Reversal of neuropathic pain by HSV-1-mediated decrease of noradrenaline in a pain facilitatory area of the brain

How do opioids control pain circuits in the brainstem during opioid-induced disorders and in chronic pain? Implications for the treatment of chronic pain

ABSTRACT

Brainstem areas involved in descending pain modulation are crucial for the analgesic actions of opioids. However, the role of opioids in these areas during tolerance, opioid-induced hyperalgesia (OIH), and in chronic pain settings remains underappreciated. We conducted a revision of the recent studies performed in the main brainstem areas devoted to descending pain modulation with a special focus on the medullary dorsal reticular nucleus (DRt), as a distinctive pain facilitatory area and a key player in the diffuse noxious inhibitory control paradigm. We show that maladaptive processes within the signaling of the µ-opioid receptor (MOR), which entail desensitization and a switch to excitatory signaling, occur in the brainstem, contributing to tolerance and OIH. In the context of chronic pain, the alterations found are complex and depend on the area and model of chronic pain. For example, the downregulation of MOR and δ-opioid receptor (DOR) in some areas, including the DRt, during neuropathic pain likely contributes to the inefficacy of opioids. However, the upregulation of MOR and DOR, at the rostral ventromedial medulla, in inflammatory pain models, suggests therapeutic avenues to explore. Mechanistically, the rationale for the diversity and complexity of alterations in the brainstem is likely provided by the alternative splicing of opioid receptors and the heteromerization of MOR. In conclusion, this review emphasizes how important it is to consider the effects of opioids at these circuits when using opioids for the treatment of chronic pain and for the development of safer and effective opioids.

Nerve injury induces transient locus coeruleus activation over time: role of the locus coeruleus-dorsal reticular nucleus pathway

ABSTRACT

The transition from acute to chronic pain results in maladaptive brain remodeling, as characterized by sensorial hypersensitivity and the ensuing appearance of emotional disorders. Using the chronic constriction injury of the sciatic nerve as a model of neuropathic pain in male Sprague-Dawley rats, we identified time-dependent plasticity of locus coeruleus (LC) neurons related to the site of injury, ipsilateral (LCipsi) or contralateral (LCcontra) to the lesion, hypothesizing that the LC→dorsal reticular nucleus (DRt) pathway is involved in the pathological nociception associated with chronic pain. LCipsi inactivation with lidocaine increased cold allodynia 2 days after nerve injury but not later. However, similar blockade of LCcontra reduced cold allodynia 7 and 30 days after inducing neuropathy but not earlier. Furthermore, lidocaine blockade of the LCipsi or LCcontra reversed pain-induced depression 30 days after neuropathy. Long-term pain enhances phosphorylated cAMP-response element binding protein expression in the DRtcontra but not in the DRtipsi. Moreover, inactivation of the LCcontra→DRtcontra pathway using dual viral-mediated gene transfer of designer receptor exclusively activated by designer drugs produced consistent analgesia in evoked and spontaneous pain 30 days postinjury. This analgesia was similar to that produced by spinal activation of α2-adrenoreceptors. Furthermore, chemogenetic inactivation of the LCcontra→DRtcontra pathway induced depressive-like behaviour in naïve animals, but it did not modify long-term pain-induced depression. Overall, nerve damage activates the LCipsi, which temporally dampens the neuropathic phenotype. However, the ensuing activation of a LCcontra→DRtcontra facilitatory pain projection contributes to chronic pain, whereas global bilateral LC activation contributes to associated depressive-like phenotype.

Brainstem Pain-Modulation Circuitry and Its Plasticity in Neuropathic Pain: Insights From Human Brain Imaging Investigations

Acute pain serves as a protective mechanism that alerts us to potential tissue damage and drives a behavioural response that removes us from danger. The neural circuitry critical for mounting this behavioural response is situated within the brainstem and is also crucial for producing analgesic and hyperalgesic responses. In particular, the periaqueductal grey, rostral ventromedial medulla, locus coeruleus and subnucleus reticularis dorsalis are important structures that directly or indirectly modulate nociceptive transmission at the primary nociceptive synapse. Substantial evidence from experimental animal studies suggests that plasticity within this system contributes to the initiation and/or maintenance of chronic neuropathic pain, and may even predispose individuals to developing chronic pain. Indeed, overwhelming evidence indicates that plasticity within this circuitry favours pro-nociception at the primary synapse in neuropathic pain conditions, a process that ultimately contributes to a hyperalgesic state. Although experimental animal investigations have been crucial in our understanding of the anatomy and function of the brainstem pain-modulation circuitry, it is vital to understand this system in acute and chronic pain states in humans so that more effective treatments can be developed. Recent functional MRI studies have identified a key role of this system during various analgesic and hyperalgesic responses including placebo analgesia, offset analgesia, attentional analgesia, conditioned pain modulation, central sensitisation and temporal summation. Moreover, recent MRI investigations have begun to explore brainstem pain-modulation circuitry plasticity in chronic neuropathic pain conditions and have identified altered grey matter volumes and functioning throughout the circuitry. Considering the findings from animal investigations, it is likely that these changes reflect a shift towards pro-nociception that ultimately contributes to the maintenance of neuropathic pain. The purpose of this review is to provide an overview of the human brain imaging investigations that have improved our understanding of the pain-modulation system in acute pain states and in neuropathic conditions. Our interpretation of the findings from these studies is often guided by the existing body of experimental animal literature, in addition to evidence from psychophysical investigations. Overall, understanding the plasticity of this system in human neuropathic pain conditions alongside the existing experimental animal literature will ultimately improve treatment options.

Predicting pain among female survivors of recent interpersonal violence: A proof-of-concept machine-learning approach

Interpersonal violence (IPV) is highly prevalent in the United States and is a major public health problem. The emergence and/or worsening of chronic pain are known sequelae of IPV; however, not all those who experience IPV develop chronic pain. To mitigate its development, it is critical to identify the factors that are associated with increased risk of pain after IPV. This proof-of-concept study used machine-learning strategies to predict pain severity and interference in 47 young women, ages 18 to 30, who experienced an incident of IPV (i.e., physical and/or sexual assault) within three months of their baseline assessment. Young women are more likely than men to experience IPV and to subsequently develop posttraumatic stress disorder (PTSD) and chronic pain. Women completed a comprehensive assessment of theory-driven cognitive and neurobiological predictors of pain severity and pain-related interference (e.g., pain, coping, disability, psychiatric diagnosis/symptoms, PTSD/trauma, executive function, neuroendocrine, and physiological stress response). Gradient boosting machine models were used to predict symptoms of pain severity and pain-related interference across time (Baseline, 1-,3-,6- follow-up assessments). Models showed excellent predictive performance for pain severity and adequate predictive performance for pain-related interference. This proof-of-concept study suggests that machine-learning approaches are a useful tool for identifying predictors of pain development in survivors of recent IPV. Baseline measures of pain, family life impairment, neuropsychological function, and trauma history were of greatest importance in predicting pain and pain-related interference across a 6-month follow-up period. Present findings support the use of machine-learning techniques in larger studies of post-IPV pain development and highlight theory-driven predictors that could inform the development of targeted early intervention programs. However, these results should be replicated in a larger dataset with lower levels of missing data.

Monoaminergic and Opioidergic Modulation of Brainstem Circuits: New Insights Into the Clinical Challenges of Pain Treatment?

The treatment of neuropathic pain remains a clinical challenge. Analgesic drugs and antidepressants are frequently ineffective, and opioids may induce side effects, including hyperalgesia. Recent results on brainstem pain modulatory circuits may explain those clinical challenges. The dual action of noradrenergic (NA) modulation was demonstrated in animal models of neuropathic pain. Besides the well-established antinociception due to spinal effects, the NA system may induce pronociception by directly acting on brainstem pain modulatory circuits, namely, at the locus coeruleus (LC) and medullary dorsal reticular nucleus (DRt). The serotoninergic system also has a dual action depending on the targeted spinal receptor, with an exacerbated activity of the excitatory 5-hydroxytryptamine 3 (5-HT3) receptors in neuropathic pain models. Opioids are involved in the modulation of descending modulatory circuits. During neuropathic pain, the opioidergic modulation of brainstem pain control areas is altered, with the release of enhanced local opioids along with reduced expression and desensitization of μ-opioid receptors (MOR). In the DRt, the installation of neuropathic pain increases the levels of enkephalins (ENKs) and induces desensitization of MOR, which may enhance descending facilitation (DF) from the DRt and impact the efficacy of exogenous opioids. On the whole, the data discussed in this review indicate the high plasticity of brainstem pain control circuits involving monoaminergic and opioidergic control. The data from studies of these neurochemical systems in neuropathic models indicate the importance of designing drugs that target multiple neurochemical systems, namely, maximizing the antinociceptive effects of antidepressants that inhibit the reuptake of serotonin and noradrenaline and preventing desensitization and tolerance of MOR at the brainstem.

Altered Brainstem Pain Modulating Circuitry Functional Connectivity in Chronic Painful Temporomandibular Disorder

There is evidence from preclinical models of chronic pain and human psychophysical investigations to suggest that alterations in endogenous brainstem pain-modulation circuit functioning are critical for the initiation and/or maintenance of pain. Whilst preclinical models have begun to explore the functioning of this circuitry in chronic pain, little is known about such functioning in humans with chronic pain. The aim of this investigation was to determine whether individuals with chronic non-neuropathic pain, painful temporomandibular disorders (TMD), display alterations in brainstem pain-modulating circuits. Using resting-state functional magnetic resonance imaging, we performed static and dynamic functional connectivity (FC) analyses to assess ongoing circuit function in 16 TMD and 45 control subjects. We calculated static FC as the correlation of functional magnetic resonance imaging signals between regions over the entire scan and dynamic FC as the correlation of signals in short (50s) windows. Compared with controls, TMD subjects showed significantly greater (static) FC between the rostral ventromedial medulla and both the subnucleus reticularis dorsalis and the region that receives orofacial nociceptive afferents, the spinal trigeminal nucleus. No differences were found in other brainstem pain-modulating regions such as the midbrain periaqueductal gray matter and locus coeruleus. We also identified that TMD subjects experience greater variability in the dynamic functional connections between the rostral ventromedial medulla and both the subnucleus reticularis dorsalis and spinal trigeminal nucleus. These changes may underlie enhanced descending pain-facilitating actions over the region that receives nociceptive afferents, ultimately leading to enhanced nociceptive transmission to higher brain regions and thus contributing to the ongoing perception of pain. PERSPECTIVE: Psychophysical studies suggest that brainstem pain-modulation circuits contribute to the maintenance of chronic pain. We report that individuals with painful TMD display altered static and dynamic FC within the brainstem pain-modulation network. Modifying this circuitry may alter an individual's ongoing pain.

Attenuation of the Diffuse Noxious Inhibitory Controls in Chronic Joint Inflammatory Pain Is Accompanied by Anxiodepressive-Like Behaviors and Impairment of the Descending Noradrenergic Modulation

The noradrenergic system is paramount for controlling pain and emotions. We aimed at understanding the descending noradrenergic modulatory mechanisms in joint inflammatory pain and its correlation with the diffuse noxious inhibitory controls (DNICs) and with the onset of anxiodepressive behaviours. In the complete Freund's adjuvant rat model of Monoarthritis, nociceptive behaviors, DNICs, and anxiodepressive-like behaviors were evaluated. Spinal alpha2-adrenergic receptors (a2-AR), dopamine beta-hydroxylase (DBH), and noradrenaline were quantified concomitantly with a2-AR pharmacologic studies. The phosphorylated extracellular signal-regulated kinases 1 and 2 (pERK1/2) were quantified in the Locus coeruleus (LC), amygdala, and anterior cingulate cortex (ACC). DNIC was attenuated at 42 days of monoarthritis while present on days 7 and 28. On day 42, in contrast to day 28, noradrenaline was reduced and DBH labelling was increased. Moreover, spinal a2-AR were potentiated and no changes in a2-AR levels were observed. Additionally, at 42 days, the activation of ERKs1/2 was increased in the LC, ACC, and basolateral amygdala. This was accompanied by anxiety- and depressive-like behaviors, while at 28 days, only anxiety-like behaviors were observed. The data suggest DNIC is attenuated in prolonged chronic joint inflammatory pain, and this is accompanied by impairment of the descending noradrenergic modulation and anxiodepressive-like behaviors.

Modulation of Voltage-Gated Sodium Channel Activity in Human Dorsal Root Ganglion Neurons by Herpesvirus Quiescent Infection

The molecular mechanisms of pain associated with alphaherpesvirus latency are not clear. We hypothesize that the voltage-gated sodium channels (VGSC) on the dorsal root ganglion (DRG) neurons controlling electrical impulses may have abnormal activity during latent viral infection and reactivation. We used herpes simplex virus 1 (HSV-1) to infect the human DRG-derived neuronal cell line HD10.6 in order to study the establishment and maintenance of viral latency, viral reactivation, and changes in the functional expression of VGSCs. Differentiated cells exhibited robust tetrodotoxin (TTX)-sensitive sodium currents, and acute infection significantly reduced the functional expression of VGSCs within 24 h and completely abolished VGSC activity within 3 days. A quiescent state of infection mimicking latency can be achieved in the presence of acyclovir (ACV) for 7 days followed by 5 days of ACV washout, and then the viruses can remain dormant for another 3 weeks. It was noted that during the establishment of HSV-1 latency, the loss of VGSC activity caused by HSV-1 infection could not be blocked by ACV treatment. However, neurons with continued ACV treatment for another 4 days showed a gradual recovery of VGSC functional expression. Furthermore, the latently infected neurons exhibited higher VGSC activity than controls. The overall regulation of VGSCs by HSV-1 during quiescent infection was proved by increased transcription and possible translation of Nav1.7. Together, these observations demonstrated a very complex pattern of electrophysiological changes during HSV infection of DRG neurons, which may have implications for understanding of the mechanisms of virus-mediated pain linked to latency and reactivation.IMPORTANCE The reactivation of herpesviruses, most commonly varicella-zoster virus (VZV) and pseudorabies virus (PRV), may cause cranial nerve disorder and unbearable pain. Clinical studies have also reported that HSV-1 causes postherpetic neuralgia and chronic occipital neuralgia in humans. The current work meticulously studies the functional expression profile changes of VGSCs during the processes of HSV-1 latency establishment and reactivation using human dorsal root ganglion-derived neuronal HD10.6 cells as an in vitro model. Our results indicated that VGSC activity was eliminated upon infection but steadily recovered during latency establishment and that latent neurons exhibited even higher VGSC activity. This finding advances our knowledge of how ganglion neurons generate uncharacteristic electrical impulses due to abnormal VGSC functional expression influenced by the latent virus.

Neuropathic Pain Induced Alterations in the Opioidergic Modulation of a Descending Pain Facilitatory Area of the Brain

Opioids play a major role at descending pain modulation but the effects of neuropathic pain on the brain opioidergic system remain understudied. Since descending facilitation is enhanced during neuropathic pain, we studied the opioidergic modulation of the dorsal reticular nucleus (DRt), a medullary pain facilitatory area, in the spared nerve injury (SNI) model of neuropathic pain. We first performed a series of behavioral experiments in naïve-animals to establish the role of μ-opioid receptor (MOR) in the effects of endogenous and exogenous opioids at the DRt. Specifically, we showed that lentiviral-mediated MOR-knockdown at the DRt increased sensitivity to thermal and mechanical stimuli while the MOR agonist DAMGO induced the opposite effects. Additionally, we showed that MOR-knockdown and the pharmacological blockade of MOR by CTAP at the DRt decreased and inhibited, respectively, the analgesic effects of systemic morphine. Then, we performed in vivo microdialysis to measure enkephalin peptides in the DRt and evaluated MOR expression in the DRt at mRNA, protein and phosphorylated form levels by quantitative real-time PCR and immunohistochemistry, respectively. SNI-animals, compared to sham control, showed higher levels of enkephalin peptides, lower MOR-labeled cells without alterations in MOR mRNA levels, and higher phosphorylated MOR-labeled cells. Finally, we performed behavioral studies in SNI animals to determine the potency of systemic morphine and the effects of the pharmacologic and genetic manipulation of MOR at the DRt. We showed a reduced potency of the antiallodynic effects of systemic morphine in SNI-animals compared to the antinociceptive effects in sham animals. Increasing MOR-cells at the DRt of SNI-animals by lentiviral-mediated MOR-overexpression produced no effects on mechanical allodynia. DAMGO induced anti-allodynia only after MOR-overexpression. These results show that MOR inhibits DRt pain facilitatory actions and that this action contributes to the analgesic effects of systemic opioids. We further show that the inhibitory function of MOR is impaired during neuropathic pain. This is likely due to desensitization and degradation of MOR which are adaptations of the receptor that can be triggered by MOR phosphorylation. Skipping counter-regulatory pathways involved in MOR adaptations might restore the opioidergic inhibition at pain facilitatory areas.

Reticular Formation and Pain: The Past and the Future

The involvement of the reticular formation (RF) in the transmission and modulation of nociceptive information has been extensively studied. The brainstem RF contains several areas which are targeted by spinal cord afferents conveying nociceptive input. The arrival of nociceptive input to the RF may trigger alert reactions which generate a protective/defense reaction to pain. RF neurons located at the medulla oblongata and targeted by ascending nociceptive information are also involved in the control of vital functions that can be affected by pain, namely cardiovascular control. The RF contains centers that belong to the pain modulatory system, namely areas involved in bidirectional balance (decrease or enhancement) of pain responses. It is currently accepted that the imbalance of pain modulation towards pain facilitation accounts for chronic pain. The medullary RF has the peculiarity of harboring areas involved in bidirectional pain control namely by the existence of specific neuronal populations involved in antinociceptive or pronociceptive behavioral responses, namely at the rostroventromedial medulla (RVM) and the caudal ventrolateral medulla (VLM). Furthermore the dorsal reticular nucleus (also known as subnucleus reticularis dorsalis; DRt) may enhance nociceptive responses, through a reverberative circuit established with spinal lamina I neurons and inhibit wide-dynamic range (WDR) neurons of the deep dorsal horn. The components of the triad RVM-VLM-DRt are reciprocally connected and represent a key gateway for top-down pain modulation. The RVM-VLM-DRt triad also represents the neurobiological substrate for the emotional and cognitive modulation of pain, through pathways that involve the periaqueductal gray (PAG)-RVM connection. Collectively, we propose that the RVM-VLM-DRt triad represents a key component of the “dynamic pain connectome” with special features to provide integrated and rapid responses in situations which are life-threatening and involve pain. The new available techniques in neurobiological studies both in animal and human studies are producing new and fascinating data which allow to understand the complex role of the RF in pain modulation and its integration with several body functions and also how the RF accounts for chronic pain.

The noradrenergic locus coeruleus as a chronic pain generator

Central noradrenergic centers such as the locus coeruleus (LC) are traditionally viewed as pain inhibitory; however, complex interactions among brainstem pathways and their receptors modulate both inhibition and facilitation of pain. In addition to the well-described role of descending pontospinal pathways that inhibit spinal nociceptive transmission, an emerging body of research now indicates that noradrenergic neurons in the LC and their terminals in the dorsal reticular nucleus (DRt), medial prefrontal cortex (mPFC), spinal dorsal horn, and spinal trigeminal nucleus caudalis participate in the development and maintenance of allodynia and hyperalgesia after nerve injury. With time after injury, we argue that the balance of LC function shifts from pain inhibition to pain facilitation. Thus, the pain-inhibitory actions of antidepressant drugs achieved with elevated noradrenaline concentrations in the dorsal horn may be countered or even superseded by simultaneous activation of supraspinal facilitating systems dependent on α₁ -adrenoreceptors in the DRt and mPFC as well as α₂ -adrenoreceptors in the LC. Indeed, these opposing actions may account in part for the limited treatment efficacy of tricyclic antidepressants and noradrenaline reuptake inhibitors such as duloxetine for the treatment of chronic pain. We propose that the traditional view of the LC as a pain-inhibitory structure be modified to account for its capacity as a pain facilitator. Future studies are needed to determine the neurobiology of ascending and descending pathways and the pharmacology of receptors underlying LC-mediated pain inhibition and facilitation. © 2016 Wiley Periodicals, Inc.

GABA acting on GABAB receptors located in a medullary pain facilitatory area enhances nociceptive behaviors evoked by intraplantar formalin injection

The dorsal reticular nucleus (DRt) plays a key role in facilitation of nociceptive transmission at the spinal cord. In this study, we evaluated the mechanisms involved in GABA-mediated control of the DRt focusing on the role of local GABAB receptors. First, we used in vivo microdialysis to study the release of GABA in the DRt during the course of the formalin test. An increase of GABA levels in comparison with baseline values was detected in the second phase of the test. Because we previously showed that GABAB receptors are expressed by opioidergic DRt neurons, which respond to nociceptive stimuli and inhibit spinally projecting DRt neurons involved in descending pronociception, we then interfered with local GABAB receptors using gene transfer and pharmacological approaches. Lentiviral-mediated knockdown of GABAB1a expression decreased nociceptive responses during the second phase of the test. Local administration of the GABAB receptor antagonist CGP 35348 also decreased nociceptive responses in the second phase of the test, whereas the opposite was detected after injection of the GABAB agonist baclofen. Finally, we determined the GABAergic afferents of the DRt, namely those arising from its main brain afferents, which are located at the telencephalon and diencephalon. For that purpose, we combined retrograde tract-tracing from the DRt with immunodetection of glutamate decarboxylase, the GABA-synthesizing enzyme. The higher numbers of retrogradely labelled glutamate decarboxylase-immunoreactive neurons were located at insular, somatosensory, and motor cortices. Collectively, the results suggest that GABA acting on GABAB receptors may enhance pain facilitation from the DRt during inflammatory pain.

Galanin-Mediated Behavioural Hyperalgesia from the Dorsomedial Nucleus of the Hypothalamus Involves Two Independent Descending Pronociceptive Pathways

Activation of the dorsomedial nucleus of the hypothalamus (DMH) by galanin (GAL) induces behavioural hyperalgesia. Since DMH neurones do not project directly to the spinal cord, we hypothesized that the medullary dorsal reticular nucleus (DRt), a pronociceptive region projecting to the spinal dorsal horn (SDH) and/or the serotoninergic raphe-spinal pathway acting on the spinal 5-HT3 receptor (5HT3R) could relay descending nociceptive facilitation induced by GAL in the DMH. Heat-evoked paw-withdrawal latency (PWL) and activity of SDH neurones were assessed in monoarthritic (ARTH) and control (SHAM) animals after pharmacological manipulations of the DMH, DRt and spinal cord. The results showed that GAL in the DMH and glutamate in the DRt lead to behavioural hyperalgesia in both SHAM and ARTH animals, which is accompanied particularly by an increase in heat-evoked responses of wide-dynamic range neurons, a group of nociceptive SDH neurones. Facilitation of pain behaviour induced by GAL in the DMH was reversed by lidocaine in the DRt and by ondansetron, a 5HT3R antagonist, in the spinal cord. However, the hyperalgesia induced by glutamate in the DRt was not blocked by spinal ondansetron. In addition, in ARTH but not SHAM animals PWL was increased after lidocaine in the DRt and ondansetron in the spinal cord. Our data demonstrate that GAL in the DMH activates two independent descending facilitatory pathways: (i) one relays in the DRt and (ii) the other one involves 5-HT neurones acting on spinal 5HT3Rs. In experimental ARTH, the tonic pain-facilitatory action is increased in both of these descending pathways.

Increased Noradrenergic Neurotransmission to a Pain Facilitatory Area of the Brain Is Implicated in Facilitation of Chronic Pain

Background:

Noradrenaline reuptake inhibitors are known to produce analgesia through a spinal action but they also act in the brain. However, the action of noradrenaline on supraspinal pain control regions is understudied. The authors addressed the noradrenergic modulation of the dorsal reticular nucleus (DRt), a medullary pronociceptive area, in the spared nerve injury (SNI) model of neuropathic pain.

Methods:

The expression of the phosphorylated cAMP response element-binding protein (pCREB), a marker of neuronal activation, was evaluated in the locus coeruleus and A5 noradrenergic neurons (n = 6 rats/group). pCREB was studied in noradrenergic DRt-projecting neurons retrogradely labeled in SNI animals (n = 3). In vivo microdialysis was used to measure noradrenaline release in the DRt on nociceptive stimulation or after DRt infusion of clonidine (n = 5 to 6 per group). Pharmacology, immunohistochemistry, and western blot were used to study α-adrenoreceptors in the DRt (n = 4 to 6 per group).

Results:

pCREB expression significantly increased in the locus coeruleus and A5 of SNI animals, and most noradrenergic DRt-projecting neurons expressed pCREB. In SNI animals, noradrenaline levels significantly increased on pinprick (mean ± SD, 126 ± 14%; P = 0.025 vs. baseline) and acetone stimulation (mean ± SD, 151 ± 12%; P < 0.001 vs. baseline), and clonidine infusion showed decreased α2-mediated inhibitory function. α1-adrenoreceptor blockade decreased nociceptive behavioral responses in SNI animals. α2-adrenoreceptor expression was not altered.

Conclusions:

Chronic pain induces brainstem noradrenergic activation that enhances descending facilitation from the DRt. This suggests that antidepressants inhibiting noradrenaline reuptake may enhance pain facilitation from the brain, counteracting their analgesic effects at the spinal cord.

Gene therapy strategies for the treatment of spinal cord injury

Spinal cord injury is a complex pathology often resulting in functional impairment and paralysis. Gene therapy has emerged as a possible solution to the problems of limited neural tissue regeneration through the administration of factors promoting axonal growth, while also offering long-term local delivery of therapeutic molecules at the injury site. Of note, gene therapy is our response to the requirements of neural and glial cells following spinal cord injury, providing, in a time-dependent manner, growth substances for axonal regeneration and eliminating axonal growth inhibitors. Herein, we explore different gene therapy strategies, including targeting gene expression to modulate the presence of neurotrophic growth or survival factors and increase neural tissue plasticity. Special attention is given to describing advances in viral and non-viral gene delivery systems, as well as the available routes of gene delivery. Finally, we discuss the future of combinatorial gene therapies and give consideration to the implementation of gene therapy in humans.

Optoactivation of locus ceruleus neurons evokes bidirectional changes in thermal nociception in rats

Pontospinal noradrenergic neurons are thought to form part of a descending endogenous analgesic system that exerts inhibitory influences on spinal nociception. Using optogenetic targeting, we tested the hypothesis that excitation of the locus ceruleus (LC) is antinociceptive. We transduced rat LC neurons by direct injection of a lentiviral vector expressing channelrhodopsin2 under the control of the PRS promoter. Subsequent optoactivation of the LC evoked repeatable, robust, antinociceptive (+4.7°C ± 1.0, p < 0.0001) or pronociceptive (-4.4°C ± 0.7, p < 0.0001) changes in hindpaw thermal withdrawal thresholds. Post hoc anatomical characterization of the distribution of transduced somata referenced against the position of the optical fiber and subsequent further functional analysis showed that antinociceptive actions were evoked from a distinct, ventral subpopulation of LC neurons. Therefore, the LC is capable of exerting potent, discrete, bidirectional influences on thermal nociception that are produced by specific subpopulations of noradrenergic neurons. This reflects an underlying functional heterogeneity of the influence of the LC on the processing of nociceptive information.

Pronociception from the dorsomedial nucleus of the hypothalamus is mediated by the rostral ventromedial medulla in healthy controls but is absent in arthritic animals

The dorsomedial nucleus of the hypothalamus (DMH) has been proposed to participate in stress-induced hyperalgesia through facilitation of pronociceptive cells in the rostroventromedial medulla (RVM). We hypothesized that the DMH participates in hyperalgesia induced by arthritis. The DMH was pharmacologically manipulated while assessing heat-evoked nociceptive behavior or the discharge rates of pronociceptive RVM ON- and antinociceptive RVM OFF-like cells in NAIVE, SHAM and monoarthritic (ARTH) animals. In NAIVE and SHAM animals, the changes in nociceptive behavior induced by activation of the DMH by glutamate and inhibition by lidocaine were in line with earlier evidence indicating that the DMH has a nociceptive facilitating role. However, in ARTH animals, neither activation nor inhibition of the DMH influenced pain-like behavior evoked by stimulation of an uninflamed skin region (paw and tail). In accordance with these behavioral results, activation or inhibition of the DMH induced pronociceptive changes in the discharge rates of RVM cells in NAIVE and SHAM animals, which suggests that the DMH has a pronociceptive role mediated by the RVM in normal animals. However, in ARTH animals, both glutamate and lidocaine in the DMH failed to influence either pain-like behavior or noxious stimulation-evoked responses of RVM cells, while blocking the DMH increased spontaneous activity in the pronociceptive RVM ON cells. Our data indicate that the DMH participates in descending facilitation of cutaneous nociception in healthy controls, but it is not engaged in the regulation of cutaneous nociception in monoarthritic animals, while a minor role in tonic suppression of nociception in arthritis cannot be discarded.

Role of glutamate receptors in the dorsal reticular nucleus in formalin-induced secondary allodynia

The role of glutamate receptors present in the medullary dorsal reticular nucleus (DRt) in the formalin test and formalin-induced secondary nociception was studied in rats. Secondary mechanical allodynia was assessed with von Frey filaments applied to the rat's hindpaw, and secondary thermal hyperalgesia was evaluated with the tail-immersion test. The selective glutamate receptor antagonists MK801 (N-methyl-D-aspartate receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (AMPA/KA receptor antagonist) and A841720 (metabotropic glutamate 1 receptor antagonist) were injected into the DRt before or 6 days after formalin injection in the rat. In the formalin test, the three antagonists significantly reduced the number of flinches in both phases of the test. DRt microinjection of MK801 or A841720, but not of CNQX, reduced both secondary nociceptive behaviors. Moreover, pre-treatment with the three antagonists injected into the DRt prevented the development of secondary mechanical allodynia and secondary thermal hyperalgesia. Similarly, in these rats, the number of c-Fos-like immunoreactive neurons were markedly reduced in both the superficial and deep lamina of the dorsal horn. Our findings support the role of DRt as a pain facilitator in acute and chronic pain states, and suggest a key role of glutamate receptors during the development and maintenance of formalin-induced secondary allodynia.

Coexistence of ipsilateral pain-inhibitory and facilitatory processes after high-frequency electrical stimulation

BACKGROUND

High-frequency electrical stimulation (HFS) of the human forearm evokes analgesia to blunt pressure in the ipsilateral forehead, consistent with descending ipsilateral inhibitory pain modulation. The aim of the current study was to further delineate pain modulation processes evoked by HFS by examining sensory changes in the arm and forehead; investigating the effects of HFS on nociceptive blink reflexes elicited by supraorbital electrical stimulation; and assessing effects of counter-irritation (electrically evoked pain at the HFS-conditioned site in the forearm) on nociceptive blink reflexes before and after HFS.

METHODS

Before and after HFS conditioning, sensitivity to heat and to blunt and sharp stimuli was assessed at and adjacent to the conditioned site in the forearm and on each side of the forehead. Nociceptive blink reflexes were also assessed before and after HFS with and without counter-irritation of the forearm.

RESULTS

HFS triggered secondary hyperalgesia in the forearm (a sign of central sensitization) and analgesia to blunt pressure in the ipsilateral forehead. Under most conditions, both HFS conditioning and counter-irritation of the forearm suppressed electrically evoked pain in the forehead, and the amplitude of the blink reflex to supraorbital stimuli decreased. Importantly, however, in the absence of forearm counter-irritation, HFS conditioning facilitated ipsilateral blink reflex amplitude to supraorbital stimuli delivered ipsilateral to the HFS-conditioned site.

CONCLUSIONS

These findings suggest that HFS concurrently triggers hemilateral inhibitory and facilitatory influences on nociceptive processing over and above more general effects of counter-irritation. The inhibitory influence may help limit the spread of sensitization in central nociceptive pathways.

How to erase memory traces of pain and fear

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Currently emerging concepts of maladaptive pain and fear suggest that they share basic neuronal circuits and cellular mechanisms of memory formation.

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Recent studies have revealed processes of erasing memory traces of pain and fear that may be promising targets for future therapies.

Pain and fear are both aversive experiences that strongly impact on behaviour and well being. They are considered protective when they lead to meaningful, adaptive behaviour such as the avoidance of situations that are potentially dangerous to the integrity of tissue (pain) or the individual (fear). Pain and fear may, however, become maladaptive if expressed under inappropriate conditions or at excessive intensities for extended durations. Currently emerging concepts of maladaptive pain and fear suggest that basic neuronal mechanisms of memory formation are relevant for the development of pathological forms of pain and fear. Thus, the processes of erasing memory traces of pain and fear may constitute promising targets for future therapies.

Phosphorylated CaMKII levels increase in rat central nervous system after large-dose intravenous remifentanil

Background

Postoperative remifentanil-induced pain sensitization is common, but its molecular mechanism remains unclear. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been shown to have a critical role in morphine-induced hyperalgesia. This study was designed to determine how CaMKII phosphorylation and protein expression levels change in the central nervous system of rats with remifentanil-induced hyperalgesia.

Material/Methods

Male Sprague-Dawley^® rats were exposed to large-dose (bolus of 6.0 μg/kg and 2.5 μg/kg/min for 2 hours) intravenous remifentanil to induce post-transfusion hyperalgesia. Levels of phosphorylated CaMKII (P-CaMKII) and total protein of CaMKII (T-CaMKII) were determined at different post-transfusion times by Western blot and immunostaining and were compared with controls.

Results

P-CaMKII increased significantly (P<0.05) at 0, 0.5, and 2 hours. However, P-CaMKII at 5 to 24 hours and T-CaMKII at 0 to 24 hours post-transfusion did not change significantly in rats’ spinal dorsal horn, hippocampus, or primary somatosensory (S1) cortex (n=6 per group). Similarly, immunostaining showed stronger P-CaMKII immunoreactants (P<0.05) and more P-CaMKII- positive cells (P<0.05) in the spinal dorsal horn, CA1 region of the hippocampus, and S1 cortex of rats 0.5 hours post-transfusion compared with the control group treated with 0.9% sodium chloride (n=3 per group).

Conclusions

These results suggest that a temporary rise in the P-CaMKII level in the central nervous system may correlate with remifentanil-induced pain sensitization in the postoperative period.

Gene-based approaches in pain research and exploration of new therapeutic targets and strategies

Large panel of gene-based techniques is used for many years specifically in the pain research field. From the first identification (cloning) of some "mythic" genes, such as those encoding opioid or capsaicin receptors allowing then the creation of first-generation knockout mice, to the today conditional (time, tissue, cell-type and even pathology-dependent) and regulatable modulation of a gene function, these approaches largely contributed to fundamental leaps forward in our understanding of the function of some proteins and of their interest as possible druggable targets. Perhaps one of the most remarkable evolution in the last years is the passage of these approaches from the bench to the patient; whether it concerns the identification of genes involved in inherited pain insensibility/susceptibility, the search for genetic markers of pain types, the individual pharmacogenomics or even the first gene therapy trials. From many possible variants of gene-grounded techniques used in pain research we focus here on gene knockouts and some recent developments, on viral vectors-based gene transfer and on transgenic models for the tracing of pain pathways. Through these selected examples we attempted to emphasize the immense potential of these approaches and their already well-recognized contribution in both the basic and clinical pain research.

A possible role of the locus coeruleus in complex regional pain syndrome

Heightened sensitivity to painful stimulation commonly spreads from the affected limb to the ipsilateral forehead in patients with complex regional pain syndrome (CRPS). In addition, acoustic startle evokes greater auditory discomfort and increases in limb pain when presented on the affected than unaffected side. In contrast, limb pain ordinarily evokes analgesia in the ipsilateral forehead of healthy participants, and acoustic startle suppresses limb pain. Together, these findings suggest that hemilateral and generalized pain control mechanisms are disrupted in CRPS, and that multisensory integrative processes are compromised. Failure to inhibit nociceptive input from the CRPS-affected limb could sensitize spinal and supraspinal neurons that receive convergent nociceptive and auditory information from hemilateral body sites. Somatosensory, auditory, and emotional inputs may then aggravate pain by feeding into this sensitized nociceptive network. In particular, a disturbance in hemilateral pain processing that involves the locus coeruleus could exacerbate the symptoms of CRPS in some patients.

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.

Decrease in the expression of N-methyl-D-aspartate receptors in the nucleus tractus solitarii induces antinociception and increases blood pressure

N-methyl-D-aspartate receptors (NMDAR) have a role in cardiovascular control at the nucleus tractus solitarii (NTS), eliciting increases or decreases in blood pressure (BP), depending on the area injected with the agonists. In spite of the association between cardiovascular control and pain modulation, the effects of manipulating NMDAR in pain responses have never been evaluated. In this study, we decreased the expression of NMDAR in the NTS using gene transfer to target receptor subunits and evaluate long-term effects. Seven days after the injection of lentiviral vectors containing the NR1a subunit cDNA of NMDAR, in antisense orientation, into the intermediate NTS of Wistar rats, BP was measured, and the formalin test of nociception was performed. The antisense vector induced a decrease of NR1 expression in the NTS and elicited BP rises and hypoalgesia. Antisense vectors inhibited formalin-evoked c-Fos expression in the spinal cord, indicating decreased nociceptive activity of spinal neurons. Using a time-course approach, we verified that the onset of both the increases in BP and the hypoalgesia was at 4 days after vector injection into the NTS. The injection of NMDA into the NTS reversed the effects of antisense vectors in pain behavioral responses and spinal neuronal activation and decreased BP and heart rate. The present study shows that the NR1 subunit of the NMDAR at the NTS is critical in the regulation of tonic cardiovascular and nociceptive control and shows an involvement of the nucleus in the modulation of sustained pain.

Gene therapy for chronic neuropathic pain: how does it work and where do we stand today?

OBJECTIVES

Chronic neuropathic pain has been an enigma to physicians and researchers for decades. A better understanding of its pathophysiology has given us more insight into its various mechanisms and possible treatment options. We now have an understanding of the role of various ionic channels, biologically active molecules involved in pain, and also the intricate pain pathways where possible interventions might lead to substantial pain relief. The recent research on laboratory animals using virus-based vectors for gene transfer at targeted sites is very promising and may lead to additional human clinical trials. However, one needs to be aware that this "novel" approach is still in its infancy and that many of its details need to be further elucidated. The purpose of this article is to thoroughly review the current available literature and analyze the deficiencies in our current knowledge.

DESIGN

Literature review.

METHODS

After an extensive online literature search, a total of 133 articles were selected to synthesize a comprehensive review about chronic neuropathic pain and gene therapy in order to understand the concepts and mechanisms.

RESULTS

Most of the studies have shown benefits of gene therapy in animal models, and recently, phase 1 human trials using herpes simplex virus vector have started for intractable cancer pain.

CONCLUSION

Although animal data have shown safety and efficacy, and initial human trials have been promising, additional studies in humans are required to more completely understand the actual benefits and risks of using gene therapy for the treatment of chronic neuropathic pain.

Inhibition of nociceptive responses after systemic administration of amidated kyotorphin

BACKGROUND AND PURPOSE

Kyotorphin (KTP; L-Tyr-L-Arg), an endogenous neuropeptide, is potently analgesic when delivered directly to the central nervous system. Its weak analgesic effects after systemic administration have been explained by inability to cross the blood-brain barrier (BBB) and detract from the possible clinical use of KTP as an analgesic. In this study, we aimed to increase the lipophilicity of KTP by amidation and to evaluate the analgesic efficacy of a new KTP derivative (KTP-amide - KTP-NH(2) ).

EXPERIMENTAL APPROACH

We synthesized KTP-NH(2) . This peptide was given systemically to assess its ability to cross the BBB. A wide range of pain models, including acute, sustained and chronic inflammatory and neuropathic pain, were used to characterize analgesic efficacies of KTP-NH(2) . Binding to opioid receptors and toxicity were also measured.

KEY RESULTS

KTP-NH(2) , unlike its precursor KTP, was lipophilic and highly analgesic following systemic administration in several acute and chronic pain models, without inducing toxic effects or affecting motor responses and blood pressure. Binding to opioid receptors was minimal. KTP-NH(2) inhibited nociceptive responses of spinal neurons. Its analgesic effects were prevented by intrathecal or i.p. administration of naloxone.

CONCLUSIONS AND IMPLICATIONS

Amidation allowed KTP to show good analgesic ability after systemic delivery in acute and chronic pain models. The indirect opioid-mediated actions of KTP-NH(2) may explain why this compound retained its analgesic effects although the usual side effects of opioids were absent, which is a desired feature in next-generation pain medications.

HSV delivery of a ligand-regulated endogenous ion channel gene to sensory neurons results in pain control following channel activation

Persistent pain remains a tremendous health problem due to both its prevalence and dearth of effective therapeutic interventions. To maximize pain relief while minimizing side effects, current gene therapy-based approaches have mostly exploited the expression of pain inhibitory products or interfered with pronociceptive ion channels. These methods do not enable control over the timing or duration of analgesia, nor titration to analgesic efficacy. Here, we describe a gene therapy strategy that potentially overcomes these limitations by providing exquisite control over therapy with efficacy in clinically relevant models of inflammatory pain. We utilize a herpes simplex viral (HSV) vector (vHGlyRα1) to express a ligand-regulated chloride ion channel, the glycine receptor (GlyR) in targeted sensory afferents; the subsequent exogenous addition of glycine provides the means for temporal and spatial control of afferent activity, and therefore pain. Use of an endogenous inhibitory receptor not normally present on sensory neurons both minimizes immunogenicity and maximizes therapeutic selectivity.