Effect of intrathecal serotonin on nociception in rats: influence of the pain test used

Involvement of the Spinal Serotonergic System in the Analgesic Effect of [6]-Shogaol in Oxaliplatin-Induced Neuropathic Pain in Mice

Oxaliplatin is a chemotherapy drug that can induce severe acute neuropathy in patients within hours of treatment. In our previous study, 10 mg/kg [6]-shogaol (i.p.) significantly alleviated cold and mechanical allodynia induced by a 6 mg/kg oxaliplatin injection (i.p.); however, the precise serotonin-modulatory effect has not been investigated. In this study, we showed that intrathecal injections of NAN-190 (5-HT1A receptor antagonist, 1 µg) and MDL-72222 (5-HT3 receptor antagonist, 15 µg), but not ketanserin (5-HT2A receptor antagonist, 1 µg), significantly blocked the analgesic effect of [6]-shogaol (10 mg/kg, i.p.). Furthermore, the gene expression of the serotonin-synthesizing enzyme tryptophan hydroxylase 2 (TPH2) and serotonin levels in the spinal cord and serum were significantly downregulated (p < 0.0001 and p = 0.0002) and upregulated (p = 0.0298 and p = 0.0099) after oxaliplatin and [6]-shogaol administration, respectively. Moreover, both the gene and protein expression of the spinal serotonin receptors 5-HT1A and 5-HT3 significantly increased after [6]-shogaol injections (p < 0.0001). Finally, intrathecal injections of both receptor agonists (8-OH-DPAT; 5-HT1A receptor agonist, 10 µg and m-CPBG; 5-HT3 receptor agonist, 15 µg) mimicked the effects of [6]-shogaol in oxaliplatin-injected mice. Taken together, these results demonstrate that [6]-shogaol attenuates oxaliplatin-induced neuropathic pain by modulating the spinal serotoninergic system.

The development of descending serotonergic modulation of the spinal nociceptive network: a life span perspective

The nociceptive network, responsible for transmission of nociceptive signals that generate the pain experience, is not fully developed at birth. Descending serotonergic modulation of spinal nociception, an important part of the pain network, undergoes substantial postnatal maturation and is suggested to be involved in the altered pain response observed in human newborns. This review summarizes preclinical data of the development of descending serotonergic modulation of the spinal nociceptive network across the life span, providing a comprehensive background to understand human newborn pain experience and treatment. Sprouting of descending serotonergic axons, originating from the rostroventral medulla, as well as changes in receptor function and expression take place in the first postnatal weeks of rodents, corresponding to human neonates in early infancy. Descending serotonergic modulation switches from facilitation in early life to bimodal control in adulthood, masking an already functional 5-HT inhibitory system at early ages. Specifically the 5-HT₃ and 5-HT₇ receptors seem distinctly important for pain facilitation at neonatal and early infancy, while the 5-HT_1a, 5-HT_1b, and 5-HT₂ receptors mediate inhibitory effects at all ages. Analgesic therapy that considers the neurodevelopmental phase is likely to result in a more targeted treatment of neonatal pain and may improve both short- and long-term effects. IMPACT: The descending serotonergic system undergoes anatomical changes from birth to early infancy, as its sprouts and descending projections increase and the dorsal horn innervation pattern changes. Descending serotonergic modulation from the rostral ventral medulla switches from facilitation in early life via the 5-HT₃ and 5-HT₇ receptors to bimodal control in adulthood. A functional inhibitory serotonergic system mainly via 5-HT_1a, 5-HT_1b, and 5-HT_2a receptors at the spinal level exists already at the neonatal phase but is masked by descending facilitation.

Anatomical changes in descending serotonergic projections from the rostral ventromedial medulla to the spinal dorsal horn following repetitive neonatal painful procedures

Excessive noxious stimulation during the critical neonatal period impacts the nociceptive network lasting into adulthood. As descending serotonergic projections from the rostral ventromedial medulla (RVM) to the spinal dorsal horn develop postnatally, this study aims to investigate the long-term effect of repetitive neonatal procedural pain on the descending serotonergic RVM-spinal dorsal horn network. A well-established rat model of repetitive noxious procedures is used in which neonatal rats received four noxious needle pricks or tactile stimulation with a cotton swab per day in the left hind paw from day of birth to postnatal day 7. Control animals were left undisturbed. When animals reached adulthood, tissue was collected for quantitative immunohistochemical analysis of serotonin (5-hydroxytryptamine, 5-HT) in the RVM and spinal dorsal horn. Both repetitive noxious and tactile procedures in the neonate decreased the 5-HT staining intensity in the adult ipsilateral, but not contralateral spinal dorsal horn. Repetitive neonatal noxious procedures resulted in an increased area covered with 5-HT staining in the adult RVM ipsilateral to the side of injury, whereas repetitive neonatal tactile stimulation resulted in increased 5-HT staining intensity in both the ipsi- and contralateral RVM. The number of 5-HT cells in adult RVM is unaffected by neonatal conditions. This detailed anatomical study shows that not only neonatal noxious procedures, but also repetitive tactile procedures result in long-lasting anatomical changes of the descending serotonergic system within the RVM and spinal dorsal horn. Future studies should investigate whether these anatomical changes translate to functional differences in descending serotonergic modulation after neonatal adverse experiences.

Roles of 5-HT3 and 5-HT7 receptors in acute pruriceptive processing in mice

The roles of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing have been demonstrated using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the involvement of 5-HT in acute pruriceptive processing has not yet been elucidated in detail. Scratching events induced by chloroquine (CQ) were attenuated by the administration of milnacipran or mirtazapine, and these effects were reversed by a treatment with ondansetron, a 5-HT₃ antagonist, or SB26970, a 5-HT₇ antagonist. CQ-induced scratching events were also ameliorated by the intrathecal administration of 5-HT, SR572227A and RS56812 (5-HT₃ agonists), and LP211 and LP44 (5-HT₇ agonists), indicating the modulation of CQ-induced scratching events by 5-HT and noradrenaline. By contrast, histamine-induced scratching events were not markedly affected by the administration of 5-HT and 5-HT₇ agonists, whereas 5-HT₃ agonists exerted attenuating effects. Similarly, they were not clearly reversed by the administration of the 5-HT₇ antagonist, unlike a 5-HT₃ antagonist. Therefore, 5-HT is involved in the attenuating effects of milnacipran and mirtazapine on CQ- and histamine-induced scratching events, and 5-HT₃ and 5-HT₇ receptors play different roles in pruriceptive processing induced by histamine or CQ.

Involvement of Serotonergic System in Oxaliplatin-Induced Neuropathic Pain

Oxaliplatin is a chemotherapeutic agent widely used against colorectal and breast cancers; however, it can also induce peripheral neuropathy that can rapidly occur even after a single infusion in up to 80–90% of treated patients. Numerous efforts have been made to understand the underlying mechanism and find an effective therapeutic agent that could diminish pain without damaging its anti-tumor effect. However, its mechanism is not yet clearly understood. The serotonergic system, as part of the descending pain inhibitory system, has been reported to be involved in different types of pain. The malfunction of serotonin (5-hydroxytryptamine; 5-HT) or its receptors has been associated with the development and maintenance of pain. However, its role in oxaliplatin-induced neuropathy has not been clearly elucidated. In this review, 16 in vivo studies focused on the role of the serotonergic system in oxaliplatin-induced neuropathic pain were analyzed. Five studies analyzed the involvement of 5-HT, while fourteen studies observed the role of its receptors in oxaliplatin-induced allodynia. The results show that 5-HT is not involved in the development of oxaliplatin-induced allodynia, but increasing the activity of the 5-HT_1A, 5-HT_2A, and 5-HT₃ receptors and decreasing the action of 5-HT_2C and 5-HT₆ receptors may help inhibit pain.

Adjunctive effect of the serotonin 5-HT2C receptor agonist lorcaserin on opioid-induced antinociception in mice

Opioid-sparing adjuncts are treatments that aim to reduce the overall dose of opioids needed to achieve analgesia, hence decreasing the burden of side effects through alternative mechanisms of action. Lorcaserin is a serotonin 5-HT_2C receptor (5-HT_2CR) agonist that has recently been reported to reduce abuse-related effects of the opioid analgesic oxycodone. The goal of our studies was to evaluate the effects of adjunctive lorcaserin on opioid-induced analgesic-like behavior using the tail-flick reflex (TFR) test as a mouse model of acute thermal nociception. We show that whereas subcutaneous (s.c.) administration of lorcaserin alone was inactive on the TFR test, adjunctive lorcaserin (s.c.) significantly increased the potency of oxycodone as an antinociceptive drug. This effect was prevented by the 5-HT_2CR antagonist SB242084. A similar lorcaserin (s.c.)-induced adjunctive phenotype was observed upon administration of the opioid analgesics morphine and fentanyl. Remarkably, we also show that, opposite to the effects observed via s.c. administration, intrathecal (i.t.) administration of lorcaserin alone induced antinociceptive TFR behavior, an effect that was not prevented by the opioid receptor antagonist naloxone. This route of administration (i.t.) also led to a significant augmentation of oxycodone-induced antinociception. Lorcaserin (s.c.) did not alter the brain or blood concentrations of oxycodone, which suggests that its adjunctive effects on opioid-induced antinociception do not depend upon changes in opioid metabolism. Together, these data indicate that lorcaserin-mediated activation of the 5-HT_2CR may represent a new pharmacological approach to augment opioid-induced antinociception. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Serotonergic mechanisms in spinal cord injury

Spinal cord injury (SCI) is a tragic event causing irreversible losses of sensory, motor, and autonomic functions, that may also be associated with chronic neuropathic pain. Serotonin (5-HT) neurotransmission in the spinal cord is critical for modulating sensory, motor, and autonomic functions. Following SCI, 5-HT axons caudal to the lesion site degenerate, and the degree of axonal degeneration positively correlates with lesion severity. Rostral to the lesion, 5-HT axons sprout, irrespective of the severity of the injury. Unlike callosal fibers and cholinergic projections, 5-HT axons are more resistant to an inhibitory milieu and undergo active sprouting and regeneration after central nervous system (CNS) traumatism. Numerous studies suggest that a chronic increase in serotonergic neurotransmission promotes 5-HT axon sprouting in the intact CNS. Moreover, recent studies in invertebrates suggest that 5-HT has a pro-regenerative role in injured axons. Here we present a brief description of 5-HT discovery, 5-HT innervation of the CNS, and physiological functions of 5-HT in the spinal cord, including its role in controlling bladder function. We then present a comprehensive overview of changes in serotonergic axons after CNS damage, and discuss their plasticity upon altered 5-HT neurotransmitter levels. Subsequently, we provide an in-depth review of therapeutic approaches targeting 5-HT neurotransmission, as well as other pre-clinical strategies to promote an increase in re-growth of 5-HT axons, and their functional consequences in SCI animal models. Finally, we highlight recent findings signifying the direct role of 5-HT in axon regeneration and suggest strategies to further promote robust long-distance re-growth of 5-HT axons across the lesion site and eventually achieve functional recovery following SCI.

Antiallodynic effect induced by [6]-gingerol in neuropathic rats is mediated by activation of the serotoninergic system and the nitric oxide-cyclic guanosine monophosphate-adenosine triphosphate-sensitive K+ channel pathway

The present study evaluated the possible antiallodynic effect induced by [6]-gingerol in rats with L5-L6 spinal nerve ligation (SNL). Moreover, we determined the possible mechanism underlying the antiallodynic effect induced by [6]-gingerol in neuropathic rats. The animals underwent L5-L6 SNL for the purpose of developing tactile allodynia. Tactile allodynia was measured with von Frey filaments. Intrathecal administration of [6]-gingerol reversed SNL-induced tactile allodynia. The [6]-gingerol-induced antiallodynic effect was prevented by the intrathecal administration of methiothepin (30 μg per rat; nonselective 5-hydroxytryptamine [5-HT] antagonist), WAY-100635 (6 μg per rat; selective 5-HT_1A receptor antagonist), SB-224289 (5 μg per rat; selective 5-HT_1B receptor antagonist), BRL-15572 (4 μg per rat; selective 5-HT_1D receptor antagonist), and SB-659551 (6 μg per rat; selective 5-HT_5A receptor antagonist), but naloxone (50 μg per rat; nonselective opioid receptor antagonist) did not prevent the [6]-gingerol-induced antiallodynic effect. Moreover, intrathecal administration of Nω-nitro-l-arginine methyl ester (100 μg per rat; nonselective nitric oxide [NO] synthase inhibitor), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 μg per rat; inhibitor of guanylate cyclase), and glibenclamide (50 μg per rat; channel blocker of adenosine triphosphate [ATP]-sensitive K⁺ channels) prevented the [6]-gingerol-induced antiallodynic effect. These data suggest that the antiallodynic effect induced by [6]-gingerol is mediated by the serotoninergic system involving the activation of 5-HT_1A/1B/1D/5A receptors, as well as the NO-cyclic guanosine monophosphate-ATP-sensitive K⁺ channel pathway but not by the opioidergic system.

Altered Gene Expression of RNF34 and PACAP Possibly Involved in Mechanism of Exercise-Induced Analgesia for Neuropathic Pain in Rats

Despite the availability of several modalities of treatment, including surgery, pharmacological agents, and nerve blocks, neuropathic pain is often unresponsive and sometimes progresses to intractable chronic pain. Although exercise therapy is a candidate for treatment of neuropathic pain, the mechanism underlying its efficacy has not been elucidated. To clarify the molecular mechanism for pain relief induced by exercise, we measured Rnf34 and Pacap mRNA levels in the spinal cord dorsal horn of SNL rats, a model of neuropathic pain. SNL model rats exhibited stable mechanical hyperalgesia for at least 6 weeks. When the rats were forced to exercise on a treadmill, mechanical and thermal hyperalgesia were significantly ameliorated compared with the non-exercise group. Accordingly, gene expression level of Rnf34 and Pacap were also significantly altered in the time course analysis after surgery. These results suggest that exercise therapy possibly involves pain relief in SNL rats by suppressing Rnf34 and Pacap expression in the spinal cord.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

Anti-allodynic effect of mangiferin in neuropathic rats: Involvement of nitric oxide-cyclic GMP-ATP sensitive K+ channels pathway and serotoninergic system

The neurobiology of neuropathic pain is caused by injury in the central or peripheral nervous system. Recent evidence points out that mangiferin shows anti-nociceptive effect in inflammatory pain. However, its role in inflammatory and neuropathic pain and the possible mechanisms of action are not yet established. The purpose of this study was to determine the possible anti-allodynic effect of mangiferin in rats with spinal nerve ligation (SNL). Furthermore, we sought to investigate the possible mechanisms of action that contribute to these effects. Mechanical allodynia to stimulation with the von Frey filaments was measured by the up and down method. Intrathecal administration of mangiferin prevented, in a dose-dependent fashion, SNL-induced mechanical allodynia. Mangiferin-induced anti-allodynia was prevented by the intrathecal administration of L-NAME (100μg/rat, non-selective nitric oxide synthase inhibitor), ODQ (10μg/rat, inhibitor of guanylate-cyclase) and glibenclamide (50μg/rat, channel blocker of ATP-sensitive K⁺ channels). Moreover, methiothepin (30μg/rat, non-selective 5-HT receptor antagonist), WAY-100635 (6μg/rat, selective 5-HT_1A receptor antagonist), SB-224289 (5μg/rat, selective 5-HT_1B receptor antagonist), BRL-15572 (4μg/rat, selective 5-HT_1D receptor antagonist) and SB-659551 (6μg/rat, selective 5-HT_5A receptor antagonist), but not naloxone (50μg/rat, non-selective opioid receptor antagonist), were able to prevent mangiferin-induced anti-allodynic effect. These data suggest that the anti-allodynic effect induced by mangiferin is mediated at least in part by the serotoninergic system, involving the activation of 5-HT_1A/1B/1D/5A receptors, as well as the nitric oxide-cyclic GMP-ATP-sensitive K⁺ channels pathway, but not by the opioidergic system, in the SNL model of neuropathic pain in rats.

Spinal cord stimulation modulates supraspinal centers of the descending antinociceptive system in rats with unilateral spinal nerve injury

BACKGROUND

The descending antinociceptive system (DAS) is thought to play crucial roles in the antinociceptive effect of spinal cord stimulation (SCS), especially through its serotonergic pathway. The nucleus raphe magnus (NRM) in the rostral ventromedial medulla is a major source of serotonin [5-hydroxytryptamine (5-HT)] to the DAS, but the role of the dorsal raphe nucleus (DRN) in the ventral periaqueductal gray matter is still unclear. Moreover, the influence of the noradrenergic pathway is largely unknown. In this study, we evaluated the involvement of these serotonergic and noradrenergic pathways in SCS-induced antinociception by behavioral analysis of spinal nerve-ligated (SNL) rats. We also investigated immunohistochemical changes in the DRN and locus coeruleus (LC), regarded as the adrenergic center of the DAS, and expression changes of synthetic enzymes of 5-HT [tryptophan hydroxylase (TPH)] and norepinephrine [dopamine β-hydroxylase (DβH)] in the spinal dorsal horn.

RESULTS

Intrathecally administered methysergide, a 5-HT1- and 5-HT2-receptor antagonist, and idazoxan, an α2-adrenergic receptor antagonist, equally abolished the antinociceptive effect of SCS. The numbers of TPH-positive serotonergic and phosphorylated cyclic AMP response element binding protein (pCREB)-positive neurons and percentage of pCREB-positive serotonergic neurons in the DRN significantly increased after 3-h SCS. Further, the ipsilateral-to-contralateral immunoreactivity ratio of DβH increased in the LC of SNL rats and reached the level seen in naïve rats, even though the number of pCREB-positive neurons in the LC was unchanged by SNL and SCS. Moreover, 3-h SCS did not increase the expression levels of TPH and DβH in the spinal dorsal horn.

CONCLUSIONS

The serotonergic and noradrenergic pathways of the DAS are involved in the antinociceptive effect of SCS, but activation of the DRN might primarily be responsible for this effect, and the LC may have a smaller contribution. SCS does not potentiate the synthetic enzymes of 5HT and norepinephrine in the neuropathic spinal cord.

Expression of the spinal 5-HT7 receptor and p-ERK pathway in the carrageenan inflammatory pain of rats

BACKGROUND

Although the inhibitory role of the 5-hydroxytrypatmine receptor 7(5-HT7R) on nociceptive processing is generally recognized, an excitatory effect associated with a reduced 5-HT7R expression has also been observed in the nerve injury model. In the carrageenan model, no significant effect is produced by the 5-HT7R activation, but the change in 5-HT7R expression has not been examined. Lesioning of the spinal serotonergic pathway enhances allodynia in the carrageenan model, but it also relieves several other pain states, including in the formalin model. While lesioning suppresses the activation of the extracellular signal-regulated kinase (ERK) of the spinal cord in the formalin model, its role in the carrageenan model has not been reported.

METHODS

Following intraplantar injections of carrageenan, the spinal 5-HT7R expression was examined using Western blotting in male Sprague-Dawley rats. The effect of serotonergic pathway lesioning with intrathecal 5,7-dihydroxytryptamine (5,7-DHT) on the expression of the phospho-ERK was measured.

RESULTS

The expression of the 5-HT7R in the carrageenan model was not significantly different from that of naive animals. The expression of the spinal p-ERK in the carrageenan model was significantly increased, but returned to the level of a naive rat 1 hour after the carrageenan injection. However, it remained significantly higher 1 hour after the injection in the animals treated with 5,7-DHT than in the naive and control rats.

CONCLUSIONS

The expression of the spinal 5-HT7R is not altered by peripheral inflammation with carrageenan, suggesting that the lack of antinociceptive effect of the 5-HT7R activation is partly attributable to the absence of changes in the expression of the 5-HT7R in the spinal cord. The extended increase of the spinal p-ERK might be related to the enhanced pain behavior in the animals with lesions of the spinal serotonergic pathway in the carrageenan model.

Serotonergic mechanism of the relieving effect of bee venom acupuncture on oxaliplatin-induced neuropathic cold allodynia in rats

Background

Oxaliplatin, an important chemotherapy drug for advanced colorectal cancer, often induces peripheral neuropathy, especially cold allodynia. Our previous study showed that bee venom acupuncture (BVA), which has been traditionally used in Korea to treat various pain symptoms, potently relieves oxaliplatin-induced cold allodynia in rats. However, the mechanism for this anti-allodynic effect of BVA remains poorly understood. We investigated whether and how the central serotonergic system, a well-known pathway for acupuncture analgesia, mediates the relieving effect of BVA on cold allodynia in oxaliplatin-injected rats.

Methods

The behavioral signs of cold allodynia in Sprague–Dawley (SD) rats were induced by a single injection of oxaliplatin (6 mg/kg, i.p.). Before and after BVA treatment, the cold allodynia signs were evaluated by immersing the rat’s tail into cold water (4°C) and measuring the withdrawal latency. For BVA treatment, a diluted BV (0.25 mg/kg) was subcutaneously administered into Yaoyangguan (GV3) acupoint, which is located between the spinous processes of the fourth and the fifth lumbar vertebra. Serotonin was depleted by a daily injection of DL-p-chlorophenylalanine (PCPA, 150 mg/kg, i.p.) for 3 days. The amount of serotonin in the spinal cord was measured by ELISA. Serotonergic receptor antagonists were administered intraperitoneally or intrathecally before BVA treatment.

Results

The serotonin levels in the spinal cord were significantly increased by BVA treatment and such increase was significantly reduced by PCPA. This PCPA pretreatment abolished the relieving effect of BVA on oxaliplatin-induced cold allodynia. Either of methysergide (mixed 5-HT₁/5-HT₂ receptor antagonist, 1 mg/kg, i.p.) or MDL-72222 (5-HT₃ receptor antagonist, 1 mg/kg, i.p) blocked the anti-allodynic effect of BVA. Further, an intrathecal injection of MDL-72222 (12 μg) completely blocked the BVA-induced anti-allodynic action, whereas NAN-190 (5-HT_1A receptor antagonist, 15 μg, i.t.) or ketanserin (5-HT_2A receptor antagonist, 30 μg, i.t.) did not.

Conclusions

These results suggest that BVA treatment alleviates oxaliplatin-induced acute cold allodynia in rats via activation of the serotonergic system, especially spinal 5-HT₃ receptors. Thus, our findings may provide a clinically useful evidence for the application of BVA as an alternative therapeutic option for the management of peripheral neuropathy, a dose-limiting side effect that occurs after an administration of oxaliplatin.

Mechanisms of electroacupuncture-induced analgesia on neuropathic pain in animal model

Neuropathic pain remains as one of the most difficult clinical pain syndromes to treat. Electroacupuncture (EA), involving endogenous opioids and neurotransmitters in the central nervous system (CNS), is reported to be clinically efficacious in various fields of pain. Although multiple experimental articles were conducted to assess the effect of EA-induced analgesia, no review has been published to assess the efficacy and clarify the mechanism of EA on neuropathic pain. To this aim, this study was firstly designed to evaluate the EA-induced analgesic effect on neuropathic pain and secondly to guide and help future efforts to advance the neuropathic pain treatment. For this purpose, articles referring to the analgesic effect of acupuncture on neuropathic pain and particularly the work performed in our own laboratory were analyzed. Based on the articles reviewed, the role of spinal opioidergic, adrenergic, serotonergic, cholinergic, and GABAergic receptors in the mechanism of EA-induced analgesia was studied. The results of this research demonstrate that μ and δ opioid receptors, α₂-adrenoreceptors, 5-HT_1A and 5-HT₃ serotonergic receptors, M₁ muscarinic receptors, and GABA_A and GABA_B GABAergic receptors are involved in the mechanisms of EA-induced analgesia on neuropathic pain.

Multiple roles of serotonin in pain control mechanisms--implications of 5-HT₇ and other 5-HT receptor types

Among monoamine neurotransmitters, serotonin (5-HT) is known to play complex modulatory roles in pain signaling mechanisms since the first reports, about forty years ago, on its essentially pro-nociceptive effects at the periphery and anti-nociceptive effects when injected directly at the spinal cord level. The discovery of multiple 5-HT receptor subtypes allowed possible explanations to this dual action at the periphery versus the central nervous system (CNS) since both excitatory and inhibitory effects can be exerted through 5-HT activation of different 5-HT receptors. However, it also appeared that activation of the same receptor subtype at CNS level might induce variable effects depending on the physiological or pathophysiological status of the animal administered with agonists. In particular, the marked neuroplastic changes induced by nerve lesion, which account for sensitization of pain signaling mechanisms, can contribute to dramatic changes in the effects of a given 5-HT receptor agonist in neuropathic rats versus intact healthy rats. This has notably been observed with 5-HT₇ receptor agonists which exert a pronociceptive action in healthy rats but alleviate hyperalgesia consecutive to nerve lesion in neuropathic animals. Analysis of cellular mechanisms underlying such dual 5-HT actions mediated by a single receptor subtype indicates that the neuronal phenotype which expresses this receptor also plays a key role in determining which modulatory action 5-HT would finally exert on pain signaling mechanisms.

Different analgesic effects of adenosine between postoperative and neuropathic pain

BACKGROUND

Adenosine is an endogenous neuromodulator in both the peripheral and central nervous systems. Adenosine inhibits pain signals by hyperpolarizing neuronal membrane.

METHODS

To clarify the effects of adenosine on pain signals, we tested intrathecal adenosine injection in two neuropathic pains (spinal cord compression and chronic constriction of sciatic nerve) and postoperative pain (plantar incision).

RESULTS

In all three kinds of pain models, significant shortening of withdrawal latencies to thermal stimulation were detected from 24 h to 1 week after the surgery. Significant improvements of pain sensation were observed in all three models after intrathecal injection of Cl-adenosine 24 h after surgery. At 72 h after surgery, intrathecal Cl-adenosine injection inhibited hyperalgesia in the two neuropathic pain models but not in the postoperative pain model. Adenosine A1R messenger RNA (mRNA) expression significantly decreased in the plantar incision model. Adenosine A1R protein levels also decreased compared with the other two models and normal control.

CONCLUSIONS

These results suggest that adenosine effectively inhibits pain signals in neuropathic pain but is less effective in postoperative pain because of the decrease in adenosine A1 receptors.

Involvement of mast cells in a mouse model of postoperative pain

Recent studies have indicated that nearly half of all surgical patients still have inadequate pain relief; therefore, it is becoming increasingly more important to understand the mechanisms involved in postoperative pain in order to be better treated. Previous studies have shown that incisions can cause mast cell degranulation. Thus, the aim of this study was to investigate the involvement of mast cells in a model of postoperative pain in mice. The depletion of mast cell mediators produced by pre-treatment with compound 48/80 (intraplantar (i.pl.)) widely (98 ± 23% of inhibition) and extensively (up to 96 h) prevented postoperative nociception and reduced histamine and serotonin levels (88 ± 4% and 68 ± 10%, respectively) in operated tissue. Furthermore, plantar surgery produced immense mast cell degranulation, as assessed by histology and confirmed by the increased levels of serotonin (three-fold higher) and histamine (fifteen-fold higher) in the perfused tissue, 1h after surgery. Accordingly, pre-treatment with the mast cell membrane stabilizer cromoglycate (200 μg/paw, i.pl.) prevented mechanical allodynia (inhibition of 96 ± 21%) and an increase in histamine (44 ± 10% of inhibition) and serotonin (73 ± 5% of inhibition) levels induced by plantar surgery. Finally, local treatment with H(1) (promethazine, 100 μg/paw, i.pl.), 5-HT(3) (ondansetron, 10 μg/paw, i.pl.) or 5-HT(2A) (ketanserin, 5 μg/paw, i.pl.) receptor antagonists partially decreased postoperative nociception in mice, but when co-administered together it completely reversed the mechanical allodynia in operated mice. Thus, mast cell activation mechanisms are interesting targets for the development of novel therapies to treat postoperative pain.

Pharmacologically enhanced spinal cord stimulation for pain: an evolving strategy

SUMMARY Spinal cord stimulation (SCS) as treatment for chronic neuropathic pain has developed into an important therapeutic strategy. However, several studies indicate that as many as 30–50% of patients do not respond sufficiently to technically well-functioning SCS. Experimental studies have revealed some of the possible neuronal systems and transmitters involved in SCS. Based on such data, a new strategy has been suggested: “pharmacologically enhanced spinal cord stimulation” using receptor active drugs to improve the therapeutic effect. The present article reviews the animal data on which clinical trials have been based and summarizes the clinical experience up to the present. Relevant data exist for intrathecal baclofen as an adjuvant to SCS, but trials with clonidine and adenosine have also been performed. Available basic studies indicate that other substances might also prove useful in future trials. The present data thus only announce the beginning of ‘drug-enhanced spinal stimulation’.

Taltirelin, a thyrotropin-releasing hormone analog, alleviates mechanical allodynia through activation of descending monoaminergic neurons in persistent inflammatory pain

Thyrotropin-releasing hormone (TRH) and its analogs have been reported to modulate descending monoaminergic inhibitory neurons, resulting in antinociception. However, it remains unknown whether TRH exerts an antiallodynic effect during persistent pain. Here, we investigated the action of taltirelin, a stable TRH analog, on mechanical allodynia in mice with inflammatory persistent pain induced by an injection of complete Freund's adjuvant into the hindpaw. Systemic administration of 1.0 mg/kg taltirelin markedly reduced mechanical allodynia. This effect was abolished by the 6-hydroxydopamine (6-OHDA)-induced depletion of central noradrenaline. While intraperitoneal injection of the α₁-adrenoceptor antagonist prazosin had no effect, intraperitoneal and intrathecal administration of the α₂-adrenoceptor antagonist yohimbine prevented the antiallodynic action of taltirelin. In addition, DL-p-chlorophenylalanine (PCPA)-induced depletion of serotonin (5-HT) and intraperitoneal and intrathecal injection of the 5-HT(1A) receptor antagonist WAY-100635 blocked the effect of taltirelin on allodynia. These findings suggest that taltirelin alleviates mechanical allodynia in inflammatory persistent pain by modulating the descending noradrenergic and serotonergic neuronal pathways via indirect activation of spinal α₂-adrenergic and 5-HT(1A) receptors.

Spinal 5-HT receptors that contribute to the pain-relieving effects of spinal cord stimulation in a rat model of neuropathy

Spinal cord stimulation (SCS) is extensively employed in the management of neuropathic pain, but the underlying mechanisms are only partially understood. Recently, we demonstrated that the pain-relieving effect of SCS appears to involve the spinal serotonin system, and the present study aimed at identifying the types of the spinal serotonin receptors involved. Experiments were performed on rats with neuropathy produced by partial ligation of the sciatic nerve. Tactile sensitivity was assessed using von Frey filaments, and cold and heat sensitivity with cold spray and radiant heat, respectively. Selective 5-HT receptor antagonists, methiothepin (5-HT(1,6,7)), ketanserin tartrate (5-HT(2A)), TICM (5-HT(3)), SDZ-205,557 (5-HT(4)), as well as receptor agonists, α-m-5-HT (5-HT(2)), m-CPBG (5-HT(3)) in per se ineffective doses, or vehicle, were administrated intrathecally 5 minutes prior to the application of SCS. Ketanserin and SDZ-205,557 significantly attenuated the suppressive effect of SCS on tactile hypersensitivity, while methiothepin and TICM were ineffective. The suppressive effect on cold hypersensitivity of SCS was counteracted by ketanserin only. None of the 5-HT receptor antagonists attenuated the suppressive effect on heat hyperalgesia of SCS. Subeffective doses of α-m-5-HT and m-CPBG enhanced the suppressive effect of SCS on tactile hypersensitivity. The enhancing effect of m-CPBG was abolished by a γ-aminobutyric acid (GABA)(A) or GABA(B) antagonist intrathecally. These results suggest that the activation of 5-HT(2A), 5-HT(3), and 5-HT(4) receptors plays an important role in SCS-induced relief of neuropathic pain. The activation of 5-HT(3) receptors appears to operate via spinal GABAergic interneurons.

Effects of 5-hydroxytryptamine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

Serotonin (5-hydroxytryptamine, 5-HT) is involved in the descending modulation of nociceptive transmission in the spinal dorsal horn. The trigeminal subnucleus caudalis (Vc; medullary dorsal horn) processes nociceptive input from the orofacial region, and 5-HT-containing axons are numerous in the superficial layers of the Vc. This study examined the actions of 5-HT on the substantia gelatinosa (SG) neurons of the Vc, using gramicidin-perforated patch-clamp recording in brainstem slice preparations from immature mice. In order to clarify the possible mechanisms underlying 5-HT actions in the SG of the Vc, the direct membrane effects of 5-HT and effects of 5-HT receptor subtype agonists were examined. 5-HT induced a hyperpolarization in the majority (64/115, 56%) of the SG neurons tested. Thirty nine (34%) SG neurons showed no response, and 12 (10%) neurons responded with depolarization. The hyperpolarizing response to 5-HT was concentration-dependent (0.1-30 μM; n=7), not desensitized by repeated application (n=22), and significantly attenuated by Ba(2+) (K(+) channel blocker; n=8). The 5-HT-induced hyperpolarization was maintained in the presence of TTX (Na(+) channel blocker), CNQX (non-NMDA glutamate receptor antagonist), AP5 (NMDA glutamate receptor antagonist), picrotoxin (GABA(A) receptor antagonist), and strychnine (glycine receptor antagonist), indicating direct postsynaptic action of 5-HT on SG neurons (n=7). The 5-HT-induced hyperpolarizing effects were mimicked by 8-OH-DPAT (5-HT(1A) receptor agonist) and α-methyl-5-HT (5-HT(2) receptor agonist) and blocked by WAY-100635 (5-HT(1A) receptor antagonist) and ketanserin (5-HT(2) receptor antagonist). Single-cell RT-PCR also revealed the presence of mRNA for 5-HT(1A) and 5-HT(2C) subtypes in the SG neurons. These results suggest that 5-HT acts directly on SG neurons and 5-HT-induced hyperpolarization is mediated, in part, by 5-HT(1A) receptors and 5-HT(2) receptors, as well as by the activation of K(+) channels, indicating an important role for 5-HT in the modulation of orofacial nociceptive processing at the level of the SG of the Vc in mice.

Subtype-specific changes in 5-HT receptor-mediated modulation of C fibre-evoked spinal field potentials are triggered by peripheral nerve injury

Neurotransmitter serotonin (5-HT) released from descending pain modulation pathways to the dorsal horn is crucial to spinal nociception processing. This study sought to gain insight into the modulatory roles of specific serotonin receptor subtypes in experimentally induced neuropathic pain. In rats subjected to spinal nerve ligation (SNL) surgery, we recorded field potentials evoked in the spinal dorsal horn by C fibre-input, during spinal superfusion with subtype-selective drugs. In neuropathic rats, subtype 5-HT1A agonist 8-OH-DPAT (100 nM) was found to potently depress evoked field potentials, as opposed to 5-HT2A or 5-HT2B subtype agonists TCB-2 (100 nM) or BW 723C86 (1 microM), respectively, which consistently enhanced evoked potentials. All three failed to alter spinal field potentials in sham operated rats. CP 94253 (1 microM), WAY 161503 (1 mM) or SR 57227 (at 1 microM in SNL rats, and 100 microM in sham rats), selective agonists for 5-HT1B, 5-HT2C and 5-HT3 receptors, respectively, significantly depressed evoked field potentials in both animal groups. The 5-HT4 agonist RS 67333 (1 microM) was depressant only in sham operated animals. Only after SNL, spinal superfusion with 5-HT1A- or 5-HT1B receptor-antagonists (S)-WAY 100135 (100 microM) or SB 224289 (100 microM), respectively, disinhibited C fibre-evoked potentials, whereas 5-HT2A or 5-HT2B receptor-antagonists 4F 4PP (100 microM) or SB 204741 (100 microM) depressed evoked potentials, suggesting tonic activity of all four subtypes as a consequence of experimental nerve injury. The present findings reveal profound subtype-specific changes in the functional modulatory activities of spinal serotonin receptors following peripheral nerve injury. In particular, spinal hyperexcitation promoted by receptors 5-HT2A and 5-HT2B is suggested as a novel pathogenic pathway contributing to neuropathic pain.

Interaction between midazolam and serotonin in spinally mediated antinociception in rats

PURPOSE

Intrathecal administration of serotonin (5-HT) is antinociceptive through the involvement of spinal cord gamma-aminobutyric acid (GABA) receptors. Therefore, 5-HT would interact with the GABA agonist, midazolam, which is well known to exert spinally mediated antinociception in the spinal cord. The present study investigated the antinociceptive interaction between spinally administered 5-HT and midazolam, using two different rat nociceptive models.

METHODS

Sprague-Dawley rats with lumbar intrathecal catheters were tested for their thermal tail withdrawal response and paw flinches induced by formalin injection after the intrathecal administration of midazolam or 5-HT, or the midazolam/ HT combination. The effects of the combination were tested by isobolographic analysis, using the combination of each 1, 1/2, 1/4, 1/8, and 1/16 of the 50% effective dose (ED50). The total fractional dose was calculated. Behavioral side effects were also examined.

RESULTS

5-HT alone and midazolam alone both showed dose-dependent antinociception in both the tail flick test and the formalin test. The ED50 of the combination was not different from the calculated additive value either in the tail flick test or in phase 2 of the formalin test, but it was significantly smaller than the calculated additive value in phase 1 of the formalin test. The total fractional dose value was 0.90 in the tail flick test, 0.093 in phase 1 of the formalin test, and 1.38 in phase 2 of the formalin test. The agitation, allodynia, or motor disturbance observed with either agent alone was not seen with the combination treatment.

CONCLUSION

The antinociceptive effects of intrathecal midazolam and 5-HT were additive on thermal acute and inflammatory facilitated stimuli, and synergistic on inflammatory acute stimulation.

Antidepressants inhibit P2X4 receptor function: a possible involvement in neuropathic pain relief

BACKGROUND

Neuropathic pain is characterized by pain hypersensitivity to innocuous stimuli (tactile allodynia) that is nearly always resistant to known treatments such as non-steroidal anti-inflammatory drugs or even opioids. It has been reported that some antidepressants are effective for treating neuropathic pain. However, the underlying molecular mechanisms are not well understood. We have recently demonstrated that blocking P2X4 receptors in the spinal cord reverses tactile allodynia after peripheral nerve injury in rats, implying that P2X4 receptors are a key molecule in neuropathic pain. We investigated a possible role of antidepressants as inhibitors of P2X4 receptors and analysed their analgesic mechanism using an animal model of neuropathic pain.

RESULTS

Antidepressants strongly inhibited ATP-mediated Ca2+ responses in P2X4 receptor-expressing 1321N1 cells, which are known to have no endogenous ATP receptors. Paroxetine exhibited the most powerful inhibition of calcium influx via rat and human P2X4 receptors, with IC50 values of 2.45 microM and 1.87 microM, respectively. Intrathecal administration of paroxetine produced a striking antiallodynic effect in an animal model of neuropathic pain. Co-administration of WAY100635, ketanserin or ondansetron with paroxetine induced no significant change in the antiallodynic effect of paroxetine. Furthermore, the antiallodynic effect of paroxetine was observed even in rats that had received intrathecal pretreatment with 5,7-dihydroxytryptamine, which dramatically depletes spinal 5-hydroxytryptamine.

CONCLUSION

These results suggest that paroxetine acts as a potent analgesic in the spinal cord via a mechanism independent of its inhibitory effect on serotonin transporters. Powerful inhibition on P2X4 receptors may underlie the analgesic effect of paroxetine, and it is possible that some antidepressants clinically used in patients with neuropathic pain show antiallodynic effects, at least in part via their inhibitory effects on P2X4 receptors.

Serotonin concentrations in the lumbosacral spinal cord of the adult rat following microinjection or dorsal surface application

Application of neuroactive substances, including monoamines, is common in studies examining the spinal mechanisms of sensation and behavior. However, affected regions and time courses of transmitter activity are uncertain. We measured the spatial and temporal distribution of serotonin [5-hydroxytryptamine (5-HT)] in the lumbosacral spinal cord of halothane-anesthetized adult rats, following its intraspinal microinjection or surface application. Carbon fiber microelectrodes (CFMEs) were positioned at various locations in the spinal cord and oxidation currents corresponding to extracellular 5-HT were measured by fast cyclic voltammetry. Intraspinal microinjection of 5-HT (100 microM, 1-3 microl) produced responses that were most pronounced at CFMEs positioned <or=800 microm from the drug micropipette: 5-HT concentration was significantly higher (1.43 vs. <0.28% of initial concentration) and response latency was shorter (67.1 vs. 598.2 s) compared with more distantly positioned CFMEs. Treatment with the selective 5-HT reuptake inhibitor clomipramine only slightly affected the spread of microinjected 5-HT. Surface application over several segments led to a transient rise in concentration that was usually apparent within 30 s and was dramatically attenuated with increasing depth: 0.25% of initial concentration (1 mM) within 400 microm of the dorsal surface and <0.001% between 1,170 and 2,000 microm. This initial response to superfusion was sometimes followed by a gradual increase to a new concentration plateau. In sum, compared with bath application, microinjection can deliver about tenfold higher transmitter concentrations, but to much more restricted areas of the spinal cord.

Activation of central 5HT2A receptors reduces the craniofacial nociception of rats

We assessed the contribution of central 5HT2A receptors to the craniofacial tissue nociception in naïve male rats. First, we tested whether activation of central 5HT2A receptors affected nociceptive neural activities recorded from superficial laminae of the trigeminal subnucleus caudalis (Vc)/upper cervical spinal cord junction (Vc/C2) region. Two types of units, such as deep-nociceptive or skin-wide dynamic range (WDR) units were identified from extracellular recordings. Topical administration of 5HT2A receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) onto the Vc/C2 region significantly reduced deep-nociceptive unit discharges evoked by formalin injection into the masseter muscle. Noxious pinch stimulation to the facial skin-evoked skin-WDR unit discharges was significantly reduced by topical administration of 0.1 mg/rat DOI onto the Vc/C2 region. Second, we tested whether i.c.v. administration of DOI affected Fos-like immunoreactivity (-LI) evoked by formalin injection into the masseter muscle. Fos-LI was significantly induced mainly at the ventrolateral (vl) area of trigeminal subnucleus interpolaris (Vi)/Vc junction (vl-Vi/Vc) region and Vc/C2 region in vehicle-treated rats. Formalin-evoked Fos-LI was significantly reduced in laminae I-II of the Vc/C2, but not vl-Vi/Vc region after i.c.v. administration of DOI. Finally, orofacial nocifensive behavioral activities evoked by formalin injection into the masseter muscle were significantly reduced by intracisternal administration of DOI. These results suggest that 5HT2A receptors in the Vc/C2 region mediate antinociceptive effects in the craniofacial nociception.

Possible involvement of spinal noradrenergic mechanisms in the antiallodynic effect of intrathecally administered 5-HT2C receptor agonists in the rats with peripheral nerve injury

Intrathecal administration of serotonin type 2C (5-HT(2C)) receptor agonists produces an antiallodynic effect in a rat model of neuropathic pain. In the present study, we characterized this effect pharmacologically. Allodynia was produced by tight ligation of the fifth (L5) and sixth (L6) lumbar spinal nerves on the left side, and was measured by applying von Frey filaments to the left hindpaw. 6-chloro-2-(1-piperazinyl)-pyrazine (MK212; 100 microg) and 1-(m-chlorophenyl)-piperazine (mCPP; 300 microg) were used as 5-HT(2C) receptor agonists. Intrathecal administration of these agonists resulted in an antiallodynic effect. Intrathecal administration of atropine (30 mug), a muscarinic receptor antagonist, and yohimbine (30 microg), an alpha(2)-adrenoceptor antagonist, reversed the effects of 5-HT(2C) receptor agonists. Intrathecal pretreatment with 6-hydroxydopamine, an adrenergic neurotoxin, inhibited the antiallodynic effect of MK212. These results suggest that spinal noradrenergic mechanisms are involved in the antiallodynic effects of intrathecally administered 5-HT(2C) receptor agonists. Previously, we demonstrated that intrathecal administration of 5-HT(2A) receptor agonists also produced antiallodynic effects, and the effects were not reversed by yohimbine. Taken together, these findings suggest that 5-HT(2A) and 5-HT(2C) receptors in the dorsal horn of the spinal cord might be involved in alleviating neuropathic pain by different mechanisms.

Roles of 5-hydroxytryptamine (5-HT) receptor subtypes in the inhibitory effects of 5-HT on C-fiber responses of spinal wide dynamic range neurons in rats

5-Hydroxytryptamine (5-HT; serotonin) plays an important role in the descending control of nociception. 5-HT and its receptors have been extensively studied in the modulation of nociceptive transmission at the spinal level using behavioral tests that may be affected by the effects of 5-HT on motor performance and skin temperature. Using electrophysiological methods, the present study aimed to systematically investigate the roles of 5-HT receptor subtypes on the inhibitory effects of 5-HT on responses of the spinal wide dynamic range (WDR) neurons to C-fiber inputs in rats. Under basal conditions, topical application of 5-HT to the spinal cord inhibited the C-fiber responses of WDR neurons dose-dependently, whereas antagonists of 5-HT(1A) [WAY 100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide maleate salt]], 5-HT(1B) [GR 55562 [3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyrid-dinyl)phenyl]benzamide dihydrochloride]], 5-HT(2A) [ketanserin [3-[2-[4-(fluorobenzoyl)-1-piperidinyl]ethyl]-2,4[1H,3H]-quinazolinedione tartrate]], 5-HT(2C) [RS 102221 [8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulfonamido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione hydrochloride]], 5-HT(3) [MDL 72222 [3-tropanyl-3,5-dichlorobenzoate]], and 5-HT(4) [GR 113808 ([1-[2-[(methylsulfonyl)-amino]ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate)] had no effect on their own. The inhibitory effects of 5-HT were reversed by antagonists of 5-HT(1B) (GR 55562), 5-HT(2A) (ketanserin), 5-HT(2C) (RS 102221), 5-HT(3) (MDL 72222), and 5-HT(4) (GR 113808) but not by 5-HT(1A) (WAY 100635) receptor antagonists. Topical administration of agonists of 5-HT(1A) [(2R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide], 5-HT(1B) [CGS 12066 [7-trifluoromethyl-4-(4-methyl-1-piperazinyl)pyrrolo-[1,2-a]quinoxaline maleate salt]], 5-HT(2A) (alpha-methyl-5-hydroxytryptamine maleate), 5-HT(2C) [MK 212 [6-chloro-2-(1-piperazinyl)pyrazine hydrochloride]], 5-HT(3) [1-(3-chlorophenyl)biguanide hydrochloride], and 5-HT(4) [2-[1-(4-piperonyl)piperazinyl]benzothiazole] also inhibited the C-responses. These results suggest that, under basal conditions, there is no tonic serotonergic inhibition on the C-responses of dorsal horn neurons, and multiple 5-HT receptor subtypes including 1B, 2A, 2C, 3, and 4 may be involved in mediating the inhibitory effects of 5-HT.

Effects of peripheral inflammation on activation of p38 mitogen-activated protein kinase in the rostral ventromedial medulla

In the present study, the activation of p38 mitogen-activated protein kinase (p38 MAPK) in the rostral ventromedial medulla (RVM) following the injection of complete Freund's adjuvant (CFA) into the rat hindpaw was examined in order to clarify the mechanisms underlying the dynamic changes in the descending pain modulatory system after peripheral inflammation. Phospho-p38 MAPK-immunoreactive (p-p38 MAPK-IR) neurons were observed in the nucleus raphe magnus (NRM) and nucleus reticularis gigantocellularis pars alpha (GiA). Inflammation induced the activation of p38 MAPK in the RVM, with a peak at 30 min after the injection of CFA into the hindpaw, which lasted for 1 h. In the RVM, the number of p-p38 MAPK-IR neurons per section in rats killed at 30 min after CFA injection (19.4+/-2.0) was significantly higher than that in the naive group (8.4+/-2.4) [p<0.05]. At 30 min after CFA injection, about 40% of p-p38 MAPK-IR neurons in the RVM were serotonergic neurons (tryptophan hydroxylase, TPH, positive) and about 70% of TPH-IR neurons in the RVM were p-p38 MAPK positive. The number of p-p38 MAPK- and TPH-double-positive RVM neurons in the rats with inflammation was significantly higher than that in naive rats [p<0.05]. These findings suggest that inflammation-induced activation of p38 MAPK in the RVM may be involved in the plasticity in the descending pain modulatory system following inflammation.

Interaction of endogenous ligands mediating antinociception

It is well known that a multitude of transmitters and receptors are involved in the nociceptive system, some of them increasing and others inhibiting the pain sensation both peripherally and centrally. These substances, which include neurotransmitters, hormones, etc., can modify the activity of nerves involved in the pain pathways. Furthermore, the organism itself can express very effective antinociception under different circumstances (e.g. stress), and, during such situations, the levels of various endogenous ligands change. A very exciting field of pain research relates to the roles of endogenous ligands. Most of them have been suggested to influence pain transmission, but only a few studies have been performed on the interactions of different endogenous ligands. This review focuses on the results of antinociceptive interactions after the co-administration of endogenous ligands. The data based on 55 situations reveal that the interactions between the endogenous ligands are very different, depending on the substances, the pain tests, the species of animals and the route of administrations. It is also revealed that only a few of the possible interactions between endogenous ligands have been investigated to date, in spite of the fact that the type of antinociceptive interaction between different endogenous ligands could hardly be predicted. The results indicate that the combination of endogenous ligands should not be omitted from the pain therapy arsenal. Attention will hopefully be drawn to the complex interdependence of endogenous ligands and their potential use in clinical practice.

Is serotonin hyperalgesic or analgesic?

Serotonin (5-hydroxtryptamine, 5-HT) is an important molecule in pain processing and modulation. Whether 5-HT has an analgesic or hyperalgesic action depends on the cell type and type of receptor it acts on. In the periphery, 5-HT sensitizes afferent nerve fibers, thus contributing to hyperalgesia in inflammation and nerve injury. In the trigeminal system, agonism at 5-HT1B/D receptors reduces neurotransmitter release, but actions through the 5-HT2A receptor may underlie chronic headache. Furthermore, genetic alterations in the 5-HT system may influence the susceptibility to migraine. In the central nervous system, 5-HT is involved in descending inhibition, but facilitatory serotonergic pathways may be functionally more important.

Effects of electroacupuncture on cold allodynia in a rat model of neuropathic pain: mediation by spinal adrenergic and serotonergic receptors

The present study was performed to examine the effects of electroacupuncture (EA) on cold allodynia and its mechanisms related to the spinal adrenergic and serotonergic systems in a rat model of neuropathic pain. For the neuropathic surgery, the right superior caudal trunk was resected at the level between S1 and S2 spinal nerves innervating the tail. Two weeks after the nerve injury, EA stimulation (2 or 100 Hz) was delivered to Zusanli (ST36) for 30 min. The behavioral signs of cold allodynia were evaluated by the tail immersion test [i.e., immersing the tail in cold water (4 degrees C) and measuring the latency to an abrupt tail movement] before and after the stimulation. And then, we examined the effects of intrathecal injection of prazosin (alpha1-adrenoceptor antagonist, 30 microg), yohimbine (alpha2-adrenoceptor antagonist, 30 microg), NAN-190 (5-HT1A antagonist, 15 microg), ketanserin (5-HT2A antagonist, 30 microg), and MDL-72222 (5-HT3 antagonist, 12 microg) on the action of EA stimulation. Although both 2 Hz and 100 Hz EA significantly relieved the cold allodynia signs, 2 Hz EA induced more robust effects than 100 Hz EA. In addition, intrathecal injection of yohimbine, NAN-190, and MDL-72222, but not prazosin and ketanserin, significantly blocked the relieving effects of 2 Hz EA on cold allodynia. These results suggest that low-frequency (2 Hz) EA is more suitable for the treatment of cold allodynia than high-frequency (100 Hz) EA, and spinal alpha2-adrenergic, 5-HT1A and 5-HT3, but not alpha1-adrenergic and 5-HT2A, receptors play important roles in mediating the relieving effects of 2 Hz EA on cold allodynia in neuropathic rats.

Effects of peripheral inflammation on activation of ERK in the rostral ventromedial medulla

In the present study, the activation of extracellular signal-regulated kinase (ERK) in the rostral ventromedial medulla (RVM) following the injection of complete Freund's adjuvant (CFA) into the rat hindpaw was examined in order to clarify the mechanisms underlying the dynamic changes in the descending pain modulatory system after peripheral inflammation. Phospho-extracellular signal-regulated kinase-immunoreactive (p-ERK-IR) neurons were observed in the nucleus raphe magnus (NRM) and nucleus reticularis gigantocellularis pars alpha (GiA). Inflammation induced the activation of ERK in the RVM, with a peak at 7 h after the injection of CFA into the hindpaw and a duration of 24 h. In the RVM, the number of p-ERK-IR neurons per section in rats killed at 7 h after CFA injection (14.2 +/- 1.7) was significantly higher than that in the control group (4.5 +/- 0.9) [P < 0.01]. At 7 h after CFA injection, about 60% of p-ERK-IR neurons in the RVM were serotonergic neurons. The percentage of RVM serotonergic neurons that are also p-ERK positive in the rats with inflammation (20.5% +/- 2.3%) was seven times higher than that in control rats (2.7% +/- 1.4%) [P < 0.01]. These findings suggest that inflammation-induced activation of ERK in the RVM may be involved in the plasticity in the descending pain modulatory system following inflammation.

Anti-allodynic efficacy of the chi-conopeptide, Xen2174, in rats with neuropathic pain

Xen2174 is a structural analogue of Mr1A, a chi-conopeptide recently isolated from the venom of the marine cone snail, Conus marmoreus. Although both chi-conopeptides are highly selective inhibitors of the norepinephrine transporter (NET), Xen2174 has superior chemical stability relative to Mr1A. It is well-known that tricyclic antidepressants (TCAs) are also potent NET inhibitors, but their poor selectivity relative to other monoamine transporters and various G-protein-coupled receptors, results in dose-limiting side-effects in vivo. As TCAs and the alpha(2)-adrenoceptor agonist, clonidine, have established efficacy for the relief of neuropathic pain, this study examined whether intrathecal (i.t.) Xen2174 alleviated mechanical allodynia in rats with either a chronic constriction injury of the sciatic nerve (CCI-rats) or an L5/L6 spinal-nerve injury. The anti-allodynic responses of i.t. Mr1A and i.t. morphine were also investigated in CCI-rats. Paw withdrawal thresholds were assessed using calibrated von Frey filaments. Bolus doses of i.t. Xen2174 produced dose-dependent relief of mechanical allodynia in CCI-rats and in spinal nerve-ligated rats. Dose-dependent anti-allodynic effects were also produced by i.t. bolus doses of Mr1A and morphine in CCI-rats, but a pronounced 'ceiling' effect was observed for i.t. morphine. The side-effect profiles were mild for both chi-conopeptides with an absence of sedation. Confirming the noradrenergic mechanism of action, i.t. co-administration of yohimbine (100 nmol) with Xen2174 (10 nmol) abolished Xen2174s anti-allodynic actions. Xen2174 appears to be a promising candidate for development as a novel therapeutic for i.t. administration to patients with persistent neuropathic pain.

The effects of acute and chronic restraint stress on activation of ERK in the rostral ventromedial medulla and locus coeruleus

Extracellular signal-regulated kinase (ERK) is a key molecule in numerous cellular and physiological processes in the CNS. Exposure to stressors causes substantial effects on the perception and response to pain. The rostral ventromedial medulla (RVM) and the locus coeruleus (LC) play crucial roles in descending pain modulation system. In the present study, the activation of ERK in the RVM and the LC in rats following acute and chronic restraint stress was examined in order to characterize the mechanisms underlying stress induced analgesic and hyperalgesic responses. Rats were stressed by restraint 6h daily for 3 weeks. The acute and chronic restraint stresses produced analgesic and hyperalgesic reactions, respectively, to thermal stimuli applied to the tail. The phospho-ERK-immunoreactive (p-ERK-IR) neurons were observed in the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis pars alpha (GiA) and LC. In the RVM, the number of p-ERK-IR neurons per section in the 3-week restraint rats (14.3+/-1.2) was significantly higher than that in the control rats (8.9+/-0.7) [P<0.01]. About 75% of p-ERK-IR neurons in the RVM in the 3-week restraint rats were serotonergic neurons. Protein levels of tryptophan hydroxylase were significantly increased in the RVM region in the 3-week restraint rats. On the other hand, the chronic restraint stress significantly decreased p-ERK-IR in the LC [P<0.05]. These findings suggest that chronic restraint stress-induced activation of ERK in the RVM and the suppression in the LC may be involved in the modulation of the pain threshold by chronic stress.

Analysis of Interactions between Serotonin and Gabapentin or Adenosine in the Spinal Cord of Rats

We evaluated the nature of the pharmacologic interaction after concurrent administration of 5-HT-gabapentin and 5-HT-adenosine at the spinal level. Intrathecal catheters were placed in the subarachnoid space of male Sprague-Dawley rats. Nociception was induced by subcutaneous injection of formalin solution (5%, 50 microl) into the hind paw. A fixed dose analysis and an isobolographic analysis were used to determine the properties of interaction. Intrathecal 5-HT dose-dependently suppressed the flinching response during phase 1 of the formalin test, while neither gabapentin nor adenosine affected the phase-1 flinching response. All three intrathecal drugs attenuated the phase-2 flinching response in a dose-dependent manner. The intrathecal combination of 5-HT with a fixed dose of gabapentin or adenosine in phase 1 had little effect or increased the antinociception of 5-HT alone. Isobolographic analysis in phase 2 revealed an additive or a synergistic interaction after intrathecal delivery of 5-HT-gabapentin or 5-HT-adenosine mixture. Taken together, the combination of 5-HT with either gabapentin or adenosine may offer a potential remedy in the treatment of the facilitated state as well as acute pain in the spinal cord.

Spinal 5-HT1A receptors differentially influence nociceptive processing according to the nature of the noxious stimulus in rats: effect of WAY-100635 on the antinociceptive activities of paracetamol, venlafaxine and 5-HT

The regulation of nociceptive processing by 5-HT at the spinal level is intricate since the neurotransmitter has been implicated in both pro and antinociception. The aim of our study was to investigate, according to the nature of the noxious stimulus, how the blockade of spinal 5-HT(1A) receptors could influence the antinociceptive actions of exogenous 5-HT as well as two analgesics involving endogenous 5-HT, paracetamol and venlafaxine. Rats were submitted either to the formalin test (tonic pain) or the paw pressure test (acute pain). WAY-100635 (40 microg/rat, i.t.), a selective 5-HT(1A) receptor antagonist, had no intrinsic action in either test. However, in the formalin test, it blocked the antinociceptive action of 5-HT (50 microg/rat, i.t.) and paracetamol (300 mg/kg, i.v.) in both phases of biting/licking behaviour and that of venlafaxine (2.5 mg/kg, s.c.) in the late phase only. In the paw pressure test, the combination of sub-effective doses of 5-HT (0.01 microg/rat, i.t.), paracetamol (50 mg/kg, i.v.) or venlafaxine (20 mg/kg, s.c.) with WAY-100635 led to a significant antinociceptive effect, which seems to depend on the reinforcement of the activity of inhibitory GABAergic interneurones. In conclusion, both direct stimulation of the spinal 5-HT(1A) receptors by 5-HT, and indirect stimulation using paracetamol or venlafaxine can differently influence pain transmission. We propose that the nature of the applied nociceptive stimulus would be responsible for the dual effect of the 5-HT(1A) receptors rather than the hyperalgesic state or the supraspinal integration of the pain message.

Lack of estrogen increases pain in the trigeminal formalin model: a behavioural and immunocytochemical study of transgenic ArKO mice

In order to examine the effect of estrogen on facial pain, we first compared the face-rubbing evoked by a formalin injection in the lip of aromatase-knockout (ArKO) mice, lacking endogenous estrogen production, 17 beta-estradiol-treated ArKO mice (ArKO-E2) and wild-type (WT) littermates. During the 'acute' phase of pain the time spent rubbing was similar in the three groups, whereas during the following 'interphase' and the second phase of pain, grooming was increased in ArKO mice. Estradiol-treatment restored a behaviour similar to WT group. To better understand estrogens modulation on pain processes, we examined changes in 5-HT and CGRP innervations of trigeminal nucleus caudalis (TNC) in ArKO, ArKO-E2 and WT groups sacrificed during the interphase. Whereas serotonin and CGRP immunoreactivities were comparable in WT and ArKO non-injected control groups, our data showed that 9 min after formalin injection, the density of serotoninergic terminals increased significantly in WT, but not in ArKO mice, while that of CGRP-immunoreactive fibers was lower in WT than in ArKO mice on the injected side. Estradiol-treatment only partially reversed these changes in ArKO-E2 mice. We conclude that estrogen deprivation in ArKO mice can be responsible for increased nociceptive response and that it is accompanied by transmitter changes favouring pro- over anti-nociceptive mechanisms in TNC during interphase of the formalin model. That estradiol-treatment completely reverses the behavioural abnormality suggests that estrogens absence produces chiefly functional activation-dependent changes. However, the fact that the immunohistochemical abnormalities were not totally normalized by estradiol-treatment suggested that some permanent developmental alterations may occur in ArKO mice.

The Different Roles of 5-HT2 and 5-HT3 Receptors on Antinociceptive Effect of Paroxetine in Chemical Stimuli in Mice

Serotonin (5-HT) is known to be an important mediator in pain modulation. Some centrally acting agents, like selective serotonin reuptake inhibitors (SSRIs), modulate pain. Activation of the endogenous opioid mechanisms or potentiation of analgesic effect by serotonergic and/or noradrenergic pathways might be involved in antinociception of SSRIs. However, peripheral mechanisms of nociception are not clear. In this study, the antinociceptive effect of paroxetine, its interaction with the opioidergic system and serotonin receptors were tested using the writhing test in mice. Paroxetine (5, 10, 20 mg/kg) induced an antinociceptive effect following i.p. administration in writhing test. For the groups in which the antagonists were tested, the dose of paroxetine that caused a significant and equipotent analgesic effect similar to 0.5 mg/kg morphine was selected. Naloxone significantly antagonized the antinociceptive effects of both paroxetine and morphine in a similar pattern and magnitude. Ketanserin (5-HT(2)-receptor antagonist) or ondansetron (5-HT(3)-receptor antagonist) alone did not alter the nociceptive action of acetic acid. While the antinociceptive effect of paroxetine was highly potentiated by ketanserin, ondansetron reduced that antinociception. In conclusion, our results indicate that the antinociceptive effect of paroxetine mainly depends on central opioidergic and serotonergic mechanisms. Peripheral serotonergic mechanisms/receptors may contribute to this antinociceptive effect, especially by 5-HT(3)-receptor subtypes.

Spinal serotonergic system is partially involved in antinociception induced by Trigonella foenum-graecum (TFG) leaf extract

It has been reported that Trigonella foenum-graecum (TFG) extract exerts analgesic, anti-inflammatory and anti-pyretic effects in different experimental models. The major objective of this paper was to investigate the site and mechanism of the analgesia induced by Trigonella foenum-graecum extract. We studied the analgesic effects of different doses of Trigonella foenum-graecum extract after i.p., i.t. and i.c.v. administration in formalin test, using male NMRI rats (200-250 g). Trigonella foenum-graecum extract showed analgesic effects in i.p. (1 g/kg) and i.t. (0.5, 1, and 2 mg/rat) (P < 0.05 in all groups) but not in i.c.v. (1 and 3 mg/rat) administrations. Based on the similarities between the effects of Trigonella foenum-graecum extract with those of nonsteroidal anti-inflammatory drugs (NSAIDs) and the role of 5-HT system in analgesic effects of NSAIDs, we tried to investigate the role of spinal 5-HT system in analgesic effects of Trigonella foenum-graecum extract. After lesioning of spinal 5-HT system by 5,7-dihydroxytryptamine (5,7-DHT), it was shown that the analgesic effect of Trigonella foenum-graecum extract (0.5 and 3 mg/rat) in the second phase of formalin test, was abolished completely and reduced relatively after using a low-dose (0.5 mg/rat) and a high-dose (3 mg/rat), respectively (P < 0.05). So, the antinociception partially remained (P < 0.05) after using the latter dose. Meanwhile, administration of naloxone (2mg/kg, i.p.) had no effect on the Trigonella foenum-graecum extract (1 g/kg, i.p.) analgesia. In conclusion, this study confirms the central action of Trigonella foenum-graecum extract and that spinal 5-HT system is partially involved in the analgesia induced by it in the second phase of formalin test and also indicates for co-existence of other analgesic mechanism(s).

Roles of serotonin receptor subtypes for the antinociception of 5-HT in the spinal cord of rats

The contribution of 5-HT (5-hydroxytryptamine) receptor subtypes to the antinociception produced by intrathecal 5-HT in the formalin test was investigated in rats. Intrathecal 5-HT suppressed both phases of behaviors produced by 5% formalin, and this was blocked by antagonists for 5-HT(1B) (3-[3-(Dimethylamino)propyl]-4-hy-droxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride, GR 55562), 5-HT(2C) (N-ormethylclozapine/8-Chloro-11-(1-piperazinyl)-5H-dibenzo[b,e][1,4]diazepine, D-MC), 5-HT3 (1-Methyl-N-(8-methyl-8-azabicyclo[3.2.1]-oct-3-yl)-1H-indazole-3-carboxamide maleate, LY-278,584) and 5-HT4 receptors (4-Amino-5-chloro-2-metho-xy-benzoic acid 2-(diethylamino)ethyl ester hydrochloride, SDZ-205,557), but not the 5-HT(1D) receptor antagonist 3-[4-(4-Chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol hydrochloride (BRL 15572). The 5-HT(1A) receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]-N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635) decreased only the second phase antinociception of 5-HT. Intrathecal administration of agonists for 5-HT(1A) (3-(N,N-Dipropylaminoethyl)-1H-indole-5-carboxamide maleate, Dipropyl-5CT), 5-HT(1B) (7-Trifluoromethyl-4(4-met-hyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline maleate, CGS-12066A), 5-HT(2C) (6-Ch-loro-2-(1-piperazinyl)pyrazine hydrochloride, MK 212), 5-HT3 (N-(3-Chlorophenyl)imidodicarbonimidic diamide hydrochloride, m-CPBG) and 5-HT4 receptors (2-[1-(4-Piperonyl)piperazinyl]benzothiazole, BZTZ) suppressed both phases of the formalin response. The results of the present study indicate that spinal 5-HT(1B,) 5-HT(2C,) 5-HT3 and 5-HT4 receptors, but not the 5-HT(1D) receptor, mediate antinociception produced by 5-HT in the formalin test. The relevance of the 5-HT(1A) receptor is less clear because of the different effects of antagonist and agonist.