Monoaminergic mechanisms of stimulation-produced analgesia

Cellular and circuit diversity determines the impact of endogenous opioids in the descending pain modulatory pathway

The descending pain modulatory pathway exerts important bidirectional control of nociceptive inputs to dampen and/or facilitate the perception of pain. The ventrolateral periaqueductal gray (vlPAG) integrates inputs from many regions associated with the processing of nociceptive, cognitive, and affective components of pain perception, and is a key brain area for opioid action. Opioid receptors are expressed on a subset of vlPAG neurons, as well as on both GABAergic and glutamatergic presynaptic terminals that impinge on vlPAG neurons. Microinjection of opioids into the vlPAG produces analgesia and microinjection of the opioid receptor antagonist naloxone blocks stimulation-mediated analgesia, highlighting the role of endogenous opioid release within this region in the modulation of nociception. Endogenous opioid effects within the vlPAG are complex and likely dependent on specific neuronal circuits activated by acute and chronic pain stimuli. This review is focused on the cellular heterogeneity within vlPAG circuits and highlights gaps in our understanding of endogenous opioid regulation of the descending pain modulatory circuits.

Comparison of pulp sensibility test responses in normotensive and hypertensive individuals: A clinical study

Background

Pulp sensibility testing is an essential part of the diagnostic process in the assessment of pulpal health. Several lines of evidence suggest an interaction between control of blood pressure and pain regulatory mechanisms.

Aim

The aim of the study is to compare pulp sensibility test responses in normotensive and hypertensive individuals.

Materials and Methods

Ninety-eight patients participated in the study, with 49 individuals each in the hypertensive and normotensive groups. A minimum of 4 and maximum of 8 sound teeth were included in the study each from the anterior, premolar, and molar, i.e., 4 teeth from either arch. A total of 832 teeth were tested. The value and time when the responses evoked for electric pulp test (EPT) and cold test were recorded respectively.

Statistical Analysis

For intergroup and intragroup analyses, independent t-test and paired t-test were utilized.

Results

A statistically significant difference was noted in values for EPT as well as cold test responses when both the groups were compared (P < 0.01). Higher values were obtained with the hypertensive group.

Conclusion

Patients with established hypertension showed an increased threshold to electric pulp testing and cold stimulus as compared to normal healthy individuals.

Paracetamol is a centrally acting analgesic using mechanisms located in the periaqueductal grey

BACKGROUND AND PURPOSE

We previously demonstrated that paracetamol has to be metabolised in the brain by fatty acid amide hydrolase enzyme into AM404 (N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide) to activate CB₁ receptors and TRPV1 channels, which mediate its analgesic effect. However, the brain mechanisms supporting paracetamol-induced analgesia remain unknown.

EXPERIMENTAL APPROACH

The effects of paracetamol on brain function in Sprague-Dawley rats were determined by functional MRI. Levels of neurotransmitters in the periaqueductal grey (PAG) were measured using in vivo ¹ H-NMR and microdialysis. Analgesic effects of paracetamol were assessed by behavioural tests and challenged with different inhibitors, administered systemically or microinjected in the PAG.

KEY RESULTS

Paracetamol decreased the connectivity of major brain structures involved in pain processing (insula, somatosensory cortex, amygdala, hypothalamus, and the PAG). This effect was particularly prominent in the PAG, where paracetamol, after conversion to AM404, (a) modulated neuronal activity and functional connectivity, (b) promoted GABA and glutamate release, and (c) activated a TRPV1 channel-mGlu₅ receptor-PLC-DAGL-CB₁ receptor signalling cascade to exert its analgesic effects.

CONCLUSIONS AND IMPLICATIONS

The elucidation of the mechanism of action of paracetamol as an analgesic paves the way for pharmacological innovations to improve the pharmacopoeia of analgesic agents.

Spinal cord stimulation in chronic pain: evidence and theory for mechanisms of action

Well-established in the field of bioelectronic medicine, Spinal Cord Stimulation (SCS) offers an implantable, non-pharmacologic treatment for patients with intractable chronic pain conditions. Chronic pain is a widely heterogenous syndrome with regard to both pathophysiology and the resultant phenotype. Despite advances in our understanding of SCS-mediated antinociception, there still exists limited evidence clarifying the pathways recruited when patterned electric pulses are applied to the epidural space. The rapid clinical implementation of novel SCS methods including burst, high frequency and dorsal root ganglion SCS has provided the clinician with multiple options to treat refractory chronic pain. While compelling evidence for safety and efficacy exists in support of these novel paradigms, our understanding of their mechanisms of action (MOA) dramatically lags behind clinical data. In this review, we reconstruct the available basic science and clinical literature that offers support for mechanisms of both paresthesia spinal cord stimulation (P-SCS) and paresthesia-free spinal cord stimulation (PF-SCS). While P-SCS has been heavily examined since its inception, PF-SCS paradigms have recently been clinically approved with the support of limited preclinical research. Thus, wide knowledge gaps exist between their clinical efficacy and MOA. To close this gap, many rich investigative avenues for both P-SCS and PF-SCS are underway, which will further open the door for paradigm optimization, adjunctive therapies and new indications for SCS. As our understanding of these mechanisms evolves, clinicians will be empowered with the possibility of improving patient care using SCS to selectively target specific pathophysiological processes in chronic pain.

Deep brain stimulation for chronic pain: intracranial targets, clinical outcomes, and trial design considerations

For over half a century, neurosurgeons have attempted to treat pain from a diversity of causes using acute and chronic intracranial stimulation. Targets of stimulation have included the sensory thalamus, periventricular and periaqueductal gray, the septum, the internal capsule, the motor cortex, posterior hypothalamus, and more recently, the anterior cingulate cortex. The current work focuses on presenting and evaluating the evidence for the efficacy of these targets in a historical context while also highlighting the major challenges to having a double-blind placebo-controlled clinical trial. Considerations for pain research in general and use of intracranial targets specifically are included.

The antihyperalgesic effects of intrathecal bupropion, a dopamine and noradrenaline reuptake inhibitor, in a rat model of neuropathic pain

BACKGROUND

Antidepressants are often used for the treatment of neuropathic pain, and their analgesic effects rely on increased noradrenaline and serotonin levels in the spinal cord. Clinical studies have also shown that bupropion, a dopamine and noradrenaline reuptake inhibitor, has strong efficacy in neuropathic pain; however, the role of spinal cord dopamine in neuropathic pain is unknown. We hypothesized that bupropion inhibits neuropathic pain by increasing noradrenaline and dopamine in the spinal cord. In the present study, we determined the efficacy and underlying mechanisms of intrathecal administration of bupropion in a rat model of neuropathic pain.

METHODS

Male Sprague-Dawley rats were anesthetized, and right L5 spinal nerve ligation (SNL) was performed to produce mechanical hyperalgesia of the hindpaw. Withdrawal threshold to a paw pressure test was measured before and after intrathecal administration of bupropion, without or with intrathecal antagonists for α2-adrenoceptors and dopamine D2 receptors. In vivo microdialysis was performed in the dorsal horn of the lumbar spinal cord to measure noradrenaline and dopamine concentrations after intrathecal injection of bupropion. We also measured the noradrenaline and dopamine contents in the ipsilateral dorsal lumbar spinal cord in normal rats and in rats 2, 3, and 4 weeks after SNL.

RESULTS

Intrathecal injection of bupropion produced a dose-dependent antihyperalgesic effect (3, 10, 30, and 100 μg, P < 0.001). The effect (30 μg) was dose-dependently reversed by intrathecal pretreatment (15 minutes before bupropion injection) with the α2-adrenoceptor antagonist idazoxan (3, 10, and 30 μg, P < 0.001) and D2 receptor antagonist sulpiride (3, 10, and 30 μg, P < 0.001). Microdialysis revealed that noradrenaline and dopamine concentrations in the spinal dorsal horn were increased after intrathecal injection of bupropion (30 μg, P < 0.001 and P = 0.001, respectively). Furthermore, the noradrenaline and dopamine contents in the spinal dorsal horn were increased 2 weeks after SNL (P < 0.001 and P = 0.044, respectively) and then decreased gradually.

CONCLUSIONS

These findings suggest that plasticity of descending inhibitory pathways such as the noradrenaline and dopamine systems contributes to the maintenance of neuropathic pain and that spinal cord noradrenaline and dopamine both play an inhibitory role in neuropathic pain.

Neuropathic pain and deep brain stimulation

Deep brain stimulation (DBS) is a neurosurgical intervention the efficacy, safety, and utility of which are established in the treatment of Parkinson's disease. For the treatment of chronic, neuropathic pain refractory to medical therapies, many prospective case series have been reported, but few have published findings from patients treated with current standards of neuroimaging and stimulator technology over the last decade . We summarize the history, science, selection, assessment, surgery, programming, and personal clinical experience of DBS of the ventral posterior thalamus, periventricular/periaqueductal gray matter, and latterly rostral anterior cingulate cortex (Cg24) in 113 patients treated at 2 centers (John Radcliffe, Oxford, UK, and Hospital de São João, Porto, Portugal) over 13 years. Several experienced centers continue DBS for chronic pain, with success in selected patients, in particular those with pain after amputation, brachial plexus injury, stroke, and cephalalgias including anesthesia dolorosa. Other successes include pain after multiple sclerosis and spine injury. Somatotopic coverage during awake surgery is important in our technique, with cingulate DBS under general anesthesia considered for whole or hemibody pain, or after unsuccessful DBS of other targets. Findings discussed from neuroimaging modalities, invasive neurophysiological insights from local field potential recording, and autonomic assessments may translate into improved patient selection and enhanced efficacy, encouraging larger clinical trials.

An overview of animal models of pain: disease models and outcome measures

Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience, including reflexive hyperalgesia measures, sensory and affective dimensions of pain, and impact of pain on function and quality of life. In this review, we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes as well as the main behavioral tests for assessing pain in each model.

PERSPECTIVE

Understanding animal models and outcome measures in animals will assist in translating data from basic science to the clinic.

Effects of 5-hydroxytryptamine applied into nucleus raphe magnus on nociceptive thresholds and neuronal firing rate

The effect of iontophoretically applied 5-hydroxytryptamine on neurones in nucleus raphe magnus, and the effect of microinjection of 5-hydroxytryptamine into nucleus raphe magnus on nociceptive thresholds were examined in the rat. Iontophoretically applied 5-hydroxytryptamine excited 66% and inhibited 6% of the neurones encountered in the nucleus raphe magnus. The excitatory response to 5-hydroxytryptamine was reduced by the putative serotonergic antagonist cinanserin in 21 of 24 cases. In 12 of these neurones the responses to iontophoretically applied glutamate were also examined. In 11 of the 12 studies the responses to glutamate were reduced by cinanserin. Microinjection of 5 microg of 5-hydroxytryptamine into the nucleus raphe magnus produced analgesia as assessed by the tail-flick response to noxious heat stimulation, but no analgesia as assessed by the paw withdrawal response to pressure. Microinjection of 5 microg of 5-hydroxytryptamine into the adjacent nucleus reticularis paragigantocellularis had no analgesic effect in either test. These results indicate that 5-hydroxytryptamine mainly excites neurones in nucleus raphe magnus and that 5-hydroxytryptamine has an action on neurones in nucleus raphe magnus which modulate the nociceptive threshold.

Central neuromodulation for refractory pain

Chronic neuropathic pain affects 8.2% of adults, extrapolated to roughly 18 million people every year in the United States. Patients who have pain that cannot be controlled with pharmacologic management or less invasive techniques can be considered for deep brain stimulation or motor cortex stimulation. These techniques are not currently approved by the Food and Drug Administration for chronic pain and are, thus, considered off-label use of medical devices for this patient population. Conclusive effectiveness studies are still needed to demonstrate the best targets as well as the reliability of the results with these approaches.

Deep brain stimulation for pain

Deep brain stimulation (DBS) is a neurosurgical intervention whose efficacy, safety, and utility have been shown in the treatment of movement disorders. For the treatment of chronic pain refractory to medical therapies, many prospective case series have been reported, but few have published findings from patients treated during the past decade using current standards of neuroimaging and stimulator technology. We summarize the history, science, selection, assessment, surgery, and personal clinical experience of DBS of the ventral posterior thalamus, periventricular/periaqueductal gray matter, and, latterly, the rostral anterior cingulate cortex (Cg24) in 100 patients treated now at two centers (John Radcliffe Hospital, Oxford, UK, and Hospital de São João, Porto, Portugal) over 12 years. Several experienced centers continue DBS for chronic pain with success in selected patients, in particular those with pain after amputation, brachial plexus injury, stroke, and cephalalgias including anesthesia dolorosa. Other successes include pain after multiple sclerosis and spine injury. Somatotopic coverage during awake surgery is important in our technique, with cingulate DBS considered for whole-body pain or after unsuccessful DBS of other targets. Findings discussed from neuroimaging modalities, invasive neurophysiological insights from local field potential recording, and autonomic assessments may translate into improved patient selection and enhanced efficacy, encouraging larger clinical trials.

Potential for Cell-Transplant Therapy with Human Neuronal Precursors to Treat Neuropathic Pain in Models of PNS and CNS Injury: Comparison of hNT2.17 and hNT2.19 Cell Lines

Effective treatment of sensory neuropathies in peripheral neuropathies and spinal cord injury (SCI) is one of the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord is a logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the potential of transplant of cells to treat chronic pain. Cell lines derived from the human neuronal NT2 cell line parentage, the hNT2.17 and hNT2.19 lines, which synthesize and release the neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5HT), respectively, have been used to evaluate the potential of cell-based release of antinociceptive agents near the lumbar dorsal (horn) spinal sensory cell centers to relieve neuropathic pain after PNS (partial nerve and diabetes-related injury) and CNS (spinal cord injury) damage in rat models. Both cell lines transplants potently and permanently reverse behavioral hypersensitivity without inducing tumors or other complications after grafting. Functioning as cellular minipumps for antinociception, human neuronal precursors, like these NT2-derived cell lines, would likely provide a useful adjuvant or replacement for current pharmacological treatments for neuropathic pain.

Subarachnoid Transplant of the Human Neuronal hNT2.19 Serotonergic Cell Line Attenuates Behavioral Hypersensitivity without Affecting Motor Dysfunction after Severe Contusive Spinal Cord Injury

Transplant of cells which make biologic agents that can modulate the sensory and motor responses after spinal cord injury (SCI) would be useful to treat pain and paralysis. To address this need for clinically useful human cells, a unique neuronal cell line that synthesizes and secretes/releases the neurotransmitter serotonin (5HT) was isolated. Hind paw tactile allodynia and thermal hyperalgesia induced by severe contusive SCI were potently reversed after lumbar subarachnoid transplant of differentiated cells, but had no effect on open field motor scores, stride length, foot rotation, base of support, or gridwalk footfall errors associated with the SCI. The sensory effects appeared 1 week after transplant and did not diminish during the 8-week course of the experiment when grafts were placed 2 weeks after SCI. Many grafted cells were still present and synthesizing 5HT at the end of the study. These data suggest that the human neuronal serotonergic hNT2.19 cells can be used as a biologic minipump for receiving SCI-related neuropathic pain, but likely requires intraspinal grafts for motor recovery.

Advances in perioperative pain management: use of medications with dual analgesic mechanisms, tramadol & tapentadol

Recovery from ambulatory surgical procedures can be limited by postoperative pain. Inadequate analgesia may delay or prevent patient discharge and can result in readmission. More frequently, postoperative pain produces discomfort and interrupts sleep, contributing to postoperative fatigue. The development of effective analgesic regimens for the management of postoperative pain is a priority especially in patients with impaired cardiorespiratory, hepatic, or renal function. Tramadol and tapentadol hydrochloride are novel in that their analgesic actions occur at multiple sites. Both agents are reported to be mu-opioid receptor agonists and monoamine-reuptake inhibitors. In contrast to pure opioid agonists, both drugs are believed to have lower risks of respiratory depression, tolerance, and dependence. The Food and Drug Administration has approved both drugs for the treatment of moderate-to-severe acute pain in adults. This article provides an evidence-based account of the role of tramadol and tapentadol in modern clinical practice.

Reserpine causes biphasic nociceptive sensitivity alteration in conjunction with brain biogenic amine tones in rats

The present study investigated the precise relationship between brain biogenic amine (dopamine, noradrenaline, and serotonin) tones and nociception. Nociceptive sensitivities to multimodal (muscle pressure, tactile, cold, and heat) stimuli were assessed in acute phase (up to 24 h after reserpine or tetrabenazine injection) and chronic phase (on day 2 or later) in rats. A single injection of reserpine (3 mg/kg s.c.) significantly decreased biogenic amines in the spinal cord (SC), thalamus (THA), and prefrontal cortex (PFC) in both acute and chronic phases, but significantly increased a dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the SC and a serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the SC and THA in acute phase. The content of all biogenic amine metabolites was at low level in chronic phase. Animals exhibited hypersensitivities to tactile and heat stimuli and hyposensitivity to muscle pressure stimulus in acute phase. In chronic phase, they manifested hypersensitivities to all modes of stimuli. Tetrabenazine (20 mg/kg i.p.) significantly decreased brain biogenic amines for a short time, although it did not significantly affect the nociceptive sensitivities. In conclusion, a single injection of reserpine causes a biphasic alteration of nociceptive sensitivities, which is in conjunction with the dynamic change of brain biogenic amine tones, in rats. Cold and heat hypersensitivities in addition to mechanical ones are induced by the reserpine treatment. Sustained modification of brain biogenic amine tones would be critical to induce a robust change in nociceptive sensitivities based on the different effects between reserpine and tetrabenazine.

Serotoninergic and non-serotoninergic effects of two tricyclic antidepressants on visceral nociception in a rat model

OBJECTIVE

Tricyclic antidepressants (TCAs) are well established in the treatment of patients with irritable bowel syndrome (IBS). The effects are believed to be linked to serotoninergic antinociceptive properties, but data on the antinociceptive effects of various TCAs with variable serotoninergic and non-serotoninergic properties have not been investigated. The aim of this study was to compare the antinociceptive effects of different TCAs.

MATERIAL AND METHODS

Colorectal distension (CRD) using a barostat device was carried out in rats and the visceromotor response (VMR) to CRD was quantified by abdominal wall electromyography. Prior to CRD, saline (control), amitriptyline (AM), desipramine (DES), reserpine (RES) or a combination of TCAs and RES (AM + RES or DES + RES) was applied intraperitoneally. Serum 5-HT levels were determined using high-performance liquid chromatography (HPLC). RES was used to antagonize the serotoninergic actions of TCAs in order to discriminate between these effects and others.

RESULTS

Both TCAs decreased the VMR compared to placebo. After RES application without TCAs, the VMR was increased compared to controls (6403 microV+/-1772 microV). Co-administration of AM and RES resulted in a modest decrease in VMR (5774 microV+/-1953 microV), while in rats treated with RES and DES the VMR again was significantly lower (3446 microV (+/-1347 microV; p <0.05)). 5-HT levels were higher in TCA pretreated rats than those in controls and significantly lower 5-HT levels were found in all rats pretreated with RES.

CONCLUSIONS

AM and DES have antinociceptive properties while RES is pro-nociceptive. The antinociceptive effects of DES are not abolished by RES pretreatment, while AM only attenuates the pro-nociceptive effects of RES. The non-serotoninergic properties of TCAs substantially contribute to the differences in the antinococeptive effects of various TCAs.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Exposure to intermittent nociceptive stimulation under pentobarbital anesthesia disrupts spinal cord function in rats

RATIONALE

Spinal cord plasticity can be assessed in spinal rats using an instrumental learning paradigm in which subjects learn an instrumental response, hindlimb flexion, to minimize shock exposure. Prior exposure to uncontrollable intermittent stimulation blocks learning in spinal rats but has no effect if given before spinal transection, suggesting that supraspinal systems modulate nociceptive input to the spinal cord, rendering it less susceptible to the detrimental consequences of uncontrollable stimulation.

OBJECTIVE

The present study examines whether disrupting brain function with pentobarbital blocks descending inhibitory systems that normally modulate nociceptive input, making the spinal cord more sensitive to the adverse effect of uncontrollable intermittent stimulation.

MATERIALS AND METHODS

Male Sprague-Dawley rats received uncontrollable intermittent stimulation during pentobarbital anesthesia after (experiment 1) or before (experiment 2) spinal cord transection. They were then tested for instrumental learning at a later time point. Experiment 3 examined whether these manipulations affected nociceptive (thermal) thresholds.

RESULTS

Experiment 1 showed that pentobarbital had no effect on the induction of the learning deficit after spinal cord transection. Experiment 2 showed that intact rats anesthetized during uncontrollable intermittent stimulation failed to learn when later transected and tested for instrumental learning. Experiment 3 found that uncontrollable intermittent stimulation induced an antinociception in intact subjects that was blocked by pentobarbital.

CONCLUSIONS

The results suggest a surgical dose of pentobarbital (50 mg/kg) suppresses supraspinal (experiment 2) but not spinal (experiment 1) systems that modulate nociceptive input to the spinal cord by blocking the antinociception that is induced by this input (experiment 3).

Interventions for relieving pain associated with panretinal photocoagulation: a prospective randomized trial

PURPOSE

To evaluate the efficacy of pain relief by oral diazepam, acetaminophen, mefenamic acid, intramuscular ketorolac tromethamine, and peribulbar anaesthesia in panretinal photocoagulation (PRP).

METHODS

A total of 220 patients with proliferative diabetic retinopathy requiring PRP treatment were enrolled in this study. Before laser treatment, the patients were allocated randomly to one of eight groups: group 1: diazepam (n=22), group 2: acetaminophen (n=21), group 3: mefenamic acid (n=21), group 4: diazepam and acetaminophen (n=22), group 5: diazepam and mefenamic acid (n=22), group 6: peribulbar anaesthesia with lidocaine (n=23), group 7: intramuscular injection of ketorolac tromethamine (n=22), group 8: placebo (n=67). Pain after the laser treatment was assessed by a verbal descriptive scale. Blood pressure and heart rate were measured before and after laser treatment.

RESULTS

Patients receiving peribulbar anaesthesia had a significantly lower pain score than the control group (P<0.0001). Additionally, the peribulbar anaesthesia-treated group had the significantly least PRP-associated rise in either systolic (P=0.043) or diastolic blood pressure rates (P=0.030). There were no significant differences in pain score using other anesthetic agents when compared with the control group. There were no significant changes in heart rate after PRP treatment.

CONCLUSION

Peribulbar anaesthesia is effective in reducing pain and blood pressure increase after PRP treatment. Oral diazepam, mefenamic acid, and acetaminophen (either alone or in combination with each other) are not effective in preventing PRP treatment-associated pain. Intramuscular injection of ketorolac tromethamine is also not effective in reducing PRP-associated pain.

Deep brain stimulation for chronic pain: results of two multicenter trials and a structured review

OBJECTIVES

A U.S. Food and Drug Administration ruling required clinical trials to evaluate the safety and efficacy of deep brain stimulation devices, thereby limiting treatment to the investigational setting.

INTRODUCTION

As an investigator in two clinical trials of deep brain stimulation, I sought to determine why pain remained an unapproved indication despite regulatory approval of the same device for tremor.

METHODS

The results of two multicenter trials of deep brain stimulation for pain were analyzed, and the pertinent literature was reviewed using published guidelines for the evaluation of clinical trial reports.

RESULTS

The first-generation Model 3380 lead trial enrolled 196 patients; the current Model 3387 trial enrolled 50 patients. Prospectively defined criteria for success included at least half of patients reporting >/=50% pain relief at 1 year. Manufacture of the Model 3380 lead was discontinued, and the 3387 trial closed early because of slow enrollment, high attrition, and low efficacy. When results were analyzed according to the study plan, neither trial was successful. Consequently, deep brain stimulation has not been approved for pain control by the U.S. Food and Drug Administration.

CONCLUSIONS

Deep brain stimulation has not been shown to produce effective long-term pain relief. Future studies of motor cortex stimulation and similar therapies will require appropriate control groups and accepted methods of data collection and analysis to support claims that predictable and reliable analgesic effects are produced in humans.

Serotonin 5-HT2 and 5-HT1A receptors in the periaqueductal gray matter differentially modulate tonic immobility in guinea pig

Tonic immobility (TI) is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. We have previously reported that cholinergic stimulation of the dorsal periaqueductal gray matter (PAG) decreases the duration of TI episodes, while stimulation of the ventrolateral region increases it. The ventrolateral PAG modulates this behavior via a similar neural circuit proposed to be involved in the antinociceptive system. Some studies have indicated that alterations in the levels of cerebral 5-hydroxytryptamine (5-HT) mediate or modulate the analgesic effect of PAG stimulation. Thus, in this study we investigated the possibility that the serotoninergic system is involved in the modulation of TI by this neural substrate. Our results showed that the effect of serotonin into the ventrolateral and dorsal PAG seems to be biphasic and dose dependent. The microinjection of low doses (0.1 microg) of 5-HT into the PAG increased the duration of TI, while high doses (1, 3 and 6 microg) decreased this behavior. Our results also showed that microinjection of a 5-HT(1A) agonist (0.003, 0.01 and 0.1 microg of 8-hydroxy-dipropylaminotretalin (8-OH-DPAT)) into the PAG increased the duration of TI episodes. However, the microinjection of 5-HT(2) agonist (0.01 and 0.1 microg of alpha-methyl-5-HT) into the PAG decreased the duration of TI and this effect could be reversed by pretreatment with an ineffective dose (0.01 microg) of ketanserin. In contrast, ketanserin (0.03 and 0.16 microg) increased this behavior in a dose-dependent manner. These results suggest that the PAG 5-HT(1A) and 5-HT(2) receptors have different roles in the modulation of TI in guinea pigs, since the 5-HT(1A) and 5-HT(2) agonists, respectively, increased and decreased the duration of TI.

Historical review: Opioid receptors

A historical review of the early phases of molecular investigations of opioid receptors is presented. The 'modern' era of molecular studies of neurotransmitter and drug receptor research commenced in the 1970s with the identification of receptors using ligand binding techniques. These findings had several ramifications. Reversible ligand binding to opioid receptors using simple, sensitive and specific techniques provided a paradigm for the study of receptors for the principal neurotransmitters in the brain. The relatively high-throughput binding techniques employed facilitated drug development in the pharmaceutical industry. Differentiation of agonist and antagonist receptor interactions by Na(+) ions and other substances helped elucidate how ligand recognition at receptors is translated into second messenger alterations. Localizations of opioid receptors clarified many of the pharmacological actions of opiate drugs. Differential binding interactions of various drugs led to the identification of opioid receptor subtypes. Receptor influences in binding paradigms and smooth muscle pharmacology permitted the identification and isolation of endogenous opioid peptides.

Striatal dopamine D1 and D2 receptors in burning mouth syndrome

Animal studies have indicated that the nigrostriatal dopaminergic system is involved in central pain modulation. In a recent positron emission tomography (PET) study, we demonstrated presynaptic dysfunction of the nigrostriatal dopaminergic pathway in burning mouth syndrome, which is a chronic pain state. The objective of the present study was to examine striatal dopamine D1 and D2 receptors in these patients. We used 11C-NNC 756 and 11C-raclopride to study D1 and D2 receptor binding in a PET study in ten burning mouth patients and 11 healthy controls. Patients underwent a structured psychiatric evaluation and an electrophysiological test for the excitability of the blink reflex. The striatal uptake of 11C-NNC 756 did not differ between patients and controls. In a voxel-level analysis, the uptake of 11C-raclopride was statistically significantly higher in the left putamen in burning mouth patients (corrected P-value 0.038 at cluster-level). In the region of interest analysis, the D1/D2 ratio was 7.7% lower in the right putamen (0.64+/-0.04 vs. 0.69+/-0.04, P=0.01) and 6.4 % lower in the left putamen (0.65+/-0.05 vs. 0.70+/-0.05, P=0.05) when compared to controls. Increased 11C-raclopride uptake and the subsequent decrease in the D1/D2 ratio may indicate a decline in endogenous dopamine levels in the putamen in burning mouth patients.

Dopamine D2 receptor binding in the human brain is associated with the response to painful stimulation and pain modulatory capacity

The pain modulatory role of dopamine D2 receptors of the human forebrain was studied by determining the association between dopamine D2 receptor binding potential and the response to experimental pain. Nineteen healthy male volunteers participated in a dopamine D2 receptor positron emission tomography study. The extrastriatal regions of interest studied with [11C]FLB 457 as radioligand (n = 11) were the anterior cingulum, the medial and lateral thalamus, the medial and lateral frontal cortex, and the medial and lateral temporal cortex. The striatal regions of interest studied with [11C]raclopride (n = 8) were the caudate nucleus and the putamen. The latency to the ice water-induced cold pain threshold and tolerance were determined in a separate psychophysical test session. Moreover, the cutaneous heat pain threshold and its elevation by concurrent cold pain in the contralateral hand were determined in each subject. Cold pain threshold was inversely correlated with D2 binding potential in the right putamen and the cold pain tolerance was inversely correlated with D2 binding potential in the right medial temporal cortex. The magnitude of heat pain threshold elevation induced by concurrent cold pain was directly correlated with D2 binding potential in the left putamen. Other correlations of D2 binding potentials in varying brain regions with sensory responses were not significant. A psychophysical control study (n = 10) showed that cold pain responses were identical in the right and left hand. The results indicate that dopamine D2 receptor binding potential in the human forebrain, particularly in the striatum, may be an important parameter in determining the individual cold pain response and the potential for central pain modulation. Accordingly, an individual with only few available D2 receptors in the forebrain is likely to have a high tonic level of pain suppression, combined with a low capacity to recruit more (dopaminergic) central pain inhibition by noxious conditioning stimulation.

Effects of dopaminergic agents on antinociception in formalin test

Morphine caused a dose-related antinociception in early phase and late phase of formalin test in mice. The D2 dopamine agonist quinpirole, but not the D1 dopamine agonist SKF 38393, increased the antinociceptive effect of morphine in both phases of the test. The antinociceptive effect of quinpirole also was decreased by sulpiride or domperidone pretreatment in the early phase of test. The D1 antagonist SCH23390, the D2 antagonist sulpiride, or the peripheral D2 dopamine antagonist domperidone, increased the morphine effect. Single administration of SKF38393, quinpirole, SCH23390, sulpiride, and domperidone also induce antinociception. The response of SCH23390, but not that of other dopamine agents, was antagonized with naloxone. The effects of the drugs alone and in combination with morphine have been discussed.

Use of levodopa to relieve pain from painful symmetrical diabetic polyneuropathy

Levodopa has been used to treat some painful conditions and found to be effective in neuropathic pain due to herpes zoster in a double-blind study. From our anecdotal observations about the efficacy of levodopa on diabetic neuropathic pain, we designed a double-blind placebo-controlled study to test levodopa in painful diabetic neuropathy. Twenty-five out-patients with painful symmetrical diabetic polyneuropathy were admitted to the study. Fourteen patients were given 100 mg levodopa plus 25 mg benserazide to be taken three times per day for 28 days. Eleven patients were given identical placebo capsules. A blinded neurologist evaluated the patients clinically and performed Visual Analogue Scale (VAS) measurement every week from day 0 to day 28. The results seemed promising and levodopa may be a choice for the control of pain in neuropathy for which we do not have many alternative treatments.

Susceptibility to pain in hypertensive and normotensive patients with coronary artery disease: response to dental pulp stimulation

An association between a decreased responsiveness to varying painful stimuli and arterial hypertension both in animals and in humans has been documented. The relationship between essential hypertension and silent myocardial ischemia in coronary artery disease (CAD) populations is not well understood. The aims of this study in CAD patients with and without essential hypertension were (1) to determine dental pain threshold and reaction to tooth pulp stimulation and (2) to ascertain whether hypertensive CAD patients differ from normotensive ones in reactivity to pain. This study involved 182 patients who were affected by mild and moderate hypertension (G1) and 174 normotensive patients (G2). The inclusion criteria were reproducible exercise-induced myocardial ischemia, CAD documented at angiography, and dental formula suitable for pulp test. All patients underwent an ergometric stress test, coronary angiography, and pulp test. Our CAD hypertensive patients showed a lower prevalence of angina during daily life (64.8% in G1 versus 81.6% in G2, P<.05) and a higher incidence of exercise-induced silent myocardial ischemia (60.4% in G1 versus 48.8% in G2, P<.05) than the normotensive group. The mean anginal pain intensity, which was suffered both during spontaneous transitory episodes of ischemia and/or during acute myocardial infarction, was significantly lower in G1 than in G2 patients (P<.05). During pulp test, 31.8% of G1 and 13.7% of G2 referred no symptoms, even at the highest current intensity of 500 mA. The hypertensive patients with symptoms during pulp test had a higher mean dental pain threshold and lower mean threshold reaction and maximal reaction than did the normotensive symptomatic ones. In patients of both groups, a positive correlation between the mean maximal reaction during pulp test and the prevalence of angina during daily life was also found. In conclusion, patients with CAD and essential hypertension differ from normotensive CAD patients in reactivity to pain. Significantly higher pain thresholds and lower reactions to tooth pulp stimulation characterized patients with increased blood pressure values.

Chronic pain-associated depression: antecedent or consequence of chronic pain? A review

OBJECTIVE

To determine the current status for the association of chronic pain and depression and to review the evidence for whether depression is an antecedent or consequence of chronic pain (CP).

DESIGN

A computer and manual literature review yielded 191 studies that related to the pain-depression association. These reports were reviewed and sorted into seven categories relating to the topic of this paper. Eighty-three studies were then selected according to inclusion criteria and subjected to a structured review.

SETTING

Any medical treatment setting including pain treatment as inclusion criteria for selection of studies.

PATIENTS

Any patients with any type of chronic pain.

RESULTS

The reviewed studies were consistent in indicating that there is a statistical relationship between chronic pain and depression. For the relationship between pain and depression, there was greater support for the consequence and scar hypotheses than the antecedent hypothesis.

CONCLUSIONS

Depression is more common in chronic pain patients (CPPs) than in healthy controls as a consequence of the presence of CP. At pain onset, predisposition to depression (the scar hypothesis) may increase the likelihood for the development of depression in some CPPS. Because of difficulties in measuring depression in the presence of CP, the reviewed studies should be interpreted with caution.

Possible mechanisms of central nervous system fatigue during exercise

Fatigue of voluntary muscular effort is a complex phenomenon. To date, relatively little attention has been placed on the role of the central nervous system (CNS) in fatigue during exercise despite the fact that the unwillingness to generate and maintain adequate CNS drive to the working muscle is the most likely explanation of fatigue for most people during normal activities. Several biological mechanisms have been proposed to explain CNS fatigue. Hypotheses have been developed for several neurotransmitters including serotonin (5-HT; 5-hydroxytryptamine), dopamine, and acetylcholine. The most prominent one involves an increase in 5-HT activity in various brain regions. Good evidence suggests that increases and decreases in brain 5-HT activity during prolonged exercise hasten and delay fatigue, respectively, and nutritional manipulations designed to attenuate brain 5-HT synthesis during prolonged exercise improve endurance performance. Other neuromodulators that may influence fatigue during exercise include cytokines and ammonia. Increases in several cytokines have been associated with reduced exercise tolerance associated with acute viral or bacterial infection. Accumulation of ammonia in the blood and brain during exercise could also negatively effect the CNS function and fatigue. Clearly fatigue during prolonged exercise is influenced by multiple CNS and peripheral factors. Further elucidation of how CNS influences affect fatigue is relevant for achieving optimal muscular performance in athletics as well as everyday life.

Evidence of the involvement of biogenic amines in the antinociceptive effect of a vouacapan extracted from Pterodon polygalaeflorus Benth

In view of the extensive use of Pterodon species in Brazilian folk medicine, the present investigation was performed to examine the involvement of biogenic amines in antinociceptive by a vouacapan (6 alpha-7 beta-dihydroxy vouacapan-17 beta-oate), extracted from seeds of Pterodon polygalaeflorus Benth), using acetic acid writhing test in mice. The alpha 2-adrenergic (yohimbine) and D2-dopaminergic (domperidone) antagonists and the pretreatment with the peripheral noradrenergic depletor, guanethidine partially inhibited the antinociceptive effect of vouacapan. Dopamine and D2 dopaminergic agonist (Ly 17155) caused antinociceptive that was not antagonized by naloxone but by domperidone, whereas noradrenaline induce pain. A synergistic analgesic effect was obtained when vouacapan was associated with clonidine or dopamine. These results indicate that vouacapan acts, at least in part, through activation of the catecholaminergic system.

A combined behavioral-physiological method for the assessment of thermal sensibility in the rat

A behavioral and physiological method has been developed to study thermal detection in the rat. Threshold temperatures signaling (1) disruption of a trained behavior (forelimb bar pressing) and (2) increased heart rate were determined using a gradually increasing thermal stimulus delivered to the hindlimb. Response thresholds for disruption of bar press activity (44.8 degrees C) and heart rate elevation (44.4 degrees C) were statistically equivalent. No differences in thresholds for either response end point were found between the left and right hindlimbs, thus validating the consistency of the technique and reproducibility of the stimulus delivery device during test periods of 3-4 weeks. Advantages of the method include the use of two independent end points that are determined objectively, performance criteria that do not rely upon motor responses from the stimulated limb and the use of quantitative descriptors that enables comparisons to be made between different test groups. The method combines behavior with cardiovascular and somatosensory function in a way that can be used for the assessment of spinal and supraspinal pathways involved in thermal detection in the unanesthetized, behaviorally active rat.

Antinociceptive effects of clebopride in the mouse

1. The effects of the substituted benzamide clebopride, an orthopramide, on nociception of chemical and thermal stimuli were investigated. 2. Clebopride (0.5, 1.0 and 2.0 mg/kg) promoted significant analgesia in the tail-flick and hot-plate tests and against abdominal constrictions produced by acetic acid or acetylcholine. 3. The analgesic effects of clebopride were not influenced by pretreatment with naltrexone (1-3 mg/kg). 4. The results suggest that clebopride induces analgesia against both thermal and chemical nociceptive stimuli, which is not mediated via opioid mechanisms.

Transcranial electrostimulation effects on rat opioid and neurotransmitter levels

A specific form of Transcranial Electrostimulation Treatment (TCET) has been shown to induce analgesia, alleviate symptoms of opiate withdrawal and alter nociceptive responses in neurons in the midbrain and hypothalamus of rats. TCET consists of a 10Hz, charge balanced, 10 mu A current passed for 30 minutes between electrodes placed in the ears. Both serotonin (5HT) and endogenous opioids have been strongly implicated in TCET responses. This study directly measured brain levels of several neurotransmitters and their metabolites in anesthetized rats stimulated with either 10 mu A TCET or 0 mu A (Sham). Neurotransmitters measured in selected homogenized brain areas by high performance liquid chromatography were 5HT and its metabolite, 5-hydroxyindolacetic acid (5HIAA); norepinephrine (NE) and its metabolite, 3-methoxy-4-hydroxyphenethyleneglycol (MHPG); and dopamine (DA). Levels of NE and DA were significantly higher in the hypothalamic region of TCET rats than of control rats. The midbrains of TCET rats contained significantly elevated levels of DA, MHPG, 5HT and 5HIAA. In the hindbrain no significant differences were observed. Thus, TCET appears to cause an increase in the synthesis or release of 5HT, DA and NE in the midbrain and DA and 5HT in the hypothalamus. In a separate experiment, beta-endorphin-like immunoreactivity was measured in blood plasma taken from rats at intervals before, during and after a 30 minute TCET treatment, but no demonstrable TCET effect was observed. The lack of change in serum endorphin levels suggests that TCET-induced opioid activity may be confined to the central nervous system, a reasonable theory because the current passes only through the head.

Short-range differential pulse voltammetry for fast, selective analysis of basal levels of cerebral compounds in vivo

Differential pulse voltammetry (DPV) with pretreated biosensors (carbon fibre microelectrodes (mCFE), 10-30 microns diameter) allows selective in vivo measurement of basal endogenous levels of dopamine (DA), serotonin (5-HT), their metabolites (dihydroxyphenylacetic acid, DOPAC; 5-hydroxyindoleacetic acid, 5-HIAA), and neuropeptides. We have now modified DPV in order to reduce the time of analysis from tens of seconds to 1-2 s without losing selectivity. We call this newly reported method short-range differential pulse voltammetry (SRDPV). Simply, while in DPV the complete oxidation peak is recorded, SRDPV measures only the top of each oxidation peak. For example, to monitor peak 2 which corresponds to the in vivo oxidation of extracellular DOPAC and occurs at approximately +85 +/- 10 mV, the initial (Ei) and final (Ef) potentials applied with DPV were -100 mV and +200 mV, respectively, while they were +75 mV (Ei) and +95 mV (Ef) with SRDPV. At the typical scan range of 10 mV.s-1, the effective time of measurement was 30 s for DPV and 2 s for SRDPV. A similar procedure was performed to analyze peak 3 (5-HIAA, occurring at +230 +/- 11 mV) with Ei + 50 mV and Ef + 350 mV for DPV, or +220 mV and +240 mV for SRDPV. DPV and SRDPV were compared in vitro by quantitating DOPAC and 5-HIAA in solutions of increasing concentrations (chosen on the basis of the suggested in vivo content of these two compounds). Data indicated that similar sensitivity and selectivity were obtained with both methods at all concentrations, supporting the applicability of SRDPV for in vitro studies. In vivo experiments were performed in anesthetized adult male rats prepared for voltammetry by inserting the electrically pretreated biosensor (mCFE) into the striatum. DPV measurements were performed automatically every 3-5 min and were alternated every 10-20 min with a sequence of 5-10 SRDPV scans performed every 10-30 s. Subsequent pharmacological or electrical manipulations of the two biogenic amine systems studied were monitored by alternate use of DPV and SRDPV. The data presented support the capability of SRDPV with pretreated biosensors to measure in vivo electroactive compounds with selectivity and sensitivity comparable to that of DPV, but with improved time resolution.

Activation of serotonin1A receptors inhibits midbrain periaqueductal gray neurons of the rat

The midbrain periaqueductal gray (PAG) is involved in a variety of functions including pain modulation, vocalization, autonomic control, fear and anxiety. This area contains serotonin receptors, particularly 5-HT1A that are known to play a role in the above functions. The goals of this study were to characterize the effects of 8-OH-DPAT, a selective 5-HT1A agonist, on the firing characteristics and membrane properties of PAG neurons. Both in vivo and in vitro preparations were used. The effects of 8-OH-DPAT on baseline activity of 91 neurons were tested in the in vivo preparation. In 50/91 cells, 8-OH-DPAT produced a decrease in the firing rate that ranged between 21 and 98% (mean +/- S.E.M. decrease of 49 +/- 1.9%). This inhibitory effect was dose dependent and could be blocked by spiperone. In 10/91 cells, 8-OH-DPAT produced an increase in the firing rate that ranged between 13 and 290%, with mean increase of 83 +/- 7.4%. The baseline firing rate of the remaining 31 cells was not affected by 8-OH-DPAT. In the PAG slice preparation, the effects of 8-OH-DPAT on synaptic and membrane properties of 17 PAG neurons were tested using whole-cell voltage clamp-recording procedures. In 14 cells, application of 8-OH-DPAT produced hyperpolarization that ranged between 6 and 21 mV, with mean of 8.4 +/- 2.0 mV. This hyperpolarization was associated with a decrease in membrane impedance that ranged between 8 and 45%, with mean decrease of 21.6 +/- 4.5%. The remaining three neurons did not respond to 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulatory role of D-1 and D-2 dopamine receptor subtypes in nociception in mice

The role of D-1 and D-2 dopamine (DA) receptors in nociception in naive as well as reserpinized mice and the modulation of the nociceptive action of morphine or naloxone by the selective D-1 and D-2 DA agonists, was investigated in mice. The D-2 DA agonists, B-HT 920 and bromocriptine produced an anti-nociceptive effect in naive mice and reversed the hyperalgesic effect of reserpine (2 mg/kg, 4 h prior) pre-treatment. The D-1 DA agonist, SKF 38393 (5 mg/kg) failed to alter the nociceptive responsiveness of naive and reserpinized mice. Apomorphine, a mixed D-1/D-2 DA agonist, produced significant analgesia in naive mice and also reversed reserpine-induced hyperalgesia. SKF 38393 (5 mg/kg) enhanced the anti-nociceptive effect of B-HT 920 (0.1 mg/kg) in naive and reserpine-pre-treated mice. The anti-nociceptive response of morphine (5 mg/kg) was enhanced by B-HT 920 while SKF 38393 reduced the same. Apomorphine (0.5 mg/kg) or the combination of B-HT 920 (0.1 mg/kg) and SKF 38393 (5 mg/kg) failed to enhance the anti-nociceptive effect of morphine. Reserpine (2 mg/kg, 4 h prior) pre-treatment significantly reduced the anti-nociceptive effect of morphine. Similarly, the hyperalgesic action of naloxone (20 mg/kg) was reversed by B-HT 920, bromocriptine and apomorphine but not by SKF 38393. The reversal of the hyperalgesic action of naloxone by B-HT 920 was blocked by pre-treatment with haloperidol (0.5 mg/kg) and sulpiride (100 mg/kg). SKF 38393 (5 mg/kg) failed to potentiate the reversal action of B-HT 920 against naloxone. These data suggest a predominant role of D-2 DA receptors in anti-nociception and the possibility of the existence of an interlink between the DAergic and opioid systems.