[Show abstract][Hide abstract] ABSTRACT: The initial reinforcing properties of drugs of abuse, such as cocaine, are largely attributed to their ability to activate the mesolimbic dopamine system. Resulting increases in extracellular dopamine in the nucleus accumbens (NAc) are traditionally thought to result from cocaine's ability to block dopamine transporters (DATs). Here we demonstrate that cocaine also interacts with the immunosurveillance receptor complex, Toll-like receptor 4 (TLR4), on microglial cells to initiate central innate immune signaling. Disruption of cocaine signaling at TLR4 suppresses cocaine-induced extracellular dopamine in the NAc, as well as cocaine conditioned place preference and cocaine self-administration. These results provide a novel understanding of the neurobiological mechanisms underlying cocaine reward/reinforcement that includes a critical role for central immune signaling, and offer a new target for medication development for cocaine abuse treatment.Molecular Psychiatry advance online publication, 3 February 2015; doi:10.1038/mp.2014.177.
[Show abstract][Hide abstract] ABSTRACT: Neuroimmune diseases have diverse symptoms and etiologies but all involve pathological inflammation that affects normal central nervous system signaling. Critically, many neuroimmune diseases also involve insufficient signaling/bioavailability of interleukin-10 (IL-10). IL-10 is a potent anti-inflammatory cytokine released by immune cells and glia, which drives the regulation of a variety of anti-inflammatory processes. This review will focus on the signaling pathways and function of IL-10, the current evidence for insufficiencies in IL-10 signaling/bioavailability in neuroimmune diseases, as well as the implications for IL-10-based therapies to treating such problems. We will review in detail four pathologies as examples of the common etiologies of such disease states, namely neuropathic pain (nerve trauma), osteoarthritis (peripheral inflammation), Parkinson's disease (neurodegeneration), and multiple sclerosis (autoimmune). A number of methods to increase IL-10 have been developed (e.g. protein administration, viral vectors, naked plasmid DNA, plasmid DNA packaged in polymers to enhance their uptake into target cells, and adenosine 2A agonists), which will also be discussed. In general, IL-10-based therapies have been effective at treating both the symptoms and pathology associated with various neuroimmune diseases, with more sophisticated gene therapy-based methods producing sustained therapeutic effects lasting for several months following a single injection. These exciting results have resulted in IL-10-targeted therapeutics being positioned for upcoming clinical trials for treating neuroimmune diseases, including neuropathic pain. Although further research is necessary to determine the full range of effects associated with IL-10-based therapy, evidence suggests IL-10 may be an invaluable target for the treatment of neuroimmune disease.
[Show abstract][Hide abstract] ABSTRACT: Despite impressive progress in understanding the molecular, cellular and circuit-level correlates of major depression, the biological mechanisms that causally underlie this disorder are still unclear, possibly due to excessive focus on the dysfunctioning of neurons, as compared with other types of brain cells. Therefore, we examined the role of dynamic alterations in microglia activation status in the development of chronic unpredictable stress (CUS)-induced depressive-like condition in rodents. We found that following an initial period (2–3 days) of stress-induced microglial proliferation and activation, some microglia underwent apoptosis, leading to reductions in their numbers within the hippocampus following 5 weeks of CUS exposure. At that time, microglia displayed reduced expression of activation markers as well as dystrophic morphology. Blockade of the initial stress-induced microglial activation by minocycline, imipramine or by transgenic interleukin-1 receptor antagonist over-expression rescued the subsequent microglial apoptosis and decline, as well as the CUS-induced depression and suppressed neurogenesis. Treatment of CUS-exposed mice with LPS (endotoxin), M-CSF or GM-CSF, which all stimulated hippocampal microglial proliferation, reversed the depressive-like behavior and dramatically increased hippocampal neurogenesis, whereas treatment with imipramine or minocycline had minimal anti-depressive effects in these mice. These findings provide direct causal evidence that disturbances in microglial functioning has an etiological role in chronic stress-induced depression, suggesting that microglia stimulators could serve as fast-acting anti-depressants in some forms of depressive and stress-related conditions.
[Show abstract][Hide abstract] ABSTRACT: Microgliosis occurs after morphine and peripheral nerve injury alone, but the behavioral and molecular impact in tandem is unknown. We hypothesized that sciatic chronic constriction injury (CCI)-allodynia would be enhanced by subsequent repeated morphine in rats, involving TLR4, P2X7 receptor (P2X7R) and caspase-1, facilitating release of interleukin (IL)-1β. Beginning 10 days after CCI, morphine (5 mg/kg b.i.d.) or saline was administered for 5 days. Compared to vehicle, morphine significantly prolonged the duration of CCI-induced allodynia (n = 6/group; p < 0.05). Morphine also significantly elevated TLR4 mRNA, P2X7R, NFkappaB, NLRP3 and caspase-1 protein levels (p < 0.05) in the ipsilateral lumbar dorsal quadrant (iLDQ), 5 weeks after dosing conclusion. Supporting a causal role for NLRP3 inflammasome activation in morphine-prolonged CCI-allodynia, continuous intrathecal infusion of inhibitors of TLR4 ([+]-naloxone; 60 micrograms/h), P2X7R (Brilliant Blue G; 30 ng/h), or caspase-1 (ac-YVAD-cmk; 1 μg/h) prevented prolonged allodynia when administered concomitantly with morphine, and abolished established morphine-prolonged CCI-allodynia when administered 5 weeks after morphine dosing (n = 6/group; p < 0.05). A single intrathecal IL-1 receptor antagonist dose (100 micrograms) also attenuated morphine-prolonged CCI-allodynia (n = 6/group; p < 0.05). In keeping with known pro-nociceptive roles for IL-1β, phosphorylation of the NR1 NMDA subunit was elevated, while GRK2 levels and GLT-1 mRNA were decreased in iLDQ 5 weeks after dosing conclusion (p < 0.05). These data suggest that morphine and the products of nerve injury interact, resulting in prolonged neuropathic pain via sustained inflammasome signaling.
[Show abstract][Hide abstract] ABSTRACT: The limited success in understanding the pathophysiology of major depression may result from excessive focus on the dysfunctioning of neurons, as compared with other types of brain cells. Therefore, we examined the role of dynamic alterations in microglia activation status in the development of chronic unpredictable stress (CUS)-induced depressive-like condition in rodents. We report that following an initial period (2-3 days) of stress-induced microglial proliferation and activation, some microglia underwent apoptosis, leading to reductions in their numbers within the hippocampus, but not in other brain regions, following 5 weeks of CUS exposure. At that time, microglia displayed reduced expression of activation markers as well as dystrophic morphology. Blockade of the initial stress-induced microglial activation by minocycline or by transgenic interleukin-1 receptor antagonist overexpression rescued the subsequent microglial apoptosis and decline, as well as the CUS-induced depressive-like behavior and suppressed neurogenesis. Similarly, the antidepressant drug imipramine blocked the initial stress-induced microglial activation as well as the CUS-induced microglial decline and depressive-like behavior. Treatment of CUS-exposed mice with either endotoxin, macrophage colony-stimulating factor or granulocyte-macrophage colony-stimulating factor, all of which stimulated hippocampal microglial proliferation, partially or completely reversed the depressive-like behavior and dramatically increased hippocampal neurogenesis, whereas treatment with imipramine or minocycline had minimal or no anti-depressive effects, respectively, in these mice. These findings provide direct causal evidence that disturbances in microglial functioning has an etiological role in chronic stress-induced depression, suggesting that microglia stimulators could serve as fast-acting anti-depressants in some forms of depressive and stress-related conditions.
[Show abstract][Hide abstract] ABSTRACT: Studies on the biological basis of major depression usually focus on abnormalities in neuronal functions. Glia cells, particularly astrocytes, have also been implicated in the pathophysiology of depression, however the role of microglia in this disease is still elusive. To elucidate the involvement of microglia in depression we examined the role of dynamic alterations in microglia activa- tion status on the development of chronic unpredictable stress (CUS)-induced depressive-like condition in rodents. We report that following an initial period (2–3 days) of stress-induced microglial activation (reflected by proliferation, assumption of activated mor- phology and mRNA expression of activation markers), some microglia underwent apoptosis (reflected by activated caspase-3 and TUNEL staining), leading to reductions in their numbers within the hippocampus (but not in other brain regions) following 5- weeks of CUS exposure. At that time, microglia displayed reduced expression of activation markers as well as dystrophic morphology. The effects of CUS on microglia were blocked by chronic treatment with the tricyclic antidepressant drug imipramine. Furthermore, blockade of the initial stress-induced microglia activation by the microglial inhibitor minocycline or by transgenic interleukin-1 receptor antagonist over-expression rescued the subsequent microglia apoptosis and decline, as well as the CUS-induced depressive-like behavior and suppressed neurogenesis. These find- ings provide direct causal evidence that disturbances in microglial functioning have an etiological role in chronic stress-induced depression.
PsychoNeuroImmunology Research Society's 20th Annual Scientific Meeting, Stockholm, Sweden; 09/2013
[Show abstract][Hide abstract] ABSTRACT: Healthy aged individuals are more likely to suffer profound memory impairments following a challenging life event such as a severe bacterial infection, surgery, or an intense psychological stressor, than are younger adults. Importantly, these peripheral challenges are capable of producing a neuroinflammatory response (e.g., increased pro-inflammatory cytokines) and in the healthy aged brain this response is exaggerated and prolonged. Normal aging primes or sensitizes microglia and this appears to be the source of this amplified response. Here, we present data suggesting that a dysregulated neuroendocrine response in the aged (24 mos) F344xBN rat is skewed toward higher brain corticosterone (CORT) levels and greater hippocampal glucocorticoid receptor activation compared to young adult rats (3 mos). In addition, we confirm others’ findings that 11beta-hydroxysteroid dehydrogenase (11beta-HSD1), the enzyme that transforms cortisone (an inactive metabolite of CORT) to the active hormone CORT, is elevated in the aged hippocampus. In concert, these factors may play a critical role in priming microglia in the aged rat brain, resulting in exaggerated and longer lasting cytokine responses in the brain and greater memory deficits following a challenge.
[Show abstract][Hide abstract] ABSTRACT: Opioid action was thought to exert reinforcing effects solely via the initial agonism of opioid receptors. Here, we present evidence for an additional novel contributor to opioid reward: the innate immune pattern-recognition receptor, toll-like receptor 4 (TLR4), and its MyD88-dependent signaling. Blockade of TLR4/MD2 by administration of the nonopioid, unnatural isomer of naloxone, (+)-naloxone (rats), or two independent genetic knock-outs of MyD88-TLR4-dependent signaling (mice), suppressed opioid-induced conditioned place preference. (+)-Naloxone also reduced opioid (remifentanil) self-administration (rats), another commonly used behavioral measure of drug reward. Moreover, pharmacological blockade of morphine-TLR4/MD2 activity potently reduced morphine-induced elevations of extracellular dopamine in rat nucleus accumbens, a region critical for opioid reinforcement. Importantly, opioid-TLR4 actions are not a unidirectional influence on opioid pharmacodynamics, since TLR4(-/-) mice had reduced oxycodone-induced p38 and JNK phosphorylation, while displaying potentiated analgesia. Similar to our recent reports of morphine-TLR4/MD2 binding, here we provide a combination of in silico and biophysical data to support (+)-naloxone and remifentanil binding to TLR4/MD2. Collectively, these data indicate that the actions of opioids at classical opioid receptors, together with their newly identified TLR4/MD2 actions, affect the mesolimbic dopamine system that amplifies opioid-induced elevations in extracellular dopamine levels, therefore possibly explaining altered opioid reward behaviors. Thus, the discovery of TLR4/MD2 recognition of opioids as foreign xenobiotic substances adds to the existing hypothesized neuronal reinforcement mechanisms, identifies a new drug target in TLR4/MD2 for the treatment of addictions, and provides further evidence supporting a role for central proinflammatory immune signaling in drug reward.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 08/2012; 32(33):11187-200. DOI:10.1523/JNEUROSCI.0684-12.2012 · 6.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Uncontrollable stress can interfere with instrumental learning and induce anxiety in humans and rodents. While evidence supports a role for serotonin (5-HT) and serotonin 2C receptors (5-HT(2C)R) in the behavioral consequences of uncontrollable stress, the specific sites of action are unknown. These experiments sought to delineate the role of 5-HT and 5-HT(2C)R in the dorsal striatum (DS) and the lateral/basolateral amygdala (BLA) in the expression of stress-induced instrumental escape deficits and exaggerated fear, as these structures are critical to instrumental learning and fear behaviors. Using in vivo microdialysis, we first demonstrated that prior uncontrollable, but not controllable, stress sensitizes extracellular 5-HT in the dorsal striatum, a result that parallels prior work in the BLA. Additionally, rats were implanted with bi-lateral cannula in either the DS or the BLA and exposed to uncontrollable tail shock stress. One day later, rats were injected with 5-HT(2C)R antagonist (SB242084) and fear and instrumental learning behaviors were assessed in a shuttle box. Separately, groups of non-stressed rats received an intra-DS or an intra-BLA injection of the 5-HT(2C)R agonist (CP809101) and behavior was observed. Intra-DS injections of the 5-HT(2C)R antagonist prior to fear/escape tests completely blocked the stress-induced interference with instrumental escape learning; a partial block was observed when injections were in the BLA. Antagonist administration in either region did not influence stress-induced fear behavior. In the absence of prior stress, intra-DS administration of the 5-HT(2C)R agonist was sufficient to interfere with escape behavior without enhancing fear, while intra-BLA administration of the 5-HT(2C)R agonist increased fear behavior but had no effect on escape learning. Results reveal a novel role of the 5-HT(2C)R in the DS in the expression of instrumental escape deficits produced by uncontrollable stress and demonstrate that the involvement of 5-HT(2C)R activation in stress-induced behaviors is regionally specific.
[Show abstract][Hide abstract] ABSTRACT: Background / Purpose:
Exposure to uncontrollable, but not controllable, stress leads to anxiety-like behaviors, such as enhanced fear conditioning, and interference with instrumental tasks, such as shuttle box escape performance. A sensitization of 5-HT release and activation of 5-HT2CR in dorsal raphe nucleus (DRN) projection sites is a likely mechanism for negative behaviors induced by uncontrollable stress. One possible site of the action of the 5-HT2CR in the expression of escape deficits is the dorsal striatum (DS), a brain area which orchestrates instrumental behaviors. The current studies investigated whether a sensitized 5-HT release occurs in the striatum after uncontrollable stress and if the 5-HT2CR is necessary and sufficient for the expression behavior.We hypothesized that 1) if 5-HT activity in the DS is involved in the expression of stress-induced shuttle box escape deficits then uncontrollable, not controllable, stress will produce an increase in extracellular 5-HT in the DS during later behavioral testing and 2) 5-HT2CR activation in the DS will be necessary and sufficient for the expression of shuttle box escape deficits.
Extracellular 5-HT in the DS is increased in uncontrollably, but not controllably, stressed rats. Intra-DS injections of the 5-HT2CR antagonist reduced shuttle box escape deficits produced by both uncontrollable stress and the 5-HT2C receptor agonist. The data indicates a novel role of the 5-HT2CR activation DS in the behavioral consequences of uncontrollable stress.
[Show abstract][Hide abstract] ABSTRACT: Control over an aversive experience can greatly impact the organism's response to subsequent stressors. We compared the effects of escapable (ES) and yoked inescapable (IS) electric tail shocks on the hypothalamic-pituitary-adrenal (HPA) axis hormonal (corticosterone and adrenocorticotropic hormone (ACTH)), neural (c-fos mRNA) and behavioral (struggling) response to subsequent restraint. We found that although the HPA axis response during restraint of both previously stressed groups were higher than stress-naïve rats and not different from each other, lack of control over the tailshock experience led to an increase in restraint-induced struggling behavior of the IS rats compared to both stress-naïve and ES rats. Additionally, c-fos expression in the basolateral amygdala was increased selectively in the IS group, and relative c-fos mRNA expression in the basolateral amygdala positively correlated with struggling behavior. Restraint-induced c-fos expression in the medial prefrontal cortex, a brain area critical for mediating some of the differential neurochemical and behavioral effects of ES and IS, was surprisingly similar in both ES and IS groups, lower than that of stress-naïve rats, and did not correlate with struggling behavior. Our findings indicate that basolateral amygdala activity may be connected with the differential effects of ES and IS on subsequent behavioral responses to restraint, without contributing to the concurrent HPA axis hormone response.
[Show abstract][Hide abstract] ABSTRACT: Glutamate neurotransmission is highly regulated, largely by glutamate transporters. In the spinal cord, the glutamate transporter GLT-1 is primarily responsible for glutamate clearance. Downregulation of GLT-1 can occur in activated astrocytes, and is associated with increased extracellular glutamate and neuroexcitation. Among other conditions, astrocyte activation occurs following repeated opioids and in models of chronic pain. If GLT-1 downregulation occurs in these states, GLT-1 could be a pharmacological target for improving opioid efficacy and controlling chronic pain. The present studies explored whether daily intrathecal treatment of rats with ceftriaxone, a beta-lactam antibiotic that upregulates GLT-1 expression, could prevent development of hyperalgesia and allodynia following repeated morphine, reverse pain arising from central or peripheral neuropathy, and reduce glial activation in these models. Ceftriaxone pre-treatment attenuated the development of hyperalgesia and allodynia in response to repeated morphine, and prevented associated astrocyte activation. In a model of multiple sclerosis (experimental autoimmune encephalomyelitis; EAE), ceftriaxone reversed tactile allodynia and halted the progression of motor weakness and paralysis. Similarly, ceftriaxone reversed tactile allodynia induced by chronic constriction nerve injury (CCI). EAE and CCI each significantly reduced the expression of membrane-bound, dimerized GLT-1 protein in lumbar spinal cord, an effect normalized by ceftriaxone. Lastly, ceftriaxone normalized CCI- and EAE-induced astrocyte activation in lumbar spinal cord. Together, these data indicate that increasing spinal GLT-1 expression attenuates opioid-induced paradoxical pain, alleviates neuropathic pain, and suppresses associated glial activation. GLT-1 therefore may be a therapeutic target that could improve available treatment options for patients with chronic pain.
[Show abstract][Hide abstract] ABSTRACT: Experience with behavioral control over tailshock (escapable shock, ES) has been shown to block the behavioral and neurochemical changes produced by later uncontrollable tail shock (inescapable shock, IS). The present experiments tested, in rats, whether the protective effect of control over tailshock extends beyond reducing the behavioral and neurochemical impact of a subsequent tailshock experience to stressors that are quite different. Social defeat (SD) was chosen as the second stress experience because it has few if any cues in common with tailshock. SD produced shuttlebox escape learning deficits ("learned helplessness") and reduced juvenile social investigation 24 h later, as does IS. IS is notable for inducing a large increase in dorsal raphe nucleus (DRN) serotonergic (5-HT) activity as measured by extracellular levels of 5-HT within the DRN, and SD did so as well. ES occurring 7 days before SD blocked this SD-induced DRN activation, as well as the SD-induced interference with shuttlebox escape and reduction in social investigation. Prior exposure to yoked IS did not reduce the DRN 5-HT activation or later behavioral effects produced by SD, and thus the proactive stress-blunting effects of ES can be attributed to it's controllability. Thus, ES confers a very general protection to the impact of a subsequent stress experience.