The effect of inflammation on Fos expression in the ferret trigeminal nucleus

Chronic tooth pulp inflammation induces persistent expression of phosphorylated ERK (pERK) and phosphorylated p38 (pp38) in trigeminal subnucleus caudalis

•

Chronic inflammation of tooth pulp activates pERK and pp38 in the trigeminal nucleus

•

Activation is persistent and bilateral, and further increased by acute stimulation

•

This altered signaling may be relevant in the development of chronic pulpitic pain

•

pERK and pp38 are more sensitive markers of central change than Fos expression

•

Sequential activation in different cell types may be linked to pain progression

Background

Extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase are transiently phosphorylated (activated) in the spinal cord and trigeminal nucleus by acute noxious stimuli. Acute stimulation of dental pulp induces short-lived ERK activation in trigeminal subnucleus caudalis (Vc), and p38 inhibition attenuates short-term sensitization in Vc induced by acute pulpal stimulation. We have developed a model to study central changes following chronic inflammation of dental pulp that induces long-term sensitization. Here, we examine the effects of chronic inflammation and acute stimulation on the expression of phosphorylated ERK (pERK), phosphorylated p38 (pp38) and Fos in Vc.

Results

Chronic inflammation alone induced bilateral expression of pERK and pp38 in Vc, but did not induce Fos expression. Stimulation of both non-inflamed and inflamed pulps significantly increased pERK and pp38 bilaterally; expression was greatest in inflamed, stimulated animals, and was similar following 10-min and 60-min stimulation. Stimulation for 60 min, but not 10 min, induced Fos in ipsilateral Vc; Fos expression was significantly greater in inflamed, stimulated animals. pERK was present in both neurons and astrocytes; pp38 was present in neurons and other non-neuronal, non-astrocytic cell types.

Conclusions

This study provides the first demonstration that chronic inflammation of tooth pulp induces persistent bilateral activation of ERK and p38 within Vc, and that this activation is further increased by acute stimulation. This altered activity in intracellular signaling is likely to be linked to the sensitization that is seen in our animal model and in patients with pulpitis. Our data indicate that pERK and pp38 are more accurate markers of central change than Fos expression. In our model, localization of pERK and pp38 within specific cell types differs from that seen following acute stimulation. This may indicate specific roles for different cell types in the induction and maintenance of pulpitic and other types of pain.

Evidence for anti-inflammatory and putative analgesic effects of a monoclonal antibody to calcitonin gene-related peptide

BACKGROUND

Calcitonin gene-related peptide (CGRP) is a powerful pro-inflammatory mediator thought to play a significant role in the development of inflammation and pain. We investigated the role of CGRP in trigeminal inflammatory pain by determining the ability of a monoclonal antibody to CGRP to modify central Fos expression in response to stimulation of the inflamed ferret tooth pulp. We also assessed the effect of the antibody on pulpal inflammation.

METHODS

Ten adult ferrets were prepared under anaesthesia to allow stimulation of the upper and lower left canine pulps, recording from the digastric muscle and intravenous injections at subsequent experiments. In all animals, pulpal inflammation was induced by introducing human caries into a deep buccal cavity. Four days later animals were treated intravenously with either CGRP antibody (n=5) or vehicle (n=5). After a further 2 days animals were re-anaesthetised and the tooth pulps stimulated at 10 times jaw-opening reflex threshold. Brainstems and tooth pulps were processed immunohistochemically for Fos and the common leucocyte marker CD45, respectively.

RESULTS

Fos was expressed in ipsilateral trigeminal subnuclei caudalis (Vc) and oralis (Vo). Significantly fewer Fos-positive nuclei were present within Vc of CGRP antibody-treated animals (p=0.003 vs vehicle-treated). Mean percentage area of staining for CD45 was significantly less in antibody-treated animals (p=0.04 vs vehicle-treated).

CONCLUSIONS

This is the first direct evidence that sequestration of CGRP has anti-inflammatory and putative analgesic effects. Previous studies using this Fos model have demonstrated that it is able to predict clinical analgesic efficacy. Thus these data indicate that this antibody may have analgesic effects in dental pain and other types of inflammatory-mediated transmission, and suggest that this is in part due to peripheral anti-inflammatory effects.

The effects of ibuprofen and the neurokinin-1 receptor antagonist GR205171A on Fos expression in the ferret trigeminal nucleus following tooth pulp stimulation

We have developed a model to study central changes following inflammation of the tooth pulp in the ferret and have examined Fos expression in the trigeminal nucleus following stimulation of non-inflamed and inflamed tooth pulps. The aim of this study was to establish the ability of this model to predict analgesic efficacy in clinical studies of inflammatory pain. We addressed this by assessing the effects of the neurokinin-1 receptor antagonist GR205171A and ibuprofen on Fos expression following stimulation of the inflamed pulp and comparing this with known analgesic efficacy. Adult ferrets were prepared under anaesthesia to allow tooth pulp stimulation, recording from the digastric muscle and intravenous injections at a subsequent experiment. In some animals pulpal inflammation was induced, by introducing human caries into a deep buccal cavity. After 5 days, animals were reanaesthetised, treated with vehicle, GR205171A or ibuprofen and the teeth were stimulated at ten times the threshold of the jaw-opening reflex. Stimulation of all tooth pulps induced ipsilateral Fos in trigeminal subnuclei caudalis and oralis. GR205171A had no significant effect on Fos expression in the trigeminal nucleus of animals with either non-inflamed or inflamed tooth pulps. Ibuprofen reduced Fos expression in the trigeminal nucleus and this effect was most marked in animals with pulpal inflammation. These results differ from those previously described using a range of other animal models, but agree with known clinical efficacy of neurokinin-1 receptor antagonists and ibuprofen. Therefore this model is likely to be of use in accurately predicting the analgesic efficacy of novel compounds.

Expression changes of K+-Cl- co-transporter 2 and Na+-K+-Cl- co-transporter1 in mouse trigeminal subnucleus caudalis following pulpal inflammation

Cation chloride co-transporters, including K(+)-Cl(-) co-transporter 2 (KCC2) and Na(+)-K(+)-Cl(-) co-transporter 1 (NKCC1), are of particular importance to GABAergic transmission and thus involved in the development of hyperalgesia at the spinal level. However, it is largely unknown whether these co-transporters in the trigeminal system contribute to dental pain. In this study, we investigated the expression of KCC2 and NKCC1 mRNAs in mouse trigeminal subnucleus caudalis (Vc) after lipopolysaccharide (LPS) application to the tooth pulp by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) method. KCC2 mRNA was found to be down-regulated at 1d after pulpal inflammation, while NKCC1 was up-regulated. Blockade of endogenous brain-derived neurotrophic factor-tyrosine receptor kinase B pathway with K252a produced pronounced antinociception as evidenced by decreased tongue protrusion behavior in LPS-treated mice. These data suggest that KCC2 and NKCC1 in Vc may play a critical role in the nociception and transmission of dental pain during pulpal inflammation.