Antinociception mediated by alpha(2)-adrenergic activation involves increasing tumor necrosis factor alpha (TNFalpha) expression and restoring TNFalpha and alpha(2)-adrenergic inhibition of norepinephrine release

Common Pathways for Pain and Depression-Implications for Practice

BACKGROUND

Pain and depression have a high impact on caring for the people who need palliative care, but both of these are neglected compared with the approach for other symptoms encountered by these patients.

AREAS OF UNCERTAINTY

There are few studies in humans that support the existence of common neural circuits between depression and pain that also explore the use of drugs with effects in both conditions. More knowledge is needed about the relationship of these clinical entities that will lead to the optimization of the treatment and improvement of quality of life.

DATA SOURCES

We conducted a search in PubMed to identify relevant articles and reviews that have been published in the last 5 years, concerning the topic of common pathways between depression and pain (2014-April 2019).

THERAPEUTIC ADVANCES

The connections between the 2 clinical entities start at the level of the cortical regions. The hippocampus is the main site of neural changes, modification of the immune system, neuromodulators, neurotransmitters, and signaling pathways implicated in both conditions. Increased levels of peripheral proinflammatory cytokines and neuroinflammatory changes are related to the physiopathology of these entities. Inflammation links depression and pain by altering neural circuits and changes in their common cortical regions. Antidepressants are used to treat depression and chronic, pain but more experimental studies are needed to determine which antidepressant drugs are the most effective in treating the 2 entities.

CONCLUSIONS

Pharmacological and nonpharmacological interventions targeting cortical changes in pain and depression are promising, but more clinical studies are needed to validate their usefulness.

Neuroinflammation, Pain and Depression: An Overview of the Main Findings

Chronic pain is a serious public health problem with a strong affective-motivational component that makes it difficult to treat. Most patients with chronic pain suffer from severe depression; hence, both conditions coexist and exacerbate one another. Brain inflammatory mediators are critical for maintaining depression-pain syndrome and could be substrates for it. The goal of our paper was to review clinical and preclinical findings to identify the neuroinflammatory profile associated with the cooccurrence of pain and depression. In addition, we aimed to explore the regulatory effect of neuronal reorganization on the inflammatory response in pain and depression. We conducted a quantitative review supplemented by manual screening. Our results revealed inflammatory signatures in different preclinical models and clinical articles regarding depression-pain syndrome. We also identified that improvements in depressive symptoms and amelioration of pain can be modulated through direct targeting of inflammatory mediators, such as cytokines and molecular inhibitors of the inflammatory cascade. Additionally, therapeutic targets that improve and regulate the synaptic environment and its neurotransmitters may act as anti-inflammatory compounds, reducing local damage-associated molecular patterns and inhibiting the activation of immune and glial cells. Taken together, our data will help to better elucidate the neuroinflammatory profile in pain and depression and may help to identify pharmacological targets for effective management of depression-pain syndrome.

Targeting tumor necrosis factor in the brain relieves neuropathic pain

Neuropathic pain is a chronic syndrome caused by direct damage to or disease of the somatosensory nervous system. The lack of safe, adequate and sustained pain relief offered by present analgesic treatments is most alarming. While many treatment options are available to manage chronic pain, such as antidepressants, non-steroidal anti-inflammatory agents, opioids, and anticonvulsants, chronic neuropathic pain remains largely unmanaged. Compounding the dilemma of ineffective chronic pain treatments is the need to provide relief from suffering and yet not contribute to the scourge of drug abuse. A recent epidemic of addiction and accidental drug prescription overdoses parallel the increased use of opioid treatment, even though opioids are rarely an effective treatment of relieving chronic pain. To make matters worse, opioids may contribute to exacerbating pain, and side-effects such as cognitive impairment, nausea, constipation, development of tolerance, as well as their potential for addiction and overdose deaths exist. Clearly, there is an urgent need for alternative, non-opiate treatment of chronic pain. Innovative discoveries of pertinent brain mechanisms and functions are key to developing effective, safe treatments. Pioneering work has revealed the essential effects of the pleiotropic mediator tumor necrosis factor (TNF) on brain functioning. These studies establish that TNF inhibits norepinephrine release from hippocampal neurons, and show that excess TNF production within the hippocampus occurs during neuropathic pain, which mobilizes additional mechanisms that further inhibit norepinephrine release. Significantly, it has been verified that elevated levels of TNF in the brain are actually required for neuropathic pain development. Since TNF decreases norepinephrine release in the brain, enhanced TNF levels would prevent engagement of the norepinephrine descending inhibitory neuronal pain pathways. Increased levels of TNF in the brain are therefore critical to the development of neuropathic pain. Therefore, strategies that decrease this enhanced TNF expression in the brain will have superior analgesic efficacy. We propose this novel approach of targeting the pathologically high levels of brain TNF as an effective strategy in the treatment of the devastating syndrome of chronic pain.

Are the emergence of affective disturbances in neuropathic pain states contingent on supraspinal neuroinflammation?

Neuro-immune interactions contribute to the pathogenesis of neuropathic pain due to peripheral nerve injury. A large body of preclinical evidence supports the idea that the immune system acts to modulate the sensory symptoms of neuropathy at both peripheral and central nervous system sites. The potential involvement of neuro-immune interactions in the highly debilitating affective disturbances of neuropathic pain, such as depression, anhedonia, impaired cognition and reduced motivation has received little attention. This is surprising given the widely accepted view that sickness behaviour, depression, cognitive impairment and other neuropsychiatric conditions can arise from inflammatory mechanisms. Moreover, there is a set of well-described immune-to-brain transmission mechanisms that explain how peripheral inflammation can lead to supraspinal neuroinflammation. In the last 5years increasing evidence has emerged that peripheral nerve injury induces supraspinal changes in cytokine or chemokine expression and alters glial cell activity. In this systematic review, based on strong preclinical evidence, we advance the argument that the emergence of affective disturbances in neuropathic pain states are contingent on pro-inflammatory mediators in the interconnected hippocampal-medial prefrontal circuitry that subserve affective behaviours. We explore how dysregulation of inflammatory mediators in these networks may result in affective disturbances through a wide variety of neuromodulatory mechanisms. There are also promising results from clinical trials showing that anti-inflammatory agents have efficacy in the treatment of a variety of neuropsychiatric conditions including depression and appear suited to sub-groups of patients with elevated pro-inflammatory profiles. Thus, although further research is required, aggressively targeting supraspinal pro-inflammatory mediators at critical time-points in appropriate clinical populations is likely to be a novel avenue to treat debilitating affective disturbances in neuropathic conditions.

Effects of alpha-7 nicotinic acetylcholine receptor positive allosteric modulator on lipopolysaccharide-induced neuroinflammatory pain in mice

Evidence indicates that microglial activation contributes to the pathophysiology and maintenance of neuroinflammatory pain involving central nervous system alpha-7 nicotinic acetylcholine receptors. The objective of the present study was to determine the effects of 3a,4,5,9b-Tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS), an alpha-7 nicotinic acetylcholine receptor positive allosteric modulator (PAM), on tactile allodynia and thermal hyperalgesia following lipopolysaccharide (LPS)-induced microglial activation in hippocampus, a neuroinflammatory pain model in mice. In addition, we examined the effects of TQS on microglial activation marker, an ionized calcium-binding adapter molecule 1 (Iba-1), in the hippocampus may be associated with neuroinflammatory pain. Pretreatment of TQS (4mg/kg) significantly reduced LPS (1mg/kg)-induced tactile allodynia and thermal hyperalgesia. Moreover, pretreatment of methyllycaconitine (3mg/kg) significantly reversed TQS-induced antiallodynic and antihyperalgesic responses indicating the involvement of alpha-7 nicotinic acetylcholine receptor. Pretreatment of TQS significantly decreased LPS-induced increased in hippocampal Iba-1 expression. Overall, these results suggest that TQS reduces LPS-induced neuroinflammatory pain like symptoms via modulating microglial activation likely in the hippocampus and/or other brain region by targeting alpha-7 nicotinic acetylcholine receptor. Therefore, alpha-7 nicotinic acetylcholine receptor PAM such as TQS could be a potential drug candidate for the treatment of neuroinflammatory pain.

Chronic constriction injury-induced nociception is relieved by nanomedicine-mediated decrease of rat hippocampal tumor necrosis factor

Neuropathic pain is a chronic pain syndrome that arises from nerve injury. Current treatments only offer limited relief, clearly indicating the need for more effective therapeutic strategies. Previously, we demonstrated that proinflammatory tumor necrosis factor-alpha (TNF) is a key mediator of neuropathic pain pathogenesis; TNF is elevated at sites of neuronal injury, in the spinal cord, and supraspinally during the initial development of pain. The inhibition of TNF action along pain pathways outside higher brain centers results in transient decreases in pain perception. The objective of this study was to determine whether specific blockade of TNF in the hippocampus, a site of pain integration, could prove efficacious in reducing sciatic nerve chronic constriction injury (CCI)-induced pain behavior. Small inhibitory RNA directed against TNF mRNA was complexed to gold nanorods (GNR-TNF siRNA; TNF nanoplexes) and injected into the contralateral hippocampus of rats 4 days after unilateral CCI. Withdrawal latencies to a noxious thermal stimulus (hyperalgesia) and withdrawal to innocuous forces (allodynia) were recorded up to 10 days and compared with baseline values and sham-operated rats. Thermal hyperalgesia was dramatically decreased in CCI rats receiving hippocampal TNF nanoplexes; and mechanical allodynia was transiently relieved. TNF levels (bioactive protein, TNF immunoreactivity) in hippocampal tissue were decreased. The observation that TNF nanoplex injection into the hippocampus alleviated neuropathic pain-like behavior advances our previous findings that hippocampal TNF levels modulate pain perception. These data provide evidence that targeting TNF in the brain using nanoparticle-protected siRNA may be an effective strategy for treatment of neuropathic pain.

The hippocampus and TNF: Common links between chronic pain and depression

Major depression and chronic pain are significant health problems that seriously impact the quality of life of affected individuals. These diseases that individually are difficult to treat often co-exist, thereby compounding the patient's disability and impairment as well as the challenge of successful treatment. The development of efficacious treatments for these comorbid disorders requires a more comprehensive understanding of their linked associations through common neuromodulators, such as tumor necrosis factor-α (TNFα), and various neurotransmitters, as well as common neuroanatomical pathways and structures, including the hippocampal brain region. This review discusses the interaction between depression and chronic pain, emphasizing the fundamental role of the hippocampus in the development and maintenance of both disorders. The focus of this review addresses the hypothesis that hippocampal expressed TNFα serves as a therapeutic target for management of chronic pain and major depressive disorder (MDD).

Increasing TNF levels solely in the rat hippocampus produces persistent pain-like symptoms

The manifestation of chronic, neuropathic pain includes elevated levels of the cytokine tumor necrosis factor-alpha (TNF). Previously, we have shown that the hippocampus, an area of the brain most notable for its role in learning and memory formation, plays a fundamental role in pain sensation. Using an animal model of peripheral neuropathic pain, we have demonstrated that intracerebroventricular infusion of a TNF antibody adjacent to the hippocampus completely alleviated pain. Furthermore, intracerebroventricular infusion of rTNF adjacent to the hippocampus induced pain behavior in naïve animals similar to that expressed during a model of neuropathic pain. These data support our premise that enhanced production of hippocampal-TNF is integral in pain sensation. In the present study, TNF gene expression was induced exclusively in the hippocampus, eliciting increased local bioactive TNF levels, and animals were assessed for pain behaviors. Male Sprague-Dawley rats received stereotaxic injection of gold nanorod (GNR)-complexed cDNA (control or TNF) plasmids (nanoplasmidexes), and pain responses (i.e., thermal hyperalgesia and mechanical allodynia) were measured. Animals receiving hippocampal microinjection of TNF nanoplasmidexes developed thermal hyperalgesia bilaterally. Sensitivity to mechanical stimulation also developed bilaterally in the rat hind paws. In support of these behavioral findings, immunoreactive staining for TNF, bioactive levels of TNF, and levels of TNF mRNA per polymerase chain reaction analysis were assessed in several brain regions and found to be increased only in the hippocampus. These findings indicate that the specific elevation of TNF in the hippocampus is not a consequence of pain, but in fact induces these behaviors/symptoms.

Peripheral nerve injury leads to working memory deficits and dysfunction of the hippocampus by upregulation of TNF-α in rodents

Patients with chronic pain usually suffer from working memory deficits, which may decrease their intellectual ability significantly. Despite intensive clinical studies, the mechanism underlying this form of memory impairment remains elusive. In this study, we investigated this issue in the spared nerve injury (SNI) model of neuropathic pain, a most common form of chronic pain. We found that SNI impaired working memory and short-term memory in rats and mice. To explore the potential mechanisms, we studied synaptic transmission/plasticity in hippocampus, a brain region critically involved in memory function. We found that frequency facilitation, a presynaptic form of short-term plasticity, and long-term potentiation at CA3-CA1 synapses were impaired after SNI. Structurally, density of presynaptic boutons in hippocampal CA1 synapses was reduced significantly. At the molecular level, we found that tumor necrosis factor-α (TNF-α) increased in cerebrospinal fluid, in hippocampal tissue and in plasma after SNI. Intracerebroventricular or intrahippocampal injection of recombinant rat TNF mimicked the effects of SNI in naive rats, whereas inhibition of TNF-α or genetic deletion of TNF receptor 1 prevented both memory deficits and synaptic dysfunction induced by SNI. As TNF-α is critical for development of neuropathic pain, we suggested that the over-production of TNF-α following peripheral nerve injury might lead to neuropathic pain and memory deficits, simultaneously.

The clinical and laboratorial evaluation of transdermal ketamine, fentanyl, clonidine or their combination in chronic low back pain

OBJECTIVES: chronic low back pain may result in central sensitization, with involvement of different receptors. The aim of this study was to evaluate the analgesic action of transdermal (T) ketamine (a NMDA antagonist), clonidine (an α2-agonist), fentanyl (an opioid agonist), or their combination in chronic low back pain. METHODS: after the institutional approval and informed consent signature, 54 patients were prospectively randomized into 6 groups. Each patient had two of the T preparations applied in different arms. The effect of either T ketamine (1 mg/h), T clonidine (25 µg/h) or T fentanyl (25 µg/h), combined with T placebo (CloG, KetG and FenG); or the combination of T ketamine and clonidine (Ket-CloG), T fentanyl and ketamine (Fen-KetG), or T fentanyl and clonidine (Fen-CloG) was searched for pain and adverse effects. Pain was evaluated by: 1) VAS pain scores, and 2) noradrenaline plasma levels at 0-h (just prior to T application), 3- and 6-h after the T application of two medications, by HPLC. RESULTS: clinically, the pain VAS score at 6-h was smaller in comparison to the 0-h in all groups (p<0.02), and lower when compared to the Fen-CloG and Fen-KetG at the 6-h in relation to the administration of each correspondent T drug alone (p<0.05). The laboratorial data revealed that administration of T fentanyl alone (FenG) resulted in plasma noradrenaline decrease at 6-h (p<0.01), while the association of T fentanyl with clonidine resulted in plasma noradrenaline decrease at 3- and 6-h as compared to the others (p<0.01). The combination of both T ketamine and clonidine (Ket-CloG) did not result in a better analgesic profile and resulted in excessive sedation during the evaluation (p<0.02). CONCLUSIONS: all the studied drugs resulted in clinical analgesia (VAS) at 6-h. However, T fentanyl analgesia was corroborated by lower plasma noradrenaline levels at 6-h when applied alone or at 3-h when combined with T clonidine.

Experimental autoimmune encephalomyelitis-induced upregulation of tumor necrosis factor-alpha in the dorsal root ganglia

Background: Multiple sclerosis (MS) is a chronic, neurological disease characterized by targeted destruction of central nervous system (CNS) myelin. The autoimmune theory is the most widely accepted explanation of disease pathology. Circulating Th1 cells become activated by exposure to CNS-specific antigens such as myelin basic protein. The activated Th1 cells secrete inflammatory cytokines, which are pivotal for inflammatory responses. We hypothesize that enhanced production of inflammatory cytokines triggers cellular events within the dorsal root ganglia (DRG) and/or spinal cord, facilitating the development of neuropathic pain (NPP) in MS. NPP, the second worst disease-induced symptom suffered by patients with MS, is normally regulated by DRG and/or spinal cord.

Objective: To determine gene and protein expression levels of tumor necrosis factor-alpha (TNFα) within DRG and/or spinal cord in an animal model of MS.

Methods: Experimental autoimmune encephalomyelitis (EAE) was induced in adolescent female Lewis rats. Animals were sacrificed every 3 days post-disease induction. DRG and spinal cords were harvested for protein and gene expression analysis.

Results: We show significant increases in TNFα expression in the DRG and of EAE animals at peak disease stage, as assessed by clinical symptoms.

Conclusion: Antigen-induced production of inflammatory cytokines such as TNFα within the DRG identifies a potential novel mechanism for MS-induced NPP.

Adjuvant therapy with intrathecal clonidine improves postoperative pain in patients undergoing coronary artery bypass graft

BACKGROUND

Alpha2 adrenergic agonists have long been employed as analgesics and to sedate patients undergoing surgical procedures. In addition, their therapeutic response synergizes that elicited by opioids. Although this response is well known, the role of alpha2 agonists, such as clonidine, during various painful surgical procedures remains to be elucidated. The goal of our study was to evaluate the effects of the intrathecal administration of clonidine on postoperative pain control and time to extubation in patients undergoing coronary artery bypass grafting.

METHODS

Eighty-five patients undergoing coronary artery bypass grafting randomly received either an intrathecal injection of preservative free morphine 0.5 mg (MOR) or a combination of morphine 0.5 mg and clonidine 100 microg (CMC) before induction of anesthesia. Anesthesia was induced and maintained using a balanced anesthesia technique. Patients were transferred to the intensive care unit while intubated and weaned from mechanical ventilation following an established weaning protocol. Postoperative pain, opioid use within the first 24 hours, and time to extubation were used as primary outcome variables. Data were analyzed by a 2-tailed t test for continuous variables and Fisher exact test for nonparametric variables.

RESULTS

There were no demographic differences between the CMC and MOR groups. Postoperative pain, as assessed by a visual analog scale, was milder in the CMC group when compared with that of the MOR group (2.2+/-0.36 vs. 3.4+/-0.33, P<0.05). Similarly, patients in the CMC group required lower doses of morphine within 24 hours compared with the MOR group (2.02+/-0.36 vs. 6.47+/-0.49 mg, P<0.0001). Time to extubation was significantly shorter in patients receiving CMC than in those who received MOR (592+/-52 vs. 887+/-75 min, P<0.05). There was no mortality in either group. There was a trend for increased vasopressin use in the CMC group compared with the MOR group, although this was not statistically significant (P=0.07).

CONCLUSIONS

Addition of clonidine to neuraxial opioids improves the quality of analgesia postoperatively and expedites the process of weaning from mechanical ventilation. There were no serious adverse events in the cohort of the patients studied. However, the safety profile of this medication remains to be examined with a larger group of patients.

Antinociception occurs with a reversal in alpha 2-adrenoceptor regulation of TNF production by peripheral monocytes/macrophages from pro- to anti-inflammatory

Tumor necrosis factor-alpha (TNF) plays a role in neuropathic pain. During neuropathic pain development in the chronic constriction injury model, elevated TNF levels in the brain occur in association with enhanced alpha 2-adrenoceptor inhibition of norepinephrine release. alpha 2-Adrenoceptors are also located on peripheral macrophage where they normally function as pro-inflammatory, since they increase the production of the cytokine TNF, a proximal mediator of inflammation. How the central increase in TNF affects peripheral alpha 2-adrenoceptor function was investigated. Male, Sprague-Dawley rats had four loose ligatures placed around the right sciatic nerve. Thermal hyperalgesia was determined by comparing hind paw withdrawal latencies between chronic constriction injury and sham-operated rats. Chronic constriction injury increased TNF immunoreactivity at the lesion and the hippocampus. Amitriptyline, an antidepressant that is used as an analgesic, was intraperitoneally administered (10 mg/kg) starting simultaneous with ligature placement (day-0) or at days-4 or -6 post-surgery. Amitriptyline treatment initiated at day-0 or day-4 post-ligature placement alleviated hyperalgesia. When initiated at day-0, amitriptyline prevented increased TNF immunoreactivity in the hippocampus and at the lesion. A peripheral inflammatory response, macrophage production of TNF, was also assessed in the current study. Lipopolysaccharide (LPS)-stimulated production of TNF by whole blood cells and peritoneal macrophages was determined following activation of the alpha 2-adrenoceptor in vitro. alpha 2-Adrenoceptor regulation of TNF production from peripheral immune-effector cells reversed from potentiation in controls to inhibition in chronic constriction injured rats. This effect is accelerated with amitriptyline treatment initiated at day-0 or day-4 post-ligature placement. Amitriptyline treatment initiated day-6 post-ligature placement did not alleviate hyperalgesia and prevented the switch from potentiation to inhibition in alpha 2-adrenoceptor regulation of TNF production. Recombinant rat TNF i.c.v. microinfusion reproduces the response of peripheral macrophages from rats with chronic constriction injury. A reversal in peripheral alpha 2-adrenoceptor regulation of TNF production from pro- to anti-inflammatory is associated with effective alleviation of thermal hyperalgesia. Thus, alpha 2-adrenoceptor regulation of peripheral TNF production may serve as a potential biomarker to evaluate therapeutic regimens.

Anticytokine therapy in neuropathic pain management

Cytokine activation or dysregulation is implied in a variety of painful disease states. Numerous experimental studies provide evidence that proinflammatory cytokines induce or facilitate neuropathic pain. Cytokine levels are rapidly and markedly upregulated in the peripheral nerves, dorsal root ganglia, spinal cord and in particular regions of the brain, after peripheral nerve injuries. Direct receptor-mediated actions on afferent nerve fibers as well as cytokine effects involving further mediators have been reported. Whereas direct application of exogenous proinflammatory cytokines induces pain, blockade of these cytokines or application of anti-inflammatory cytokines reduces pain behavior in most experimental paradigms. Cytokine measurements may identify patients at risk of developing chronic pain associated with their neuropathic conditions, as in the examples of peripheral neuropathies and postherpetic neuralgia. Anticytokine agents currently on the market are effective for the treatment of mostly inflammatory pain conditions, and are starting to be introduced for neuropathic pain states; however, their use is limited by potential life-threatening complications. Owing to the pleiotropy and redundancy of the cytokine system, the successful approach may not be inhibition of one particular cytokine but strategies shifting the balance between pro- and anti-inflammatory cytokines in properly selected patients. Agents that specifically target downstream signaling molecules may provide hope for safer and more specific therapies.