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

Nerve Injury Triggers Time-dependent Activation of the Locus Coeruleus, Influencing Spontaneous Pain-like Behavior in Rats

BACKGROUND

Dynamic changes in neuronal activity and in noradrenergic locus coeruleus (LC) projections have been proposed during the transition from acute to chronic pain. Thus, the authors explored the cellular cFos activity of the LC and its projections in conjunction with spontaneous pain-like behavior in neuropathic rats.

METHODS

Tyrosine hydroxylase:Cre and wild-type Long-Evans rats, males and females, were subjected to chronic constriction injury (CCI) for 2 (short-term, CCI-ST) or 30 days (long-term, CCI-LT), evaluating cFos and Fluoro-Gold expression in the LC, and its projections to the spinal cord (SC) and rostral anterior cingulate cortex (rACC). These tests were carried out under basal conditions (unstimulated) and after noxious mechanical stimulation. LC activity was evaluated through chemogenetic and pharmacologic approaches, as were its projections, in association with spontaneous pain-like behaviors.

RESULTS

CCI-ST enhanced basal cFos expression in the LC and in its projection to the SC, which increased further after noxious stimulation. Similar basal activation was found in the neurons projecting to the rACC, although this was not modified by stimulation. Strong basal cFos expression was found in CCI-LT, specifically in the projection to the rACC, which was again not modified by stimulation. No cFos expression was found in the CCI-LT LCipsilateral (ipsi)/contralateral (contra)→SC. Chemogenetics showed that CCI-ST is associated with greater spontaneous pain-like behavior when the LCipsi is blocked, or by selectively blocking the LCipsi→SC projection. Activation of the LCipsi or LCipsi/contra→SC dampened pain-like behavior. Moreover, Designer Receptor Exclusively Activated by Designer Drugs (DREADDs)-mediated inactivation of the CCI-ST LCipsi→rACC or CCI-LT LCipsi/contra→rACC pathway, or intra-rACC antagonism of α-adrenoreceptors, also dampens pain-like behavior.

CONCLUSIONS

In the short term, activation of the LC after CCI attenuates spontaneous pain-like behaviors via projections to the SC while increasing nociception via projections to the rACC. In the long term, only the projections from the LC to the rACC contribute to modulate pain-like behaviors in this model.

EDITOR’S PERSPECTIVE

The differential in vivo contribution of spinal α2A- and α2C-adrenoceptors in tonic and acute evoked nociception in the rat

Spinal α₂-adrenoceptor induces analgesia by neuronal inhibition of primary afferent fibers. This family receptor coupled to G _i/o proteins can be subdivided into three functional subtypes: α_2A, α_2B, and α_2C-adrenoceptors, and current evidence on spinal analgesia supports the relevance of α_2A and seems to exclude the role of α_2B, but the functional contribution of α_2C-adrenoceptors remains elusive. The present study was designed to pharmacologically dissect the contribution of spinal α₂-adrenoceptor subtypes modulating tonic or acute peripheral nociception. Using male Wistar rats, we analyzed the effect of spinal clonidine (a non-selective α_2A/α_2B/α_2C-adrenoceptor agonist) and/or selective subtype α₂-adrenoceptor antagonists on: 1) tonic nociception induced by subcutaneous formalin (flinching behavior) or 2) acute nociception induced by peripheral electrical stimulus in in vivo extracellular recordings of spinal dorsal horn second-order wide dynamic range (WDR) neurons. Clonidine inhibited the nocifensive behavior induced by formalin, an effect blocked by BRL 44408 (α_2A-adrenoceptor antagonist) but not by imiloxan (α_2B-adrenoceptor antagonist) or JP 1302 (α_2C-adrenoceptor antagonist). Similarly, spinal BRL 44408 reversed the clonidine-induced inhibition of nociceptive WDR activity. Interestingly, spinal JP 1302 per se produced behavioral antinociception (an effect blocked by bicuculline, a preferent GABA_A channel blocker), but no correlation was found with the electrophysiological experiments. These data imply that, at the spinal level, 1) presynaptic α_2A-adrenoceptor activation produces antinociception during acute or tonic nociceptive stimuli; and 2) under tonic nociceptive (inflammatory) input, spinal α_2C-adrenoceptors are pronociceptive, probably by the inactivation of GABAergic transmission. This result supports a differential role of α_2A and α_2C-adrenoceptors modulating nociception.

Cannabigerol (CBG) attenuates mechanical hypersensitivity elicited by chemotherapy-induced peripheral neuropathy

BACKGROUND

Cannabigerol (CBG) is a non-psychoactive phytocannabinoid produced by the plant Cannabis sativa with affinity to various receptors involved in nociception. As a result, CBG is marketed as an over-the-counter treatment for many forms of pain. However, there is very little research-based evidence for the efficacy of CBG as an anti-nociceptive agent.

METHODS

To begin to fill this knowledge gap, we assessed the anti-nociceptive effects of CBG in C57BL/6 mice using three different models of pain; cisplatin-induced peripheral neuropathy, the formalin test, and the tail-flick assay.

RESULTS

Using the von Frey test, we found that CBG-attenuated mechanical hypersensitivity evoked by cisplatin-induced peripheral neuropathy in both male and female mice. Additionally, we observed that this CBG-induced reduction in mechanical hypersensitivity was attenuated by the α₂ -adrenergic receptor antagonist atipamezole (3 mg/kg, i.p.) and the CB₁ R antagonist, AM4113 (3 mg/kg, i.p.), and blocked by the CB₂ R antagonist/inverse agonist, SR144528 (10 mg/kg, i.p.). We found that the TRPV1 antagonist, SB705498 (20 mg/kg, i.p.) was unable to prevent CBG actions. Furthermore, we show that CBG:CBD oil (10 mg/kg, i.p.) was more effective than pure CBG (10 mg/kg) at reducing mechanical hypersensitivity in neuropathic mice. Lastly, we show that pure CBG and CBG:CBD oil were ineffective at reducing nociception in other models of pain, including the formalin and tail flick assays.

CONCLUSIONS

Our findings support the role of CBG in alleviating mechanical hypersensitivity evoked by cisplatin-induced peripheral neuropathy, but highlight that these effects may be limited to specific types of pain.

SIGNIFICANCE

There are few effective treatments for neuropathic pain and neuropathic pain is projected to increase with the aging population. We demonstrate that CBG (cannabigerol) and CBG:CBD oil attenuate neuropathy-induced mechanical hypersensitivity mice. Second, we identify receptor targets that mediate CBG-induced reduction in mechanical hypersensitivity in neuropathic mice. Third, we demonstrate that an acute injection of CBG is anti-nociceptive specifically for neuropathic pain rather than other forms of pain, including persistent pain and thermal pain.

Dexmedetomidine alleviates hyperalgesia in arthritis rats through inhibition of the p38MAPK signaling pathway

BACKGROUND

Dexmedetomidine (DEX) has showed significant analgesic effects in neuropathic pain, but the underlying mechanism has remained elusive. Our present study aimed to explore the effect of DEX on hyperalgesia with the involvement of p38MAPK signaling pathway a rat model of monoarthritis (MA).

METHODS

MA rat model was induced by injection of Complete Freund's Adjuvant (CFA). Pathological changes of ma rats were observed by HE staining and Safranin-O/Fast Green staining. Ankle circumference, paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) was measured to judge the degree of hyperalgesia in MA rats. Immunohistochemistry and ELISA were applied to observe the degree of inflammation in rats. Western blot analysis was conducted to detect expression of p38MAPK signaling pathway-related factors. The mechanism of p38MAPK signaling pathway in MA rats was observed via treatment of Anisomycin or SB203580 combined with DEX.

RESULTS

After 8 h of CFA induction, joint swelling and hyperalgesia occurred in rats. There were obvious pathological changes in the joint cavity, the joint cavity space became narrow and synovial bursa became rough. A large number of inflammatory cell infiltration was observed under microscope. After injection of DEX and SB203580, PWT and PWL was prolonged, the expression of serum inflammatory factors was decreased, and the expression of p38MAPK signaling pathway-related factors was decreased; while all the detected indexes were recovered in MA rats after treated with DEX and Anisomycin.

CONCLUSIONS

Our study provided evidence that DEX could alleviate hyperalgesia in arthritis rats through inhibition of the p38MAPK signaling pathway.

Effects of norepinephrine on microglial neuroinflammation and neuropathic pain

Norepinephrine (NE) is an important neurotransmitter in the central nervous system. NE is released from locus coeruleus neurons and is involved in a variety of physiological and pathological processes. Neuroinflammation is a common manifestation of many kinds of neurological diseases. The activation of microglia directly affects the status of neuroinflammation. Several kinds of adrenergic receptors, which anchor on microglia and can be regulated by NE, affect the activation of microglia and neuroinflammation. NE influences chronic pain, anxiety, and depression by regulating the activation of microglia.

Role of neuroglia in neuropathic pain and depression

Patients with neuropathic pain induced by nerve injury usually present with co-morbid affective changes, such as depression. Neuroglia was reported to play an important role in the development and maintenance of neuropathic pain both centrally and peripherally. Meanwhile, there have been studies showing that neuroglia participated in the development of depression. However, the specific role of neuroglia in neuropathic pain and depression has not been reviewed comprehensively. Therefore, we summarized the recent findings on the role of neuroglia in neuropathic pain and depression. Based on this review, we found a bridge-like role of neuroglia in neuropathic pain co-morbid with depression. This review may provide therapeutic implications in the treatment of neuropathic pain and offer potential help in the studies of mechanisms in the future.

Central sensitization of dorsal root potentials and dorsal root reflexes: An in vitro study in the mouse spinal cord

BACKGROUND

Axo-axonic contacts onto central terminals of primary afferents modulate sensory inputs to the spinal cord. These contacts produce primary afferent depolarization (PAD), which serves as a mechanism for presynaptic inhibition, and also produce dorsal root reflexes (DRRs), which may regulate the excitability of peripheral terminals and second order neurons. We aimed to identify changes in these responses as a consequence of peripheral inflammation.

METHODS

In vitro spinal cord recordings of spontaneous activities in dorsal and ventral roots were performed in control mice and following paw inflammation. We also used pharmacological assays to define the neurotransmitter systems implicated in such responses.

RESULTS

Paw inflammation increased the frequency and amplitude of spontaneous dorsal root depolarizations, the occurrence of DRRs and the amplitude of ventral roots depolarizations. PAD was classified in two different patterns based on their relation to ventral activity: time-locked and independent events. Both patterns increased in amplitude after paw inflammation, and independent events also increased in frequency. The circuits that were responsible for this activity implicated both glutamatergic and GABAergic transmission. Adrenergic modulation differentially affected both types of PAD, and this modulation changed after paw inflammation.

CONCLUSIONS

Our findings suggest the existence of independent spinal circuits at the origin of PAD and DRRs. Inflammation modulates these circuits differentially, unveiling varied mechanisms of spinal sensitization. This in vitro approach provides an isolated model for the study of the mechanisms of central sensitization and for the performance of pharmacological assays with the purpose of identifying and testing novel antinociceptive targets.

SIGNIFICANCE

Spinal circuits modulate activity of primary afferents acting on central terminals. Under in vitro conditions, dorsal roots show spontaneous activity in the form of depolarizations and action potentials. Our findings are consistent with the existence of several independent generator circuits. Experimental paw inflammation reduced mechanical withdrawal threshold and significantly increased the spontaneous activity of dorsal roots, which may be secondary to an enhanced output of spinal generators. This can be considered as a novel sign of central sensitization.

Migraine in Children Under 7 Years of Age: a Review

PURPOSE OF REVIEW

Despite the accumulation of a significant amount of data on pediatric headache, few studies have been conducted on its occurrence in children under 7 years of age. Within primary headaches in this age, migraine especially, turns out to be a disorder affecting up to 4% of the general population. An underestimate of its true prevalence can be due to lack of specific diagnostic markers, the frequent difficulty of describing pain in childhood, and the necessity of reliable parents' reports. Thus, migraine in children under 7 years of age represents an important challenge for clinicians. The objective of this manuscript is to provide a comprehensive review of epidemiologic, clinic, and therapeutic aspects of migraine in this age.

RECENT FINDINGS

Current literature data show that migraine has some differences, especially in clinical and therapeutic terms, in this age group compared to subsequent ages. Furthermore, some evidences showing that an early onset of migraine may play an unfavorable role in its natural history, suggest an early identification and management of migraine in younger children. Moreover, we highlight the role that factors of prenatal and perinatal development can play in the predisposition and anticipation of migraine onset. Finally, open questions related to the several undefined features of migraine in this age are reported. Migraine in this pediatric population is absolutely not rare, represents an importan clinical challenge and probably has a negative predictive role.

What goes up must come down: insights from studies on descending controls acting on spinal pain processing

Descending controls link higher processing of noxious signals to modulation of spinal cord responses to their noxious inputs. It has become possible to study one key inhibitory system in animals and humans using one painful stimulus to attenuate another distant response and so eliciting diffuse noxious inhibitory controls (DNIC) or the human counterpart, conditioned pain modulation (CPM). Here, we discuss the neuronal pathways in both species, their pharmacology and examine changes in descending controls with a focus on osteoarthritis. We will also discuss the opposing descending facilitatory system. Strong parallels between DNIC and CPM emphasize the possibility of forward and reverse translation.

The Operant Plantar Thermal Assay: A Novel Device for Assessing Thermal Pain Tolerance in Mice

Pain is a multidimensional experience of sensory-discriminative, cognitive, and affective processes; however, current basic research methods rely heavily on response to threshold stimuli, bypassing the supraspinal processing that ultimately gives rise to the pain experience. We developed the operant plantar thermal assay (OPTA), which utilizes a novel, conflict-based operant task requiring evaluation and active decision-making to obtain reward under thermally aversive conditions to quantify thermal pain tolerance. In baseline measures, male and female mice exhibited similar temperature preferences, however in the OPTA, female mice exhibited greater temperature-dependent tolerance, as defined by choice time spent in an adverse thermal condition to obtain reward. Increasing reward salience (4% vs 10% sucrose solution) led to increased thermal tolerance for males but not females. To determine whether neuropathic and inflammatory pain models alter thermal tolerance, animals with chronic constriction injury (CCI) or complete Freund's adjuvant (CFA), respectively, were tested in the OPTA. Surprisingly, neuropathic animals exhibited increased thermal tolerance, as shown by greater time spent in the reward zone in an adverse thermal condition, compared with sham animals. There was no effect of inflammation on thermal tolerance. Administration of clonidine in the CCI model led to increased thermal tolerance in both injured and sham animals. In contrast, the non-steroidal anti-inflammatory meloxicam was anti-hyperalgesic in the CFA model, but reduced thermal pain tolerance. These data support the feasibility of using the OPTA to assess thermal pain tolerance to gain new insights into complex pain behaviors and to investigate novel aspects of analgesic efficacy.

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.

Role of Spinal Cord α2-Adrenoreceptors in Noradrenergic Inhibition of Nociceptive Transmission During Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a problem during cancer treatment and for cancer survivors but the central mechanisms underlying CIPN remain understudied. This study aims to determine if CIPN is associated with alterations of noradrenergic modulation of nociceptive transmission at the spinal cord. CIPN was induced in male Wistar rats by paclitaxel injections. One month after CIPN induction, the behavioral effects of the administration of reboxetine (noradrenaline reuptake inhibitor), clonidine (agonist of α₂-adrenoreceptors; α_2–AR) and atipamezole (antagonist of α_2–AR) were evaluated using the von Frey and cold plate tests. Furthermore, we measured the expression of the noradrenaline biosynthetic enzyme dopamine-β-hydroxylase (DBH) and of α_2–AR in the spinal dorsal horn. Reboxetine and clonidine reversed the behavioral signs of CIPN whereas the opposite occurred with atipamezole. In the 3 pharmacological approaches, a higher effect was detected in mechanical allodynia, the pain modality which is under descending noradrenergic control. DBH expression was increased at the spinal dorsal horn of paclitaxel-injected animals. The enhanced noradrenergic inhibition during CIPN may represent an adaptation of the descending noradrenergic pain control system to the increased arrival of peripheral nociceptive input. A potentiation of the α_2–AR mediated antinociception at the spinal cord may represent a therapeutic opportunity to face CIPN.

Activation of the Intrinsic Pain Inhibitory Circuit from the Midcingulate Cg2 to Zona Incerta Alleviates Neuropathic Pain

Neuropathic pain is one of the most common and notorious neurological diseases. The changes in cerebral structures after nerve injury and the corresponding contributions to neuropathic pain are not well understood. Here we found that the majority of glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2^Glu) were inhibited by painful stimulation in male mice. Optogenetic manipulation revealed that these neurons were tonically involved in the inhibitory modulation of multimodal nociception. We further identified the projections to GABAergic neurons in the zona incerta (ZI^GABA) mediated the pain inhibitory role. However, MCC Cg2^Glu became hypoactive after nerve injury. Although a brief activation of the MCC Cg2^Glu to ZI^GABA circuit was able to relieve the aversiveness associated with spontaneous ongoing pain, consecutive activation of the circuit was required to alleviate neuropathic allodynia. In contrast, glutamatergic neurons in the area 1 of MCC played opposite roles in pain modulation. They became hyperactive after nerve injury and only consecutive inhibition of their activity relieved allodynia. These results demonstrate that MCC Cg2^Glu constitute a component of intrinsic pain inhibitory circuitry and their hypoactivity underlies neuropathic pain. We propose that selective and persistent activation of the MCC Cg2^Glu to ZI^GABA circuit may serve as a potential therapeutic strategy for this disease.SIGNIFICANCE STATEMENT Glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2^Glu) are tonically involved in the intrinsic pain inhibition via projecting to GABAergic neurons in the zona incerta. They are hypoactive after nerve injury. Selective activation of the circuit compensates the reduction of its analgesic strength and relieves neuropathic pain. Therefore, MCC Cg2^Glu and the related analgesic circuit may serve as therapeutic targets for neuropathic pain. In contrast, MCC Cg1^Glu have an opposite role in pain modulation and become hyperactive after nerve injury. The present study provides novel evidence for the concept that neuropathic pain is associated with the dysfunction of endogenous pain modulatory system and new perspective on the treatment of neuropathic pain.

A study of cortical and brainstem mechanisms of diffuse noxious inhibitory controls in anaesthetised normal and neuropathic rats

Diffuse noxious inhibitory controls (DNIC) are a mechanism of endogenous descending pain modulation and are deficient in a large proportion of chronic pain patients. However, the pathways involved remain only partially determined with several cortical and brainstem structures implicated. This study examined the role of the dorsal reticular nucleus (DRt) and infralimbic (ILC) region of the medial prefrontal cortex in DNIC. In vivo electrophysiology was performed to record from dorsal horn lamina V/VI wide dynamic range neurones with left hind paw receptive fields in anaesthetised sham‐operated and L5/L6 spinal nerve‐ligated (SNL) rats. Evoked neuronal responses were quantified in the presence and absence of a conditioning stimulus (left ear clamp). In sham rats, DNIC were reproducibly recruited by a heterotopically applied conditioning stimulus, an effect that was absent in neuropathic rats. Intra‐DRt naloxone had no effect on spinal neuronal responses to dynamic brush, punctate mechanical, evaporative cooling and heat stimuli in sham and SNL rats. In addition, intra‐DRt naloxone blocked DNIC in sham rats, but had no effect in SNL rats. Intra‐ILC lidocaine had no effect on spinal neuronal responses to dynamic brush, punctate mechanical, evaporative cooling and heat stimuli in sham and SNL rats. However, differential effects were observed in relation to the expression of DNIC; intra‐ILC lidocaine blocked activation of DNIC in sham rats but restored DNIC in SNL rats. These data suggest that the ILC is not directly involved in mediating DNIC but can modulate its activation and that DRt involvement in DNIC requires opioidergic signalling.

Rescue of Noradrenergic System as a Novel Pharmacological Strategy in the Treatment of Chronic Pain: Focus on Microglia Activation

Different types of pain can evolve toward a chronic condition characterized by hyperalgesia and allodynia, with an abnormal response to normal or even innocuous stimuli, respectively. A key role in endogenous analgesia is recognized to descending noradrenergic pathways that originate from the locus coeruleus and project to the dorsal horn of the spinal cord. Impairment of this system is associated with pain chronicization. More recently, activation of glial cells, in particular microglia, toward a pro-inflammatory state has also been implicated in the transition from acute to chronic pain. Both α2- and β2-adrenergic receptors are expressed in microglia, and their activation leads to acquisition of an anti-inflammatory phenotype. This review analyses in more detail the interconnection between descending noradrenergic system and neuroinflammation, focusing on drugs that, by rescuing the noradrenergic control, exert also an anti-inflammatory effect, ultimately leading to analgesia. More specifically, the potential efficacy in the treatment of neuropathic pain of different drugs will be analyzed. On one side, drugs acting as inhibitors of the reuptake of serotonin and noradrenaline, such as duloxetine and venlafaxine, and on the other, tapentadol, inhibitor of the reuptake of noradrenaline, and agonist of the µ-opioid receptor.

Anti-IL-6 Receptor Antibody Inhibits Spontaneous Pain at the Pre-onset of Experimental Autoimmune Encephalomyelitis in Mice

Chronic pain is a significant symptom in patients with autoimmune encephalomyelitis, such as multiple sclerosis and neuromyelitis optica. The most commonly used animal model of these diseases is experimental autoimmune encephalomyelitis (EAE). We previously reported that evoked pain, such as mechanical allodynia, was improved by an anti-IL-6 receptor antibody in EAE mice. However, few reports have evaluated spontaneous pain in EAE mice. Here, we assessed spontaneous pain in EAE mice by utilizing the Mouse Grimace Scale (MGS, a standardized murine facial expression-based coding system) and evaluated the influence of an anti-IL-6 receptor antibody (MR16-1). EAE was induced in female C57BL/6J mice by subcutaneous immunization with myelin oligodendrocyte glycoprotein 35–55 emulsified in adjuvant and administration of pertussis toxin. Mice were placed individually in cubicles and filmed for about 10 min. Ten clear head shots per mouse from the video recording were given a score of 0, 1, or 2 for each of three facial action units: orbital tightening, nose bulge, and ear position. Clinical symptoms of EAE were also scored. Measurement of 5-HT in the spinal cord and functional imaging of the periaqueductal gray (PAG) were also performed. Compared with control mice, MGS score was significantly higher in EAE mice. MR16-1 prevented this increase, especially in pre-onset EAE mice. Promotion of spinal 5-HT turnover and reduction of PAG activity were observed in pre-onset EAE mice. These results suggest that MR16-1 prevented spontaneous pain developed before EAE onset.

Spinal α2 -adrenoceptors and neuropathic pain modulation; therapeutic target

Neuropathic pain can arise from disease or damage to the nervous system. The most common symptoms of neuropathic pain include spontaneous pain, allodynia, and hyperalgesia. There is still limited knowledge about the factors that initiate and maintain neuropathic pain. However, ample evidence has proved the antinociceptive role of spinal α-adrenoceptors following nerve injury. It is well-documented that noradrenergic descending pathways from supraspinal loci exert an inhibitory influence on the spinal cord nociceptive neurons, mostly through the activation of spinal α₂ -adrenoceptors. This, in turn, suppresses transmission of pain input and the hyperexcitability of spinal dorsal horn neurons. There is considerable evidence demonstrating that spinal application of α₂ -adrenoceptor agonists leads to analgesic effects in animal models of neuropathic pain. Today, despite the recent rapid development of neuroscience and drug discovery, effective drugs with clear basic mechanisms have remained a mystery. Here, we give an overview of the cellular mechanisms through which brainstem adrenergic descending inhibitory processing can alter spinal pain transmission to the higher centres, and how these pathways change in neuropathic pain conditions focusing on the role of spinal α₂ -adrenoceptors in the spinal dorsal horn. We then suggest that α₂ -adrenoceptor agonist may be useful to treat neuropathic pain. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Comparison of ex vivo and in vitro actions of gabapentin in superficial dorsal horn and the role of extra-spinal sites of drug action

Although gabapentin (GBP) is a first-line treatment in the management of neuropathic pain, its mechanism of action is incompletely understood. We have previously shown, in rats made neuropathic following sciatic chronic constriction injury, that IP injection of 100 mg/kg GBP decreases overall excitability of spinal cord slices obtained ex vivo. Excitability was assessed using confocal imaging to monitor the amplitude of K⁺- induced increases in cytoplasmic Ca²⁺. This decrease in excitability involved a reduction in the frequency and amplitude of spontaneous EPSC's (sEPSC) in putative excitatory substantia gelatinosa neurons and an increase in sEPSC frequency in putative inhibitory neurons. We used have whole-cell recording to compare these ex vivo actions of GBP with its acute in vitro effects on spinal cord slices obtained from neuropathic but drug-free rats. While GBP (100μM) decreased sEPSC amplitude and frequency in excitatory neurons in vitro in a similar fashion to effects observed ex vivo, sEPSC frequency in inhibitory neurons was decreased in vitro rather than increased. Acute in vitro application of GBP also failed to decrease the overall excitability of slices from neuropathic animals as monitored by confocal Ca²⁺ imaging. Since spinal cord slices in vitro are disconnected from the periphery and higher brain centres, the GBP-induced increase in sEPSC frequency in inhibitory neurons previously reported and seen ex vivo must result from extra-spinal actions. It may be attributable to alterations in descending neurotrophic control of dorsal horn circuitry.

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.