Human psychophysics and rodent spinal neurones exhibit peripheral and central mechanisms of inflammatory pain in the UVB and UVB heat rekindling models

A Parallel Human and Rat Investigation of the Interaction Between Descending and Spinal Modulatory Mechanisms

BACKGROUND

Healthy individuals demonstrate considerable heterogeneity upon dynamic quantitative sensory testing assessment of endogenous pain modulatory mechanisms. For those who stratify into a 'pro-nociceptive profile' cohort, consisting of inefficient conditioned pain modulation (CPM) and elevated temporal summation of pain (TSP), the optimal approach for balancing the net output of pain modulatory processes towards anti-nociception remains unresolved. In this translational healthy human and rat study, we examined whether descending modulation countered spinal amplification during concurrent application of a CPM and TSP paradigm alongside pupillometry since pontine activity was previously linked to functionality of endogenous pain modulatory mechanisms and pupil dilation.

METHODS

Perceptual (quantitative sensory testing) and spinal neuronal (in vivo electrophysiology) assessment was performed in healthy humans and rats respectively upon application of parallel CPM/diffuse noxious inhibitory controls (cuff algometry) and TSP/wind-up (pinprick) paradigms alongside pupillometry.

RESULTS

In humans, repetitive pinprick stimulation produced TSP while concurrent application of a noxious conditioning stimulus did not affect pain ratings to a single pinprick stimulus, repetitive stimulation or the wind-up ratio. In rats, repetitive pinprick produced neuronal wind-up while concurrent application of a noxious conditioning stimulus inhibited neuronal responses to a single stimulus and repetitive stimulation but not the wind-up ratio. For pupillometry experiments, dilatory responses did not increase during application of a TSP or CPM paradigm in humans, while reliable rat responses were not obtained.

CONCLUSIONS

Under the conditions of our study, spinal amplification mechanisms surpassed descending inhibitory controls while pupillometry did not offer a reliable indicator of endogenous pain modulatory mechanism function.

SIGNIFICANCE

In this translational healthy human and rat study, activity in descending inhibitory controls did not counter spinal amplification processes underpinned by wind up. Despite pupil dilation being previously linked to modulatory mechanisms, dilatory responses did not offer a reliable indicator of functionality. For pro-nociceptive individuals exhibiting inefficient conditioned pain modulation and/or high temporal summation of pain, dampening faciliatory mechanisms rather than augmenting top-down inhibitory processes may be a more effective pain-relief strategy.

A back-translational study of descending interactions with the induction of hyperalgesia by high-frequency electrical stimulation in rats and humans

ABSTRACT

In humans and animals, high-frequency electrocutaneous stimulation (HFS) induces an "early long-term potentiation-like" sensitisation, where synaptic plasticity is underpinned by an ill-defined interaction between peripheral input and central modulatory processes. The relative contributions of these processes to the initial pain or nociceptive response likely differ from those that underpin development of the heightened response. To investigate the impact of HFS-induced hyperalgesia on pain and nociception in perception and neural terms, respectively, and to explore the impact of descending inhibitory pathway activation on the development of HFS-induced hyperalgesia, we performed parallel studies utilising identical stimuli to apply HFS concurrent to (1) a conditioned pain modulation paradigm during psychophysical testing in healthy humans or (2) a diffuse noxious inhibitory controls paradigm during in vivo electrophysiological recording of spinal neurones in healthy anaesthetised rats. High-frequency electrocutaneous stimulation alone induced enhanced perceptual responses to pinprick stimuli in cutaneous areas secondary to the area of electrical stimulation in humans and increased the excitability of spinal neurones which exhibited stimulus intensity-dependent coded responses to pinprick stimulation in a manner that tracked with human psychophysics, supporting their translational validity. Application of a distant noxious conditioning stimulus during HFS did not alter perceived primary or secondary hyperalgesia in humans or the development of primary or secondary neuronal hyperexcitability in rats compared with HFS alone, suggesting that, upon HFS-response initiation in a healthy nervous system, excitatory signalling escapes inhibitory control. Therefore, in this model, dampening facilitatory mechanisms rather than augmenting top-down inhibitions could prevent pain development.

Effects of systemic oxytocin administration on ultraviolet B-induced nociceptive hypersensitivity and tactile hyposensitivity in mice

Ultraviolet B (UVB) radiation induces cutaneous inflammation, leading to thermal and mechanical hypersensitivity. Here, we examine the mechanical properties and profile of tactile and nociceptive peripheral afferents functionally disrupted by this injury and the role of oxytocin (OXT) as a modulator of this disruption. We recorded intracellularly from L4 afferents innervating the irradiated area (5.1 J/cm2) in 4-6 old week male mice (C57BL/6J) after administering OXT intraperitoneally, 6 mg/Kg. The distribution of recorded neurons was shifted by UVB radiation to a pattern observed after acute and chronic injuries and reduced mechanical thresholds of A and C- high threshold mechanoreceptors while reducing tactile sensitivity. UVB radiation did not change somatic membrane electrical properties or fiber conduction velocity. OXT systemic administration rapidly reversed these peripheral changes toward normal in both low and high-threshold mechanoreceptors and shifted recorded neuron distribution toward normal. OXT and V1aR receptors were present on the terminals of myelinated and unmyelinated afferents innervating the skin. We conclude that UVB radiation, similar to local tissue surgical injury, cancer metastasis, and peripheral nerve injury, alters the distribution of low and high threshold mechanoreceptors afferents and sensitizes nociceptors while desensitizing tactile units. Acute systemic OXT administration partially returns all of those effects to normal.

Sensory defunctionalization induced by 8% topical capsaicin treatment in a model of ultraviolet-B-induced cutaneous hyperalgesia

Subpopulations of primary nociceptors (C- and Aδ-fibers), express the TRPV1 receptor for heat and capsaicin. During cutaneous inflammation, these afferents may become sensitized, leading to primary hyperalgesia. It is known that TRPV1⁺ nociceptors are involved in heat hyperalgesia; however, their involvement in mechanical hyperalgesia is unclear. This study explored the contribution of capsaicin-sensitive nociceptors in the development of mechanical and heat hyperalgesia in humans following ultraviolet-B (UVB) irradiation. Skin areas in 18 healthy volunteers were randomized to treatment with 8% capsaicin/vehicle patches for 24 h. After patches removal, one capsaicin-treated area and one vehicle area were irradiated with 2xMED (minimal erythema dose) of UVB. 1, 3 and 7 days post-UVB exposure, tests were performed to evaluate the development of UVB-induced cutaneous hyperalgesia: thermal detection and pain thresholds, pain sensitivity to supra-threshold heat stimuli, mechanical pain threshold and sensitivity, touch pleasantness, trans-epidermal water loss (TEWL), inflammatory response, pigmentation and micro-vascular reactivity. Capsaicin pre-treatment, in the UVB-irradiated area (Capsaicin + UVB area), increased heat pain thresholds (P < 0.05), and decreased supra-threshold heat pain sensitivity (P < 0.05) 1, 3 and 7 days post-UVB irradiation, while mechanical hyperalgesia resulted unchanged (P > 0.2). No effects of capsaicin were reported on touch pleasantness (P = 1), TEWL (P = 0.31), inflammatory response and pigmentation (P > 0.3) or micro-vascular reactivity (P > 0.8) in response to the UVB irradiation. 8% capsaicin ablation predominantly defunctionalizes TRPV1⁺-expressing cutaneous nociceptors responsible for heat pain transduction, suggesting that sensitization of these fibers is required for development of heat hyperalgesia following cutaneous UVB-induced inflammation but they are likely only partially necessary for the establishment of robust primary mechanical hyperalgesia.

Mechanical sensitization, increased axonal excitability, and spontaneous activity in C-nociceptors after ultraviolet B irradiation in pig skin

ABSTRACT

Ultraviolet B (UVB) irradiation induces hyperalgesia in human and animal pain models. We investigated mechanical sensitization, increase in axonal excitability, and spontaneous activity in different C-nociceptor classes after UVB in pig skin. We focused on units with receptive fields covering both irradiated and nonirradiated skin allowing intraindividual comparisons. Thirty-five pigs were irradiated in a chessboard pattern, and extracellular single-fibre recordings were obtained 10 to 28 hours later (152 fibers). Units from the contralateral hind limb served as a control (n = 112). Irradiated and nonirradiated parts of the same innervation territory were compared in 36 neurons; low threshold C-touch fibers (n = 10) and sympathetic efferents (n = 2) were unchanged, but lower mechanical thresholds and higher discharge frequency at threshold were found in mechanosensitive nociceptors (n = 12). Half of them could be activated with nonnoxious brush stimuli in the sunburn. Four of 12 mechanoinsensitive nociceptors were found sensitized to mechanical stimulation in the irradiated part of the receptive field. Activity-dependent slowing of conduction was reduced in the irradiated and in the nonirradiated skin as compared with the control leg, whereas increased ability to follow high stimulation frequencies was restricted to the sunburn (108.5 ± 37 Hz UVB vs 6.3 ± 1 Hz control). Spontaneous activity was more frequent in the sunburn (72/152 vs 31/112). Mechanical sensitization of primary nociceptors and higher maximum after frequency are suggested to contribute to primary hyperalgesia, whereas the spontaneous activity of silent nociceptors might offer a mechanistic link contributing to ongoing pain and facilitated induction of spinal sensitization.

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Challenges and opportunities in translational pain research - An opinion paper of the working group on translational pain research of the European pain federation (EFIC)

For decades, basic research on the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need. In this opinion paper bringing together pain researchers from very different disciplines, the opportunities and challenges of translational pain research are discussed. The many factors that may prevent the successful translation of bench observations into useful and effective clinical applications are reviewed, including interspecies differences, limited validity of currently available preclinical disease models of pain, and limitations of currently used methods to assess nociception and pain in non-human and human models of pain. Many paths are explored to address these issues, including the backward translation of observations made in patients and human volunteers into new disease models that are more clinically relevant, improved generalization by taking into account age and sex differences, and the integration of psychobiology into translational pain research. Finally, it is argued that preclinical and clinical stages of developing new treatments for pain can be improved by better preclinical models of pathological pain conditions alongside revised methods to assess treatment-induced effects on nociception in human and non-human animals. Significance: For decades, basic research of the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need.

UVB irradiation induces contralateral changes in galanin, substance P and c-fos immunoreactivity in rat dorsal root ganglia, dorsal horn and lateral spinal nucleus

The selection of control group is crucial, as the use of an inadequate group may strongly affect the results. In this study we examine the effect on contralateral tissue protein levels, in a model of unilateral UVB irradiation, as the contralateral side is commonly used as a control. Previous studies have shown that UVB irradiation increases immunoreactivity for inflammatory regulated neuropeptides. Unilateral UVB irradiation of rat hind paw was performed and corresponding contralateral spinal cord and dorsal root ganglia (DRG) were collected 2-96 h after and investigated for changes in galanin, substance P and c-fos immunoreactivity. Control tissue was collected from naïve rats. Measurement of skin blood flow from contralateral heel hind paws (Doppler), revealed no change compared to naïve rats. However, UVB irradiation caused a significant reduction in the contralateral proportion of galanin immunopositive DRG neurons, at all-time points, as well as an increase in the contralateral spinal cord dorsal horn, around the central canal and in the lateral spinal nucleus (2-48 h). The contralateral proportion of SP positive DRG neurons and dorsal horn immunoreactivity was unchanged, whereas the lateral spinal nucleus area showed increased immunoreactivity (48 h). UVB irradiation also induced a slight contralateral upregulation of c-fos in the dorsal horn/central canal area (24 and 48 h). In summary, unilateral UVB irradiation induced contralateral changes in inflammatory/nociceptive neuropeptides in spinal cord and afferent pathways involved in pain signaling already within 24 h, a time point when also ipsilateral neurochemical/physiological changes have been reported for rats and humans.

Supraspinal Opioid Circuits Differentially Modulate Spinal Neuronal Responses in Neuropathic Rats

BACKGROUND

The anterior cingulate cortex and central nucleus of the amygdala connect widely with brainstem nuclei involved in descending modulation, including the rostral ventromedial medulla. Endogenous opioids in these circuits participate in pain modulation. The hypothesis was that a differential opioidergic role for the brain nuclei listed in regulation of spinal neuronal responses because separable effects on pain behaviors in awake animals were previously observed.

METHODS

This study utilized in vivo electrophysiology to determine the effects of morphine microinjection into the anterior cingulate cortex, right or left central nucleus of the amygdala, or the rostral ventromedial medulla on spinal wide dynamic range neuronal responses in isoflurane-anesthetized, male Sprague-Dawley rats. Ongoing activity in the ventrobasal thalamus was also measured. In total, 33 spinal nerve ligated and 26 control age- and weight-matched control rats were used.

RESULTS

Brainstem morphine reduced neuronal firing to 60-g von Frey stimulation in control rats (to 65 ± 12% of control response (means ± 95% CI), P < 0.001) with a greater inhibition in neuropathic rats (to 53 ± 17% of control response, P < 0.001). Contrasting anterior cingulate cortex morphine had only marginal modulatory effects on spinal neuronal responses with limited variance in effect between control and neuropathic rats. The inhibitory effects of morphine in the central nucleus of the amygdala were dependent on pain state and laterality; only right-side morphine reduced neuronal firing to 60-g stimulation in neuropathic rats (to 65 ± 14% of control response, P = 0.001). In addition, in neuropathic rats elevated ongoing neuronal activity in the ventral posterolateral thalamus was not inhibited by anterior cingulate cortex morphine, in contrast to evoked responses.

CONCLUSIONS

Cumulatively the data support opioid modulation of evoked responses predominately through a lateralized output from the right amygdala, as well as from the brainstem that is enhanced in injured conditions. Minimal modulation of dorsal horn responses was observed after anterior cingulate cortex opioid administration regardless of injury state.

Activation of the descending pain modulatory system using cuff pressure algometry: Back translation from man to rat

BACKGROUND

Diffuse noxious inhibitory controls (DNIC) as measured in rat and conditioned pain modulation (CPM), the supposed psychophysical paradigm of DNIC measured in humans, are unique manifestations of an endogenous descending modulatory pathway that is activated by the application of a noxious conditioning stimulus. The predictive value of the human CPM processing is crucial when deliberating the translational worth of the two phenomena.

METHODS

For CPM or DNIC measurement, test and conditioning stimuli were delivered using a computer-controlled cuff algometry system or manual inflation of neonate blood pressure cuffs, respectively. In humans (n = 20), cuff pain intensity (for pain detection and pain tolerance thresholds) was measured using an electronic visual analogue scale. In isoflurane-anaesthetized naïve rats, nociception was measured by recording deep dorsal horn wide dynamic range (WDR) neuronal firing rates (n = 7) using in vivo electrophysiology.

RESULTS

A painful cuff-pressure conditioning stimulus on the leg increased pain detection and pain tolerance thresholds recorded by cuff stimulation on the contralateral leg in humans by 32% ± 3% and 24% ± 2% (mean ± SEM) of baseline responses, respectively (p < .001). This finding was back-translated by revealing that a comparable cuff-pressure conditioning stimulus (40 kPa) on the hind paw inhibited the responses of WDR neurons to noxious contralateral cuff test stimulation to 42% ± 9% of the baseline neuronal response (p = .003).

CONCLUSIONS

These data substantiate that the noxious cuff pressure paradigm activates the descending pain modulatory system in rodent (DNIC) and man (CPM), respectively. Future back and forward translational studies using cuff pressure algometry may reveal novel mechanisms in varied chronic pain states.

SIGNIFICANCE

This study provides novel evidence that a comparable noxious cuff pressure paradigm activates a unique form of endogenous inhibitory control in healthy rat and man. This has important implications for the forward translation of bench and experimental pain research findings to the clinical domain. If translatable mechanisms underlying dysfunctional endogenous inhibitory descending pathway expression (previously evidenced in painful states in rat and man) were revealed using cuff pressure algometry, the identification of new analgesic targets could be expedited.

Translational issues in precision medicine in neuropathic pain

Neuropathic pain remains poorly treated, with most new drugs falling through the translational gap. The traditional model of bench-to-bedside research has relied on identifying new mechanisms/targets in animal models and then developing clinical applications. Several have advocated bridging the translational gap by beginning with clinical observations and back-translating to animal models for further investigation of mechanisms. There is good evidence that phenotyping of patients through quantitative sensory testing can lead to improved treatment selection and hence improved patient outcomes. This practice has been widely adopted in clinical investigations, but its application in preclinical research is not mainstream. In this review, we retrospectively examine our historical rodent data sets with the aim of reconsidering drug effects on sensory neuronal endpoints, their alignment with clinical observations, and how these might guide future clinical studies.

Issues in the future development of new analgesic drugs

PURPOSE OF REVIEW

There is a clear unmet need for either the development of new drugs for the treatment of painful pathologies or the better use of the existing agents denoted by the lack of efficacy of many existing drugs in a number of patients, limitations of their use due to severity of side effects, and by the high number of drugs that fail to reach clinical efficacy from preclinical development. This account considers the efforts being made to better validate new analgesic components and to improve translational efficacy of existing drugs.

RECENT FINDINGS

A better use of the available models and tools can improve the predictive validity of new analgesic drugs, as well as using intermediate steps when translating drugs to clinical context such as characterizing drugs using stem cell-sensory derived neurones. Profiling patient sensory phenotypes can decrease the number of failed clinical trials and improve patient outcome.

SUMMARY

An integrative approach, comprising the use of complementary techniques to fully characterize drug profiles, is necessary to improve translational success of new analgesics.

Selective deficiencies in descending inhibitory modulation in neuropathic rats: implications for enhancing noradrenergic tone

Supplemental Digital Content is Available in the Text.

Descending noradrenergic pathways modulate spontaneous but not evoked thalamic neuronal hyperexcitability in neuropathic pain states. Spinal clonidine inhibits evoked and spontaneous firing, whereas reboxetine selectively inhibits evoked firing.

Pontine noradrenergic neurones form part of a descending inhibitory system that influences spinal nociceptive processing. Weak or absent descending inhibition is a common feature of chronic pain patients. We examined the extent to which the descending noradrenergic system is tonically active, how control of spinal neuronal excitability is integrated into thalamic relays within sensory-discriminative projection pathways, and how this inhibitory control is altered after nerve injury. In vivo electrophysiology was performed in anaesthetised spinal nerve–ligated (SNL) and sham-operated rats to record from wide dynamic range neurones in the ventral posterolateral thalamus (VPL). In sham rats, spinal block of α₂-adrenoceptors with atipamezole resulted in enhanced stimulus-evoked and spontaneous firing in the VPL, and produced conditioned place avoidance. However, in SNL rats, these conditioned avoidance behaviours were absent. Furthermore, inhibitory control of evoked neuronal responses was lost, but spinal atipamezole markedly increased spontaneous firing. Augmenting spinal noradrenergic tone in neuropathic rats with reboxetine, a selective noradrenergic reuptake inhibitor, modestly reinstated inhibitory control of evoked responses in the VPL but had no effect on spontaneous firing. By contrast, clonidine, an α₂ agonist, inhibited both evoked and spontaneous firing, and exhibited increased potency in SNL rats compared with sham controls. These data suggest descending noradrenergic inhibitory pathways are tonically active in sham rats. Moreover, in neuropathic states, descending inhibitory control is diminished, but not completely absent, and distinguishes between spontaneous and evoked neuronal activity. These observations may have implications for how analgesics targeting the noradrenergic system provide relief.

Neuropathy following spinal nerve injury shares features with the irritable nociceptor phenotype: A back-translational study of oxcarbazepine

BACKGROUND

The term 'irritable nociceptor' was coined to describe neuropathic patients characterized by evoked hypersensitivity and preservation of primary afferent fibres. Oxcarbazepine is largely ineffectual in an overall patient population, but has clear efficacy in a subgroup with the irritable nociceptor profile. We examine whether neuropathy in rats induced by spinal nerve injury shares overlapping pharmacological sensitivity with the irritable nociceptor phenotype using drugs that target sodium channels.

METHODS

In vivo electrophysiology was performed in anaesthetized spinal nerve ligated (SNL) and sham-operated rats to record from wide dynamic range (WDR) neurones in the ventral posterolateral thalamus (VPL) and dorsal horn.

RESULTS

In neuropathic rats, spontaneous activity in the VPL was substantially attenuated by spinal lidocaine, an effect that was absent in sham rats. The former measure was in part dependent on ongoing peripheral activity as intraplantar lidocaine also reduced aberrant spontaneous thalamic firing. Systemic oxcarbazepine had no effect on wind-up of dorsal horn neurones in sham and SNL rats. However, in SNL rats, oxcarbazepine markedly inhibited punctate mechanical-, dynamic brush- and cold-evoked neuronal responses in the VPL and dorsal horn, with minimal effects on heat-evoked responses. In addition, oxcarbazepine inhibited spontaneous activity in the VPL. Intraplantar injection of the active metabolite licarbazepine replicated the effects of systemic oxcarbazepine, supporting a peripheral locus of action.

CONCLUSIONS

We provide evidence that ongoing activity in primary afferent fibres drives spontaneous thalamic firing after spinal nerve injury and that oxcarbazepine through a peripheral mechanism exhibits modality-selective inhibitory effects on sensory neuronal processing.

SIGNIFICANCE

The inhibitory effects of lidocaine and oxcarbazepine in this rat model of neuropathy resemble the clinical observations in the irritable nociceptor patient subgroup and support a mechanism-based rationale for bench-to-bedside translation when screening novel drugs.

Modality selective roles of pro-nociceptive spinal 5-HT2A and 5-HT3 receptors in normal and neuropathic states

Descending brainstem control of spinal nociceptive processing permits a dynamic and adaptive modulation of ascending sensory information. Chronic pain states are frequently associated with enhanced descending excitatory drive mediated predominantly through serotonergic neurones in the rostral ventromedial medulla. In this study, we examine the roles of spinal 5-HT_2A and 5-HT₃ receptors in modulating ascending sensory output in normal and neuropathic states. In vivo electrophysiology was performed in anaesthetised spinal nerve ligated (SNL) and sham-operated rats to record from wide dynamic range neurones in the ventral posterolateral thalamus. In sham rats, block of spinal 5-HT₃Rs with ondansetron revealed tonic facilitation of noxious punctate mechanical stimulation, whereas blocking 5-HT_2ARs with ketanserin had minimal effect on neuronal responses to evoked stimuli. The inhibitory profiles of both drugs were altered in SNL rats; ondansetron additionally inhibited neuronal responses to lower intensity punctate mechanical stimuli and noxious heat evoked responses, whereas ketanserin inhibited innocuous and noxious evaporative cooling evoked responses. Neither drug had any effect on dynamic brush evoked responses nor on spontaneous firing rates in both sham and SNL rats. These data identify novel modality and intensity selective facilitatory roles of spinal 5-HT_2A and 5-HT₃ receptors on sensory neuronal processing within the spinothalamic-somatosensory cortical pathway.

Painful stimulation of a sensitized site in the forearm inhibits ipsilateral trigeminal nociceptive blink reflexes

Exposure to moderate levels of ultraviolet B radiation (UVB) is painless but nevertheless induces an inflammatory response that sensitizes primary afferent nociceptors. Subsequently, heating the UVB-treated site can sensitize spinal nociceptors. We used a repeated-measures design to determine whether heating the UVB-treated site also triggers ipsilateral inhibitory controls. Specifically, a 2-cm diameter site on the forearm of 20 participants was exposed to UVB at twice the minimum erythema dose. 48 h later mechanical and thermal sensitivity had increased at the UVB-treated site, indicating primary hyperalgesia. In addition, sensitivity to blunt pressure had increased in the ipsilateral forehead, implying activation of an ipsilateral supra-spinal pro-nociceptive mechanism. Despite this, the area under the curve of the ipsilateral nociceptive blink reflex decreased when the UVB-treated site was heated to induce moderate pain. Together, these findings suggest that the UVB treatment sensitized primary nociceptive afferents and generated an ipsilateral supra-spinal pro-nociceptive mechanism. In addition, sensitization to heat induced by the UVB treatment strengthened an ipsilateral anti-nociceptive process elicited by heat-pain. Infrequent but enduring discharge of sensitized primary nociceptive afferents, driven by inflammation after UVB exposure, might initiate a lateralized supra-spinal pro-nociceptive influence that heightens awareness of impending harm on the sensitized side of the body. In addition, a lateralized anti-nociceptive response triggered by an intense barrage of nociceptive signals may help to differentiate stronger from weaker sources of pain.

Brain-derived neurotrophic factor derived from sensory neurons plays a critical role in chronic pain

Many studies support the pro-nociceptive role of brain-derived neurotrophin factor (BDNF) in pain processes in the peripheral and central nervous system. We have previously shown that nociceptor-derived BDNF is involved in inflammatory pain. Microglial-derived BDNF has also been shown to be involved in neuropathic pain. However, the distinct contribution of primary afferent-derived BNDF to chronic pain processing remains undetermined. In this study, we used Avil-CreERT2 mice to delete Bdnf from all adult peripheral sensory neurons. Conditional BDNF knockouts were healthy with no sensory neuron loss. Behavioural assays and in vivo electrophysiology indicated that spinal excitability was normal. Following formalin inflammation or neuropathy with a modified Chung model, we observed normal development of acute pain behaviour, but a deficit in second phase formalin-induced nocifensive responses and a reversal of neuropathy-induced mechanical hypersensitivity during the later chronic pain phase in conditional BDNF knockout mice. In contrast, we observed normal development of acute and chronic neuropathic pain in the Seltzer model, indicating differences in the contribution of BDNF to distinct models of neuropathy. We further used a model of hyperalgesic priming to examine the contribution of primary afferent-derived BDNF in the transition from acute to chronic pain, and found that primed BDNF knockout mice do not develop prolonged mechanical hypersensitivity to an inflammatory insult. Our data suggest that BDNF derived from sensory neurons plays a critical role in mediating the transition from acute to chronic pain.

Hopes for the Future of Pain Control

Here we aim to present an accessible review of the pharmacological targets for pain management, and succinctly discuss the newest trends in pain therapy. A key task for current pain pharmacotherapy is the identification of receptors and channels orchestrating nociception. Notwithstanding peripheral alterations in the receptors and channels following pathophysiological events, the modulatory mechanisms in the central nervous system are also fundamental to the regulation of pain perception. Bridging preclinical and clinical studies of peripheral and central components of pain modulation, we present the different types of pain and relate these to pharmacological interventions. We firstly highlight the roles of several peripheral nociceptors, such as NGF, CGRP, sodium channels, and TRP-family channels that may become novel targets for therapies. In the central nervous system, the roles of calcium channels and gabapentinoids as well as NMDA receptors in generating excitability are covered including ideas on central sensitization. We then turn to central modulatory systems and discuss opioids and monoamines. We aim to explain the importance of central sensitization and the dialogue of the spinal circuits with the brain descending modulatory controls before discussing a mechanism-based effectiveness of antidepressants in pain therapy and their potential to modulate the descending controls. Emphasizing the roles of conditioned pain modulation and its animal's equivalent, diffuse noxious inhibitory controls, we discuss these unique descending modulations as a potential tool for understanding mechanisms in patients suffering from pain. Mechanism-based therapy is the key to picking the correct treatments and recent clinical studies using sensory symptoms of patients as surrogates for underlying mechanisms can be used to subgroup patients and reveal actions of drugs that may be lost when studying heterogenous groups of patients. Key advances in the understanding of basic pain principles will impact our thinking about therapy targets. The complexity of pain syndromes will require tailored pharmacological drugs, often in combination or through drugs with more than one action, and often psychotherapy, to fully control pain.

UVB irradiation induces rapid changes in galanin, substance P and c-fos immunoreactivity in rat dorsal root ganglia and spinal cord

Recent studies have shown that UVB irradiation induces primary and secondary hyperalgesia in rats and humans peaking about 24h after UVB exposure. In the present study we investigated the changes in galanin, substance P and c-fos immunoreactivity in rat DRG and spinal cord at the L5 level 2-96h after UVB irradiation. UVB irradiation of the heel area in rats almost increased the skin blood flow two-fold 24h after irradiation as measured by laser Doppler technique. UVB irradiation induced a significant reduction of the proportion of galanin positive DRG neurons for all time points, except at 12h. In the spinal cord, UVB irradiation induced increased immunoreactivity for galanin in the dorsal horn, the area around the central canal and interestingly also in the lateral spinal nucleus 12-96h after exposure. For substance P the proportion of substance P positive neurons was unchanged but UVB irradiation induced increased substance P immunoreactivity in the dorsal part of the spinal cord 48h after irradiation. UVB irradiation also induced c-fos immunoreactivity in the dorsal horn and the area around the central canal 24 and 48h after exposure. This translational model of UVB irradiation will induce rapid changes of neuropeptides implicated in nociceptive signaling in areas known to be of importance for nociception in a time frame, about 24h after exposure, where also neurophysiological alteration have been described in humans and rats.

Discrepancies between cortical and behavioural long-term readouts of hyperalgesia in awake freely moving rats

BACKGROUND

It is still unclear to what extent the most common animal models of pain and analgesia, based on indirect measures such as nocifensive behaviours, provide valid measures of pain perception.

METHODS

To address this issue, we developed a novel animal model comprising a more direct readout via chronically (>1 month) implanted multichannel electrodes (MCE) in rat primary somatosensory cortex (S1; known to be involved in pain perception in humans) and compared this readout to commonly used behavioural pain-related measures during development of hyperalgesia. A translational method to induce hyperalgesia, UVB irradiation of the skin, was used. Localized CO2 laser stimulation was made of twenty skin sites (20 stimulations/site/observation day) on the plantar hind paw, before and during the time period when enhanced pain perception is reported in humans after UVB irradiation.

RESULTS

We demonstrate a 2-10 fold significant enhancement of cortical activity evoked from both irradiated and adjacent skin and a time course that corresponds to previously reported enhancement of pain magnitude during development of primary and secondary hyperalgesia in humans. In contrast, withdrawal reflexes were only significantly potentiated from the irradiated skin area and this potentiation was significantly delayed as compared to activity in S1.

CONCLUSIONS

The present findings provide direct evidence that chronic recordings in S1 in awake animals can offer a powerful, and much sought for, translational model of the perception of pain magnitude during hyperalgesia. WHAT DOES THIS STUDY ADD?: In a novel animal model, chronic recordings of nociceptive activity in primary somatosensory cortex (S1) in awake freely moving rats are compared to behavioural readouts during UVB-induced hyperalgesia. Evoked activity in rat S1 replicates altered pain perception in humans during development of hyperalgesia, but withdrawal reflexes do not.

Neuronal hyperexcitability in the ventral posterior thalamus of neuropathic rats: modality selective effects of pregabalin

Studies on brain mechanisms of neuropathic pain are lacking. This study characterizes the properties of rat ventral posterior thalamic wide dynamic range (WDR) and nociceptive-specific (NS) neurons, the latter of which are uncharacterized in a neuropathic state. We provide evidence of phenotypic changes in neuronal sensitivity that may underlie cold and brush hypersensitivity, and that WDR neurons, and not NS neurons, encode hypersensitivity to low-intensity stimuli. Pregabalin reversed neuronal hyperexcitability in spinal nerve-ligated rats in a modality-selective manner.

Neuropathic pain represents a substantial clinical challenge; understanding the underlying neural mechanisms and back-translation of therapeutics could aid targeting of treatments more effectively. The ventral posterior thalamus (VP) is the major termination site for the spinothalamic tract and relays nociceptive activity to the somatosensory cortex; however, under neuropathic conditions, it is unclear how hyperexcitability of spinal neurons converges onto thalamic relays. This study aimed to identify neural substrates of hypersensitivity and the influence of pregabalin on central processing. In vivo electrophysiology was performed to record from VP wide dynamic range (WDR) and nociceptive-specific (NS) neurons in anesthetized spinal nerve-ligated (SNL), sham-operated, and naive rats. In neuropathic rats, WDR neurons had elevated evoked responses to low- and high-intensity punctate mechanical stimuli, dynamic brushing, and innocuous and noxious cooling, but less so to heat stimulation, of the receptive field. NS neurons in SNL rats also displayed increased responses to noxious punctate mechanical stimulation, dynamic brushing, noxious cooling, and noxious heat. Additionally, WDR, but not NS, neurons in SNL rats exhibited substantially higher rates of spontaneous firing, which may correlate with ongoing pain. The ratio of WDR-to-NS neurons was comparable between SNL and naive/sham groups, suggesting relatively few NS neurons gain sensitivity to low-intensity stimuli leading to a “WDR phenotype.” After neuropathy was induced, the proportion of cold-sensitive WDR and NS neurons increased, supporting the suggestion that changes in frequency-dependent firing and population coding underlie cold hypersensitivity. In SNL rats, pregabalin inhibited mechanical and heat responses but not cold-evoked or elevated spontaneous activity.

Ionic mechanisms of spinal neuronal cold hypersensitivity in ciguatera

Cold hypersensitivity is evident in a range of neuropathies and can evoke sensations of paradoxical burning cold pain. Ciguatoxin poisoning is known to induce a pain syndrome caused by consumption of contaminated tropical fish that can persist for months and include pruritus and cold allodynia; at present no suitable treatment is available. This study examined, for the first time, the neural substrates and molecular components of Pacific ciguatoxin-2-induced cold hypersensitivity. Electrophysiological recordings of dorsal horn lamina V/VI wide dynamic range neurones were made in non-sentient rats. Subcutaneous injection of 10 nm ciguatoxin-2 into the receptive field increased neuronal responses to innocuous and noxious cooling. In addition, neuronal responses to low-threshold but not noxious punctate mechanical stimuli were also elevated. The resultant cold hypersensitivity was not reversed by 6-({2-[2-fluoro-6-(trifluoromethyl)phenoxy]-2-methylpropyl}carbamoyl)pyridine-3-carboxylic acid, an antagonist of transient receptor potential melastatin 8 (TRPM8). Both mechanical and cold hypersensitivity were completely prevented by co-injection with the Nav 1.8 antagonist A803467, whereas the transient receptor potential ankyrin 1 (TRPA1) antagonist A967079 only prevented hypersensitivity to innocuous cooling and partially prevented hypersensitivity to noxious cooling. In naive rats, neither innocuous nor noxious cold-evoked neuronal responses were inhibited by antagonists of Nav 1.8, TRPA1 or TRPM8 alone. Ciguatoxins may confer cold sensitivity to a subpopulation of cold-insensitive Nav 1.8/TRPA1-positive primary afferents, which could underlie the cold allodynia reported in ciguatera. These data expand the understanding of central spinal cold sensitivity under normal conditions and the role of these ion channels in this translational rat model of ciguatoxin-induced hypersensitivity.