Activation of ERK in the rostral ventromedial medulla is involved in hyperalgesia during peripheral inflammation

The rostral ventromedial medulla modulates pain and depression-related behaviors caused by social stress

ABSTRACT

The rostral ventromedial medulla (RVM) is a crucial structure in the descending pain modulatory system, playing a key role as a relay for both the facilitation and inhibition of pain. The chronic social defeat stress (CSDS) model has been widely used to study stress-induced behavioral impairments associated with depression in rodents. Several studies suggest that CSDS also causes changes related to chronic pain. In this study, we aimed to investigate the involvement of the RVM in CSDS-induced behavioral impairments, including those associated with chronic pain. We used chemogenetics to activate or inhibit the RVM during stress. The results indicated that the RVM is a vital hub influencing stress outcomes. Rostral ventromedial medulla activation during CSDS ameliorates all the stress outcomes, including social avoidance, allodynia, hyperalgesia, anhedonia, and behavioral despair. In addition, RVM inhibition in animals exposed to a subthreshold social defeat stress protocol induces a susceptible phenotype, facilitating all stress outcomes. Finally, chronic RVM inhibition-without any social stress stimulus-induces chronic pain but not depressive-like behaviors. Our findings provide insights into the comorbidity between chronic pain and depression by indicating the involvement of the RVM in establishing social stress-induced behavioral responses associated with both chronic pain and depression.

Altered glial glutamate transporter expression in descending circuitry and the emergence of pain chronicity

BACKGROUND

The glutamate type 1 transporter (GLT1) plays a major role in glutamate homeostasis in the brain. Although alterations of GLT1 activity have been linked to persistent pain, the significance of these changes is poorly understood. Focusing on the rostral ventromedial medulla, a key site in pain modulation, we examined the expression and function of GLT1 and related transcription factor kappa B-motif binding phosphoprotein (KBBP) in rats after adjuvant-induced hind paw inflammation.

RESULTS

After inflammation, GLT1 and KBBP showed an early upregulation and gradual transition to downregulation that lasted throughout the eight-week observation period. Nitration of GLT1 was reduced at 30 min and increased at eight weeks after inflammation, suggesting an initial increase and later decrease in transporter activity. Mechanical hyperalgesia and paw edema exhibited an initial developing phase with peak hyperalgesia at 4 to 24 h, a subsequent attenuating phase, followed by a late persistent phase that lasted for months. The downregulation of GLT1 occurred at a time when hyperalgesia transitioned into the persistent phase. In the rostral ventromedial medulla, pharmacological block with dihydrokainic acid and RNAi of GLT1 and KBBP increased nociception and overexpression of GLT1 reversed persistent hyperalgesia. Further, the initial upregulation of GLT1 and KBBP was blocked by local anesthetic block, and pretreatment with dihydrokainic acid facilitated the development of hyperalgesia.

CONCLUSIONS

These results suggest that the initial increased GLT1 activity depends on injury input and serves to dampen the development of hyperalgesia. However, later downregulation of GLT1 fosters the net descending facilitation as injury persists, leading to the emergence of persistent pain.

Increased neuronal expression of neurokinin-1 receptor and stimulus-evoked internalization of the receptor in the rostral ventromedial medulla of the rat after peripheral inflammatory injury

This study examined possible mechanisms by which Substance P (Sub P) assumes a pronociceptive role in the rostral ventromedial medulla (RVM) under conditions of peripheral inflammatory injury, in this case produced by intraplantar (ipl) injection of complete Freund's adjuvant (CFA). In saline- and CFA-treated rats, neurokinin-1 receptor (NK1R) immunoreactivity was localized to neurons in the RVM. Four days after ipl injection of CFA, the number of NK1R-immunoreactive neurons in the RVM was increased by 30%, and there was a concomitant increase in NK1R-immunoreactive processes in CFA-treated rats. Although NK1R immunoreactivity was increased, tachykinin-1 receptor (Tacr1) mRNA was not increased in the RVM of CFA-treated rats. To assess changes in Sub P release, the number of RVM neurons that exhibited NK1R internalization was examined in saline- and CFA-treated rats following noxious heat stimulation of the hind paws. Only CFA-treated rats that experienced noxious heat stimulation exhibited a significant increase in the number of neurons showing NK1R internalization. These data suggest that tonic Sub P release is not increased as a simple consequence of peripheral inflammation, but that phasic or evoked release of Sub P in the RVM is increased in response to noxious peripheral stimulation in a persistent inflammatory state. These data support the proposal that an upregulation of the NK1R in the RVM, as well as enhanced release of Sub P following noxious stimulation, underlie the pronociceptive role of Sub P under conditions of persistent inflammatory injury.

ERK1/2: Function, signaling and implication in pain and pain-related anxio-depressive disorders

Despite the increasing knowledge regarding pain modulation, the understanding of the mechanisms behind a complex and pathologic chronic pain condition is still insufficient. These knowledge gaps might result in ineffective therapeutic approaches to relieve painful sensations. As a result, severe untreated chronic pain frequently triggers the onset of new disorders such as depression and/or anxiety, and therefore, both the diagnosis and treatment of patients suffering from chronic pain become seriously compromised, prompting a self-perpetuating cycle of symptomatology. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are molecules strongly implicated in the somatic component of pain at the spinal cord level and have been emerging as mediators of the emotional-affective component as well. Although these molecules might represent good biomarkers, their use as pharmacological targets is still open to discussion as paradoxical information has been obtained. Here we review the current scientific literature regarding ERK1/2 signaling in the modulation of pain, depression and anxiety, including the emotional-affective spheres of the pain experience.

Expression of the spinal 5-HT7 receptor and p-ERK pathway in the carrageenan inflammatory pain of rats

BACKGROUND

Although the inhibitory role of the 5-hydroxytrypatmine receptor 7(5-HT7R) on nociceptive processing is generally recognized, an excitatory effect associated with a reduced 5-HT7R expression has also been observed in the nerve injury model. In the carrageenan model, no significant effect is produced by the 5-HT7R activation, but the change in 5-HT7R expression has not been examined. Lesioning of the spinal serotonergic pathway enhances allodynia in the carrageenan model, but it also relieves several other pain states, including in the formalin model. While lesioning suppresses the activation of the extracellular signal-regulated kinase (ERK) of the spinal cord in the formalin model, its role in the carrageenan model has not been reported.

METHODS

Following intraplantar injections of carrageenan, the spinal 5-HT7R expression was examined using Western blotting in male Sprague-Dawley rats. The effect of serotonergic pathway lesioning with intrathecal 5,7-dihydroxytryptamine (5,7-DHT) on the expression of the phospho-ERK was measured.

RESULTS

The expression of the 5-HT7R in the carrageenan model was not significantly different from that of naive animals. The expression of the spinal p-ERK in the carrageenan model was significantly increased, but returned to the level of a naive rat 1 hour after the carrageenan injection. However, it remained significantly higher 1 hour after the injection in the animals treated with 5,7-DHT than in the naive and control rats.

CONCLUSIONS

The expression of the spinal 5-HT7R is not altered by peripheral inflammation with carrageenan, suggesting that the lack of antinociceptive effect of the 5-HT7R activation is partly attributable to the absence of changes in the expression of the 5-HT7R in the spinal cord. The extended increase of the spinal p-ERK might be related to the enhanced pain behavior in the animals with lesions of the spinal serotonergic pathway in the carrageenan model.

Activation of p38 MAPK in the rostral ventromedial medulla by visceral noxious inputs transmitted via the dorsal columns may contribute to pelvic organ cross-sensitization in rats with endometriosis

Whether visceral organ cross-sensitization is involved in endometriosis-associated pain remains elusive. Previous studies have shown that visceral noxious stimuli may trigger a cascade of signal transductions in the rostral ventromedial medulla (RVM) via the spinal dorsal column (DC) pathway and the RVM plays a critical role in the descending control of visceral nociception. In the current study, we hypothesized that the p38 mitogen-activated protein kinase (MAPK) activation in the RVM by noxious visceral inputs from ectopic growths via the DC was involved in the development of pelvic organ cross-sensitization in established endometriosis. A rat model of experimental endometriosis was established. To examine ectopic growths-to-colon cross-sensitization, graded colorectal distention (CRD) was performed and abdominal withdrawal reflex (AWR) scores were recorded in female rats at 8weeks after the uterine or fat (control) auto-transplantation. Western blot study was carried out to examine the phosphorylated form and the total level of p38 MAPK protein in the RVM. Our results showed that lesions of bilateral DCs immediately following uterine or fat auto-transplantation in female rats significantly attenuated the later development of ectopic growths-to-colon cross-sensitization and the increased p38 MAPK activation in the RVM, as compared to sham DC lesions. Furthermore, intra-RVM microinjection of a p38 MAPK inhibitor (SB 203580), but not vehicle, in female rats with established endometriosis significantly attenuated ectopic growths-to-colon cross-sensitization and the increased activation of p38 MAPK in the RVM. These findings suggest that the noxious inputs from ectopic growths may activate p38 MAPK in the RVM via the DC, which may contribute to the development of ectopic growths-to-colon cross-sensitization in established endometriosis.

BDNF-dependent plasticity induced by peripheral inflammation in the primary sensory and the cingulate cortex triggers cold allodynia and reveals a major role for endogenous BDNF as a tuner of the affective aspect of pain

Painful experiences are multilayered, composed of sensory, affective, cognitive and behavioral facets. Whereas it is well accepted that the development of chronic pain is due to maladaptive neuronal changes, the underlying molecular mechanisms, their relationship to the different pain modalities, and indeed the localization of these changes are still unknown. Brain-derived neurotrophic factor (BDNF) is an activity-dependent neuromodulator in the adult brain, which enhances neuronal excitability. In the spinal cord, BDNF underlies the development and maintenance of inflammatory and neuropathic pain. Here, we hypothesized that BDNF could be a trigger of some of these plastic changes. Our results demonstrate that BDNF is upregulated in the anterior cingulate cortex (ACC) and the primary sensory cortex (S1) in rats with inflammatory pain. Injections of recombinant BDNF (into the ACC) or a viral vector synthesizing BDNF (into the ACC or S1) triggered both neuronal hyperexcitability, as shown by elevated long-term potentiation, and sustained pain hypersensitivity. Finally, pharmacological blockade of BDNF-tropomyosin receptor kinase B (TrkB) signaling in the ACC, through local injection of cyclotraxin-B (a novel, highly potent, and selective TrkB antagonist) prevented neuronal hyperexcitability, the emergence of cold hypersensitivity, and passive avoidance behavior. These findings show that BDNF-dependent neuronal plasticity in the ACC, a structure known to be involved in the affective-emotional aspect of pain, is a key mechanism in the development and maintenance of the emotional aspect of chronic pain.

Changes in response properties of rostral ventromedial medulla neurons during prolonged inflammation: modulation by neurokinin-1 receptors

Activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM) can facilitate pain transmission in conditions such as inflammation, and thereby contribute to hyperalgesia. Since blockade of NK-1 receptors in the RVM can attenuate hyperalgesia produced by prolonged inflammation, we examined the role of NK-1 receptors in changes of response properties of RVM neurons following four days of hind paw inflammation with complete Freund's adjuvant. Recordings were made from functionally identified ON, OFF and NEUTRAL cells in the RVM. Spontaneous activity and responses evoked by a series of mechanical (10, 15, 26, 60, 100, and 180 g) and heat (34-50 °C) stimuli applied to the inflamed and non-inflamed hind paws were determined before and at 15 and 60 min after injection of the NK-1-antagonist L-733,060 or vehicle into the RVM. Prolonged inflammation did not alter the proportions of functionally-identified ON, OFF and NEUTRAL cells. ON cells exhibited enhanced responses to mechanical (60-100g) and heat (48-50 °C) stimuli applied to the inflamed paw, which were attenuated by L-733,060 but not by vehicle. Inhibitory responses of OFF cells evoked by mechanical stimuli applied to the inflamed paw were also inhibited by L-733,060, but responses evoked by stimulation of the contralateral paw were increased. Heat-evoked responses of OFF cells were not altered by L-733,060. Also, neither L-733,060 nor vehicle altered spontaneous ongoing discharge rate of RVM neurons. These data indicate that NK-1 receptors modulate excitability of ON cells which contribute to both mechanical and heat hyperalgesia, whereas NK-1 modulation of OFF cells contributes to mechanical hyperalgesia during prolonged inflammation.

Stress-induced hyperalgesia is associated with a reduced and delayed GABA inhibitory control that enhances post-synaptic NMDA receptor activation in the spinal cord

GABA and glutamate are both affected by stress and are involved in nociception. Thus, we determined whether stress-induced enhancement of inflammatory hyperalgesia is mediated by an imbalance between glutamate and GABA neurotransmission. Male rats were subjected daily to 10 to 20 minutes per day of either forced swimming (FS) or sham swimming for 3 consecutive days; nonconditioned rats served as controls. Some rats were treated i.p. with ketamine (5 mg/kg), diazepam (2 mg/kg), flumazenil (0.1 mg/kg), or vehicle (0.9% NaCl), 30 to 60 minutes before each conditioning session or nociception assessment. Pain behavior, spinal nociceptive neuronal activation and GABA and glutamate release were respectively evaluated by the formalin test, the expression of c-Fos and in vivo microdialysis of superficial laminae of the lumbar spinal cord, 48 hours after the last conditioning session. Nitric oxide metabolites (NO(x)) were determined as markers of post-synaptic NMDA receptor activation. FS stress enhanced formalin-induced hyperalgesia, increased pain-elicited c-Fos expression, decreased basal and delayed pain-induced GABA release, and increased basal and induced glutamate release. Hyperalgesia and c-Fos overexpression were blocked only by prestress treatment with diazepam and post-stress treatment with ketamine, whereas changes in GABA and glutamate release were reversed by prestress treatment with diazepam. Diazepam effects were blocked by flumazenil. NO(x) increased in lumbar spinal cord of FS rats by a mechanism antagonized by ketamine. Thus, stress-induced hyperalgesia is initiated by a decreased and delayed GABA release and GABA-A receptor activation, whereas it is maintained by increased glutamate release and NMDA glutamate receptor activation at the spinal level.

Activation of mitogen-activated protein kinase in descending pain modulatory system

The descending pain modulatory system is thought to undergo plastic changes following peripheral tissue injury and exerts bidirectional (facilitatory and inhibitory) influence on spinal nociceptive transmission. The mitogen-activated protein kinases (MAPKs) superfamily consists of four main members: the extracellular signal-regulated protein kinase1/2 (ERK1/2), the c-Jun N-terminal kinases (JNKs), the p38 MAPKs, and the ERK5. MAPKs not only regulate cell proliferation and survival but also play important roles in synaptic plasticity and memory formation. Recently, many studies have demonstrated that noxious stimuli activate MAPKs in several brain regions that are components of descending pain modulatory system. They are involved in pain perception and pain-related emotional responses. In addition, psychophysical stress also activates MAPKs in these brain structures. Greater appreciation of the convergence of mechanisms between noxious stimuli- and psychological stress-induced neuroplasticity is likely to lead to the identification of novel targets for a variety of pain syndromes.

Extracellular signal-regulated kinases in pain of peripheral origin

Activation of members of the family of enzymes known as extracellular signal-regulated kinases (ERKs) is now known to be involved in the development and/or maintenance of the pain associated with many inflammatory conditions, such as herniated spinal disc pain, chronic inflammatory articular pain, and the pain associated with bladder inflammation. Moreover, ERKs are implicated in the development of neuropathic pain signs in animals which are subjected to the lumbar 5 spinal nerve ligation model and the chronic constriction injury model of neuropathic pain. The position has now been reached where all scientists working on pain subjects ought to be aware of the importance of ERKs, if only because certain of these enzymes are increasingly employed as experimental markers of nociceptive processing. Here, we introduce the reader, first, to the intracellular context in which these enzymes function. Thereafter, we consider the involvement of ERKs in mediating nociceptive signalling to the brain resulting from noxious stimuli at the periphery which will be interpreted by the brain as pain of peripheral origin.

Injury induced activation of extracellular signal-regulated kinase (ERK) in the rat rostral ventromedial medulla (RVM) is age dependant and requires the lamina I projection pathway

Descending controls originating in part from the rostral ventromedial medulla (RVM) regulate the excitability of dorsal horn neurons and maintain peripheral pain states. Activation of extracellular signal regulated kinase (ERK) in RVM neurons has been shown following peripheral inflammation and is involved in generating the accompanying inflammatory hyperalgesia. Here, we show that spared nerve injury (SNI), a model of neuropathic pain, results in an increase in ERK activity in RVM neurons of adult rats 3 and 8 days following surgery. We carried out two experimental procedures to demonstrate that this increase in ERK activation was related to the increased mechanical sensitivity associated with SNI. First, we showed that lesions of the lamina I/III ascending pathway from the dorsal horn attenuated both mechanical hyperalgesia and ERK activation in the RVM. Second, we performed SNI in P10 rats. At this age, SNI did not result in mechanical hypersensitivity, as previously shown, and did not activate ERK in the RVM. Finally, the percentage of pERK expressing neurones that were also serotonergic was always around 60%, independent of pain state and age, indicating an important role for serotonin in descending controls of pain states.

Involvement of descending facilitation from the rostral ventromedial medulla in the enhancement of formalin-evoked nocifensive behavior following repeated forced swim stress

In the present study we examined whether the descending facilitation from the rostral ventromedial medulla (RVM) is required for the enhancement of formalin-evoked nocifensive behavior following repeated forced swim stress. Rats were subjected to forced or sham swim stress for 3days. Withdrawal latency to noxious thermal stimuli and mechanical withdrawal threshold to von Frey filaments did not change significantly in both groups at 24h after the last stress session. The forced swim stress showed significantly enhanced nocifensive behavior to the subcutaneous administration of formalin at 2days after the last stress session (1330.1+/-62.8s), compared to the sham swim (1076+/-102.4s, p<0.05) and naive groups (825.9+/-83.2s, p<0.01). The destruction of the RVM with ibotenic acid led to prevent the enhancement of formalin-evoked nocifensive behavior in the forced swim group. These findings suggest that the descending facilitation from the RVM may be involved in the enhancement of formalin-evoked nocifensive behavior following the forced swim stress.

Activation of ERK in the locus coeruleus following acute noxious stimulation

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.

Supraspinal glial-neuronal interactions contribute to descending pain facilitation

Spinal glial reaction and proinflammatory cytokine induction play an important role in the development of chronic pain states after tissue and nerve injury. The present study investigated the cellular and molecular mechanisms underlying descending facilitation of neuropathic pain with an emphasis on supraspinal glial-neuronal relationships. An early and transient reaction of microglia and prolonged reaction of astrocytes were found after chronic constriction injury (CCI) of the rat infraorbital nerve in the rostral ventromedial medulla (RVM), a major component of brainstem descending pain modulatory circuitry. There were prolonged elevations of cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) after CCI, and they were expressed in RVM astrocytes at 14 d after injury. Intra-RVM injection of microglial and astrocytic inhibitors attenuated mechanical hyperalgesia and allodynia at 3 and 14 d after CCI, respectively. Moreover, TNFR1 and IL-1R, receptors for TNF-alpha and IL-1beta, respectively, were expressed primarily in RVM neurons exhibiting immunoreactivity to the NMDA receptor (NMDAR) subunit NR1. CCI increased TNFR1 and IL-1R levels and NR1 phosphorylation in the RVM. Neutralization of endogenous TNF-alpha and IL-1beta in the RVM significantly reduced CCI-induced behavioral hypersensitivity and attenuated NR1 phosphorylation. Finally, intra-RVM administration of recombinant TNF-alpha or IL-1beta upregulated NR1 phosphorylation and caused a reversible and NMDAR-dependent allodynia in normal rats, further suggesting that TNF-alpha and IL-1beta couple glial hyperactivation with NMDAR function. These studies have addressed a novel contribution of supraspinal astrocytes and associated cytokines as well as central glial-neuronal interactions to the enhancement of descending facilitation of neuropathic pain.