Elsevier

Brain Research

Volume 1263, 31 March 2009, Pages 50-57
Brain Research

Research Report
Activation of ERK in the locus coeruleus following acute noxious stimulation

https://doi.org/10.1016/j.brainres.2009.01.052Get rights and content

Abstract

In the present study, the activation of extracellular signal-regulated kinase (ERK) in the locus coeruleus (LC) following injection of formalin or complete Freund's adjuvant (CFA) into the rat hindpaw was examined in order to clarify the mechanisms underlying the dynamic changes in the descending pain modulatory system after acute noxious stimulation or chronic inflammation. In naive rats there were few phospho-extracellular signal-regulated kinase-immunoreactive (p-ERK-IR) neurons in the LC. Formalin-, CFA- and saline-injections induced an increase in p-ERK-IR in the LC. The number of p-ERK-IR neurons in the LC in the formalin group was significantly higher than those in all other groups from 5 min to 1 h after the injection (p < 0.05). CFA injection induced only a transient significant increase in the number of p-ERK-IR neurons and there was no significant difference in the number of p-ERK-IR neurons between the CFA and saline groups. At 5 min after formalin injection, almost all p-ERK-IR neurons in the LC were tyrosine hydroxylase (TH) -positive. These findings suggest that activation of ERK in the LC is induced by acute noxious stimulation, such as formalin injection, but not by CFA-induced chronic inflammation. The activation of ERK in the LC may be involved in the plasticity of the descending pain modulatory systems following acute noxious stimulation.

Introduction

The locus coeruleus (LC) in the dorsolateral pons sends noradrenergic projection to the spinal dorsal horn and is a critical component of the descending pain modulatory system. Electrical or chemical stimulation of the LC produces release of noradrenaline (NA) in the spinal cord (Hentall et al., 2003), inhibits the responses of spinal dorsal horn neurons to noxious stimuli (Jones and Gebhart, 1986, Mokha et al., 1985) and induces antinociception (West et al., 1993). Noxious stimuli activate the LC neurons, which is evidenced by Fos expression, a measure of neural activation, in the previous studies (Han et al., 2003, Tsuruoka et al., 2003b). Since the LC neurons activated by noxious stimuli project to somatosensory thalamic nuclei, it is suggested that the ascending projections from the LC may also modulate nociceptive sensory processing within the thalamus (Voisin et al., 2005).

Coeruleospinal pain inhibitory system exhibits dynamic changes in response to nociceptive inputs following inflammation and peripheral nerve injury. Spinal antinociception induced by electrical stimulation of the LC was significantly weaker in nerve-injured than control animals (Viisanen and Pertovaara, 2007). In contrast to this finding, the LC has a strong spinal antinociceptive influence in animals with inflammation (Tsuruoka and Willis, 1996, Tsuruoka et al., 2003a).

Extracellular signal-regulated kinases (ERK1 and ERK2) are members of the serine/threonine protein kinases implicated in the transduction of neurotrophic and neurochemical signals from the cell surface to the nucleus (Grewal et al., 1999). ERK plays important roles in synaptic plasticity and memory formation (Sweatt, 2001), and phosphorylated ERK (p-ERK), an activated form of ERK, has been used as a marker of neural activation (Ji et al., 2002, Kawasaki et al., 2004, Shimizu et al., 2006).

Many reports have revealed that ERK activation in the spinal dorsal horn and primary sensory neurons contributes to nociception (Dai et al., 2002, Ji et al., 2002) and we have recently documented significant roles of ERK activation in the supraspinal structures including the RVM on nociceptive sensory processing. Activation of ERK in the RVM is involved in thermal hyperalgesia during peripheral inflammation (Imbe et al., 2005, Imbe et al., 2008). However, mechanisms underlying the dynamic changes in another main structure of descending pain modulatory system, the LC, remain unknown.

Several studies have demonstrated that ERK is also activated in the LC under several conditions including immobilization stress, opioid withdrawal, and anesthesia (Hebert et al., 2005, Kwon et al., 2006, Schulz and Hollt, 1998, Springell et al., 2005); however, no studies have assessed if noxious stimulation could induce ERK activation in the LC as was seen in the RVM.

In the present study, we examined the activation of ERK in the LC following injection of formalin or complete Freund's adjuvant (CFA) into the rat hindpaw in order to clarify changes of signal transduction system in the LC after acute noxious stimulation or chronic inflammation. The results show that formalin injection into the rat hindpaw significantly increases phospho-ERK-immunoreactive (p-ERK-IR) neurons in the LC.

Section snippets

Nocifensive behavior following injection of formalin or CFA into the hindpaw

Formalin injection induced the intense nocifensive behaviors, such as licking or biting of the injected paw, for the observation period. In contrast, CFA injection did not induce those behaviors except for the first 3-min interval after the injection (Fig. 1). The repeated ANOVA revealed a significant group difference between the formalin and CFA groups (P < 0.0001, Fig. 1).

Activation of ERK in the LC following injection of formalin or CFA into the hindpaw

A number of p-ERK-IR neurons were observed in the LC of rats with injections of formalin, CFA and saline or restraint (Fig. 2

Discussion

The present study clearly showed that acute noxious stimulation induced by formalin injection into the hindpaw resulted in a significant increase in p-ERK-IR in the LC for 1 h after the injection. We observed bilateral activation of ERK in the LC after unilateral formalin injection. Our finding is in accordance with the previous studies showing bilateral Fos expression in the LC after unilateral injection of carrageenan or formalin into the rat paw (Han et al., 2003, Tsuruoka et al., 2003b).

Animals

Male Sprague–Dawley rats (Japan SLC, Shizuoka, Japan) weighing 238–450 g were used in all experiments. The animals were housed two per cage, maintained under a 12-hour light–dark cycle, and allowed free access to food and water. The experiments were approved by the Animal Care Committee of Wakayama Medical University. All experiments conformed to the National Institutes of Health Guide for the Care and Use of the Laboratory Animals (NIH Publications No. 99-158 revised 2002). All efforts were

Acknowledgments

This study was supported in part by a Grant-in-Aid for Scientific Research (C) from Japan Society for the Promotion of Science (18613021).

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