Mechanisms for ovariectomy-induced hyperalgesia and its relief by calcitonin: participation of 5-HT1A-like receptor on C-afferent terminals in substantia gelatinosa of the rat spinal cord

Effect of gonadectomy on pain assessment in dogs undergoing orthopaedic stifle surgery

OBJECTIVE

To compare pain perception between gonadectomized and intact dogs.

STUDY DESIGN

Blinded, prospective, cohort study.

ANIMALS

A group of 74 client-owned dogs.

METHODS

Dogs were divided into four groups: group 1-female/neutered (F/N), group 2-female/intact (F/I), group 3-male/neutered (M/N) and group 4-male/intact (M/I). Premedication consisted of intramuscularly administered acepromazine (0.05 mg kg-1) and morphine (0.2 mg kg-1), and subcutaneously administered carprofen (4 mg kg-1). Anaesthesia was induced with propofol (1 mg kg-1 intravenously and supplementary doses to effect) and maintained with isoflurane in 100% oxygen. Intraoperative analgesia was achieved with fentanyl infusion (0.1 μg kg-1 minute-1). Pain assessments [using the University of Melbourne Pain Scale (UMPS) and an algometer at the incision site (IS), parallel to the incision site (NIS), and on the contralateral healthy limb] were performed preoperatively, and at 1, 2, 4, 6, 9 and 20 hours after extubation. The time-standardised area under the curve (AUCst) for measurements was calculated and compared by performing a one-way multivariate analysis of variance (manova). Statistical significance was set at p < 0.05.

RESULTS

Postoperatively, F/N exhibited higher pain than F/I, with estimated marginal means (95% confidence intervals) AUCstISGroup1 909 (672-1146) versus AUCstISGroup2 1385 (1094-1675) (p = 0.014), AUCstNISGroup1 1122 (823-1420) versus AUCstNISGroup2 1668 (1302-2033) (p = 0.024) and AUCstUMPSGroup1 5.30 (4.58-6.02) versus AUCstUMPSGroup2 4.1 (3.2-5.0) (p = 0.041). Similarly, M/N showed higher pain than M/I with AUCstISGroup3 686 (384-987) versus AUCstISGroup4 1107 (871-1345) (p = 0.031) and AUCstNISGroup3 856 (476-1235) versus AUCstNISGroup4 1407 (1109-1706) (p = 0.026), and AUCstUMPSGroup3 6.0 (5.1-6.9) versus AUCstUMPSGroup4 4.4 (3.7-5.2) (p = 0.008).

CONCLUSIONS AND CLINICAL RELEVANCE

Gonadectomy affects pain sensitivity in dogs undergoing stifle surgery. Neutering status should be taken into consideration when planning individualized anaesthetic/analgesic protocols.

A case report of effective intra-articular elcatonin administration in a patient with osteonecrosis of the lunate

INTRODUCTION AND IMPORTANCE

Although corticosteroids are effective for bronchial asthma, they are associated with various side effects, such as osteonecrosis of the femoral head and tibial condyle and osteoporosis. Here, we report a patient who was on corticosteroids for a long period of time due to severe asthma, which was thought to have led to lunate osteonecrosis of both wrist joints. Calcitonin (elcatonin), an osteoporosis drug, was administered to the wrist joint.

CASE PRESENTATION

Allergy to anesthetics and various non-steroidal anti-inflammatory drugs (NSAIDs) made surgical treatment not possible and pain control difficult. In addition, pain in the wrist joint interfered with activities of daily living (ADLs). When calcitonin was administered intra-articularly into the wrist joint, the pain in the wrist joint was relieved and ADLs were improved. However, the pain-suppressing effect lasted only 1 week, and pain returned to the original state by the second week after treatment. Repeated injections were necessary every 2 weeks.

CLINICAL DISCUSSION

This is the first report of the effectiveness of intra-articular calcitonin in a patient with osteonecrosis of the lunate. This treatment may be applicable to patients with allergies to anesthesia and NSAIDs.

CONCLUSION

In the present case, calcitonin administered intra-articularly to the wrist appeared to be effective in pain control for steroid-induced lunate osteonecrosis.

Inhibition of Spinal 5-HT3 Receptor and Spinal Dorsal Horn Neuronal Excitability Alleviates Hyperalgesia in a Rat Model of Parkinson's Disease

Pain in Parkinson's disease (PD) is increasingly recognized as a major factor associated with poor life quality of PD patients. However, classic therapeutic drugs supplying dopamine have limited therapeutic effects on PD-related pain. This suggests that there is a mechanism outside the dopamine system that causes pain in PD. Our previous study demonstrated that 6-OHDA induced PD model manifested hyperalgesia to thermal and mechanical stimuli and decreased serotonin (5-hydroxytryptamine; 5-HT) in the spinal dorsal horn (SDH). Several 5-HT receptor subtypes have been confirmed to be associated with nociception in the spinal cord, such as 5-HT1A receptor, 5-HT1B receptor, 5-HT2 receptor, 5-HT3 receptor, and 5-HT7 receptor. Most research has shown that 5-HT1A receptor and 5-HT3 receptor play a key role in pain transmission in the spinal cord. We hypothesized that hyperalgesia of 6-OHDA rats may be related to increased excitability of SDH neurons, and functional change of 5-HT3 receptor may reverse the hyperalgesia of 6-OHDA lesioned rats and decrease cell excitability of SDH neurons. To test this hypothesis, we used whole-cell patch-clamp and pharmacological methods to evaluate the effect of 5-HT3 receptor and 5-HT1A receptor on the hyperalgesia of 6-OHDA rats. The results suggested that increased excitability in SDH neurons could be reversed by 5-HT3 receptor antagonist ondansetron (20 μmol/L) and palosetron (10 μmol/L), but not 5-HT3 receptor agonist m-CPBG (30 μmol/L) and SR 57,727 (10 μmol/L), 5-HT1A receptor agonist 8-OH DPAT (10 μmol/L) and eptapirone (10 μmol/L) and 5-HT1A receptor antagonist WAY-100635 (10 μmol/L) and p-MPPI (10 μmol/L). Intrathecal injection of ondansetron (0.1 mg/kg) but not m-CPBG (0.1 mg/kg), 8-OH DPAT (0.1 mg/kg), and WAY-100635 (0.1 mg/kg) significantly attenuated the mechanical hyperalgesia and thermal hyperalgesia in 6-OHDA lesioned rats. In conclusion, the present study suggests that inhibition of spinal 5-HT3 receptor and SDH neuronal excitability alleviates hyperalgesia in PD rats. Our study provides a novel mechanism or therapeutic strategy for pain in patients with PD.

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.

Identification of a Potent and Selective 5-HT1A Receptor Agonist with In Vitro and In Vivo Antinociceptive Activity

Opioids are the gold standard drugs for the treatment of acute and chronic severe pain, although their serious side effects constitute a big limitation. In the search for new and safer drugs, 5-HT_1AR agonists are emerging as potential candidates in pain relief therapy. In this work, we evaluated the affinity and activity of enantiomers of the two newly synthesized, potent 5-HT_1AR agonists N-[(2,2-diphenyl-1,3-dioxolan-4-yl)methyl]-2-[2-(pyridin-4-yl)phenoxy]ethan-1-ammonium hydrogenoxalate (rac-1) and N-((2,2-diphenyl-1,3-dioxolan-4-yl)methyl)-2-(2-(1-methyl-1H-imidazol-5-yl)phenoxy)ethan-1-ammonium hydrogenoxalate (rac-2) in vitro and in vivo. The role of chirality in the interaction with 5-HT_1AR was evaluated by molecular docking. The activity of the rac-1 was tested in mouse models of acute pain (hot plate) and severe tonic nociceptive stimulation (intraplantar formalin test). Rac-1 was active in the formalin test with a reduction in paw licking in both phases at 10 mg/kg, and its effect was abolished by the selective 5-HT_1AR antagonist, WAY-100635. The eutomer (S)-1, but not the racemate, was active during the hot plate test at 10 and 20 mg/kg, and this effect was abolished by 30 min treatment with WAY-100635 at 30 min. Similarly to 8-OH-DPAT, (S)-1 evoked a slow outward current and depressed spontaneous glutamatergic transmission in superficial dorsal horn neurons, more effectively than rac-1. The eutomer (S)-1 showed promising developability properties, such as high selectivity over 5-HT subtypes, no interaction with the μ receptors, and low hepato- and cardiotoxicity. Therefore, (S)-1 may represent a potential candidate for the treatment of acute and chronic pain without having the adverse effects that are commonly associated with the classic opioid drugs.

Role of 5-HT1A and 5-HT3 receptors in serotonergic activation of sensory neurons in relation to itch and pain behavior in the rat

Serotonin (5-hydroxytryptamine, 5-HT) released by platelets, mast cells, and immunocytes is a potent inflammatory mediator which modulates pain and itch sensing in the peripheral nervous system. The serotonergic receptors expressed by primary afferent neurons involved in these sensory functions are not fully identified and appear to be to a large extent species dependent. Moreover, the mechanisms through which 5-HT receptor activation is coupled to changes in neuronal excitability have not been completely revealed. Using a combination of in vitro (calcium and voltage imaging and patch-clamp) and in vivo behavioral methods, we used both male and female Wistar rats to provide evidence for the involvement of two 5-HT receptor subtypes, 5-HT1A and 5-HT3, in mediating the sustained and transient effects, respectively, of 5-HT on rat primary afferent neurons involved in pain and itch processing. In addition, our results are consistent with a model in which sustained serotonergic responses triggered via the 5-HT1A receptor are due to closure of background potassium channels, followed by membrane depolarization and action potentials, during which the activation of voltage-gated calcium channels leads to calcium entry. Our results may provide a better understanding of mammalian serotonergic itch signaling.

Inflammation induces developmentally regulated sumatriptan inhibition of spinal synaptic transmission

BACKGROUND AND PURPOSE

While triptans are used to treat migraine, there is evidence that they also reduce inflammation-induced pain at the spinal level. The cellular mechanisms underlying this spinal enhancement are unknown. We examined whether inflammation alters sumatriptan modulation of synaptic transmission in the rat spinal dorsal horn.

EXPERIMENTAL APPROACH

Three to four days following intraplantar injection of complete Freund's adjuvant (CFA) or saline, whole cell recordings of evoked glutamatergic EPSCs were made from lumbar lamina I-II dorsal horn neurons in rat spinal slices KEY RESULTS: In 2- to 3-week-old animals, sumatriptan reduced the amplitude of evoked EPSCs and this was greater in slices from CFA, compared to saline-injected rats. In CFA-injected animals, sumatriptan increased the paired pulse ratio of evoked EPSCs and reduced the rate of spontaneous miniature EPSCs. The 5-HT_1B and 5-HT_1D agonists CP9 3129 and PNU109291 both inhibited evoked EPSCs in CFA but not saline-injected rats. By contrast, the 5-HT_1A agonist R(+)-8-OH-DPAT inhibited evoked EPSCs in saline but not CFA-injected rats. In CFA-injected rats, the sumatriptan-induced inhibition of evoked EPSCs was reduced by the 5-HT_1B and 5-HT_1D antagonists NAS181 and BRL-15572. Intriguingly, the difference in sumatriptan inhibition between CFA and saline-injected animals was only observed in animals less than 4 weeks old.

CONCLUSION AND IMPLICATIONS

These findings indicate that inflammation induces a developmentally regulated 5-HT_1B/1D presynaptic inhibition of excitatory transmission into the rat superficial dorsal horn. Thus, triptans could potentially act as spinal analgesic agents for inflammatory pain in the juvenile setting.

Serotonergic Modulation of Nociceptive Circuits in Spinal Cord Dorsal Horn

Background:

Despite the extensive number of studies performed in the last 50 years, aimed at describing the role of serotonin and its receptors in pain modulation at the spinal cord level, several aspects are still not entirely understood. The interpretation of these results is often complicated by the use of different pain models and animal species, together with the lack of highly selective agonists and antagonists binding to serotonin receptors.

Method:

In this review, a search has been conducted on studies investigating the modulatory action exerted by serotonin on specific neurons and circuits in the spinal cord dorsal horn. Particular attention has been paid to studies employing electro-physiological techniques, both in vivo and in vitro.

Conclusion:

The effects of serotonin on pain transmission in dorsal horn depend on several factors, including the type of re-ceptors activated and the populations of neurons involved. Recently, studies performed by activating and/or recording from identified neurons have importantly contributed to the understanding of serotonergic modulation on dorsal horn circuits.

Teriparatide relieves ovariectomy-induced hyperalgesia in rats, suggesting the involvement of functional regulation in primary sensory neurons by PTH-mediated signaling

Clinical studies have reported that teriparatide (TPTD), a human parathyroid hormone analog, reduces back pain in osteoporotic patients. However, the mechanistic insights of this pharmacological action remain elusive. This study investigated the antinociceptive effect of TPTD mainly on primary sensory neurons in ovariectomized (OVX) rats. The plantar test showed thermal hyperalgesia in the OVX rats, which was significantly, but not fully, recovered immediately after the initial TPTD administration. The von Frey test also demonstrated reduced withdrawal threshold in the OVX rats. This was partially recovered by TPTD. Consistently, the number and size of spinal microglial cells were significantly increased in the OVX rats, while TPTD treatment significantly reduced the number but not size of these cells. RNA sequencing-based bioinformatics of the dorsal root ganglia (DRG) demonstrated that changes in neuro-protective and inflammatory genes were involved in the pharmacological effect of TPTD. Most neurons in the DRG expressed substantial levels of parathyroid hormone 1 receptor. TPTD treatment of the cultured DRG-derived neuronal cells reduced the cAMP level and augmented the intracellular calcium level as the concentration increased. These findings suggest that TPTD targets neuronal cells as well as bone cells to exert its pharmacological action.

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

Potentiation of the glycine response by serotonin on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice

The lamina II, also called the substantia gelatinosa (SG), of the trigeminal subnucleus caudalis (Vc), is thought to play an essential role in the control of orofacial nociception. Glycine and serotonin (5-hydroxytryptamine, 5-HT) are the important neurotransmitters that have the individual parts on the modulation of nociceptive transmission. However, the electrophysiological effects of 5-HT on the glycine receptors on SG neurons of the Vc have not been well studied yet. For this reason, we applied the whole-cell patch clamp technique to explore the interaction of intracellular signal transduction between 5-HT and the glycine receptors on SG neurons of the Vc in mice. In nine of 13 neurons tested (69.2%), pretreatment with 5-HT potentiated glycine-induced current (I_Gly). Firstly, we examined with a 5-HT₁ receptor agonist (8-OH-DPAT, 5-HT_1/7 agonist, co-applied with SB-269970, 5-HT₇ antagonist) and antagonist (WAY-100635), but 5-HT₁ receptor agonist did not increase I_Gly and in the presence of 5-HT₁ antagonist, the potentiation of 5-HT on I_Gly still happened. However, an agonist (α-methyl-5-HT) and antagonist (ketanserin) of the 5-HT₂ receptor mimicked and inhibited the enhancing effect of 5-HT on I_Gly in the SG neurons, respectively. We also verified the role of the 5-HT₇ receptor by using a 5-HT₇ antagonist (SB-269970) but it also did not block the enhancement of 5-HT on I_Gly. Our study demonstrated that 5-HT facilitated I_Gly in the SG neurons of the Vc through the 5-HT₂ receptor. The interaction between 5-HT and glycine appears to have a significant role in modulating the transmission of the nociceptive pathway.

Molecular-Based Treatment Strategies for Osteoporosis: A Literature Review

Osteoporosis is an unavoidable public health problem in an aging or aged society. Anti-resorptive agents (calcitonin, estrogen, and selective estrogen-receptor modulators, bisphosphonates, anti-receptor activator of nuclear factor κB ligand antibody along with calcium and vitamin D supplementations) and anabolic agents (parathyroid hormone and related peptide analogs, sclerostin inhibitors) have major roles in current treatment regimens and are used alone or in combination based on the pathological condition. Recent advancements in the molecular understanding of bone metabolism and in bioengineering will open the door to future treatment paradigms for osteoporosis, including antibody agents, stem cells, and gene therapies. This review provides an overview of the molecular mechanisms, clinical evidence, and potential adverse effects of drugs that are currently used or under development for the treatment of osteoporosis to aid clinicians in deciding how to select the best treatment option.

Inhibition by O-desmethyltramadol of glutamatergic excitatory transmission in adult rat spinal substantia gelatinosa neurons

To reveal cellular mechanisms for antinociception produced by clinically used tramadol, we investigated the effect of its metabolite O-desmethyltramadol (M1) on glutamatergic excitatory transmission in spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons. The whole-cell patch-clamp technique was applied at a holding potential of −70 mV to SG neurons of an adult rat spinal cord slice with an attached dorsal root. Under the condition where a postsynaptic action of M1 was inhibited, M1 superfused for 2 min reduced the frequency of spontaneous excitatory postsynaptic current in a manner sensitive to a μ-opioid receptor antagonist CTAP; its amplitude and also a response of SG neurons to bath-applied AMPA were hardly affected. The presynaptic effect of M1 was different from that of noradrenaline or serotonin which was examined in the same neuron. M1 also reduced by almost the same extent the peak amplitudes of monosynaptic primary-afferent Aδ-fiber and C-fiber excitatory postsynaptic currents evoked by stimulating the dorsal root. These actions of M1 persisted for >10 min after its washout. These results indicate that M1 inhibits the quantal release of L-glutamate from nerve terminals by activating μ-opioid but not noradrenaline and serotonin receptors; this inhibition is comparable in extent between monosynaptic primary-afferent Aδ-fiber and C-fiber transmissions. Considering that the SG plays a pivotal role in regulating nociceptive transmission, the present findings could contribute to at least a part of the inhibitory action of tramadol on nociceptive transmission together with its hyperpolarizing effect as reported previously.

A randomized, double-blind, placebo-controlled study of once weekly elcatonin in primary postmenopausal osteoporosis

OBJECTIVE

Very few reports have described changes in bone mineral density (BMD) with long-term, once weekly administration of elcatonin, and its effects in reducing incident fractures remain unverified. Therefore, the efficacy and safety of once weekly elcatonin were examined over a 3 year period.

METHODS

This was a multicenter, double-blinded, randomized, placebo-controlled study. Postmenopausal women with primary osteoporosis received either 20 units of elcatonin (EL group, n = 433) or placebo (P group, n = 436) once a week for 144 weeks (3 years) intramuscularly. The primary endpoint was the incidence of new vertebral fractures at 24, 48, 72, 96, 120, and 144 weeks after the start. Secondary endpoints were the incidence of non-vertebral fractures, changes in lumbar, hip total and femoral neck BMD, and the incidence of adverse drug reactions (ADRs).

RESULTS

No significant reduction in the incidence of new vertebral fractures was found in the EL group. The percentage increase in lumbar BMD was significantly higher in the EL group from 24 weeks to the last administration. Although the EL group showed tendencies toward smaller decreased hip total and femoral neck BMD, no significant differences were observed between groups. The incidence of ADRs was significantly greater in the EL group, although these have all been previously reported and no new safety concerns were identified.

CONCLUSIONS

Once weekly injection of 20 units of elcatonin significantly increased lumbar BMD over a 3 year period and did not cause any safety problems, but no significant reduction in the incidence of vertebral or non-vertebral fractures was demonstrated.

Effects of High Concentrations of Naftopidil on Dorsal Root-Evoked Excitatory Synaptic Transmissions in Substantia Gelatinosa Neurons In Vitro

Purpose

Naftopidil ((±)-1-[4-(2-methoxyphenyl) piperazinyl]-3-(1-naphthyloxy) propan-2-ol) is prescribed in several Asian countries for lower urinary tract symptoms suggestive of benign prostatic hyperplasia. Previous animal experiments showed that intrathecal injection of naftopidil abolished rhythmic bladder contraction in vivo. Naftopidil facilitated spontaneous inhibitory postsynaptic currents in substantia gelatinosa (SG) neurons in spinal cord slices. These results suggest that naftopidil may suppress the micturition reflex at the spinal cord level. However, the effect of naftopidil on evoked excitatory postsynaptic currents (EPSCs) in SG neurons remains to be elucidated.

Methods

Male Sprague-Dawley rats at 6 to 8 weeks old were used. Whole-cell patch-clamp recordings were made using SG neurons in spinal cord slices isolated from adult rats. Evoked EPSCs were analyzed in Aδ or C fibers. Naftopidil or prazosin, an α1-adrenoceptor blocker, was perfused at 100 μM or 10 μM, respectively.

Results

Bath-applied 100 μM naftopidil significantly decreased the peak amplitudes of Aδ and C fiber-evoked EPSCs to 72.0%±7.1% (n=15) and 70.0%±5.5% (n=20), respectively, in a reversible and reproducible manner. Bath application of 10μM prazosin did not inhibit Aδ or C fiber-evoked EPSCs.

Conclusions

The present study suggests that a high concentration of naftopidil reduces the amplitude of evoked EPSCs via a mechanism that apparently does not involve α1-adrenoceptors. Inhibition of evoked EPSCs may also contribute to suppression of the micturition reflex, together with nociceptive stimulation.

Orexin B Modulates Spontaneous Excitatory and Inhibitory Transmission in Lamina II Neurons of Adult Rat Spinal Cord

Cellular mechanisms underlying the antinociceptive properties of orexins, a group of neuropeptides produced by the hypothalamus, in the spinal dorsal horn have not been thoroughly investigated. We examined how orexin B affects spontaneous synaptic transmission in lamina II neurons, which play a pivotal role in regulating nociceptive transmission, by applying a whole-cell patch-clamp technique to lamina II neurons in adult rat spinal cord slices. In 66% of neurons tested, bath-applied orexin B concentration dependently produced an inward current at -70 mV and/or increased the frequency of glutamatergic spontaneous excitatory postsynaptic current (sEPSC) without changing its amplitude, in a manner resistant to the voltage-gated Na⁺-channel blocker tetrodotoxin (TTX). Glycinergic spontaneous inhibitory transmission was enhanced by orexin B in a TTX-sensitive manner in 71% of neurons examined, whereas GABAergic transmission was unaffected in the majority of these neurons. These activities were inhibited by an orexin-2 receptor antagonist (JNJ10397049) but not an orexin-1 receptor antagonist (SB334867). While the effects of orexin B in orexin B-sensitive neurons were mimicked by orexin A, another hypothalamic neuropeptide, oxytocin, produced an inward current but no increase in sEPSC frequency. These results indicate that orexin B produces membrane depolarization and/or increased spontaneous l-glutamate release in lamina II neurons by activating orexin-2 receptors, leading to increased excitability of these neurons. Such increases potentially produce an action potential, resulting in enhancement of glycinergic transmission in lamina II neurons. This activity of orexin B, and possibly orexin A, may contribute to its antinociceptive effects, which are partly shared by oxytocin.

Mechanisms of the analgesic effect of calcitonin on chronic pain by alteration of receptor or channel expression

The polypeptide hormone calcitonin is well known clinically for its ability to relieve osteoporotic back pain and neuropathic pain such as spinal canal stenosis, diabetic neuropathy, chemotherapy-induced neuropathy, and complex regional pain syndrome. Because the analgesic effects of calcitonin have a broad range, the underlying mechanisms of pain relief by calcitonin are largely unknown. However, recent studies using several types of chronic pain models combined with various methods have been gradually clarifying the mechanism. Here, we review the mechanisms of the analgesic action of calcitonin on ovariectomy-induced osteoporotic and neuropathic pain. The analgesic action of calcitonin may be mediated by restoration of serotonin receptors that control selective glutamate release from C-afferent fibers in ovariectomized rats and by normalization of sodium channel expression in damaged peripheral nerves. Serotonin receptors are reduced or eliminated by the relatively rapid reduction in estrogen during the postmenopausal period, and damaged nerves exhibit hyperexcitability due to abnormal expression of Na⁺ channel subtypes. In addition, in chemotherapy-induced peripheral neuropathy, inhibition of signals related to transient receptor potential ankyrin-1 and melastatin-8 is proposed to participate in the anti-allodynic action of calcitonin. Further, an unknown calcitonin-dependent signal appears to be present in peripheral nervous tissues and may be activated by nerve injury, resulting in regulation of the excitability of primary afferents by control of sodium channel transcription in dorsal root ganglion neurons. The calcitonin signal in normal conditions may be non-functional because no target is present, and ovariectomy or nerve injury may induce a target. Moreover, it has been reported that calcitonin reduces serotonin transporter but increases serotonin receptor expression in the thalamus in ovariectomized rats. These data suggest that calcitonin could alleviate lower back pain in patients with osteoporosis or neuropathic pain by the alteration in receptor or channel expression.

Osteoporotic Pain is Associated with Increased Transient Receptor Vanilloid 4 Expression in the Dorsal Root Ganglia of Ovariectomized Osteoporotic Rats: A Pilot Basic Study

INTRODUCTION

Osteoporosis can produce a persistent state of pain known as osteoporotic pain. One proposed mechanism of this pathology is increased calcitonin gene-related peptide (CGRP; a marker related to inflammatory pain) expression in the dorsal root ganglia (DRG) innervating osteoporotic vertebrae. Alternatively, a previous study revealed that axial loading caused osteoporotic pain in a rodent model of coccygeal vertebrae compression. Because this compression model is associated with trauma, additional mechanistic studies of osteoporotic pain in the absence of trauma are required. The current study aimedto evaluate the expression and relative distribution of transient receptor potential vanilloid 4 (TRPV4), a pain-related mechanoreceptor, in ovariectomized (OVX) osteoporotic rats.

METHODS

CGRP-immunoreactive (-ir) and TRPV4-ir DRG neurons innervating the L3 vertebrae of Sprague-Dawley rats were labeled with a neurotracer, FluoroGold. Intravertebral pH was also measured during the neurotracer procedure. TRPV4-ir/CGRP-ir FluoroGold-positive DRG neurons were quantified in sham control and OVX rats (n = 10, ea). The threshold for statistical significance was set at P < 0.05.

RESULTS

There was no statistical difference in the number of FluoroGold-positive DRG neurons between groups; however, there were significantly more CGRP-ir/TRPV4-ir FluoroGold-positive DRG neurons in the OVX group compared with the sham control group (P < 0.05) as well as the significantly increased molecular production of each peptide. Intravertebral pH was also lower in the OVX group compared with the sham control group (P < 0.05).

CONCLUSION

Sensory neurons innervating osteoporotic vertebrae exhibited increased expression of co-localized CGRP and TRPV4 in OVX osteoporotic rats. Additionally, intravertebral pH was low in the vicinity osteoporotic vertebrae. Considering that TRPV4 is a mechanosensitive nociceptor that is activated in acidic environments, its upregulation may be associated with the pathology of osteoporotic pain derived from microinflammation involved in osteoporosis.

Pathomechanisms and management of osteoporotic pain with no traumatic evidence

Introduction

Osteoporosis is a pathological state with an unbalanced bone metabolism mainly caused by accelerated osteoporotic osteoclast activity due to a postmenopausal estrogen deficiency, and it causes some kinds of pain, which can be divided into two types: traumatic pain due to a fragility fracture from impaired rigidity, and pain derived from an osteoporotic pathology without evidence of fracture. We aimed to review the concepts of osteoporosis-related pain and its management.

Methods

We reviewed clinical and basic articles on osteoporosis-related pain, especially with a focus on the mechanism of pain derived from an osteoporotic pathology (i.e., osteoporotic pain) and its pharmacological treatment.

Results

Osteoporosis-related pain tends to be robust and acute if it is due to fracture or collapse, whereas pathology-related osteoporotic pain is vague and dull. Non-traumatic osteoporotic pain can originate from an undetectable microfracture or structural change such as muscle fatigue in kyphotic patients. Furthermore, basic studies have shown that the osteoporotic state itself is related to pain or hyperalgesia with increased pain-related neuropeptide expression or acid-sensing channels in the local tissue and nervous system. Traditional treatment for osteoporotic pain potentially prevents possible fracture-induced pain by increasing bone mineral density and affecting related mediators such as osteoclasts and osteoblasts. The most common agent for osteoporotic pain management is a bisphosphonate. Other non-osteoporotic analgesic agents such as celecoxib have also been reported to have a suppressive effect on osteoporotic pain.

Conclusions

Osteoporotic pain has traumatic and non-traumatic factors. Anti-osteoporotic treatments are effective for osteoporotic pain, as they improve bone structure and the condition of the pain-related sensory nervous system. Physicians should always consider these matters when choosing a treatment strategy that would best benefit patients with osteoporotic pain.

Sarcopenia in elderly patients with chronic low back pain

Objectives

The prevalence of chronic low back pain (CLBP) increases with age and several mechanisms are involved in the development of CLBP, including osteoporosis; however, no associations with sarcopenia have yet been identified.

Methods

In total, 100 patients with CLBP and 560 patients without CLBP (nCLBP) aged over 65 years were studied. Skeletal muscle mass index (SMI) and percentage of body fat were evaluated using whole-body dual-energy X-ray absorptiometry. Sarcopenia was diagnosed when the relative SMI was more than 2 standard deviations below the mean in young adults. Thus, the cutoff value for sarcopenia was defined according to Sanada's Japanese population data. Paraspinal muscle cross-sectional areas of the lumbar multifidus and the erector spinae muscles were calculated using magnetic resonance imaging.

Results

Forty patients (40.0%) from the CLBP group and 149 (26.6%) from the nCLBP group met the criteria of sarcopenia. SMI was significantly lower and the body fat ratio was significantly higher in the CLBP group compared with the nCLBP group. Sarcopenic obesity was significantly observed in the CLBP group. Lumbar multifidus and the erector spinae muscle cross sectional area were significantly lower in the CLBP group.

Conclusions

Elderly patients with CLBP have significantly lower skeletal muscle mass, and age-related mechanisms in sarcopenia are considered to be associated with chronic pain. Therapeutic procedures that are used to treat elderly aging muscle, including muscle strengthening and performance training, can possibly be a treatment for or used to prevent elderly CLBP.

Effect of elcatonin versus nonsteroidal anti-inflammatory medications for acute back pain in patients with osteoporotic vertebral fracture: a multiclinic randomized controlled trial

The aim of this study was to compare the efficacy of elcatonin injections and oral nonsteroidal anti-inflammatory drugs (NSAIDs) for patients with osteoporosis who have acute lumbar pain after experiencing new vertebral compression fractures. Two hundred twenty-eight Japanese female patients (mean age 77.3 years) with acute lumbar pain from osteoporotic vertebral fractures were randomly divided into two groups. Patients in one group were given an NSAID (NSAIDs group) and patients in the other group were given weekly intramuscular injections of 20 units of elcatonin (elcatonin group). All patients underwent follow-up examinations up to 6 weeks from the start of the trial. Outcome measures were the level of functional impairment according to the Japan Questionnaire for Osteoporotic Pain (JQ22), the Roland-Morris Disability Questionnaire (RDQ), and a visual analog scale (VAS) of pain intensity. Statistical analyses focused on (1) the time course of pain and functional level using linear mixed effects models to analyze the longitudinal data and (2) the effectiveness of elcatonin injection with mean difference values and 95 % confidence intervals. Significant differences were seen over time between the initial values and the postintervention values (4 and 6 weeks) in JQ22, RDQ, and VAS scores (effect size d > 0.4) in each group. The mean differences between the elcatonin group and the NSAIDs group in each measure at 4 and 6 weeks were -4.8 and -8.3 for the JQ22, -1.3 and -2.6 for the RDQ, and -11.3 and -11.5 for the VAS, shifted to elcatonin. Once weekly elcatonin injection was more effective than NSAIDs for treating acute lumbar pain and improving mobility in Japanese women with osteoporotic vertebral fractures.

Platypus and opossum calcitonins exhibit strong activities, even though they belong to mammals

In mammalian assay systems, calcitonin peptides of non-mammalian species exhibit stronger activity than those of mammals. Recently, comparative analyses of a wide-range of species revealed that platypus and opossum, which diverged early from other mammals, possess calcitonins that are more similar in amino acid sequence to those of non-mammals than mammals. We herein determined whether platypus and opossum calcitonins exhibit similar biological activities to those of non-mammalian calcitonins using an assay of actin ring formation in mouse osteoclasts. We also compared the dose-dependent effects of each calcitonin on cAMP production in osteoclasts. Consistent with the strong similarities in their primary amino acid sequences, platypus and opossum calcitonins disrupted actin rings with similar efficacies to that of salmon calcitonin. Human calcitonin exhibited the weakest inhibitory potency and required a 100-fold higher concentration (EC₅₀=3×10^-11M) than that of salmon calcitonin (EC₅₀=2×10^-13M). Platypus and opossum calcitonins also induced cAMP production in osteoclast cultures with the same efficacies as that of salmon calcitonin. Thus, platypus and opossum calcitonins exhibited strong biological activities, similar to those of the salmon. In addition, phylogenetic analysis revealed that platypus and opossum calcitonins clustered with the salmon-type group but not human- or porcine-type group. These results suggest that platypus and opossum calcitonins are classified into the salmon-type group, in terms of the biological activities and amino acid sequences.

Chronic Osteoporotic Pain in Mice: Cutaneous and Deep Musculoskeletal Pain Are Partially Independent of Bone Resorption and Differentially Sensitive to Pharmacological Interventions

Although the pathological changes in osteoporotic bones are well established, the characterization of the osteoporotic pain and its appropriate treatment are not fully elucidated. We investigated the behavioral signs of cutaneous and deep musculoskeletal pain and physical function; time-dependent changes in bone mineral density (BMD) and the emergence of the behavioral phenotype; and the effects of pharmacological interventions having different mechanisms of action (chronic intraperitoneal administration of pamidronate [0.25 mg/kg, 5x/week for 5 weeks] versus acute treatment with intraperitoneal morphine [10 mg/kg] and pregabalin [100 mg/kg]) in a mouse model of ovariectomized or sham-operated mice 6 months following surgery. We observed reduced BMD associated with weight gain, referred cutaneous hypersensitivity, and deep musculoskeletal pain that persisted for 6 months. Chronic bisphosphonate treatment, 6 months after ovariectomy, reversed bone loss and hypersensitivity to cold, but other behavioral indices of osteoporotic pain were unchanged. While the efficacy of acute morphine on cutaneous pain was weak, pregabalin was highly effective; deep musculoskeletal pain was intractable. In conclusion, the reversal of bone loss alone is insufficient to manage pain in chronic osteoporosis. Additional treatments, both pharmacological and nonpharmacological, should be implemented to improve quality of life for osteoporosis patients.

Changes in synaptic transmission of substantia gelatinosa neurons after spinal cord hemisection revealed by analysis using in vivo patch-clamp recording

BACKGROUND

After spinal cord injury, central neuropathic pain develops in the majority of spinal cord injury patients. Spinal hemisection in rats, which has been developed as an animal model of spinal cord injury in humans, results in hyperexcitation of spinal dorsal horn neurons soon after the hemisection and thereafter. The hyperexcitation is likely caused by permanent elimination of the descending pain systems. We examined the change in synaptic transmission of substantia gelatinosa neurons following acute spinal hemisection by using an in vivo whole-cell patch-clamp technique.

RESULTS

An increased spontaneous action potential firings of substantia gelatinosa neurons was detected in hemisected rats compared with that in control animals. The frequencies and amplitudes of spontaneous excitatory postsynaptic currents and of evoked excitatory postsynaptic currentss in response to non-noxious and noxious stimuli were not different between hemisected and control animals. On the contrary, the amplitude and frequency of spontaneous inhibitory postsynaptic currents of substantia gelatinosa neurons in hemisected animals were significantly smaller and lower, respectively, than those in control animals (P < 0.01). Large amplitude and high-frequency spontaneous inhibitory postsynaptic currents, which could not be elicited by mechanical stimuli, were seen in 44% of substantia gelatinosa neurons in control animals but only in 17% of substantia gelatinosa neurons in hemisected animals. In control animals, such large amplitude spontaneous inhibitory postsynaptic currents were suppressed by spinal application of tetrodotoxin (1 µM). Cervical application of lidocaine (2%, 10 µl) also inhibited such large amplitude of inhibitory postsynaptic currents. The proportion of multi-receptive substantia gelatinosa neurons, which exhibit action potential firing in response to non-noxious and noxious stimuli, was much larger in hemisected animals than in control animals.

CONCLUSIONS

These suggest that substantia gelatinosa neurons receive tonic inhibition by spinal inhibitory interneurons which generate persistent action potentials. Spinal hemisection results in hyperexcitation of substantia gelatinosa neurons at least in part by eliminating the tonic descending control of spinal inhibitory interneurons from supraspinal levels.

Differential Activation of TRP Channels in the Adult Rat Spinal Substantia Gelatinosa by Stereoisomers of Plant-Derived Chemicals

Activation of TRPV1, TRPA1 or TRPM8 channel expressed in the central terminal of dorsal root ganglion (DRG) neuron increases the spontaneous release of l-glutamate onto spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons which play a pivotal role in regulating nociceptive transmission. The TRP channels are activated by various plant-derived chemicals. Although stereoisomers activate or modulate ion channels in a distinct manner, this phenomenon is not fully addressed for TRP channels. By applying the whole-cell patch-clamp technique to SG neurons of adult rat spinal cord slices, we found out that all of plant-derived chemicals, carvacrol, thymol, carvone and cineole, increase the frequency of spontaneous excitatory postsynaptic current, a measure of the spontaneous release of l-glutamate from nerve terminals, by activating TRP channels. The presynaptic activities were different between stereoisomers (carvacrol and thymol; (-)-carvone and (+)-carvone; 1,8-cineole and 1,4-cineole) in the extent or the types of TRP channels activated, indicating that TRP channels in the SG are activated by stereoisomers in a distinct manner. This result could serve to know the properties of the central terminal TRP channels that are targets of drugs for alleviating pain.

Elcatonin attenuates disuse osteoporosis after fracture fixation of tubular bone in rats

BACKGROUND

Elcatonin (ECT) is used to prevent and treat osteoporosis. However, little is known about its effect on the disuse osteoporosis (DOP). The aim of this study is to evaluate the effect of ECT on DOP caused by fracture fixation.

METHODS

Forty-five male Sprague-Dawley (SD) rats, aged 6 weeks, were randomly allocated into three groups: the control group without surgery and elcatonin treatment (CTR, n = 15), the surgery group without elcatonin treatment (SUR, n = 15), and the surgery group which received elcatonin subcutaneously (SUR + ECT, n = 15). Surgery was produced by cutting the midshaft of the right femur transversely, fixing with stainless intramedullary needle, and immobilizing the right leg. All the proximal tibias from the random five rats in each group were harvested and investigated by evaluating bone mineral density (BMD), X-ray images, and histological staining respectively at the 4th, 8th, and 12th weeks after surgery.

RESULTS

Both of the SUR and SUR + ECT groups obviously exhibited lower BMD values compared to the CTR group; however, the SUR + ECT group showed significantly higher BMD values (p < 0.001, p < 0.05, and p < 0.05) than the SUR group at each time point after surgery. Moreover, similar changes were observed between these groups when examining the radiographs and hematoxylin and eosin (HE) staining.

CONCLUSIONS

Elcatonin attenuates disuse osteoporosis after fractures in rats, which may provide a new avenue to prevent and treat disuse osteoporosis after surgery in clinic.

Additive effect of elcatonin to risedronate for chronic back pain and quality of life in postmenopausal women with osteoporosis: a randomized controlled trial

Calcitonin has been reported to reduce acute and chronic back pain in osteoporotic patients. The additive effect of calcitonin with a bisphosphonate on chronic back pain remains unclear. The purpose of this study was to evaluate the effect of combining elcatonin (eel calcitonin) with risedronate for patients with chronic back pain. Forty-five postmenopausal women diagnosed as having osteoporosis with chronic back pain persisting for more than 3 months, after excluding women with fresh vertebral fractures within the last 6 months, were randomly allocated to a risedronate group (risedronate alone, n = 22) and a combined group (risedronate and elcatonin, n = 23). The study period was 6 months. Pain was evaluated with a visual analogue scale (VAS) and the Roland-Morris questionnaire (RDQ). Back extensor strength, bone mineral density, and quality of life on the SF-36 and the Japanese osteoporosis quality of life score were also evaluated. Significant improvements were found in the combined group for VAS at final follow-up compared with baseline and 3 months, mental health status on the SF-36, and JOQOL domains for back pain and general health. The JOQOL domain for back pain improved significantly, but no change was found in the VAS or other domains in the risedronate group. Bone mineral density increased significantly in the two groups, but no significant difference was found between the groups. Back extensor strength did not change in both groups. In conclusion, the use of elcatonin in addition to risedronate for more than 3 months reduced chronic back pain. The additional therapy of risedronate with elcatonin may be a useful and practical choice for the treatment of osteoporosis with chronic back pain persisting more than 3 months.

Comparison of the effects of calcium carbonate and ossein-hydroxyapatite complex on back and knee pain and quality of life in osteopenic perimenopausal women

BACKGROUND

Back and knee pain (BP; KP), which often accompanies osteoporosis, is a prevalent health problem affecting quality of life (QoL) in middle-aged women.

AIM

To compare the effects of calcium carbonate (CC) and ossein-hydroxyapatite complex (OHC) on BP and KP and QoL in perimenopausal osteopenic women.

SUBJECTS

74 perimenopausal women were randomized to receive 1,200 mg/day of CC (n=38) or 1,660 mg/day of OHC (n=36) for 6 months.

METHODS

This was a randomized, open-label, parallel-group, controlled, prospective study. Back and knee basal pain was recorded using a visual analogue scale (VAS) at each control and exercise-induced pain was recorded using a visual rating system (VRS). Changes in QoL were evaluated using the SF-36 questionnaire.

RESULTS

In patients treated with OHC, mean VAS and VRS pain scores decreased significantly after 5 and 6 months of treatment, indicating a significant analgesic effect. In the CC group, however, changes were minor and non-significant. Two-way analysis of variance using treatment group and time as independent variables revealed a significantly greater effect of OHC over CC on VAS and VRS scores. SF-36 showed significant improvement for OHC on the physical component summary score and no changes for CC. Responses to items assessing emotional and social aspects of QoL showed only a significant improvement in vitality for OHC and no significant changes for CC in any of the four dominions constituting the SF-36 mental component.

CONCLUSION

OHC has a significant analgesic effect and improves the physical component of QoL to a greater extent than CC.

Bone mineral density is not associated with musculoskeletal pain in postmenopausal Korean women aged ≥50 years

Although many studies reported improvement of back pain after osteoporosis treatment, there is insufficient evidence to determine whether osteoporosis is painful. We investigated whether bone mineral density correlated with musculoskeletal pain in postmenopausal Korean women aged ≥50 years. Data for postmenopausal women aged ≥50 years were obtained from the fifth Korea National Health and Nutrition Examination Survey database. Demographics, Kellgren-Lawrence grade, and numeric rating scale for pain in the hip and knee joints, presence of back pain, and activity level were analyzed. Only subjects with dual-energy X-ray absorptiometry scans and hip and knee radiographs were included. Those with malignancy, pain medication use, or a history of fragility fractures were excluded. After univariate analysis, multiple linear regression analysis was performed to examine the significant factors correlated with the degree of hip and knee pain. Binary logistic regression analysis was performed to identify factors significantly associated with the presence of back pain. In total, 387 women were included in the data analysis. Age (p = 0.005) was the only significant factor correlated with the intensity of hip pain, while Kellgren-Lawrence grade (p < 0.001) was the only significant factor correlated with knee pain intensity in multiple regression analysis. Binary logistic regression analysis showed that age (p = 0.002) was the only significant factor associated with the presence of back pain. Musculoskeletal pain was not affected by or associated with the bone mineral density (BMD) of the affected body part in postmenopausal Korean women aged ≥50 years after adjusting for the degree of osteoarthritis.

Comparisons on efficacy of elcatonin and limaprost alfadex in patients with lumbar spinal stenosis and concurrent osteoporosis: a preliminary study using a crossover design

STUDY DESIGN

Multicenter prospective study with a crossover design.

PURPOSE

The objective of this study is to compare the efficacy of limaprost alfadex (LP) and elcatonin (EL) for lumbar spinal stenosis (LSS) patients with concurrent osteoporosis.

OVERVIEW OF LITERATURE

It has been increasingly important to improve quality of life by establishing appropriate conservative treatments for LSS patients with concurrent osteoporosis who will presumably continue to increase due to the percentage of the aging elevations, however there is no prospective study.

METHODS

A total of 19 patients with LSS and concurrent osteoporosis were enrolled in this study. The patients were divided into two groups and compared using a crossover design. The Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ) and short-form (SF)-8 health survey scale were used for clinical evaluations.

RESULTS

There was a significant improvement of buttock-leg pain and numbness in the EL group. A significant improvement of impaired walking function was noted for the LP group according to the JOABPEQ while the rest of the items in the JOABPEQ showed no significant differences. The SF-8 health survey revealed that somatic pains and physical summary scores in the EL group and physical functioning and physical summary scores in the LP group tended to improve but not to any statistically significant extents.

CONCLUSIONS

Concomitant uses of EL may be useful in patients who do not respond satisfactorily to the treatments of LP for 6-8 weeks.

TRP Channels Involved in Spontaneous L-Glutamate Release Enhancement in the Adult Rat Spinal Substantia Gelatinosa

The spinal substantia gelatinosa (SG) plays a pivotal role in modulating nociceptive transmission through dorsal root ganglion (DRG) neurons from the periphery. TRP channels such as TRPV1 and TRPA1 channels expressed in the SG are involved in the regulation of the nociceptive transmission. On the other hand, the TRP channels located in the peripheral terminals of the DRG neurons are activated by nociceptive stimuli given to the periphery and also by plant-derived chemicals, which generates a membrane depolarization. The chemicals also activate the TRP channels in the SG. In this review, we introduce how synaptic transmissions in the SG neurons are affected by various plant-derived chemicals and suggest that the peripheral and central TRP channels may differ in property from each other.

Blockade of substance P receptor attenuates osteoporotic pain, but not bone loss, in ovariectomized mice

OBJECTIVE

The aim of this study was to investigate the effect of a substance P (SP) receptor (NK1 receptor [NK1-R]) antagonist on hyperalgesia and bone metabolism in ovariectomized mice.

METHODS

Thirty-six 9-week-old mice were subjected to either bilateral ovariectomy or sham surgery. Three weeks after the operation, the mice were treated with either a single-dose injection or 2-week repeated daily administration of L-703606, an NK1-R antagonist. Behavioral tests were performed for pain assessment; tibiae and the third lumbar vertebrae were dissected and assessed for microarchitectural or biomechanical properties. The expressions of SP and NK1-R in the dorsal root ganglia and spinal cord were also evaluated.

RESULTS

Both single-dose injection and 2-week repeated injections of L-703606 led to a significant increase in nociceptive threshold in ovariectomized mice. However, the antihyperalgesic effect faded at 2 hours and almost disappeared at 5 hours after a single-dose injection. With the 14-day repeated treatment of ovariectomized mice, the effect was not detectable at 24 hours after the first injection but was obvious at 24 hours after 1-week and 2-week administrations and still existed at 48 hours after the last injection. Ovariectomized mice at the hyperalgesic state had enhanced SP immunoreactivity in the dorsal root ganglia and up-regulated SP and NK1-R expressions in the spinal cord. However, no significant change in serum SP level was detected. Two-week treatment with L-703606 could down-regulate these expressions but failed to salvage the deteriorated trabecular microstructure and reduced compressive strength in ovariectomized mice.

CONCLUSIONS

Estrogen deficiency-induced hyperalgesia is achieved through up-regulation of SP and NK1-R expressions. Blockade of SP receptor can alleviate pain but cannot ameliorate bone loss. NK1-R antagonist is not recommended for the treatment of estrogen deficiency osteoporosis.

Synaptic modulation and inward current produced by oxytocin in substantia gelatinosa neurons of adult rat spinal cord slices

Cellular mechanisms for antinociception produced by oxytocin in the spinal dorsal horn have not yet been investigated thoroughly. We examined how oxytocin affects synaptic transmission in substantia gelatinosa neurons, which play a pivotal role in regulating nociceptive transmission, by applying the whole-cell patch-clamp technique to the substantia gelatinosa neurons of adult rat spinal cord slices. Bath-applied oxytocin did not affect glutamatergic spontaneous, monosynaptically-evoked primary-afferent Aδ-fiber and C-fiber excitatory transmissions. On the other hand, oxytocin produced an inward current at −70 mV and enhanced GABAergic and glycinergic spontaneous inhibitory transmissions. These activities were repeated with a slow recovery from desensitization, concentration-dependent and mimicked by oxytocin-receptor agonist. The oxytocin current was inhibited by oxytocin-receptor antagonist, intracellular GDPβS, U-73122, 2-aminoethoxydiphenyl borate, but not dantrolene, chelerythrine, dibutyryl cyclic-AMP, CNQX, Ca2+-free and tetrodotoxin, while the spontaneous inhibitory transmission enhancements were depressed by tetrodotoxin. Current-voltage relation for the oxytocin current reversed at negative potentials more than the equilibrium potential for K+, or around 0 mV. The oxytocin current was depressed in high-K+, low-Na+ or Ba2+-containing solution. Vasopressin V1A-receptor antagonist inhibited the oxytocin current, but there was no correlation in amplitude between a vasopressin-receptor agonist [Arg8]vasopressin and oxytocin responses. It is concluded that oxytocin produces a membrane depolarization mediated by oxytocin but not vasopressin-V1A receptors, which increases neuronal activity, resulting in the enhancement of inhibitory transmission, a possible mechanism for antinociception. This depolarization is due to a change in membrane permeabilities to K+ and/or Na+, which is possibly mediated by phospholipase C and inositol 1,4,5-triphosphate-induced Ca2+-release.

Antinociceptive activity of methanolic extract of Muntingia calabura leaves: further elucidation of the possible mechanisms

Background

Muntingia calabura (Elaecoparceae) is a medicinal plant traditionally used, particularly, by the Peruvian people to alleviate headache and cold, pain associated with gastric ulcers or to reduce the prostate gland swelling. Following the recent establishment of antinociceptive activity of M. calabura leaf, the present study was performed to further elucidate on the possible mechanisms of antinociception involved.

Methods

The methanol extract of M. calabura (MEMC) was prepared in the doses of 100, 250 and 500 mg/kg. The role of bradykinin, protein kinase C, pottasium channels, and various opioid and non-opioid receptors in modulating the extract’s antinociceptive activity was determined using several antinociceptive assays. Results are presented as Mean ± standard error of mean (SEM). The one-way ANOVA test with Dunnett's multiple comparison was used to analyze and compare the data, with P < 0.05 as the limit of significance.

Results

The MEMC, at all doses, demonstrated a significant (p < 0.05) dose-dependent antinociceptive activity in both the bradykinin- and phorbol 12-myristate 13-acetate (PMA)-induced nociception. Pretreatment of the 500 mg/kg MEMC with 10 mg/kg glibenclamide (an ATP-sensitive K⁺ channel inhibitor), the antagonist of μ-, δ- and κ-opioid receptors (namely 10 mg/kg β-funaltrexamine, 1 mg/kg naltrindole and 1 mg/kg nor-binaltorphimine), and the non-opioid receptor antagonists (namely 3 mg/kg caffeine (a non-selective adenosinergic receptor antagonist), 0.15 mg/kg yohimbine (an α₂-noradrenergic antagonist), and 1 mg/kg pindolol (a β-adrenoceptor antagonist)) significantly (p < 0.05) reversed the MEMC antinociception. However, 10 mg/kg atropine (a non-selective cholinergic receptor antagonist), 0.15 mg/kg prazosin (an α₁-noradrenergic antagonist) and 20 mg/kg haloperidol (a non-selective dopaminergic antagonist) did not affect the extract's antinociception. The phytochemicals screening revealed the presence of saponins, flavonoids, tannins and triterpenes while the HPLC analysis showed the presence of flavonoid-based compounds.

Conclusions

The antinociceptive activity of MEMC involved activation of the non-selective opioid (particularly the μ-, δ- and κ-opioid) and non-opioid (particularly adenosinergic, α₂-noradrenergic, and β-adrenergic) receptors, modulation of the ATP-sensitive K⁺ channel, and inhibition of bradikinin and protein kinase C actions. The discrepancies in MEMC antinociception could be due to the presence of various phytochemicals.

Vertebral compression exacerbates osteoporotic pain in an ovariectomy-induced osteoporosis rat model

STUDY DESIGN

Basic pain study using osteoporotic rodent models.

OBJECTIVE

To examine alterations in distribution of pain-related neuropeptides after compressive force on osteoporotic vertebrae and their chronic pain-related properties.

SUMMARY OF BACKGROUND DATA

We previously reported significantly increased production of calcitonin gene-related peptide (CGRP), a marker of inflammatory pain, in the dorsal root ganglia (DRG) of vertebrae in osteoporosis-model ovariectomized (OVX) rats. Here, we hypothesized that longitudinal compressive force on vertebrae can affect osteoporotic pain properties, which has not been examined yet.

METHODS

OVX rats were used as the osteoporosis model. Female Sprague-Dawley rats were prepared and Fluoro-Gold (FG) neurotracer was applied to the periosteal surface of the Co5 vertebra. After FG labeling, the animals were divided into 4 groups: Control, Control + compression, OVX, and OVX + compression. The Control groups were not ovariectomized. In the compression groups, K-wires were stabbed transversely through Co4 and Co6 with Co5 compressed longitudinally by rubber bands bridged between the 2. One, 2, 4, and 8 weeks after surgery, bilateral S1 to S3 DRGs were excised for immunofluorescence assays. Expression of CGRP and activating transcription factor 3, a marker of neuronal injury, were compared among the 4 groups.

RESULTS

Sustained upregulation of CGRP in DRG neurons was observed after compression of the Co5 vertebra, and Co5 compression caused significant increase in CGRP production in DRG neurons, whereas a greater level of activating transcription factor 3 upregulation was observed in DRGs in OVX rats after dynamic vertebral compression 8 weeks after surgery, implying potential neuropathic pain.

CONCLUSION

There was sustained upregulation of CGRP and activating transcription factor 3 in DRGs in osteoporotic model rats compared with controls, and levels were further enhanced by dynamic vertebral compression. These findings imply that dynamic compression stress on vertebrae can exacerbate osteoporotic pain by inducing both inflammatory and neuropathic pain mediators.

LEVEL OF EVIDENCE

N/A.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Inhalation therapy of calcitonin relieves osteoarthritis of the knee

This study was conducted to determine if nasal salmon calcitonin has additional beneficial effects on clinical symptoms, serum NO, IL-1β, matrix metalloproteinase 3, urinary C-terminal telopeptide type II collagen (CTX-II) levels and MRI findings in knee osteoarthritis (OA) when used concomitantly with exercise therapy. Fifty female patients with knee OA were randomized into two groups. The first group (n = 30) received 200 IU/day nasal salmon calcitonin and a home exercise program; the second group (n = 20) received a home exercise program for 6 months. Compared with baseline,while significant improvements were observed in visual analogue scale (VAS), WOMAC pain, physical function scores, 20-m walking time (P < 0.001) and WOMAC stiffness score (P = 0.041) in the first group, walking and resting VAS, and WOMAC physical function scores were improved (P = 0.029) in the second group after treatment. Significantly increased levels of serum NO and urinary CTX-II (P < 0.001) and significant improvements in the area of medial femoral condyle (P < 0.05) were noted only in the first group. There were significant differences in VAS activation values (P = 0.032) and NO levels (P < 0.001) in the favor of the first group. In conclusion, nasal salmon calcitonin may have possible chondroprotective effects besides its known effects on symptoms in patients with knee OA.

Effect of synthetic eel calcitonin, elcatonin, on cold and mechanical allodynia induced by oxaliplatin and paclitaxel in rats

Oxaliplatin and paclitaxel are commonly used anti-cancer drugs, but they frequently cause peripheral neuropathic pain. In this study, we investigated the effect of elcatonin, a synthetic eel calcitonin, on oxaliplatin- and paclitaxel-induced neuropathy in rats. The rats were treated with a single dose of oxaliplatin (6 mg/kg, i.p.) or repeated doses of paclitaxel (2 mg/kg, i.p.) on 4 alternate days. Both treatments resulted in cold and mechanical allodynia. We assessed the anti-allodynic effects of subcutaneously administered elcatonin (20 U/kg/day) by using a newly developed method to provide cold stimulation (8°C) directly to the hind paw of the rats and by using the von Frey test. Elcatonin almost completely reversed the effects of both cold and mechanical allodynia. To determine the mechanism of this anti-allodynic effect, we examined the effect of elcatonin on neuropathy induced by intraplantar injection of two organic compounds: allyl isothiocyanate (1 nmol/paw), which activates transient receptor potential ankyrin-1 channels, and menthol (1.28 μmol/paw), which activates transient receptor potential ankyrin-1 and melastatin-8. Pre-administration of elcatonin almost completely prevented cold and mechanical allodynia from being induced by both compounds. These results suggest that elcatonin attenuates oxaliplatin- and paclitaxel-induced neuropathic pain by inhibiting the cellular signaling related to transient receptor potential ankyrin-1 and melastatin-8. Thus, we conclude that administration of elcatonin may improve the quality of life of cancer patients receiving chemotherapy.

Identification of 5-HT receptor subtypes enhancing inhibitory transmission in the rat spinal dorsal horn in vitro

Background

5-hydroxytryptamine (5-HT) is one of the major neurotransmitters widely distributed in the CNS. Several 5-HT receptor subtypes have been identified in the spinal dorsal horn which act on both pre- and postsynaptic sites of excitatory and inhibitory neurons. However, the receptor subtypes and sites of actions as well as underlying mechanism are not clarified rigorously. Several electrophysiological studies have been performed to investigate the effects of 5-HT on excitatory transmission in substantia gelatinosa (SG) of the spinal cord. In the present study, to understand the effects of 5-HT on the inhibitory synaptic transmission and to identify receptor subtypes, the blind whole cell recordings were performed from SG neurons of rat spinal cord slices.

Results

Bath applied 5-HT (50 μM) increased the frequency but not amplitudes of spontaneous inhibitory postsynaptic currents (sIPSCs) in 58% of neurons, and both amplitude and frequency in 23% of neurons. The frequencies of GABAergic and glycinergic mIPSCs were both enhanced. TTX (0.5 μM) had no effect on the increasing frequency, while the enhancement of amplitude of IPSCs was eliminated. Evoked-IPSCs (eIPSCs) induced by focal stimulation near the recording neurons in the presence of CNQX and APV were enhanced in amplitude by 5-HT. In the presence of Ba²⁺ (1 mM), a potassium channel blocker, 5-HT had no effect on both frequency and amplitude. A 5-HT_2A receptor agonist, TCB-2 mimicked the 5-HT effect, and ketanserin, an antagonist of 5-HT_2A receptor, inhibited the effect of 5-HT partially and TCB-2 almost completely. A 5-HT_2C receptor agonist WAY 161503 mimicked the 5-HT effect and this effect was blocked by a 5-HT_2C receptor antagonist, N-desmethylclozapine. The amplitudes of sIPSCs were unaffected by 5-HT_2A or 5-HT_2C agonists. A 5-HT₃ receptor agonist mCPBG enhanced both amplitude and frequency of sIPSCs. This effect was blocked by a 5-HT₃ receptor antagonist ICS-205,930. The perfusion of 5-HT_2B receptor agonist had no effect on sIPSCs.

Conclusions

Our results demonstrated that 5-HT modulated the inhibitory transmission in SG by the activation of 5-HT_2A and 5-HT_2C receptors subtypes located predominantly at inhibitory interneuron terminals, and 5-HT₃ receptors located at inhibitory interneuron terminals and soma-dendrites, consequently enhanced both frequency and amplitude of IPSCs.

Role of 5-HT(1) receptor subtypes in the modulation of pain and synaptic transmission in rat spinal superficial dorsal horn

BACKGROUND AND PURPOSE

5-HT receptor agonists have variable nociceptive effects within the spinal cord. While there is some evidence for 5-HT(1A) spinally-mediated analgesia, the role of other 5-HT(1) receptor subtypes remains unclear. In the present study, we examined the spinal actions of a range of 5-HT(1) agonists, including sumatriptan, on acute pain, plus their effect on afferent-evoked synaptic transmission onto superficial dorsal horn neurons.

EXPERIMENTAL APPROACH

For in vivo experiments, 5-HT agonists were injected via chronically implanted spinal catheters to examine their effects in acute mechanical and thermal pain assays using a paw pressure analgesymeter and a Hargreave's device. For in vitro experiments, whole-cell patch-clamp recordings of primary afferent-evoked glutamatergic EPSC were made from lamina II neurons in rat lumbar spinal slices.

KEY RESULTS

Intrathecal (i.t.) delivery of the 5-HT(1A) agonist R ± 8-OH-DPAT (30-300 nmol) produced a dose-dependent thermal, but not mechanical, analgesia. Sumatriptan and the 5-HT(1B), 5-HT(1D), 5-HT(1F) agonists CP93129, PNU109291 and LY344864 (100 nmol) had no effect on either acute pain assay. R ± 8-OH-DPAT (1 µM) and sumatriptan (3 µM) both reduced the amplitude of the evoked EPSC. In contrast, CP93129, PNU109291 and LY344864 (0.3-3 µM) had no effect on the evoked EPSC. The actions of both R ± 8-OH-DPAT and sumatriptan were abolished by the 5-HT(1A) antagonist WAY100635 (3 µM).

CONCLUSIONS AND IMPLICATIONS

These findings indicate that the 5-HT(1A) receptor subtype predominantly mediates the acute antinociceptive and cellular actions of 5-HT(1) ligands within the rat superficial dorsal horn.

Milnacipran inhibits glutamatergic N-methyl-D-aspartate receptor activity in spinal dorsal horn neurons

Background

Antidepressants, which are widely used for treatment of chronic pain, are thought to have antinociceptive effects by blockade of serotonin and noradrenaline reuptake. However, these drugs also interact with various receptors such as excitatory glutamatergic receptors. Thermal hyperalgesia was induced by intrathecal injection of NMDA in rats. Paw withdrawal latency was measured after intrathecal injection of antidepressants. The effects of antidepressants on the NMDA and AMPA-induced responses were examined in lamina II neurons of rat spinal cord slices using the whole-cell patch-clamp technique. The effects of milnacipran followed by application of NMDA on pERK activation were also investigated in the spinal cord.

Results

Intrathecal injection of milnacipran (0.1 μmol), but not citalopram (0.1 μmol) and desipramine (0.1 μmol), followed by intrathecal injection of NMDA (1 μg) suppressed thermal hyperalgesia. Milnacipran (100 μM) reduced the amplitude of NMDA (56 ± 3 %, 64 ± 5 % of control)-, but not AMPA (98 ± 5 %, 97 ± 5 % of control)-mediated currents induced by exogenous application and dorsal root stimulation, respectively. Citalopram (100 μM) and desipramine (30 μM) had no effect on the amplitude of exogenous NMDA-induced currents. The number of pERK-positive neurons in the group treated with milnacipran (100 μM), but not citalopram (100 μM) or desipramine (30 μM), followed by NMDA (100 μM) was significantly lower compared with the NMDA-alone group.

Conclusions

The antinociceptive effect of milnacipran may be dependent on the drug’s direct modulation of NMDA receptors in the superficial dorsal horn. Furthermore, in addition to inhibiting the reuptake of monoamines, glutamate NMDA receptors are also important for analgesia induced by milnacipran.

Anti-hyperalgesic effects of calcitonin on neuropathic pain interacting with its peripheral receptors

Background

The polypeptide hormone calcitonin is clinically well known for its ability to relieve neuropathic pain such as spinal canal stenosis, diabetic neuropathy and complex regional pain syndrome. Mechanisms for its analgesic effect, however, remain unclear. Here we investigated the mechanism of anti-hyperalgesic action of calcitonin in a neuropathic pain model in rats.

Results

Subcutaneous injection of elcatonin, a synthetic derivative of eel calcitonin, relieved hyperalgesia induced by chronic constriction injury (CCI). Real-time reverse transcriptase-polymerase chain reaction analysis revealed that the CCI provoked the upregulation of tetrodotoxin (TTX)-sensitive Nav.1.3 mRNA and downregulation of TTX-resistant Nav1.8 and Nav1.9 mRNA on the ipsilateral dorsal root ganglion (DRG), which would consequently increase the excitability of peripheral nerves. These changes were reversed by elcatonin. In addition, the gene expression of the calcitonin receptor and binding site of ¹²⁵I-calcitonin was increased at the constricted peripheral nerve tissue but not at the DRG. The anti-hyperalgesic effect and normalization of sodium channel mRNA by elcatonin was parallel to the change of the calcitonin receptor expression. Elcatonin, however, did not affect the sensitivity of nociception or gene expression of sodium channel, while it suppressed calcitonin receptor mRNA under normal conditions.

Conclusions

These results suggest that the anti-hyperalgesic action of calcitonin on CCI rats could be attributable to the normalization of the sodium channel expression, which might be exerted by an unknown signal produced at the peripheral nerve tissue but not by DRG neurons through the activation of the calcitonin receptor. Calcitonin signals were silent in the normal condition and nerve injury may be one of triggers for conversion of a silent to an active signal.

Improvement of Quality of Life (QOL) in Osteoporotic Patients by Elcatonin Treatment: A Trial Taking the Participants' Preference into Account

Osteoporosis is associated with compromised quality of life (QOL), to which pain has the most important contribution. Elcatonin, a derivative of calcitonin, is widely used in the treatment of osteoporosis in two ways. One is as the inhibitor of osteoclastic bone resorption. The other is for osteoporosis-related pain based on the unique analgesic effects of elcatonin. Since pain is subjective in nature, and QOL is the only clinical outcome representing the patients’ subjective perception of health status, pain associated with osteoporosis would be best evaluated based on QOL assessment. Evidence based medicine gives the highest remarks to the double-blinded, randomized controlled trial, which, however, cannot be free from methodological problems on some occasions. For example, it is practically impossible to remain blinded in the trial of a potent analgesia, which in turn causes biases. Thus, the significance of taking the patients’ preference into account is increasingly acknowledged. In this study, 45 osteoporotic patients were given brochures describing the pros and cons on the three treatment choices; calcium and alfacalcidol, additional use of elcatonin, and additional use of bisphosphonate. Those who favored elcatonin were older, had more vertebral fractures, and lower QOL scores. QOL was evaluated before and three months after the treatment using SF-8; the most widely used generic questionnaire, and RDQ; a lumbago-specific measure. Elcatonin treatment improved physical function, general health, and vitality of SF-8, and RDQ score. Although this is a preliminary study, our results suggest that patients with vertebral fracture(s) have impaired QOL and more likely to favor elcatonin treatment expecting analgesia.

Comparison of the effects of elcatonin and risedronate on back and knee pain by electroalgometry using fall of skin impedance and quality of life assessment using SF-36

Back and knee pain is a widespread health problem and a serious threat to the quality of life (QOL) in middle-aged and older adults, as it frequently accompanies osteoporosis and osteoarthritis. In order to compare the effects of elcatonin and risedronate on such pain, 20 units of elcatonin was intramuscularly injected to 18 patients, and 5 mg of risedronate was orally administered daily to 20 others with similar backgrounds. Exercise-induced pain was analyzed by measuring the fall of skin impedance by electroalgometry (EAM), and subjective pain was recorded by a visual rating system (VRS) on a scale of 0 (no pain) to 100 (unbearable pain). In patients treated with elcatonin, the mean EAM-estimated pain was significantly reduced after 4, 5 and 6 months of treatment, and the VRS score after 3, 5 and 6 months, indicating a significant analgesic effect. In the risedronate group, however, improvement was less remarkable. Two-way analysis of variance using pain as a dependent variable and treatment group and time as independent variables revealed a significantly greater effect of elcatonin over risedronate on both the EAM and VRS scores, and the influence of treatment time on pain was indistinguishable between the two treatment groups. Effect of exercise load on pain was less on knee load than knee and spine load and spine load, but indistinguishable between the two groups. Changes in QOL were evaluated by the SF-36 system. Norm-based scoring showed significant improvements in 3 of 4 categories for elcatonin and in 2 of 4 for risedronate, suggesting comparable effects on the physical aspects of QOL, whereas responses to emotionally and socially directed questions indicated significant improvements in all 4 categories for risedronate, but none for elcatonin, suggesting a more physical than emotional component in elcatonin effects compared to risedronate.

Blocking TNF-α with infliximab alleviates ovariectomy induced mechanical and thermal hyperalgesia in rats

Studies have proved an increased expression of tumor necrosis factor alpha (TNF-α) in estrogen deficiency animals, and TNF-α also plays a role in inflammation and neuropathic pain. This study aimed to explore the relationship between TNF-α and ovariectomy induced hyperalgesia. 36 female Sparague-Dawley were included, estrogen depletion models were established by ovariectomy. Then infliximab (a TNF-α blocker) was administrated to the ovariectomized rats for 8 weeks. Pain behavioral tests were performed once a week. The bone mineral density (BMD), serum estradiol and TNF-α level were determined at the 8th week after ovariectomy. The expression of TNF-α in lumbar 5 dorsal root ganglions (L5 DRGs) was examined by immunofluorescence method. Significant hyperalgesia to mechanical and thermal stimuli in groups Ovx-1 and Ovx-2 was observed 1 week after the operation. After treated with infliximab, the pain threshold of Ovx-2 was partially restored, although still lower than the Sham group. The serum TNF-α level of Ovx-1 was significantly higher than Sham and Ovx-2. TNF-α immunofluorescence indicated a significant increase in the expression of TNF-α at L5 DRGs in group Ovx-1 when compared with groups Sham and Ovx-2. The BMD of group Ovx-2 was significantly higher than group Ovx-1 and lower than group Sham. In conclusion, TNF-α plays an important role in estrogen deficiency induced mechanical and thermal hyperalgesia, and DRG may be one site on which TNF-α acts to cause hyperalgesia. Blocking the effect of TNF-α could partially alleviate the estrogen deficiency induced hyperalgesia in rats. Thus, TNF-α may contribute to chronic pain in postmenopausal women.

Synaptic connections of the neurokinin 1 receptor-like immunoreactive neurons in the rat medullary dorsal horn

The synaptic connections between neurokinin 1 (NK1) receptor-like immunoreactive (LI) neurons and γ-aminobutyric acid (GABA)-, glycine (Gly)-, serotonin (5-HT)- or dopamine-β-hydroxylase (DBH, a specific marker for norepinephrinergic neuronal structures)-LI axon terminals in the rat medullary dorsal horn (MDH) were examined under electron microscope by using a pre-embedding immunohistochemical double-staining technique. NK1 receptor-LI neurons were observed principally in laminae I and III, only a few of them were found in lamina II of the MDH. GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were densely encountered in laminae I and II, and sparsely in lamina III of the MDH. Some of these GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were observed to make principally symmetric synapses with NK1 receptor-LI neuronal cell bodies and dendritic processes in laminae I, II and III of the MDH. The present results suggest that neurons expressing NK1 receptor within the MDH might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons located in the MDH and 5-HT- or norepinephrine (NE)-containing descending fibers originated from structures in the brainstem.