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Physiology and Pathophysiology of Pain
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Burrowing behaviour is employed to assess pain-associated behaviour in laboratory rodents. To gain insight in how models of disease associated persistent pain and analgesics affect burrowing behaviour, we performed a systematic review and meta-analysis of studies that assessed burrowing behaviour. A systematic search in March 2020 and update in September 2020 was conducted in 4 databases. Study design characteristics and experimental data were extracted, followed by a random-effects meta-analysis. We explored the association between burrowing and monofilament-induced limb withdrawal. Dose response relationship was investigated for some analgesics. Forty-five studies were included in the meta-analysis, in which 16 model types and 14 drug classes were used. Most experiments used rat (79%) and male (72%) animals. Somatic inflammation and trauma induced neuropathy models were associated with reduced burrowing behaviour. Analgesics (NSAID and gabapentinoids) attenuated burrowing deficits in these models. Reporting of measures to reduce risk of bias was unclear except for randomisation which was high. There was not a correlation (R2 = 0.1421) between burrowing and monofilament-induced limb withdrawal. Opioids, gabapentin and naproxen showed reduced burrowing behaviour at high doses, while ibuprofen and celecoxib showed opposite trend. The findings indicate that burrowing could be used to assess pain-associated behaviour. We support the use of a portfolio of composite measures including spontaneous and stimulus-evoked tests. The information collected here could help in designing experiments involving burrowing assessment in models of disease associated pain.
Animals continuously sense the temperature in their environment, which is crucial for survival and for maintaining an optimal energy expenditure. Thermal perception is enabled by sensory afferent neurons that innervate the skin and express molecules that transduce thermal stimuli into electrical signals, which are later processed by the nervous system. In the recent years, studies in genetically modified mice have found sensory afferents and ion channels that transduce cooling in mammals, but the perceptual ability of mice to sense warmth and the underlying encoding mechanisms remain unknown.
In this work, I have investigated the neurobiological mechanisms that underlie the perception of warmth. To do so, I have employed the mouse (Mus musculus) as a model system due to both the phylogenetic proximity to humans and the availability of genetic tools for mechanistic studies.
Using a sensory detection behavior, I first show that mice perceive tiny (0.5oC) changes in temperature of the forepaw. Like humans, mice are able to discriminate warming from cooling, they are less sensitive to warmth and the baseline temperature strongly impacts the perceptual saliency. Together, these data indicate that mice and humans share many features of thermal perception, suggesting common sensory coding mechanisms.
Cooling perception in mice is known to require cool-activated sensory afferents that express the channel TRPM8, but the neurons encoding warmth are unknown. Here, warming recruited two polymodal afferent populations: one fired upon warming (warm-activated) and the other was both warm-silenced and cool-activated. To investigate their role in perception, I used gene knockouts and optogenetic afferent stimulation and found that mice sense and encode warming without the warm-activated ion channels TRPV1, TRPM3, TRPA1 and TRPM2. However, surprisingly, despite TRPM8+ afferent stimulation evoked a cooling percept, TRPM8-null mice cannot detect warming. Trpm8-/- mice possess warm-activated afferents but lack warm-silenced neurons, suggesting that cooling input from warm-inhibited fibers is required for warmth perception.
In preliminary work I have also investigated the role of primary somatosensory cortex in warm perception. Using intrinsic optical imaging I observed that cooling and touch, but not warming, elicits robust responses; but silencing of this brain region impaired the perception of warming stimuli.
Altogether, the data from my thesis suggest that warming perception is an integrative process and requires input from both warm- and cool-activated sensory pathways.
Cauterization by hot iron and application of caustic paste are 2 common methods of disbudding calves. In this study, we compared the affective experience of these 2 procedures on young dairy calves using conditioned place aversion. Male dairy calves (n = 14; 7 ± 2 d old) were disbudded by both thermal and chemical methods (1 horn bud at a time, 48 h apart). Calves received treatments in pens made visually distinct with either red squares or blue triangles on the walls. Calves were restricted to these treatment pens for 6 h following disbudding. For all treatments, calves received a sedative (xylazine, 0.2 mg/kg), local anesthetic (lido-caine, 5 mL), and analgesic (meloxicam, 0.5 mg/kg). Calves were then tested for conditioned place aversion at 48, 72, and 96 h after their last treatment. During tests, calves were placed in a neutral pen connected to both treatment pens where they had previously been disbudded. Time spent in each treatment pen was recorded until calves chose to lie down for 1 min (latency to lie down: 31.0 ± 8.6 min). During the first test (48 h after last disbudding), calves spent more time in the pen associated with hot-iron disbudding compared with what would be expected by chance (intercept: 73.5%, 95% CI: 56.5, 90.5) and fewer calves lay down in the caustic paste pen than in the hot-iron pen (3 vs. 10 lying events). No evidence of preference for the hot-iron pen was found in the following test sessions (72 and 96 h since last disbudding). These results suggest that calves initially remember caustic paste disbudding as a more negative experience than hot-iron disbudding, even with the use of sedation, local anesthesia, and analgesia.
The current International Association for the Study of Pain (IASP) definition of pain as “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” was recommended by the Subcommittee on Taxonomy and adopted by the IASP Council in 1979. This definition has become accepted widely by health care professionals and researchers in the pain field and adopted by several professional, governmental, and nongovernmental organizations, including the World Health Organization. In recent years, some in the field have reasoned that advances in our understanding of pain warrant a reevaluation of the definition and have proposed modifications. Therefore, in 2018, the IASP formed a 14-member, multinational Presidential Task Force comprising individuals with broad expertise in clinical and basic science related to pain, to evaluate the current definition and accompanying note and recommend whether they should be retained or changed. This review provides a synopsis of the critical concepts, the analysis of comments from the IASP membership and public, and the committee’s final recommendations for revisions to the definition and notes, which were discussed over a 2-year period. The task force ultimately recommended that the definition of pain be revised to “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage,” and that the accompanying notes be updated to a bulleted list that included the etymology. The revised definition and notes were unanimously accepted by the IASP Council early this year.
Reproducible science requires transparent reporting. The ARRIVE guidelines (Animal Research: Reporting of In Vivo Experiments) were originally developed in 2010 to improve the reporting of animal research. They consist of a checklist of information to include in publications describing in vivo experiments to enable others to scrutinise the work adequately, evaluate its methodological rigour, and reproduce the methods and results. Despite considerable levels of endorsement by funders and journals over the years, adherence to the guidelines has been inconsistent, and the anticipated improvements in the quality of reporting in animal research publications have not been achieved. Here, we introduce ARRIVE 2.0. The guidelines have been updated and information reorganised to facilitate their use in practice. We used a Delphi exercise to prioritise and divide the items of the guidelines into 2 sets, the “ARRIVE Essential 10,” which constitutes the minimum requirement, and the “Recommended Set,” which describes the research context. This division facilitates improved reporting of animal research by supporting a stepwise approach to implementation. This helps journal editors and reviewers verify that the most important items are being reported in manuscripts. We have also developed the accompanying Explanation and Elaboration document, which serves (1) to explain the rationale behind each item in the guidelines, (2) to clarify key concepts, and (3) to provide illustrative examples. We aim, through these changes, to help ensure that researchers, reviewers, and journal editors are better equipped to improve the rigour and transparency of the scientific process and thus reproducibility.
Reproducible science requires transparent reporting. The ARRIVE guidelines (Animal Research: Reporting of In Vivo Experiments) were originally developed in 2010 to improve the reporting of animal research. They consist of a checklist of information to include in publications describing in vivo experiments to enable others to scrutinise the work adequately, evaluate its methodological rigour, and reproduce the methods and results. Despite considerable levels of endorsement by funders and journals over the years, adherence to the guidelines has been inconsistent, and the anticipated improvements in the quality of reporting in animal research publications have not been achieved. Here, we introduce ARRIVE 2.0. The guidelines have been updated and information reorganised to facilitate their use in practice. We used a Delphi exercise to prioritise and divide the items of the guidelines into 2 sets, the “ARRIVE Essential 10,” which constitutes the minimum requirement, and the “Recommended Set,” which describes the research context. This division facilitates improved reporting of animal research by supporting a stepwise approach to implementation. This helps journal editors and reviewers verify that the most important items are being reported in manuscripts. We have also developed the accompanying Explanation and Elaboration (E&E) document, which serves (1) to explain the rationale behind each item in the guidelines, (2) to clarify key concepts, and (3) to provide illustrative examples. We aim, through these changes, to help ensure that researchers, reviewers, and journal editors are better equipped to improve the rigour and transparency of the scientific process and thus reproducibility.
Humans detect skin temperature changes that are perceived as warm or cool. Like humans, mice report forepaw skin warming with perceptual thresholds of less than 1°C and do not confuse warm with cool. We identify two populations of polymodal C-fibers that signal warm. Warm excites one population, whereas it suppresses the ongoing cool-driven firing of the other. In the absence of the thermosensitive TRPM2 or TRPV1 ion channels, warm perception was blunted, but not abolished. In addition, trpv1:trpa1:trpm3−/− triple-mutant mice that cannot sense noxious heat detected skin warming, albeit with reduced sensitivity. In contrast, loss or local pharmacological silencing of the cool-driven TRPM8 channel abolished the ability to detect warm. Our data are not reconcilable with a labeled line model for warm perception, with receptors firing only in response to warm stimuli, but instead support a conserved dual sensory model to unambiguously detect skin warming in vertebrates.
Pig cryptorchidism is a congenital anomaly, which requires surgical intervention. Pain assessment in pig farming has some limitations and consumers are increasingly concerned about suffering linked to on-farm procedures. The PGS seems to be a promising tool for pain assessment in piglets, although it has not been investigated whether this tool is applicable to growing pigs. This study was designed to determine whether the PGS could be useful to assess pain in growing pigs undergoing on-farm cryptorchidectomy. Ten mixed-breed cryptorchid pigs were pre-medicated with azaperone and ketamine. Anesthesia was induced and maintained using IV sodium based. Pigs were filmed pre- and 6 h post-surgery to evaluate their behavior (scan sampling every minute). Besides, 36 pictures of the face expressions (18 pre- and 18 6 h post-surgery) were scored with the PGS by three treatment-blind observers. The pre-surgery pig’s activity ratio was 59%, while it was 2% 6 h post-surgery. While the PGS inter-observer reliability was excellent (Interclass Correlation Coefficient value of 0.87), the PGS score increased significantly in pigs 6 h post-surgery (Paired sample t-test, p = 0.02). The PGS proved to be a potentially effective method to assess pain associated with cryptorchidectomy. However, further validation studies are required to validate this tool for other potentially painful procedures.
Hot-iron disbudding, a routine procedure that prevents horn bud growth through cauterization, is painful for calves. The resulting burns remain sensitive to touch for weeks, but it is unknown whether calves experience ongoing, non-evoked pain. We evaluated conditioned place preference for analgesia in 44 calves disbudded or sham-disbudded 6 hours (Day 0) or 20 days (Day 20) before testing (n = 11/treatment). Calves were conditioned to associate the effects of a lidocaine cornual nerve block with the location and pattern of a visual stimulus, and a control injection of saline with the contrasting stimulus. On Day 0, disbudded calves tended to prefer the lidocaine-paired stimulus over the saline-paired one, suggesting that they found analgesia rewarding. On Day 20, sham calves avoided the lidocaine-paired stimulus, consistent with humans’ experience of this drug being painful. Disbudded calves on Day 20 did not show this aversion, suggesting that they traded off the short-term pain of the lidocaine with the longer-term analgesia provided. Day 0 sham calves did not avoid the lidocaine-paired stimulus, likely because they received less than half the dose of Day 20 calves during conditioning. Thus, higher doses of lidocaine are aversive to uninjured animals, but disbudded calves are willing to engage in this cost. We conclude that calves experience ongoing pain 3 weeks after disbudding, raising additional welfare concerns about this procedure.
TRP channels of the transient receptor potential ion channel superfamily are involved in a wide variety of mechanosensory processes, including touch sensation, pain, blood pressure regulation, bone loading, and detection of cerebrospinal fluid flow. However, it is unclear in many instances whether TRP channels are the primary transducers of mechanical force in these processes. In this study, we tested stretch activation of eleven TRP channels from six subfamilies. We found that these TRP channels were insensitive to short membrane stretch in cellular systems. Furthermore, we purified TRPC6 and demonstrated its insensitivity to stretch in liposomes, an artificial bilayer system free from cellular components. Additionally, we demonstrated that when expressed in C. elegans neurons, mouse TRPC6 restores the mechanoresponse of a touch insensitive mutant but requires diacylglycerol for activation. These results strongly suggest that the mammalian members of the TRP ion channel family are insensitive to tension induced by cell membrane stretching and thus they are more likely activated by cytoplasmic tethers or downstream components and act as amplifiers of cellular mechanosensory signaling cascades.
Significance
The cellular correlate for cold sensing has been ascribed to either Trpm8-expressing or Na V 1.8-expressing neurons. Importantly, transcriptomic analysis shows that these neuronal populations are nonoverlapping. Using in vivo GCaMP imaging in live mice we show that the vast majority of acute cold-sensing neurons activated at ≥1 °C do not express Na V 1.8, and that the loss of Na V 1.8 does not affect acute cold-sensing behavior in mice. Instead, we show that cold-responding neurons are enriched with Trpm8 as well as numerous potassium channels, including Kcnk9. By contrast, Na V 1.8-positive neurons signal prolonged extreme cold. These observations highlight the complexity of cold sensing in DRG neurons, and the role of Na V 1.8-negative neurons in cold sensing down to 1 °C.
Burrowing, or the removal of material from an enclosed tube, is emerging as a prominent means of testing changes in a voluntary behavior in rodent models of various pain states. Here, we report no significant differences between male and female mice in terms of burrowing performance, in a substantially shorter time frame than previous reports. We found that the color of the burrow tube affects the variability of burrowing performance when tested in a lit room, suggesting that light aversion is at least a partial driver of this behavior. Spared nerve injury (SNI; as a model of neuropathy) impairs burrowing performance and correlates with enhanced mechanical sensitivity as assessed by von Frey filaments, as well as being pharmacologically reversed by an analgesic, gabapentin. Loss of the SNI-induced burrowing deficit was observed with daily testing post-surgery, but not when the testing interval was increased to 5 days, suggesting a confounding effect of daily repeat testing in this paradigm. Intraplantar complete Freund’s adjuvant (as a model of inflammatory pain) and systemic nitroglycerin (as a model of migraine-like symptoms) administration did not induce any burrowing deficit, indicating that assessment of burrowing behavior may not be universally suitable for the detection of behavioral changes across all rodent pain models.
Burrowing, or the removal of material from an enclosed tube, is emerging as a prominent means of testing changes in a voluntary behavior in rodent models of various pain states. Here, we report no significant differences between male and female mice in terms of burrowing performance, in a substantially shorter time frame than previous reports. We found that the color of the burrow tube affects the variability of burrowing performance when tested in a lit room, suggesting that light aversion is at least a partial driver of this behavior. Spared nerve injury (SNI; as a model of neuropathy) impairs burrowing performance and correlates with enhanced mechanical sensitivity as assessed by von Frey filaments, as well as being pharmacologically reversed by an analgesic, gabapentin. Loss of the SNI-induced burrowing deficit was observed with daily testing post-surgery, but not when the testing interval was increased to 5 days, suggesting a confounding effect of daily repeat testing in this paradigm. Intraplantar complete Freund’s adjuvant (as a model of inflammatory pain) and systemic nitroglycerin (as a model of migraine-like symptoms) administration did not induce any burrowing deficit, indicating that assessment of burrowing behavior may not be universally suitable for the detection of behavioral changes across all rodent pain models.
Worldwide, osteoarthritis (OA) is one of the leading causes of chronic pain, for which adequate relief is not available. Ongoing peripheral input from the affected joint is a major factor in OA-associated pain. Therefore, this review focuses predominantly on peripheral targets emerging in the preclinical and clinical arena. Nerve growth factor is the most advanced of these targets, and its blockade has shown tremendous promise in clinical trials in knee OA. A number of different types of ion channels, including voltage-gated sodium channels and calcium channels, transient receptor potential channels, and acid-sensing ion channels, are important for neuronal excitability and play a role in pain genesis. Few channel blockers have been tested in preclinical models of OA, with varying results. Finally, we discuss some examples of G-protein coupled receptors, which may offer attractive therapeutic strategies for OA pain, including receptors for bradykinin, calcitonin gene-related peptide, and chemokines. Since many of the pathways described above can be selectively and potently targeted, they offer an exciting opportunity for pain management in OA, either systemically or locally.
Purpose of review:
Here, we give a topical overview of the ways in which brain processing can alter spinal pain transmission through descending control pathways, and how these change in pain states. We link preclinical findings on the transmitter systems involved and discuss how the monoamines, noradrenaline, 5-hydroxytryptamine (5-HT), and dopamine, can interact through inhibitory and excitatory pathways.
Recent findings:
Descending pathways control sensory events and the actions of the neurotransmitters noradrenaline and 5-HT in the dorsal horn of the spinal cord are chiefly implicated in nociception or antinociception according to the receptor that is activated. Abnormalities in descending controls effect central pain processing. Following nerve injury a noradrenaline-mediated control of spinal excitability is lost, whereas its restoration reduces neuropathic hypersensitivity. The story with 5-HT remains more complex because of the myriad of receptors that it can act upon; however the most recent findings support that facilitations may dominate over inhibitions.
Summary:
The monoaminergic system can be manipulated to great effect in the clinic resulting in improved treatment outcomes and is the basis for the actions of the antidepressant drugs in pain. Looking to the future, prediction of treatment responses will possible by monitoring a form of inhibitory descending control for optimized pain relief.
Wide-dynamic-range (WDR) neurons are found in the spinal dorsal horn and may be projection neurons and/or interneurons for polysynaptic reflexes. The cutaneous receptive field of a WDR neuron exhibits a gradient of sensitivity with the center responding to any mechanical stimulus while the periphery responds only to noxious stimuli. These neurons also receive signals from viscera, muscles, and joints. Thus WDR neurons continuously capture information from both the interface with the external environment and the internal milieu. This information constitutes a basic somesthetic activity, which may help to build a representation of the whole body. The sizes of the peripheral fields of WDR neurons may change as a result of plasticity in both excitatory and inhibitory mechanisms. The activity of a WDR neuron can be inhibited by noxious stimulation of most of the body outside its excitatory field. Thus a noxious stimulus will both activate a segmental subset of WDR neurons and inhibit the remaining population, thus disrupting the basic somesthetic activity and distorting the body representation in favor of the painful focus.
Neurons in the anterior cingulate cortex (ACC) are assumed to play important roles in the perception of nociceptive signals and the associated emotional responses. However, the neuronal types within the ACC that mediate these functions are poorly understood. In the present study, we used optogenetic techniques to selectively modulate excitatory pyramidal neurons and inhibitory interneurons in the ACC and to assess their ability to modulate peripheral mechanical hypersensitivity in freely moving mice. We found that selective activation of pyramidal neurons rapidly and acutely reduced nociceptive thresholds and that this effect was occluded in animals made hypersensitive using Freund's Complete Adjuvant (CFA). Conversely, inhibition of ACC pyramidal neurons rapidly and acutely reduced hypersensitivity induced by CFA treatment. A similar analgesic effect was induced by activation of parvalbumin (PV) expressing interneurons, whereas activation of somatostatin (SOM) expressing interneurons had no effect on pain thresholds. Our results provide direct evidence of the pivotal role of ACC excitatory neurons, and their regulation by PV expressing interneurons, in nociception.
Electronic supplementary material
The online version of this article (doi:10.1186/s13041-015-0170-6) contains supplementary material, which is available to authorized users.
Proprioception, the perception of body and limb position, is mediated by proprioceptors, specialized mechanosensory neurons that convey information about the stretch and tension experienced by muscles, tendons, skin and joints. In mammals, the molecular identity of the stretch-sensitive channel that mediates proprioception is unknown. We found that the mechanically activated nonselective cation channel Piezo2 was expressed in sensory endings of proprioceptors innervating muscle spindles and Golgi tendon organs in mice. Two independent mouse lines that lack Piezo2 in proprioceptive neurons showed severely uncoordinated body movements and abnormal limb positions. Moreover, the mechanosensitivity of parvalbumin-expressing neurons that predominantly mark proprioceptors was dependent on Piezo2 expression in vitro, and the stretch-induced firing of proprioceptors in muscle-nerve recordings was markedly reduced in Piezo2-deficient mice. Together, our results indicate that Piezo2 is the major mechanotransducer of mammalian proprioceptors.
Pain compromises the welfare of animals. A prerequisite for being able to alleviate pain is that we are able to recognize it. Potential behavioural signs of pain were investigated for dairy cattle with the aim of constructing a pain scale for use under production conditions. Forty-three cows were selected and fifteen different behaviours were scored, subsequently a clinical examination was performed to allocate the cows to a pain and non-pain group. The animals were then treated with an analgesic or a placebo and after a resting period the cows were re-scored by two observers blinded to the treatment. Six behaviours were found to be significantly different between the pain and non-pain group and robust enough to be included in the pain scale: ‘attention towards the surroundings’ ‘head position’, ‘ears position’, ‘facial expressions’, ‘response to approach’ and ‘back position’ (a seventh, piloerection, was also significant but seemed difficult to use as it changed rapidly; p < 0.05 for all measures). The Cow Pain Scale is the sum of the score for the aforementioned behaviours. For each individual animal before and after treatment, it was significantly lower after analgesic treatment (p = 0.003) in the ClinPain group but not after placebo treatment (p = 0.06); the pain score did not differ significantly before compared to after treatment with analgesic or placebo for the non-pain group (p = 0.2; p = 0.1). A second study was conducted to further validate the Cow Pain Scale. Cows from two herds were randomly selected (n = 119) and their behaviour scored by two observers. Subsequently the cows were clinically examined and allocated to a pain and non-pain group (n = 96, 23 cows were excluded because of incomplete examination). The cows from the pain group scored higher on The Cow Pain Scale compared to the non-pain group for both observer I (p < 0.0001) and observer II (p = 0.0001). For the two observers the sensitivity of the Cow Pain Scale was calculated to 0.61/0.75 and the specificity to 0.75/0.75 with a weighted Kappa of 0.62. In conclusion the Cow Pain Scale has the potential to be applied for the assessment of pain in dairy cattle under production conditions.
Cumulative evidence from both humans and animals suggests that the anterior cingulate cortex (ACC) is important for pain-related perception, and thus a likely target for pain relief therapy. However, use of existing electrode based ACC stimulation has not significantly reduced pain, at least in part due to the lack of specificity and likely co-activation of both excitatory and inhibitory neurons. Herein, we report a dramatic reduction of pain behavior in transgenic mice by optogenetic stimulation of the inhibitory neural circuitry of the ACC expressing channelrhodopsin-2. Electrophysiological measurements confirmed that stimulation of ACC inhibitory neurons is associated with decreased neural activity in the ACC. Further, a distinct optogenetic stimulation intensity and frequency-dependent inhibition of spiking activity in the ACC was observed. Moreover, we confirmed specific electrophysiological responses from different neuronal units in the thalamus, in response to particular types of painful stimuli (i,e., formalin injection, pinch), which we found to be modulated by optogenetic control of the ACC inhibitory neurons. These results underscore the inhibition of the ACC as a clinical alternative in inhibiting chronic pain, and leads to a better understanding of the pain processing circuitry of the cingulate cortex.
Somatosensory perception is dependent on a variety of primary sensory neurons that are able to detect and transmit sensations, which range from hedonic to painful (Basbaum et al., 2009, Lallemend and Ernfors, 2012 and Liu and Ma, 2011). Deciphering this complex neuronal diversity is a key challenge in sensory neurobiology. One intriguing facet is the role of the cutaneous mechanosensors that discriminate innocuous and noxious mechanical forces and respectively translate them into touch or pain sensations. Moreover, in the development of pathological pain, normal tactile signaling of low-threshold mechanoreceptors (LTMRs) is converted into sensations of pain. As LTMRs encompass different neuron subclasses, decoding the mechanisms that determine their sensitivity and their contribution to pain pathologies is critical to the development of future analgesic treatments.
The rostral ventromedial medulla (RVM) is a key brainstem structure that conveys powerful descending influence of the central pain-modulating system on spinal pain transmission and processing. Serotonergic (5-HT) neurons are a major component in the heterogeneous populations of RVM neurons and in the descending pathways from RVM. However, the descending influence of RVM 5-HT neurons on pain behaviors remains unclear.
In this study using optogenetic stimulation in tryptophan hydroxylase 2 (TPH2)- Channelrhodopsin 2 (ChR2) transgenic mice, we determined the behavioral effects of selective activation of RVM 5-HT neurons on mechanical and thermal pain behaviors in vivo. We found that ChR2-EYFP-positive neurons strongly co-localized with TPH2-positive (5-HT) neurons in RVM. Optogenetic stimulation significantly increased c-fos expression in 5-HT cells in the RVM of TPH2-ChR2 mice, but not in wild type mice. Behaviorally, the optogenetic stimulation decreased both mechanical and thermal pain threshold in an intensity-dependent manner, with repeated stimulation producing sensitized pain behavior for up to two weeks.
These results suggest that selective activation of RVM 5-HT neurons exerts a predominant effect of pain facilitation under control conditions.
At normal body temperature, the two-pore potassium channels TREK-1 (K2P2.1/KCNK2), TREK-2 (K2P10.1/KCNK10), and TRAAK (K2P4.1/KCNK2) regulate cellular excitability by providing voltage-independent leak of potassium. Heat dramatically potentiates K2P channel activity and further affects excitation. This review focuses on the current understanding of the physiological role of heat-activated K2P current, and discusses the molecular mechanism of temperature gating in TREK-1, TREK-2, and TRAAK.
Most pre-clinical analgesic efficacy assays still involve nociceptive testing in rodents. This is despite concerns as to the relevance of these tests for evaluating the pain-preventative properties of drugs. More appropriate methods would target pain rather than nociception, but these are currently not available, so it remains unknown whether animal pain equates to the negatively affective and subjective/emotional state it causes in humans. Mouse cancer models are common despite the likelihood of substantial pain. We used Conditioned Place Preference (CPP) testing, assessments of thermal hyperalgesia and behaviour to determine the likelihood that MBT-2 bladder cancer impacts negatively on mouse welfare, such as by causing pain. There was no CPP to saline, but morphine preference in tumour bearing mice exceeded that seen in tumour-free controls. This occurred up to 10 days before the study end-point alongside reduced body weight, development of hyperalgesia and behaviour changes. These effects indicated mice experienced a negative welfare state caused by malaise (if not pain) before euthanasia. Due to the complexity of the assessments needed to demonstrate this, it is unlikely that this approach could be used for routine welfare assessment on a study-by-study basis. However, our results show mice in sufficiently similar studies are likely to benefit from more intensive severity assessment and re-evaluation of end-points with a view to implementing appropriate refinements. In this particular case, a refinement would have been to have euthanased mice at least 7 days earlier or possibly by provision of end-stage pain relief. CPP testing was found to be a helpful method to investigate the responses of mice to analgesics, possibly on a subjective level. These findings and those of other recent studies show it could be a valuable method of screening candidate analgesics for efficacy against cancer pain and possibly other pain or disease models.
The spinal cord receives, relays and processes sensory information from the periphery and integrates this information with descending inputs from supraspinal centres to elicit precise and appropriate behavioural responses and orchestrate body movements. Understanding how the spinal cord circuits that achieve this integration are wired during development is the focus of much research interest. Several families of proteins have well-established roles in guiding developing spinal cord axons, and recent findings have identified new axon guidance molecules. Nevertheless, an integrated view of spinal cord network development is lacking, and many current models have neglected the cellular and functional diversity of spinal cord circuits. Recent advances challenge the existing spinal cord axon guidance dogmas and have provided a more complex, but more faithful, picture of the ontogenesis of vertebrate spinal cord circuits. Spinal neural circuits are established through the navigation of multiple types of neuronal axon to their appropriate synaptic targets. Chédotal reviews the cellular and molecular mechanisms that control this complex wiring, incorporating recent discoveries of new guidance factors.
Specific primary afferent fibres termed nociceptors are responsible for transmitting nociceptive information. Centrally the axonal terminals of these fibres synapse with secondary projection neurones in the spinal dorsal horn to transmit nociceptive information to the higher centres in the brain. Irrespective of the presence or absence of nociceptive inflow the activity of dorsal horn neurones is modulated by, amongst other things, local interneurones and descending midbrain and brainstem networks which can inhibit or facilitate dorsal horn transmission. These pathways therefore set the threshold for information inflow to the CNS. This review article summarises the anatomy, physiology and pharmacology particularly of these descending inhibitory and facilitatory pathways and explains why the study of descending modulation is essential if we are to develop more efficacious interventions for treating pain and relieving suffering.
This is the protocol for a review and there is no abstract. The objectives are as follows: To assess the analgesic efficacy of topical clonidine for chronic neuropathic pain in adults To assess the adverse events associated with the clinical use of topical clonidine for chronic neuropathic pain.
Significance:
Cannabinoids consistently produced antinociceptive effects in preclinical models, whereas they heterogeneously influenced the perception of experimentally induced pain in humans and did not provide robust clinical analgesia, which jeopardizes the translation of preclinical research on cannabinoid-mediated antinociception into the human setting.
Considerable evidence supports the notion that on- and off-cells of the rostral ventromedial medulla (RVM) facilitate and depress, respectively, spinal nociceptive transmission. This notion stems from a covariation of on- or off-cell activities and spinal nocifensive reflexes. Such covariation could theoretically be due to their independently responding to a common source, or to an RVM-derived modulation of ventral horn neurons. Here, we tested whether on- and off-cells indeed modulate spinal nociceptive neurons. In deeply anesthetized rats, unitary recordings were simultaneously made from an RVM on-like or off-like cell and a spinal nociceptive neuron that shared a receptive field (RF) at a hind paw. Action potential firing in RVM/spinal neuron pairs was highly correlated, positively for on-like cells and negatively for off-like cells, both during ongoing activity and during application of calibrated noxious pressure to the RF. Microinjection of morphine into RVM induced a correlated decrease in on-like cell/spinal neuron ongoing activity and response to noxious stimulation. RVM morphine induced changes in off-like cell activity that were not correlated with spinal neuronal activity. These results suggest that on-cells exert a positive modulation upon spinal nociceptive neurons, upstream to ventral horn circuits and plausibly at the origin of nociceptive information that eventually reaches the cerebral cortex. On-cells may in this manner contribute to inflammation- and neuropathy-induced increases in withdrawal reflexes. Most significantly, on-cell modulation of nociceptive neurons may be a key factor in clinical pain conditions such as hyperalgesia and allodynia.
The noradrenergic system is crucial for several activities in the body, including the modulation of pain. As the major producer of noradrenaline in the central nervous system, the Locus Coeruleus (LC) is a nucleus that has been studied in several pain conditions, mostly due to its strategic location. Indeed, apart from a well-known descending LC-spinal pathway that is important for pain control, an ascending pathway passing through this nucleus may be responsible for the noradrenergic inputs to higher centers of the pain processing, such as the limbic system and frontal cortices. Thus, the noradrenergic system appears to modulate different components of the pain experience and accordingly, its manipulation has distinct behavioral outcomes. The main goal of this review is to bring together the data available regarding the noradrenergic system in relation to pain, particularly focusing on the ascending and descending LC projections in different conditions. How such findings influence our understanding of these conditions is also discussed.
The rostroventral medial medulla (RVM) is part of a rapidly acting spino-bulbo-spinal loop that is activated by ascending nociceptive inputs and drives descending feedback modulation of spinal nociception. In the adult rat, the RVM can facilitate or inhibit dorsal horn neuron inputs but in young animals descending facilitation dominates. It is not known whether this early life facilitation is part of a feedback loop. We hypothesized that the newborn RVM functions independently of sensory input, before the maturation of feedback control. We show here that noxious hind paw pinch evokes no fos activation in the RVM or the periaqueductal gray at postnatal day (P) 4 or P8, indicating a lack of nociceptive input at these ages. Significant fos activation was evident at P12, P21, and in adults. Furthermore, direct excitation of RVM neurons with microinjection of DL-homocysteic acid did not alter the net activity of dorsal horn neurons at P10, suggesting an absence of glutamatergic drive, whereas the same injections caused significant facilitation at P21. In contrast, silencing RVM neurons at P8 with microinjection of lidocaine inhibited dorsal horn neuron activity, indicating a tonic descending spinal facilitation from the RVM at this age. The results support the hypothesis that early life descending facilitation of spinal nociception is independent of sensory input. Since it is not altered by RVM glutamatergic receptor activation, it is likely generated by spontaneous brainstem activity. Only later in postnatal life can this descending activity be modulated by ascending nociceptive inputs in a functional spinal-bulbo-spinal loop.
Descending influences on the spinal nociceptive tail-flick (TF) reflex produced by focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGCα) were examined and characterized in rats lightly anesthetized with pentobarbital. Both inhibition and facilitation of the TF reflex were produced by electrical stimulation at identical sites in the NGC/NGCα; glutamate microinjection only inhibited the TF reflex.
The chronaxie of stimulation for inhibition of the TF reflex was 169 ± 28 μsec. Inhibition of the TF reflex by stimulation was produced throughout the NGC and NGCα; intensities of stimulation for inhibition were least in the ventral NGC and in the NGCα. At threshold intensities of stimulation, inhibition of the TF reflex did not outlast the period of stimulation.
Facilitation of the TF reflex was produced at many of the same sites at which stimulation inhibited the TF reflex, but always at lesser intensities of stimulation (mean, 10 μA vs. 43 μA for inhibition, n = 25). Stimulation in the NGC/NGCα at threshold intensities for facilitation or inhibition of the TF reflex did not significantly affect blood pressure.
Strength-duration characterization of electrical stimulation and microinjection of glutamate into identical sites in the NGC and NGCα suggest that descending inhibition of the TF reflex results from activation of cell bodies in the NGC and NGCα.
Chronic pain is a major medical problem that is resistant to conventional medical intervention. It also causes emotional changes such as anxiety and fear. Furthermore, anxiety or fear often enhances the suffering of pain. Based on recent studies, I propose chronic anxiety triggered by injury or chronic pain is mediated through presynaptic long-term potentiation (LTP) in the anterior cingulate cortex (ACC), a key cortical region for pain perception. Conversely, NMDA receptor-dependent postsynaptic LTP plays a more important role in behavioral sensitization in chronic pain. Thus, postsynaptic and presynaptic LTP in ACC neurons are likely the key cellular mechanisms for causing chronic pain and its associated anxiety, respectively. This suggests potential targets for treating chronic pain and related anxiety.
Purpose
Many studies have examined the postoperative analgesic effects of nefopam in various settings. However, although nefopam is expected to be useful in bimaxillary osteotomy, no published data are available.
Material and methods
We divided 42 patients into nefopam [n = 21, nefopam 20 mg intravenous (i.v.) 30 min before surgery, followed by an i.v. infusion (5 mg/h) beginning immediately postoperatively for 24 h] and control [n = 21, normal saline] groups. Then we compared the analgesic efficacy, side effects, and degree of patient satisfaction with postoperative analgesia.
Results
Pain was lower in the nefopam group than in the controls in the recovery room [4.6 (3.0–6.0) vs. 6.0 (5.5–7.0), median (interquartile range), P = 0.002] and on the ward. Fewer patients in the nefopam group required rescue analgesics, and the degree of patient satisfaction was significantly higher in the nefopam group (P < 0.001). There were no significant differences in other side effects between the groups. However, the control group showed more sedation 1 h postoperatively (P = 0.009).
Conclusion
Nefopam is an effective analgesic in bimaxillary osteotomy in that it can reduce the use of opioids and nonsteroidal anti-inflammatory drugs, thereby reducing the side effects of conventional analgesics.
(Trial Registration: ClinicalTrials.gov (NCT 01461031))
All mammals possess the capacity to affect appropriate responses to “escapable” or “inescapable” stressors, and to facilitate recovery and healing once the stress passes. Different stressors possess, to varying degrees, physical and psychological components. A substantial body of evidence has been reviewed, which supports the concept that the Periaqueductal Gray (PAG) is divisible into a number of distinct, longitudinal neuronal columns each of which lies embedded within a circuit that extends rostrally to include specific prefrontal cortical (PFC) and hypothalamic areas. These PFC-PAG/hypothalamic circuits, in turn project caudally to affect somatic and autonomic pre-motor neurons within the ventrolateral medulla and and ventromedial (raphe) and paramedian medullary neural pools. The evidence reviewed suggests that different PAG columns (and their distinctive forebrain and brainstem connections) play important roles in coordinating distinct emotional coping strategies for dealing with different classes of stress. Specifically, it has been proposed: (i) that the lPAG column (and its associated circuit) is activated preferentially by “escapable” physical stressors to which an active defensive reaction(s) is the primary response; (ii) that the dlPAG column (and its associated circuit) is activated preferentially by “escapable” psychological stressors to which an active defensive response is the primary response; and (iii) that the vlPAG column (and its associated circuit) is activated: (a) by “inescapable” physical or psychological stressors for which passive coping behavior is the primary response; or (b) as a delayed/secondary response, to promote recovery and healing following any stressor.
This chapter deals with projections that originate from the spinal cord and descent that brain to spinal cord. The ascending spinal projections connect the spinal cord to supraspinal levels and transmit sensory information such as pain, temperature, position sense, and touch from somatic structures and pressure, pain and visceral information from internal organs. Spinal cord neurons project to the brainstem, cerebellum, midbrain, diencephalons, and telencephalon. Some projections, such as the postsynaptic dorsal column pathway and the spinocervical pathway, synapse with second-order neurons, which in turn project to higher centers. Some of the projections are directly to supraspinal structures, such as the spinothalamic, spinomesencephalic, and spinohypothalamic tracts. Ascending spinal projections are composed of mostly myelinated dorsal root fibers from the dorsal root ganglia and axons of spinal neurons. Anatomically, the ascending spinal projections are located in the ventral, lateral and dorsal funiculi on each side of the spinal cord. In experimental animals, electrophysiological studies and anatomical tract tracing methods using tracers and Wallerian degeneration technique have collectively provided extensive and accurate information on ascending spinal projections. In humans, on the contrary, information is limited to data obtained from patients with localized traumatic and inflammatory spinal lesions, or surgical interventions.
Two-pore-domain K(+) (K(2P)) channel subunits are made up of four transmembrane segments and two pore-forming domains that are arranged in tandem and function as either homo- or heterodimeric channels. This structural motif is associated with unusual gating properties, including background channel activity and sensitivity to membrane stretch. Moreover, K(2P) channels are modulated by a variety of cellular lipids and pharmacological agents, including polyunsaturated fatty acids and volatile general anaesthetics. Recent in vivo studies have demonstrated that TREK1, the most thoroughly studied K(2P) channel, has a key role in the cellular mechanisms of neuroprotection, anaesthesia, pain and depression.
Salient events activate the midbrain dopaminergic system and have important impacts on various aspects of mnemonic function, including the stability of hippocampus-dependent memories. Dopamine is also central to modulation of neocortical memory processing, particularly during prefrontal cortex-dependent working memory. Here, we review the current state of the circuitry and physiology underlying dopamine's actions, suggesting that-alongside local effects within hippocampus and prefrontal cortex-dopamine released from the midbrain ventral tegmental area is well positioned to dynamically tune interactions between limbic-cortical circuits through modulation of rhythmic network activity.
© 2015 Elsevier B.V. All rights reserved.
Individuals diagnosed with fibromyalgia syndrome (FMS) report chronic pain that is frequently worsened by physical activity and improved by rest. Palpation of muscle and tendinous structures suggests that nociceptors in deep tissues are abnormally sensitive in FMS, but methods of controlled mechanical stimulation of muscles are needed to better characterize the sensitivity of deep tissues. Accordingly, force‐controlled mechanical stimulation was applied to the flexor digitorum muscle of the forearm in a series of brief contacts (15 stimuli, each of 1 s duration, at 3 or 5 s interstimulus intervals). Repetitive stimulation was utilized to determine whether temporal summation of deep muscular pain would occur for normal subjects and would be enhanced for FMS subjects. Moderate temporal summation of deep pain was observed for normal controls (NC), and temporal summation was greatly exaggerated for FMS subjects. Temporal summation for FMS subjects occurred at substantially lower forces and at a lower frequency of stimulation. Furthermore, painful after‐sensations were greater in amplitude and more prolonged for FMS subjects. These observations complement a previous demonstration that temporal summation of pain and after‐sensations elicited by thermal stimulation of the skin are moderately enhanced for FMS subjects. Abnormal input from muscle nociceptors appears to underlie production of central sensitization in FMS that generalizes to input from cutaneous nociceptors.
Inhibition of spinal and trigeminal withdrawal reflexes by morphine and by the cannabinoid agonist HU 210 has been studied in anaesthetized and in decerebrated rabbits. In intact, pentobarbitone-anaesthetized animals, the jaw-depressor reflex (JDR) evoked by stimulation of the tongue, and the reflex elicited in the ankle flexor tibialis anterior (TA) by stimulation of the toes were inhibited to the same extent by morphine (1–30 mg kg−1 i.v. cumulative). In spinalized, anaesthetized rabbits morphine depressed the JDR to the same level as in non-spinal preparations, but the effect of the opioid on the TA reflex was significantly reduced. All effects of morphine were reversed by naloxone (0.25 mg kg−1, i.v.). In anaesthetised intact animals, HU 210 depressed the JDR at a dose of 100 nmol kg−1 i.v. cumulative, reduced reflexes evoked in the knee flexor muscle semitendinosus (ST) by stimulation at the toes at a dose of 30 nmol kg−1 i.v. cumulative, but had no consistent or significant effects on the TA reflex to toe stimulation. The same results were obtained in spinalized, anaesthetised animals. In decerebrated, spinalized rabbits with no anaesthesia, HU 210 (30 nmol kg−1) depressed both ST and TA reflexes evoked by toe stimulation. These data reveal that trigeminal and spinal withdrawal reflexes are equally sensitive to morphine provided the spinal cord is intact, suggesting that at least part of the action of systemic morphine is due to activation of descending inhibition. The present results also show for the first time that cannabinoid agonists can inhibit trigeminal withdrawal reflexes. HU 210 had differential effects on the three reflexes studied depending on the presence or absence of anaesthesia. This is the first occasion on which we have found pharmacological distinctions between withdrawal reflexes, and indicates that spinal sensorimotor processing is more heterogeneous than has been suspected previously.
The descending noradrenergic (NAergic) projection to the spinal cord forms part of an endogenous analgesic system. After nerve injury, a localised failure in this compensatory system has been implicated as a permissive factor in the development of neuropathic sensitisation. We investigated whether restoring descending NAergic tone with intrathecal reboxetine can oppose the development of the neuropathic pain phenotype after tibial nerve transection (TNT). Rats had a lumbar intrathecal catheter implanted at the time of nerve injury for administration of reboxetine (10 μg) in both acute and chronic dosing experiments. In acute dosing experiments, both intrathecal and systemic (30 mg/kg) reboxetine partially reversed mechanical allodynia. This antiallodynic effect of intrathecal reboxetine was blocked by prior administration of yohimbine (α2-adrenoceptor antagonist, 30 μg) but not by prazosin (α1-adrenoceptor antagonist, 30 μg) or propranolol (β-adrenoceptor antagonist, 100 μg). Chronic intrathecal reboxetine (10 μg, intrathecally, twice daily for 2 weeks) suppressed the development of cold and mechanical allodynia. Nerve-injured animals demonstrated a place preference for intrathecal reboxetine, suggesting that it also reduced spontaneous pain. In contrast, an equivalent antiallodynic dose of systemic reboxetine (30 mg/kg) was aversive in both naive and TNT rats. On cessation of chronic intrathecal reboxetine, there was a gradual development of allodynic sensitisation that was indistinguishable from control TNT animals by 7 days after the end of dosing. Our results suggest that pharmacological restoration of spinal NAergic tone with intrathecal reboxetine can suppress both allodynia and spontaneous pain in the TNT model.
Key points
Electrical stimulation of the rostral ventromedial medulla (RVM) facilitates pain behaviours in neonates but inhibits these behaviours in adults. The cellular mechanisms underlying these changes in RVM modulation of pain behaviours are not known.
We optimized whole‐cell patch‐clamp recordings for RVM neurons in animals older than postnatal day 30 and compared the results to postnatal day 10–21 animals.
Our results demonstrate that the γ‐aminobutyric acid (GABA) release is lower and opioid effects are more evident in adult rats compared to early postnatal rats.
A cannabinoid receptor antagonist significantly increased GABA release in mature but not in immature RVM neurons suggesting the presence of local endocannabinoid tone in mature RVM.
Abstract
Neurons in the rostral ventromedial medulla (RVM) play critical and complex roles in pain modulation. Recent studies have shown that electrical stimulation of the RVM produces pain facilitation in young animals (postnatal (PN) day < 21) but predominantly inhibits pain behaviours in adults. The cellular mechanisms underlying these changes in RVM modulation of pain behaviours are not known. This is in part because whole‐cell patch‐clamp studies in RVM to date have been in young (PN day < 18) animals because the organization and abundance of myelinated fibres in this region make the RVM a challenging area for whole‐cell patch‐clamp recording in adults. Several neurotransmitter systems, including GABAergic neurotransmission, undergo developmental changes that mature by PN day 21. Thus, we focused on optimizing whole‐cell patch‐clamp recordings for RVM neurons in animals older than PN day 30 and compared the results to animals at PN day 10–21. Our results demonstrate that the probability of GABA release is lower and that opioid and endocannabinoid effects are more evident in adult rats (mature) compared to early postnatal (immature) rats. Differences in these properties of RVM neurons may contribute to the developmental changes in descending control of pain from the RVM to the spinal cord.
Two-pore domain background K(+) channels (K2p or KCNK) produce hyperpolarizing currents that control cell-membrane polarity and neuronal excitability throughout the nervous system. The TREK2 channel as well as the related TREK1 and TRAAK channels are mechanical-, thermal- and lipid-gated channels that share many regulatory properties. TREK2 is one of the major background channels expressed in rodent nociceptive neurons of the dorsal root ganglia that innervate the skin and deep body tissues, but its role in somatosensory perception and nociception remained poorly understood. We now report that TREK2 is a regulatory channel that controls the perception of non aversive warm, between 40°C and 46°C, and moderate ambient cool-temperatures, between 20°C and 25°C, in mice. TREK2 controls the firing activity of peripheral sensory C-fibers in response to changes in temperature. The role of TREK2 in thermosensation is different from that of TREK1 and TRAAK channels. Rather, TREK2, TREK1 and TRAAK channels appear to have complementary roles in thermosensation. TREK2 is also involved in mechanical pain perception and in osmotic pain after sensitization by prostaglandin E2. TREK2 is involved in the cold allodynia that characterizes the neuropathy commonly associated with treatments with the anticancer drug oxaliplatin. These results suggest that positive modulation of the TREK2 channel may have beneficial analgesic effects in these neuropathic conditions.