Aging sensitizes male mice to cognitive dysfunction induced by central HIV-1 gp120.

Although highly active antiretroviral therapy has led to improved prognosis and alleviation of some HIV-related disease complications, it has not provided complete protection against HIV-associated dementia. As the population of persons living with HIV grows older and aged persons represent a significant number of new infections, it is important to understand how HIV may affect the aged brain. In the current study, both adult and aged mice were treated with HIV gp120 and trained in a reference memory version of the water maze. Analysis of probe data revealed that aged animals treated with gp120 demonstrated profound decrements in water maze performance compared to gp120 treated young animals and saline treated aged or young animals. Additionally, we examined the neuroinflammatory responses in the aged and adult brain 4 h after treatment with gp120. Pro-inflammatory cytokines associated with neuroinflammation are known to be antagonistic to learning and memory processes and aged and adult animals treated with gp120 demonstrated similar increases in IL-1ß and IL-6 in the hippocampus and cortex. Additionally, gp120 treatment was associated with an increase in MHCII gene expression, a marker of microglial activation, in the hippocampus. Although, the aged brain demonstrated a similar inflammatory profile at the time point measured, aged animals were more sensitive to cognitive dysfunction related to gp120 treatment. This finding supports the theory that aging may be a significant risk factor in the development of HIV-associated dementia.

A drug-drug conditioning paradigm reveals multiple antipsychotic-nicotine interactions.

Clinical studies indicate a reciprocal impact between nicotine use and antipsychotic medications in patients with schizophrenia. The present study used a conditioned avoidance response (CAR) test (a behavioral test of antipsychotic effect) and examined the specific drug-drug interactions between nicotine and haloperidol or clozapine. Following acquisition of the avoidance response, rats were first tested under either vehicle, nicotine (0.2, 0.4 mg/kg, sc), haloperidol (0.025, 0.05 mg/kg, sc), clozapine (5.0, 10.0 mg/kg, sc), or a combination of nicotine and haloperidol or nicotine and clozapine for seven consecutive days. Afterward, they were challenged with nicotine (0.2 mg/kg), haloperidol (0.025 mg/kg), or clozapine (5.0 mg/kg) in the CAR to assess if haloperidol or clozapine affected the behavioral effect of nicotine on avoidance response and if nicotine altered the avoidance suppressive effect of haloperidol and clozapine. During the seven avoidance drug test days, nicotine did not alter the avoidance suppressive effect of haloperidol or clozapine. However, in the challenge test, prior nicotine treatment (0.2 mg/kg) attenuated haloperidol's (0.05 mg/kg) sensitized effect on avoidance response. On the other hand, prior haloperidol treatment increased nicotine's (0.2 mg/kg) avoidance disruptive effect, and even engendered nicotine 0.4 mg/kg to exhibit an "acquired" avoidance suppressive effect. The combined nicotine and clozapine treatment did not produce any detectable interactive effects on avoidance response and motor activity. These findings suggest that nicotine is capable of altering the long-term antipsychotic efficacy of haloperidol, while haloperidol can alter the behavioral effects of nicotine. Clozapine and nicotine are less likely to influence each other.

Drug-drug conditioning between citalopram and haloperidol or olanzapine in a conditioned avoidance response model: implications for polypharmacy in schizophrenia.

Patients with schizophrenia often have anxiety and depression, and thus are treated with multiple psychotherapeutic medications. This practice of polypharmacy increases the possibility for drug-drug interactions. However, the pharmacological and behavioral mechanisms underlying drug-drug interactions in schizophrenia remain poorly understood. In the present study, we adopted a preclinical approach and examined a less known behavioral mechanism, drug-drug conditioning (DDC) between haloperidol (a typical antipsychotic) or olanzapine (atypical antipsychotic) and citalopram (a selective serotonin reuptake inhibitor). A rat two-way conditioned avoidance response paradigm was used to measure antipsychotic activity and determine how DDC may alter the antipsychotic efficacy in this model. Following acquisition of the avoidance response, rats were then randomly assigned to receive vehicle, citalopram (10.0 mg/kg, intraperitoneally), haloperidol (0.05 mg/kg, subcutaneously), olanzapine (1.0 mg/kg, subcutaneously), combined haloperidol with citalopram, or combined olanzapine with citalopram treatment for seven avoidance test sessions. In comparison with antipsychotic treatment alone, combined treatment with citalopram potentiated the antiavoidance effect of olanzapine or haloperidol (to a lesser extent) during the seven drug-test sessions. In addition, repeated pairing of citalopram with haloperidol or olanzapine caused citalopram to show a newly acquired avoidance-disruptive effect. This effect was context specific because citalopram paired with haloperidol or olanzapine outside the avoidance testing context (i.e. home cages) did not show such an effect. These findings indicate that concurrent antidepressant and antipsychotic treatments may engender a DDC process that follows the general Pavlovian associative conditioning principles. They also indicate that adjunctive citalopram treatment may enhance the antipsychotic efficacy of haloperidol and olanzapine in the treatment of schizophrenia.

Inhibition of interleukin-6 trans-signaling in the brain facilitates recovery from lipopolysaccharide-induced sickness behavior.

BACKGROUND: Interleukin (IL)-6 is produced in the brain during peripheral infection and plays an important but poorly understood role in sickness behavior. Therefore, this study investigated the capacity of soluble gp130 (sgp130), a natural inhibitor of the IL-6 trans-signaling pathway to regulate IL-6 production in microglia and neurons in vitro and its effects on lipopolysaccharide (LPS)-induced sickness behavior in vivo. METHODS: A murine microglia (BV.2) and neuronal cell line (Neuro.2A) were used to study the effects of stimulating and inhibiting the IL-6 signaling pathway in vitro. In vivo, adult (3-6 mo) BALB/c mice received an intracerebroventricular (ICV) injection of sgp130 followed by an intraperitoneal (i.p.) injection of LPS, and sickness behavior and markers of neuroinflammation were measured. RESULTS: Soluble gp130 attenuated IL-6- and LPS-stimulated IL-6 receptor (IL-6R) activation along with IL-6 protein release in both microglial (BV.2) and neuronal (Neuro.2A) cell types in vitro. Moreover, in vivo experiments showed that sgp130 facilitated recovery from LPS-induced sickness, and this sgp130-associated recovery was paralleled by reduced IL-6 receptor signaling, mRNA, and protein levels of IL-6 in the hippocampus. CONCLUSIONS: Taken together, the results show that sgp130 may exert an anti-inflammatory effect on microglia and neurons by inhibiting IL-6 binding. These data indicate that sgp130 inhibits the LPS-induced IL-6 trans-signal and show IL-6 and its receptor are involved in maintaining sickness behavior.

A neurotoxic regimen of methamphetamine exacerbates the febrile and neuroinflammatory response to a subsequent peripheral immune stimulus.

Methamphetamine (MA) use is associated with activation of microglia and, at high doses, can induce neurotoxicity. Given the changes in the neuroinflammatory environment associated with MA, we investigated whether MA administration would interfere with the thermoregulatory and neuroinflammatory response to a subsequent peripheral immune stimulus. C57BL6/J mice were given four i.p. injections of either 5 mg/kg MA or saline at two hour intervals. Twenty-four hours following the first MA injection, mice were given 100 µg/kg LPS or saline i.p. and blood and brains were collected. Here we report that mice exposed to MA developed higher fevers in response to LPS than did those given LPS alone. MA also exacerbated the LPS-induced increase in central cytokine mRNA. MA alone increased microglial Iba1 expression and expression was further increased when mice were exposed to both MA and LPS, suggesting that MA not only activated microglia but also influenced their response to a peripheral immune stimulus. Taken together, these data show that MA administration exacerbates the normal central immune response, most likely by altering microglia.

Cognitive deficits in interleukin-10-deficient mice after peripheral injection of lipopolysaccharide.

Interleukin (IL)-10 is important for regulating inflammation but whether it protects against infection-related deficits in cognitive function is unknown. Therefore, the current study evaluated sickness behavior, hippocampal-dependent matching-to-place performance and several inflammatory cytokines and neurotrophins in wild-type (IL-10(+/+)) and IL-10-deficient (IL-10(-/-)) mice after i.p. injection of lipopolysaccharide (LPS). Additionally, morphology of dendrites of pyramidal neurons in the dorsal CA1 hippocampus was assessed. Treatment with LPS increased IL-1beta, IL-6, and tumor necrosis factor alpha (TNFalpha) mRNA in all brain areas examined including the hippocampus, in both IL-10(+/+) and IL-10(-/-) mice but the increase was largest in IL-10(-/-) mice. Plasma IL-1beta, IL-6 and TNFalpha were also higher in IL-10(-/-) mice compared to IL-10(+/+) mice after LPS. Consistent with increased inflammatory cytokines in IL-10(-/-) mice after LPS treatment, were a more lengthy sickness behavior syndrome and a more prominent reduction in hippocampal levels of nerve growth factor mRNA; brain-derived neurotrophic factor mRNA was reduced similarly in both genotypes after LPS. In a test of hippocampal-dependent learning and memory that required mice to integrate new information with previously learned information and switch strategies to master a task, IL-10(-/-) mice were found to be less efficient after LPS than were similarly treated wild-type mice. LPS did not affect morphology of dendrites of pyramidal neurons in the dorsal CA1 hippocampus in either genotype. Taken together the results are interpreted to suggest that during peripheral infection IL-10 inhibits sickness behavior and tribulations in hippocampal-dependent working memory via its propensity to mitigate inflammation. We conclude that IL-10 is critical for maintaining normal neuro-immune communication during infection.

Neuroinflammation associated with aging sensitizes the brain to the effects of infection or stress.

The aging brain is characterized by a shift from the homeostatic balance of inflammatory mediators to a proinflammatory state. This increase in neuroinflammation is marked by increased numbers of activated and primed microglia, increased steady-state levels of inflammatory cytokines and decreases in anti-inflammatory molecules. These conditions sensitize the aged brain to produce an exaggerated response to the presence of an immune stimulus in the periphery or following exposure to a stressor. In the brain, proinflammatory cytokines can have profound effects on behavioral and neural processes. As the aged brain is primed to respond to inflammatory stimuli, infection or stress may produce more severe detriments in cognitive function in the aged. Typically after an immune stimulus, aged animals display prolonged sickness behaviors, increased cytokine induction and greater cognitive impairments compared to adults. Additionally, aging can also augment the central response to stressors leading to exaggerated cytokine induction and increased decrements in learning and memory. This alteration in neuroinflammation and resultant sensitization to extrinsic and intrinsic stressors can have considerable effects upon the elderly's recovery and coping during disease and stress.

Neonatal endotoxin exposure impairs avoidance learning and attenuates endotoxin-induced sickness behavior and central IL-1beta gene transcription in adulthood.

Infection during infancy, a time of critical neural development, may have long-term implications. Infection or exposure to an immune stimulus such as lipopolysaccharide (LPS) early in life leads to alterations in the reactivity of the hypothalamic-pituitary-adrenal axis (HPA) and febrile response in adulthood. Relatively few studies have assessed the behavioral and cognitive alterations induced by perinatal immune challenge. The data indicate that neonatal immune activation may alter adulthood behavior with, or sometimes without, subsequent adulthood exposure, depending on the study. The current study investigated the behavioral effects and IL-1beta transcription following intraperitoneal LPS administration on postnatal days (PNDs) 4 and 5, and subsequent LPS or saline administration in adulthood. Alterations in anxiety, motor behavior, and learning were assessed in male and female subjects. The results indicate that neonatal endotoxin exposure attenuated the LPS-induced decrease in motor behavior in female, but not male, subjects. Furthermore, perinatal immune activation disrupted avoidance learning in male, but not female, subjects in the absence of adulthood LPS administration. In addition, for male subjects, neonatal LPS exposure diminished central IL-1beta gene transcription following adulthood LPS administration. These findings indicate that perinatal endotoxin exposure may lead to alterations in the behavioral response to adulthood LPS administration, and provide evidence that early immune activation alone may trigger alterations in adulthood learning ability.

Neuroinflammation and disruption in working memory in aged mice after acute stimulation of the peripheral innate immune system.

Acute cognitive disorders are common in elderly patients with peripheral infections but it is not clear why. Here, we injected old and young mice with Escherichia coli lipopolysaccharide (LPS) to mimic an acute peripheral infection and separated the hippocampal neuronal cell layers from the surrounding hippocampal tissue by laser capture microdissection and measured mRNA for several inflammatory cytokines (IL-1 beta, IL-6, and TNFalpha) that are known to disrupt cognition. The results showed that old mice had an increased inflammatory response in the hippocampus after LPS compared to younger cohorts. Immunohistochemistry further showed more microglial cells in the hippocampus of old mice compared to young adults, and that more IL-1 beta-positive cells were present in the dentate gyrus and in the CA1, CA2, and CA3 regions of LPS-treated old mice compared to young adults. In a test of cognition that required animals to effectively integrate new information with a preexisting schema to complete a spatial task, we found that hippocampal processing is more easily disrupted in old animals than in younger ones when the peripheral innate immune system is stimulated. Collectively, the results suggest that aging can facilitate neurobehavioral complications associated with peripheral infections probably by allowing the over expression of inflammatory cytokines in brain areas that mediate cognitive processing.

Aging exacerbates depressive-like behavior in mice in response to activation of the peripheral innate immune system.

Exposure to peripheral infections may be permissive to cognitive and behavioral complications in the elderly. We have reported that peripheral stimulation of the innate immune system with lipopolysaccharide (LPS) causes an exaggerated neuroinflammatory response and prolonged sickness behavior in aged BALB/c mice. Because LPS also causes depressive behavior, the purpose of this study was to determine whether aging is associated with an exacerbated depressive-like response. We confirmed that LPS (0.33 mg/kg intraperitoneal) induced a protracted sickness response in aged mice with reductions in locomotor and feeding activities 24 and 48 h postinjection, when young adults had fully recovered. When submitted to the forced swim test 24 h post-LPS, both young adult and aged mice exhibited an increased duration of immobility. However, when submitted to either the forced swim test or the tail suspension test 72 h post-LPS, an increased duration of immobility was evident only in aged mice. This prolonged depressive-like behavior in aged LPS-treated mice was associated with a more pronounced induction of peripheral and brain indoleamine 2,3-dioxygenase and a markedly higher turnover rate of brain serotonin (as measured by the ratio of 5-hydroxy-indoleacetic acid over 5-hydroxy-tryptamine) compared to young adult mice at 24 post-LPS injection. These results provide the first evidence that age-associated reactivity of the brain cytokine system could play a pathophysiological role in the increased prevalence of depression observed in the elderly.

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.

Bacterial endotoxin-induced behavioral alterations in two variations of the Morris water maze.

Several studies report that lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) may affect behavior in a variety of learning tasks, including the Morris water maze (MWM), though the nature of these effects varies with testing parameters. The present study used C57BL/6J mice to evaluate the effect of a single intraperitoneal LPS injection 4 h prior to day 1 of testing, LPS before each day of testing, or saline prior to each test day, on performance in two variations of the MWM. In the first experiment, one that utilized a standard hidden platform, LPS clearly affected performance, as shown by increased latencies and greatly decreased swimming speeds. However, a modest effect on distance swam was only present during later test days. These data show a clear deficit in performance (driven by decreased swim speed), and some evidence for learning decrements on later test days. To explore to what degree the effects of LPS in the water maze were the result of alterations in performance factors such as motor behavior, a second experiment was conducted in which a highly visible jet-black platform was utilized. Despite eliminating the need for spatial learning, mice administered LPS still exhibited significantly increased latency scores and decreased swim speed. However, there was no difference between treatment groups in distance swam. These results reinforce the idea that, even when present, potential learning effects of LPS may sometimes be difficult to untangle from performance effects unrelated to learning, and underscore the need for utilizing behavioral tests that offer suitable control for LPS-induced performance effects.

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.

Effects of intraperitoneal lipopolysaccharide on Morris maze performance in year-old and 2-month-old female C57BL/6J mice.

Several studies have shown that systemic lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) may affect performance in various learning tasks, including the Morris water maze. In the current study, female C57BL/6J mice, either 2 months or 1 year of age, were given 5 days of testing followed by 3 days of rest, and then three additional days of testing. Mice either received a single LPS injection on day 1 and saline on days 2-5, LPS injections on days 1-5, or saline injections on days 1-5. Daily LPS administration significantly prolonged latency for the animals to find the platform, and decreased their swimming speed. Year-old mice treated with LPS each day also exhibited significantly higher levels of thigmotaxis in the maze. Despite effects on latency and swim speed, no effect of LPS treatment was observed for distance traveled to the platform or other measures that clearly indicate disruption of learning in the maze. On the other hand, age was a significant factor affecting both latency and distance, with older animals swimming greater distances to find the platform. Additionally, older animals were more adversely affected by daily LPS treatment. In this study, although LPS-induced performance impairments in the Morris water maze were noted, particularly in older animals, these effects were not clearly indicative of learning impairment per se.