Release of calcitonin gene-related peptide (CGRP), substance P (SP), and vasoactive intestinal polypeptide (VIP) from rat spinal cord: modulation by alpha 2 agonists

Substance P release from rat dura mater is inversely correlated with CGRP release- experiments using glycerol trinitrate and anti-CGRP antibodies

BACKGROUND

The neuropeptides calcitonin gene-related peptide (CGRP) and substance P are important mediators of neurogenic inflammation when they are released from activated primary nociceptive afferents. It is long evident that neuropeptides play an important role in migraine pathophysiology, but the significance of neurogenic inflammation is still debated.

METHODS

In an approved hemisected rodent head preparation, we measured CGRP release from the cranial dura mater in parallel with substance P release using animals pre-treated with anti-CGRP antibodies or control solutions.

RESULTS

Apart from the known decrease in CGRP release following antibody treatment, we found a significant inverse correlation of basal and stimulated CGRP versus substance P release across all experiments. The results are discussed in connection with our previously published data.

CONCLUSIONS

An increase in CGRP release seems to inhibit substance P release in meningeal structures possibly decreasing substance P-dependent plasma extravasation, which argues against a significant role of neurogenic inflammation in migraine.

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.

Inhibition of transient receptor potential vanilloid type 1 through α2 adrenergic receptors at peripheral nerve terminals relieves pain

The activation of α₂ adrenergic receptors contributes to analgesia not only in the central nervous system but also in the peripheral nervous system. We reported that noradrenaline inhibits the activity of transient receptor potential vanilloid 1 (TRPV1) evoked by capsaicin through α₂ receptors in cultured rat dorsal root ganglion (DRG) neurons. However, it is unclear whether activation of TRPV1 expressed in peripheral nerve terminals is inhibited by α₂ receptors and whether this phenomenon contributes to analgesia. Therefore, we examined effects of clonidine, an α₂ receptor agonist, on several types of nociceptive behaviors, which may be caused by TRPV1 activity, and subtypes of α₂ receptors expressed with TRPV1 in primary sensory neurons in rats. Capsaicin injected into hind paws evoked nociceptive behaviors and clonidine preinjected into the same site inhibited capsaicin-evoked responses. This inhibition was not observed when clonidine was injected into the contralateral hind paws. Preinjection of clonidine into the plantar surface of ipsilateral, but not contralateral, hind paws reduced the sensitivity to heat stimuli. Clonidine partially reduced formalin-evoked responses when it was preinjected into ipsilateral hind paws. The expression level of α_2C receptor mRNA quantified by real-time PCR was highest followed by those of α_2A and α_2B receptors in DRGs. α_2A and α_2C receptor-like immunoreactivities were detected with TRPV1-like immunoreactivities in the same neurons. These results suggest that TRPV1 and α₂ receptors are coexpressed in peripheral nerve terminals and that the functional association between these two molecules causes analgesia.

Hemokinin-1 is an important mediator of pain in mouse models of neuropathic and inflammatory mechanisms

The Tac4 gene-derived hemokinin-1 (HK-1) is present in pain-related regions and activates the tachykinin NK1 receptor, but with binding site and signaling pathways different from Substance P (SP). NK1 receptor is involved in nociception, but our earlier data showed that it has no role in chronic neuropathic hyperalgesia, similarly to SP. Furthermore, NK1 antagonists failed in clinical trials as analgesics due to still unknown reasons. Therefore, we investigated the role of HK-1 in pain conditions of distinct mechanisms using genetically modified mice. Chronic neuropathic mechanical and cold hyperalgesia after partial sciatic nerve ligation (PSL) were determined by dynamic plantar aesthesiometry and withdrawal latency from icy water, motor coordination on the accelerating Rotarod. Peripheral nerve growth factor (NGF) production was measured by ELISA, neuronal and glia cell activation by immunohistochemistry in pain-related regions. Acute somatic and visceral chemonocifensive behaviors were assessed after intraplantar formalin or intraperitoneal acetic-acid injection, respectively. Resiniferatoxin-induced inflammatory mechanical and thermal hyperalgesia by aesthesiometry and increasing temperature hot plate. Chronic neuropathic mechanical and cold hypersensitivity were significantly decreased in HK-1 deficient mice. NGF level in the paw homogenates of intact mice were significantly lower in case of HK-1 deletion. However, it significantly increased under neuropathic condition in contrast to wildtype mice, where the higher basal concentration did not show any changes. Microglia, but not astrocyte activation was observed 14 days after PSL in the ipsilateral spinal dorsal horn of wildtype, but not HK-1-deficient mice. However, under neuropathic conditions, the number of GFAP-positive astrocytes was significantly smaller in case of HK-1 deletion. Acute visceral, but not somatic nocifensive behavior, as well as neurogenic inflammatory mechanical and thermal hypersensitivity were significantly reduced by HK-1 deficiency similarly to NK1, but not to SP deletion. We provide evidence for pro-nociceptive role of HK-1, via NK1 receptor activation in acute inflammation models, but differently from SP-mediated actions. Identification of its targets and signaling can open new directions in pain research.

Effect of dexmedetomidine on the development of mechanical allodynia and central sensitization in chronic post-ischemia pain rats

PURPOSE

Complex regional pain syndrome type 1 (CRPS I) is an intractable neuropathic pain syndrome. Chronic post-ischemia pain (CPIP) model is an animal model of CRPS I which is produced by ischemia-reperfusion (IR) injury of the hind limb. Dexmedetomidine (DEX) is a selective and potent α2 adrenergic receptor agonist with analgesic and protective effects following an IR injury. We hypothesized that DEX protects the development of mechanical allodynia and central sensitization in CRPS I. Therefore, we evaluated the dose-related protective effect of DEX in a CPIP model.

METHODS

We divided 45 rats into five groups: sham, CPIP, CPIP + DEX 10 µg/kg, CPIP + DEX 50 µg/kg, and CPIP + DEX 100 µg/kg. Rats in the sham group underwent sham surgery, and the other rats received CPIP injury. One hour before reperfusion or end of sham surgery, normal saline was injected into the rats in the sham and CPIP groups, and DEX (designated dose) was injected into the rats in the other groups. All rats were evaluated for the withdrawal threshold of both hind paws before surgery and 1, 3, and 7 days after surgery. Phosphorylation of N-methyl-d-aspartate receptor subunits (pGluN1) and phosphorylation of extracellular signal-regulated kinases (pERK) in the spinal cord were measured 3 days after surgery.

RESULTS

Administration of DEX before reperfusion showed a significant increase in the withdrawal threshold in both hind paws and a significant decrease of the expressions of pGluN1 and pERK in CPIP rats dose dependently (P<0.05).

CONCLUSION

DEX may inhibit the development of mechanical allodynia and central sensitization in CPIP rats.

Antinociceptive effects of dexmedetomidine via spinal substance P and CGRP

The aim of this study was to examine the role played by substance P and calcitonin gene-related peptide (CGRP) within the dorsal horn of the spinal cord in engagement of antinociception evoked by dexmedetomidine (DEX). Paw withdrawal threshold (PWT) to mechanical stimulation was determined after chronic intrathecal infusion of DEX and enzyme-linked immunosorbent assay (ELISA) was employed to examine the levels of spinal substance P and CGRP. Our results show that PWT was significantly increased by intrathecal administration of DEX in rats (P < 0.05 vs. vehicle control, n = 20 in each group). Also, intrathecal infusion of DEX significantly decreased the concentrations of substance P and CGRP as compared with vehicle control (P < 0.05 DEX vs. vehicle control, n = 20 in each group). Blocking α₂-adrenoreceptors (α₂-AR) blunted the decreases of substance P and CGRP levels and the enhancement of PWT evoked by DEX. Additionally, a linear relationship was observed between PWT and the levels of spinal substance P (r = 0.87; P < 0.005) and CGRP (r = 0.85; P < 0.005). Moreover, blocking individual substance P and CGRP receptors amplified PWT without altering substance P and CGRP levels. Thus, DEX plays a role in stimulating α₂-AR receptors, which thereby decreases substance P and CGRP levels within the dorsal horn. This contributes to DEX-evoked antinociception.

Endomorphin-2 Inhibition of Substance P Signaling within Lamina I of the Spinal Cord Is Impaired in Diabetic Neuropathic Pain Rats

Opiate analgesia in the spinal cord is impaired in diabetic neuropathic pain (DNP), but until now the reason is unknown. We hypothesized that it resulted from a decreased inhibition of substance P (SP) signaling within the dorsal horn of the spinal cord. To investigate this possibility, we evaluated the effects of endomorphin-2 (EM2), an endogenous ligand of the μ-opioid receptor (MOR), on SP release within lamina I of the spinal dorsal horn (SDH) in rats with DNP. We established the DNP rat model and compared the analgesic efficacy of EM2 between inflammation pain and DNP rat models. Behavioral results suggested that the analgesic efficacy of EM2 was compromised in the condition of painful diabetic neuropathy. Then, we measured presynaptic SP release induced by different stimulating modalities via neurokinin-1 receptor (NK1R) internalization. Although there was no significant change in basal and evoked SP release between control and DNP rats, EM2 failed to inhibit SP release by noxious mechanical and thermal stimuli in DNP but not in control and inflammation pain model. We also observed that EM2 decreased the number of FOS-positive neurons within lamina I of the SDH but did not change the amount of FOS/NK1R double-labeled neurons. Finally, we identified a remarkable decrease in MORs within the primary afferent fibers and dorsal root ganglion (DRG) neurons by Western blot (WB) and immunohistochemistry (IHC). Taken together, these data suggest that reduced presynaptic MOR expression might account for the loss of the inhibitory effect of EM2 on SP signaling, which might be one of the neurobiological foundations for decreased opioid efficacy in the treatment of DNP.

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.

Selective blockade of N-methyl-D-aspartate channels in combination with dopamine receptor antagonism induces loss of the righting reflex in mice, but not immobility

RATIONALE

The selective N-methyl-D-aspartate (NMDA) channel blocker MK-801 is known to induce no loss of the righting reflex (LORR) and to stimulate catecholaminergic (CAergic) neurons in rodents, playing a crucial role in arousal.

OBJECTIVES

We examined whether MK-801 in combination with CA receptor ligands, which inhibit CAergic neuronal activities, could induce anesthesia including LORR.

METHODS

All drugs were administered systemically to mice. To assess anesthesia, three different behaviors were used: loss of nociceptive response (analgesia in the free-moving state without LORR), LORR, and loss of movement in response to noxious stimulation (immobility under LORR).

RESULTS

A very large dose of MK-801 (50 mg/kg) induced neither analgesia nor LORR. In contrast, MK-801 in combination with a small dose of the dopamine (DA) receptor antagonist haloperidol (0.2 mg/kg) dose-dependently produced LORR with a 50 % effective dose (ED50) of 1.6 (0.9-3.0; 95 % confidence limit) mg/kg, but not immobility. The α2-adrenoceptor agonist dexmedetomidine induced not only analgesia, but also immobility in animals treated with MK-801 (5 mg/kg) plus haloperidol (0.2 mg/kg), which then lost their righting reflex. The ED50 value of 0.26 (0.10-0.66) mg/kg (various doses of dexmedetomidine plus a fixed dose of MK-801 and haloperidol) for immobility was approximately three-fold larger than that of 0.09 (0.03-0.23) mg/kg (dexmedetomidine plus vehicle saline) for analgesia. This may occur, as LORR induced by MK-801 plus haloperidol inhibits the pain suppression system. The other ligands had little or no effect.

CONCLUSIONS

The DAergic stimulant actions of MK-801 may mask its LORR effects by NMDA channel blockade.

Tapentadol inhibits calcitonin gene-related peptide release from rat brainstem in vitro

We have previously developed an in vitro model of rat brainstem explants. The latter release sizable amounts of calcitonin gene-related peptide (CGRP); basal release can be stimulated by such secretagogues as high KCl concentrations, veratridine or capsaicine. In this paradigm we investigated the activity of the analgesic agent tapentadol; the effects of tapentadol were compared to those of a classical opioid receptor agonist, morphine, and the selective noradrenaline reuptake inhibitor reboxetine. Morphine inhibited basal CGRP release, with statistical significance from 1 nM onward and maximal (-44%) inhibition at 100 μM. Morphine also inhibited K(+)-stimulated peptide release, with a significant effect from 1 μM and maximal (-39%) decrease at 100 μM, but failed to inhibit release stimulated by 10 μM capsaicin. At variance, reboxetine had no effect on baseline CGRP outflow, but was able to inhibit both K(+)-stimulated [significant inhibition from 1 μM onward and maximal (-37%) decrease at 100 μM], and capsaicin-stimulated release [significant effect from 1 μM and maximal (-31%) decrease at 100 μM]. Likewise, tapentadol had no effect on baseline CGRP release up to 100 μM, but decreased secretion stimulated by 56 mM KCl or capsaicin, with significant effects from 0.1 and 1 μM respectively; maximal inhibition over 56 mM KCl and capsaicin stimuli was -29% and -31%, respectively. Naloxone antagonized the effect of morphine, but not those of reboxetine and tapentadol, on K(+)-stimulated CGRP secretion. In conclusion the present study provides consistent pharmacological evidence that tapentadol acts as a noradrenaline reuptake inhibitor agent in this experimental model.

Highlights in opioid agonists and antagonists

This review highlights new insights in to opioid agonists and antagonists, focusing on their mechanism of action with spinal and systemic administration, chronic use and main adverse effects. Short-cuts on some opioid agonists and antagonists of clinical interest are also presented, revealing potential clinical implications and future clinical directions as part of multimodal analgesia.

The interaction of octopamine and neuropeptides to slow aversive responses in C. elegans mimics the modulation of chronic pain in mammals

Octopamine (OA) appears to function as the invertebrate counterpart of norepinephrine (NE) in the modulation of a number of key behaviors. In C. elegans, OA signaling is complex, mediated by at least three distinct α-adrenergic-like receptors and appears to activate more global peptidergic signaling cascades that have the potential to dramatically amplify the octopaminergic signal. These OA-dependent peptidergic signaling cascades involve an array of neuropeptides that activate receptors throughout the nervous system and have the potential to both directly and indirectly modulate locomotory decision-making. In this commentary we highlight the use of C. elegans as a model to expand our understanding of noradrenergic signaling in mammals, specifically as it relates to the role of NE in anti-nociception.

Effects of milnacipran, a 5-HT and noradrenaline reuptake inhibitor, on C-fibre-evoked field potentials in spinal long-term potentiation and neuropathic pain

BACKGROUND AND PURPOSE

The analgesic action of 5-HT and noradrenaline reuptake inhibitors (SNRIs) on nociceptive synaptic transmission in the spinal cord is poorly understood. We investigated the effects of milnacipran, an SNRI, on C-fibre-evoked field potentials (FPs) in spinal long-term potentiation (LTP), a proposed synaptic mechanism of hypersensitivity, and on the FPs in a neuropathic pain model.

EXPERIMENTAL APPROACH

C-fibre-evoked FPs by electrical stimulation of the sciatic nerve fibres were recorded in the spinal dorsal horn of anaesthetized adult rats, and LTP was induced by high-frequency stimulation of the sciatic nerve fibres. A rat model of neuropathic pain was produced by L5 spinal nerve ligation and transection.

KEY RESULTS

Milnacipran produced prolonged inhibition of C-fibre-evoked FPs when applied spinally after the establishment of LTP of C-fibre-evoked FPs in naïve animals. In the neuropathic pain model, spinal administration of milnacipran clearly reduced the basal C-fibre-evoked FPs. These inhibitory effects of milnacipran were blocked by spinal administration of methysergide, a 5-HT½ receptor antagonist, and yohimbine or idazoxan, α₂-adrenoceptor antagonists. However, spinal administration of milnacipran in naïve animals did not affect the basal C-fibre-evoked FPs and the induction of spinal LTP.

CONCLUSION AND IMPLICATIONS

Milnacipran inhibited C-fibre-mediated nociceptive synaptic transmission in the spinal dorsal horn after the establishment of spinal LTP and in the neuropathic pain model, by activating both spinal 5-hydroxytryptaminergic and noradrenergic systems. The condition-dependent inhibition of the C-fibre-mediated transmission by milnacipran could provide novel evidence regarding the analgesic mechanisms of SNRIs in chronic pain.

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.

Pharmacological profiles of alpha 2 adrenergic receptor agonists identified using genetically altered mice and isobolographic analysis

Endogenous, descending noradrenergic fibers impose analgesic control over spinal afferent circuitry mediating the rostrad transmission of pain signals. These fibers target alpha 2 adrenergic receptors (alpha(2)ARs) on both primary afferent terminals and secondary neurons, and their activation mediates substantial inhibitory control over this transmission, rivaling that of opioid receptors which share a similar pattern of distribution. The terminals of primary afferent nociceptive neurons and secondary spinal dorsal horn neurons express alpha(2A)AR and alpha(2C)AR subtypes, respectively. Spinal delivery of these agents serves to reduce their side effects, which are mediated largely at supraspinal sites, by concentrating the drugs at the spinal level. Targeting these spinal alpha(2)ARs with one of five selective therapeutic agonists, clonidine, dexmedetomidine, brimonidine, ST91 and moxonidine, produces significant antinociception that can work in concert with opioid agonists to yield synergistic antinociception. Application of several genetically altered mouse lines had facilitated identification of the primary receptor subtypes that likely mediate the antinociceptive effects of these agents. This review provides first an anatomical description of the localization of the three subtypes in the central nervous system, second a detailed account of the pharmacological history of each of the six primary agonists, and finally a comprehensive report of the specific interactions of other GPCR agonists with each of the six principal alpha(2)AR agonists featured.

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.

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.

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.

Adrenergic and cholinergic compounds

Adrenergic and cholinergic signalling contributes significantly to the endogenous antinociceptive system. Exogenous alpha 2 adrenergic agonists have a well-established analgesic profile; however, recent investigations suggest that this class of agents is underused, and herein we highlight the potential for both current application and future development of these agents. Nicotinic and muscarinic cholinergic ligands represent a novel class of agents with much promise for the management of problematic pain. In this chapter we review advances in both preclinical and clinical arenas and highlight potential avenues for further research.

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.

Neuropeptides as synaptic transmitters

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.

The influence of the time course of inflammation and spinalization on the antinociceptive activity of the α2-adrenoceptor agonist medetomidine

The purpose of the present study was to investigate the influence of the time course of inflammation and the implication of spinal and supraspinal sites on the antihyperalgesic effects of the alpha(2)-adrenoceptor agonist medetomidine. Behavioral experiments showed a more intense antihyperalgesia in the phase of maintenance of inflammation than in the early or resolution stages. Maximum effect, without sedation, was observed with a dose of 40 microg/kg (66+/-12% and 76+/-15% reduction of mechanical and thermal hyperalgesia). No change was observed in the paw swelling, indicating that its effects were not secondary to a reduction of inflammation. In electrophysiological experiments, the effect was more pronounced in animals with an intact spinal cord than in spinalized animals (max. effects of 2+/-0.7% vs. 48+/-11% of control, noxious mechanical stimulation). We conclude that the antihyperalgesic effect of medetomidine depends on the time course of inflammation and that it is mainly located supraspinally.

Synergistic and additive interactions of the cannabinoid agonist CP55,940 with mu opioid receptor and alpha2-adrenoceptor agonists in acute pain models in mice

1. Cannabinoid receptor agonists elicit analgesic effects in acute and chronic pain states via spinal and supraspinal pathways. We investigated whether the combination of a cannabinoid agonist with other classes of antinociceptive drugs exerted supra-additive (synergistic) or additive effects in acute pain models in mice. 2. The interactions between the cannabinoid agonist CP55,940, alpha2-adrenoceptor agonist dexmedetomidine and mu-opioid receptor agonist morphine were evaluated by isobolographic analysis of antinociception in hot plate (55 degrees C) and tail flick assays in conscious male Swiss mice. Drug interactions were examined by administering fixed-ratio combinations of agonists (s.c.) in 1:1, 3:1 and 1:3 ratios of their respective ED50 fractions. 3. CP55,940, dexmedetomidine and morphine all caused dose-dependent antinociception. In the hot plate and tail flick assays, ED50 values (mg kg(-1)) were CP55,940 1.13 and 0.51, dexmedetomidine 0.066 and 0.023, and morphine 29.4 and 11.3, respectively. Synergistic interactions existed between CP55,940 and dexmedetomidine in the hot plate assay, and CP55,940 and morphine in both assays. Additive interactions were found for CP55,940 and dexmedetomidine in the tail flick assay, and dexmedetomidine and morphine in both assays. 4. Thus, an alpha2-adrenoceptor agonist or mu opioid receptor agonist when combined with a cannabinoid receptor agonist showed significant synergy in antinociception in the hot plate test. However, for the tail flick nociceptive response to heat, only cannabinoid and mu opioid receptor antinociceptive synergy was demonstrated. If these results translate to humans, then prudent selection of dose and receptor-specific agonists may allow an improved therapeutic separation from unwanted side effects.

alpha2-adrenoceptors inhibit the intracellular Ca2+ response to electrical stimulation in normal and injured sensory neurons, with increased inhibition of calcitonin gene-related peptide expressing neurons after injury

Nerve injury resulting in chronic pain is associated with novel excitatory effects of norepinephrine on injured peripheral nerve terminals and their cell bodies, due to actions on alpha2-adrenoceptors. Paradoxically, alpha2-adrenoceptor agonists administered near peripheral terminals or their cell bodies results in analgesia, not pain. This study tested, using intracellular Ca2+ response to stimulation, the effects of alpha2-adrenoceptor agonists on injured sensory neurons and classified their neuronal phenotype. Dorsal root ganglion cells from normal and spinal nerve-ligated rats were dissociated and activated twice with electrical field stimulation, while measuring Fura-2 fluorescence. Cells were perfused between stimulations with vehicle or alpha2-adrenoceptor agonists alone or with antagonists. Cells were considered inhibited if the ratio of their peak Ca2+ response to the second stimulus divided by the first was less than the 2.5th percentile for vehicle controls. alpha2-, But not alpha1-adrenoceptor agonists inhibited the Ca2+ response in a concentration related fashion, and this inhibition was blocked by alpha2-adrenoceptor antagonists. Clonidine inhibited a similar percentage of cells in the normal and spinal nerve-ligated group. In both groups, the large majority of clonidine-inhibited cells stained for isolectin B4. Spinal nerve ligation resulted in a 4-10-fold increase in the percentage of clonidine inhibited cells which immunostained for calcitonin gene-related peptide. These data are consistent with the known inhibition of Ca2+ currents by alpha2-adrenoceptors and suggest that, at the level of intracellular Ca2+, the key determinant of neurotransmitter release, alpha2-adrenoceptors are inhibitory after nerve injury, not excitatory. There is a shift in phenotype of sensory neurons which are inhibited by clonidine after nerve injury, which may explain clonidine's increased potency in the treatment of neuropathic compared with acute pain.

The evolving role of spinal agents in acute pain

Local anesthetics and opioids were among the earliest pharmacologic agents used for intrathecal injection. Side-effect profiles have limited the use of additional agents and, after a century of clinical use, local anesthetics and opioids remain the most widely used intrathecal or epidural medications.

Synergy between intrathecal omega-conotoxin CVID and dexmedetomidine to attenuate mechanical hypersensitivity in the rat

The analgesic effects of intrathecal (i.t.) omega-conotoxin CVID, an N-type Ca2+ channel antagonist, and the alpha2-adrenoceptor agonist, dexmedetomidine, were tested alone and in combination following unilateral ligation of L (lumbar) 5/6 spinal nerves in rats. Mechanical allodynia was observed prior to insertion of an i.t. catheter. Effects and interactions of omega-conotoxin CVID (0.01-10 microg/kg) and dexmedetomidine (0.1-10 microg/kg) were tested on allodynic and tail flick (thermal stimulus) responses. Only dexmedetomidine increased the latency of the tail flick response. Both dexmedetomidine and omega-conotoxin CVID completely inhibited allodynia (ED50 0.78+/-0.02 and 0.35+/-0.08 microg/kg, respectively; n=63, 41). Dexmedetomidine and omega-conotoxin CVID combined in dose ratios 0.7 and 1.3 (adjusted for ED50) were synergistic in decreasing mechanical hypersensitivity; interaction index (gamma) 0.39 (confidence interval [CI] 0.33, 0.46) and 0.3 (CI 0.23, 0.38). Despite the necessity for i.t. administration, these data suggest that the synergistic combination confers enhanced potency (lower doses) of both drugs that may avoid clinical toxicity of single drug therapy.

NK1 receptor activation leads to enhancement of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Substance P (SP) is a well-established pain messenger in the spinal cord, although its role in substantia gelatinosa (lamina II) still remains elusive. We carried out patch-clamp recordings on lamina II neurons from transverse mouse spinal cord slices (P8-12), using the selective NK1 receptor agonist [Sar9,Met(O2)11]-SP (SM-SP, 3-5 microM) in the presence of NBQX. Activation of NK1 receptors was confirmed after pre-incubation with selective NK1 antagonist L732,138 (4 microM) that consistently blocked the effects of SM-SP (nine neurons). After SM-SP challenge and spontaneous inhibitory post-synaptic current (sIPSC) analysis, 50% of recorded neurons (15 out of 30) were found to display a transient increase in frequency; in five neurons this was also associated with increase of peak amplitude. Five out of eight neurons displayed pure GABAA microM) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After miniature IPSC analysis, a significant increase in frequency was observed in three out of 14 SM-SP responsive neurons. At least four different morphological types were apparent among NK1-responsive neurons after filling with Lucifer Yellow/biocytin: fusiform with dorso-ventral dendritic arbors (i); round-to-oval with dendritic arborization mainly directed to lamina I (ii) or III (iii), and round-to-oval with dendrites sparsely distributed all around the cell body (iv). Thus, there was no correlation between morphology and electrophysiological properties of responsive neurons. Our observations provide new insights on the processing of sensory neurotransmission in spinal cord, and indicate that activation of NK1 receptors is involved in the maintenance of the inhibitory tone of substantia gelatinosa interneurons.

Antihyperalgesic effects of cizolirtine in diabetic rats: behavioral and biochemical studies

Although clinically well controlled at the metabolic level, type I diabetes resulting from an insufficient insulin secretion remains the cause of severe complications. In particular, diabetes can be associated with neuropathic pain which fails to be treated by classical analgesics. In this study, we investigated the efficacy of a novel non opioid analgesic, cizolirtine, to reduce mechanical hyperalgesia associated with streptozotocin (STZ)-induced diabetes, in the rat. Cizolirtine was compared to paroxetine, an antidepressant drug with proven efficacy to relieve painful diabetic neuropathy. Under acute conditions, cizolirtine (30 and 80 mg/kgi.p.) significantly increased paw withdrawal and vocalization thresholds in the paw pressure test in diabetic rats displaying mechanical hyperalgesia. The antihyperalgesic effects of cizolirtine persisted under chronic treatment conditions, since pre-diabetes thresholds were recovered after a two week-treatment with the drug (3 mg/kg/day, s.c.). In this respect, cizolirtine was as efficient as paroxetine (5 mg/kg per day, s.c.) which, however, was inactive under acute treatment conditions. Measurements of the spinal release of calcitonin gene-related peptide (CGRP) through intrathecal perfusion under halothane-anesthesia showed that acute administration of cizolirtine (80 mg/kg, i.p.) significantly diminished (-36%) the peptide outflow in diabetic rats suffering from neuropathic pain. This effect as well as the antihyperalgesic effect of cizolirtine were prevented by the alpha(2)-adrenoreceptor antagonist idazoxan (2 mg/kg, i.p.). These data suggest that the antihyperalgesic effect of cizolirtine in diabetic rats suffering from neuropathic pain implies an alpha(2)-adrenoceptor-dependent presynaptic inhibition of CGRP-containing primary afferent fibers.

Neurokinin release produced by capsaicin acting on the central terminals and axons of primary afferents: relationship with n-methyl-d-aspartate and gabab receptors

Capsaicin stimulates neurokinin release in the spinal cord when applied both centrally and peripherally. To determine whether these two actions have different mechanisms, we measured neurokinin 1 receptor (NK1R) internalization in rat spinal cord slices elicited by incubating the whole slice or just the dorsal root with capsaicin. NK1R internalization produced by incubating the slices with capsaicin was abolished by the NK1R antagonist RP-67580, by the vanilloid receptor 1 (VR1) antagonist capsazepine, and by eliminating Ca(2+) from the medium, but was not affected by the Na(+) channel blocker lidocaine. Therefore, the internalization was due to neurokinin release mediated by Ca(2+) entry through VR1 receptors, but did not require the firing of action potentials. Incubating the root with capsaicin produced NK1R internalization in the ipsilateral dorsal horn that was abolished when capsazepine or lidocaine was included in, or when Ca(2+) was omitted from, the medium surrounding the root. Therefore, the internalization was mediated by Ca(2+) entry in the axons through VR1, and required firing of action potentials. The efficacy of capsaicin when applied to the root (36+/-3%) was lower than when applied to the slice (91+/-3%), but its potency was the same (0.49 microM and 0.37 microM, respectively). We also investigated whether presynaptic N-methyl-D-aspartate (NMDA) and GABA(B) receptors modulate these two actions of capsaicin. Neither the NMDA receptor blocker MK-801 nor the GABA(B) agonist baclofen decreased NK1R internalization produced by 1 microM capsaicin applied to the slices, but they inhibited the internalization produced by 0.3 microM capsaicin applied to the slices or 1 microM capsaicin applied to the root. Therefore, capsaicin can produce neurokinin release from primary afferents 1) by a direct action on their central terminals and 2) by increasing the firing of action potentials on their axons. The first effect largely bypasses other modulatory mechanism, but the second does not.

Alpha-2 adrenoceptor mediating the facilitatory effect of norepinephrine on the glycine response in the spinal dorsal horn neuron of the rat

Effects of norepinephrine (NE) on the glycine-mediated inhibitory response were investigated in neurons acutely dissociated from the rat spinal dorsal horn, using nystatin perforated patch recording mode under voltage-clamp conditions. NE reversibly and concentration dependently facilitated Cl(-) current induced by 3 x 10(-5) M glycine. NE neither changed the reversal potential of the glycine response nor affected the affinity of glycine to its receptor. This effect could be mimicked by clonidine (10(-7) M) and blocked by yohimbine (10(-6) M), respectively. N-[2(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-89), an inhibitor of protein kinase A, effectively mimicked the effect of NE on glycine response, whereas chelerythrine (an inhibitor of protein kinase C) failed. NE further enhanced glycine response even in the presence of chelerythrine or stearoylcarnitine chloride (another inhibitor of protein kinase C) or chelerythrine together with stearoylcarnitine chloride. The present results suggest that alpha2-adrenoceptor is involved in the potentiation of NE on glycine response in freshly isolated spinal dorsal horn neurons. Activation of alpha2-adrenoceptor down-regulates the activity of protein kinase A that results in the potentiation of the glycinergic inhibitory effects within the spinal dorsal horn.

Relationship between capsaicin-evoked substance P release and neurokinin 1 receptor internalization in the rat spinal cord

The relationship between substance P release and the activation of its receptor in the spinal cord remains unclear. Substance P release is usually measured by radioimmunoassay, whereas the internalization of the neurokinin 1 (NK1) receptor has been used to assess its activation by noxious stimuli. Our objective was to compare substance P release and NK1 receptor internalization produced by capsaicin in rat spinal cord slices. Superfusion of the slices with capsaicin for 3 min produced a gradual increase in substance P release that peaked 3-7 min afterward, and then decreased to baseline levels. The concentration-response curve for capsaicin was biphasic, with concentrations above 10 microM producing significantly less release. The effective concentration for 50% of response (EC(50)) for capsaicin, calculated from its stimulatory phase, was 2.3 microM. However, the potency of capsaicin to elicit NK1 receptor internalization in the same slices was one order of magnitude higher (EC(50)=0.37 microM) in lamina I, probably because NK1 receptors become saturated at relatively low concentrations of substance P. The potency of capsaicin to produce internalization was progressively lower in lamina III (EC(50)=1.9 microM) and lamina IV (EC(50)=14.5 microM), suggesting that neurokinins released in laminae I-II become diluted as they diffuse to the inner dorsal horn. To study the correlation between these two measures, we plotted substance P release against NK1 receptor internalization and fitted a saturation binding function to the points. The correlation was good for laminae I (R(2)=0.82) and III (R(2)=0.78), but it was poor (R(2)=0.35) for lamina IV because NK1 receptor internalization kept on increasing at high concentrations of capsaicin, whereas substance P release decreased. In conclusion, amounts of substance P able to activate NK1 receptors may fall under the threshold of detection of radioimmunoassay. Conversely, radioimmunoassay often detects levels of substance P release well over those required to saturate NK1 receptors in the superficial dorsal horn, but that may be able to activate these receptors in nearby regions of the spinal cord.

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.

Costorage and coexistence of neuropeptides in the mammalian CNS

The term neuropeptides commonly refers to a relatively large number of biologically active molecules that have been localized to discrete cell populations of central and peripheral neurons. I review here the most important histological and functional findings on neuropeptide distribution in the central nervous system (CNS), in relation to their role in the exchange of information between the nerve cells. Under this perspective, peptide costorage (presence of two or more peptides within the same subcellular compartment) and coexistence (concurrent presence of peptides and other messenger molecules within single nerve cells) are discussed in detail. In particular, the subcellular site(s) of storage and sorting mechanisms within neurons are thoroughly examined in the view of the mode of release and action of neuropeptides as neuronal messengers. Moreover, the relationship of neuropeptides and other molecules implicated in neural transmission is discussed in functional terms, also referring to the interactions with novel unconventional transmitters and trophic factors. Finally, a brief account is given on the presence of neuropeptides in glial cells.

The NK1 receptor mediates both the hyperalgesia and the resistance to morphine in mice lacking noradrenaline

Noradrenaline (NA), a key neurotransmitter of the endogenous pain inhibitory system, acutely inhibits nociceptive transmission (including that mediated by substance P), potentiates opioid analgesia, and underlies part of the antinociceptive effects of the widely prescribed tricyclic antidepressants. Lesions of noradrenergic neurons, however, result in either normal or reduced pain behavior and variable changes in morphine antinociception, undermining the proposed association between noradrenaline (NA) deficiency and chronic pain (hyperalgesia). We used mice lacking the gene coding for dopamine beta-hydroxylase, the enzyme responsible for synthesis of NA from dopamine, to reexamine the consequences of a lack of NA on pain behavior. Here, we show that absence of NA in the central nervous system results in a substance P-mediated chronic hyperalgesia (decreased nociceptive threshold) to thermal, but not mechanical, stimuli and decreased efficacy of morphine. Contrary to studies that show substance P-mediated hyperalgesia requires intense stimuli, we found that even a mild stimulus is sufficient to evoke substance P-dependent hyperalgesia in the NA-deficient mice. Restoring central NA normalized both the nociceptive threshold and morphine efficacy, which is consistent with a tonic inhibitory effect of NA on nociceptive transmission. Unexpectedly, however, antagonists to the substance P receptor (the NK1 receptor) could achieve the same effect as NA replacement. We conclude that when unopposed by NA, substance P acting at the NK1 receptor causes chronic thermal hyperalgesia, and that the reduced opioid efficacy associated with a lack of NA is due to increased NK1-receptor stimulation.

Contribution of alpha(2) receptor subtypes to nerve injury-induced pain and its regulation by dexmedetomidine

There is evidence that noradrenaline contributes to the development and maintenance of neuropathic pain produced by trauma to a peripheral nerve. It is, however, unclear which subtype(s) of alpha adrenergic receptors (AR) may be involved. In addition to pro-nociceptive actions of AR stimulation, alpha(2) AR agonists produce antinociceptive effects. Here we studied the contribution of the alpha(2) AR subtypes, alpha(2A), alpha(2B) and alpha(2C) to the development of neuropathic pain. We also examined the antinociceptive effect produced by the alpha(2) AR agonist dexmedetomidine in nerve-injured mice. The studies were performed in mice that carry either a point (alpha(2A)) or a null (alpha(2B) and alpha(2C)) mutation in the gene encoding the alpha(2) AR. To induce a neuropathic pain condition, we partially ligated the sciatic nerve and measured changes in thermal and mechanical sensitivity. Baseline mechanical and thermal withdrawal thresholds were similar in all mutant and wild-type mice; and, after peripheral nerve injury, all mice developed comparable hypersensitivity (allodynia) to thermal and mechanical stimulation. Dexmedetomidine reversed the allodynia at a low dose (3 microg kg(-1), s.c.) and produced antinociceptive effects at higher doses (10 - 30 microg kg(-1)) in all groups except in alpha(2A) AR mutant mice. The effect of dexmedetomidine was reversed by intrathecal, but not systemic, injection of the alpha(2) AR antagonist RS 42206. These results suggest that neither alpha(2A), alpha(2B) nor alpha(2C) AR is required for the development of neuropathic pain after peripheral nerve injury, however, the spinal alpha(2A) AR is essential for the antinociceptive effects of dexmedetomidine.

The novel analgesic, cizolirtine, inhibits the spinal release of substance P and CGRP in rats

Although previous studies have established that cizolirtine (5-([(N,N-dimethylaminoethoxy)phenyl]methyl)-1-methyl-1H-pyrazol citrate) is a potent analgesic in rodents, its mechanism(s) of action remain(s) unclear. In vitro and in vivo approaches were used to assess whether cizolirtine could affect the spinal release of two pain-related neuropeptides, substance P (SP) and calcitonin gene-related peptide (CGRP), in rats. Cizolirtine significantly reduced the K(+)-evoked overflow of both the SP-like material (SPLM; -25% at 0.1 microM--0.1 mM) and CGRPLM (-20% at 0.1--1.0 microM) from slices of the dorsal half of the lumbar enlargement of the spinal cord. Intrathecal perfusion in halothane-anaesthetized rats showed that local application of cizolirtine markedly diminished the spinal outflow of SPLM (up to -50% at 0.1 mM) but only marginally that of CGRPLM. Systemic administration of cizolirtine at an analgesic dose (80 mg/kg i.p.) also reduced spinal SPLM outflow (-50%) but not that of CGRPLM. Under both in vitro and in vivo conditions, idazoxan (10 microM) antagonized the effects of cizolirtine on SPLM and CGRPLM release, suggesting their mediation through alpha(2) adrenoceptors.

Substance P

Substance P is considered to be an important neuropeptide in nociceptive processes. Although substance P was described more than 60 years ago, there is still controversy about its exact role in nociception. This article reviews the current knowledge about the function of substance P in pain. Special emphasis is put on how to use this knowledge in the development of new ways to treat pain.

Synergistic interactions between two alpha(2)-adrenoceptor agonists, dexmedetomidine and ST-91, in two substrains of Sprague-Dawley rats

Several lines of evidence indicate that the antinociception produced by intrathecal administration of the alpha(2)-adrenoceptor agonists dexmedetomidine or ST-91 is mediated by different subtypes of the alpha(2)-adrenoceptor. We recently provided additional pharmacologic evidence for this idea, as well as for differences in the function of these receptors between Harlan and Sasco rats, two widely-used outbred substrains of Sprague-Dawley rat. The present study used isobolographic analysis to further characterize the receptors at which intrathecally administered ST-91 and dexmedetomidine act in these two substrains. The rationale for these studies derives from the assumption that if dexmedetomidine and ST-91 act as agonists at the same receptor then they should interact in an additive manner. However, if they interact in a supra-additive manner, then they must act at different subtypes of the alpha(2)-adrenoceptor. In the tail-flick test, the dose-effect relationship for a 1:3 mixture of dexmedetomidine and ST-91 was shifted significantly to the left of the theoretical dose-additive line in both Harlan and Sasco Sprague-Dawley rats. A similar finding was made in the hot-plate test despite the fact that the dose-response characteristics of the agonists were different in this test. Thus, in Harlan rats, in which ST-91 is a full agonist and dexmedetomidine is essentially inactive, the dose-effect relationship for the mixture of dexmedetomidine and ST-91 was shifted far to the left of the dose-additive line. Similarly, in Sasco rats, in which ST-91 is a partial agonist and dexmedetomidine is inactive, co-administration of the two agonists also shifted the dose-response relationship to the left of the dose-additive line. The consistent finding that these two alpha(2)-adrenoceptor agonists interact in a supra-additive manner provides strong evidence that dexmedetomidine and ST-91 produce antinociception by acting at different alpha(2)-adrenoceptor subtypes in the spinal cord. This conclusion is consistent with the earlier proposal that dexmedetomidine acts predominantly at alpha(2A)-adrenoceptors whereas ST-91 acts predominantly at non-alpha(2A)-adrenoceptors. Recent anatomical evidence indicates that these non-alpha(2A) adrenoceptors may be of the alpha(2C) type. The synergistic combination of an alpha(2A)- and an alpha(2C)-adrenoceptor agonist may provide a unique and highly effective drug combination for the treatment of pain without the sedation produced by an equianalgesic dose of a single alpha(2)-adrenoceptor agonist.

Calcitonin gene-related peptide: distribution and effects on spontaneous rhythmic activity in embryonic chick spinal cord

Immunohistochemical and in vitro electrophysiological techniques were utilized to examine the distribution and possible role of calcitonin gene-related peptide (CGRP) in the spinal cord of the developing chick. CGRP-like immunoreactivity first appeared in the lateral motor column of the lumbosacral spinal cord at embryonic day 6 followed by the emergence of fiber immunoreactivity in the dorsal horn at embryonic day 11. A rostrocaudal survey of the cervical to lumbosacral spinal cord in embryonic day 18 chick demonstrated robust CGRP-like immunoreactivity at all levels in both putative motor neurons and dorsal horn fibers. Additionally, small immunoreactive lamina VII neurons were observed in sections of lumbosacral cord. In the embryonic day 10 (E10) in vitro reduced spinal cord preparation, bath application of the calcitonin gene-related peptide antagonist human alpha-CGRP fragment 8-37 decreased the frequency and increased the duration of episodes of spontaneously occurring rhythmic activity. Conversely, application of alpha or beta forms of calcitonin gene-related peptide increased the frequency of the rhythmic episodes. The electrophysiological results suggest there is a constitutive release of calcitonin gene-related peptide contributing to the spontaneous rhythmic activity. Immunohistochemical results from E10 animals suggest that CGRP-like immunoreactive putative motoneurons may be the source of the released CGRP.

Locus coeruleus modulates thalamic nociceptive responses via adrenoceptors

This study investigated the parafascicular (PF) neuronal nociceptive responses and their modulation following electrical stimulation of the locus coeruleus (LC) and intrathecal (i.t.) or intracerebroventricular (i.c.v.) administration of two alpha-adrenoceptor antagonists, the alpha2-antagonist, yohimbine, and the alpha1-antagonist, prazosin. The main results were as follows: (1) the nociceptive evoked discharges in PF neurons were suppressed by preceding stimulation of LC; (2) the suppressive effect of LC stimulation on PF neurons was replaced by a facilitatory effect following pretreatment of i.t. yohimbine in 14 units tested, while i.t. prazosin failed to alter the LC-induced suppression, even when the prazosin dose was doubled; (3) i.c.v. pretreatment with prazosin strengthened the suppressive effect of LC stimulation on PF neurons; (4) i.c.v. norepinephrine (NE) administration induced, in PF neurons, a biphasic response to noxious stimulation; an early, brief (about 10 min) inhibitory effect followed by a late, long-lasting facilitatory effect; and (5) i.c.v. pretreatment of yohimbine or prazosin prevented the inhibitory or facilitatory responses released by NE, respectively. These results provide evidence that: (1) the LC-descending projections exhibit a suppressive effect on nociceptive transmission at the spinal level through alpha2-receptors; and (2) the LC-ascending projections exhibit dual effects, facilitatory and inhibitory, at the medial thalamus (PF) level through alpha1- and alpha2-receptors, respectively.