CNS penetration of small molecules following local inflammation, widespread systemic inflammation or direct injury to the nervous system

Hypercholesterolemia and the Increased Risk of Vascular Dementia: a Cholesterol Perspective

PURPOSE OF REVIEW

Vascular dementia (VaD) is the second most prevalent type of dementia after Alzheimer's disease.Hypercholesterolemia may increase the risk of dementia, but the association between cholesterol and cognitive function is very complex. From the perspective of peripheral and brain cholesterol, we review the relationship between hypercholesterolemia and increased risk of VaD and how the use of lipid-lowering therapies affects cognition.

RECENT FINDINGS

Epidemiologic studies show since 1980, non-HDL-C levels of individuals has increased rapidly in Asian countries.The study has suggested that vascular risk factors increase the risk of VaD, such as disordered lipid metabolism. Dyslipidemia has been found to interact with chronic cerebral hypoperfusion to promote inflammation resulting in cognitive dysfunction in the brain.Hypercholesterolemia may be a risk factor for VaD. Inflammation could potentially serve as a link between hypercholesterolemia and VaD. Additionally, the potential impact of lipid-lowering therapy on cognitive function is also worth considering. Finding strategies to prevent and treat VaD is critical given the aging of the population to lessen the load on society. Currently, controlling underlying vascular risk factors is considered one of the most effective methods of preventing VaD. Understanding the relationship between abnormal cholesterol levels and VaD, as well as discovering potential serum biomarkers, is important for the early prevention and treatment of VaD.

Microbial neuraminidase induces TLR4-dependent long-term immune priming in the brain

Innate immune memory explains the plasticity of immune responses after repeated immune stimulation, leading to either enhanced or suppressed immune responses. This process has been extensively reported in peripheral immune cells and also, although modestly, in the brain. Here we explored two relevant aspects of brain immune priming: its persistence over time and its dependence on TLR receptors. For this purpose, we used an experimental paradigm consisting in applying two inflammatory stimuli three months apart. Wild type, toll-like receptor (TLR) 4 and TLR2 mutant strains were used. The priming stimulus was the intracerebroventricular injection of neuraminidase (an enzyme that is present in various pathogens able to provoke brain infections), which triggers an acute inflammatory process in the brain. The second stimulus was the intraperitoneal injection of lipopolysaccharide (a TLR4 ligand) or Pam3CSK4 (a TLR2 ligand). One day after the second inflammatory challenge the immune response in the brain was examined. In wild type mice, microglial and astroglial density, as well as the expression of 4 out of 5 pro-inflammatory genes studied (TNFα, IL1β, Gal-3, and NLRP3), were increased in mice that received the double stimulus compared to those exposed only to the second one, which were initially injected with saline instead of neuraminidase. Such enhanced response suggests immune training in the brain, which lasts at least 3 months. On the other hand, TLR2 mutants under the same experimental design displayed an enhanced immune response quite similar to that of wild type mice. However, in TLR4 mutant mice the response after the second immune challenge was largely dampened, indicating the pivotal role of this receptor in the establishment of immune priming. Our results demonstrate that neuraminidase-induced inflammation primes an enhanced immune response in the brain to a subsequent immune challenge, immune training that endures and that is largely dependent on TLR4 receptor.

TAK-242, Toll-Like Receptor 4 Antagonist, Attenuates Brain Edema in Subarachnoid Hemorrhage Mice

BACKGROUND

Brain edema is a common and critical pathology following subarachnoid hemorrhage (SAH). Toll-like receptor 4 (TLR4) activation may exacerbate brain edema. The purpose of this study was to clarify if TAK-242, a TLR4 antagonist, suppresses brain edema formation and neurological impairments after SAH in mice.

METHODS

A total of 46 mice underwent endovascular perforation to induce SAH or sham operation and were classified as Sham+TAK-242, SAH+ phosphate-buffered saline (PBS), and SAH + TAK-242 groups. The PBS or TAK-242 was administered intracerebroventricularly to mice at 30 min from the operation. Neurobehavioral tests, SAH severity, and brain water content were evaluated at 24 h from the operation.

RESULTS

The SAH + PBS group was significantly worse in neurological tests (P < 0.001) and brain water content of the cerebral hemisphere in the bleeding side (p = 0.005) compared with the Sham+PBS group, while there were no differences between the SAH + TAK-242 and Sham+PBS groups. SAH severity in the SAH + PBS group was similar to that in the SAH + TAK-242 group.

CONCLUSIONS

Intracerebroventricular administration of TAK-242 possibly prevents neurological impairments at least via suppression of brain edema.

Effect of High-Fat Diets on Oxidative Stress, Cellular Inflammatory Response and Cognitive Function

Cognitive dysfunction is linked to chronic low-grade inflammatory stress that contributes to cell-mediated immunity in creating an oxidative environment. Food is a vitally important energy source; it affects brain function and provides direct energy. Several studies have indicated that high-fat consumption causes overproduction of circulating free fatty acids and systemic inflammation. Immune cells, free fatty acids, and circulating cytokines reach the hypothalamus and initiate local inflammation through processes such as microglial proliferation. Therefore, the role of high-fat diet (HFD) in promoting oxidative stress and neurodegeneration is worthy of further discussion. Of particular interest in this article, we highlight the associations and molecular mechanisms of HFD in the modulation of inflammation and cognitive deficits. Taken together, a better understanding of the role of oxidative stress in cognitive impairment following HFD consumption would provide a useful approach for the prevention of cognitive dysfunction.

A role for pericytes in chronic pain?

PURPOSE OF REVIEW

The importance of the blood-brain barrier (BBB) and neuroinflammation in neurodegenerative conditions is becoming increasingly apparent, yet very little is known about these neurovascular functions in nonmalignant disease chronic pain. Neural tissue pericytes play critical roles in the formation and maintenance of the BBB. Herein, we review the important roles of neural pericytes and address their potential role in chronic pain.

RECENT FINDINGS

Pericytes are implicated in the function of neural microvasculature, including BBB permeability, neuroimmune factor secretion and leukocyte transmigration. In addition, the multipotent stem cell nature of pericytes affords pericytes the ability to migrate into neural parenchyma and differentiate into pain-associated cell types. These recent findings indicate that pericytes are key players in pathological BBB disruption and neuroinflammation, and as such pericytes may be key players in chronic pain states.

SUMMARY

Pericytes play key roles in pathological processes associated with chronic pain. We propose that pericytes may be a therapeutic target for painful diseases that have associated neural vascular dysfunction. Given the paucity of new pharmacotherapies for chronic pain conditions, we hope that this review inspires researchers to unearth the potential role(s) of pericytes in chronic pain sowing the seeds for future new chronic pain therapies.

Oral Ultramicronized Palmitoylethanolamide: Plasma and Tissue Levels and Spinal Anti-hyperalgesic Effect

Palmitoylethanolamide (PEA) is a pleiotropic lipid mediator with established anti-inflammatory and anti-hyperalgesic activity. Ultramicronized PEA (PEA-um) has superior oral efficacy compared to naïve (non-micronized) PEA. The aim of the present study was two-fold: (1) to evaluate whether oral PEA-um has greater absorbability compared to naïve PEA, and its ability to reach peripheral and central tissues under healthy and local inflammatory conditions (carrageenan paw edema); (2) to better characterize the molecular pathways involved in PEA-um action, particularly at the spinal level. Rats were dosed with 30 mg/kg of [13C]4-PEA-um or naïve [13C]4-PEA by oral gavage, and [13C]4-PEA levels quantified, as a function of time, by liquid chromatography/atmospheric pressure chemical ionization/mass spectrometry. Overall plasma levels were higher in both healthy and carrageenan-injected rats administered [13C]4-PEA-um as compared to those receiving naïve [13C]4-PEA, indicating the greater absorbability of PEA-um. Furthermore, carrageenan injection markedly favored an increase in levels of [13C]4-PEA in plasma, paw and spinal cord. Oral treatment of carrageenan-injected rats with PEA-um (10 mg/kg) confirmed beneficial peripheral effects on paw inflammation, thermal hyperalgesia and tissue damage. Notably, PEA-um down-regulated distinct spinal inflammatory and oxidative pathways. These last findings instruct on spinal mechanisms involved in the anti-hyperalgesic effect of PEA-um in inflammatory pain.

Uraemia: an unrecognized driver of central neurohumoral dysfunction in chronic kidney disease?

Chronic kidney disease (CKD) carries a large cardiovascular burden in part due to hypertension and neurohumoral dysfunction - manifesting as sympathetic overactivity, baroreflex dysfunction and chronically elevated circulating vasopressin. Alterations within the central nervous system (CNS) are necessary for the expression of neurohumoral dysfunction in CKD; however, the underlying mechanisms are poorly defined. Uraemic toxins are a diverse group of compounds that accumulate as a direct result of renal disease and drive dysfunction in multiple organs, including the brain. Intensive haemodialysis improves both sympathetic overactivity and cardiac baroreflex sensitivity in renal failure patients, indicating that uraemic toxins participate in the maintenance of autonomic dysfunction in CKD. In rodents exposed to uraemia, immediate early gene expression analysis suggests upregulated activity of not only pre-sympathetic but also vasopressin-secretory nuclei. We outline several potential mechanisms by which uraemia might drive neurohumoral dysfunction in CKD. These include superoxide-dependent effects on neural activity, depletion of nitric oxide and induction of low-grade systemic inflammation. Recent evidence has highlighted superoxide production as an intermediate for the depolarizing effect of some uraemic toxins on neuronal cells. We provide preliminary data indicating augmented superoxide production within the hypothalamic paraventricular nucleus in the Lewis polycystic kidney rat, which might be important for mediating the neurohumoral dysfunction exhibited in this CKD model. We speculate that the uraemic state might serve to sensitize the central actions of other sympathoexcitatory factors, including renal afferent nerve inputs to the CNS and angiotensin II, by way of recruiting convergent superoxide-dependent and pro-inflammatory pathways.

Peripheral nerve injury induces aquaporin-4 expression and astrocytic enlargement in spinal cord

Aquaporin-4 (AQP4), a water channel protein, is expressed mainly in the perivascular end-feet of astrocytes in the brain and spinal cord. Dysregulation of AQP4 is critically associated with abnormal water transport in the astrocytes. We aimed to examine whether peripheral nerve injury (PNI) could induce the changes of AQP4 expression and astrocytic morphology in the spinal cord. Two different PNI models [partial sciatic nerve transection (PST) and chronic constriction injury (CCI)] were established on the left sciatic nerve in Sprague-Dawley rats, which decreased the pain withdrawal threshold in the ipsilateral hind paws. Both PNI models were associated with a persistent up-regulation of AQP4 in the ipsilateral dorsal horn at the lower lumbar region over 3 weeks, despite an absence of direct injury to the spinal cord. Three-dimensional reconstruction of astrocytes was made and morphometric analysis was done. Up-regulation of AQP4 was accompanied by a significant increase in the length and volume of astrocytic processes and the number of branch points. The most prominent changes were present in the distal processes of the astrocytes and the changes were maintained throughout the whole experimental period. Extravasation of systemically administered tracers Evans Blue and sodium fluorescein was not seen in both models. Taken together, PNI was associated with a long-lasting AQP4 up-regulation and enlargement of astrocytic processes in the spinal cord in rats, both of which were not related to the disruption of blood-spinal cord barrier. The findings could provide novel insights on the understanding of pathophysiology of spinal cords after PNI.

Modality and sex differences in pain sensitivity during human endotoxemia

Systemic inflammation can induce pain hypersensitivity in animal and human experimental models, and has been proposed to be central in clinical pain conditions. Women are overrepresented in many chronic pain conditions, but experimental studies on sex differences in pain regulation during systemic inflammation are still scarce. In two randomized and double blind placebo controlled experiments, we used low doses of lipopolysaccharide (LPS) as an experimental model of systemic inflammation. The first study employed 0.8ng/kg LPS in a within-subject design of 8 individuals (1 woman), and the second study 0.6ng/kg LPS in a between-subject design of 52 participants (29 women). We investigated the effect on (a) pressure, heat, and cold pain thresholds, (b) suprathreshold noxious heat and cold sensitivity, and (c) conditioned pain modulation (CPM), and differences between men and women. LPS induced significantly lower pressure pain thresholds as compared to placebo (mean change with the 0.8ng/kg dose being -64±30kPa P=.04; with the 0.6ng/kg dose -58±55kPa, P<.01, compared to before injection), whereas heat and cold pain thresholds remained unaffected (P's>.70). Suprathreshold noxious pain was not affected by LPS in men (P's⩾.15). However, LPS made women rated suprathreshold noxious heat stimuli as more painful (P=.01), and showed a tendency to rate noxious cold pain as more painful (P=.06) as compared to placebo. Furthermore, LPS impaired conditioned pain modulation, a measure of endogenous pain inhibition, but this effect was also restricted to women (P<.01, for men P=.27). Pain sensitivity correlated positively with plasma IL-6 and IL-8 levels. The results show that inflammation more strongly affects deep pain, rather than cutaneous pain, and suggest that women's pain perception and modulation is more sensitive to immune activation than men's.

The inhibitory effect of kakkonto, Japanese traditional (kampo) medicine, on brain penetration of oseltamivir carboxylate in mice with reduced blood-brain barrier function

Oseltamivir phosphate (OP) is used to treat influenza virus infections. However, its use may result in central nervous system (CNS) adverse effects. In Japan, OP is used with Kampo formulations to improve clinical effectiveness. We evaluated the potential for using Kampo formulations to reduce CNS adverse effects by quantifying the CNS distribution of oseltamivir and its active metabolite oseltamivir carboxylate (OC) when administered with maoto and kakkonto. We administered lipopolysaccharide (LPS) by intraperitoneal injection to C57BL/6 mice to reduce blood-brain barrier function. Saline, maoto, and kakkonto were administered orally at the same time as LPS. OP was orally administered 4 hours after the last LPS injection and the migration of oseltamivir and OC was examined. Additionally, we examined the brain distribution of OC following intravenous administration. Changes in OC concentrations in the brain suggest that, in comparison to LPS-treated control mice, both Kampo formulations increased plasma levels of OC, thereby enhancing its therapeutic effect. Additionally, our findings suggest kakkonto may not only improve the therapeutic effect of oseltamivir but also reduce the risk of CNS-based adverse effects. Considering these findings, it should be noted that administration of kakkonto during periods of inflammation has led to increased OAT3 expression.

Obesity and neuroinflammation: a pathway to cognitive impairment

Obesity is a growing problem worldwide and is associated with a range of comorbidities, including cognitive dysfunction. In this review we will address the evidence that obesity and high fat feeding can lead to cognitive dysfunction. We will also examine the idea that obesity-associated systemic inflammation leads to inflammation within the brain, particularly the hypothalamus, and that this is partially responsible for these negative cognitive outcomes. Thus, obesity, and high fat feeding, lead to systemic inflammation and excess circulating free fatty acids. Circulating cytokines, free fatty acids and immune cells reach the brain at the level of the hypothalamus and initiate local inflammation, including microglial proliferation. This local inflammation likely causes synaptic remodeling and neurodegeneration within the hypothalamus, altering internal hypothalamic circuitry and hypothalamic outputs to other brain regions. The result is disruption to cognitive function mediated by regions such as hippocampus, amygdala, and reward-processing centers. Central inflammation is also likely to affect these regions directly. Thus, central inflammation in obesity leads not just to disruption of hypothalamic satiety signals and perpetuation of overeating, but also to negative outcomes on cognition.

The role of the blood-brain barrier in the development and treatment of migraine and other pain disorders

The function of the blood-brain barrier (BBB) related to chronic pain has been explored for its classical role in regulating the transcellular and paracellular transport, thus controlling the flow of drugs that act at the central nervous system, such as opioid analgesics (e.g., morphine) and non-steroidal anti-inflammatory drugs. Nonetheless, recent studies have raised the possibility that changes in the BBB permeability might be associated with chronic pain. For instance, changes in the relative amounts of occludin isoforms, resulting in significant increases in the BBB permeability, have been demonstrated after inflammatory hyperalgesia. Furthermore, inflammatory pain produces structural changes in the P-glycoprotein, the major efflux transporter at the BBB. One possible explanation for these findings is the action of substances typically released at the site of peripheral injuries that could lead to changes in the brain endothelial permeability, including substance P, calcitonin gene-related peptide, and interleukin-1 beta. Interestingly, inflammatory pain also results in microglial activation, which potentiates the BBB damage. In fact, astrocytes and microglia play a critical role in maintaining the BBB integrity and the activation of those cells is considered a key mechanism underlying chronic pain. Despite the recent advances in the understanding of BBB function in pain development as well as its interference in the efficacy of analgesic drugs, there remain unknowns regarding the molecular mechanisms involved in this process. In this review, we explore the connection between the BBB as well as the blood-spinal cord barrier and blood-nerve barrier, and pain, focusing on cellular and molecular mechanisms of BBB permeabilization induced by inflammatory or neuropathic pain and migraine.

Neuronal CC chemokines: the distinct roles of CCL21 and CCL2 in neuropathic pain

The development of neuropathic pain in response to peripheral nerve lesion for a large part depends on microglia located at the dorsal horn of the spinal cord. Thus the injured nerve initiates a response of microglia, which represents the start of a cascade of events that leads to neuropathic pain development. For long it remained obscure how a nerve injury in the periphery would initiate a microglia response in the dorsal horn of the spinal cord. Recently, two chemokines have been suggested as potential factors that mediate the communication between injured neurons and microglia namely CCL2 and CCL21. This assumption is based on the following findings. Both chemokines are not found in healthy neurons, but are expressed in response to neuronal injury. In injured dorsal root ganglion cells CCL2 and CCL21 are expressed in vesicles in the soma and transported through the axons of the dorsal root into the dorsal horn of the spinal cord. Finally, microglia in vitro are known to respond to CCL2 and CCL21. Whereas the microglial chemokine receptor involved in CCL21-induced neuropathic pain is not yet defined the situation concerning the receptors for CCL2 in microglia in vivo is even less clear. Recent results obtained in transgenic animals clearly show that microglia in vivo do not express CCR2 but that peripheral myeloid cells and neurons do. This suggests that CCL2 expressed by injured dorsal root neurons does not act as neuron-microglia signal in contrast to CCL21. Instead, CCL2 in the injured dorsal root ganglia (DRG) may act as autocrine or paracrine signal and may stimulate first or second order neurons in the pain cascade and/or attract CCR2-expressing peripheral monocytes/macrophages to the spinal cord.

Systemic injection of low-dose lipopolysaccharide fails to break down the blood-brain barrier or activate the TLR4-MyD88 pathway in neonatal rat brain

We aimed to investigate whether peripheral low-dose lipopolysaccharide (LPS) induces the breakdown of the blood–brain barrier (BBB) and/or the activation of toll-like receptor 4 (TLR4) in the neonatal rat brain. Neonatal rats received intraperitoneal injections of low-dose LPS (0.3 mg/kg∙bw), and the BBB compromise was detected by Evans Blue extravasation and electron microscopy. Meanwhile, TLR4, adaptin myeloid differentiation factor 88 (MyD88), nuclear transcription factor kappa-B (NF-κB) p50 and tumor necrosis factor alpha (TNFα) in the neonatal rat brain were determined by quantitative real-time polymerase chain reaction (PCR) and Western Blot. Immunohistochemistry was used to determine the distribution and activation of microglia in the brain after LPS administration. It was demonstrated that Evans Blue extravasation was not observed in the brain parenchyma, and that tight junctions of cerebral endothelial cells remained intact after systemic injections of LPS in neonatal rats. Although intracerebroventricular injections of LPS activated microglia and up-regulated the expression of TLR4, MyD88, NF-κB p50 and TNFα in the neonatal rat brain, systemic LPS did not induce these responses. These findings indicate that while the neonatal rat brain responds to the direct intra-cerebral administration of LPS through robust TLR4 activation, systemic low-dose LPS does not induce the innate immune reaction or compromise the BBB in neonatal rats.

Neurovascular unit in chronic pain

Chronic pain is a debilitating condition with major socioeconomic impact, whose neurobiological basis is still not clear. An involvement of the neurovascular unit (NVU) has been recently proposed. In particular, the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB), two NVU key players, may be affected during the development of chronic pain; in particular, transient permeabilization of the barrier is suggested by several inflammatory- and nerve-injury-based pain models, and we argue that the clarification of molecular BBB/BSCB permeabilization events will shed new light in understanding chronic pain mechanisms. Possible biases in experiments supporting this theory and its translational potentials are discussed. Moving beyond an exclusive focus on the role of the endothelium, we propose that our understanding of the mechanisms subserving chronic pain will benefit from the extension of research efforts to the NVU as a whole. In this view, the available evidence on the interaction between analgesic drugs and the NVU is here reviewed. Chronic pain comorbidities, such as neuroinflammatory and neurodegenerative diseases, are also discussed in view of NVU changes, together with innovative pharmacological solutions targeting NVU components in chronic pain treatment.

Assessment of body cell mass at bedside in critically ill patients

Critical illness affects body composition profoundly, especially body cell mass (BCM). BCM loss reflects lean tissue wasting and could be a nutritional marker in critically ill patients. However, BCM assessment with usual isotopic or tracer methods is impractical in intensive care units (ICUs). We aimed to modelize the BCM of critically ill patients using variables available at bedside. Fat-free mass (FFM), bone mineral (Mo), and extracellular water (ECW) of 49 critically ill patients were measured prospectively by dual-energy X-ray absorptiometry and multifrequency bioimpedance. BCM was estimated according to the four-compartment cellular level: BCM = FFM - (ECW/0.98) - (0.73 × Mo). Variables that might influence the BCM were assessed, and multivariable analysis using fractional polynomials was conducted to determine the relations between BCM and these data. Bootstrap resampling was then used to estimate the most stable model predicting BCM. BCM was 22.7 ± 5.4 kg. The most frequent model included height (cm), leg circumference (cm), weight shift (Δ) between ICU admission and body composition assessment (kg), and trunk length (cm) as a linear function: BCM (kg) = 0.266 × height + 0.287 × leg circumference + 0.305 × Δweight - 0.406 × trunk length - 13.52. The fraction of variance explained by this model (adjusted r(2)) was 46%. Including bioelectrical impedance analysis variables in the model did not improve BCM prediction. In summary, our results suggest that BCM can be estimated at bedside, with an error lower than ±20% in 90% subjects, on the basis of static (height, trunk length), less stable (leg circumference), and dynamic biometric variables (Δweight) for critically ill patients.

Effects of peripheral inflammation on the blood-spinal cord barrier

Background

Changes in the blood-central nervous system barriers occur under pathological conditions including inflammation and contribute to central manifestations of various diseases. After short-lasting peripheral and neurogenic inflammation, the evidence is mixed whether there are consistent blood-spinal cord changes. In the current study, we examine changes in the blood-spinal cord barrier after intraplantar capsaicin and λ-carrageenan using several methods: changes in occludin protein, immunoglobulin G accumulation, and fluorescent dye penetration. We also examine potential sex differences in male and female adult rats.

Results

After peripheral carrageenan inflammation, but not capsaicin inflammation, immunohistochemistry shows occludin protein in lumbar spinal cord to be significantly altered at 72 hours post-injection. In addition, there is also significant immunoglobulin G detected in lumbar and thoracic spinal cord at this timepoint in both male and female rats. However, acute administration of sodium fluorescein or Evans Blue dyes is not detected in the parenchyma at this timepoint.

Conclusions

Our results show that carrageenan inflammation induces changes in tight junction protein and immunoglobulin G accumulation, but these may not be indicative of a blood-spinal cord barrier breakdown. These changes appear transiently after peak nociception and may be indicative of reversible pathology that resolves together with inflammation.

Ghrelin-mediated sympathoinhibition and suppression of inflammation in sepsis

Sepsis, a systemic inflammatory response to infection, continues to carry a high mortality despite advances in critical care medicine. Elevated sympathetic nerve activity in sepsis has been shown to contribute to early hepatocellular dysfunction and subsequently multiple organ failure, resulting in a poor prognosis, especially in the elderly. Thus, suppression of sympathetic nerve activity represents a novel therapeutic option for sepsis. Ghrelin is a 28-amino acid peptide shown to inhibit sympathetic nerve activity and inflammation in animal models of tissue injury. Age-related ghrelin hyporesponsiveness has also been shown to exacerbate sepsis. However, the mechanistic relationship between ghrelin-mediated sympathoinhibition and suppression of inflammation remains poorly understood. This review assesses the therapeutic potential of ghrelin in sepsis in the context of the neuroanatomical and molecular basis of ghrelin-mediated suppression of inflammation through inhibition of central sympathetic outflow.

Tempol modulates changes in xenobiotic permeability and occludin oligomeric assemblies at the blood-brain barrier during inflammatory pain

Our laboratory has shown that λ-carrageenan-induced peripheral inflammatory pain (CIP) can alter tight junction (TJ) protein expression and/or assembly leading to changes in blood-brain barrier xenobiotic permeability. However, the role of reactive oxygen species (ROS) and subsequent oxidative stress during CIP is unknown. ROS (i.e., superoxide) are known to cause cellular damage in response to pain/inflammation. Therefore, we examined oxidative stress-associated effects at the blood-brain barrier (BBB) in CIP rats. During CIP, increased staining of nitrosylated proteins was detected in hind paw tissue and enhanced presence of protein adducts containing 3-nitrotyrosine occurred at two molecular weights (i.e., 85 and 44 kDa) in brain microvessels. Tempol, a pharmacological ROS scavenger, attenuated formation of 3-nitrotyrosine-containing proteins in both the hind paw and in brain microvessels when administered 10 min before footpad injection of λ-carrageenan. Similarly, CIP increased 4-hydroxynoneal staining in brain microvessels and this effect was reduced by tempol. Brain permeability to [(14)C]sucrose and [(3)H]codeine was increased, and oligomeric assemblies of occludin, a critical TJ protein, were altered after 3 h CIP. Tempol attenuated both [(14)C]sucrose and [(3)H]codeine brain uptake as well as protected occludin oligomers from disruption in CIP animals, suggesting that ROS production/oxidative stress is involved in modulating BBB functional integrity during pain/inflammation. Interestingly, tempol administration reduced codeine analgesia in CIP animals, indicating that oxidative stress during pain/inflammation may affect opioid delivery to the brain and subsequent efficacy. Taken together, our data show for the first time that ROS pharmacological scavenging is a viable approach for maintaining BBB integrity and controlling central nervous system drug delivery during acute inflammatory pain.

Targeting blood-brain barrier changes during inflammatory pain: an opportunity for optimizing CNS drug delivery

The blood-brain barrier (BBB) is the most significant obstacle to effective CNS drug delivery. It possesses structural and biochemical features (i.e., tight-junction protein complexes and, influx and efflux transporters) that restrict xenobiotic permeation. Pathophysiological stressors (i.e., peripheral inflammatory pain) can alter BBB tight junctions and transporters, which leads to drug-permeation changes. This is especially critical for opioids, which require precise CNS concentrations to be safe and effective analgesics. Recent studies have identified molecular targets (i.e., endogenous transporters and intracellular signaling systems) that can be exploited for optimization of CNS drug delivery. This article summarizes current knowledge in this area and emphasizes those targets that present the greatest opportunity for controlling drug permeation and/or drug transport across the BBB in an effort to achieve optimal CNS opioid delivery.

Following nerve injury neuregulin-1 drives microglial proliferation and neuropathic pain via the MEK/ERK pathway

Following peripheral nerve injury microglia accumulate within the spinal cord and adopt a proinflammatory phenotype a process which contributes to the development of neuropathic pain. We have recently shown that neuregulin-1, a growth factor released following nerve injury, activates erbB 2, 3, and 4 receptors on microglia and stimulates proliferation, survival and chemotaxis of these cells. Here we studied the intracellular signaling pathways downstream of neuregulin-1-erbB activation in microglial cells. We found that neuregulin-1 in vitro induced phosphorylation of ERK1/2 and Akt without activating p38MAPK. Using specific kinase inhibitors we found that the mitogenic effect of neuregulin-1 on microglia was dependant on MEK/ERK1/2 pathway, the chemotactic effect was dependant on PI3K/Akt signaling and survival was dependant on both pathways. Intrathecal treatment with neuregulin-1 was associated with microgliosis and development of mechanical and cold pain related hypersensitivity which was dependant on ERK1/2 phosphorylation in microglia. Spinal nerve ligation results in a robust microgliosis and sustained ERK1/2 phosphorylation within these cells. This pathway is downstream of neuregulin-1/erbB signaling since its blockade resulted in a significant reduction in microglial ERK1/2 phosphorylation. Inhibition of the MEK/ERK1/2 pathway resulted in decreased spinal microgliosis and in reduced mechanical and cold hypersensitivity after peripheral nerve damage. We conclude that neuregulin-1 released after nerve injury activates microglial erbB receptors which consequently stimulates the MEK/ERK1/2 pathway that drives microglial proliferation and contributes to the development of neuropathic pain.

Interactions between the immune and nervous systems in pain

Immune cells and glia interact with neurons to alter pain sensitivity and to mediate the transition from acute to chronic pain. In response to injury, resident immune cells are activated and blood-borne immune cells are recruited to the site of injury. Immune cells not only contribute to immune protection but also initiate the sensitization of peripheral nociceptors. Through the synthesis and release of inflammatory mediators and interactions with neurotransmitters and their receptors, the immune cells, glia and neurons form an integrated network that coordinates immune responses and modulates the excitability of pain pathways. The immune system also reduces sensitization by producing immune-derived analgesic and anti-inflammatory or proresolution agents. A greater understanding of the role of the immune system in pain processing and modulation reveals potential targets for analgesic drug development and new therapeutic opportunities for managing chronic pain.

Neuregulin-ErbB signaling promotes microglial proliferation and chemotaxis contributing to microgliosis and pain after peripheral nerve injury

A key component in the response of the nervous system to injury is the proliferation and switch to a "proinflammatory" phenotype by microglia (microgliosis). In situations where the blood-brain barrier is intact, microglial numbers increase via the proliferation and chemotaxis of resident microglia; however, there is limited knowledge regarding the factors mediating this response. After peripheral nerve injury, a dorsal horn microgliosis develops, which directly contributes to the development of neuropathic pain. Neuregulin-1 (NRG-1) is a growth and differentiation factor with a well characterized role in neural and cardiac development. Microglia express the NRG1 receptors erbB2, 3, and 4, and NRG1 signaling via the erbB2 receptor stimulated microglial proliferation, chemotaxis, and survival, as well as interleukin-1beta release in vitro. Intrathecal treatment with NRG1 resulted in microglial proliferation within the dorsal horn, and these cells developed an activated morphology. This microglial response was associated with the development of both mechanical and cold pain-related hypersensitivity. Primary afferents express NRG1, and after spinal nerve ligation (SNL) we observed both an increase in NRG1 within the dorsal horn as well as activation of erbB2 specifically within microglia. Blockade of the erbB2 receptor or sequestration of endogenous NRG after SNL reduced the proliferation, the number of microglia with an activated morphology, and the expression of phospho-P38 by microglia. Furthermore, consequent to such changes, the mechanical pain-related hypersensitivity and cold allodynia were reduced. NRG1-erbB signaling therefore represents a novel pathway regulating the injury response of microglia.

Contributions of peripheral inflammation to seizure susceptibility: cytokines and brain excitability

Inflammation is an important factor in the pathophysiology of seizure generation and epileptogenesis. While the role of CNS inflammation is well acknowledged as an important factor in seizure pathophysiology, less is known about the role of peripheral inflammation. Systemic inflammation induces a mirror inflammatory response in the brain that might have transient or long-term effects on seizure susceptibility. The focus of our laboratory research is the study of the interaction of systemic inflammatory events with neuronal excitability and seizure susceptibility. In this paper we provide a review of our findings and discuss possible mechanisms.