Interactions of descending serotonergic systems with other neurotransmitters in the modulation of nociception

Tension-type headache

Tension-type headache (TTH) is the most prevalent neurological disorder worldwide and is characterized by recurrent headaches of mild to moderate intensity, bilateral location, pressing or tightening quality, and no aggravation by routine physical activity. Diagnosis is based on headache history and the exclusion of alternative diagnoses, with clinical criteria provided by the International Classification of Headache Disorders, third edition. Although the biological underpinnings remain unresolved, it seems likely that peripheral mechanisms are responsible for the genesis of pain in TTH, whereas central sensitization may be involved in transformation from episodic to chronic TTH. Pharmacological therapy is the mainstay of clinical management and can be divided into acute and preventive treatments. Simple analgesics have evidence-based effectiveness and are widely regarded as first-line medications for the acute treatment of TTH. Preventive treatment should be considered in individuals with frequent episodic and chronic TTH, and if simple analgesics are ineffective, poorly tolerated or contraindicated. Recommended preventive treatments include amitriptyline, venlafaxine and mirtazapine, as well as some selected non-pharmacological therapies. Despite the widespread prevalence and associated disability of TTH, little progress has been made since the early 2000s owing to a lack of attention and resource allocation by scientists, funding bodies and the pharmaceutical industry.

The Mediating Effect of Sleep Disturbance on the Relationship Between Nonmalignant Chronic Pain and Suicide Death

IMPORTANCE

Few studies have examined the relationship between nonmalignant chronic pain (NMCP) and suicide death, and even fewer have specifically explored what role sleep disturbance might play in the association between NMCP and suicide death.

OBJECTIVE

To assess whether sleep disturbance mediates the relationship between NMCP and suicide death.

DESIGN

This case-control study included 2,674 individuals who died by suicide between 2000 and 2013 (cases) and 267,400 matched individuals (controls).

SETTING

Eight Mental Health Research Network (MHRN)-affiliated healthcare systems.

PARTICIPANTS

All cases and matched controls were health plan members for at least 10 months during the year prior to the index date.

MAIN OUTCOMES AND MEASURES

Sociodemographic data and diagnosis codes for NMCP and sleep disorders were extracted from the MHRN's Virtual Data Warehouse. Suicide mortality was identified using International Statistical Classification of Diseases and Related Health Problems (ICD)-10 codes from official government mortality records matched to health system records.

RESULTS

After accounting for covariates, there was a significant relationship between NMCP and sleep disturbance; those who were diagnosed with NMCP were more likely to develop subsequent sleep disturbance. Similarly, sleep disturbance was significantly associated with suicide death. Finally, a significant indirect effect of NMCP on suicide death, through sleep disturbance, and a nonsignificant direct effect of NMCP on suicide death provide support for a fully mediated model.

CONCLUSIONS AND RELEVANCE

There is a need for clinicians to screen for both sleep disturbance and suicidal ideation in NMCP patients and for health systems to implement more widespread behavioral treatments that address comorbid sleep problems and NMCP.

5-HT3 receptor signaling in serotonin transporter-knockout rats: a female sex-specific animal model of visceral hypersensitivity

The irritable bowel syndrome (IBS) is a functional gastrointestinal motor and visceral sensation disorder that is more common in women than men. Female serotonin transporter (SERT)-gene knockout (KO) rats exhibit hypersensitivity to colorectal balloon distention (CRD) that mimics colonic hypersensitivity occurring in female IBS patients. Alosetron (5-HT₃ receptor antagonist) is used to treat diarrhea-predominant IBS in female patients. Other 5-HT₃ receptor antagonists are ineffective at treating IBS symptoms. The visceromotor response (VMR) to CRD in SERT-KO and wild-type (WT) rats was measured following subcutaneous (sc), intracerobroventricular (icv), or intrathecal (it) treatment with 5-HT₃ receptor antagonists and an agonist. Alosetron (sc) and granisetron (antagonists) caused a paradoxical increase in the VMR to CRD in SERT-KO female rats. Alosetron (sc) increased the VMR to CRD in WT male rats. Alosetron (it) increased the VMR to CRD in SERT-KO female rats only, and the 5-HT₃ receptor agonist SR-52772 increased the VMR to CRD in SERT-KO male rats. Depletion of spinal 5-HT using 5,7-dihydroxytryptamine prevented the increase in VMR to CRD in SERT-KO female and male rats treated it with alosetron and SR-52772, respectively. Alosetron (icv) did not affect the VMR to CRD in WT or KO female rats, but it increased the VMR in male SERT-KO but not WT male rats. These data suggest that 5-HT₃ receptor signaling at the dorsal spinal cord mediates visceral hypersensitivity in female SERT-KO rats. Such differences could facilitate development of sex-specific drug treatments for visceral pain. NEW & NOTEWORTHY We studied a model of female sex-specific visceral hypersensitivity using rats that had a loss of function of the serotonin transporter (SERT) caused by gene truncation. Female SERT-KO rats exhibited visceral hypersensitivity in response to colorectal balloon distention. We found that increased 5-HT signaling at dorsal spine 5-HT₃ receptors was responsible for visceral hypersensitivity in female but not male SERT-KO rats.

Cervical spine disorders and its association with tinnitus: The "triple" hypothesis

Subjective tinnitus and cervical spine disorders (CSD) are among the most common complaints encountered by physicians. Although the relationship between tinnitus and CSD has attracted great interest during the past several years, the pathogenesis of tinnitus induced by CSD remains unclear. Conceivably, CSD could trigger a somatosensory pathway-induced disinhibition of dorsal cochlear nucleus (DCN) activity in the auditory pathway; furthermore, CSD can cause inner ear blood impairment induced by vertebral arteries hemodynamic alterations and trigeminal irritation. In genetically -predisposed CSD patients with reduced serotoninergic tone, signals from chronically stimulated DCNs could activate specific cortical neuronal networks and plastic neural changes resulting in tinnitus. Therefore, an early specific tailored CSD treatments and/or boosting serotoninergic activity may be required to prevent the creation of 'tinnitus memory circuits' in CSD patients.

Spinal 5-HT3 receptors mediate descending facilitation and contribute to behavioral hypersensitivity via a reciprocal neuron-glial signaling cascade

BACKGROUND

It has been recently recognized that the descending serotonin (5-HT) system from the rostral ventromedial medulla (RVM) in the brainstem and the 5-HT3 receptor subtype in the spinal dorsal horn are involved in enhanced descending pain facilitation after tissue and nerve injury. However, the mechanisms underlying the activation of the 5-HT3 receptor and its contribution to facilitation of pain remain unclear.

RESULTS

In the present study, activation of spinal 5-HT3 receptors by intrathecal injection of a selective 5-HT3 receptor agonist SR 57227 induced spinal glial hyperactivity, neuronal hyperexcitability and pain hypersensitivity in rats. We found that there was neuron-to-microglia signaling via the chemokine fractalkine, microglia to astrocyte signaling via cytokine IL-18, astrocyte to neuronal signaling by IL-1β, and enhanced activation of NMDA receptors in the spinal dorsal horn. Glial hyperactivation in spinal dorsal horn after hindpaw inflammation was also attenuated by molecular depletion of the descending 5-HT system by intra-RVM Tph-2 shRNA interference.

CONCLUSIONS

These findings offer new insights into the cellular and molecular mechanisms at the spinal level responsible for descending 5-HT-mediated pain facilitation during the development of persistent pain after tissue and nerve injury. New pain therapies should focus on prime targets of descending facilitation-induced glial involvement, and in particular the blocking of intercellular signaling transduction between neurons and glia.

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.

Mechanisms of the antinociceptive action of (-) epicatechin obtained from the hydroalcoholic fraction of Combretum leprosum Mart & Eic in rodents

BACKGROUND

The mechanisms of the antinociceptive activity of (-) epicatechin (EPI), a compound isolated from the hydroalcoholic fraction of Combreum leprosum Mart & Eicher.

METHODS

were assessed in the model of chemical nociception induced by glutamate (20 μmol/paw). To evaluate the mechanisms involved, the animals , male Swiss mice (25-30 g), received EPI (50 mg/kg p.o.) after pretreatment with naloxone (2 mg/kg s.c. opioid antagonist), glibenclamide (2 mg/kg s.c. antagonist K + channels sensitive to ATP), ketanserin (0.3 mg/kg s.c. antagonist of receptor 5-HT(2A)), yoimbine (0.15 mg/kg s.c. α2 adrenergic receptor antagonist), pindolol (1 mg/kg s.c. 5-HT1(a)/1(b) receptor antagonist), atropine (0.1 mg/kg s.c. muscarinic antagonist) and caffeine (3 mg/kg s.c. adenosine receptor antagonist), ondansetron (0.5 mg/kg s.c. for 5-HT(3) receptor) and L-arginine (600 mg/kg i.p.).

RESULTS

The antinociceptive effect of EPI was reversed by pretreatment with naloxone and glibenclamide, ketanserin, yoimbine, atropine and pindolol, which demonstrates the involvement of opioid receptors and potassium channels sensitive to ATP, the serotoninergic (receptor 5HT(1A) and 5HT(2A)), adrenergic (receptor alpha 2) and cholinergic (muscarinic receptor) systems in the activities that were observed. The effects of EPI, however, were not reversed by pretreatment with caffeine, L-arginine or ondansetron, which shows that there is no involvement of 5HT(3) receptors or the purinergic and nitrergic systems in the antinociceptive effect of EPI. In the Open Field and Rotarod test, EPI had no significant effect, which shows that there was no central nervous system depressant or muscle relaxant effect on the results.

CONCLUSIONS

This study demonstrates that the antinociceptive activity of EPI in the glutamate model involves the participation of the opioid system, serotonin, adrenergic and cholinergic.

An increase in spinal cord noradrenaline is a major contributor to the antihyperalgesic effect of antidepressants after peripheral nerve injury in the rat

Antidepressants are often used for the treatment of neuropathic pain. Clinical studies suggest that the efficacy of serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs) for neuropathic pain is greater than that of selective 5-HT reuptake inhibitors (SSRIs). In the present study, we determined the efficacy and mechanisms involved in the antihyperalgesic effects of milnacipran, an SNRI, compared with paroxetine, an SSRI, and maprotiline, a selective NA reuptake inhibitor, using a rat model of neuropathic pain. Male Sprague-Dawley rats underwent spinal nerve ligation (SNL), and the withdrawal threshold to paw pressure was measured. Intraperitoneal injection of milnacipran (3-30mg/kg) produced a dose-dependent antihyperalgesic effect. The effect was reversed by intrathecal injection of the α(2)-adrenoceptor antagonist idazoxan (30μg), but not by various 5-HT receptor antagonists. Paroxetine produced an antihyperalgesic effect only at the highest dose tested (10mg/kg). This effect was reversed by intrathecal injection of both idazoxan and ondansetron (30μg), a 5-HT3 receptor antagonist. Maprotiline produced an antihyperalgesic effect (10 and 30mg/kg), and the effect was reversed by intrathecal idazoxan. In microdialysis studies, NA and 5-HT concentrations in the spinal dorsal horn were increased after injection of either milnacipran or paroxetine, and only NA was increased after maprotiline. Furthermore, the NA content in the spinal cord of SNL rats was greater than that in normal animals. These findings suggest that an increase in NA in the spinal cord plays an important role in the antihyperalgesic effects of not only NA reuptake inhibitors but also SSRIs.

Spinal 5-HT(3) receptor activation induces behavioral hypersensitivity via a neuronal-glial-neuronal signaling cascade

Recent studies indicate that the descending serotonin (5-HT) system from the rostral ventromedial medulla (RVM) in the brainstem and the 5-HT(3) receptor subtype in the spinal dorsal horn are involved in enhanced descending pain facilitation after tissue and nerve injury. However, the mechanisms underlying the activation of the 5-HT(3) receptor and its contribution to facilitation of pain remain unclear. In the present study, activation of spinal 5-HT(3) receptor by intrathecal injection of a selective 5-HT(3) receptor agonist, SR57227, induced spinal glial hyperactivity, neuronal hyperexcitability, and pain hypersensitivity in rats. We found that there was neuron-to-microglia signaling via chemokine fractalkine, microglia to astrocyte signaling via the cytokine IL-18, astrocyte to neuronal signaling by IL-1β, and enhanced activation of GluN (NMDA) receptors in the spinal dorsal horn. In addition, exogenous brain-derived neurotrophic factor-induced descending pain facilitation was accompanied by upregulation of CD11b and GFAP expression in the spinal dorsal horn after microinjection in the RVM, and these events were significantly prevented by functional blockade of spinal 5-HT(3) receptors. Enhanced expression of spinal CD11b and GFAP after hindpaw inflammation was also attenuated by molecular depletion of the descending 5-HT system by intra-RVM Tph-2 shRNA interference. Thus, these findings offer new insights into the cellular and molecular mechanisms at the spinal level responsible for descending 5-HT-mediated pain facilitation during the development of persistent pain after tissue and nerve injury. New pain therapies should focus on prime targets of descending facilitation-induced glial involvement, and in particular the blocking of intercellular signaling transduction between neuron and glia.

Geranylgeraniol and 6α,7β-dihydroxyvouacapan-17β-oate methyl ester isolated from Pterodon pubescens Benth.: Further investigation on the antinociceptive mechanisms of action

The crude alcoholic extracts obtained from Pterodon pubescens Benth. seeds are widely used in Brazilian folk medicine as anti-inflammatory, analgesic, anti-rheumatic tonics and depurative preparations. We previously demonstrated the antinociceptive activity on writhing capsaicin, glutamate, and hot-plate tests of two compounds isolated from P. pubescens: geranylgeraniol (C1) and 6α,7β-dihydroxyvouacapan-17β-oate methyl ester (C2). This work is a continuation of the previous study investigating the possible mechanisms of action for compounds C1 and C2, and the differences between them. The present study demonstrated that when administered intraperitoneally (i.p.): i), compounds C1 and C2 produced significant anti-allodynic activity during the acute phase of the Complete Freund's Adjuvant (CFA)-induced persistent pain model; ii) compound C1 produced significant anti-hypernociception activity in the carrageenan-induced pain model; iii) compound C2 presented a significant loss of activity after p-chlorophenylalanine methyl ester hydrochloride (PCPA) [5-HT synthesis inhibitor] treatment, suggesting that the mechanisms of action could be related to either the synthesis or release of serotonin; iv) compound C1 presented a significant loss of activity after ondansetron (5-HT(3) receptor antagonist) treatment suggesting activity upon 5-HT(3) serotonin receptors; v) compound C1 presented a significant loss of activity after efaroxan (mixed I(1) imidazoline/α(2)-adrenoceptor antagonist) treatment suggesting the participation of this compound upon imidazoline I(1) receptors; and vi) both compounds C1 and C2 did not appear to exert their activity via 5-HT(1A), 5-HT(2A), imidazoline I(2), α(2)-adrenoceptor, nitric oxide, GABA(A), acetylcholine muscarinic, and nicotinic receptors when evaluated in acetic acid-induced nociception.

5-HT2C receptor agonists attenuate pain-related behaviour in a rat model of trigeminal neuropathic pain

Peripheral branches of the trigeminal nerve may be damaged during maxillofacial injury or surgical procedures and trigeminal trauma may induce severe pain that is very challenging to treat. Chronic constriction injury to the infraorbital nerve (ION-CCI) by loose ligatures has proven a useful model for some types of trigeminal neuropathic pain disorder. Using ION-CCI rats, we examined the antiallodynic effects of intrathecally administered agents which are selective for 5-HT2C receptors. Allodynia was evaluated by applying von Frey filaments to skin innervated by the injured ION. Dose-dependent antiallodynic effects followed administration of three 5-HT2C receptor agonists, 6-chloro-2-(1-piperazinyl)-pyrazine (MK212: 10, 30, and 100 μg); (S)-2-(chloro-5-fluoro-indol-l-yl)-1-methyamine fumarate (RO 60-0175: 10, 30, and 100 μg); (AaR)-8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one (WAY-161503: 10, 30, and 100 μg). ED50 values for antiallodynic effects of MK212, RO 60-0175, and WAY-161503 were 39.62, 46.67, and 51.22 μg, respectively. Intrathecal administration of the 5-HT2C receptor antagonist, 8-[5-2,4-dimethoxy-5-(4-trifluoromethylphenylsulphonamido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4,5]decane-2,4-dione (RS-102221: 30 μg) did not alter the mechanical threshold. Intrathecal pretreatment with RS-102221 (10 and 30 μg) reduced the antiallodynic effects of the highest dose of 5-HT2C agonists. These results indicated that, in this rat model, the 5-HT2C receptor plays a role in spinal inhibition of trigeminal neuropathic pain.

Spinal microglial motility is independent of neuronal activity and plasticity in adult mice

Microglia are the resident macrophages in the central nervous system. In the spinal cord dorsal horn, microglia stay in resting condition during physiological sensory processing, and are activated under pathological conditions such as peripheral nerve injury. In cases such as this, the nearby resting microglia increase their motility and accumulate at the site of injury. However, direct evidence to support that nerve activity can enhance the motility of microglia has not yet to be reported. In this study we investigated whether the activation of spinal microglia under in vivo nerve injury may be mimicked by neuronal activity in the spinal cord slice preparation. We found that local application of spinal excitatory neurotransmitters, such as glutamate and substance P did not cause any change in the motility of microglial cells in the spinal cord dorsal horn. The motility of microglial cells is unlikely modulated by other transmitters, neuromodulators and chemokines, because similar applications such as GABA, serotonin, noradrenaline, carbachol, fractalkine or interleukin did not produce any obvious effect. Furthermore, low or high frequency stimulation of spinal dorsal root fibers at noxious intensities failed to cause any enhanced extension or retraction of the microglia processes. By contrast, focal application of ATP triggered rapid and robust activation of microglial cells in the spinal dorsal horn. Our results provide the first evidence that the activation of microglia in the spinal cord after nerve injury is unlikely due solely to neuronal activity, non-neuronal factors are likely responsible for the activation of nerve injury-related microglial cells in the spinal dorsal horn.

Evidence for a role of 5-HT(1A) receptor on antinociceptive action from Geissospermum vellosii

ETHNOPHARMACOLOGICAL RELEVANCE

Geissospermum vellosii is a tree widely found throughout the Amazonic forest and frequently used by the native population for painful disorders.

AIM OF THE STUDY

The present study examined the antinociceptive effects of Geissospermum vellosii in behavioral models of nociception.

MATERIALS, METHODS AND RESULTS

Oral administration of crude extract of Geissospermum vellosii or its dichloromethane fraction (1-100 mg/kg) inhibited formalin-induced inflammatory nociception and acetic acid-induced visceral nociception. The antinociceptive effect of Geissospermum vellosii was unrelated with motor dysfunctions. Furthermore, the alkaloid 12-metoxy-1-methyl-aspidospermidine (0.001-1 mg/kg), isolated from the dichloromethane fraction, also produced antinociception. The antinociception caused by the dichloromethane fraction was significantly attenuated by pre-treatment of mice with p-chlorophenylalanine methyl ester (PCPA, an inhibitor of serotonin synthesis, 100 mg/kg once a day for 4 consecutive days) and WAY-100635 (a 5-HT(1A) receptor antagonist, 0.3 mg/kg). In contrast, dichloromethane fraction antinociception was not affected by pre-treatment of animals with ketanserin (a 5-HT(2) receptor antagonist, 0.3 mg/kg) or ondansetron (a 5-HT(3) receptor antagonist, 0.5 mg/kg).

CONCLUSIONS

Together, these results indicate that Geissospermum vellosii produces antinociception through an interaction with 5-HT(1A) receptors. Furthermore, the alkaloid 12-metoxy-1-methyl-aspidospermidine contributes to the antinociceptive properties reported for Geissospermum vellosii.

Further analysis of the antinociceptive action caused by p-methoxyl-diphenyl diselenide in mice

The objective of this study was to extend our previous findings by investigating in greater detail the mechanisms that might be involved in the antinociceptive action of p-methoxyl-diphenyl diselenide, (MeOPhSe)(2), in mice. The pretreatment with nitric oxide precursor, l-arginine (600 mg/kg, intraperitoneal, i.p.), reversed antinociception caused by (MeOPhSe)(2) (10 mg/kg, p.o.) or N(G)-nitro-l-arginine (l-NOARG, 75 mg/kg, i.p.) in the glutamate test. Ondansetron (0.5 mg/kg, i.p., a 5-HT(3) receptor antagonist) and SCH23390 (0.05 mg/kg, i.p.., a D(1) receptor antagonist) blocked the antinociceptive effect caused by (MeOPhSe)(2). Conversely, pindolol (1 mg/kg, i.p., a 5-HT(1A)/(1B) receptor/beta adrenoceptor antagonist), WAY 100635 (0.7 mg/kg, i.p., a selective 5-HT(1A) receptor antagonist), ketanserin (0.3 mg/kg, i.p., a selective 5-HT(2A) receptor antagonist), prazosin (0.15 mg/kg, i.p., an alpha(1)-adrenoreceptor antagonist), yohimbine (1.0 mg/kg, i.p., an alpha(2)-adrenoreceptor antagonist), sulpiride (5 mg/kg, i.p., a D(2) receptor antagonist), naloxone (1 mg/kg, i.p., a non-selective opioid receptor antagonist) and caffeine (3 mg/kg, i.p., a non-selective adenosine receptor antagonist) did not change the antinociceptive effect of (MeOPhSe)(2). (MeOPhSe)(2) significantly inhibited nociception induced by intraplantar (i.pl.) injection of bradykinin (10 nmol/paw) and Des-Arg(9)-bradykinin (10 nmol/paw, a B(1) receptor agonist). (MeOPhSe)(2) significantly inhibited phorbol myristate acetate (PMA, 0.03 mug/paw, a protein kinase C (PKC) activator)-induced licking response. These results indicate that (MeOPhSe)(2) produced antinociception in mice through mechanisms that involve an interaction with nitrergic system, 5-HT(3) and D(1) receptors. The antinociceptive effect is related to (MeOPhSe)(2) ability to interact with kinin B(1) and B(2) receptors and PKC pathway mediated mechanisms.

Antinociceptive action of ethanolic extract obtained from roots of Humirianthera ampla Miers

Humirianthera ampla Miers is a member of the Icacinaceae family and presents great amounts of di and triterpenoids. These chemical constituents in roots of Humirianthera ampla sustain not only the ethnopharmacological use against snake venom, but also some anti-inflammatory and analgesic properties of the plant. In this study we investigated the antinociceptive action of the ethanolic extract (EE) from roots of the Humirianthera ampla in chemical and thermal models of pain in mice. The oral treatment with ethanolic extract dose-dependently inhibited glutamate-, capsaicin- and formalin-induced licking. However, it did not prevent the nociception caused by radiant heat on the tail-flick test. The ethanolic extract (30 mg/kg) caused marked inhibition of the nociceptive biting response induced by glutamate, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), N-methyl-d-aspartate (NMDA) and substance P. The antinociception caused by ethanolic extract was significantly attenuated by naloxone, l-arginine, WAY100635, ondansetron or ketanserin, but not by caffeine or naloxone methiodide. In conclusion, the ethanolic extract from roots of Humirianthera ampla produces antinociception against neurogenic and inflammatory models of nociception. The mechanisms of antinociception involve nitric oxide, opioid, serotonin and glutamate pathways. Therefore, our results support the ethnopharmacological use of the Humirianthera ampla against inflammatory and painful process caused by snake venom.

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.

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.

Central serotonergic and noradrenergic receptors in functional dyspepsia

Functional dyspepsia is a symptom complex characterised by upper abdominal discomfort or pain, early satiety, motor abnormalities, abdominal bloating and nausea in the absence of organic disease. The central nervous system plays an important role in the conducting and processing of visceral signals. Alterations in brain processing of pain, perception and affective responses may be key factors in the pathogenesis of functional dyspepsia. Central serotonergic and noradrenergic receptor systems are involved in the processing of motor, sensory and secretory activities of the gastrointestinal tract. Visceral hypersensitivity is currently regarded as the mechanism responsible for both motor alterations and abdominal pain in functional dyspepsia. Some studies suggest that there are alterations in central serotonergic and noradrenergic systems which may partially explain some of the symptoms of functional dyspepsia. Alterations in the autonomic nervous system may be implicated in the motor abnormalities and increases in visceral sensitivity in these patients. Noradrenaline is the main neurotransmitter in the sympathetic nervous system and again alterations in the functioning of this system may lead to changes in motor function. Functional dyspepsia causes considerable burden on the patient and society. The pathophysiology of functional dyspepsia is not fully understood but alterations in central processing by the serotonergic and noradrenergic systems may provide plausible explanations for at least some of the symptoms and offer possible treatment targets for the future.

Possible involvement of cholinergic and opioid receptor mechanisms in fluoxetine mediated antinociception response in streptozotocin-induced diabetic mice

Clinical and experimental studies have been reported that antidepressant drugs can be used as co-analgesics in the management of neuropathic pain. However, the mechanism through which they alleviate pain still remains unclear. The aim of the present study was to investigate the possible mechanism of action of fluoxetine-induced antinociceptive effect in streptozotocin-induced diabetic mice, especially the involvement of non-serotonergic neurotransmitters and their receptors. Diabetes was induced in male Laka mice with a single intraperitoneal injection of streptozotocin (200 mg/kg). Four weeks after streptozotocin, diabetic mice were tested for pain responses in the tail-immersion and hot-plate assays. Diabetic mice exhibited significant hyperalgesia as compared with control mice. Fluoxetine (10 and 20 mg/kg, i.p) injected into diabetic mice produced an antinociceptive effect in both tail-immersion and hot-plate assays. The antinociceptive effect of fluoxetine in diabetic mice was significantly lower as compared with that in control mice. Pretreatment with a muscarinic receptor antagonist, atropine (2 and 5 mg/kg, i.p) and an opioid receptor antagonist, naloxone (2 and 5 mg/kg, i.p), but not the alpha(2)-adrenoreceptor antagonist, yohimbine (2 and 5 mg/kg, i.p) reversed the antinociceptive effect of fluoxetine (20 mg/kg). These results suggest that apart from serotonin pathway, muscarinic and opioid receptors also participate in fluoxetine-induced antinociception in diabetic neuropathic pain.

Pathophysiology of tension-type headache

Tension-type headache is one of the most common primary headache disorders. Advances in basic pain and clinical research have improved our understanding of pathophysiologic mechanisms of tension-type headache. Increased excitability of the central nervous system generated by repetitive and sustained pericranial myofascial input may be responsible for the transformation of episodic tension-type headache into the chronic form. Studies of nitric oxide (NO) mechanisms suggest that NO may play a key role in the pathophysiology of tension-type headache and that the antinociceptive effect of nitric oxide synthase inhibitors may become a novel principle in the future treatment of chronic headache. Future studies should focus on investigation of the source of peripheral nociception, the role of descending pain modulation, and the development of an animal model of tension-type headache to support the pathophysiologic importance of central sensitization in tension-type headache.

Antinociceptive properties of the ethanolic extract and of the triterpene 3β,6β,16β-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in mice

The present study examined the antinociceptive effects of the ethanolic extract (EE) and of the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in chemical and thermal behavioural models of pain in mice. The EE (10-1000 mg/kg) given orally (p.o.), 1 h prior to testing, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 131.9 mg/kg. In the formalin test, the EE (10-300 mg/kg, p.o.) also caused significant inhibition of both the early (neurogenic pain) and the late (inflammatory pain) phases of formalin-induced licking, however, it was more potent and efficacious in relation to the late phase of the formalin test, with mean ID50 values for the neurogenic and the inflammatory phases of approximately 300 and 88.8 mg/kg, respectively. The EE (10-1000 mg/kg, p.o.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 160.5 and 38.3 mg/kg, respectively. Furthermore, the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene (1-30 mg/kg), given p.o., 1 h prior to testing, also produced dose-related inhibition of glutamate-induced pain, with a mean ID50 value of 5.6 mg/kg. When assessed in a thermal model of pain, the EE (10-300 mg/kg, p.o.) and fentanyl (100 microg/kg, s.c.) caused a significant and marked increase in the latency response on the hot-plate test (50 degrees C). The antinociception caused by EE (100 mg/kg, p.o.) in the glutamate test was significantly attenuated by intraperitoneal (i.p.) treatment of mice with naloxone (opioid receptor antagonist, 1 mg/kg), pindolol (a 5-HT 1A/1B receptor/beta adrenoceptor antagonist, 1 mg/kg), WAY100635 (a 5-HT 1A receptor antagonist, 0.7 mg/kg) or ketanserin (a 5-HT 2A receptor antagonist, 0.3 mg/kg). In contrast, EE (100 mg/kg, p.o.) antinociception was affected neither by L-arginine (precursor of nitric oxide, 600 mg/kg) nor by ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg) i.p. treatment. It was not associated with non-specific effects such as muscle relaxation or sedation. Together, these results indicate that EE produces dose-related antinociception in several models of chemical and thermal pain through mechanisms that involve an interaction with opioid and serotonergic (i.e., through 5-HT 1A/1B and 5-HT 2A receptors) systems.

The monoamine-mediated antiallodynic effects of intrathecally administered milnacipran, a serotonin noradrenaline reuptake inhibitor, in a rat model of neuropathic pain

Antidepressants are often used to treat neuropathic pain. In the present study, we determined the antiallodynic effects of selective monoamine reuptake inhibitors in the spinal cord in a rat model of neuropathic pain. Mechanical allodynia was produced by tight ligation of the left L5 and L6 spinal nerves and determined by applying von Frey filaments to the left hindpaw. A serotonin noradrenaline reuptake inhibitor, milnacipran, a selective serotonin reuptake inhibitor, paroxetine, or a selective noradrenaline reuptake inhibitor, maprotiline, was administered intrathecally via a chronically implanted catheter. Milnacipran produced dose-dependent antiallodynic effects at doses between 3 microg and 100 microg. The effect lasted for 7 h after injection of 100 microg (P < 0.05). The antiallodynic effect of 30 microg of milnacipran was attenuated by intrathecal coadministration of 30 microg of yohimbine, an alpha(2)-adrenoceptor antagonist, 30 microg of methysergide, a serotonin receptor antagonist, or 30 microg of atropine, a muscarinic receptor antagonist (P < 0.01, respectively). Intraperitoneal administration of milnacipran had no antiallodynic effects at doses of 3 to 30 mg/kg. Antiallodynic effects were not produced by intrathecal administration of paroxetine (10 to 100 microg) or maprotiline (10 to 100 microg). These findings suggest that simultaneous inhibition of serotonin and noradrenaline reuptake in the spinal cord is essential to mediate antiallodynic effects. Milnacipran might be effective for suppression of neuropathic pain.

Reduced serotonin synthesis during early embryogeny changes effect of subsequent prenatal stress on persistent pain in the formalin test in adult male and female rats

The considerable evidence supporting a role for serotonin (5-HT) in the embryonic formation of CNS, mediation of prenatal stress, and pain processing is reviewed. Long-term influences of prenatal 5-HT depletion as well as its combination with prenatal stress effects on tonic nociceptive system in 90-day-old Wistar rats were studied in the formalin test. Pregnant dams were injected with para-chlorophenylalanine (pCPA, 400 mg/kg/2 ml, ip), producing 5-HT depletion during the early period of fetal serotonergic system development. The adult offspring from pCPA-treated dams revealed changes in behavioral indices of persistent pain (flexing + shaking and licking) in the formalin test (2.5%, 50 microl) that were accompanied by irreversible morphological alterations in the dorsal raphe nuclei. In the other series of experiments, the role of 5-HT in the mediation of prenatal stress on the behavioral indices of persistent pain was investigated in the adult offspring from dams with 5-HT depletion followed by restraint stress. Stress during the last embryonic week caused much more increase in flexing + shaking and licking in the second tonic phase of the response to formalin in offspring from pCPA- than saline-treated (control) dams. The former was characterized by alterations in the durations of the interphase, the second phase, and the whole behavioral response too. In offspring from pCPA-treated dams, sex dimorphism was revealed in tonic pain evaluated by licking. Together with our previous results in juvenile rats demonstrating the necessity of definite level of prenatal 5-HT for normal development of tonic nociceptive system, the present pioneering findings obtained in adult rats indicate that prenatal 5-HT depletion causes long-term morphological abnormalities in the dorsal raphe nuclei accompanied by alterations in behavioral indices of tonic pain. Early prenatal 5-HT depletion increases vulnerability of tonic nociceptive circuits to the following prenatal stress.

Serotonergic fiber sprouting to external anal sphincter motoneurons after spinal cord contusion

The present study analyzed the anatomical plasticity of serotonergic immunoreactive projections to external anal sphincter (EAS) motoneurons, and the behavioral plasticity of EAS reflexes, penile erection, and locomotion in rats with spinal contusion injury (SCI) or complete spinal cord transection (TX). Electromyographic activity of the EAS, penile erection latency, and BBB locomotor score exhibited parallel recovery over the 6-week recovery period after contusion SCI. This pattern of recovery was not observed in TX animals. While locomotor scores demonstrated a small increase after TX, erectile and anorectal function remained at abnormal levels established immediately after injury. Serotonergic immunofluorescent (5-HT-IF) staining at the lesion site identified a small number of fibers spared after SCI that may provide a substrate for functional recovery. Pixel density measurements of 5-HT-IF in the vicinity of retrogradely labeled EAS and unlabeled pudendal motoneurons necessary for penile erection provide indirect evidence of serotonergic sprouting that parallels the observed functional recovery in animals with SCI. No 5-HT-IF was detected caudal to the injury site in TX animals. These studies indicate: (1) lumbosacral eliminative and reproductive reflexes provide a valid means of studying the mechanisms of post-SCI plasticity; (2) the similar recovery curves suggest similar return of descending control, perhaps through sprouting of descending serotonergic fibers; (3) the observed deficits after TX likely represent the permanent removal of descending inhibition and reflect reorganization of segmental circuitry.

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.

Pain reactivity in 2-month-old infants after prenatal and postnatal serotonin reuptake inhibitor medication exposure

OBJECTIVE

In this prospective study, we examined biobehavioral responses to acute procedural pain at 2 months of age in infants with prenatal and postnatal selective serotonin reuptake inhibitor (SSRI) medication exposure. Based on previous findings showing reduced pain responses in newborns after prenatal exposure, we hypothesized that altered pain reactivity would also be found at 2 months of age.

METHODS

Facial action (Neonatal Facial Coding System) and cardiac autonomic reactivity derived from the respiratory activity and heart rate variability (HRV) responses to a painful event (heel-lance) were compared between 3 groups of infants: (1) infants with prenatal SSRI exposure alone (n = 11; fluoxetine, n = 2; paroxetine, n = 9); (2) infants with prenatal and postnatal SSRI (via breast milk) exposure (total n = 30; fluoxetine, n = 6; paroxetine, n = 20; sertraline, n = 4); and (3) control infants (n = 22; nonexposed) during baseline, lance, and recovery periods. Measures of maternal mood and drug levels were also obtained, and Bayley Scales of Infant Development-II were administered at ages 2 and 8 months.

RESULTS

Facial action increased in all groups immediately after the lance but was significantly lower in the pSE group during the lance period. HR among infants in the pSE and ppSE groups was significantly lower during recovery. Using measures of HRV and the transfer relationship between heart rate and respiration, exposed infants had a greater return of parasympathetic cardiac modulation in the recovery period, whereas a sustained sympathetic response continued in control infants. Although postnatal exposure via breast milk was extremely low when infant drug levels could be detected in ppSE infants, changes in HR and HRV from lance to recovery were greater compared among infants with levels too low to be quantified. Neither maternal mood nor the presence of clonazepam influenced pain responses.

CONCLUSIONS

Blunted facial-action responses were observed among infants with prenatal SSRI exposure alone, whereas both prenatal and postnatal exposure was associated with reduced parasympathetic withdrawal and increased parasympathetic cardiac modulation during recovery after an acute noxious event. These findings are consistent with patterns of pain reactivity observed in the newborn period in the same cohort. Given that postnatal exposure via breast milk was extremely low and altered biobehavioral pain reactivity was not associated with levels of maternal reports of depression, these data suggest possible sustained neurobehavioral outcomes beyond the newborn period. This is the first study of pain reactivity in infants with prenatal and postnatal SSRI exposure, and our findings were limited by the lack of a depressed nonmedicated control group, small sample size, and understanding of infant behaviors associated with pain reactivity that could have also have been influenced by prenatal SSRI exposure. The developmental and clinical implications of our findings remain unclear, and the mechanisms that may have altered 5-hydroxytryptamine-mediated pain modulation in infants after SSRI exposure remain to be studied. Treating maternal depression with antidepressants during and after pregnancy and promoting breastfeeding in this setting should remain a key goal for all clinicians. Additional study is needed to understand the long-term effects of prenatal and early postnatal SSRI exposure.

Loss of amitriptyline analgesia in alpha 2A-adrenoceptor deficient mice

Tricyclic antidepressants have analgesic and sedative effects in addition to their antidepressive properties. We tested the acute analgesic and locomotor inhibitory effects of the tricyclic antidepressant amitriptyline and the alpha(2)-adrenoceptor agonist clonidine in wild-type control and in alpha(2A)-adrenoceptor knockout mice in hot-plate and tail-flick tests. Amitriptyline-induced analgesia was lost in alpha(2A)-adrenoceptor knockout mice. The locomotor inhibitory effect of amitriptyline was reduced, but not fully abolished in alpha(2A)-adrenoceptor knockout mice. Similar results were obtained with clonidine. We conclude that alpha(2A)-adrenoceptors appear to have a significant role in amitriptyline-induced acute analgesia in mice, and that alpha(2A)-adrenoceptors also participate in the sedative effects of amitriptyline.

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.

Antiallodynic effects of intrathecally administered 5-HT(2C) receptor agonists in rats with nerve injury

Intrathecal administration of serotonin type 2 (5-HT(2)) receptor agonists, alpha-methyl-5-hydroxytryptamine maleate (alpha-m-5-HT) or (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI), produces antiallodynic effects in a rat model of neuropathic pain. In the present study, we examined the antiallodynic effects of intrathecally administered agents which are selective for 5-HT(2C) receptors. Allodynia was produced by tight ligation of the left L5 and L6 spinal nerves, and was measured by applying von Frey filaments to the left hindpaw. Administration of the 5-HT(2C) receptor agonist, 6-chloro-2-(1-piperazinyl)-pyrazine (MK212; 3-100 microg), 1-(m-chlorophenyl)-piperazine (mCPP; 30-300 microg), or 1-(m-trifluoromethylphenyl)-piperazine (TFMPP; 30-300 microg), produced antiallodynic effects in a dose-dependent manner with no associated motor weakness. The ED(50) values of MK212, mCPP, and TFMPP were 39.2, 119.9, and 191.9 microg, respectively. Intrathecal pretreatment with the selective 5-HT(2C) receptor antagonist RS-102221 (30 microg) diminished the effects of the highest doses of 5-HT(2C) receptor agonists. The preferential 5-HT(2A) receptor antagonist ketanserin (30 microg) did not reverse the effects. In contrast to 5-HT(2C) receptor agonists, the antiallodynic effects of intrathecally administered alpha-m-5-HT (30 microg) and DOI (100 microg) were reversed by ketanserin, but not by RS-102221. These results indicate that 5-HT(2C) receptors have a role in spinal inhibition of neuropathic pain, and the effects produced by intrathecal administration of 5-HT(2C) receptor agonists are mediated by a mechanism different from that of alpha-m-5-HT or DOI, which seem to produce their effects through 5-HT(2A) receptors.

Suicidal ideation in outpatients with chronic musculoskeletal pain: an exploratory study of the role of sleep onset insomnia and pain intensity

OBJECTIVES

Sleep disturbance, depression, and heightened risk of suicide are among the most clinically significant sequelae of chronic pain. While sleep disturbance is associated with suicidality in patients with major depression and is a significant independent predictor of completed suicide in psychiatric patients, it is not known whether sleep disturbance is associated with suicidal behavior in chronic pain. This exploratory study evaluates the importance of insomnia in discriminating suicidal ideation in chronic pain relative to depression severity and other pain-related factors.

METHODS

Fifty-one outpatients with non-cancer chronic pain were recruited. Subjects completed a pain and sleep survey, the Pittsburgh Sleep Quality Index, the Beck Depression Inventory, and the Multidimensional Pain Inventory. Subjects were classified as "suicidal ideators" or "non-ideators" based on their responses to BDI-Item 9 (Suicide). Bivariate analyses and multivariate discriminant function analyses were conducted.

RESULTS

Twenty-four percent reported suicidal ideation (without intent). Suicidal ideators endorsed higher levels of: sleep onset insomnia, pain intensity, medication usage, pain-related interference, affective distress, and depressive symptoms (P < 0.03). These 6 variables were entered into stepwise discriminant function analyses. Two variables predicted group membership: Sleep Onset Insomnia Severity and Pain Intensity, respectively. The discriminant function correctly classified 84.3% of the cases (P < 0.0001).

DISCUSSION

Chronic pain patients who self-reported severe and frequent initial insomnia with concomitant daytime dysfunction and high pain intensity were more likely to report passive suicidal ideation, independent from the effects of depression severity. Future research aimed at determining whether sleep disturbance is a modifiable risk factor for suicidal ideation in chronic pain is warranted.

Effects of m-CPP in altering neuronal function: blocking depolarization in invertebrate motor and sensory neurons but exciting rat dorsal horn neurons

The compound m-chlorophenylpiperazine (m-CPP) is used clinically to manipulate serotonergic function, though its precise mechanisms of actions are not well understood. m-CPP alters synaptic transmission and neuronal function in vertebrates by non-selective agonistic actions on 5-HT(1) and 5-HT(2) receptors. In this study, we demonstrated that m-CPP did not appear to act through a 5-HT receptor in depressing neuronal function in the invertebrates (crayfish and Drosophila). Instead, m-CPP likely decreased sodium influx through voltage-gated sodium channels present in motor and primary sensory neurons. Intracellular axonal recordings showed that m-CPP reduced the amplitude of the action potentials in crayfish motor neurons. Quantal analysis of excitatory postsynaptic currents, recorded at neuromuscular junctions (NMJ) of crayfish and Drosophila, indicated a reduction in the number of presynaptic vesicular events, which produced a decrease in mean quantal content. m-CPP also decreased activity in primary sensory neurons in the crayfish. In contrast, serotonin produces an increase in synaptic strength at the crayfish NMJ and an increase in activity of sensory neurons; it produces no effect at the Drosophila NMJ. In the rat spinal cord, m-CPP enhances the occurrence of spontaneous excitatory postsynaptic potentials with no alteration in evoked currents.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

The neuropharmacology of centrally-acting analgesic medications in fibromyalgia

As demonstrated above, the anatomy and neuropharmacology of the pain pathways within the CNS, even to the level of the midbrain, are extraordinarily complex. Indeed, discussions of the effects of these agents on the neuropharmacology of the thalamus, hypothalamus, and cortex were excluded from this review owing to their adding further to this complexity. Also, the dearth of data regarding FMS pain pathophysiology necessitated a relatively generic analysis of the pain pathways. As mentioned in the introduction, the current thought is that central sensitization plays an important role in FMS. However, we see in this chapter that the behavioral state of central sensitization may be a result of alterations in either the ascending systems or in one or more descending systems. Studies to assess the presence or relative importance of such changes in FMS are difficult to perform in humans, and to date there are no animal models of FMS. Accepting these limitations, it is apparent that many drugs considered to date for the treatment of FMS do target a number of appropriate sites within both the ascending and descending pain pathways. The data regarding clinical efficacy on some good candidate agents, however, is extremely preliminary. For example, it is evident from the present analysis that SNRIs, alpha 2 agonists, and NK1 antagonists may be particularly well suited to FMS, although current data supporting their use is either anecdotal or from open-label trials [114,149]. Other sites within the pain pathways have not yet been targeted. Examples of these include the use of CCKB antagonists to block on-cell activation or of nitric oxide synthetase antagonists to block the downstream mediators of NMDA activation. Efficacy of such agents may give considerable insight into the pathophysiology of FMS. Finally, as indicated previously, FMS consists of more than just chronic pain, and the question of how sleep abnormalities, depression, fatigues, and so forth tie into disordered pain processing is being researched actively. Future research focusing on how the various manifestations of FMS relate to one another undoubtedly will lead to a more rational targeting of drugs in this complex disorder.

Spinal noradrenergic activation mediates allodynia reduction from an allosteric adenosine modulator in a rat model of neuropathic pain

Activation of adenosine A1 receptors by endogenous adenosine or synthetic agonists produces anti-nociception in animal models of acute pain and also reduces hypersensitivity in models of inflammatory and nerve-injury pain. Allosteric adenosine modulators facilitate adenosine agonist binding to the A1 receptor. The purpose of the current study was to examine the effect, mechanisms of action, and interaction with noradrenergic systems of intrathecal (i.t.) or oral administration of the allosteric adenosine modulator T62 in a rat model of neuropathic pain. A spinal nerve ligation rat model (SNL; ligation of left L5 and L6 spinal nerve roots) was used. One week after SNL surgery, an i.t. catheter was inserted. Withdrawal threshold to mechanical stimulation of the left hind paw was determined before and after surgery, confirming mechanical hypersensitivity. Oral or i.t. T62 reduced hypersensitivity induced by SNL. The effects of i.t. T62 were inhibited by i.t. injection of an A1 receptor antagonist and by an 2-adrenergic antagonist but not by an A2 adenosine receptor antagonist. Anti-dopamine hydroxylase (DH)-saporin treatment reduce spinal norepinephrine content by 97%, accompanied by an almost complete loss of DH immunoreactive axons in the spinal dorsal horn and neurons in the locus coeruleus. The effect of T62 was completely lost in animals treated with anti-DH-saporin. These data support the hypothesis that activation of the A1 receptor by the allosteric modulator, T62, produces anti-nociception via spinal noradrenergic activation.

Possible involvement of a muscarinic receptor in the anti-allodynic action of a 5-HT2 receptor agonist in rats with nerve ligation injury

Intrathecal administration of 5-HT(2) receptor agonists produces an anti-allodynic effect in a rat model of neuropathic pain. Several non-serotonergic neurotransmitters have been implicated these anti-nociceptive effects. In the present study, intrathecal pre-treatment with the muscarinic receptor antagonist atropine (10 and 30 microg) and pirenzepine (10 microg) reversed the anti-allodynic effect of the 5-HT(2) receptor agonist alpha-methyl-5-hydroxytryptamine, unlike various other antagonists. Thus, muscarinic receptors may be involved in the anti-allodynic action of intrathecally injected 5-HT(2) receptor agonist.

Pain modulation role of melatonin in eating disorders

The objective of this study was to test the effect of melatonin on thermal pain threshold in female patients with eating disorders. Fourteen patients were included in the study. We found a parabolic relation between pain threshold and the content of urine sulfatoxymelatonin (r = 0.6299, P < 0.05). We can speculate that increase in severity of eating disorder pathology may decrease both the melatonin level and pain sensitivity. In contrast with expected melatonin analgesic effect, our results showed its possible normalizing influence as well on pathologically decreased pain sensitivity.

A review of evidence in support of a role for 5-HT in the perception of tinnitus

Tinnitus is a debilitating condition from which some 0.5-1% of the population of the Western world suffer sufficiently badly as to interfere with their normal working and leisure life. There is no satisfactory treatment at the present time and the uncertainty surrounding the mechanism of its generation makes it difficult to devise an effective cure. After much debate, the consensus of opinion amongst researchers regarding its site of origin is that it is primarily a central nervous system pathology although there certainly exists a class of patients whose tinnitus is peripherally based. In this paper, we provide some speculative ideas on how an initial auditory insult can be translated into central neurological substrates that represent tinnitus. Plastic changes arising from sensory deprivation trigger a change in synaptology and neurotransmission with a consequent change in receptor configuration. From neuroanatomical considerations and analogies with other clinical conditions, we postulate the involvement of serotonin (5-HT) in these plastic changes and consider the evidence available from the use of serotonergic drugs in different conditions. A possible relationship between 5-HT and lidocaine is also discussed.

Transplantation of embryonic Raphe cells regulates the modifications of the gabaergic phenotype occurring in the injured spinal cord

Transection of the spinal cord yields a permanent deficit due to the interruption of descending and ascending tracts which subserve the supraspinal control of spinal cord functions. We have shown previously that transplantation below the level of the section of embryonic monoaminergic neurons can promote the recovery of some segmental functions via a local serotonergic and noradrenergic reinnervation. Moreover, the up-regulation of the corresponding receptors resulting from the section was corrected by the transplants. The aim of the present work was to determine whether such a graft could also influence non-monoaminergic local neurons, the GABAergic interneurons of the spinal cord. Following spinal cord transection, the number of cells which express glutamate decarboxylase (mol. wt 67,000) messenger RNA--a marker of GABA synthesis--increased significantly below the lesion compared with the intact animal. In contrast, in lesioned animals which had been grafted one week later with raphe neuroblasts, this number was close to control level. These post-grafting modifications were further associated with increased GABA immunoreactivity in the host tissue. These data suggest that the graft of embryonic raphe cells which compensates the deficit of serotonin in the distal segment also regulates the expression of the GABAergic phenotype in the host spinal cord. This regulation could be mediated by the re-establishment of a local functional innervation by both serotonin and GABAergic transplanted neurons and/or by trophic factors released from the embryonic cells. It appears then that grafted cells influence the host tissue in a complex manner, through the release and/or regulation of several neurotransmitter systems.

Differential effects of (R)- and (S)-8-hydroxy-2-(di-n-propylamino)tetralin on the monosynaptic spinal reflex in rats

We examined the effects of (R)- and (S)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) on the monosynaptic spinal reflex in rats. In intact rats, (R)-8-OH-DPAT (10 microg/kg, i.v.) enhanced the amplitude of the monosynaptic reflex, whereas at 100 microg/kg, it reduced the amplitude. (S)-8-OH-DPAT enhanced the monosynaptic reflex dose-dependently. In spinalized rats, (R)-8-OH-DPAT produced dose-dependent inhibition, but the (S)-enantiomer did not affect the monosynaptic reflex. Pretreatment with spiroxatrine or 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190) inhibited (R)-8-OH-DPAT-induced monosynaptic reflex enhancement in intact rats, as did 5-hydroxytryptamine (5-HT) depletion. Ketanserin reduced the effect of (R)-8-OH-DPAT. These pretreatment regimens had no effect on the monosynaptic reflex depression produced by the (R)-enantiomer in intact and spinalized rats. Pretreatment with prazosin inhibited (S)-8-OH-DPAT-induced monosynaptic reflex enhancement in intact rats, as did noradrenaline and 5-HT depletion. These results suggest that supraspinal 5-HT1A receptors and the descending serotonergic system are involved in the stimulatory effect of (R)-8-OH-DPAT on the monosynaptic reflex, while both the descending serotonergic and noradrenergic systems, the latter acting via alpha1-adrenoceptors, are involved in the effect of the (S)-enantiomer on this reflex.

Morphine tolerance in arthritic rats and serotonergic system

To understand whether chronic inflammation alters the development of morphine tolerance, the tail-flick test was used to evaluate the analgesic effect of morphine (75 mg tablet, s.c.) in the arthritic rats at the day 9-12 after the inoculation with Freund's adjuvant. Spinal cord monoamines and amino acid neurotransmitters were concomitantly measured. Chronic inflammation attenuated the antinociceptive effect of morphine as tolerance developed faster in the arthritic rats compared to the vehicle-treated controls. In addition, ratio of 5-hydroxyindole-3-acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) increased in the lumbar spinal cord of arthritic rats without any change in the concentrations of norepinephrine, glutamate, aspartate or GABA. Interestingly, increased serotonin turnover in the spinal cord was observed in both control and arthritic rats 24 hours after morphine treatment. Overall, the results suggest a significant role of serotonin up-regulation in the spinal cord during chronic pain and the development of morphine tolerance.

Transmitters involved in antinociception in the spinal cord

The possible physiological and pathophysiological role of monoamines-adrenergic transmitter (norepinephrine), serotonin; cholinergic transmitter (acetylcholine); inhibitory (gamma-aminobutyric acid) and excitatory (glutamate) amino acids; opioid and nonopioid peptides, enkephalins, beta-endorphin and substance P, neurokinin-A, neurokinin-B, neurotensin, cytokines, calcitonine gene-related peptide, galanin, neuropeptide Y, nerve growth factor, cholecystokinin; purines; nitric oxide; vanilloid receptor agonists (capasaicin); and nociceptin-in spinal transmission of pain is reviewed. The role of substance P, neurokinin-A and neurokinin-B in the dorsal horn has been identified. These were suggested to be primary afferent transmitters mediating or facilitating the expression of nociceptive inputs. Pronociceptive modulators will be discussed later. Recent findings showing that N-methyl-D-aspartate (NMDA) receptor activation generates nitric oxide and prostanoids that enhance pain transmission whereas adenosine release acts to control these NMDA-mediated events are also mentioned. The clinical importance of centrally acting alpha2-adrenoceptor agonists (clonidine and dexmedetomidine) is also discussed. Antinociceptive and morphine-potentiating drugs are ideal adjuvants for anesthesia; their application in spinal anesthesia is highlighted. The recent development in understanding the importance of noradrenergic transmission and subtypes of alpha2-adrenoceptors (alpha2A and alpha2B) for the first time is reviewed.

Antinociception by adenosine analogs and an adenosine kinase inhibitor: dependence on formalin concentration

Spinal administration of adenosine analogs and an adenosine kinase inhibitor produces antinociception in thermal threshold tests. In the present study, we determined the effects of N6-cyclohexyladenosine (adenosine A1 receptor selective), 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethyl-carboxamidoadeno sine (CGS-21680) (adenosine A2A receptor selective), and 5'-N-ethylcarboxamidoadenosine (NECA) (non-selective), on formalin induced nociceptive responses (flinching/lifting and licking/biting) using two concentrations of formalin (2% and 5%). We also examined the antinociceptive effects of 5'-amino-5'-deoxyadenosine, an adenosine kinase inhibitor, and deoxycoformycin, an adenosine deaminase inhibitor, under these conditions. Adenosine A1 receptor agonists, but not the A2A selective agent, produced significant antinociception, as did 5'-amino-5'-deoxyadenosine, but not deoxycoformycin. The extent of antinociception produced was greater with the lower stimulus intensity. The effects of NECA and 5'-amino-5'-deoxyadenosine were inhibited by caffeine, indicating the involvement of cell surface adenosine receptors in their actions. We conclude (a) that the adenosine A1, but not the A2A, receptor is involved in spinally mediated antinociception, (b) that adenosine kinase is more important than adenosine deaminase in regulating endogenous adenosine levels in the spinal cord, and (c) that stimulus intensity is an important determinant of the efficacy of purines in the spinal cord.