Impaired noradrenaline homeostasis in rats with painful diabetic neuropathy as a target of duloxetine analgesia

Influence of the descending pain-inhibiting serotonergic pathway on the antihyperalgesic effect of gabapentin in neuropathic pain model rats

Gabapentinoids are used worldwide as first-line agents for the treatment of neuropathic pain. Accumulating evidence indicates that one of the antihyperalgesic mechanisms of gabapentinoids is through activation of the noradrenergic pathway of the descending pain inhibition system. However, the involvement of the serotonin pathway is unclear. We investigated the effects of gabapentin (GBP) on the serotonergic pathway of the descending inhibitory system using the spinal nerve ligation (SNL) rat model. As in previous reports, administration of GBP to SNL rats improved paw withdrawal thresholds (PWT). Intrathecally administered serotonin receptor antagonists abolished GBP's amelioration in PWT. GBP did not ameliorate PWT in noradrenaline-depleted SNL rats by DSP-4. However, GBP ameliorated PWT in serotonin-depleted SNL rats by para-chlorophenylalanine, which was not inhibited by intrathecal administration of a serotonin receptor antagonist. Immunohistochemical analysis of serotonin in the spinal dorsal horn revealed a slight, albeit statistically insignificant, increase in 5-HT levels in SNL rats compared to naive rats. However, no apparent changes were observed before or after GBP administration in naive and SNL rats. In conclusion, the involvement of the serotonergic pathway in the antihyperalgesic effects of GBP on the spinal cord is secondary, although it cooperates with the noradrenergic system to produce analgesia.

Diabetic Neuropathic Pain and Serotonin: What Is New in the Last 15 Years?

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is involved in numerous physiological functions and plays a key role in pain modulation including neuropathic pain. Diabetic neuropathy is a common complication of diabetes mellitus often accompanied by chronic neuropathic pain. Animal models of diabetes offer relevant tools for studying the pathophysiological mechanisms and pharmacological sensitivity of diabetic neuropathic pain and for identifying new therapeutic targets. In this review, we report data from preclinical work published over the last 15 years on the analgesic activity of drugs acting on the serotonergic system, such as serotonin and noradrenaline reuptake inhibitor (SNRI) antidepressants, and on the involvement of certain serotonin receptors-in particular 5-HT_1A, 5-HT_2A/2c and 5-HT₆ receptors-in rodent models of painful diabetic neuropathy.

Spinal cord vascular degeneration impairs duloxetine penetration

Introduction

Chronic pain is a prevalent physically debilitating health-related morbidity. Frontline analgesics are inadequate, providing only partial pain relief in only a proportion of the patient cohort. Here, we explore whether alterations in spinal cord vascular perfusion are a factor in reducing the analgesic capability of the noradrenaline reuptake inhibitor, duloxetine.

Method

An established rodent model of spinal cord vascular degeneration was used. Endothelial-specific vascular endothelial growth factor receptor 2 knockout mouse was induced via hydroxytamoxifen administered via intrathecal injection. Duloxetine was administered via intraperitoneal injection, and nociceptive behavioural testing was performed in both WT and VEGFR2KO mice. LC-MS/MS was performed to explore the accumulation of duloxetine in the spinal cord in WT and VEGFR2KO mice.

Results

Spinal cord vascular degeneration leads to heat hypersensitivity and a decline in capillary perfusion. The integrity of noradrenergic projections (dopa - hydroxylase labelled) in the dorsal horn remained unaltered in WT and VEGFR2KO mice. There was an association between dorsal horn blood flow with the abundance of accumulated duloxetine in the spinal cord and analgesic capacity. In VEGFR2KO mice, the abundance of duloxetine in the lumbar spinal cord was reduced and was correlated with reduced anti-nociceptive capability of duloxetine.

Discussion

Here, we show that an impaired vascular network in the spinal cord impairs the anti-nociceptive action of duloxetine. This highlights that the spinal cord vascular network is crucial to maintaining the efficacy of analgesics to provide pain relief.

A randomized control trial of duloxetine and gabapentin in painful diabetic neuropathy

BACKGROUND

To analyze the efficacy and safety of duloxetine and gabapentin in painful diabetic neuropathy (PDN).

METHODS

A randomized, open-label, active control, 12-week trial was conducted. A total of 86 participants were randomized in 1:1 ratio into gabapentin 300 mg and duloxetine 60 mg groups. The primary efficacy objective was comparison of mean change in Visual Analogue Scale (VAS) (0-100 points) scores between duloxetine and gabapentin. The symptom scores and adverse events were assessed as secondary outcomes.

RESULTS

Statistically significant (P value<.001) improvement was observed in VAS scores in both duloxetine group and gabapentin group at 12 weeks as compared to baseline. However, no significant difference in VAS scores between duloxetine and gabapentin. Similar improvement in diabetic neuropathy symptoms (DNS), diabetic neuropathy examination (DNE), and neuropathic disability score (NDS) was observed in either group over 12 weeks. There were no significant differences in DNS (P = 0.578), DNE (P = 0.410), and NDS (P = 0.071) scores between the two treatment groups. The overall safety evaluation of both duloxetine and gabapentin were similar. The most common adverse events reported were gastrointestinal.

CONCLUSION

The results indicated that both drugs were effective for the symptomatic relief from PDN and had similar efficacy. Follow-up of patients was only for 12 weeks and therefore the long-term efficacy and safety of the study drugs could not be assessed.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

Attenuation of the Diffuse Noxious Inhibitory Controls in Chronic Joint Inflammatory Pain Is Accompanied by Anxiodepressive-Like Behaviors and Impairment of the Descending Noradrenergic Modulation

The noradrenergic system is paramount for controlling pain and emotions. We aimed at understanding the descending noradrenergic modulatory mechanisms in joint inflammatory pain and its correlation with the diffuse noxious inhibitory controls (DNICs) and with the onset of anxiodepressive behaviours. In the complete Freund's adjuvant rat model of Monoarthritis, nociceptive behaviors, DNICs, and anxiodepressive-like behaviors were evaluated. Spinal alpha2-adrenergic receptors (a2-AR), dopamine beta-hydroxylase (DBH), and noradrenaline were quantified concomitantly with a2-AR pharmacologic studies. The phosphorylated extracellular signal-regulated kinases 1 and 2 (pERK1/2) were quantified in the Locus coeruleus (LC), amygdala, and anterior cingulate cortex (ACC). DNIC was attenuated at 42 days of monoarthritis while present on days 7 and 28. On day 42, in contrast to day 28, noradrenaline was reduced and DBH labelling was increased. Moreover, spinal a2-AR were potentiated and no changes in a2-AR levels were observed. Additionally, at 42 days, the activation of ERKs1/2 was increased in the LC, ACC, and basolateral amygdala. This was accompanied by anxiety- and depressive-like behaviors, while at 28 days, only anxiety-like behaviors were observed. The data suggest DNIC is attenuated in prolonged chronic joint inflammatory pain, and this is accompanied by impairment of the descending noradrenergic modulation and anxiodepressive-like behaviors.

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.

Expression of the noradrenaline transporter in the peripheral nervous system

The noradrenaline transporter (NAT) transfers noradrenaline released into the synaptic cleft back into the presynaptic terminal, thus terminating neurotransmission. Although the distribution of NAT within the central nervous system has been well-characterized, less is known about its distribution elsewhere in the peripheral nervous system and in organs such as the skin. To address this in the present study, NAT expression was investigated using immunohistochemistry in the hind paw skin and more proximally in the sciatic nerve, dorsal root ganglia and spinal cord of five male Wistar rats. It was hypothesised that NAT would be expressed exclusively on nerve fibres labelled by dopamine beta hydroxylase (DβH), an enzyme involved in the conversion of dopamine to noradrenaline. NAT co-localised with DβH in neurons in the spinal cord, dorsal root ganglia and sciatic nerve. Unexpectedly, however, NAT-like immunoreactivity was not observed in DβH immuno-reactive fibres that innervated dermal blood vessels, suggesting that a mechanism other than presynaptic re-uptake of noradrenaline through NAT regulates transmission at neurovascular junctions in the skin. Furthermore, a novel association between NAT-like immunoreactivity and the myelin marker myelin basic protein (MBP) was identified in peripheral nerves. Specifically, NAT and MBP appeared to congregate around primary afferent nerve fibres labelled by neurofilament 200, a marker of neurons with medium- and large-diameter axons. NAT-like immunoreactivity was also detected in cultured Schwann cells immunohistochemically and at the mRNA level. Together, these findings imply a hitherto unrecognised role of Schwann cells in clearance of noradrenaline in the peripheral nervous system.

The role of the endogenous neurotransmitters associated with neuropathic pain and in the opioid crisis: The innate pain-relieving system

Neuropathic pain is a chronic pain caused by central and peripheral nerve injury, long-term diabetes or treatment with chemotherapy drugs, and it is dissimilar to other chronic pain conditions. Chronic pain usually seriously affects the quality of life, and its drug treatment may result in increased costs of social and medical care. As in the USA and Canada, in Europe, the demand for pain-relieving medicines used in chronic pain has also significantly increased, but most European countries are not experiencing an opioid crisis. In this review, the role of various endogenous transmitters (noradrenaline, dopamine, serotonin, met- and leu-enkephalins, β-endorphin, dynorphins, cannabinoids, ATP) and various receptors (α₂, μ, etc.) in the innate pain-relieving system will be discussed. Furthermore, the modulation of pain processing pathways by transmitters, focusing on neuropathic pain and the role of the sympathetic nervous system in the side effects of excessive opioid treatment, will be explained.

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.

Comparison of the Efficacy of Duloxetine and Pregabalin in Pain Relief Associated with Diabetic Neuropathy

Introduction

Painful diabetic peripheral neuropathy (PDPN) complicates 25% of type II diabetes mellitus. It has a profound impact on diabetes-related morbidity and worsens the quality of life. Both pregabalin and duloxetine may be indicated for PDPN. In this study, the efficacy of duloxetine and pregabalin was compared in patients with PDPN.

Methods

It was a single-centre open-label study conducted with patients of diabetes mellitus type II diagnosed with PDPN. Patients were randomized to receive 60 mg/daily duloxetine or 300 mg/daily pregabalin. Pain scores were recorded using visual analogue scale (VAS) on day 0, week 4, and week 12. Data was entered and analysed using SPSS version 22.0 (IBM Corp., Armonk, NY).

Results

In the duloxetine group, the mean VAS score decreased from 6.81 ± 0.91 to 4.01 ± 1.12 with 12 weeks of therapy (p <0.0001). In the pregabalin group, the mean VAS score decreased from 6.99 ± 1.12 to 4.91 ± 0.82 with 12 weeks of therapy (p <0.0001). At 12 weeks, duloxetine showed lower VAS scores than pregabalin (p <0.0001). In the duloxetine group, the mean change in VAS score over time was - 2.80 and in the pregabalin group, the mean change was - 2.80. Adverse events were reported in 17.9% of the participants. Lethargy/somnolence (8.1%) and peripheral edema (3.4%) were commonly reported in the pregabalin group and constipation (6.9%) and orthostatic hypotension (4.6%) were commonly reported in the duloxetine group.

Conclusions

Duloxetine at a daily fixed dose of 60 mg is efficacious in the relief of neuropathic pain. Pregabalin also showed a comparable outcome. Both duloxetine and pregabalin have a promising safety profile and are well-tolerated.

Renal and cerebral RAS interaction contributes to diabetic kidney disease

The diabetes mellitus has posed a grave threat on human health, and is bound to result in renal trauma by uncertain mechanisms. Increasing evidences indicated that the activation of the renin-angiotensin system plays a pivotal role during the progression of diabetic kidney disease. In streptozotocin (STZ)-induced type 1 diabetic rat model, the losartan (a selective angiotensin II type 1 (AT1) receptor antagonist) and tempol (4-Hydroxy-TEMPO, reactive oxygen species scavenger) were administrated through intracerebroventricular injection or intragastric gavage. Intracerebroventricular administration of clonidine or renal denervation was carried out to block sympathetic nerve traffic. Compared with non-diabetic rats, the reno-cerebral axis was over-activated, including activity of renin-angiotensin system (RAS), oxidative stress, and sympathetic activity in diabetic rats. Central blockade of RAS inhibited the central oxidative stress and sympathetic activity, which led to decrease of intrarenal RAS activity and oxidative stress. Meanwhile, central administration of tempol reduced brain RAS, thus downregulated renal RAS activity and oxidative stress. Importantly, oral administration by intragastric gavage of high dose of losartan and tempol achieved the same effect. The results suggested that there is a cross-talk between renal and cerebral RAS/reactive oxygen species, contributing to the progression of diabetic kidney disease. The subfornical organ, paraventricular nucleus, and supraoptic nucleus in the forebrain also play a key role in development and progression of renal trauma through reno-cerebral reflex axis.

Duloxetine in patients with diabetic peripheral neuropathic pain in Japan: a randomized, doubleblind, noninferiority comparative study with pregabalin

Purpose

Duloxetine and pregabalin are recommended as first-line treatments for diabetic peripheral neuropathic pain (DPNP). However, studies have not reported a direct comparison between duloxetine and pregabalin. We conducted a postmarketing, randomized, double-blind study to assess the noninferiority of duloxetine compared with pregabalin after 12 weeks of treatment in adult patients with DPNP in Japan (NCT02417935).

Patients and methods

Patients (N = 303) with distal symmetrical DPNP were randomized to and were administered duloxetine (40–60 mg/day) or pregabalin (300–600 mg/day). The primary endpoint was the change from baseline in weekly mean of the 24-hour average pain score (numeric rating scale [NRS]). Noninferiority of duloxetine compared with pregabalin was assessed with the primary endpoint at week 12. Secondary measures, including night pain and worst pain, Brief Pain Inventory-Severity and Interference rating short form (BPI-SF), Clinical Global Impression of Improvement (CGI-I), Patient Global Impression of Improvement (PGI-I), and Neuropathic Pain Symptom Inventory (NPSI), health outcome measures (EuroQol 5-Dimension index and VAS), and safety were also assessed.

Results

For the 24-hour NRS average pain score, the difference between the duloxetine and pregabalin groups was 0.072 (95% CI: – 0.295, 0.439), and the upper bound of the 95% CI (0.439) did not exceed the predefined noninferiority margin (0.51), at the end of the study period. For secondary outcome measures (night pain, worst pain, BPI-SF, CGI-I, PGI-I, NPSI) and health outcome measures, both the duloxetine and pregabalin treatment groups showed an improvement from baseline with no significant between-group difference. Duloxetine and pregabalin were well tolerated and the safety profiles were consistent with previously reported results.

Conclusion

This study demonstrated the noninferior efficacy of duloxetine compared with pregabalin in the treatment of adult patients with DPNP. The safety analyses showed an acceptable tolerability based on safety profiles of duloxetine and pregabalin.

The Neurotoxin DSP-4 Induces Hyperalgesia in Rats that is Accompanied by Spinal Oxidative Stress and Cytokine Production

Central neuropathic pain (CNP) a significant problem for many people, is not well-understood and difficult to manage. Dysfunction of the central noradrenergic system originating in the locus coeruleus (LC) may be a causative factor in the development of CNP. The LC is the major noradrenergic nucleus of the brain and plays a significant role in central modulation of nociceptive neurotransmission. Here, we examined CNS pathophysiological changes induced by intraperitoneal administration of the neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride). Administration of DSP-4 decreased levels of norepinephrine in spinal tissue and cerebrospinal fluid (CSF) and led to the development of thermal and mechanical hyperalgesia over 21 days, that was reversible with morphine. Hyperalgesia was accompanied by significant increases in noradrenochrome (oxidized norepinephrine) and expression of 4-hydroxynonenal in CSF and spinal cord tissue respectively at day 21, indicative of oxidative stress. In addition, spinal levels of pro-inflammatory cytokines (interleukins 6 and 17A, tumor necrosis factor-α), as well as the anti-inflammatory cytokine interleukin10 were also significantly elevated at day 21, indicating that an inflammatory response occurred. The inflammatory effect of DSP-4 presented in this study that includes oxidative stress may be particularly useful in elucidating mechanisms of CNP in inflammatory disease states.

Systemic dexmedetomidine attenuates mechanical allodynia through extracellular sign db type 2 diabetic mice

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus. However, the treatment for PDN is limited in clinical practice. In the present study, we investigated the effect of systemic administration dexmedetomidine (DEX), a selective alpha 2 adrenergic receptor (α2AR) agonist, on mechanical allodynia and its underlying mechanism in db/db mice, an animal model of type 2 diabetes mellitus. Our data demonstrated that db/db mice develop mechanical allodynia at the early stage of diabetes. During the period of mechanical allodynia, we detected increased release of norepinephrine (NE) and decreased levels of α_2A-Adrenoceptors in db/db mice. Immunohistochemistry showed that the α_2A-Adrenoceptor is predominantly expressed in neurons in the spinal cord. Acute injection of dexmedetomidine significantly decreased mechanical allodynia, which was blocked by its selective antagonist BRL44408. Furthermore, the upregulation of pERK1 and pERK2 in db/db mice were attenuated by preadministration of dexmedetomidine. We provide the first evidence that the functional alternation of spinal noradrenergic system might underlie exaggerated nociception in PDN. Systemic dexmedetomidine inhibits the mechanical allodynia which is related to ERK signaling pathway in type 2 diabetes, implying that the α₂-Adrenoceptor might be a potential therapeutic strategy for PDN.

Levetiracetam synergizes with gabapentin, pregabalin, duloxetine and selected antioxidants in a mouse diabetic painful neuropathy model

RATIONALE

We have reported that levetiracetam, a novel anticonvulsant with analgesic properties, synergizes with ibuprofen/aspirin/paracetamol in a model of diabetic painful neuropathy (DPN). Most guidelines recommend gabapentin, pregabalin, and duloxetine as first- or second-line agents for DPN.

OBJECTIVE

We examined the effects of combination treatment of first-/second-line analgesics with levetiracetam in a model of DPN. Additionally, the levetiracetam's combinations with antioxidants, low dose of aspirin, coenzyme Q10, or α-lipoic acid were evaluated.

METHODS

Diabetes was induced in C57BL/6 mice with a single high dose of streptozotocin. The antinociceptive effects of orally administered levetiracetam, gabapentin, pregabalin, duloxetine (acute treatment) and aspirin, coenzyme Q10, and α-lipoic acid (preventive 7-day treatment), as well as combinations of levetiracetam with individual drugs were examined in the tail-flick test. In combination experiments, the drugs were coadministered in fixed-dose fractions of single-drug ED₅₀; the type of interaction was determined by isobolographic analysis.

RESULTS

About 60-, 32-, 30-, 26-, 18-, and 6-fold reductions of doses of both drugs in levetiracetam combinations with pregabalin, gabapentin, coenzyme Q10, aspirin, duloxetine, and α-lipoic acid, respectively, were detected.

CONCLUSIONS

Combinations of levetiracetam with gabapentin/pregabalin/duloxetine that target different mechanisms/sites of action involved in DPN, as well as combinations of levetiracetam and low-dose aspirin/coenzyme Q10/α-lipoic acid that target underlying causes of DPN, produce marked synergistic interactions in reducing nociception in diabetic mice. This suggests that these combination treatments might be of great benefit for diabetic patients and should be explored further in clinical trials.

Noradrenergic inhibition of spinal hyperexcitation elicited by cutaneous cold stimuli in rats with oxaliplatin-induced allodynia: electrophysiological and behavioral assessments

We investigated the spinal action of noradrenaline on cold-elicited hyperexcitation detected in dorsal horn neurons of rats with allodynia induced by an oxaliplatin (6 mg/kg, i.p.) injection. In vivo extracellular recordings from the spinal dorsal horn showed that wide dynamic range neurons responded to cutaneous acetone (10 μl) stimulation in normal rats, and cold-elicited firings in oxaliplatin-administered rats were increased with a longer duration, correlated with behavioral responses. These responses were significantly attenuated by spinal administration (50 μM) of noradrenaline or its agonists, clonidine (α2), phenylephrine (α1) and isoprenaline (β), in descending order of efficacy. Thus, the inhibitory effect of noradrenaline on spinal oxaliplatin-induced cold hyperexcitation is mediated mainly by activation of α2- and/or α1-adrenoceptors.

Duloxetine Inhibits Microglial P2X4 Receptor Function and Alleviates Neuropathic Pain after Peripheral Nerve Injury

P2X4 receptors (P2X4R) are a family of ATP-gated non-selective cation channels. We previously demonstrated that activation of P2X4R in spinal microglia is crucial for neuropathic pain, a highly debilitating chronic pain condition, suggesting that P2X4R is a potential therapeutic target for treating neuropathic pain. Thus, the identification of a compound that has a potent inhibitory effect on P2X4R is an important clinical challenge. In the present study, we screened a chemical library of clinically approved drugs and show for the first time that duloxetine, a serotonin and noradrenaline reuptake inhibitor, has an inhibitory effect on rodent and human P2X4R. In primary cultured microglial cells, duloxetine also inhibited P2X4R-, but not P2X7R-, mediated responses. Moreover, intrathecal administration of duloxetine in a model of neuropathic pain produced a reversal of nerve injury-induced mechanical allodynia, a cardinal symptom of neuropathic pain. In rats that were pretreated with a serotonin-depleting agent and a noradrenaline neurotoxin, the antiallodynic effect of duloxetine was reduced, but still remained. Based on these results, we suggest that, in addition to duloxetine’s primary inhibitory action on serotonin and noradrenaline transporters, an inhibitory effect on P2X4R may be involved at least in part in an antiallodynic effect of intrathecal duloxetine in a model of neuropathic pain.

Restoring Spinal Noradrenergic Inhibitory Tone Attenuates Pain Hypersensitivity in a Rat Model of Parkinson's Disease

In the present study, we investigated whether restoring descending noradrenergic inhibitory tone can attenuate pain in a PD rat model, which was established by stereotaxic infusion of 6-hydroxydopamine (6-OHDA) into the bilateral striatum (CPu). PD rats developed thermal and mechanical hypersensitivity at the 4th week after surgery. HPLC analysis showed that NE content, but not dopamine or 5-HT, significantly decreased in lumbar spinal cord in PD rats. Additional noradrenergic depletion by injection of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) aggravated pain hypersensitivity in PD rats. At the 5th week after injection of 6-OHDA, systemic treatment with pharmacological norepinephrine (NE) precursor droxidopa (L-DOPS) or α2 adrenoceptor agonist clonidine significantly attenuated thermal and mechanical pain hypersensitivity in PD rats. Furthermore, application of norepinephrine (NE) and 5-hydroxytryptamine (5-HT) reuptake inhibitors duloxetine, but not 5-HT selective reuptake inhibitors sertraline, significantly inhibited thermal and mechanical pain hypersensitivity in PD rats. Systemic administration of Madopar (L-DOPA) or the D2/D3 agonist pramipexole slightly inhibited the thermal, but not mechanical, hypersensitivity in PD rats. Thus, our study revealed that impairment of descending noradrenergic system may play a key role in PD-associated pain and restoring spinal noradrenergic inhibitory tone may serve as a novel strategy to manage PD-associated pain.

Alleviation of pain in painful diabetic neuropathy

INTRODUCTION

Painful diabetic neuropathy (PDN) is a disabling pain condition. Its pathomechanism remains unknown, but a sensitization and neuronal hyperexcitabilty have been suggested. Only symptomatic pharmacological pain management treatment is currently available.

AREAS COVERED

The origin of PDN is enigmatic, and the evidence-based therapeutic guidelines therefore consist only of antidepressants and antiepileptics as first-line recommended drugs. This article relates to a MEDLINE/PubMed systematic search (2005-2015).

EXPERT OPINION

The results of the meta-analysis from the aspect of the efficacy of amitriptyline, duloxetine, venlafaxine, gabapentin and pregabalin are favorable, but the placebo response rate is relatively high in patients with neuropathic pain. For personalization of the medication of PDN patients, the optimum dosing, the genotyping of the metabolizing enzymes and optimum biomarkers are needed. As concerns the future perspectives, specific sodium channel subtype inhibitors acting on peripheral nociceptive neurons or modified T-type voltage-gated calcium channel blockers may be promising targets for pharmaceutical innovations. Another attractive strategy for the treatment is based on the effects of monoclonal antibodies against nerve growth factor, sodium channels, specific receptor and cytokines. Botulinum toxin A, capsaicin patch and spinal cord stimulation therapies are the nearest future therapeutic options for the treatment of PDN patients.

Synaptic and network consequences of monosynaptic nociceptive inputs of parabrachial nucleus origin in the central amygdala

A large majority of neurons in the superficial layer of the dorsal horn projects to the lateral parabrachial nucleus (LPB). LPB neurons then project to the capsular part of the central amygdala (CeA; CeC), a key structure underlying the nociception-emotion link. LPB-CeC synaptic transmission is enhanced in various pain models by using electrical stimulation of putative fibers of LPB origin in brain slices. However, this approach has limitations for examining direct monosynaptic connections devoid of directly stimulating fibers from other structures and local GABAergic neurons. To overcome these limitations, we infected the LPB of rats with an adeno-associated virus vector expressing channelrhodopsin-2 and prepared coronal and horizontal brain slices containing the amygdala. We found that blue light stimulation resulted in monosynaptic excitatory postsynaptic currents (EPSCs), with very small latency fluctuations, followed by a large polysynaptic inhibitory postsynaptic current in CeC neurons, regardless of the firing pattern type. Intraplantar formalin injection at 24 h before slice preparation significantly increased EPSC amplitude in late firing-type CeC neurons. These results indicate that direct monosynaptic glutamatergic inputs from the LPB not only excite CeC neurons but also regulate CeA network signaling through robust feed-forward inhibition, which is under plastic modulation in response to persistent inflammatory pain.

Effects of the Fourth Ventricle Compression in the Regulation of the Autonomic Nervous System: A Randomized Control Trial

Introduction. Dysfunction of the autonomic nervous system is an important factor in the development of chronic pain. Fourth ventricle compression (CV-4) has been shown to influence autonomic activity. Nevertheless, the physiological mechanisms behind these effects remain unclear. Objectives. This study is aimed at evaluating the effects of fourth ventricle compression on the autonomic nervous system. Methods. Forty healthy adults were randomly assigned to an intervention group, on whom CV-4 was performed, or to a control group, who received a placebo intervention (nontherapeutic touch on the occipital bone). In both groups, plasmatic catecholamine levels, blood pressure, and heart rate were measured before and immediately after the intervention. Results. No effects related to the intervention were found. Although a reduction of norepinephrine, systolic blood pressure, and heart rate was found after the intervention, it was not exclusive to the intervention group. In fact, only the control group showed an increment of dopamine levels after intervention. Conclusion. Fourth ventricle compression seems not to have any effect in plasmatic catecholamine levels, blood pressure, or heart rate. Further studies are needed to clarify the CV-4 physiologic mechanisms and clinical efficacy in autonomic regulation and pain treatment.

Upregulation of glutamatergic transmission in anterior cingulate cortex in the diabetic rats with neuropathic pain

Peripheral neuropathic pain is a common complication in the diabetic patients, and the underlying central mechanism remains unclear. Forebrain anterior cingulate cortex (ACC) is critically involved in the supraspinal perception of physical and affective components of noxious stimulus and pain modulation. Excitatory glutamatergic transmission in the ACC extensively contributed to the maintenance of negative affective component of chronic pain. The present study examined the adaptation of glutamatergic transmission in the ACC in rats with diabetic neuropathic pain. Injection with streptozotocin (STZ) induced hyperglycemia, thermal hyperalgesia and mechanical allodynia in the rats. In these rats, significant enhanced basal glutamatergic transmission was observed in the ACC neurons. The increased presynaptic glutamate release and enhanced conductance of postsynaptic glutamate receptors were also observed in the ACC neurons of these modeled rats. Increased phosphorylation of PKMζ, but not the expression of total PKMζ, was also observed in the ACC. Microinjection of PKMζ inhibitor ZIP into ACC attenuated the upregulation of glutamate transmission and painful behaviors in STZ-injected rats. These results revealed a substantial central sensitization in the ACC neurons in the rodents with diabetic neuropathic pain, which may partially underlie the negative affective components of patients with diabetic neuropathic pain.