Spinal opioid analgesia: how critical is the regulation of substance P signaling?

Neuropeptide Y release in the rat spinal cord measured with Y1 receptor internalization is increased after nerve injury

Neuropeptide Y (NPY) modulates nociception in the spinal cord, but little is known about its mechanisms of release. We measured NPY release in situ using the internalization of its Y1 receptor in dorsal horn neurons. Y1 receptor immunoreactivity was normally localized to the cell surface, but addition of NPY to spinal cord slices increased the number of neurons with Y1 internalization in a biphasic fashion (EC₅₀s of 1 nM and 1 μM). Depolarization with KCl, capsaicin, or the protein kinase A activator 6-benzoyl-cAMP also induced Y1 receptor internalization, presumably by releasing NPY. NMDA receptor activation in the presence of BVT948, an inhibitor of protein tyrosine phosphatases, also released NPY. Electrical stimulation of the dorsal horn frequency-dependently induced NPY release; and this was decreased by the Y1 antagonist BIBO3304, the Nav channel blocker lidocaine, or the Cav2 channel blocker ω-conotoxin MVIIC. Dorsal root immersion in capsaicin, but not its electrical stimulation, also induced NPY release. This was blocked by CNQX, suggesting that part of the NPY released by capsaicin was from dorsal horn neurons receiving synapses from primary afferents and not from the afferent themselves. Mechanical stimulation in vivo, with rub or clamp of the hindpaw, elicited robust Y1 receptor internalization in rats with spared nerve injury but not sham surgery. In summary, NPY is released from dorsal horn interneurons or primary afferent terminals by electrical stimulation and by activation of TRPV1, PKA or NMDA receptors in. Furthermore, NPY release evoked by noxious and tactile stimuli increases after peripheral nerve injury.

The Neurokinins: Peptidomimetic Ligand Design and Therapeutic Applications

The neurokinins are indisputably essential neurotransmitters in numerous pathoand physiological events. Being widely distributed in the Central Nervous System (CNS) and peripheral tissues, their discovery rapidly promoted them to drugs targets. As a necessity for molecular tools to understand the biological role of this class, endogenous peptides and their receptors prompted the scientific community to design ligands displaying either agonist and antagonist activity at the three main neurokinin receptors, called NK1, NK2 and NK3. Several strategies were implemented for this purpose. With a preference to small non-peptidic ligands, many research groups invested efforts in synthesizing and evaluating a wide range of scaffolds, but only the NK1 antagonist Aprepitant (EMENDT) and its prodrug Fosaprepitant (IVEMENDT) have been approved by the Food Drug Administration (FDA) for the treatment of Chemotherapy-Induced and Post-Operative Nausea and Vomiting (CINV and PONV, respectively). While non-peptidic drugs showed limitations, especially in side effect control, peptidic and pseudopeptidic compounds progressively regained attention. Various strategies were implemented to modulate affinity, selectivity and activity of the newly designed ligands. Replacement of canonical amino acids, incorporation of conformational constraints, and fusion with non-peptidic moieties gave rise to families of ligands displaying individual or dual NK1, NK2 and NK3 antagonism, that ultimately were combined with non-neurokinin ligands (such as opioids) to target enhanced biological impact.

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

Dissociation of morphine analgesic effects in the sensory and affective components of formalin-induced spontaneous pain in male and female rats

Sex differences in the analgesic effects of morphine have been previously reported in various models that represent the sensory component of pain. However, pain sensation is a complex process that consists of both sensory and affective components. It is presently unclear whether the analgesic effects of morphine between the sensory and affective components of pain are sexually dimorphic. Moreover, differences in morphine dose-response in the two components of pain have not been examined in male and female rats. Therefore, we examined the analgesic effects of morphine on the sensory and affective components of formalin-induced pain behaviors in male and female rats. To discern the sensory component, rats were pretreated with varying doses of morphine and then intraplantar formalin-induced paw flinches were measured. Morphine reduced the number of formalin-induced paw flinches at a treatment dose of 4.0mg/kg. Morphine analgesia was similar across the sexes in the early (phase 1) and late phase (phase 2) of the formalin test. To examine the affective component, rats were pretreated with varying doses of morphine, and then intraplantar formalin-induced conditioned place aversion (CPA) was examined. Formalin produced CPA, which was blocked by morphine at doses of 1.0mg/kg and higher in male and female rats. Lastly, formalin-induced cFos expression and the effects of systemic morphine were examined in the superficial dorsal horn of the spinal cord. Intraplantar formalin produced robust expression of cFos; however, morphine did not attenuate the cFos expression. These results demonstrate a notable dissociation of the analgesic effects of morphine by detecting a fourfold shift in the minimum effective dose between the sensory and affective components of formalin-induced spontaneous pain, that were similar between male and female rats. The findings further suggest disparate mechanisms involved in systemic morphine-induced analgesia in the two components of formalin-induced pain.

Hydrazone Linker as a Useful Tool for Preparing Chimeric Peptide/Nonpeptide Bifunctional Compounds

The area of multitarget compounds, joining two pharmacophores within one molecule, is a vivid field of research in medicinal chemistry. Not only pharmacophoric elements are essential for the design and activity of such compounds, but the type and length of linkers used to connect them are also crucial. In the present contribution, we describe compound 1 in which a typical opioid peptide sequence is combined with a fragment characteristic for neurokinin-1 receptor (NK1R) antagonists through a hydrazone bridge. The compound has a high affinity for μ- and δ-opioid receptors (IC₅₀= 12.7 and 74.0 nM, respectively) and a weak affinity for the NK1R. Molecular modeling and structural considerations explain the observed activities. In in vivo test, intrathecal and intravenous administrations of 1 exhibited a strong analgesic effect, which indicates potential BBB penetration. This letter brings an exemplary application of the hydrazone linker for fast, facile, and successful preparation of chimeric compounds.

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.

Contribution of the respiratory network to rhythm and motor output revealed by modulation of GIRK channels, somatostatin and neurokinin-1 receptors

Breathing is generated by a respiratory network in the brainstem. At its core, a population of neurons expressing neurokinin-1 receptors (NK1R) and the peptide somatostatin (SST) form the preBötzinger Complex (preBötC), a site essential for the generation of breathing. PreBötC interneurons generate rhythm and follower neurons shape motor outputs by activating upper airway respiratory muscles. Since NK1R-expressing preBötC neurons are preferentially inhibited by μ-opioid receptors via activation of GIRK channels, NK1R stimulation may also involve GIRK channels. Hence, we identify the contribution of GIRK channels to rhythm, motor output and respiratory modulation by NK1Rs and SST. In adult rats, GIRK channels were identified in NK1R-expressing preBötC cells. Their activation decreased breathing rate and genioglossus muscle activity, an important upper airway muscle. NK1R activation increased rhythmic breathing and genioglossus muscle activity in wild-type mice, but not in mice lacking GIRK2 subunits (GIRK2^−/−). Conversely, SST decreased rhythmic breathing via SST₂ receptors, reduced genioglossus muscle activity likely through SST₄ receptors, but did not involve GIRK channels. In summary, NK1R stimulation of rhythm and motor output involved GIRK channels, whereas SST inhibited rhythm and motor output via two SST receptor subtypes, therefore revealing separate circuits mediating rhythm and motor output.

Neurokinin 1 and opioid receptors: relationships and interactions in nervous system

Opioid receptors and neurokinin 1 receptor (NK1R) are found highly expressed in the central nervous system. The co-localization of these two kinds of receptors suggests that they might interact with each other in both the transmission and modulation of the pain signal. In this review, we explore the relationships between opioid receptors and NK1R. Substance P (SP) plays a modulatory role in the pain transmission by activating the NK1R. Opioid receptor activation can inhibit SP release. NK1R is found participating in the mechanisms of the side effects of the opioids, including opioid analgesic tolerance, hyperalgesia, anxiety behaviors of morphine reward and opioids related respiratory depression. A series of compounds such as NK1R antagonists and ligands works on both mu/delta opioid receptor (MOR/DOR) and NK1R were synthesized as novel analgesics that enhance the clinical pain management efficacy and reduce the dosage and side effects. The current status of these novel ligands and the limitations are discussed in this review. Although the working mechanisms of these ligands remained unclear, they could be used as research tool for developing novel analgesic drugs in the future.

Neurotrophin selectivity in organizing topographic regeneration of nociceptive afferents

Neurotrophins represent some of the best candidates to enhance regeneration. In the current study, we investigated the effects of artemin, a member of the glial derived neurotrophic factor (GDNF) family, on sensory axon regeneration following a lumbar dorsal root injury and compared these effects with that observed after either NGF or GDNF expression in the rat spinal cord. Unlike previously published data, artemin failed to induce regeneration of large-diameter myelinated sensory afferents when expressed within either the spinal cord or DRG. However, artemin or NGF induced regeneration of calcitonin gene related peptide positive (CGRP(+)) axons only when expressed within the spinal cord. Accordingly, artemin or NGF enhanced recovery of only nociceptive behavior and showed a cFos distribution similar to the topography of regenerating axons. Artemin and GDNF signaling requires binding to different co-receptors (GFRα3 or GFRα1, respectively) prior to binding to the signaling receptor, cRet. Approximately 70% of DRG neurons express cRet, but only 35% express either co-receptor. To enhance artemin-induced regeneration, we co-expressed artemin with either GFRα3 or GDNF. Co-expression of artemin and GFRα3 only slightly enhanced regeneration of IB4(+) non-peptidergic nociceptive axons, but not myelinated axons. Interestingly, this co-expression also disrupted the ability of artemin to produce topographic targeting and lead to significant increases in cFos immunoreactivity within the deep dorsal laminae. This study failed to demonstrate artemin-induced regeneration of myelinated axons, even with co-expression of GFRα3, which only promoted mistargeted regeneration.

Functional distinction between NGF-mediated plasticity and regeneration of nociceptive axons within the spinal cord

Successful regeneration after injury requires either the direct reformation of the circuit or the formation of a bridge circuit to provide partial functional return through a more indirect route. Presently, little is known about the specificity of how regenerating axons reconnect or reconstruct functional circuits. We have established an in vivo Dorsal root entry zone (DREZ) model, which in the presence of Nerve Growth Factor (NGF), shows very robust regeneration of peptidergic nociceptive axons, but not other sensory axons. Expression of NGF in normal, non-injured animals leads to robust sprouting of only the peptidergic nociceptive axons. Interestingly, NGF-induced sprouting of these axons leads to severe chronic pain, whereas, regeneration leads to protective-like pain without chronic pain. Using this model we set out to compare differences in behavioral outcomes and circuit features between these two groups. In this study, we examined pre-synaptic and post-synaptic markers to evaluate the relationship between synaptic connections and behavioral responses. NGF-induced sprouting of calcitonin gene-related peptide (CGRP) axons resulted in a significant redistribution of synapses and cFos expression into the deeper dorsal horn. Regeneration of only the CGRP axons showed a general reduction in synapses and cFos expression within laminae I and II; however, inflammation of the hindpaw induced peripheral sensitization. These data show that although NGF-induced sprouting of peptidergic axons induces robust chronic pain and cFos expression throughout the entire dorsal horn, regeneration of the same axons resulted in normal protective pain with a synaptic and cFos distribution similar, albeit significantly less than that shown by the sprouting of CGRP axons.

Transmitting pain and itch messages: a contemporary view of the spinal cord circuits that generate gate control

The original formulation of Gate Control Theory (GCT) proposed that the perception of pain produced by spinal cord signaling to the brain depends on a balance of activity generated in large (nonnociceptive)- and small (nociceptive)-diameter primary afferent fibers. The theory proposed that activation of the large-diameter afferent "closes" the gate by engaging a superficial dorsal horn interneuron that inhibits the firing of projection neurons. Activation of the nociceptors "opens" the gate through concomitant excitation of projection neurons and inhibition of the inhibitory interneurons. Sixty years after publication of the GCT, we are faced with an ever-growing list of morphologically and neurochemically distinct spinal cord interneurons. The present Review highlights the complexity of superficial dorsal horn circuitry and addresses the question whether the premises outlined in GCT still have relevance today. By examining the dorsal horn circuits that underlie the transmission of "pain" and "itch" messages, we also address the extent to which labeled lines can be incorporated into a contemporary view of GCT.

BDNF released during neuropathic pain potentiates NMDA receptors in primary afferent terminals

NMDA receptors in primary afferent terminals can contribute to hyperalgesia by increasing neurotransmitter release. In rats and mice, we found that the ability of intrathecal NMDA to induce neurokinin 1 receptor (NK1R) internalization (a measure of substance P release) required a previous injection of BDNF. Selective knock-down of NMDA receptors in primary afferents decreased NMDA-induced NK1R internalization, confirming the presynaptic location of these receptors. The effect of BDNF was mediated by tropomyosin-related kinase B (trkB) receptors and not p75 neurotrophin receptors (p75(NTR) ), because it was not produced by proBDNF and was inhibited by the trkB antagonist ANA-12 but not by the p75(NTR) inhibitor TAT-Pep5. These effects are probably mediated through the truncated form of the trkB receptor as there is little expression of full-length trkB in dorsal root ganglion (DRG) neurons. Src family kinase inhibitors blocked the effect of BDNF, suggesting that trkB receptors promote the activation of these NMDA receptors by Src family kinase phosphorylation. Western blots of cultured DRG neurons revealed that BDNF increased Tyr(1472) phosphorylation of the NR2B subunit of the NMDA receptor, known to have a potentiating effect. Patch-clamp recordings showed that BDNF, but not proBDNF, increased NMDA receptor currents in cultured DRG neurons. NMDA-induced NK1R internalization was also enabled in a neuropathic pain model or by activating dorsal horn microglia with lipopolysaccharide. These effects were decreased by a BDNF scavenger, a trkB receptor antagonist and a Src family kinase inhibitor, indicating that BDNF released by microglia potentiates NMDA receptors in primary afferents during neuropathic pain.

μ-Opioid receptor inhibition of substance P release from primary afferents disappears in neuropathic pain but not inflammatory pain

Opiate analgesia in the spinal cord is impaired during neuropathic pain. We hypothesized that this is caused by a decrease in μ-opioid receptor inhibition of neurotransmitter release from primary afferents. To investigate this possibility, we measured substance P release in the spinal dorsal horn as neurokinin 1 receptor (NK1R) internalization in rats with chronic constriction injury (CCI) of the sciatic nerve. Noxious stimulation of the paw with CCI produced inconsistent NK1R internalization, suggesting that transmission of nociceptive signals by the injured nerve was variably impaired after CCI. This idea was supported by the fact that CCI produced only small changes in the ability of exogenous substance P to induce NK1R internalization or in the release of substance P evoked centrally from site of nerve injury. In subsequent experiments, NK1R internalization was induced in spinal cord slices by stimulating the dorsal root ipsilateral to CCI. We observed a complete loss of the inhibition of substance P release by the μ-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in CCI rats but not in sham-operated rats. In contrast, DAMGO still inhibited substance P release after inflammation of the hind paw with complete Freund's adjuvant and in naïve rats. This loss of inhibition was not due to μ-opioid receptor downregulation in primary afferents, because their colocalization with substance P was unchanged, both in dorsal root ganglion neurons and primary afferent fibers in the dorsal horn. In conclusion, nerve injury eliminates the inhibition of substance P release by μ-opioid receptors, probably by hindering their signaling mechanisms.

Inflammation enhances Y1 receptor signaling, neuropeptide Y-mediated inhibition of hyperalgesia, and substance P release from primary afferent neurons

Neuropeptide Y (NPY) is present in the superficial laminae of the dorsal horn and inhibits spinal nociceptive processing, but the mechanisms underlying its anti-hyperalgesic actions are unclear. We hypothesized that NPY acts at neuropeptide Y1 receptors in the dorsal horn to decrease nociception by inhibiting substance P (SP) release, and that these effects are enhanced by inflammation. To evaluate SP release, we used microdialysis and neurokinin 1 receptor (NK1R) internalization in rat. NPY decreased capsaicin-evoked SP-like immunoreactivity in the microdialysate of the dorsal horn. NPY also decreased non-noxious stimulus (paw brush)-evoked NK1R internalization (as well as mechanical hyperalgesia and mechanical and cold allodynia) after intraplantar injection of carrageenan. Similarly, in rat spinal cord slices with dorsal root attached, [Leu(31), Pro(34)]-NPY inhibited dorsal root stimulus-evoked NK1R internalization. In rat dorsal root ganglion neurons, Y1 receptors colocalized extensively with calcitonin gene-related peptide (CGRP). In dorsal horn neurons, Y1 receptors were extensively expressed and this may have masked the detection of terminal co-localization with CGRP or SP. To determine whether the pain inhibitory actions of Y1 receptors are enhanced by inflammation, we administered [Leu(31), Pro(34)]-NPY after intraplantar injection of complete Freund's adjuvant (CFA) in rat. We found that [Leu(31), Pro(34)]-NPY reduced paw clamp-induced NK1R internalization in CFA rats but not uninjured controls. To determine the contribution of increased Y1 receptor-G protein coupling, we measured [(35)S]GTPγS binding simulated by [Leu(31), Pro(34)]-NPY in mouse dorsal horn. CFA inflammation increased the affinity of Y1 receptor G-protein coupling. We conclude that Y1 receptors contribute to the anti-hyperalgesic effects of NPY by mediating the inhibition of SP release, and that Y1 receptor signaling in the dorsal horn is enhanced during inflammatory nociception.

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

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

Increased spinal prodynorphin gene expression in reinflammation-associated hyperalgesia after neonatal inflammatory insult

Background

Neuroplasticity induced by neonatal inflammation is the consequence of a combination of activity-dependent changes in neurons. We investigated neuronal sensitivity to a noxious stimulus in a rat model of neonatal hind-paw peripheral inflammation and assessed changes in pain behaviour at the physiological and molecular levels after peripheral reinflammation in adulthood.

Results

A decrease in paw withdrawal latency (PWL) after a heat stimulus was documented in rats that received inflammatory injections in their left hind paws on postnatal day one (P1) and a reinflammation stimulus at postnatal 6-8 weeks of age, compared with normal rats. An increase in the expression of the prodynorphin (proDYN) gene was noted after reinflammation in the spinal cord ipsilateral to the afferents of the neonatally treated hind paw. The involvement of the activation of extracellular signal-regulated kinases (ERK) in peripheral inflammatory pain hypersensitivity was evidenced evident by the increase in phospho-ERK (pERK) activity after reinflammation.

Conclusions

Our results indicate that peripheral inflammation in neonates can permanently alter the pain processing pathway during the subsequent sensory stimulation of the region. Elucidation of the mechanism underlying the developing pain circuitry will provide new insights into the understanding of the early pain behaviours and the subsequent adaptation to pain.

Pain behavior in the formalin test persists after ablation of the great majority of C-fiber nociceptors

Although the formalin test is a widely used model of persistent pain, the primary afferent fiber types that underlie the cellular and behavioral responses to formalin injection are largely unknown. Here we used a combined genetic and pharmacological approach to investigate the effect of ablating subsets of primary afferent nociceptors on formalin-induced nocifensive behaviors and spinal cord Fos protein expression. Intrathecal capsaicin-induced ablation of the central terminals of TRPV1+neurons greatly reduced the behavioral responses and Fos elicited by low-dose (0.5%) formalin. In contrast, genetic ablation of the MrgprD-expressing subset of non-peptidergic unmyelinated afferents, which constitute a largely non-overlapping population, altered neither the behavior nor the Fos induced by low-dose formalin. Remarkably, nocifensive behavior following high-dose (2%) formalin was unchanged in mice lacking either afferent population, or even in mice lacking both populations, which together make up the great majority of C-fiber nociceptors. Thus, at high doses, which are routinely used in the formalin test, formalin-induced "pain" behavior persists in the absence of the vast majority of C-fiber nociceptors, which points to a contribution of a large spectrum of afferents secondary to non-specific formalin-induced tissue and nerve damage.

Intra-articular opioid analgesia is effective in reducing pain and inflammation in an equine LPS induced synovitis model

REASONS FOR PERFORMING STUDY

Intra-articular administration of morphine as a local analgesic and anti-inflammatory drug is widely used in human medicine. In equids, little is known about its clinical analgesic and anti-inflammatory efficacy.

OBJECTIVES

To use an inflammatory orthopaedic pain model to investigate the analgesic and anti-inflammatory effects of intra-articularly administered morphine as a new treatment modality in horses with acute arthritis.

METHODS

In a crossover study design, synovitis was induced in the left or right talocrural joint by means of intra-articular injection of 0.5 ng lipopolyssacharide (LPS). The effect of 120 mg morphine, intra-articularly administered at 1 h after induction of synovitis, was evaluated using both physiological and behavioural pain variables. Synovial fluid was sampled at 0, 4, 8, 28 and 52 h after induction of synovitis and analysed for total protein concentration, leucocyte count and for prostaglandin E(2), bradykinin and substance P concentrations by ELISA. Ranges of motion of metatarsophalangeal and talocrural joints were measured as kinematic variables with the horses walking and trotting on a treadmill under sound and lame conditions. Clinical lameness scores and several behavioural variables related to the perception of pain were obtained.

RESULTS

LPS injection caused marked transient synovitis, resulting in increased concentrations of inflammatory synovial fluid markers, clinical lameness, joint effusion and several behavioural changes, such as increased time spent recumbent, decreased limb loading at rest and decreased time spent eating silage. Intra-articular morphine resulted in a significant decrease in synovial white blood cell count, prostaglandin E(2) and bradykinin levels and improvement in clinical lameness, kinematic and behavioural parameters, compared to placebo treatment.

CONCLUSIONS

Intra-articular morphine offers potent analgesic and anti-inflammatory effects in horses suffering from acute synovitis.

POTENTIAL RELEVANCE

Local administration of opioids may be useful for horses with acute inflammatory joint pain and offers possibilities for multimodal analgesic therapies without opioid-related systemic side effects.

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.

Cannabinoid CB1 receptor facilitation of substance P release in the rat spinal cord, measured as neurokinin 1 receptor internalization

The contribution of CB1 receptors in the spinal cord to cannabinoid analgesia is still unclear. The objective of this study was to investigate the effect of CB1 receptors on substance P release from primary afferent terminals in the spinal cord. Substance P release was measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. It was induced in spinal cord slices by dorsal root stimulation and in live rats by a noxious stimulus. In spinal cord slices, the CB1 receptor antagonists AM251, AM281 and rimonabant partially but potently inhibited NK1 receptor internalization induced by electrical stimulation of the dorsal root. This was due to an inhibition of substance P release and not of NK1 receptor internalization itself, because AM251 and AM281 did not inhibit NK1 receptor internalization induced by exogenous substance P. The CB1 receptor agonist ACEA increased NK1 receptor internalization evoked by dorsal root stimulation. The effects of AM251 and ACEA cancelled each other. In vivo, AM251 injected intrathecally decreased NK1 receptor internalization in spinal segments L5 and L6 induced by noxious hind paw clamp. Intrathecal AM251 also produced analgesia to radiant heat stimulation of the paw. The inhibition by AM251 of NK1 receptor internalization was reversed by antagonists of mu-opioid and GABA(B) receptors. This indicates that CB1 receptors facilitate substance P release by inhibiting the release of GABA and opioids next to primary afferent terminals, producing disinhibition. This results in a pronociceptive effect of CB1 receptors in the spinal cord.

NMDA receptors in primary afferents require phosphorylation by Src family kinases to induce substance P release in the rat spinal cord

The function of N-methyl-d-aspartate (NMDA) receptors in primary afferents remains controversial, in particular regarding their ability to evoke substance P release in the spinal cord. The objective of this study was, first, to confirm that substance P release evoked by NMDA is mediated by NMDA receptors in primary afferent terminals. Second, we investigated whether these NMDA receptors are inactivated in some conditions, which would explain why their effect on substance P release was not observed in some studies. Substance P release was induced in spinal cord slices and measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. NMDA (combined with d-serine) induced NK1 receptor internalization with a half of the effective concentration (EC50) of 258 nM. NMDA-induced NK1 receptor internalization was abolished by the NK1 receptor antagonist L-703,606, confirming that is was caused by substance P release, by NMDA receptor antagonists (MK1801 and ifenprodil), showing that it was mediated by NMDA receptors containing the NR2B subunit, and by preincubating the slices with capsaicin, showing that the substance P release was from primary afferents. However, it was not affected by lidocaine and omega-conotoxin MVIIA, which block Na+ channels and voltage-dependent Ca2+ channels, respectively. Therefore, NMDA-induced substance P release does not require firing of primary afferents or the opening of Ca2+ channels, which is consistent with the idea that NMDA receptors induce substance P directly by letting Ca2+ into primary afferent terminals. Importantly, NMDA-induced substance P release was eliminated by preincubating the slices for 1 h with the Src family kinase inhibitors PP1 and dasatinib, and was substantially increased by the protein tyrosine phosphatase inhibitor BVT948. In contrast, PP1 did not affect NK1 receptor internalization induced by capsaicin. These results show that tyrosine-phosphorylation of these NMDA receptors is regulated by the opposite actions of Src family kinases and protein tyrosine phosphatases, and is required to induce substance P release.

Substance P release and neurokinin 1 receptor activation in the rat spinal cord increase with the firing frequency of C-fibers

Both the firing frequency of primary afferents and neurokinin 1 receptor (NK1R) internalization in dorsal horn neurons increase with the intensity of noxious stimulus. Accordingly, we studied how the pattern of firing of primary afferent influences NK1R internalization. In rat spinal cord slices, electrical stimulation of the dorsal root evoked NK1R internalization in lamina I neurons by inducing substance P release from primary afferents. The stimulation frequency had pronounced effects on NK1R internalization, which increased up to 100 Hz and then diminished abruptly at 200 Hz. Peptidase inhibitors increased NK1R internalization at frequencies below 30 Hz, indicating that peptidases limit the access of substance P to the receptor at moderate firing rates. NK1R internalization increased with number of pulses at all frequencies, but maximal internalization was substantially lower at 1-10 Hz than at 30 Hz. Pulses organized into bursts produced the same NK1R internalization as sustained 30 Hz stimulation. To determine whether substance P release induced at high stimulation frequencies was from C-fibers, we recorded compound action potentials in the sciatic nerve of anesthetized rats. We observed substantial NK1R internalization when stimulating at intensities evoking a C-elevation, but not at intensities evoking only an Adelta-elevation. Each pulse in trains at frequencies up to 100 Hz evoked a C-elevation, demonstrating that C-fibers can follow these high frequencies. C-elevation amplitudes declined progressively with increasing stimulation frequency, which was likely caused by a combination of factors including temporal dispersion. In conclusion, the instantaneous firing frequency in C-fibers determines the amount of substance P released by noxious stimuli.

Acute inflammation induces segmental, bilateral, supraspinally mediated opioid release in the rat spinal cord, as measured by mu-opioid receptor internalization

The objective of this study was to measure opioid release in the spinal cord during acute and long-term inflammation using mu-opioid receptor (MOR) internalization. In particular, we determined whether opioid release occurs in the segments receiving the noxious signals or in the entire spinal cord, and whether it involves supraspinal signals. Internalization of neurokinin 1 receptors (NK1Rs) was measured to track the intensity of the noxious stimulus. Rats received peptidase inhibitors intrathecally to protect opioids from degradation. Acute inflammation of the hind paw with formalin induced moderate MOR internalization in the L5 segment bilaterally, whereas NK1R internalization occurred only ipsilaterally. MOR internalization was restricted to the lumbar spinal cord, regardless of whether the peptidase inhibitors were injected in a lumbar or thoracic site. Formalin-induced MOR internalization was substantially reduced by isoflurane anesthesia. It was also markedly reduced by a lidocaine block of the cervical-thoracic spinal cord (which did not affect the evoked NK1R internalization) indicating that spinal opioid release is mediated supraspinally. In the absence of peptidase inhibitors, formalin and hind paw clamp induced a small amount of MOR internalization, which was significantly higher than in controls. To study spinal opioid release during chronic inflammation, we injected complete Freund's adjuvant (CFA) in the hind paw and peptidase inhibitors intrathecally. Two days later, no MOR or NK1R internalization was detected. Furthermore, CFA inflammation decreased MOR internalization induced by clamping the inflamed hind paw. These results show that acute inflammation, but not chronic inflammation, induces segmental opioid release in the spinal cord that involves supraspinal signals.

Spinal N-methyl-D-aspartate receptors and nociception-evoked release of primary afferent substance P

Dorsal horn N-methyl-D-aspartate (NMDA) receptors contribute significantly to spinal nociceptive processing through an effect postsynaptic to non-primary glutamatergic axons, and perhaps presynaptic to the primary afferent terminals. The present study sought to examine the regulatory effects of NMDA receptors on primary afferent release of substance P (SP), as measured by neurokinin 1 receptor (NK1r) internalization in the spinal dorsal horn of rats. The effects of intrathecal NMDA alone or in combination with D-serine (a glycine site agonist) were initially examined on basal levels of NK1r internalization. NMDA alone or when co-administered with D-serine failed to induce NK1r internalization, whereas activation of spinal TRPV1 receptors by capsaicin resulted in a notable NK1r internalization. To determine whether NMDA receptor activation could potentiate NK1r internalization or pain behavior induced by a peripheral noxious stimulus, intrathecal NMDA was given prior to an intraplantar injection of formalin. NMDA did not alter the formalin-induced NK1r internalization nor did it enhance the formalin paw flinching behavior. To further characterize the effects of presynaptic NMDA receptors, the NMDA antagonists DL-2-amino-5-phosphonopentanoic acid (AP-5) and MK-801 were intrathecally administered to assess their regulatory effects on formalin-induced NK1r internalization and pain behavior. AP-5 had no effect on formalin-induced NK1r internalization, whereas MK-801 produced only a modest reduction. Both antagonists, however, reduced the formalin paw flinching behavior. In subsequent in vitro experiments, perfusion of NMDA in spinal cord slice preparations did not evoke basal release of SP or calcitonin gene-related peptide (CGRP). Likewise, perfusion of NMDA did not enhance capsaicin-evoked release of the two peptides. These results suggest that presynaptic NMDA receptors in the spinal cord play little if any role on the primary afferent release of SP.

Noxious mechanical stimulation evokes the segmental release of opioid peptides that induce mu-opioid receptor internalization in the presence of peptidase inhibitors

The internalization of mu-opioid receptors (MORs) provides an ideal way to locate areas of opioid peptide release. We used this method to study opioid release in the spinal cord evoked by noxious stimuli in anesthetized rats. Previous studies have shown that opioids released in the spinal cord produce MOR internalization only when they are protected from peptidase degradation. Accordingly, rats were implanted with chronic intrathecal catheters that were used to inject a mixture of peptidase inhibitors (amastatin, captopril and phosphoramidon) onto the lumbar spinal cord. Five minutes later, a noxious stimulus was delivered to the paw. Lumbar spinal segments were double-stained with antibodies against MORs and neurokinin 1 receptors (NK1Rs) using immunofluorescence. Mechanical stimulation of the hindpaw consisted of repeated 10 s clamps with a hemostat for 10 min. In the ipsilateral dorsal horn, the stimulus produced abundant NK1R internalization in segments L3-L6, and a more modest but significant MOR internalization in segments L5 and L6. In the contralateral dorsal horn, NK1R was substantially lower and MOR internalization was negligible. The same mechanical stimulus applied to a forepaw did not produce NK1R or MOR internalization in the lumbar spinal cord. Thermal stimulation consisted of immersing a hindpaw in water at 52 degrees C for 2 min. It produced substantial NK1R internalization ipsilaterally in segment L6, but no MOR internalization. These results show that mechanical stimulation induces segmental opioid release, i.e., in the dorsal horn receiving the noxious signals and not in other spinal segments.

The impact of opioid-induced hyperalgesia for postoperative pain

Clinical evidence suggests that--besides their well known analgesic activity - opioids can increase rather than decrease sensitivity to noxious stimuli. Based on the observation that opioids can activate pain inhibitory and pain facilitatory systems, this pain hypersensitivity has been attributed to a relative predominance of pronociceptive mechanisms. Acute receptor desensitization via uncoupling of the receptor from G-proteins, upregulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA)-receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days in rodents, and also in humans large-doses of intraoperative micro-receptor agonists were found to increase postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients is often associated with a requirement for increasing doses and the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha2-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor/selectivity.

Analgesic effects of subarachnoidally administered hyperbaric opioids in horses

OBJECTIVE

To evaluate the effects of subarachnoidally administered hyperbaric morphine, buprenorphine, and methadone on avoidance threshold to noxious electrical stimulation of the perineal, sacral, lumbar, and thoracic regions in horses.

ANIMALS

6 healthy adult horses.

PROCEDURES

Horses were assigned to receive subarachnoid administration of hyperbaric morphine (0.01 mg/kg), buprenorphine (0.001 mg/kg), methadone (0.01 mg/kg), or 10% dextrose solution in equal volumes (5 mL). Electrical stimulation was applied every 10 minutes for 60 minutes and every 30 minutes for 120 minutes after subarachnoid injection over the dermatomes of the perineal, sacral, lumbar, and thoracic regions, and the avoidance threshold voltage was recorded. Heart and respiratory rate, blood gas tensions, serum electrolyte concentrations, and sedative effects were also evaluated.

RESULTS

Administration of 10% dextrose solution did not change the avoidance threshold. Morphine and methadone significantly increased the avoidance threshold by 10 minutes after injection, which lasted until 120 minutes after subarachnoid administration in the perineal, sacral, lumbar, and thoracic regions. Profound analgesia (avoidance threshold > 40 V) was achieved in all regions. Buprenorphine also significantly increased the avoidance threshold by 10 minutes (36 V) after injection, which lasted 60 minutes and was considered moderate. Heart rate, blood pressure, respiratory rate, and blood gas tensions stayed within reference range. No ataxia, signs of sedation, or CNS excitement were observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Subarachnoid administration of hyperbaric morphine or methadone produces intense analgesia for 120 minutes over the dermatomes of the perineal, sacral, lumbar, and thoracic areas without cardiorespiratory depression, ataxia, or CNS excitement in horses.

Role of NK-1 neurotransmission in opioid-induced hyperalgesia

Opiates are among the most important drugs for treatment of moderate to severe pain and prolonged opiate administration is often required to treat chronic pain states. We investigated the neurobiological actions of sustained opiate administration revealing paradoxical pronociceptive adaptations associated with NK-1 receptor function. Sustained morphine delivered over 6 days elicited hyperalgesia in rats and mice during the period of opiate delivery. Sustained morphine administration increased substance P (SP) and NK-1 receptor expression in the spinal dorsal horn. Sustained morphine treatment also enhanced capsaicin-evoked SP release in vitro, and increased internalization of NK-1 receptors in response to noxious stimulation. While NK-1 receptor internalization was observed primarily in the superficial laminae of placebo-treated rats, NK-1 receptor internalization was seen in both superficial and deep lamina of the dorsal horn in morphine-treated animals. Morphine-induced hyperalgesia was reversed by spinal administration of an NK-1 receptor antagonist in rats and mice, and was observed in wildtype (NK-1(+/+)), but not NK-1 receptor knockout (NK-1(-/-)), mice. These data support a critical role for the NK-1 receptor in the expression of sustained morphine-induced hyperalgesia. Additionally, these data indicate that sustained opiate administration induces changes reminiscent of those associated with inflammatory pain. These opiate-induced changes might produce unintended deleterious actions in the course of pain treatment in patients. Understanding of sustained morphine-induced neurochemical changes will help identify approaches that limit the deleterious actions of opiates.

Resting and evoked spinal substance P release during chronic intrathecal morphine infusion: parallels with tolerance and dependence

Spinal opiate analgesia is associated with presynaptic inhibition of release of excitatory neurotransmitters/neuromodulators, e.g., substance P (SP), from primary afferent terminals. Chronic intrathecal (i.t.) administration of opiates such as morphine results in an initial analgesia followed by tolerance and a state of dependence. In this study, we examined the resting and evoked neurokinin 1 receptor (NK1r) internalization, indicative of endogenous SP release, in dorsal horn neurons of the lumbar spinal cord by immunocytochemistry during chronic i.t. infusion of morphine in rats. Noxious mechanical stimulation (compression) applied to unilateral hind paw evoked a significant increase in NK1r internalization in lamina I neurons in the ipsilateral dorsal horn. Intrathecal morphine infusion (40 nmol/microl/h) for 1 day possessed similar analgesic efficacy as acute morphine and blocked compression-induced spinal NK1r internalization. After 5 days of morphine infusion, thermal escape latencies were the same as in preinfusion animals or saline-infused controls, and compression-evoked NK1r internalization was no longer suppressed. Systemic administration of naloxone to rats on day 6 of morphine infusion resulted in prominent withdrawal behaviors and a concomitant increase in NK1r internalization in dorsal horn. The naloxone-induced internalization was blocked by NK1r antagonist L-703,606 [cis-2-(diphenylmethyl)-N-[(2-iodophenyl)methyl]-1 azabicyclo[2.2.2]octan-3-amine] or pretreatment with capsaicin, confirming that the internalization is due to the endogenous SP release from the primary afferents. We conclude that inability to suppress release of excitatory neurotransmitters/neuromodulators from primary afferents by morphine after chronic exposure is an important component in spinal morphine tolerance, and excessive release from these afferents contributes to the spinal morphine withdrawal syndrome.

Rat trigeminal lamina I neurons that project to thalamic or parabrachial nuclei contain the mu-opioid receptor

Ligands of the mu-opioid receptor are known to inhibit nociceptive transmission in the dorsal horn, yet the cellular site(s) of action for this inhibition remain to be fully elucidated. Neurons located in lamina I of the dorsal horn are involved in distinct aspects of nociceptive transmission. Neurons projecting to the thalamus are thought to be involved in sensory-discriminative aspects of pain perception, while neurons projecting to the parabrachial nucleus are thought to be important for emotional and/or autonomic responses to noxious stimuli. The present study examined these two populations of lamina I projection neurons in the trigeminal dorsal horn to determine if the mu-opioid receptor protein (MOR1) is differentially located in these populations of neurons. Lamina I projection neurons were identified using the retrograde tracer FluoroGold (FGold). FGold was injected into either the contralateral thalamus (ventral posterolateral (VPM)/ventral posterolateral (VPL) thalamic region) or into the ipsilateral parabrachial nuclei. The distribution of MOR1 in these neurons was determined using immunocytochemistry. The distribution of MOR1-ir within these two populations of lamina I projection neurons was examined by both confocal and electron microscopy. We found that both populations of projection neurons contained MOR1. Immunogold analyses revealed the presence of MOR1-ir at membrane sites and within the cytoplasm of these neurons. Cytoplasmic receptor labeling may represent sites of synthesis, recycling or reserve populations of receptors. MOR1 was primarily found in the somata and proximal dendrites of projection neurons. In addition, these neurons rarely received synaptic input from MOR1-containing axon terminals. These results indicate that lamina I neurons in trigeminal dorsal horn that project to the thalamic and parabrachial nuclei contain MOR1 and are likely sites of action for MOR ligands that modulate sensory and/or autonomic aspects of pain transmission in the trigeminal dorsal horn.

GABAA receptor facilitation of neurokinin release from primary afferent terminals in the rat spinal cord

Our goal was to test the following hypotheses: 1) GABA(A) receptors facilitate neurokinin release from primary afferent terminals; 2) they do this by suppressing an inhibitory effect of GABA(B) receptors; 3) the activation of these two receptors is controlled by the firing frequency of primary afferents. We evoked neurokinin release by stimulating the dorsal root attached to spinal cord slices, and measured it using neurokinin 1 receptor (NK1R) internalization. Internalization evoked by root stimulation at 1 Hz (but not at 100 Hz) was increased by the GABA(A) receptor agonists muscimol (effective concentration of drug for 50% of the increase [EC50] 3 microM) and isoguvacine (EC50 4.5 microM). Internalization evoked by root stimulation at 100 Hz was inhibited by the GABA(A) receptor antagonists bicuculline (effective concentration of drug for 50% of the inhibition [IC50] 2 microM) and picrotoxin (IC50 243 nM). Internalization evoked by incubating the root with capsaicin (to selectively recruit nociceptive fibers) was increased by isoguvacine and abolished by picrotoxin. Therefore, GABA(A) receptors facilitate neurokinin release. Isoguvacine-facilitated neurokinin release was inhibited by picrotoxin, low Cl-, low Ca2+, Ca2+ channel blockers and N-methyl-D-aspartate receptor antagonists. Bumetanide, an inhibitor of the Na(+)-K(+)-2Cl- cotransporter, inhibited isoguvacine-facilitated neurokinin release, but this could be attributed to a direct inhibition of GABA(A) receptors. The GABA(B) agonist baclofen inhibited NK1R internalization evoked by 100 Hz root stimulation (IC50 1.5 microM), whereas the GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl) phosphinic acid (CGP-55845) increased NK1R internalization evoked by 1 Hz root stimulation (EC50 21 nM). Importantly, baclofen inhibited isoguvacine-facilitated neurokinin release, and CGP-55845 reversed the inhibition of neurokinin release by bicuculline. In conclusion, 1) GABA(B) receptors located presynaptically in primary afferent terminals inhibit neurokinin release; 2) GABA(A) receptors located in GABAergic interneurons facilitate neurokinin release by suppressing GABA release onto these GABA(B) receptors; 3) high frequency firing of C-fibers stimulates neurokinin release by activating GABA(A) receptors and inhibiting GABA(B) receptors, whereas low frequency firing inhibits neurokinin release by the converse mechanisms.

Spinal dorsal horn neurone targets for nociceptive primary afferents: do single neurone morphological characteristics suggest how nociceptive information is processed at the spinal level

It has become increasingly clear that nociceptive information is signalled by several anatomically distinct populations of primary afferents that target different populations of neurones in the spinal cord. It is probable that these different systems all give rise to the sensation pain and hence, an understanding of their separate roles and the processes that they employ, may offer ways of selectively targeting pain arising from different causes. The review focuses on what is known of the anatomy of neurones in LI-III of the spinal dorsal horn that are implicated in nociception. The dendritic geometry and synaptic input of the large LI neurones that receive input from primary afferents containing substance P that express neurokinin 1 (NK(1)) receptors suggests that these neurones may monitor the extent of injury rather than the specific localisation of a discrete noxious stimulus. This population of neurones is also critically involved in hyperalgesia. In contrast neurones in LII with the morphology of stalked cells that receive primary afferent input from glomerular synapses may be more suitable for fine discrimination of the exact location of a noxious event such as a sting or parasite attack. The review focuses as far as possible on precisely defined anatomy in the belief that only by understanding these anatomical relationships will we eventually be able to interpret the complex processes occurring in the dorsal horn. The review attempts to be an accessible guide to a sometimes complex and highly specialised literature in this field.

The contribution of spinal cord neurokinin-1 receptor signaling to pain

Discovery of the occurrence of neurokinin-1 (NK-1) receptor internalization in response to agonist activation has provided researchers with a new tool for studying tachykinin actions. Using the readily observable end point of NK-1 receptor internalization as an activity marker, this observation has allowed for more detailed study of tachykinin systems in vivo and in vitro. What has this technique taught us about tachykinin function and activity in the spinal cord? Here we discuss recent findings, which shed light on the functional relevance of receptor internalization, the regulation of neuropeptide release from primary afferent nociceptors, and the signaling produced by tachykinins during nociception and injury. The potential consequences of these discoveries for the treatment of pain and understanding of the role of tachykinins in nociception are discussed.

Cardiovascular and behavioural effects induced by naloxone-precipitated morphine withdrawal in rat: characterization with tachykinin antagonists

This study examined the intracerebroventricular (i.c.v.) effects of three selective tachykinin receptor antagonists on the cardiovascular and behavioural responses induced by naloxone-precipitated morphine withdrawal in rats. I.c.v. injection of naloxone (10 microg) to morphine pre-treated rats (i.c.v. for 5 days) induced an immediate increase in blood pressure ( approximately 10 mmHg) and behavioural activity (sniffing > rearing > face washing approximately grooming approximately wet dog shake) without causing significant heart rate changes. The prior i.c.v. injection of the NK(1) receptor antagonist (6.5 nmol LY306740) reduced face washing and grooming during morphine withdrawal. NK(2) and NK(3) receptor antagonists (6.5 nmol SR48968 and R820) did not affect behavioural effects, yet the co-injection of the three tachykinin antagonists reduced all behavioural activity. The pressor response was not affected by the selective inhibition of NK(1) and NK(3) receptors while both blood pressure and heart rate were markedly enhanced by SR48968 during morphine withdrawal. The potentiating effect of SR48968 was prevented following simultaneous blockade of the three tachykinin receptors. In addition to confirming the involvement of central tachykinins in behavioural manifestations to morphine withdrawal, data suggest a modulatory function for tachykinins, especially the NK(2) receptor, in brain autonomic control of blood pressure and heart rate in supraspinal noloxone-precipitated withdrawal.

Effect of peptidases on the ability of exogenous and endogenous neurokinins to produce neurokinin 1 receptor internalization in the rat spinal cord

The ability of peptidases to restrict neurokinin 1 receptor (NK1R) activation by exogenously applied or endogenously released neurokinins was investigated by measuring NK1R internalization in rat spinal cord slices. Concentration-response curves for substance P and neurokinin A were obtained in the presence and absence of 10 microm thiorphan, an inhibitor of neutral endopeptidase (EC 3.4.24.11), plus 10 microm captopril, an inhibitor of dipeptidyl carboxypeptidase (EC 3.4.15.1). These inhibitors significantly decreased the EC50 of substance P to produce NK1R internalization from 32 to 9 nm, and the EC50 of neurokinin A from 170 to 60 nm. Substance P was significantly more potent than neurokinin A, both with and without these peptidase inhibitors. In the presence of peptidase inhibitors, neurokinin B was 10 times less potent than neurokinin A and 64 times less potent than substance P (EC50=573 nm). Several aminopeptidase inhibitors (actinonin, amastatin, bacitracin, bestatin and puromycin) failed to further increase the effect of thiorphan plus captopril on the NK1R internalization produced by 10 nm substance P. Electrical stimulation of the dorsal root produced NK1R internalization by releasing endogenous neurokinins. Thiorphan plus captopril increased NK1R internalization produced by 1 Hz stimulation, but not by 30 Hz stimulation. Therefore, NEN and DCP restrict NK1R activation by endogenous neurokinins when they are gradually released by low-frequency firing of primary afferents, but become saturated or inhibited when primary afferents fire at a high frequency.

Dorsal horn neurons firing at high frequency, but not primary afferents, release opioid peptides that produce micro-opioid receptor internalization in the rat spinal cord

To determine what neural pathways trigger opioid release in the dorsal horn, we stimulated the dorsal root, the dorsal horn, or the dorsolateral funiculus (DLF) in spinal cord slices while superfusing them with peptidase inhibitors to prevent opioid degradation. Internalization of mu-opioid receptors (MOR) and neurokinin 1 receptors (NK1R) was measured to assess opioid and neurokinin release, respectively. Dorsal root stimulation at low, high, or mixed frequencies produced abundant NK1R internalization but no MOR internalization, indicating that primary afferents do not release opioids. Moreover, capsaicin and NMDA also failed to produce MOR internalization. In contrast, dorsal horn stimulation elicited MOR internalization that increased with the frequency, being negligible at <10 Hz and maximal at 500 Hz. The internalization was abolished by the MOR antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), in the presence of low Ca2+ and by the Na+ channel blocker lidocaine, confirming that it was caused by opioid release and neuronal firing. DLF stimulation in "oblique" slices (encompassing the DLF and the dorsal horn of T11-L4) produced MOR internalization, but only in areas near the stimulation site. Moreover, cutting oblique slices across the dorsal horn (but not across the DLF) eliminated MOR internalization in areas distal to the cut, indicating that it was produced by signals traveling in the dorsal horn and not via the DLF. These findings demonstrate that some dorsal horn neurons release opioids when they fire at high frequencies, perhaps by integrating signals from the rostral ventromedial medulla, primary afferents, and other areas of the spinal cord.

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.

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.

Neurokinin release in the rat nucleus of the solitary tract via NMDA and AMPA receptors

Neurokinins (substance P, neurokinin A and neurokinin B) and the neurokinin receptors, the NK1 and NK3 receptors, are largely expressed in the nucleus of the solitary tract (NST) where they are involved in the central regulation of visceral function. Studying the mechanisms that control neurokinin release can provide valuable information concerning the control of autonomic functions subserved by the NST. Glutamate is the principal excitatory neurotransmitter in the NST and the main neurotransmitter of afferent vagal fibers. Neurokinins and glutamate may interact within the NST. In the present study, we have examined the contribution of the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) subtypes of glutamate receptors on the release of the endogenous neurokinins in the NST. We used internalization of the NK1 or NK3 receptor as an index of endogenous neurokinin release assessed by immunocytochemical visualization of the NK1 or NK3 receptor endocytosis. Experiments were performed in vitro using rat brainstem slices. A first series of experiments were done in order to validate our in vitro preparation. Application of substance P, neurokinin A or neurokinin B induced dose-dependent internalization of NK1 and NK3 receptor. This was blocked by the endocytosis inhibitor, phenylarzine oxide. The NK1 receptor antagonist SR140333 blocked internalization of NK1 receptor induced by the three neurokinins. In addition, the internalization NK1 or NK3 receptor was reversible. These results demonstrate that internalization and recycling mechanisms of NK1 or NK3 receptor were preserved in in vitro brainstem slices. Application of NMDA or AMPA induced internalization of NK1 receptor. This was blocked by the application of SR140333 suggesting that NK1 receptor internalization is due to the binding of endogenous neurokinin released under the effects of NMDA and AMPA. Application of NMDA or AMPA had no effect on NK3 receptor. Application of tetrodotoxin blocked NK1 receptor internalization induced by NMDA, demonstrating that the release of neurokinins is dependent of axon potential propagation. This result excludes the hypothesis of a release on neurokinins via pre-synaptic NMDA receptors located on neurokinin-containing axon terminals. NMDA or AMPA may directly induce neurokinin release in the NST by acting on receptors located on the cell bodies and dendrites of neurokinin-containing neurons. Release of neurokinins may also be the result of a general activation of neuron networks of the NST by NMDA or AMPA. To conclude, our results suggest that glutamate, through activation of post-synaptic NMDA and AMPA receptors, contributes to neurokinin signaling in the NST.

Peptidases prevent mu-opioid receptor internalization in dorsal horn neurons by endogenously released opioids

To evaluate the effect of peptidases on mu-opioid receptor (MOR) activation by endogenous opioids, we measured MOR-1 internalization in rat spinal cord slices. A mixture of inhibitors of aminopeptidases (amastatin), dipeptidyl carboxypeptidase (captopril), and neutral endopeptidase (phosphoramidon) dramatically increased the potencies of Leu-enkephalin and dynorphin A to produce MOR-1 internalization, and also enhanced the effects of Met-enkephalin and alpha-neoendorphin, but not endomorphins or beta-endorphin. The omission of any one inhibitor abolished Leu-enkephalin-induced internalization, indicating that all three peptidases degraded enkephalins. Amastatin preserved dynorphin A-induced internalization, and phosphoramidon, but not captopril, increased this effect, indicating that the effect of dynorphin A was prevented by aminopeptidases and neutral endopeptidase. Veratridine (30 microm) or 50 mm KCl produced MOR-1 internalization in the presence of peptidase inhibitors, but little or no internalization in their absence. These effects were attributed to opioid release, because they were abolished by the selective MOR antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) and were Ca(2+) dependent. The effect of veratridine was protected by phosphoramidon plus amastatin or captopril, but not by amastatin plus captopril or by phosphoramidon alone, indicating that released opioids are primarily cleaved by neutral endopeptidase, with a lesser involvement of aminopeptidases and dipeptidyl carboxypeptidase. Therefore, because the potencies of endomorphin-1 and endomorphin-2 to elicit internalization were unaffected by peptidase inhibitors, the opioids released by veratridine were not endomorphins. Confocal microscopy revealed that MOR-1-expressing neurons were in close proximity to terminals containing opioids with enkephalin-like sequences. These findings indicate that peptidases prevent the activation of extrasynaptic MOR-1 in dorsal horn neurons.

Spinal lamina I neurones that express neurokinin 1 receptors: II. Electrophysiological characteristics, responses to primary afferent stimulation and effects of a selective mu-opioid receptor agonist

Intracellular recordings were made from neurones in laminae I and II of the dorsal horn of a longitudinal, parasagittal spinal cord slice from the neonatal rat. Their responses to peripheral nerve stimulation were first tested. Then the responses to bath application of [Sar(9),Met(O(2))(11)]-substance P and [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin, neurokinin 1 (NK(1)) and mu-opioid receptor agonists respectively, were studied. Finally, the structure of each neurone was investigated by injecting neurobiotin intracellularly following recording, and immunocytochemical studies were performed on post-fixed tissues to reveal whether they expressed the NK(1) receptor. Nine lamina I neurones where shown to express NK(1) receptor and these were depolarised by [Sar(9),Met(O(2))(11)]-substance P. These neurones typically received a powerful C-fibre input that was strongly inhibited, presynaptically, by the mu-opioid receptor agonist.The structure, afferent input, opioid sensitivity and intrinsic properties of these neurones are all consistent with the view that they are a major relay for nociceptive information leading to intense pain. The characteristics of 10 other neurones studied in which the NK(1) receptor was not found to be expressed at levels detectable by immunocytochemistry are briefly described for comparison. These results contribute to the emergent view that the large neurones in the most dorsal neuronal layer (lamina I) of the spinal cord, which express the principal receptor for substance P (NK(1)) over their entire soma and dendrites, are a major relay for information leading to intense pain. Inhibition of the relay of information by these neurones would be predicted to result in analgesia and hence, a detailed knowledge of their unique neurochemical characteristics is of paramount importance.

The role of descending fibers from the rostral ventromedial medulla in opioid analgesia in rats

There has been controversy as to whether the contribution of descending fibers from the rostral ventromedial medulla to opioid analgesia depends on the nature of the noxious stimulus eliciting pain. In the present study, inactivation of descending fibers by microinjection of muscimol (50 ng) in the rostral ventromedial medulla abolished morphine analgesia in the tail immersion and hot plate tests but decreased morphine analgesia by 60% in the formalin test. Analysis of the dose-response relation for morphine after inactivation of descending fibers revealed that, except for the tail immersion test, high doses of morphine could not overcome the block induced by muscimol. Also, morphine analgesia elicited supraspinally was not detectable when descending fibers were inactivated, suggesting that the analgesic effect of morphine in the brain requires a relay via the rostral ventromedial medulla. The analgesic effect of buprenorphine also depends on the integrity of descending fibers from the rostral ventromedial medulla. The results indicate that descending fibers from the rostral ventromedial medulla are critically important to the analgesic effect of opioids, regardless of the type of noxious stimulation eliciting pain. Residual analgesic effects of opioids after inactivation of descending fibers may be due to peripheral effects in the presence of inflammation.

Time-course of the internalization and recycling of neurokinin 1 receptors in rat dorsal horn neurons

Neurokinin 1 receptor (NK1R) internalization in dorsal horn neurons is important for intracellular signaling in nociception. Since the rates of NK1R internalization and recycling vary substantially, particularly between cultured and native cells, it is imperative to characterize them in dorsal horn neurons. When rat spinal cord slices were incubated at 35 degrees C with 1 microM substance P (SP), NK1Rs in lamina I neurons internalized rapidly following apparent exponential association kinetics (half-life=71 s). Confocal images of neuronal somas at different incubation times revealed that NK1Rs were uniformly distributed at the cell surface up to 30 s and formed aggregates at the membrane by 60 s. NK1R-containing endosomes migrated to the cell interior at 90-120 s, and were found throughout the cytoplasm at 300 s and thereafter. Upon elimination of SP, NK1Rs recycled back to the cell surface following an apparent linear time-course. Recycling was slower than internalization, being completed in 60-90 min. Confocal microscopy revealed that NK1R-containing endosomes docked at the cell surface 45 min after the elimination of SP. NK1Rs still formed aggregates at the cell surface at 60 min, but were once again uniformly distributed along the membrane by 90 min. NK1R internalization and recycling also occurred in lamina I dendrites. NK1R-containing endosomes in dendrites did not migrate to the cytoplasm. These results show that NK1R internalization and recycling are considerably faster in dorsal horn neurons than in cultured cells, and that most NK1Rs in dorsal horn neurons are internalized when NK1R-mediated hyperalgesia is more severe.

MOR-1-immunoreactive neurons in the dorsal horn of the rat spinal cord: evidence for nonsynaptic innervation by substance P-containing primary afferents and for selective activation by noxious thermal stimuli

A direct action of mu-opioid agonists on neurons in the spinal dorsal horn is thought to contribute to opiate-induced analgesia. In this study we have investigated neurons that express the mu-opioid receptor MOR-1 in rat spinal cord to provide further evidence about their role in nociceptive processing. MOR-1-immunoreactive cells were largely restricted to lamina II, where they comprised approximately 10% of the neuronal population. The cells received few contacts from nonpeptidergic unmyelinated afferents, but many from substance P-containing afferents. However, electron microscopy revealed that most of these contacts were not associated with synapses. None of the MOR-1 cells in lamina II expressed the neurokinin 1 receptor; however, the mu-selective opioid peptide endomorphin-2 was present in the majority (62-82%) of substance P axons that contacted them. Noxious thermal stimulation of the foot induced c-Fos expression in approximately 15% of MOR-1 cells in the medial third of the ipsilateral dorsal horn at mid-lumbar level. However, following pinching of the foot or intraplantar injection of formalin very few MOR-1 cells expressed c-Fos, and for intraplantar formalin injection this result was not altered significantly by pretreatment with systemic naloxone. Although these findings indicate that at least some of the neurons in lamina II with MOR-1 are activated by noxious thermal stimulation, the results do not support the hypothesis that the cells have a role in transmitting nociceptive information following acute mechanical or chemical noxious stimuli.

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.

Presynaptic localization of the carboxy-terminus epitopes of the mu opioid receptor splice variants MOR-1C and MOR-1D in the superficial laminae of the rat spinal cord

Opioids inhibit nociceptive transmission at the level of the spinal cord, possibly through inhibition of neurotransmitter release by presynaptic mu opioid receptors (MORs) thus preventing the activation of ascending pathways and the perception of pain. Most nociceptive primary afferents are unmyelinated fibers containing peptides such as substance P and/or calcitonin gene-related peptide. However, few terminals contain both substance P and MOR. Recently, we identified new carboxy-terminal MOR splice variants that are localized in the superficial laminae of the dorsal horn. We now report the precise cellular distribution of two of these MOR-1 variants, MOR-1C (exon 7/8/9 epitope) and MOR-1D (exon 8/9 epitope), at the ultrastructural level. In the superficial laminae of the dorsal horn, the majority of the labeling of MOR-1C and MOR-1D was found in unmyelinated axons. This distribution contrasts with that of MOR-1 (exon 4 epitope), in which labeling is equally found in dendrites and soma, as well as in axons. The presence of dense core vesicles in many of the MOR-1C-like immunoreactive terminals implies that this splice variant might be involved in presynaptic inhibition of transmitter release from peptide-containing afferents to the dorsal horn. Consistent with this finding, confocal microscopy analyses showed that many MOR-1C profiles in laminae I-II also contained calcitonin gene-related peptide, whereas fewer MOR-1 profiles contained either substance P or calcitonin gene-related peptide in this same region. From these findings we suggest that there are differential distributions of MOR-1 splice variants as well as distinct peptide colocalizations in the dorsal horn.

Tooth extraction-induced internalization of the substance P receptor in trigeminal nucleus and spinal cord neurons: imaging the neurochemistry of dental pain

Although pains arising from the craniofacial complex can be severe and debilitating, relatively little is known about the peripheral and central mechanisms that generate and maintain orofacial pain. To better understand the neurons in the trigeminal complex and spinal cord that are activated following nociceptive stimuli to the orofacial complex, we examined substance P (SP) induced internalization of substance P receptors (SPR) in neurons following dental extraction in the rat. Unilateral gingival reflection or surgical extraction of a rat maxillary incisor or molar was performed and tissues harvested at various time points post-extraction. Immunohistochemical analysis of brainstem and cervical spinal cord sections was performed using an anti-SPR antibody and confocal imaging. Both the number and location of neurons showing SPR internalization was dependent on the location and extent of tissue injury. Whereas extraction of the incisor induced internalization of SPR in neurons bilaterally in nucleus caudalis and the spinal cord, extraction of the molar induced strictly unilateral internalization of SPR-expressing neurons in the same brain structures. Minor tissue injury (retraction of the gingiva) activated SPR neurons located in lamina I whereas more extensive and severe tissue injury (incisor or molar extraction) induced extensive SPR internalization in neurons located in both laminae I and III-V. The rostrocaudal extent of the SPR internalization was also correlated with the extent of tissue injury. Thus, following relatively minor tissue injury (gingival reflection) neurons showing SPR internalization were confined to the nucleus caudalis while procedures which cause greater tissue injury (incisor or molar extraction), neurons showing SPR internalization extended from the interpolaris/caudalis transition zone through the C7 spinal level. Defining the population of neurons activated in orofacial pain and whether analgesics modify the activation of these neurons should provide insight into the mechanisms that generate and maintain acute and chronic orofacial pain.

Presynaptic regulation of spinal cord tachykinin signaling via GABA(B) but not GABA(A) receptor activation

Internalization of spinal cord neurokinin-1 receptors following noxious stimulation provides a reliable measure of tachykinin signaling. In the present study, we examined the contribution of GABAergic mechanisms to the control of nociceptor processing involving tachykinins. Spinal administration of the GABA(B) receptor agonist R(+)-baclofen in the rat, at antinociceptive doses, significantly reduced the magnitude of neurokinin-1 receptor internalization in neurons of lamina I in response to acute noxious mechanical or thermal stimulation. By contrast, administration of even high doses of the GABA(A) receptor agonists, muscimol or isoguvacine, were without effect. CGP55845, a selective GABA(B) receptor antagonist, completely blocked the effects of baclofen, but failed to increase the incidence of internalization when administered alone. These results provide evidence for a presynaptic control of nociceptive primary afferent neurons by GABA(B) but not GABA(A) receptors in the superficial laminae of the spinal cord, limiting tachykinin release. Because CGP5584 alone did not increase the magnitude of neurokinin-1 receptor internalization observed following noxious stimulation, there appears to be little endogenous activation of GABA(B) receptors on tachykinin-releasing nociceptors under acute stimulus conditions. The contribution of pre- and postsynaptic regulatory mechanisms to GABA(B) receptor-mediated antinociception was also investigated by comparing the effect of baclofen on Fos expression evoked by noxious stimulation to that induced by intrathecal injection of substance P. In both instances, baclofen reduced Fos expression not only in neurons that express the neurokinin-1 receptor, but also in neurons that do not. We conclude that baclofen acts at presynaptic sites to reduce transmitter release from small-diameter nociceptive afferents. Presynaptic actions on non-tachykinin-containing nociceptors could similarly account for the reduction by baclofen of noxious stimulus-induced Fos expression in neurokinin-1 receptor-negative neurons. However, the inhibition of Fos expression induced by exogenous substance P indicates that actions at sites postsynaptic to tachykinin- and/or non-tachykinin-containing primary afferent terminals must also contribute to the antinociceptive actions of GABA(B) receptor agonists.

Differential contribution of substance P and neurokinin A to spinal cord neurokinin-1 receptor signaling in the rat

Although the tachykinins substance P (SP) and neurokinin A (NKA) are coreleased from primary afferent nociceptors and act via neurokinin (NK) receptors, their differential effects in vivo are not known. Despite pharmacological evidence that NKA preferentially binds NK-2 receptors, this receptor is not found in spinal cord neurons. Thus, in the present studies, we compared the extent to which SP and NKA contribute to spinal nociceptive processing via the NK-1 receptor. We found that SP and NKA induce NK-1 receptor internalization with identical dose dependence and induce increases in intracellular calcium at the same concentrations, suggesting that SP and NKA equally activate the NK-1 receptor. We found, however, that the selective NK-1 receptor antagonist GR 205171 blocked NKA but not SP-induced NK-1 receptor internalization in the rat spinal cord in vivo and in embryonic day 19 rat spinal neurons in vitro. Using this selectivity of GR 205171 for NKA-induced NK-1 receptor activation, we examined the relative contribution of SP and NKA to noxious stimulus-induced activation of spinal NK-1 receptors. We estimate that NKA contributes to at least 50% of the NK-1 receptor activation in lamina I. Under inflammatory conditions, all noxious stimulus-induced NK-1 receptor internalization in deep dorsal horn neurons was blocked by GR 205171, suggesting that it is entirely NKA-mediated. Substance P-mediated NK-1 receptor internalization was focused at the site of termination of stimulated nociceptors but NKA also activated NK-1 receptors at more distant sites. We conclude that NKA not only targets the NK-1 receptor but may be a predominant pronociceptive primary afferent neurotransmitter.