinergic component did not result in any general suppression of ingestive behavior since no change in saccharin intake was recorded during a dessert test in animals receiving central injections of IL-1beta. Furthermore, the inhibitory effects of IL-1beta on water intake cannot be attributed to sickness-like effects induced by these compounds, since an aversion test excluded this possibility. In summary, the data shown in the present study clearly show that the antidipsogenic effect observed in rats following third ventricle injections of IL-1beta depend on the functional integrity of a brain kappa-opioid-dependent component.

Movement-evoked hyperalgesia induced by lipopolysaccharides is not suppressed by glucocorticoids

Systemic exposure to lipopolysaccharides (LPS) produces a variety of effects, including movement-evoked hyperalgesia that can be measured using the grip force assay in mice. Because both lethality and enhanced sensitivity to cutaneous pain following exposure to endotoxins have each been attributed to inflammatory mediators, we explored the possibility that LPS-induced movement-evoked hyperalgesia is also sensitive to manipulations of glucocorticoids that regulate these other LPS responses. We found that the hyperalgesic effect of LPS (5mg/kg s.c.) in mice that were adrenalectomized did not differ from that in control mice that were sham operated, even though mortality after LPS was potentiated by adrenalectomy. The development of tolerance to the movement-evoked hyperalgesic effect of LPS also did not differ between adrenalectomized and sham-operated control mice. In addition, mifepristone (25mg/kg s.c.), a glucocorticoid antagonist, did not attenuate the hyperalgesic effect of LPS (2mg/kg s.c.), yet this dose of mifepristone was sufficient to enhance the incidence of lethality induced by LPS. Enhancement of glucocorticoid activity by two injections of dexamethasone (1mg/kg s.c.) had no effect on the degree of hyperalgesia in mice injected with LPS (5mg/kg s.c.), yet this dose of dexamethasone was sufficient to attenuate the incidence of mortality induced by LPS in adrenalectomized mice. Finally, morphine (10mg/kg i.p.) reversed the decrease in grip force caused by LPS (5mg/kg i.p.), supporting the interpretation that decreases in grip force produced by LPS reflect muscle hyperalgesia that is not sensitive to glucocorticoids.

Inhibition of water intake by the central administration of IL-1beta in rats: role of the central opioid system

In the present study we investigated, the effect of third ventricle injections of IL-1beta on water intake, in rats, induced by three different physiological stimuli: dehydration induced by water deprivation, hypernatremia associated with hyperosmolarity induced by intragastric salt load, and hypovolemia produced by subcutaneous polytethyleneglycol administration. Central administration of IL-1beta at the doses of 4 and 8 ng reduced water intake in all three conditions studied. Third ventricle injections of IL-1beta (8 ng) were unable to diminish water intake in the groups of rats pretreated with central injections of the non-selective opioid antagonist naloxone (10 microg) in the three different conditions studied. Furthermore, the central administration of IL-1beta was neither able to modify the intake of a 0.1% saccharin solution when the animals were submitted to a "dessert test" nor to induce any significant locomotor deficit in the open-field test. These results suggest that the central activation of interleukin-1 receptors by IL-1beta is able to impair the thirst-inducing mechanisms triggered by the physiological stimuli represented by dehydration, hyperosmolarity and hypovolemia. These results lead us to conclude that the antidipsogenic effects observed following central administration of IL-1beta require the functional integrity of the brain opiatergic system.

Role of the central opioid system in the inhibition of water and salt intake induced by central administration of IL-1β in rats

In the present study we investigated, the effect of third ventricle injections of IL-1beta on water and salt intake in fluid-deprived and sodium-depleted rats. Central administration of IL-1beta significantly reduced water and salt intake in fluid-deprived animals and decreased salt intake in sodium-depleted rats. The antidipsogenic and antinatriorexic effects elicited by the central administration of IL-1beta were suppressed by pretreatment with central injections of the non-selective opioid antagonist naloxone (10 mug) in the two different experimental protocols used here (water deprivation and sodium depletion). In addition, central administration of IL-1beta failed to modify the intake of a 0.1% saccharin solution when the animals were submitted to a "dessert test" or to induce any significant locomotor deficit in the open-field test. The present results suggest that the activation of the central interleukinergic component by IL-1beta impairs the increase in water and salt intake induced by water deprivation and the enhancement in sodium appetite that follows sodium depletion. The data also support the conclusion that the antidipsogenic and antinatriorexic effects resulting from the activation of the central interleukinergic component rely on an opioid-dependent, naloxone-blockable system.

Tolerance develops to the effect of lipopolysaccharides on movement-evoked hyperalgesia when administered chronically by a systemic but not an intrathecal route

Single exposures to lipopolysaccharides (LPS) produce deep tissue pain in humans and cutaneous hyperalgesia in rodents. While tolerance develops to many effects of LPS, sensitization to hyperalgesia is documented after a single injection. To determine the effect of long-term exposure to LPS, we explored the chronic effect of LPS on movement-evoked pain using a new assay based on grip force in mice. We found that a single systemic injection of LPS (i.p. or s.c.) induced a dose-related decrease in forelimb grip force responses beginning 6-8 h after injection and peaking between 9 and 24 h. The consequence of LPS is likely hyperalgesia rather than weakness as these decreases were rapidly attenuated by either 10 mg/kg of morphine i.p. or 10 microg of morphine injected intrathecally (i.t.). Complete tolerance to this hyperalgesia developed after repeated injections of LPS at doses of 0.9 mg/kg i.p. or 5 mg/kg s.c. Tolerance began after a single injection and was fully developed after as few as four injections of 5 mg/kg of LPS delivered s.c. The concentration of circulating LPS 5 h after a single parenteral injection was less in LPS-tolerant mice than naïve controls, suggesting that tolerance may result from a more efficient clearance of LPS from the circulation. Injected i.t., LPS also induced hyperalgesia, however, tolerance did not develop to multiple injections by this route. There was no cross-tolerance between s.c. and i.t. injections of LPS. These data indicate that decreases in grip force are a sensitive measure of LPS-induced movement-evoked hyperalgesia and that tolerance develops to parenteral but not central hyperalgesic effects of LPS.

Melatonin reduces anxiety induced by lipopolysaccharide in the rat

In male Sprague-Dawley rats intraperitoneal (i.p.) injection of Escherichia coli lipopolysaccharide (0.25, 0.50 and 1 mg/kg) increased anxiety levels. This effect was reversed by a prior, concomitant, and subsequent i.p. treatment with melatonin (4 and 6 mg/kg). As the effects of melatonin upon the actions induced by lipopolysaccharide were reversed by the melatonin receptor antagonist luzindole (30 and 60 mg/kg, i.p.), we argued that they are, but not only, melatonin receptor mediated. These findings, in accordance with our previous works, suggest that melatonin could be useful in the treatment of sickness behaviour associated with systemic infection diseases or as adjuvant in the anti-anxiety therapy.

Repeated lipopolysaccharide administration produces tolerance to anorexia and fever but not to inhibition of thirst in rat

In 24 h water and food deprived rats, a single lipopolysaccharide treatment (0.25, 0.50 and 1 mg/kg, i.p.) induced inhibition of thirst and hunger as well as fever. Moreover, the same treatment increased serum cytokines, plasma nitrite/nitrate and corticosterone and urinary prostaglandin levels. In another group of 24 h water and food deprived rats, a repeated lipopolysaccharide treatment (0.25, 0. 50 and 1 mg/kg, i.p.), given at 0, 2, 6, 12 and 24 h, induced tolerance to inhibition of food intake and fever, but not to antidipsogenic effect. Moreover, the same repeated treatment stopped the increase in serum cytokines, plasma corticosterone and urinary prostaglandin concentrations and failed to reduce plasma nitrite/nitrate levels. This data, together with the evidence that a pretreatment with N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) (5 and 10 microg per rat) reverses the antidipsogenic effects in lipopolysaccharide tolerant rats, suggests that the persistent reduction of water intake after a repeated lipopolysaccharide treatment is due to the antidipsogenic action of nitric oxide in the brain.

Human immunodeficiency virus-1 coat protein gp120 impairs contextual fear conditioning: a potential role in AIDS related learning and memory impairments

Many AIDS patients suffer from cognitive impairments including deficits in learning and memory. The Human Immunodeficiency Virus-1 (HIV-1) envelope glycoprotein gp120 is one possible mediator of these impairments. This is because gp120 activates brain microglial cells and astrocytes, and in vivo activation of glia leads to the release of the proinflammatory cytokine interleukin-1 beta (IL-1beta). gp120 induced IL-1beta release could be involved in producing memory impairments associated with AIDS because central IL-1beta activity adversely affects cognitive function. The reported experiments evaluated the effects of i.c.v. gp120 administration and subsequent IL-1beta activity on learning and memory processes in the rat. Intracerebroventricular gp120 produced memory impairments on hippocampally dependent contextual fear conditioning, but not hippocampally independent auditory-cue fear conditioning following post-conditioning gp120 administration. Central gp120 administration also caused increases in IL-1beta protein levels in the hippocampus and frontal cortex but not in the hypothalamus. gp120 induced memory impairments were blocked by 2 different IL-1 antagonists, alpha melanocyte stimulating hormone (alphaMSH) and interleukin-1 receptor antagonist (IL-1ra). Finally, heat denaturation of the tertiary structure of gp120 abolished its effects on fear conditioning, suggesting that gp120 impairs contextual fear conditioning by binding to its receptors on glia.

Role of interleukin-1 beta in impairment of contextual fear conditioning caused by social isolation

Isolating rats immediately after conditioning impairs contextual but not auditory-cue fear conditioning. The reported experiments examine the involvement of brain interleukin-1beta (IL-1beta) in the impairment in contextual fear conditioning caused by social isolation. As measured by the conditioned freezing response, 5 h of social isolation after conditioning, impaired contextual but not auditory-cue fear conditioning in adult male Sprague-Dawley rats. Social isolation for 1 or 3 h after conditioning also increased IL-1beta protein in the hippocampus and cerebral cortex. No differences in IL-1beta protein levels were found in the pituitary or the hypothalamus. Intracerebroventricular (ICV) IL-1 receptor antagonist (IL-1ra) given after conditioning prevented the impairment in contextual fear conditioning caused by isolation. ICV IL-1ra had no effect on auditory-cue fear conditioning in these same animals, nor did it affect the level of contextual fear conditioning displayed by home cage controls. Like isolation, ICV IL-1beta (10 or 20 ng) after conditioning also impaired contextual but not auditory-cue fear conditioning. These results suggest that increased levels of brain IL-1beta play a role in producing the impairment in contextual fear conditioning produced by social isolation. These findings also add to the generality of the idea that stressors induce IL-1beta activity in the brain and that IL-1beta may play physiological roles in the uninjured brain.

Role of centrally administered melatonin and inhibitors of COX and NOS in LPS-induced hyperthermia and adipsia

In the present study we have examined the effect of centrally administered non-steroidal anti-inflammatory drugs (NSAIDS), nitric oxide synthase (NOS) inhibitor and melatonin on lipopolysaccharide (LPS)-induced hyperthermia and its anti-dipsogenic effect. Intracerebroventricular (i.c.v.) administration of LPS (100-200 ng/rat) induces a dose dependent elevation in body temperature and decreases water consumption in 24 h water deprived rats. Coadministration of NSAIDS (indomethacin and nimesulide: 10 nM/rat each) with LPS (100 ng) reversed, whereas NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME: 10-20 microg/rat) enhanced LPS-induced hyperthermia. In contrast L-NAME reversed the LPS-induced anti-dipsogenic effect in a dose dependent manner, whereas NSAIDS showed no change in the effect of LPS. Further, centrally administered prostaglandin E2 (PGE2, 0.5-1 microg/rat) produced hyperthermia without affecting the drinking behavior, suggesting that two independent mechanisms operate in LPS-induced hyperthermia and in the anti-dipsogenic effect. The pineal hormone melatonin is known to inhibit cellular damage caused by LPS, produced dose dependent (5-10 nM i.c.v.) inhibition of LPS-induced hyperthermia and adipsia, but failed to reverse the PGE2-induced hyperthermia, shows reversal of LPS-induced hyperthermia by melatonin is due to inhibition of prostaglandin synthesis rather than antagonism of prostaglandin action. The overall study reveals that inhibition of both NO and prostaglandin production by melatonin might be responsible for its reversal of LPS-induced hyperthermia and adipsia.

Central effects of cromoglycate sodium salt in rats treated with lipopolysaccharide

In 24-h water- and food-deprived rats, we have evaluated the effects of cromoglycate sodium salt, an inhibitor of the mast cell degranulation with anti-inflammatory and membrane-stabilizating activity, on the central effects induced by Escherichia coli lipopolysaccharide (LPS). Intraperitoneal (i.p.) injection of LPS (0.25, 0.50 and 1 mg/kg) induced a dose-dependent inhibition of water and food intake, fever, reduction in locomotor activity as well as increased anxiety levels. All these LPS effects were antagonized by a prior intracerebroventricular (i.c.v.) injection of cromoglycate sodium salt (100, 150 and 200 microg/rat). Our findings suggest that peripheral LPS administration may activate brain mast cells and indicate an involvement of these cells in brain pathophysiology.

Role of hypothalamic interleukin-1beta in fever induced by cecal ligation and puncture in rats

Bacterial endotoxin induces fever by causing the release of interleukin (IL)-1beta into the circulation or the brain. IL-1beta is believed to mediate fever via triggering the production and/or release of IL-6 in the hypothalamus. The present study examined whether IL-1beta and IL-6 in the hypothalamus of the rat are also involved in fever during bacterial sepsis caused by cecal ligation and puncture (CLP). CLP induces fever for 2 days. Polyclonal rabbit antibody against rat IL-1beta (anti-IL-1beta, 2 microg/microl) or control rabbit IgG (2 microg/microl) was unilaterally microinjected into the hypothalamus of rats immediately after or 24 h after CLP or sham-CLP surgery. Anti-IL-1beta injected 24 h after CLP (when fever was already present) or sham-CLP surgery did not affect fever. Microinjection of anti-IL-1beta into the hypothalamus immediately after surgery caused a significant decrease in body temperature during the night after CLP surgery and a 48% reduction of fever on the following day. Although blood plasma levels of IL-6 were significantly elevated 1.5, 6, 24, and 48 h after CLP surgery, there were no differences in IL-6 concentrations in the extracellular fluid of the anterior hypothalamus (collected by push-pull perfusion). These data suggest that fever due to bacterial sepsis is initiated by IL-1beta within the hypothalamus, and this febrile response, unlike endotoxin-induced fever, is not accompanied by elevation in the hypothalamic concentration of IL-6.

Melatonin effects on inhibition of thirst and fever induced by lipopolysaccharide in rat

In 24 h water deprived rats we have evaluated the effects of melatonin on the inhibition of thirst and on fever induced by Escherichia coli lipopolysaccharide. Intraperitoneal (i.p) injection of lipopolysaccharide (0.32, 0.64 and 0.96 mg/kg) alone induced, a dose-dependent and significant inhibition of water intake as well as fever. In addition, lipopolysaccharide at the same concentrations increased urinary prostaglandins and serum cytokines levels. On the contrary, lipopolysaccharide treatment had no effects on cerebral brain nitric oxide synthase activity. All lipopolysaccharide effects were reverted by a prior, concomitant and subsequent i.p. treatment with melatonin (2, 4 and 6 mg/kg), whereas they were still present when melatonin was injected in combination with the melatonin receptor antagonist luzindole (15, 30 and 60 mg/kg, i.p.). We suggest that melatonin could exert its dipsogenic effects through a reduction of the free radical nitric oxide (NO.) whereas it may reduce body temperature by preventing an excessive formation of prostaglandins and cytokines.

Interleukin-1 inhibits drinking behaviour through prostaglandins, but not by nitric oxide formation

Interleukin-1 beta (IL-1 beta) causes inhibition of drinking behaviour. Moreover it induces formation of prostaglandins (PGs) and nitric oxide (NO). Both PGs and NO are able to inhibit drinking stimulated by water deprivation or by intracerebroventricular (i.c.v.) administration of angiotensin II. In this study, we studied in the preoptic area (POA) the possible role of PGs and NO in the antidipsogenic action induced by IL-1 beta. IL-1 beta was injected in the lateral cerebral ventricle (i.c.v.) (2.5, 10, 20, and 40 ng/rat) or into POA (0.625, 1.25, 2.5, and 10 ng/rat). L-arginine (12.5, 25, 50, and 100 ng/rat), the precursor of NO, or NG-nitro-L-arginine methyl ester (L-NAME) (25, 50, and 100 ng/rat), an inhibitor of nitric oxide synthase (NOS), were injected only into POA. Drinking behaviour was induced by water deprivation (24 h). IL-1 beta injected either i.c.v. or into POA caused dose dependent inhibition of drinking. In the POA a treatment with acetylsalicylic acid (ASA) (33, 66, and 135 micrograms/rat), but not with L-NAME, antagonized the inhibition of drinking behaviour induced by the highest doses of IL-1 beta in the POA. In the POA, a treatment with ASA or L-NAME antagonized the inhibition of drinking behaviour caused by injection of the highest doses of L-arginine. Our data suggest that the central inhibition of drinking behaviour of IL-1 beta is mediated through the formation of PGs, but not NO, in the POA.

Effects of interleukin-10 on water intake, locomotory activity, and rectal temperature in rat treated with endotoxin

Intracerebroventricular (i.c.v.) interleukin-10 (25, 50, and 100 ng/rat) effects on water intake, exploratory behaviour, and rectal temperature were evaluated in rats treated intraperitoneally (i.p.) with lipopolysaccharide (0.32, 0.64, and 0.96 mg/kg). Endotoxin administration induced fever and inhibition of thirst in water-deprived rats, and a decrease in lococomotory activity in normohydrated and water-deprived animals. Our data show that interleukin-10 during lipopolysaccharide administration controlled fever, increases exploratory behaviour, but did not reverse lipopolysaccharide inhibition of thirst. These effects suggest that fever, depression in locomotory activity but not inhibition of thirst, induced by endotoxin are influenced by interleukin-10 levels.