Noradrenergic inhibition of the release of substance P from the primary afferents in the rabbit spinal dorsal horn

Novel drug treatments for pain in advanced cancer and serious illness: a focus on neuropathic pain and chemotherapy-induced peripheral neuropathy

Drugs that are commercially available but have novel mechanisms of action should be explored as analgesics. This review will discuss haloperidol, miragabalin, palmitoylethanolamide (PEA), and clonidine as adjuvant analgesics or analgesics. Haloperidol is a sigma-1 receptor antagonist. Under stress and neuropathic injury, sigma-1 receptors act as a chaperone protein, which downmodulates opioid receptor activities and opens several ion channels. Clinically, there is only low-grade evidence that haloperidol improves pain when combined with morphine, methadone, or tramadol in patients who have cancer, pain from fibrosis, radiation necrosis, or neuropathic pain. Miragabalin is a gabapentinoid approved for the treatment of neuropathic pain in Japan since 2019. In randomized trials, patients with diabetic neuropathy have responded to miragabalin. Its long binding half-life on the calcium channel subunit may provide an advantage over other gabapentinoids. PEA belongs to a group of endogenous bioactive lipids called ALIAmides (autocoid local injury antagonist amides), which have a sense role in modulating numerous biological processes in particular non-neuronal neuroinflammatory responses to neuropathic injury and systemic inflammation. Multiple randomized trials and meta-analyses have demonstrated PEA's effectiveness in reducing pain severity arising from diverse pain phenotypes. Clonidine is an alpha2 adrenoceptor agonist and an imidazoline2 receptor agonist, which is U.S. Federal Drug Administration approved for attention deficit hyperactivity disorder in children, Tourette's syndrome, adjunctive therapy for cancer-related pain, and hypertension. Clonidine activation at alpha2 adrenoceptors causes downstream activation of inhibitory G-proteins (Gi/Go), which inhibits cyclic Adenosine monophosphate (AMP) production and hyperpolarizes neuron membranes, thus reducing allodynia. Intravenous clonidine has been used in terminally ill patients with poorly controlled symptoms, in particular pain and agitation.

A Comparative Study of the Antiemetic Effects of α2-Adrenergic Receptor Agonists Clonidine and Dexmedetomidine against Diverse Emetogens in the Least Shrew (Cryptotis parva) Model of Emesis

In contrast to cats and dogs, here we report that the α2-adrenergic receptor antagonist yohimbine is emetic and corresponding agonists clonidine and dexmedetomidine behave as antiemetics in the least shrew model of vomiting. Yohimbine (0, 0.5, 0.75, 1, 1.5, 2, and 3 mg/kg, i.p.) caused vomiting in shrews in a bell-shaped and dose-dependent manner, with a maximum frequency (0.85 ± 0.22) at 1 mg/kg, which was accompanied by a key central contribution as indicated by increased expression of c-fos, serotonin and substance P release in the shrew brainstem emetic nuclei. Our comparative study in shrews demonstrates that clonidine (0, 0.1, 1, 5, and 10 mg/kg, i.p.) and dexmedetomidine (0, 0.01, 0.05, and 0.1 mg/kg, i.p.) not only suppress yohimbine (1 mg/kg, i.p.)-evoked vomiting in a dose-dependent manner, but also display broad-spectrum antiemetic effects against diverse well-known emetogens, including 2-Methyl-5-HT, GR73632, McN-A-343, quinpirole, FPL64176, SR141716A, thapsigargin, rolipram, and ZD7288. The antiemetic inhibitory ID50 values of dexmedetomidine against the evoked emetogens are much lower than those of clonidine. At its antiemetic doses, clonidine decreased shrews' locomotor activity parameters (distance moved and rearing), whereas dexmedetomidine did not do so. The results suggest that dexmedetomidine represents a better candidate for antiemetic potential with advantages over clonidine.

Applications of Dexmedetomidine in Palliative and Hospice Care

Although the use of dexmedetomidine is currently approved by the US Food and Drug Administration in the adult population for monitored anesthesia care and sedation during mechanical ventilation, clinical experience suggests the potential application of dexmedetomidine in the palliative care arena. The medication can provide sedation with lower risk of delirium, control or minimize the adverse effects of other medications, and augment analgesia from opioids. We conducted a computerized bibliographic search of the literature regarding dexmedetomidine use for the treatment of pain and provision of sedation during palliative and hospice care in adult and pediatric patients. The objective was to provide a general descriptive account of the physiologic effects of dexmedetomidine and review its potential applications in the field of palliative and hospice care in adult and pediatric patients. The sedative and analgesic effects of dexmedetomidine have been well studied in animal and human models. Published experience from both single case reports and small case series has demonstrated the potential therapeutic applications of dexmedetomidine in palliative and hospice care. In addition to intravenous administration, case reports have demonstrated its successful use by both the intranasal and subcutaneous routes. Although these experiences have suggested its safety and efficacy, larger series and additional clinical experience with prospective comparison to other agents are needed to further define its efficacy and role in palliative and hospice care.

Recombinant peptide derived from the venom the Phoneutria nigriventer spider relieves nociception by nerve deafferentation

The avulsion of nerve roots of the brachial plexus that is commonly seen in motorcycle accidents is a type of neuropathy due to deafferentation. This type of pain is clinically challenging since therapeutical protocols fail or have severe side effects. Thus, it is proposed to evaluate the antinociceptive activity of the recombinant CTK 01512-2 peptide that is derived from the venom of the Phoneutria nigriventer spider, as a future new therapeutical option. The neuropathic pain was surgically induced by avulsion of the upper brachial plexus trunk in groups of male Wistar rats and after 17 days, they were treated intrathecally with morphine, ziconotide, and CTK 01512-2. Behavioral tests were performed to evaluate mechanical and thermal hyperalgesia, cold allodynia, the functional activity of the front paw, and exploratory locomotion after the treatments. The peripheral blood samples were collected 6 h after the treatments and a comet assay was performed. The spinal cord was removed for the lipoperoxidation dosing of the membranes. The cerebrospinal fluid was analyzed for the dosage of glutamate. The recombinant peptide showed an antinociceptive effect when compared to the other drugs, without affecting the locomotor activity of the animals. Mechanical and thermal hyperalgesia, as well as cold allodynia, were reduced in the first hours of treatment. The levels of glutamate and the damage by membrane lipoperoxidation were shown to be improved, and genotoxicity was not demonstrated. In a scenario of therapeutical failures in the treatment of this type of pain, CTK 01512-2 was shown as a new effective alternative protocol. However, further testing is required to determine pharmacokinetics.

Managing Procedural Pain in the Neonate Using an Opioid-sparing Approach

PURPOSE

Pain in the neonate is often challenging to assess but important to control. Physicians often must balance the need for optimal pain control with the need to minimize oversedation and prolonged opioid use. Both inadequate pain control and overuse of opioids can have long-term consequences, including poor developmental outcomes. The aim of this review is to introduce a comprehensive approach to pain management for physicians, nurses, and surgeons caring for critically ill neonates, focusing on nonopioid alternatives to manage procedural pain.

FINDINGS

After review, categories of opioid-sparing interventions identified included (1) nonopioid pharmacologic agents, (2) local and regional anesthesia, and (3) nonpharmacologic alternatives. Nonopioid pharmacologic agents identified for neonatal use included acetaminophen, NSAIDs, dexmedetomidine, and gabapentin. Local and regional anesthesia included neuraxial blockade (spinals and epidurals), subcutaneous injections, and topical anesthesia. Nonpharmacologic agents uniquely available in the neonatal setting included skin-to-skin care, facilitated tucking, sucrose, breastfeeding, and nonnutritive sucking.

IMPLICATIONS

The use of various pharmacologic and interventional treatments for neonatal pain management allows for the incorporation of opioid-sparing techniques in neonates who are already at risk for poor neurodevelopmental outcomes. A multifactorial approach to pain control is paramount to optimize periprocedural comfort and to minimize the negative sequelae of uncontrolled pain in the neonate.

Increased levels of substance P in patients taking beta-blockers are linked with a protective effect on oropharyngeal dysphagia

BACKGROUND

We have recently found a potential protective effect of beta-blockers on oropharyngeal dysphagia (OD). The action mechanism by which beta-blockers could wield this protective effect is unknown, but the neurotransmitter substance P (SP) could play a key role. The aim of this study was to analyze serum and saliva SP levels in patients taking beta-blockers (TBB), and to explore its relationship with OD.

METHODS

Adult (>50 year) patients TBB were randomly recruited from the primary care setting and 1:1 matched by age, sex, and Barthel Index (BI) with patients not taking beta-blockers (NTBB). Serum and saliva samples were taken and analyzed for their SP levels using an enzyme-linked immunosorbent assay (ELISA). Socio-demographic and clinical variables were collected. Dysphagia was evaluated in all patients using the clinical volume-viscosity swallow test (V-VST).

KEY RESULTS

We studied 28 patients TBB (64.96 ± 7.31 years, 57.1% women, BI 99.6 ± 1.31, carvedilol-equivalent dose 24.11 ± 18.12 mg) and 28 patients NTBB (65.61 ± 6.43 years, 57.1% women, BI 99.6 ± 1.31). SP serum levels were significantly higher in patients TBB (260.68 ± 144.27 vs 175.46 ± 108.36 pg/mL, P = .009) as were SP saliva levels (170.34 ± 146.48 vs 102.73 ± 52.28 pg/mL, P < .001) compared with patients NTBB. The prevalence of OD was 32.1% in patients TBB and 67.9% in patients NTBB (P = .015). Moreover, patients with OD had significantly lower SP saliva levels in comparison with patients without clinical signs of OD (98.39 ± 43.25 vs 174.69 ± 147.21 pg/mL) P < .001.

CONCLUSIONS & INFERENCES

We have found that serum and saliva SP levels are greater in patients TBB. This increase in SP levels could be the action mechanism by which beta-blockers protect patients from OD.

An updated understanding of the mechanisms involved in chemotherapy-induced neuropathy

The burdensome condition of chemotherapy-induced peripheral neuropathy occurs with various chemotherapeutics, including bortezomib, oxaliplatin, paclitaxel and vincristine. The symptoms, which include pain, numbness, tingling and loss of motor function, can result in therapy titrations that compromise therapy efficacy. Understanding the mechanisms of chemotherapy-induced peripheral neuropathy is therefore essential, yet incompletely understood. The literature presented here will address a multitude of molecular and cellular mechanisms, beginning with the most well-understood cellular and molecular-level changes. These modifications include alterations in voltage-gated ion channels, neurochemical transmission, organelle function and intracellular pathways. System-level alterations, including changes to glial cells and cytokine activation are also explored. Finally, we present research on the current understanding of genetic contributions to this condition. Suggestions for future research are provided.

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

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

Neuroprotection and neurotoxicity in the developing brain: an update on the effects of dexmedetomidine and xenon

Growing and consistent preclinical evidence, combined with early clinical epidemiological observations, suggest potentially neurotoxic effects of commonly used anesthetic agents in the developing brain. This has prompted the FDA to issue a safety warning for all sedatives and anesthetics approved for use in children under three years of age. Recent studies have identified dexmedetomidine, the potent α2-adrenoceptor agonist, and xenon, the noble gas, as effective anesthetic adjuvants that are both less neurotoxic to the developing brain, and also possess neuroprotective properties in neonatal and other settings of acute ongoing neurologic injury. Dexmedetomidine and xenon are effective anesthetic adjuvants that appear to be less neurotoxic than other existing agents and have the potential to be neuroprotective in the neonatal and pediatric settings. Although results from recent clinical trials and case reports have indicated the neuroprotective potential of xenon and dexmedetomidine, additional randomized clinical trials corroborating these studies are necessary. By reviewing both the existing preclinical and clinical evidence on the neuroprotective effects of dexmedetomidine and xenon, we hope to provide insight into the potential clinical efficacy of these agents in the management of pediatric surgical patients.

Fibromyalgia syndrome pathology and environmental influences on afflictions with medically unexplained symptoms

Fibromyalgia syndrome (FMS) is a clinical disorder predominant in females with unknown etiology and medically unexplained symptoms (MUS), similar to other afflictions, including irritable bowel syndrome (IBS), chronic fatigue syndrome (CFS), post-traumatic stress disorder (PTSD), Gulf War illness (GFI), and others. External environmental stimuli drive behavior and impact physiologic homeostasis (internal environment) via autonomic functioning. These environments directly impact the individual affective state (mind), which feeds back to regulate physiology (body). FMS has emerged as a complex disorder with pathologies identified among neurotransmitter and enzyme levels, immune/cytokine functionality, cortical volumes, cutaneous innervation, as well as an increased frequency among people with a history of traumatic and/or emotionally negative events, and specific personality trait profiles. Yet, quantitative physical evidence of pathology or disease etiology among FMS has been limited (as with other afflictions with MUS). Previously, our group published findings of increased peptidergic sensory innervation associated with the arterio-venous shunts (AVS) in the glabrous hand skin of FMS patients, which provides a plausible mechanism for the wide-spread FMS symptomology. This review focuses on FMS as a model affliction with MUS to discuss the implications of the recently discovered peripheral innervation alterations, explore the role of peripheral innervation to central sensitization syndromes (CSS), and examine possible estrogen-related mechanisms through which external and internal environmental factors may contribute to FMS etiology and possibly other afflictions with MUS.

Involvement of spinal α2 -adrenoceptors in prolonged modulation of hind limb withdrawal reflexes following acute noxious stimulation in the anaesthetized rabbit

The role of spinal α₂‐adrenoceptors in mediating long‐lasting modulation of hind limb withdrawal reflexes following acute noxious chemical stimulation of distant heterotopic and local homotopic locations has been investigated in pentobarbitone‐anaesthetized rabbits. Reflexes evoked in the ankle extensor muscle medial gastrocnemius (MG) by electrical stimulation of the ipsilateral heel, and reflexes elicited in the ankle flexor tibialis anterior and the knee flexor semitendinosus by stimulation at the base of the ipsilateral toes, could be inhibited for over 1 h after mustard oil (20%) was applied to either the snout or into the contralateral MG. The heel–MG response was also inhibited after applying mustard oil across the plantar metatarsophalangeal joints of the ipsilateral foot, whereas this homotopic stimulus facilitated both flexor responses. Mustard oil also caused a significant pressor effect when applied to any of the three test sites. The selective α₂‐adrenoceptor antagonist, RX 821002 (100–300 μg, intrathecally), had no effect on reflexes per se, but did cause a decrease in mean arterial blood pressure. In the presence of the α₂‐blocker, inhibitory and facilitatory effects of mustard oil on reflexes were completely abolished. These data imply that long‐lasting inhibition of spinal reflexes following acute noxious stimulation of distant locations involves activation of supraspinal noradrenergic pathways, the effects of which are dependent on an intact α₂‐adrenoceptor system at the spinal level. These pathways and receptors also appear to be involved in facilitation (sensitization) as well as inhibition of reflexes following a noxious stimulus applied to the same limb.

Clonidine, an alpha-2 adrenoceptor agonist relieves mechanical allodynia in oxaliplatin-induced neuropathic mice; potentiation by spinal p38 MAPK inhibition without motor dysfunction and hypotension

Cancer chemotherapy with platinum-based antineoplastic agents including oxaliplatin frequently results in a debilitating and painful peripheral neuropathy. We evaluated the antinociceptive effects of the alpha-2 adrenoceptor agonist, clonidine on oxaliplatin-induced neuropathic pain. Specifically, we determined if (i) the intraperitoneal (i.p.) injection of clonidine reduces mechanical allodynia in mice with an oxaliplatin-induced neuropathy and (ii) concurrent inhibition of p38 mitogen-activated protein kinase (MAPK) activity by the p38 MAPK inhibitor SB203580 enhances clonidine's antiallodynic effect. Clonidine (0.01-0.1 mg kg(-1), i.p.), with or without SB203580(1-10 nmol, intrathecal) was administered two weeks after oxaliplatin injection(10 mg kg(-1), i.p.) to mice. Mechanical withdrawal threshold, motor coordination and blood pressure were measured. Postmortem expression of p38 MAPK and ERK as well as their phosphorylated forms(p-p38 and p-ERK) were quantified 30 min or 4 hr after drug injection in the spinal cord dorsal horn of treated and control mice. Clonidine dose-dependently reduced oxaliplatin-induced mechanical allodynia and spinal p-p38 MAPK expression, but not p-ERK. At 0.1 mg kg(-1), clonidine also impaired motor coordination and decreased blood pressure. A 10 nmol dose of SB203580 alone significantly reduced mechanical allodynia and p-p38 MAPK expression, while a subeffective dose(3 nmol) potentiated the antiallodynic effect of 0.03 mg kg(-1) clonidine and reduced the increased p-p38 MAPK. Coadministration of SB203580 and 0.03 mg kg(-1) clonidine decreased allodynia similar to that of 0.10 mg kg(-1) clonidine, but without significant motor or vascular effects. These findings demonstrate that clonidine treatment reduces oxaliplatin-induced mechanical allodynia. The concurrent administration of SB203580 reduces the dosage requirements for clonidine, thereby alleviating allodynia without producing undesirable motor or cardiovascular effects.

Clonidine Reduces Nociceptive Responses in Mouse Orofacial Formalin Model: Potentiation by Sigma-1 Receptor Antagonist BD1047 without Impaired Motor Coordination

Although the administration of clonidine, an alpha-2 adrenoceptor agonist, significantly attenuates nociception and hyperalgesia in several pain models, clinical trials of clonidine are limited by its side effects such as drowsiness, hypotension and sedation. Recently, we determined that the sigma-1 receptor antagonist BD1047 dose-dependently reduced nociceptive responses in a mouse orofacial formalin model. Here we examined whether intraperitoneal injection of clonidine suppressed the nociceptive responses in the orofacial formalin test, and whether co-administration with BD1047 enhances lower-dose clonidine-induced anti-nociceptive effects without the disruption of motor coordination and blood pressure. Formalin (5%, 10 µL) was subcutaneously injected into the right upper lip, and the rubbing responses with the ipsilateral fore- or hind-paw were counted for 45 min. Clonidine (10, 30 or 100 µg/kg) was intraperitoneally administered 30 min before formalin injection. Clonidine alone dose-dependently reduced nociceptive responses in both the first and second phases. Co-localization for alpha-2A adrenoceptors and sigma-1 receptors was determined in trigeminal ganglion cells. Interestingly, the sub-effective dose of BD1047 (3 mg/kg) significantly potentiated the anti-nociceptive effect of lower-dose clonidine (10 or 30 µg/kg) in the second phase. In particular, the middle dose of clonidine (30 µg/kg) in combination with BD1047 produced an anti-nociceptive effect similar to that of the high-dose clonidine, but without a significant motor dysfunction or hypotension. In contrast, mice treated with the high dose of clonidine developed severe impairment in motor coordination and blood pressure. These data suggest that a combination of low-dose clonidine with BD1047 may be a novel and safe therapeutic strategy for orofacial pain management.

Analgesic topical capsaicinoid therapy increases somatostatin-like immunoreactivity in the human plasma

The aim of the present study was to evaluate the therapeutic potential of local capsaicinoid (EMSPOMA(®) cream) treatment on chronic low back pain in patients with degenerative spine diseases and to investigate the possible mechanism of action of the therapy. The qualitative and quantitative analyses of capsaicinoids in EMSPOMA(®) cream were performed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). In the clinical study 20 patients with degenerative spine diseases were involved in a self-controlled examination. During the 21 day therapy they received 30 min daily treatment with capsaicinoid (EMSPOMA(®)) cream to the lumbar region of the back. The pain (VASs, Oswestry Disability Index) and the mobility of the lumbar region of the spine (Schober's, Domján's L and R test) were detected at baseline and at the end of the 1st, 2nd and 3rd weeks. The plasma level of somatostatin-like immunoreactivity (SST-LI) was measured by radioimmunoassay (RIA) before and after the treatment on the first and the last day of the therapy. Nonivamide (0.01%) was identified as the only capsaicinoid molecule in the cream. In the clinical study the 21 day local nonivamide treatment reduced the pain sensation. Oswestry Disability Index decreased from 39 ± 3.9% to 32.5 ± 4.4%. VASs showed 37.29%-59.51% improvement. In the plasma level of SST-LI threefold elevation was observed after the first nonivamide treatment. We conclude that nonivamide treatment exerts analgesic action in chronic low back pain and causes the release of the antinociceptive and anti-inflammatory neuropeptide somatostatin which may play pivotal role in the pain-relieving effect.

[A memoir of my research on pain and analgesia for 39 years]

This review describes my research for the past 39 years regarding the pharmacology of pain and analgesia. We have demonstrated that the descending noradrenergic system is involved in the analgesic effect of morphine injected into the nucleus reticularis gigantocellularis, and that noradrenaline exerts antinociception mediated by α-adrenoceptors. We have found that noxious mechanical and thermal stimuli to the skin increase the release of substance P and somatostatin, respectively, from the dorsal horn in situ, and that noradrenaline inhibits the release of substance P and glutamate from primary afferents. We developed an animal model of cancer pain using melanoma cells. We have shown that the suppression of cancer pain results in the inhibition of tumor growth and lung metastasis, and that melanoma cells release several algogenic substances including ATP, endothelin-1, and bradykinin. We investigated neuropathic allodynia induced by the chemotherapeutic drugs paciltaxel, oxaliplatin, vincristine, and bortezomib. Single administration of these drugs caused allodynia with similar time-courses. However, antiallodynic actions of adjuvant analgesics, including gabapentin and limaprost, were dependent on the chemotherapeutic drugs used. Limaprost experiments have revealed that a decrease in peripheral blood flow is involved in allodynia exacerbation after the administration of paciltaxel and oxaliplatin. We have developed animal models of herpetic pain and postherpetic neuralgia using herpes simplex virus 1. We have demonstrated that nitric oxide, prostaglandin E2, and galectin-3 are involved in herpetic allodynia, that risk factors associated with postherpetic allodynia include severe herpetic pain, nociceptin, and major histocompatibility complex, and that deafferentation and nitric oxide are involved in postherpetic allodynia.

Effects of the selective sigma-1 receptor antagonist S1RA on formalin-induced pain behavior and neurotransmitter release in the spinal cord in rats

We have previously shown that the selective sigma-1 receptor (σ1 R) antagonist S1RA (E-52862) inhibits neuropathic pain and activity-induced spinal sensitization in various pre-clinical pain models. In this study we characterized both the behavioral and the spinal neurochemical effects of S1RA in the rat formalin test. Systemic administration of S1RA produced a dose-related attenuation of flinching and lifting/licking behaviors in the formalin test. Neurochemical studies using concentric microdialysis in the ipsilateral dorsal horn of awake, freely moving rats revealed that the systemic S1RA-induced antinociceptive effect occurs concomitantly with an enhancement of noradrenaline levels and an attenuation of formalin-evoked glutamate release in the spinal dorsal horn. Intrathecal pre-treatment with idazoxan prevented the systemic S1RA antinociceptive effect, suggesting that the S1RA antinociception depends on the activation of spinal α2 -adrenoceptors which, in turn, could induce an inhibition of formalin-evoked glutamate release. When administered locally, intrathecal S1RA inhibited only the flinching behavior, whereas intracerebroventricularly or intraplantarly injected also attenuated the lifting/licking behavior. These results suggest that S1RA supraspinally activates the descending noradrenergic pain inhibitory system, which may explain part of its antinociceptive properties in the formalin test; however, effects at other central and peripheral sites also account for the overall effect. Formalin-induced nociceptive effect occurs concomitantly with an enhancement of glutamate (Glu) level in the dorsal horn spinal cord. The selective σ1 receptor antagonist S1RA results in inhibition of formalin-evoked Glu release, no modification of GABA levels, and enhancement of noradrenaline (NA) levels. This increased spinal NA activates spinal α2-adrenoceptors producing the attenuation of the formalin-induced pain behaviour.

The antinociceptive effect of milnacipran in the monosodium iodoacetate model of osteoarthritis pain and its relation to changes in descending inhibition

Osteoarthritis (OA) is a chronic joint disorder whose principal symptom is chronic pain. Current analgesics are inadequate and the mechanisms contributing to this pain are poorly understood but likely to include both local joint changes and central consequences. These studies used monoamine receptor agents combined with behavioral studies and single-unit dorsal horn recordings to examine whether descending noradrenergic and serotonergic inhibitions are altered in the monosodium iodoacetate model of OA pain, and whether increasing these inhibitions with the serotonin/noradrenaline reuptake inhibitor milnacipran can attenuate the attendant hypersensitivity. Early and late in the course of this model, milnacipran (s.c.) reduced behavioral hypersensitivity, and inhibited evoked responses from sensitized dorsal horn neurons. In naïve animals and the early, but not late, phase of the model, spinal administration of the α(2)-adrenoceptor antagonist atipamezole fully reversed this neuronal inhibition, whereas atipamezole administered alone revealed that endogenous noradrenergic inhibition was reduced in the late phase. Blocking spinal 5-hydroxytryptamine-7 receptors with (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine hydrochloride suggested that the effects of milnacipran in the late phase were partly mediated by these receptors, and that descending serotonergic inhibition was increased in this phase. An opioidergic mechanism behind the effects of milnacipran was indicated by a partial reversal of these effects with naloxone. These studies demonstrate antinociceptive effects for milnacipran in a model of OA pain, whose effects come via descending serotonergic and noradrenergic, as well as opioidergic, pathways. Variations in the activity of these pathways over the course of this model may contribute to the presence of behavioral hypersensitivity and determine through which endogenous systems milnacipran exerts its effects.

The puerperium alters spinal cord plasticity following peripheral nerve injury

Tissue and nerve damage can result in chronic pain. Yet, chronic pain after cesarean delivery is remarkably rare in women and hypersensitivity from peripheral nerve injury in rats resolves rapidly if the injury occurs in the puerperium. Little is known regarding the mechanisms of this protection except for a reliance on central nervous system oxytocin signaling. Here we show that the density of inhibitory noradrenergic fibers in the spinal cord is greater when nerve injury is performed in rats during the puerperium, whereas the expression of the excitatory regulators dynorphin A and neuregulin-1 in the spinal cord is reduced. The puerperium did not alter spinal cord microgial and astrocyte activation. Astrocyte activation, as measured by glial fibrillary acidic protein (GFAP) expression, was not evident in female rats with injury, regardless of delivery status suggesting sex differences in spinal astrocyte activation after injury. These results suggest a change in the descending inhibitory/facilitating balance on spinal nociception neurotransmission during the puerperium, as mechanisms for its protective effect against injury-induced hypersensitivity.

Tonic inhibition of allergic itch signaling by the descending noradrenergic system in mice

We investigated whether the descending noradrenergic system regulates allergic itch. Mosquito allergy of the hind paw elicited biting, an itch-related response, in sensitized mice. The biting was inhibited by intrathecal clonidine and reversed by yohimbine, an α(2)-adrenoceptor antagonist. The biting was increased by intrathecal pretreatment with the catecholaminergic neurotoxin 6-hydroxydopamine and the α-adrenoceptor antagonist phentolamine but not the serotonergic neurotoxin 5,7-dihydroxytryptamine. We propose that α(2)-adrenoceptors are involved in the inhibition of allergic itch in the spinal cord and that the descending noradrenergic system exerts a tonic inhibition on the itch signaling. The serotonergic system may not be involved.

Src family kinases mediate the inhibition of substance P release in the rat spinal cord by μ-opioid receptors and GABA(B) receptors, but not α2 adrenergic receptors

GABA(B) , μ-opioid and adrenergic α(2) receptors inhibit substance P release from primary afferent terminals in the dorsal horn. Studies in cell expression systems suggest that μ-opioid and GABA(B) receptors inhibit transmitter release from primary afferents by activating Src family kinases (SFKs), which then phosphorylate and inhibit voltage-gated calcium channels. This study investigated whether SFKs mediate the inhibition of substance P release by these three receptors. Substance P release was measured as neurokinin 1 receptor (NK1R) internalization in spinal cord slices and in vivo. In slices, NK1R internalization induced by high-frequency dorsal root stimulation was inhibited by the μ-opioid agonist DAMGO and the GABA(B) agonist baclofen. This inhibition was reversed by the SFK inhibitor PP1. NK1R internalization induced by low-frequency stimulation was also inhibited by DAMGO, but PP1 did not reverse this effect. In vivo, NK1R internalization induced by noxious mechanical stimulation of the hind paw was inhibited by intrathecal DAMGO and baclofen. This inhibition was reversed by intrathecal PP1, but not by the inactive PP1 analog PP3. PP1 produced no effect by itself. The α(2) adrenergic agonists medetomidine and guanfacine produced a small but statistically significant inhibition of NK1R internalization induced by low-frequency dorsal root stimulation. PP1 did not reverse the inhibition by guanfacine. These results show that SFKs mediate the inhibition of substance P release by μ-opioid and GABA(B) receptors, but not by α(2) receptors, which is probably mediated by the binding of G protein βγ subunits to calcium channels.

Physiologic effects of electroacupuncture combined with intramuscular administration of xylazine to provide analgesia in goats

OBJECTIVE

To investigate physiologic effects of electroacupuncture (EA) combined with xylazine administration in goats.

ANIMALS

48 healthy crossbred goats.

PROCEDURES

Goats were randomly allotted to 8 groups of 3 (nonpregnant and nonlactating) female goats and 3 male goats each. The 8 treatment groups were as follows: 1 EA group, 3 xylazine (0.1, 0.2, and 0.4 mg/kg, IM) groups, 3 EA plus xylazine (0.1, 0.2, and 0.4 mg/kg, IM) groups, and 1 control group. Electroacupuncture was performed for 90 minutes. Xylazine was administered 20 minutes after EA was performed. Pain threshold, heart rate, mean arterial pressure (MAP), respiration rate, and rectal temperature were observed at 0, 5, 25, 45, 65, and 85 minutes after xylazine administration.

RESULTS

Xylazine administered at 0.4 mg/kg increased the pain threshold and reduced MAP. Xylazine administered at 0.1, 0.2, or 0.4 mg/kg reduced heart rate, respiration rate, and temperature. Electroacupuncture increased the pain threshold but had no effect on heart rate, MAP, respiratory rate, or rectal temperature. Pain threshold in goats that underwent EA plus xylazine administration was higher than in goats that received EA or xylazine alone. Electroacupuncture combined with xylazine at 0.1 mg/kg did not affect heart rate, MAP, respiratory rate, or rectal temperature. Pain threshold in goats that underwent EA plus xylazine administration at 0.1 mg/kg was higher than in goats given xylazine at 0.4 mg/kg alone.

CONCLUSIONS AND CLINICAL RELEVANCE

Electroacupuncture combined with xylazine, even at 0.1 mg/kg, provided analgesia without significantly affecting cardiorespiratory parameters or rectal temperature in goats.

Coexpression of alpha 2A-adrenergic and delta-opioid receptors in substance P-containing terminals in rat dorsal horn

Agonists acting at alpha(2)-adrenergic and opioid receptors (alpha(2)ARs and ORs, respectively) inhibit pain transmission in the spinal cord. When coadministered, agonists activating these receptors interact in a synergistic manner. Although the existence of alpha(2)AR/OR synergy has been well characterized, its mechanism remains poorly understood. The formation of heterooligomers has been proposed as a molecular basis for interactions between neuronal G-protein-coupled receptors. The relevance of heterooligomer formation to spinal analgesic synergy requires demonstration of the expression of both receptors within the same neuron as well as the localization of both receptors in the same neuronal compartment. We used immunohistochemistry to investigate the spatial relationship between alpha(2)ARs and ORs in the rat spinal cord to determine whether coexpression could be demonstrated between these receptors. We observed extensive colocalization between alpha(2A)-adrenergic and delta-opioid receptors (DOP) on substance P (SP)-immunoreactive (-ir) varicosities in the superficial dorsal horn of the spinal cord and in peripheral nerve terminals in the skin. alpha(2A)AR- and DOP-ir elements were colocalized in subcellular structures of 0.5 mum or less in diameter in isolated nerve terminals. Furthermore, coincubation of isolated synaptosomes with alpha(2)AR and DOP agonists resulted in a greater-than-additive increase in the inhibition of K(+)-stimulated neuropeptide release. These findings suggest that coexpression of the synergistic receptor pair alpha(2A)AR-DOP on primary afferent nociceptive fibers may represent an anatomical substrate for analgesic synergy, perhaps as a result of protein-protein interactions such as heterooligomerization.

Low doses of alpha 2-adrenoceptor antagonists augment spinal morphine analgesia and inhibit development of acute and chronic tolerance

BACKGROUND AND PURPOSE

Ultra-low doses of opioid receptor antagonists augment spinal morphine antinociception and block the induction of tolerance. Considering the evidence demonstrating functional and physical interactions between the opioid and alpha(2)-adrenoceptors, this study investigated whether ultra-low doses of alpha(2)-adrenoceptor antagonists also influence spinal morphine analgesia and tolerance.

EXPERIMENTAL APPROACH

Effects of low doses of the competitive alpha(2)-adrenoceptor antagonists-atipamezole (0.08, 0.8 ng), yohimbine (0.02, 2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) were investigated on intrathecal morphine analgesia, as well as acute and chronic morphine antinociceptive tolerance using the rat tail flick and paw pressure tests.

KEY RESULTS

At doses markedly lower than those producing alpha(2)-adrenoceptor blockade, atipamezole, yohimbine, mirtazapine and idazoxan, prolonged the antinociceptive effects of morphine. When co-administered with repeated acute spinal injections of morphine, all four agents blocked the induction of acute tolerance. Co-injection of atipamezole with morphine for 5 days inhibited the development of tolerance in a chronic treatment paradigm. Spinal administration of atipamezole also reversed established antinociceptive tolerance to morphine as indicated by the restoration of morphine antinociceptive potency. The effects of atipamezole on spinal morphine tolerance were not influenced by treatment with 6-hydroxydopamine.

CONCLUSIONS AND IMPLICATIONS

Low doses of competitive alpha(2)-adrenoceptor antagonists can augment acute morphine analgesia and block or reverse tolerance to spinal administration of morphine. These actions are interpreted in terms of their interaction with an opioid-alpha(2)-adrenoceptor complex, whose activity may have a function in the genesis of analgesic tolerance.

Dexmedetomidine and ST-91 analgesia in the formalin model is mediated by alpha2A-adrenoceptors: a mechanism of action distinct from morphine

BACKGROUND AND PURPOSE

Intrathecal administration of alpha(2)-adrenoceptor agonists produces potent analgesia. This study addressed the subtype of spinal alpha(2)-adrenoceptor responsible for the analgesic effects of i.t. dexmedetomidine and ST-91 in the formalin behavioural model and their effects on primary afferent substance P (SP) release and spinal Fos activation.

EXPERIMENTAL APPROACH

The analgesic effects of i.t. dexmedetomidine and ST-91 (alpha(2) agonists) were tested on the formalin behavioural model. To determine the subtype of alpha(2)-adrenoceptor involved in the analgesia, i.t. BRL44408 (alpha(2A) antagonist) or ARC239 (alpha(2B/C) antagonist) were given before dexmedetomidine or ST-91. Moreover, the ability of dexmedetomidine and ST-91 to inhibit formalin-induced release of SP from primary afferent terminals was measured by the internalization of neurokinin(1) (NK(1)) receptors. Finally, the effects of dexmedetomidine on formalin-induced Fos expression were assessed in the dorsal horn.

KEY RESULTS

Intrathecal administration of dexmedetomidine or ST-91 dose-dependently reduced the formalin-induced paw-flinching behaviour in rats. BRL44408 dose-dependently blocked, whereas ARC239 had no effect on the analgesic actions of dexmedetomidine and ST-91. Dexmedetomidine and ST-91 had no effect on the formalin-induced NK(1) receptor internalization, while morphine significantly reduced the NK(1) receptor internalization. On the other hand, both dexmedetomidine and morphine diminished the formalin-induced Fos activation. The effect of dexmedetomidine on formalin-induced Fos activation was reversed by BRL44408, but not ARC239.

CONCLUSION AND IMPLICATIONS

These findings suggest that alpha(2A)-adrenoceptors mediate dexmedetomidine and ST-91 analgesia. This effect could be through a mechanism postsynaptic to primary afferent terminals, distinct from that of morphine.

Intrathecal clonidine suppresses phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit in spinal dorsal horn neurons of rats with neuropathic pain

BACKGROUND

Intrathecal (IT) administration of the alpha-2 adrenoceptor agonist, clonidine, produces significant analgesic effects. Although several mechanisms underlying clonidine-induced analgesia have been proposed, the possible interaction with N-methyl-D-aspartate (NMDA) receptors as a major antinociceptive mechanism has not been addressed. We designed the present study to determine whether clonidine or other analgesics can affect spinal NMDA receptor activation in rats with chronic constriction injury (CCI)-induced neuropathy.

METHODS

Rats underwent unilateral CCI, and received IT clonidine (1, 5, 20 microg/rat), [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO, mu opioid receptor agonist, 1 microg/rat), gabapentin (anticonvulsant, 100 microg/rat) or vehicle 2 wks later. After drug injection, we measured the pain response to thermal or mechanical stimuli and used immunohistochemistry to evaluate spinal cord phosphorylated NMDA-receptor subunit 1 (pNR1) expression.

RESULTS

Two weeks after CCI surgery, rats displayed significant mechanical allodynia and thermal hyperalgesia, and the spinal cord dorsal horn showed a significant increase in the number of pNR1 immunoreactive neurons. IT injection of clonidine (20 microg/rat), DAMGO and gabapentin potently reduced mechanical allodynia and thermal hyperalgesia. Importantly, IT clonidine, but not IT DAMGO or gabapentin, dose-dependently reduced CCI-induced pNR1 expression in all lamina of the spinal cord dorsal horn by 30 min after injection. In addition, IT injection of the alpha-2 adrenoceptor antagonist, idazoxan (40 microg/rat) 10 min before clonidine injection completely reversed clonidine's antihyperalgesic and antiallodynic effects, as well as clonidine's suppressive effect on CCI-induced NR1 phosphorylation in the spinal cord dorsal horn.

CONCLUSIONS

Our data indicate that IT clonidine's antihyperalgesic/antiallodynic effect on neuropathic pain is associated with a significant reduction in spinal NMDA receptor phosphorylation and suggests a potentially novel mechanism of clonidine's action.

Inhibition of opioid release in the rat spinal cord by alpha2C adrenergic receptors

Neurotransmitter receptors that control the release of opioid peptides in the spinal cord may play an important role in pain modulation. Norepinephrine, released by a descending pathway originating in the brainstem, is a powerful inducer of analgesia in the spinal cord. Adrenergic alpha2C receptors are present in opioid-containing terminals in the dorsal horn, where they could modulate opioid release. The goal of this study was to investigate this possibility. Opioid release was evoked from rat spinal cord slices by incubating them with the sodium channel opener veratridine in the presence of peptidase inhibitors (actinonin, captopril and thiorphan), and was measured in situ through the internalization of mu-opioid receptors in dorsal horn neurons. Veratridine produced internalization in 70% of these neurons. The alpha2 receptor agonists clonidine, guanfacine, medetomidine and UK-14304 inhibited the evoked mu-opioid receptor internalization with IC50s of 1.7 microM, 248 nM, 0.3 nM and 22 nM, respectively. However, inhibition by medetomidine was only partial, and inhibition by UK-14304 reversed itself at concentrations higher than 50 nM. None of these agonists inhibited mu-opioid receptor internalization produced by endomorphin-2, showing that they inhibited opioid release and not the internalization itself. The inhibitions produced by clonidine, guanfacine or UK-14304 were completely reversed by the selective alpha2C antagonist JP-1203. In contrast, inhibition by guanfacine was not prevented by the alpha2A antagonist BRL-44408. These results show that alpha2C receptors inhibit the release of opioids in the dorsal horn. This action may serve to shut down the opioid system when the adrenergic system is active.

Electroacupuncture-induced analgesia in a rat model of ankle sprain pain is mediated by spinal alpha-adrenoceptors

In a previous study, we showed that electroacupuncture (EA) applied to the SI-6 point on the contralateral forelimb produces long-lasting and powerful analgesia in pain caused by ankle sprain in a rat model. To investigate the underlying mechanism of EA analgesia, the present study tested the effects of various antagonists on known endogenous analgesic systems in this model. Ankle sprain was induced in anesthetized rats by overextending their right ankle with repeated forceful plantar flexion and inversion of the foot. When rats developed pain behaviors (a reduction in weight-bearing of the affected hind limb), EA was applied to the SI-6 point on the contralateral forelimb for 30 min under halothane anesthesia. EA significantly improved the weight-bearing capacity of the affected hind limb for 2h, suggesting an analgesic effect. The alpha-adrenoceptor antagonist phentolamine (2mg/kg, i.p. or 30 microg, i.t.) completely blocked the EA-induced analgesia, whereas naloxone (1mg/kg, i.p.) failed to block the effect. These results suggest that EA-induced analgesia is mediated by alpha-adrenoceptor mechanisms. Further experiments showed that intrathecal administration of yohimbine, an alpha(2)-adrenergic antagonist, reduced the EA-induced analgesia in a dose-dependent manner, whereas terazosin, an alpha(1)-adrenergic antagonist, did not produce any effect. These data suggest that the analgesic effect of EA in ankle sprain pain is, at least in part, mediated by spinal alpha(2)-adrenoceptor mechanisms.

Mechanisms involved in the antinociception caused by melatonin in mice

The present study assesses the antinociceptive effect of melatonin in chemical behavioral models of nociception and investigates some of the mechanisms underlying this effect. Melatonin administered by intraperitoneal (i.p., 10-100 mg/kg), intracerebroventricular (i.c.v., 250-500 pmol/site) and intraplantar (i.pl., 30-100 ng/i.pl.) routes, reduced in a dose-dependent manner the nociception caused by i.pl. injection of glutamate (10 micromol/paw), with mean ID50 values of 32.6 mg/kg, 200 pmol/site and 59 ng/i.pl., respectively. Furthermore, melatonin in the dose range of 10-100 mg/kg, i.p., reduced the neurogenic pain caused by i.pl. injection of capsaicin (5.2 nmol/paw) with inhibition of 48 +/- 4%. The antinociceptive effect of melatonin (100 mg/kg, i.p.) on glutamate-induced nociception was completely prevented by the pretreatment of animals with naloxone (a nonselective opioid receptor antagonist, 1 mg/kg, i.p.), ketanserin (a preferential 5-HT2A receptor antagonist, 1 mg/kg, i.p.), sulpiride (a D2 receptor antagonist, 50 mg/kg, i.p.), L-arginine (a precursor of nitric oxide, 600 mg/kg, i.p.), yohimbine (an alpha2-adrenoceptor antagonist, 0.15 mg/kg, i.p.) and luzindole (a preferential MT2 receptor antagonist, 10 mg/kg, i.p.), but was not affected by the pretreatment with D-arginine (an inactive isomer of L-arginine, 600 mg/kg, i.p.), prazosin (an alpha1-adrenoceptor antagonist, 0.15 mg/kg, i.p.) or after bilateral adrenalectomy. Collectively, present results suggest that melatonin produces peripheral and central antinociception when assessed on capsaicin- or glutamate-induced pain in mice through mechanisms that are likely mediated by interaction with plasma membrane-bound melatonin receptors and modulated by opioid, serotonergic (5-HT2A receptors), dopaminergic (D2-receptors), adrenergic (alpha2-adrenoceptors) systems as well as the L-arginine-nitric oxide pathway.

Inhibitory effect of somatostatin on inflammation and nociception

The present review focuses on promising new opportunities for anti-inflammatory and analgesic therapy. The theoretical background is an original observation based on our own experimental results. These data demonstrate that somatostatin is released from capsaicin-sensitive, peptidergic sensory nerve endings in response to noxious heat and chemical stimuli such as vanilloids, protons or lipoxygenase products. It reaches distant parts of the body via the circulation and exerts systemic anti-inflammatory and analgesic effects. Somatostatin binds to G-protein-coupled membrane receptors (sst(1)-sst(5)) and diminishes neurogenic inflammation by prejunctional action on sensory-efferent nerve terminals, as well as by postjunctional mechanisms on target cells. It decreases the release of pro-inflammatory neuropeptides from sensory nerve endings and also acts on receptors of vascular endothelial, inflammatory and immune cells. Analgesic effect is mediated by an inhibitory action on peripheral terminals of nociceptive neurons, since circulating somatostatin cannot exert central action. Somatostatin itself is not suitable for drug development because of its broad spectrum and short elimination half life, stable, receptor-selective agonists have been synthesized and investigated. The present overview is aimed at summarizing the physiological importance of somatostatin and sst receptors, pharmacological significance of synthetic agonists and their potential in the development of novel anti-inflammatory and analgesic drugs. These compounds might provide novel perspectives in the pharmacotherapy of acute and chronic painful inflammatory diseases, as well as neuropathic conditions.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Activation of alpha2-adrenoceptors in the trigeminal region produces sex-specific modulation of nociception in the rat

Sex-related differences in the sensitivity to pain and in the response to analgesics have been reported including higher perceptual responses to experimentally induced pain and the higher prevalence of many pain syndromes in women compared with men. This study examines whether alpha2-adrenoceptor-mediated antinociceptive effects are reduced by estrogen which could account for the sex-related differences in pain perception and modulation. Clonidine, an alpha2-adrenoceptor agonist, has been shown to inhibit noxious stimulus-evoked nociceptive behavior as well as the responses of nociceptive neurons in the medullary dorsal horn. Intracisternal microinjection of clonidine (7 microg/5 microl) through the implanted PE-10 cannulae dorsal to the trigeminal region in male, ovariectomized (OVX), and diestrous (DiE) Sprague-Dawley rats produced a strong antinociceptive effect on N-methyl-D-aspartic acid (NMDA)-induced nociceptive scratching behavior and heat-induced face withdrawal nociceptive tests. However, it failed to produce any inhibition in the estradiol-treated ovariectomized (OVX+E) group regardless of the dose of estradiol (1, 10 or 100 microg/100 microl sesame oil) or in the proestrous (ProE) group. Further, clonidine produced dose-dependent effects in male and OVX groups but not in the OVX+E group on the NMDA-induced nociceptive behavior. Finally, the effect of clonidine was reversed by yohimbine, an alpha2-adrenoceptor antagonist, in male and OVX groups on thermal nociceptive test. These results lead us to conclude that activation of alpha2-adrenoceptors produces sex-specific, estrogen dependent modulation of nociception in the trigeminal region of the rat. A decreased alpha2-adrenoceptor-mediated inhibition could be one of the factors responsible for the higher prevalence of pain syndromes in females.

Dexmedetomidine: a novel sedative-analgesic agent

Since the first report of clonidine, an alpha2-adrenoceptor agonist, the indications for this class of drugs have continued to expand. In December 1999, dexmedetomidine was approved as the most recent agent in this group and was introduced into clinical practice as a short-term sedative (<24 hours). Alpha2-adrenoceptor agonists have several beneficial actions during the perioperative period. They decrease sympathetic tone, with attenuation of the neuroendocrine and hemodynamic responses to anesthesia and surgery; reduce anesthetic and opioid requirements; and cause sedation and analgesia. They allow psychomotoric function to be preserved while letting the patient rest comfortably. With this combination of effects, alpha2-adrenoceptor agonists may offer benefits in the prophylaxis and adjuvant treatment of perioperative myocardial ischemia. Furthermore, their role in pain management and regional anesthesia is expanding. Side effects consist of mild to moderate cardiovascular depression, with slight decreases in blood pressure and heart rate. The development of new, more selective alpha2-adrenoceptor agonists with improved side effect profiles may provide a new concept for the administration of perioperative anesthesia and analgesia. This review aims to give background information to improve understanding of the properties and applications of the novel alpha2-adrenoceptor agonist, dexmedetomidine.

Spinal nerve ligation increases alpha2-adrenergic receptor G-protein coupling in the spinal cord

Intrathecal and epidural administration of the alpha2-adrenergic receptor agonist clonidine in humans results in analgesia to both acute nociceptive and chronic neuropathic pain. The potency of clonidine increases with hypersensitivity to mechanical stimuli after nerve injury, although the reasons for this change are unknown. In the present study, we tested the hypothesis that peripheral nerve injury alters either spinal alpha2-adrenergic receptor-mediated G-protein activity or alpha2-adrenergic receptor number. Rats were randomized to left spinal nerve ligation (SNL) or sham surgery. Tactile hypersensitivity in the hindpaw was confirmed and lumbar spinal cords were removed for binding assays. To examine agonist-induced G-protein coupling, [35S]GTP gamma S binding experiments were performed in spinal cord membranes and sections using norepinephrine as an alpha2-adrenergic agonist. SNL was associated with an increase in maximal efficacy, but not potency, of norepinephrine-stimulated [35S]GTP gamma S binding in dorsal horn. SNL had no effect on basal [35S]GTP gamma S binding or on muscarinic cholinergic-stimulated [35S]GTP gamma S binding. [35S]GTP gamma S autoradiography showed that this increase in alpha2-adrenergic-activated G-proteins occurred both ipsilateral and contralateral to SNL surgery. SNL did not alter total alpha2-adrenergic receptor number or affinity to [3H]-rauwolscine binding, and displacement studies with the alpha2A-adrenergic antagonist BRL44408 revealed that most of the binding was associated with the alpha2A-adrenergic subtype. These data suggest that the increased potency of clonidine in neuropathic pain could reflect increased efficiency of G-protein coupling from spinal alpha2-adrenergic receptors.

Noradrenergic receptor mRNA expression in adult rat superficial dorsal horn and dorsal root ganglion neurons

Noradrenaline (NAdr) has well documented analgesic actions at the level of the spinal cord. Released from bulbospinal projections onto superficial dorsal horn (SDH) neurons, NAdr modulates the excitability of these neurons through the activation of alpha1, alpha2 or beta adrenoceptors. This study utilised in situ hybridisation to determine the specific expression of adrenoceptors within adult rat lumbar SDH and dorsal root ganglion (DRG) neurons, and reports the presence of alpha1A, alpha1B, alpha2B, beta1 and beta2 adrenoceptor mRNA within SDH neurons, and the presence of alpha1A, alpha1B and alpha2C adrenoceptor mRNA within DRG neurons. The present study provides an insight into the modulation of sensory processing at the level of the spinal cord following adrenoceptor activation.

Dextromethorphan potentiates morphine-induced antinociception at both spinal and supraspinal sites but is not related to the descending serotoninergic or adrenergic pathways

Morphine is a strong and widely used opioid analgesic in pain management, but some adverse effects limit its clinical use at high doses. The clinically available non-opioid antitussive, dextromethorphan (DM) can potentiate the analgesic effect of morphine and decrease the dose of morphine in acute postoperative pain. However, the mechanism underlying this synergistic phenomenon is still not clear. To examine if the potentiation by DM occurs through the descending pain-inhibitory pathways, ketanserin (a 5-HT2 receptor antagonist) and yohimbine (an alpha2-adrenergic receptor antagonist) were employed and found to have no significant effect on the potentiation by DM. Using local delivery of drugs in rats in the present study, potentiation of morphine-induced antinociception by DM was observed via both intrathecal and intracerebroventricular routes, suggesting that both spinal and supraspinal sites are involved. This suggests that the potentiation of morphine-induced antinociception by DM is not mediated by the serotoninergic or adrenergic descending pain-inhibitory pathways. The present results are consistent with findings in clinical studies, which showed that DM can effectively decrease the consumption of morphine in patients suffering from pain. Since DM has excellent clinical potential as a synergistic agent with morphine, further investigating and clarifying the possible pharmacological mechanism of DM are of great importance for future studies.

Clonidine and guanfacine-induced antinociception in visceral pain: possible role of α2/I2 binding sites

Visceral pain is one of the most common forms of pain which is poorly understood. We now studied the influence of imidazoline/guanidinium compounds such as clonidine and guanfacine on visceral pain in the presence or absence of yohimbine and benazoline. To produce visceral pain-related behaviours, formalin (10%) was administered by inserting a fine cannula into the colon via the anus. Each experiment took 1 h. Clonidine (0.001, 0.01 and 0.1 mg/kg, i.p.) and guanfacine (2.5, 5 and 10 mg/kg, i.p.) produced analgesia dose dependently. The clonidine response was inhibited by yohimbine (0.2 mg/kg, i.p.). On the other hand, benazoline (5 mg/kg, i.p.) blocked the antinociceptive effect of guanfacine (5 mg/kg). Benazoline (2.5 and 5 mg/kg) itself also induced analgesia in inflammatory colonic pain. In this study, we used morphine to ensure that the behavioural responses were pain-related. Our results showed that morphine (2.5, 5 and 10 mg/kg, s.c.) produced a dose-dependent antinociception. The morphine (7 mg/kg, s.c.) response was reduced by naloxone (2 mg/kg, i.p.). However, we concluded that both imidazoline (I(2)) and alpha(2)-adrenoceptors may play a role in producing analgesia in visceral pain.

Substance P release in the cat spinal cord upon afferent C-fibre stimulation is not attenuated by clonidine at analgesic doses

In anaesthetized cats, antibody microprobes were used to measure the release of immunoreactive substance P (irSP) in the lumbar dorsal horn during electrical stimulation of primary afferent fibres at intensities suprathreshold for unmyelinated fibres. Release of irSP was detected in the region of the superficial dorsal horn. This evoked release was not reduced by clonidine hydrochloride, administered intravenously or by superfusion of the dorsal cord surface. Microprobes inserted during cord superfusion with lignocaine hydrochloride detected less irSP along their entire length, including in the region of evoked release. The results suggest that the analgesic action of clonidine does not involve reduced release of SP from the central terminals of nociceptors in the spinal cord.