Global gene expression analysis in the mouse brainstem after hyperalgesia induced by facial carrageenan injection--evidence for a form of neurovascular coupling?

Role of prefrontal cortical calcium-independent phospholipase A2 in antinociceptive effect of the norepinephrine reuptake inhibitor antidepresssant maprotiline

The prefrontal cortex is essential for executive functions such as decision-making and planning. There is also accumulating evidence that it is important for the modulation of pain. In this study, we investigated a possible role of prefrontal cortical calcium-independent phospholipase A₂ (iPLA₂) in antinociception induced by the norepinephrine reuptake inhibitor (NRI) and tetracyclic (tricyclic) antidepressant, maprotiline. Intraperitoneal injections of maprotiline increased iPLA₂ mRNA and protein expression in the prefrontal cortex. This treatment also reduced grooming responses to von-Frey hair stimulation of the face after facial carrageenan injection, indicating decreased sensitivity to pain. The antinociceptive effect of maprotiline was abrogated by iPLA₂ antisense oligonucleotide injection to the prefrontal cortex, indicating a role of this enzyme in antinociception. In contrast, injection of iPLA₂ antisense oligonucleotide to the somatosensory cortex did not reduce the antinociceptive effect of maprotiline. Lipidomic analysis of the prefrontal cortex showed decrease in phosphatidylcholine species, but increase in lysophosphatidylcholine species, indicating increased PLA2 activity, and release of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) after maprotiline treatment. Differences in sphingomyelin/ceramide were also detected. These changes were not observed in maprotiline-treated mice that received iPLA₂ antisense oligonucleotide to the prefrontal cortex. Metabolites of DHA and EPA may help to strengthen a known supraspinal antinociceptive pathway from the prefrontal cortex to the periaqueductal gray. Together, results indicate a role of prefrontal cortical iPLA₂ and its enzymatic products in the antinociceptive effect of maprotiline.

The Effect of Acute and Chronic Social Stress on the Hippocampal Transcriptome in Mice

Psychogenic stress contributes to the formation of brain pathology. Using gene expression microarrays, we analyzed the hippocampal transcriptome of mice subjected to acute and chronic social stress of different duration. The longest period of social stress altered the expression of the highest number of genes and most of the stress-induced changes in transcription were reversible after 5 days of rest. Chronic stress affected genes involved in the functioning of the vascular system (Alas2, Hbb-b1, Hba-a2, Hba-a1), injury response (Vwf, Mgp, Cfh, Fbln5, Col3a1, Ctgf) and inflammation (S100a8, S100a9, Ctla2a, Ctla2b, Lcn2, Lrg1, Rsad2, Isg20). The results suggest that stress may affect brain functions through the stress-induced dysfunction of the vascular system. An important issue raised in our work is also the risk of the contamination of brain tissue samples with choroid plexus. Such contamination would result in a consistent up- or down-regulation of genes, such as Ttr, Igf2, Igfbp2, Prlr, Enpp2, Sostdc1, 1500015O10RIK (Ecrg4), Kl, Clic6, Kcne2, F5, Slc4a5, and Aqp1. Our study suggests that some of the previously reported, supposedly specific changes in hippocampal gene expression, may be a result of the inclusion of choroid plexus in the hippocampal samples.

A novel operant-based behavioral assay of mechanical allodynia in the orofacial region of rats

BACKGROUND

Detecting behaviors related to orofacial pain in rodent models often relies on subjective investigator grades or methods that place the animal in a stressful environment. In this study, an operant-based behavioral assay is presented for the assessment of orofacial tactile sensitivity in the rat.

NEW METHODS

In the testing chamber, rats are provided access to a sweetened condensed milk bottle; however, a 360° array of stainless steel wire loops impedes access. To receive the reward, an animal must engage the wires across the orofacial region. Contact with the bottle triggers a motor, requiring the animal to accept increasing pressure on the face during the test. To evaluate this approach, tolerated bottle distance was measured for 10 hairless Sprague Dawley rats at baseline and 30 min after application of capsaicin cream (0.1%) to the face. The experiment was repeated to evaluate the ability of morphine to reverse this effect.

RESULTS

The application of capsaicin cream reduced tolerated bottle distance measures relative to baseline (p<0.05). As long as morphine did not cause reduced participation due to sedation, subcutaneous morphine dosing reduced the effects of capsaicin (p<0.001). Comparison with existing method: For behavioral tests, experimenters often make subjective decisions of an animal's response. Operant methods can reduce these effects by measuring an animal's selection in a reward-conflict decision. Herein, a method to measure orofacial sensitivity is presented using an operant system.

CONCLUSIONS

This operant device allows for consistent measurement of heightened tactile sensitivity in the orofacial regions of the rat.

Activation of cyclin-dependent kinase 5 mediates orofacial mechanical hyperalgesia

Background

Cyclin-dependent kinase 5 (Cdk5) is a unique member of the serine/threonine kinase family. This kinase plays an important role in neuronal development, and deregulation of its activity leads to neurodegenerative disorders. Cdk5 also serves an important function in the regulation of nociceptive signaling. Our previous studies revealed that the expression of Cdk5 and its activator, p35, is upregulated in nociceptive neurons during peripheral inflammation. The aim of the present study was to characterize the involvement of Cdk5 in orofacial pain. Since mechanical hyperalgesia is the distinctive sign of many orofacial pain conditions, we adapted an existing orofacial stimulation test to assess the behavioral responses to mechanical stimulation in the trigeminal region of the transgenic mice with either reduced or increased Cdk5 activity.

Results

Mice overexpressing or lacking p35, an activator of Cdk5, showed altered phenotype in response to noxious mechanical stimulation in the trigeminal area. Mice with increased Cdk5 activity displayed aversive behavior to mechanical stimulation as indicated by a significant decrease in reward licking events and licking time. The number of reward licking/facial contact events was significantly decreased in these mice as the mechanical intensity increased. By contrast, mice deficient in Cdk5 activity displayed mechanical hypoalgesia.

Conclusions

Collectively, our findings demonstrate for the first time the important role of Cdk5 in orofacial mechanical nociception. Modulation of Cdk5 activity in primary sensory neurons makes it an attractive potential target for the development of novel analgesics that could be used to treat multiple orofacial pain conditions.

P-selectin is required for neutrophils and macrophage infiltration into injured site and contributes to generation of behavioral hypersensitivity following peripheral nerve injury in mice

Growing evidence suggests that leukocyte extravasation is initiated by the interaction of selectins with their ligands; as well as an essential role for P-selectin in the initial recruitment of inflammatory cells to sites of inflammation. In this study, P-selectin-deficient (P-sel-/-) mice were used to test the hypothesis that lack of P-selectin would attenuate the recruitment of inflammatory cells to the site of inflammation, thereby modulating pain in a murine chronic neuropathic pain model. Nociceptive sensitization and the microenvironment of the peripheral injury site were studied in wild-type (P-sel+/+) and P-selectin-deficient (P-sel-/-) mice after partial sciatic nerve ligation (PSNL). Variables measured included myeloperoxidase (MPO) activity, several inflammatory cell infiltration profiles, cytokines, and endogenous opioid peptide expression in damaged nerves. Results indicate that behavioral hypersensitivity, MPO activity, and infiltration of neutrophils and macrophages were attenuated in P-sel-/- mice after PSNL. Proinflammatory cytokines, tumor necrosis factor α, and interleukin (IL)-6, were reduced in damaged nerves following PSNL; however, several antiinflammatory cytokines - IL-1Ra, IL-4, and IL-10 - were significantly increased in P-sel-/- mice. In addition, endogenous opioid peptides mRNA was significantly lower in P-sel-/- mice compared with P-sel +/+ mice. The current results demonstrated that the absence of P-selectin in mice leads to an altered microenvironment that attenuated behavioral hypersensitivity. The specific role of P-selectin could have been a result of decreased neutrophils, as well as the accumulation of macrophages at the site of injury, which may subsequently modulate the inflammatory cytokine expression and impact behavioral hypersensitivity within the injured nerve.

Behavioral testing in rodent models of orofacial neuropathic and inflammatory pain

Orofacial pain conditions are often very debilitating to the patient and difficult to treat. While clinical interest is high, the proportion of studies performed in the orofacial region in laboratory animals is relatively low, compared with other body regions. This is partly due to difficulties in testing freely moving animals and therefore lack of reliable testing methods. Here we present a comprehensive review of the currently used rodent models of inflammatory and neuropathic pain adapted to the orofacial areas, taking into account the difficulties and drawbacks of the existing approaches. We examine the available testing methods and procedures used for assessing the behavioral responses in the face in both mice and rats and provide a summary of some pharmacological agents used in these paradigms to date. The use of these agents in animal models is also compared with outcomes observed in the clinic.

Models of inflammation: Carrageenan- or complete Freund's Adjuvant (CFA)-induced edema and hypersensitivity in the rat

Animal models of inflammation are used to assess the production of inflammatory mediators at sites of inflammation, the anti-inflammatory properties of agents such as nonsteroidal anti-inflammatory drugs (NSAIDs), and the efficacy of putative analgesic compounds in reversing cutaneous hypersensitivity. This unit details methods to elicit and measure carrageenan- and complete Freund's adjuvant (CFA)-induced cutaneous inflammation. Due to possible differences between the dorsal root sensory system and the trigeminal sensory system, injections of either the footpad or vibrissal pad are described. In this manner, cutaneous inflammation can be assessed in tissue innervated by the lumbar dorsal root ganglion neurons (footpad) and by the trigeminal ganglion neurons (vibrissal pad).

Comprehensive gene expression profiling in the prefrontal cortex links immune activation and neutrophil infiltration to antinociception

Functional neuroimaging studies have implicated the prefrontal cortex (PFCTX) in descending modulation of pain and the placebo effect. This study was performed to elucidate comprehensive PFCTX gene expression in an animal model of persistent trigeminal pain. Adult male C57BL/6J mice received facial carrageenan injection and showed sustained increase in nociceptive responses. Microarray analyses of differentially expressed genes in the PFCTX at 3 d after injection showed "immune system process" as the dominant ontology term and increased mRNA expression of S100a8, S100a9, Lcn2, Il2rg, Fcgr1, Fcgr2b, C1qb, Ptprc, Ccl12, and Cd52 were verified by RT-PCR. Upregulation of S100A8, S100A9, and lipocalin 2 (LCN2) were confirmed by Western blots, and cells in the PFCTX were double immunolabeled with MPO, indicating they were neutrophils. Analyses of blood of facial carrageenan-injected mice also showed increased mRNA expression of these markers, suggesting transmigration of activated neutrophils into the brain. Other immune-related genes, Il2rg, Fcgr2b, C1qb, Ptprc, and Ccl12 were upregulated in the PFCTX but not blood. Approximately 70% of S100A9-positive cells in the PFCTX of carrageenan-injected mice were located in capillaries adherent to endothelial cells, whereas 30% were within the brain parenchyma. Carrageenan-injected mice showed significantly reduced nociceptive responses after injection of C terminus of murine S100A9 protein in the lateral ventricles and PFCTX but not somatosensory barrel cortex. Together, these findings demonstrate activation of immune-related genes in the PFCTX during inflammatory pain and highlight an exciting role of neutrophils in linking peripheral inflammation with immune activation of the PFCTX and antinociception.

Assessing nociceptive sensitivity in mouse models of inflammatory and neuropathic trigeminal pain

Chronic orofacial pain encompasses a range of debilitating conditions, however in contrast to other body regions, few animal models are available to investigate mechanisms and treatments in the trigeminal area. Particularly, there is a lack of reliable models and testing methods in mice. We have behaviourally tested C57BL/6 mice subjected to unilateral chronic constriction injury (CCI) of the infraorbital nerve (IoN) or unilateral injections of Complete Freunds Adjuvant (CFA) into the vibrissal pad region with the aid of von Frey filaments and air-puffs and the use of a newly designed restraining device. These models were validated by suppressing the pain responses with appropriate drugs. The IoN-CCI group showed significant hyperalgesia on the ipsilateral side in comparison to baseline values for up to 20 days post-CCI following von Frey and air-puff stimulation. Gabapentin (60mg/kg), but not saline, temporarily reversed the hyperalgesia. Animals that received a CFA injection showed hyperresponsivity to both von Frey and air-puff stimulation for up to 4 days post injection. These effects were transiently reversed with 3mg/kg i.p. morphine but not saline. Our study proposes a new restraining device for mice, and validates a behavioural testing procedure of several facial pain models in mice, allowing for reproducible and robust assessment of the effects of pain-related agents and treatments, or phenotyping of genetically modified animals.

MicroRNA changes in the mouse prefrontal cortex after inflammatory pain

Activation of the prefrontal cortex occurs during acute and chronic pain and models of experimental hyperalgesia. The present study was carried out to determine possible miRNA changes in the prefrontal cortex, after inflammatory pain induced by facial carrageenan injection in mice. miRNA microarray analyses showed significantly increased levels of miR-155 and miR-223 in the prefrontal cortex of carrageenan-injected mice. The changes were verified by real-time RT-PCR, and shown to occur bilaterally. The potential targets of the two miRNAs were predicted, and changes in two of the miRNA targets, c/ebp Beta and granulocyte colony-stimulating factor (GCSF) verified by real-time RT-PCR. Significantly downregulated c/ebp Beta but upregulated GCSF, accompanied by increased immunolabeling with an antibody to myeloperoxidase were found in the prefrontal cortex of facial carrageenan treated mice. It is postulated that this could lead to increased inflammation and activation of the prefrontal cortex. Further studies are necessary to determine if specific miRNAs could be useful as therapeutic molecules for pain.