The role of 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptors in modulating spinal nociceptive transmission in normal and carrageenan-injected rats

Bespoke library docking for 5-HT2A receptor agonists with antidepressant activity

There is considerable interest in screening ultralarge chemical libraries for ligand discovery, both empirically and computationally1-4. Efforts have focused on readily synthesizable molecules, inevitably leaving many chemotypes unexplored. Here we investigate structure-based docking of a bespoke virtual library of tetrahydropyridines-a scaffold that is poorly sampled by a general billion-molecule virtual library but is well suited to many aminergic G-protein-coupled receptors. Using three inputs, each with diverse available derivatives, a one pot C-H alkenylation, electrocyclization and reduction provides the tetrahydropyridine core with up to six sites of derivatization5-7. Docking a virtual library of 75 million tetrahydropyridines against a model of the serotonin 5-HT2A receptor (5-HT2AR) led to the synthesis and testing of 17 initial molecules. Four of these molecules had low-micromolar activities against either the 5-HT2A or the 5-HT2B receptors. Structure-based optimization led to the 5-HT2AR agonists (R)-69 and (R)-70, with half-maximal effective concentration values of 41 nM and 110 nM, respectively, and unusual signalling kinetics that differ from psychedelic 5-HT2AR agonists. Cryo-electron microscopy structural analysis confirmed the predicted binding mode to 5-HT2AR. The favourable physical properties of these new agonists conferred high brain permeability, enabling mouse behavioural assays. Notably, neither had psychedelic activity, in contrast to classic 5-HT2AR agonists, whereas both had potent antidepressant activity in mouse models and had the same efficacy as antidepressants such as fluoxetine at as low as 1/40th of the dose. Prospects for using bespoke virtual libraries to sample pharmacologically relevant chemical space will be considered.

Structure-Based Design of a Chemical Probe Set for the 5-HT5A Serotonin Receptor

The 5-HT_5A receptor (5-HT_5AR), for which no selective agonists and a few antagonists exist, remains the least understood serotonin receptor. A single commercial antagonist, SB-699551, has been widely used to investigate the 5-HT_5AR function in neurological disorders, including pain, but this molecule has substantial liabilities as a chemical probe. Accordingly, we sought to develop an internally controlled probe set. Docking over 6 million molecules against a 5-HT_5AR homology model identified 5 mid-μM ligands, one of which was optimized to UCSF678, a 42 nM arrestin-biased partial agonist at the 5-HT_5AR with a more restricted off-target profile and decreased assay liabilities versus SB-699551. Site-directed mutagenesis supported the docked pose of UCSF678. Surprisingly, analogs of UCSF678 that lost the 5-HT_5AR activity revealed that 5-HT_5AR engagement is nonessential for alleviating pain, contrary to studies with less-selective ligands. UCSF678 and analogs constitute a selective probe set with which to study the function of the 5-HT_5AR.

The central effects of buspirone on abdominal pain in rats

BACKGROUND

Buspirone, a partial agonist of the 5-HT_1a receptor (5-HT_1a R), owing to potential antinociceptive properties may be useful in treatment of abdominal pain in IBS patients. The pain-related effects of buspirone are mediated via the 5-HT_1a Rs, specifically located within the ventrolateral medulla (VLM). The most animal studies of the 5-HT_1a R involvement in pain control have been carried out with somatic behavioral tests. The 5-HT_1a R contribution in visceral pain transmission within the VLM is unclear. The objective of our study was to evaluate the 5-HT_1a R contribution in abdominal pain transmission within the VLM.

METHODS

Using animal model of abdominal pain (urethane-anaesthetized rats), based on the noxious colorectal distension (CRD) as pain stimulus we studied effects of buspirone (1.0-4.0 mg kg^-1 , iv) on the CRD-induced VLM neuron and blood pressure responses as markers of abdominal pain before and after the 5-HT_1a R blockade by antagonist, WAY 100,635.

RESULTS

The CRD induced a significant increase in VLM neuron activity up to 201.5 ± 18.0% and depressor reactions up to 68 ± 1.8% of baseline. Buspirone (1.0-4.0 mg kg^-1 , iv) resulted in an inhibition of the CRD-induced neuron responses which were changed inversely with dose increase and decreased depressor reactions directly with dose increase. These effects were antagonized by intracerebroventricular WAY 100,635.

CONCLUSION

Buspirone exerts complex biphasic action on the pain-related VLM neuron activity inversely depending on dose. The final effect of buspirone depends on the functional balance between of activation the pre- and postsynaptic 5-HT_1a Rs in mediating pain control networks.

Auricular vagus nerve stimulation enhances central serotonergic function and inhibits diabetic neuropathy development in Zucker fatty rats

Painful neuropathy is a frequent comorbidity in diabetes. Zucker diabetic fatty (fa/fa) rats develop type 2 diabetes spontaneously with aging and show nociceptive hypersensitivity at the age of 13 weeks. In preclinical and clinical studies, the treatment of diabetic neuropathy is challenging, but complementary medicine such as transcutaneous auricular vagus nerve stimulation (taVNS) appears beneficial to the relief of neuropathic pain. However, the mechanism behind the effectiveness of taVNS remains unclear. In this study, we show that daily 30-min taVNS (2/15 Hz, 2 mA) for consecutive 27 days effectively inhibited the development of nociceptive hypersensitivity in Zucker diabetic fatty rats as detected by thermal hyperalgesia and mechanical allodynia in hindpaw. We also demonstrated that this beneficial effect in nociceptive behavior is related to an elevated serotonin (5-HT) plasma concentration and an upregulated expression of 5-HT receptor type 1A (5-HT1AR) in hypothalamus. We conclude that daily 30-min taVNS sessions lessen diabetic neuropathy development by enhancing serotonergic function in genetically diabetes prone individuals.

Perspective

This article presents taVNS as a new approach to inhibit the development of diabetic neuropathy in genetically prone individuals. This approach could potentially help clinicians who seek to avoid the complication of neuropathic pain in diabetic patient or to relieve the pain if there was one.

Regular physical activity prevents development of chronic muscle pain through modulation of supraspinal opioid and serotonergic mechanisms

The current study shows that blockade of opioid receptors systemically in the periaqueductal gray and the rostral ventromedial medulla prevents analgesia by 8 weeks of wheel running in a chronic muscle pain model. We further show increases in serotonin transporter expression and reversal of hyperalgesia with a selective reuptake inhibitor in the rostral ventromedial medulla in the chronic muscle pain model, and exercise normalizes serotonin transporter expression.

Introduction:

It is generally believed that exercise produces its effects by activating central opioid receptors; there are little data that support this claim. The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are key nuclei in opioid-induced analgesia, and opioids interact with serotonin to produce analgesia.

Objectives:

The purpose was to examine central inhibitory mechanisms involved in analgesia produced by wheel running.

Methods:

C57/Black6 mice were given access to running wheels in their home cages before induction of chronic muscle hyperalgesia and compared with those without running wheels. Systemic, intra-PAG, and intra-RVM naloxone tested the role of central opioid receptors in the antinociceptive effects of wheel running in animals with muscle insult. Immunohistochemistry for the serotonin transporter (SERT) in the spinal cord and RVM, and pharmacological blockade of SERT, tested whether the serotonin system was modulated by muscle insult and wheel running.

Results:

Wheel running prevented the development of muscle hyperalgesia. Systemic naloxone, intra-PAG naloxone, and intra-RVM naloxone reversed the antinociceptive effect of wheel running in animals that had received muscle insult. Induction of chronic muscle hyperalgesia increased SERT in the RVM, and blockade of SERT reversed the hyperalgesia in sedentary animals. Wheel running reduced SERT expression in animals with muscle insult. The serotonin transporter in the superficial dorsal horn of the spinal cord was unchanged after muscle insult, but increased after wheel running.

Conclusion:

These data support the hypothesis that wheel running produced analgesia through central inhibitory mechanisms involving opioidergic and serotonergic systems.

Antinociceptive, antiallodynic and antihyperalgesic effects of the 5-HT1A receptor selective agonist, NLX-112 in mouse models of pain

BACKGROUND AND PURPOSE

NLX-112 (a.k.a. befiradol, F13640) is a drug candidate intended for the treatment of l-DOPA-induced dyskinesia. It is a highly selective serotonin 5-HT_1A receptor full agonist which has been previously tested in a variety of models of CNS effects including analgesic activity in rat. Its activity in mouse models of pain has not been previously investigated.

EXPERIMENTAL APPROACH

The activity of NLX-112 was tested in mouse models of acute pain (hot plate), tonic pain (intraplantar formalin test), in the oxaliplatin-induced neuropathic pain model of chemotherapy-induced peripheral neuropathy and in the streptozotocin (STZ)-induced model of painful diabetic neuropathy.

KEY RESULTS

The main findings indicate that (i) NLX-112 was markedly active in the formalin test with potent reduction of paw licking in both phases of the test (minimal effective dose (MED) 0.5 mg/kg i.p. and p.o. in acute phase, and 0.1 mg/kg i.p. and 1 mg/kg p.o. in late phase). The effects of NLX-112 in this test were completely abolished by the selective 5-HT_1A receptor antagonist, WAY100635; (ii) NLX-112 was active in the hot plate test and in the oxaliplatin-induced neuropathic pain model of chemotherapy-induced peripheral neuropathy, but at markedly higher doses (MED 2.5 mg/kg i.p.); (iii) NLX-112 was least active in the STZ-induced model of painful diabetic neuropathy (MED 5 mg/kg i.p.); (iv) NLX-112 did not affect locomotor activity.

CONCLUSIONS AND IMPLICATIONS

NLX-112 may have significant potential for treatment of tonic pain but may be less promising as a candidate for treatment of chemotherapy-induced or diabetic neuropathic pain.

Regional-specific effect of fluoxetine on rapidly dividing progenitors along the dorsoventral axis of the hippocampus

Hippocampus-dependent cognitive and emotional function appears to be regionally dissociated along the dorsoventral (DV) axis of the hippocampus. Recent observations that adult hippocampal neurogenesis plays a critical role in both cognition and emotion raised an interesting question whether adult neurogenesis within specific subregions of the hippocampus contributes to these distinct functions. We examined the regional-specific and cell type-specific effects of fluoxetine, which requires adult hippocampal neurogenesis to function as an antidepressant, on the proliferation of hippocampal neural stem cells (NSCs). Fluoxetine specifically increased proliferation of NSCs located in the ventral region of the hippocampus while the mitotic index of NSCs in the dorsal portion of the hippocampus remained unaltered. Moreover, within the ventral hippocampus, type II NSC and neuroblast populations specifically responded to fluoxetine, showing increased proliferation; however, proliferation of type I NSCs was unchanged in response to fluoxetine. Activation or inhibition of serotonin receptor 1A (5-HTR1A) recapitulated or abolished the effect of fluoxetine on proliferation of type II NSCs and neuroblast populations in the ventral hippocampus. Our study showed that the effect of fluoxetine on proliferation is dependent upon the type and the position of the NSCs along the DV axis of the hippocampus.

Spinal 5-HT1A, not the 5-HT1B or 5-HT3 receptors, mediates descending serotonergic inhibition for late-phase mechanical allodynia of carrageenan-induced peripheral inflammation

Previous electrophysiological studies demonstrated a limited role of 5-hydroxytryptamine 3 receptor (5-HT3R), but facilitatory role of 5-HT1AR and 5-HT1BR in spinal nociceptive processing of carrageenan-induced inflammatory pain. The release of spinal 5-HT was shown to peak in early-phase and return to baseline in late-phase of carrageenan inflammation. We examined the role of the descending serotonergic projections involving 5-HT1AR, 5-HT1BR, and 5-HT3R in mechanical allodynia of early- (first 4h) and late-phase (24h after) carrageenan-induced inflammation. Intrathecal administration of 5-HT produced a significant anti-allodynic effect in late-phase, but not in early-phase. Similarly, intrathecal 5-HT1AR agonist (8-OH-DPAT) attenuated the intensity of late-phase allodynia in a dose dependent fashion which was antagonized by 5-HT1AR antagonist (WAY-100635), but produced no effect on the early-phase allodynia. However, other agonists or antagonists of 5-HT1BR (CP-93129, SB-224289) and 5-HT3R (m-CPBG, ondansetron) did not produce any anti- or pro-allodynic effect in both early- and late- phase allodynia. These results suggest that spinal 5-HT1A, but not 5-HT1B or 5-HT3 receptors mediate descending serotonergic inhibition on nociceptive processing of late-phase mechanical allodynia in carrageenan-induced inflammation.

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.

The degree of acute descending control of spinal nociception in an area of primary hyperalgesia is dependent on the peripheral domain of afferent input

Descending controls of spinal nociceptive processing play a critical role in the development of inflammatory hyperalgesia. Acute peripheral nociceptor sensitization drives spinal sensitization and activates spino–supraspinal–spinal loops leading to descending inhibitory and facilitatory controls of spinal neuronal activity that further modify the extent and degree of the pain state. The afferent inputs from hairy and glabrous skin are distinct with respect to both the profile of primary afferent classes and the degree of their peripheral sensitization. It is not known whether these differences in afferent input differentially engage descending control systems to different extents or in different ways. Injection of complete Freund's adjuvant resulted in inflammation and swelling of hairy hind foot skin in rats, a transient thermal hyperalgesia lasting <2 h, and longlasting primary mechanical hyperalgesia (≥7 days). Much longer lasting thermal hyperalgesia was apparent in glabrous skin (1 h to >72 h). In hairy skin, transient hyperalgesia was associated with sensitization of withdrawal reflexes to thermal activation of either A- or C-nociceptors. The transience of the hyperalgesia was attributable to a rapidly engaged descending inhibitory noradrenergic mechanism, which affected withdrawal responses to both A- and C-nociceptor activation and this could be reversed by intrathecal administration of yohimbine (α-2-adrenoceptor antagonist). In glabrous skin, yohimbine had no effect on an equivalent thermal inflammatory hyperalgesia. We conclude that acute inflammation and peripheral nociceptor sensitization in hind foot hairy skin, but not glabrous skin, rapidly activates a descending inhibitory noradrenergic system. This may result from differences in the engagement of descending control systems following sensitization of different primary afferent classes that innervate glabrous and hairy skin.

Evidence for the participation of peripheral 5-HT₂A, 5-HT₂B, and 5-HT₂C receptors in formalin-induced secondary mechanical allodynia and hyperalgesia

The role of 5-HT₂A/₂B/₂C receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia. Pre-treatment for five consecutive days with compound 48/80 (1, 3, 10, 10, and 10 μg/paw) prevented formalin-induced secondary allodynia and hyperalgesia. Ipsilateral, but not contralateral, peripheral pre-treatment (nmol/paw) with the 5-HT₂ receptor agonist DOI (3-30), 5-HT (10-100) or fluoxetine (0.3-3) significantly increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of DOI (10 nmol/paw), 5-HT (100 nmol/paw) and fluoxetine (1 nmol/paw) was blocked by selective 5-HT₂A (ketanserin), 5-HT₂B (RS-127445), and 5-HT₂C (RS-102221) receptor antagonists. Furthermore, ipsilateral pre-treatment (nmol/paw) with ketanserin (1, 10, and 100), RS-127445 (0.01, 0.1 and 1) or RS-102221 (1, 10 and 100) prevented while post-treatment reversed 1% formalin-induced secondary allodynia and hyperalgesia in both paws. In marked contrast, contralateral injection of the greatest tested dose of 5-HT₂A/₂B/₂C receptor antagonists did not modify long-lasting secondary allodynia and hyperalgesia. These results suggest that 5-HT released from mast cells after formalin injection sensitizes primary afferent neurons via 5-HT₂A/₂B/₂C receptors leading to the development and maintenance of secondary allodynia and hyperalgesia.

Pronociceptive effect of 5-HT(1A) receptor agonist on visceral pain involves spinal N-methyl-D-aspartate (NMDA) receptor

The functional role of serotonergic 5-HT(1A) receptors in the modulation of visceral pain is controversial. The objective of this study was to systematically examine the mechanism and site of action of a selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (DPAT) on visceral pain. In the behavioral model of visceral pain, systemic injection (5-250 μg/kg) of DPAT produced a significant increase in the viscero-motor response (VMR) to colorectal distension (CRD) and this effect was blocked by the selective 5-HT(1A) receptor antagonist WAY-100135 (5 mg/kg, s.c.). Similarly, intrathecal (i.t.) injection (5 μmol) of DPAT into the lumbo-sacral (L6-S1) spinal cord produced a significant increase in VMR. The administration of N-methyl D-aspartate (NMDA) receptor antagonist AP5 (50 μg/kg) prior to DPAT injection completely blocked the pronociceptive effect of DPAT. Similarly, DPAT failed to increase VMR in rats chronically treated with NR1 subunit-targeted antisense oligonucleotide (ON), whereas the drug increased VMR in rats treated with mismatched-ON. Chronic i.t. injection of allylglycine (AG), a γ-amino decarboxylase (GAD) enzyme inhibitor, produced significant increase in VMRs, suggesting that the inhibition of GABA synthesis produces pronociception. In AG-treated rats, i.t. injection of DPAT failed to further increase in VMR, suggesting that the DPAT action is linked to GABA release. Similarly, WAY-100135 failed to attenuate VMR in AG-treated rats, suggesting that unlike DPAT, AG action is not via the activation of 5-HT(1A) receptors. In electrophysiology experiments, DPAT (50 μg/kg) significantly increased the responses of spinal neurons to CRD, but did not influence the mechanotransduction property of CRD-sensitive pelvic nerve afferent fibers. The effect of DPAT on spinal neurons remained unaffected when tested in spinal-transected (C1-C2) rats. These results indicate that the 5-HT(1A) receptor agonist DPAT produces pronociceptive effects, primarily via the activation of presynaptic 5-HT(1A) receptors in GABAergic neuron to restrict GABA release and thereby disinhibits the excitatory glutamatergic neurons in the spinal cord.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

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

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

Serotonin receptors are involved in the spinal mediation of descending facilitation of surgical incision-induced increase of Fos-like immunoreactivity in rats

Background

Descending pronociceptive pathways may be implicated in states of persistent pain. Paw skin incision is a well-established postoperative pain model that causes behavioral nociceptive responses and enhanced excitability of spinal dorsal horn neurons. The number of spinal c-Fos positive neurons of rats treated intrathecally with serotonin, noradrenaline or acetylcholine antagonists where evaluated to study the descending pathways activated by a surgical paw incision.

Results

The number of c-Fos positive neurons in laminae I/II ipsilateral, lamina V bilateral to the incised paw, and in lamina X significantly increased after the incision. These changes: remained unchanged in phenoxybenzamine-treated rats; were increased in the contralateral lamina V of atropine-treated rats; were inhibited in the ipsilateral lamina I/II by 5-HT_1/2B/2C (methysergide), 5-HT_2A (ketanserin) or 5-HT_{1/2A/2C/5/6/7} (methiothepin) receptors antagonists, in the ipsilateral lamina V by methysergide or methiothepin, in the contralateral lamina V by all the serotonergic antagonists and in the lamina X by LY 278,584, ketanserin or methiothepin.

Conclusions

We conclude: (1) muscarinic cholinergic mechanisms reduce incision-induced response of spinal neurons inputs from the contralateral paw; (2) 5-HT_1/2A/2C/3 receptors-mediate mechanisms increase the activity of descending pathways that facilitates the response of spinal neurons to noxious inputs from the contralateral paw; (3) 5-HT_1/2A/2C and 5-HT_1/2C receptors increases the descending facilitation mechanisms induced by incision in the ipsilateral paw; (4) 5-HT_2A/3 receptors contribute to descending pronociceptive pathways conveyed by lamina X spinal neurons; (5) α-adrenergic receptors are unlikely to participate in the incision-induced facilitation of the spinal neurons.

Nociceptin/Orphanin FQ in PAG modulates the release of amino acids, serotonin and norepinephrine in the rostral ventromedial medulla and spinal cord in rats

High density Nociceptin/Orphanin FQ (N/OFQ) and its receptor (NOPr) have been found in the ventrolateral periaqueductal gray (vlPAG), a main output pathway involved in the descending pain-control system. Our previous study demonstrated that the microinjection of N/OFQ into the vlPAG markedly facilitated nociceptive responses of spinal dorsal horn neurons. The aim of the present work was to further provide evidence for the supraspinal mechanisms of action for N/OFQ-mediated nociceptive facilitation by examining the effect of N/OFQ in the vlPAG on neurotransmitter release in the descending pain-control system, including the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC) and dorsal horn of the spinal cord. The results showed that the microinjection of N/OFQ into the vlPAG produced robust decreases in 5-hydroxytryptamine (5-HT, serotonin), norepinephrine (NE), and gamma-aminobutyric acid (GABA), and increase in glutamate (Glu) release in the spinal dorsal horn. Spinal application of 5-HT, 2-Me-5-HT (5-HT(3) receptor agonist), muscimol (GABA(A) receptor agonist), and baclofen (GABA(B) receptor agonist) significantly blocked intra-vlPAG-induced facilitation on nociceptive responses. However, the extracellular concentrations of these neurotransmitters in the NRM and NGC exhibited diversity following intra-vlPAG of N/OFQ. In the NRM, intra-vlPAG injection of N/OFQ significantly decreased 5-HT, NE, and Glu, but increased GABA release. Differently, in the NGC, both NE and GABA releases were attenuated by intra-vlPAG of N/OFQ, whereas the concentration of 5-HT and Glu exhibited a trend to increase. These findings provide direct support for the hypothesis that intra-PAG of N/OFQ-induced facilitation of nociceptive responses is associated with the release of 5-HT, NE, and amino acids.

Interaction between midazolam and serotonin in spinally mediated antinociception in rats

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Role of the spinal cord NR2B-containing NMDA receptors in the development of neuropathic pain

Activation of N-methyl-d-aspartate (NMDA) receptors in the spinal dorsal horn has been shown to be essential for the initiation of central sensitization and the hyperexcitability of dorsal horn neurons in chronic pain. However, whether the spinal NR2B-containing NMDA (NMDA-2B) receptors are involved still remains largely unclear. Using behavioral test and in vivo extracellular electrophysiological recording in L5 spinal nerve-ligated (SNL) neuropathic rats, we investigate the roles of spinal cord NMDA-2B receptors in the development of neuropathic pain. Our study showed that intrathecal (i.t.) injection of Ro 25-6981, a selective NMDA-2B receptor antagonist, had a dose-dependent anti-allodynic effect without causing motor dysfunction. Furthermore, i.t. application of another NMDA-2B receptor antagonist ifenprodil prior to SNL also significantly inhibited the mechanical allodynia but not the thermal hyperalgesia. These data suggest that NMDA-2B receptors at the spinal cord level play an important role in the development of neuropathic pain, especially at the early stage following nerve injury. In addition, spinal administration of Ro 25-6981 not only had a dose-dependent inhibitory effect on the C-fiber responses of dorsal horn wide dynamic range (WDR) neurons in both normal and SNL rats, but also significantly inhibited the long-term potentiation (LTP) in the C-fiber responses of WDR neurons induced by high-frequency stimulation (HFS) applied to the sciatic nerve. These results indicate that activation of the dorsal horn NMDA-2B receptors may be crucial for the spinal nociceptive synaptic transmission and for the development of long-lasting spinal hyperexcitability following nerve injury. In conclusion, the spinal cord NMDA-2B receptors play a role in the development of central sensitization and neuropathic pain via the induction of LTP in dorsal horn nociceptive synaptic transmission. Therefore, the spinal cord NMDA-2B receptor is likely to be a target for clinical pain therapy.

Role of serotonin 5-HT1A and opioid receptors in the antiallodynic effect of tramadol in the chronic constriction injury model of neuropathic pain in rats

RATIONALE

Tramadol (1RS, 2RS)-2-[(dimethylamino)-methyl]-1-(3-methoxyphenyl)-cyclohexanol) is an atypical centrally acting analgesic agent with weak opioid receptor affinity that, like some antidepressants, enhances the extraneuronal concentrations of the monoamine neurotransmitters, noradrenaline and serotonin, by interfering with their re-uptake and release mechanisms.

OBJECTIVES

The present study was undertaken to evaluate the potential role of 5-HT(1A) receptors and opioids receptors in the analgesic effect of tramadol in neuropathic pain. With this aim, the effect of either a selective 5-HT(1A) receptor antagonist (WAY-100635, N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT(1A) receptor agonist (8-OH-DPAT, 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) or an opioid receptor antagonist (naloxone; naloxone hydrochloride dihydrate) was investigated in combination with tramadol by means of the cold-plate test in the chronic constriction injury model in rats.

RESULTS

The results showed that WAY-100635 (0.8 mg/kg) significantly enhanced the antiallodynic effect of non-effective doses of tramadol (5-10 mg/kg). In contrast, 8-OH-DPAT (0.5 mg/kg) counteracted the antiallodynic effect of an effective dose of tramadol (22 mg/kg). Naloxone (0.5 mg/kg) partially counteracted the antiallodynic effect of tramadol (22 mg/kg).

CONCLUSIONS

These findings suggest the involvement of opioid and 5-HT(1A) receptors in the antinociceptive effect of tramadol and support the idea that the combination of tramadol with compounds having 5-HT(1A) antagonist properties could be a new strategy to improve tramadol-induced analgesia in neuropathic pain.

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

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

Distinct effects of D-serine on spinal nociceptive responses in normal and carrageenan-injected rats

Single unit extracellular recordings from dorsal horn neurons were performed with glass micropipettes in pentobarbital-anesthetized rats. A total of 60 wide dynamic range (WDR) neurons were obtained from 34 rats. In normal rats (20/34), spinally administered D-serine (10 nmol), a putative endogenous agonist of glycine site of NMDA receptors, significantly enhanced the C- but not Abeta-, and Adelta-fiber responses of WDR neurons in the spinal dorsal horn. When 1 nmol of the glycine site antagonist 7-chlorokynurenic acid (7-CK) was co-administered with 10 nmol D-serine, the facilitation of D-serine on C-fiber response was completely blocked. 7-CK (1 nmol) alone failed to influence Abeta-, Adelta-, and C-fiber responses of WDR neurons. In contrast, in carrageenan-injected rats (14/34), 10 nmol D-serine had no effect on C-fiber response, while 1 nmol 7-CK per se markedly depressed C-fiber response of WDR neurons. These findings suggest that under physiological conditions, glycine sites in the spinal cord were available but became saturated following peripheral inflammation. Thus, increased endogenous d-serine or glycine may be involved in nociceptive transmission by modulating NMDA receptor activities. The glycine site of NMDA receptors may become a target for the prevention of inflammatory pain.

5-HT1A receptors in endogenous regulation of neuropathic hypersensitivity in the rat

The role of medullary and spinal 5-HT(1A) receptors in endogenous regulation of neuropathic hypersensitivity was studied. When administered in the rostroventromedial medulla or subcutaneously, WAY-100635, a 5-HT(1A) receptor antagonist, attenuated mechanical hypersensitivity in rats with a spinal nerve injury. Thermal or mechanical nociception outside of the injured area was not influenced by medial medullary or subcutaneous administration of WAY-100635. Intrathecal administration of WAY-100635 had no significant effect on pain-related behavior. Suppression of mechanical hypersensitivity induced by medial medullary administration of WAY-100635 was reversed by intrathecal administration of WAY-100635 or atipamezole, an alpha2-adrenoceptor antagonist, but not by naloxone, an opioid receptor antagonist. The results indicate that endogenous release of 5-HT, via action on medial medullary 5-HT(1A) receptors, tonically suppresses descending inhibition in neuropathic animals. Following medial medullary administration of a 5-HT(1A) receptor antagonist, descending pain regulatory pathways are disinhibited. This leads to selective attenuation of neuropathic hypersensitivity, due to action on spinal 5-HT(1A) receptors and alpha2-adrenoceptors.

Effects of the high-efficacy 5-HT1A receptor agonist, F 13640 in the formalin pain model: a c-Fos study

We studied the effects of the high-efficacy 5-hydroxytryptamine1A (5-HT1A) receptor agonist, F 13640 on both formalin-induced spinal cord c-Fos protein expression and pain behaviours in the rat. Replicating earlier data, F 13640 (0.63 mg/kg, i.p.; t(-15 min)) completely inhibited the elevation and licking of the formalin-injected paw. In the same animals, and in spite of the agent as in earlier data increasing the number of c-Fos labelled nuclei when it was administered alone, F 13640 markedly reduced the number of formalin-induced c-Fos labelled nuclei. This was found in both the superficial (I-II) and deep (V-VI) dorsal horn laminae (2 h post-injection: 72+/-2% and 92+/-1% of reduction, respectively; P<0.001 in either case), spinal areas that contain neurons responsive to nociceptive stimulation. Co-operation occurred so that after the co-administration of F 13640 and formalin, c-Fos expression was inferior to that induced when either stimulation was administered alone. The data provide initial evidence for the agent's inhibitory effects on noxiously evoked c-Fos expression. The results indicate that co-operation between 5-HT1A receptor activation and nociceptive stimulation powerfully inhibits responses to severe, tonic nociception.

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

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

The novel analgesic and high-efficacy 5-HT1A receptor agonist F 13640 inhibits nociceptive responses, wind-up, and after-discharges in spinal neurons and withdrawal reflexes

Evidence shows that serotonin (5-HT) is involved in the transmission of nociception in the central nervous system. Using a new electrophysiological method of simultaneous recordings in rats we examined the actions of the novel analgesic and high-efficacy 5-HT1A receptor agonist F 13640 as well as those of the opioid receptor agonist fentanyl on simultaneously evoked responses of spinal dorsal horn (DH) wide-dynamic range (WDR) neurons and spinal withdrawal reflexes. Spinal withdrawal reflexes were studied by assessing the activity of single motor units (SMUs) electromyographically (EMG). Like that of 0.02 mg/kg fentanyl, intraperitoneal injection of 0.31 mg/kg of F 13640 markedly inhibited nociceptive pinch-evoked responses as well as C-fiber-mediated late responses including wind-up of both DH WDR neurons and SMUs to suprathreshold (1.5 x T) repeated (3 Hz) electrical stimulation. Specifically, in contrast to no significant depressive effects by fentanyl on 20 Hz electrically evoked after-discharge of DH WDR neurons, the after-discharges of DH WDR neurons and SMUs were significantly inhibited by F 13640 (P < 0.05 and P < 0.001, respectively). The inhibitory effects of F 13640 and fentanyl on responses of DH WDR neurons and SMUs were reversed by the specific antagonists WAY 100635 and naloxone, respectively, further indicating that this 5-HT1A receptor-modulated anti-nociception is mu-opioid receptor independent. For the first time, 5-HT1A receptors are clearly proved to be involved in the progressive wind-up to 3-Hz frequency of electrical stimulation as well as after-discharges of sensory input of DH WDR neurons, and simultaneously recorded motor output of spinal reflexes to 20-Hz frequency of electrical stimulation; this suggests that serotonin, through 5-HT1A receptors, exerts an inhibitory role in the control of obstinate pathological pain.

Activation of Cannabinoid CB2Receptors Suppresses C-Fiber Responses and Windup in Spinal Wide Dynamic Range Neurons in the Absence and Presence of Inflammation

Effects of the CB2-selective cannabinoid agonist AM1241 on activity evoked in spinal wide dynamic range (WDR) neurons by transcutaneous electrical stimulation were evaluated in urethane-anesthetized rats. Recordings were obtained in both the absence and the presence of carrageenan inflammation. AM1241, administered intravenously or locally in the paw, suppressed activity evoked by transcutaneous electrical stimulation during the development of inflammation. Decreases in WDR responses resulted from a suppression of C-fiber–mediated activity and windup. Aβ- and Aδ-fiber–mediated responses were not reliably altered. The AM1241-induced suppression of electrically evoked responses was blocked by the CB2antagonist SR144528 but not by the CB1antagonist SR141716A. AM1241 (33 μg/kg intraplantar [ipl]), administered to the carrageenan-injected paw, suppressed activity evoked in WDR neurons relative to groups receiving vehicle in the same paw or AM1241 in the opposite (noninflamed) paw. The electrophysiological effects of AM1241 (330 μg/kg intravenous [iv]) were greater in rats receiving ipl carrageenan compared with noninflamed rats receiving an ipl injection of vehicle. AM1241 failed to alter the activity of purely nonnociceptive neurons recorded in the lumbar dorsal horn. Additionally, AM1241 (330 μg/kg iv and ipl; 33 μg/kg ipl) reduced the diameter of the carrageenan-injected paw. The AM1241-induced decrease in peripheral edema was blocked by the CB2but not by the CB1antagonist. These data demonstrate that activation of cannabinoid CB2receptors is sufficient to suppress neuronal activity at central levels of processing in the spinal dorsal horn. Our findings are consistent with the ability of AM1241 to normalize nociceptive thresholds and produce antinociception in inflammatory pain states.

5-hydroxytryptamine1A receptor is involved in the bee venom induced inflammatory pain

Injection of bee venom into one hindpaw of rat can elicit acute inflammation together with spontaneous pain, heat hyperalgesia and mechanical hyperalgesia/allodynia in the injected paw. 5-hydroxytryptamine (5-HT)1A receptor is the predominant receptor subtype in the spinal dorsal horn mediating the function of 5-HT in nociception. The goal of the present study is to assess the role of 5-HT1A receptor in the pain associated with the bee venom induced inflammation. Here we showed that 1 or 4 h after a subcutaneous bee venom challenge, expression of 5-HT1A receptor mRNA in the ipsilateral lumbar spinal cord increased significantly by 80.94 or 37.86%, respectively. Antisense oligodeoxynucleotide knockdown of spinal 5-HT1A receptor attenuated spontaneous pain and reversed heat hyperalgesia in rats injected with bee venom. Thus, the present data suggest a facilitating role for 5-HT1A receptor in bee venom induced inflammatory pain.

Joint manipulation reduces hyperalgesia by activation of monoamine receptors but not opioid or GABA receptors in the spinal cord

Joint manipulation has long been used for pain relief. However, the underlying mechanisms for manipulation-related pain relief remain largely unexplored. The purpose of the current study was to determine which spinal neurotransmitter receptors mediate manipulation-induced antihyperalgesia. Rats were injected with capsaicin (50 microl, 0.2%) into one ankle joint and mechanical withdrawal threshold measured before and after injection. The mechanical withdrawal threshold decreases 2 h after capsaicin injection. Two hours after capsaicin injection, the following drugs were administered intrathecally: bicuculline, blocks gamma-aminobutyric acid (GABAA) receptors; naloxone, blocks opioid receptors; yohimbine blocks, alpha2-adrenergic receptors; and methysergide, blocks 5-HT(1/2) receptors. In addition, NAN-190, ketanserin, and MDL-72222 were administered to selectively block 5-HT1A, 5-HT2A, and 5-HT3 receptors, respectively. Knee joint manipulation was performed 15 min after administration of drug. The knee joint was flexed and extended to end range of extension while the tibia was simultaneously translated in an anterior to posterior direction. The treatment group received three applications of manipulation, each 3 min in duration separated by 1 min of rest. Knee joint manipulation after capsaicin injection into the ankle joint significantly increases the mechanical withdrawal threshold for 45 min after treatment. Spinal blockade of 5-HT(1/2) receptors with methysergide prevented, while blockade of alpha2-adrenergic receptors attenuated, the manipulation-induced antihyperalgesia. NAN-190 also blocked manipulation-induced antihyperalgesia suggesting that effects of methysergide are mediated by 5-HT1A receptor blockade. However, spinal blockade of opioid or GABAA receptors had no effect on manipulation induced-antihyperalgesia. Thus, the antihyperalgesia produced by joint manipulation appears to involve descending inhibitory mechanisms that utilize serotonin and noradrenaline.

Expression of 5-HT1A receptor mRNA in rat lumbar spinal dorsal horn neurons after peripheral inflammation

The present study observed the expression of the 5-hydroxytryptamine (5-HT) (1A) receptor mRNA in the lumbar spinal dorsal horn neurons following carrageenan inflammation using in situ hybridization (ISH). We also studied the co-localization of 5-HT(1A) receptor mRNA and gamma-amino butyric acid (GABA) or enkephalin (ENK) immunoreactivities using a combined fluorescent ISH and immunofluorescent histochemical double-staining technique. The finding of this study demonstrated that 5-HT(1A) receptor mRNA was widely distributed in the spinal dorsal horn with the highest density in laminae III-VI. Following carrageenan-induced inflammation, the 5-HT(1A) receptor mRNA expression in all layers of ipsilateral dorsal horn was significantly enhanced, and the peak occurred after 8h. Furthermore, the number of 5-HT(1A) receptor mRNA and GABA or ENK immunoreactive double-labeled cells was also markedly increased 8h after carrageenan injection. These findings suggested that following peripheral inflammation, the synthesis of 5-HT(1A) receptor was increased in the lumbar spinal dorsal horn neurons, especially in spinal GABA and ENK neurons.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.