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

Neurokinin-2 receptor negatively modulates substance P responses by forming complex with Neurokinin-1 receptor

BACKGROUND

Tachykinins and their cognate receptors, neurokinin receptors (NKs) including NK1, NK2, and NK3 play vital roles in regulating various physiological processes including neurotransmission, nociception, inflammation, smooth muscle contractility, and stimulation of endocrine and exocrine gland secretion. Their abnormal expression has been reported to be associated with neurological disorders, inflammation, and cancer. Even though NKs are expressed in the same cells with their expression being inversely correlated in some conditions, there is no direct evidence to prove their interaction. Understanding the functional crosstalk between NKs in mediated downstream signaling and cellular responses may elucidate the roles of each receptor in pathophysiology.

RESULTS

In this study, we showed that NKs were co-expressed in some cells. However, different from NK3, which only forms homodimerization, we demonstrated a direct interaction between NK1 and NK2 at the protein level using co-immunoprecipitation and NanoBiT-based protein interaction analysis. Through heterodimerization, NK2 downregulated substance P-stimulated NK1 signals, such as intracellular Ca2+ mobilization and ERK phosphorylation, by enhancing β-arrestin recruitment, even at the ligand concentration that could not activate NK2 itself or in the presence of NK1 specific antagonist, aprepitant. In A549 cells with receptors deleted and reconstituted, NK2 exerted a negative effect on substance P/NK1-mediated cell migration.

CONCLUSION

Our study has provided the first direct evidence of an interaction between NK1 and NK2, which highlights the functional relevance of their heterodimerization in cellular responses. Our findings demonstrated that through dimerization, NK2 exerts negative effects on downstream signaling and cellular response mediated by NK1. Moreover, this study has significant implications for understanding the complexity of GPCR dimerization and its effect on downstream signaling and cellular responses. Given the important roles of tachykinins and NKs in pathophysiology, these insights may provide clues for developing NKs-targeting drugs.

Nociceptor-Macrophage Interactions in Apical Periodontitis: How Biomolecules Link Inflammation with Pain

Periradicular tissues have a rich supply of peripheral afferent neurons, also known as nociceptive neurons, originating from the trigeminal nerve. While their primary function is to relay pain signals to the brain, these are known to be involved in modulating innate and adaptive immunity by initiating neurogenic inflammation (NI). Studies have investigated neuroanatomy and measured the levels of biomolecules such as cytokines and neuropeptides in human saliva, gingival crevicular fluid, or blood/serum samples in apical periodontitis (AP) to validate the possible role of trigeminal nociceptors in inflammation and tissue regeneration. However, the contributions of nociceptors and the mechanisms involved in the neuro-immune interactions in AP are not fully understood. This narrative review addresses the complex biomolecular interactions of trigeminal nociceptors with macrophages, the effector cells of the innate immune system, in the clinical manifestations of AP.

Intrathecally administered substance P activated the spinal defecation center and enhanced colorectal motility in anesthetized rats

Noxious stimuli on the colorectum cause colorectal contractions through activation of descending monoaminergic pathways projecting from the supraspinal defecation center to the spinal defecation center. Since it is known that substance P is involved in the response to peripheral noxious stimuli in the spinal cord, we investigated the effects of intrathecally administered substance P at L6-S1 levels on colorectal motility in rats that were anesthetized with α-chloralose and ketamine. Intrathecally administered substance P enhanced colorectal motility, even after transection of the thoracic spinal cord at the T4 level. Severing the pelvic nerves, but not the colonic nerves, abolished substance P enhanced colorectal motility. In the spinal cord at L6-S1 levels, expression of mRNA coding neurokinin (NK) 1-3 receptors was detected by RT-PCR. Immunohistological experiments revealed that preganglionic neurons of the pelvic nerves express NK1 receptors, whereas expression of NK2 receptors was not found. In addition, substance P-containing fibers densely innervated around the preganglionic neurons expressing NK1 receptors. An intrathecally administered NK1 receptor antagonist (spantide) attenuated capsaicin-induced colorectal contractions. These results suggest that the colokinetic action of substance P is mediated by the NK1 receptor in the spinal defecation center. Our findings indicate that substance P may function as a neurotransmitter in the spinal defecation center.NEW & NOTEWORTHY We found that intrathecally administered substance P enhanced colorectal motility in anesthetized rats. Neurokinin (NK) 1 receptors, but not NK2 receptors, were detected in preganglionic neurons of the pelvic nerves. Blockade of NK1 receptors in the spinal cord attenuated the enhanced colorectal motility in response to intracolonic noxious stimuli. The findings indicate that substance P may function as a neurotransmitter in the spinal reflex pathway controlling defecation.

Selective G protein signaling driven by substance P-neurokinin receptor dynamics

The neuropeptide substance P (SP) is important in pain and inflammation. SP activates the neurokinin-1 receptor (NK1R) to signal via Gq and Gs proteins. Neurokinin A also activates NK1R, but leads to selective Gq signaling. How two stimuli yield distinct G protein signaling at the same G protein-coupled receptor remains unclear. We determined cryogenic-electron microscopy structures of active NK1R bound to SP or the Gq-biased peptide SP6-11. Peptide interactions deep within NK1R are critical for receptor activation. Conversely, interactions between SP and NK1R extracellular loops are required for potent Gs signaling but not Gq signaling. Molecular dynamics simulations showed that these superficial contacts restrict SP flexibility. SP6-11, which lacks these interactions, is dynamic while bound to NK1R. Structural dynamics of NK1R agonists therefore depend on interactions with the receptor extracellular loops and regulate G protein signaling selectivity. Similar interactions between other neuropeptides and their cognate receptors may tune intracellular signaling.

Neurokinin receptor mechanisms in forebrain medial septum modulate nociception in the formalin model of inflammatory pain

The present study has explored the hypothesis that neurokinin1 receptors (NK1Rs) in medial septum (MS) modulate nociception evoked on hind paw injection of formalin. Indeed, the NK1Rs in MS are localized on cholinergic neurons which have been implicated in nociception. In anaesthetized rat, microinjection of L-733,060, an antagonist at NK1Rs, into MS antagonized the suppression of CA1 population spike (PS) evoked on peripheral injection of formalin or on intraseptal microinjection of substance P (SP), an agonist at NK1Rs. The CA1 PS reflects the synaptic excitability of pyramidal cells in the region. Furthermore, microinjection of L-733,060 into MS, but not LS, attenuated formalin-induced theta activation in both anaesthetized and awake rat, where theta reflects an oscillatory information processing by hippocampal neurons. The effects of L-733,060 on microinjection into MS were nociceptive selective as the antagonist did not block septo-hippocampal response to direct MS stimulation by the cholinergic receptor agonist, carbachol, in anaesthetized animal or on exploration in awake animal. Interestingly, microinjection of L-733,060 into both MS and LS attenuated formalin-induced nociceptive flinches. Collectively, the foregoing novel findings highlight that transmission at NK1R provide an affective valence to septo-hippocampal information processing and that peptidergic transmission in the septum modulates nociceptive behaviours.

PACAP and Other Neuropeptide Targets Link Chronic Migraine and Opioid-induced Hyperalgesia in Mouse Models

Chronic use of opioids can produce opioid-induced hyperalgesia (OIH), and when used to treat migraine, these drugs can result in increased pain and headache chronicity. We hypothesized that overlapping mechanisms between OIH and chronic migraine occur through neuropeptide dysregulation. Using label-free, non-biased liquid chromatography-mass spectrometry to identify and measure changes in more than 1500 neuropeptides under these two conditions, we observed only 16 neuropeptides that were altered between the two conditions. The known pro-migraine molecule, calcitonin-gene related peptide, was among seven peptides associated with chronic migraine, with several pain-processing neuropeptides among the nine other peptides affected in OIH. Further, composite peptide complements Pituitary adenylate cyclase-activating polypeptide (PACAP), Vasoactive intestinal peptide (VIP) and Secretogranin (SCG) showed significant changes in both chronic migraine and OIH. In a follow-up pharmacological study, we confirmed the role of PACAP in models of these two disorders, validating the effectiveness of our peptidomic approach, and identifying PACAP as a mechanistic link between chronic migraine and OIH. Data are available via ProteomeXchange with identifier PXD013362.

Neuropeptide-induced modulation of carcinogenesis in a metastatic breast cancer cell line (MDA-MB-231LUC+)

Background

Metastatic cancer to bone is well-known to produce extreme pain. It has been suggested that the magnitude of this perceived pain is associated with disease progression and poor prognosis. These data suggest a potential cross-talk between cancer cells and nociceptors that contribute not only to pain, but also to cancer aggressiveness although the underlying mechanisms are yet to be stablished.

Methods

The in vitro dose dependent effect of neuropeptides (NPs) (substance P [SP], calcitonin gene-related peptide and neurokinin A [NKA]) and/or its combination, on the migration and invasion of MDA-MB-231^LUC+ were assessed by wound healing and collagen-based cell invasion assays, respectively. The effect of NPs on the expression of its receptors (SP [NK1] and neurokinin A receptors [NK2], CALCRL and RAMP1) and kininogen (high-molecular-weight kininogen) release to the cell culture supernatant of MDA-MB-231^LUC+, were measured using western-blot analysis and an ELISA assay, respectively. Statistical significance was tested using one-way ANOVA, repeated measures ANOVA, or the paired t-test. Post-hoc testing was performed with correction for multiple comparisons as appropriate.

Results

Our data show that NPs strongly modify the chemokinetic capabilities of a cellular line commonly used as a model of metastatic cancer to bone (MDA-MB-231^LUC+) and increased the expression of their receptors (NK1R, NK2R, RAMP1, and CALCRL) on these cells. Finally, we demonstrate that NPs also trigger the acute release of HMWK (Bradykinin precursor) by MDA-MB-231^LUC+, a molecule with both tumorigenic and pro-nociceptive activity.

Conclusions

Based on these observations we conclude that NPs exposure modulates this breast cancer cellular line aggressiveness by increasing its ability to migrate and invade new tissues. Furthermore, these results also support the pro nociceptive and cancer promoter role of the peripheral nervous system, during the initial stages of the disease.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Neuropeptide imaging in rat spinal cord with MALDI-TOF MS: Method development for the application in pain-related disease studies

Spinal cord as a connection between brain and peripheral nervous system is an essential material for studying neural transmission, especially in pain-related research. This study was the first to investigate pain-related neuropeptide distribution in rat spinal cord using a matrix-assisted laser desorption ionization-time of flight imaging mass spectrometry (MALDI TOF MS) approach. The imaging workflow was evaluated and showed that MALDI TOF MS provides efficient resolution and robustness for neuropeptide imaging in rat spinal cord tissue. The imaging result showed that in naive rat spinal cord the molecular distribution of haeme, phosphatidylcholine, substance P and thymosin beta 4 were well in line with histological features. Three groups of pain-related neuropeptides, which are cleaved from prodynorphin, proenkephalin and protachykinin-1 proteins were detected. All these neuropeptides were found predominantly localized in the dorsal spinal cord and each group had unique distribution pattern. This study set the stage for future MALDI TOF MS application to elucidate signalling mechanism of pain-related diseases in small animal models.

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.

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.

Decision-making using fMRI in clinical drug development: revisiting NK-1 receptor antagonists for pain

Substance P (SP) and neurokinin-1 receptors (NK-1R) are localized within central and peripheral sensory pain pathways. The roles of SP and NK-1R in pain processing, the anatomical distribution of NK-1R and efficacy observed in preclinical pain studies involving pain and sensory sensitization models, suggested that NK-1R antagonists (NK-1RAs) would relieve pain in patient populations. Despite positive data available in preclinical tests for a role of NK-1RAs in pain, clinical studies across several pain conditions have been negative. In this review, we discuss how functional imaging-derived information on activity in pain-processing brain regions could have predicted that NK-1RAs would have a low probability of success in this therapeutic domain.

Substance P and calcitonin gene related peptide mediate pain in chronic pancreatitis and their expression is driven by nerve growth factor

CONTEXT

Calcitonin gene-related peptide (CGRP), substance P and nerve growth factor play an important role in inflammatory pain in various somatic pain models but their role in chronic pancreatitis has not been well studied.

OBJECTIVES

The aim of this study was to investigate the effects of intrathecal administration of calcitonin gene-related peptide antagonist and substance P receptor antagonist on pain behavior in a rat model of chronic pancreatitis and to determine whether nerve growth factor drives the up-regulation of expression of these neuropeptides in sensory neurons.

METHODS

Pancreatitis was induced by retrograde infusion of trinitobenzene sulfonic acid into the pancreatic duct of adult rats. Three weeks post infusion continuous intrathecal infusion of the calcitonin gene-related peptide antagonist alpha CGRP8-37 or neurokinin-1 receptor antagonist CP-96345 or its inactive enantiomer CP-96344 was administered for seven days. The effects of treatment on pancreatic hyperalgesia were assessed by sensitivity of the abdominal wall to von Frey filament probing as well as by the nocifensive response to electrical stimulation of the pancreas. In a separate experiment chronic pancreatitis was induced and pancreas specific dorsal root ganglion neurons labeled with DiI were assessed for calcitonin gene-related peptide and substance P immunoreactivity.

RESULTS

Intrathecal infusion of calcitonin gene-related peptide and neurokinin-1 receptor antagonists significantly attenuated behavioral pain responses in rats with chronic pancreatitis. Further, treatment of chronic pancreatitis rats with nerve growth factor antibody significantly reduced pancreas specific neurons expressing calcitonin gene-related peptide and substance P in thoracic dorsal root ganglion.

CONCLUSIONS

Calcitonin gene-related peptide and substance P mediate pancreatic hyperalgesia in chronic pancreatitis and nerve growth factor in turn sustains the up-regulation of these neuropeptides in pancreatic sensory neurons.

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.

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.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Involvement of voltage-gated sodium channel Na(v)1.8 in the regulation of the release and synthesis of substance P in adult mouse dorsal root ganglion neurons

This study was conducted to determine whether Na(v)1.8 contributes to the release and/or synthesis of substance P (SP) in adult mice dorsal root ganglion (DRG) neurons. The SP released from cultured DRG neurons of Na(v)1.8 knock-out mice exposed to either capsaicin or KCl was significantly lower than that from wild-type (C57BL/6) mice based on a radioimmunoassay. The SP level of L6 DRG in Na(v)1.8 knock-out mice was also lower than that in wild-type mice. After chronic constriction injury (CCI) of the sciatic nerve, the level of SP decreased in the L6 ipsilateral DRG of wild-type but not Na(v)1.8 knock-out mice. The preprotachykinin-A (PPT-A) mRNAs in L4 - 6 DRGs of Na(v)1.8 knock-out mice also fell to half their normally abundant levels of expression. There were significant increases in Na(v)1.8 expression of the L6 contralateral DRG from wild-type mice and in the percentage of neurons expressing neurokinin-1 receptor in the cytosol of L6 DRGs from wild-type or Na(v)1.8 knock-out mice. These findings suggest that Na(v)1.8 is involved in the regulation of the release and synthesis of SP in the DRG neurons of wild-type mice.

Neurokinin A engages neurokinin-1 receptor to induce NF-κB-dependent gene expression in murine macrophages: implications of ERK1/2 and PI 3-kinase/Akt pathways

Neurokinin A (NKA) belongs to the tachykinin neuropeptide family. Its biological functions are primarily mediated by the neurokinin (NK)-2 receptor. NKA has been implicated in several inflammatory conditions. However, there are limited data about the mechanism of its pathogenetic action. Here, we investigated proinflammatory effects of NKA on peripheral immune cells using the mouse macrophage/monocyte cell line RAW 264.7 and primary peritoneal macrophages. The signaling mechanistic pathways involved were also studied. In mouse macrophages with no detectable NK-2 receptors, NKA induces the upregulation of NK-1 but not NK-2 receptor expression. Furthermore, NKA engages this NK-1 receptor, resulting in inflammatory-like responses involving activation of the transcription factor nuclear factor (NF)-κB and induction of NF-κB-responsive proinflammatory chemokine expression. NKA activates NF-κB as evidenced by induced phosphorylation (leading to degradation) of its inhibitory protein IκBα, increased cellular levels of the transactivation-active phospho(Ser276)-p65 and its nuclear translocation, as well as enhanced DNA-binding activity of NF-κB. These responses are specifically inhibited by selective NK-1 receptor antagonists but not NK-2 receptor antagonists, thereby excluding the role of NK-2 receptor. Further investigation on the upstream signaling mechanisms suggests that two NF-κB-activating pathways (extracellular signal-regulated kinase 1/2 and phosphatidylinositol 3-kinase/protein kinase B) are activated by NKA. Specific inhibitors of the two pathways block NF-κB-dependent chemokine expression. The inhibitory effects are mediated through regulation of nuclear translocation, DNA-binding activity, and/or transactivation activity of NF-κB. Together, we provide novel evidence that NKA engages NK-1 receptors on mouse macrophages to elicit NF-κB-dependent cellular responses. The findings reveal cellular mechanisms that may underlie NKA-mediated inflammatory and immunological conditions.

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 role of neurokinin 1 receptors in the maintenance of visceral hyperalgesia induced by repeated stress in rats

BACKGROUND & AIMS

The neurokinin 1 receptors (NK(1)Rs) and substance P (SP) have been implicated in the stress and/or pain pathways involved in chronic pain conditions. Here we examined the participation of NK(1)Rs in sustained visceral hyperalgesia observed in rats exposed to chronic psychological stress.

METHODS

Male Wistar rats were exposed to daily 1-hour water avoidance stress (WA) or sham WA for 10 consecutive days. We tested intraperitoneal or intrathecal injection of the NK(1)R antagonist SR140333 on the visceromotor reflex to colorectal distention in both groups at day 11. Real-time reverse-transcription polymerase chain reaction, Western blot, and immunohistochemistry were used to assess the expression of NK(1)Rs and/or SP in samples of colon, spinal cord, and dorsal root ganglia.

RESULTS

Both intraperitoneal and intrathecal SR140333 injection diminished the enhanced visceromotor reflex to colorectal distention at day 11 in stressed rats but did not affect the response in control animals. Real-time polymerase chain reaction and Western blotting demonstrated stress-induced up-regulation of spinal NK(1)Rs. Immunohistochemistry showed an increased number of NK(1)R-expressing neurons in the laminae I of the dorsal horn in stressed rats. The expression of NK(1)Rs was decreased in colon from stressed rats compared with control. The expression of SP gene precursor in dorsal root ganglia was unchanged in stressed rats compared with controls.

CONCLUSIONS

Stress-induced increased NK(1)R expression on spinal neurons and the inhibitory effect of intrathecal NK(1)R antagonist on visceral hyperalgesia support the key contribution of spinal NK(1)Rs in the molecular pathways involved in the maintenance of visceral hyperalgesia observed after chronic WA.

Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human

Tachykinins (TKs), substance P (SP), neurokinin A (NKA) and B (NKB) are important peptide modulators of intestinal motility in animal species studied so far, including humans. Modulation of motility by TKs can occur at various levels, since these peptides are expressed in cholinergic excitatory motor neurons projecting to both circular and longitudinal muscle, interneurons, and intramural and extramural sensory neurons. The effects of SP, NKA and NKB are preferentially mediated through the stimulation of NK1, NK2 and NK3 receptors, respectively; however, the selectivity of natural TKs for their preferred receptors is relative. In addition, SP and NKA are expressed in similar quantities in the human intestine and adequate stimuli can release similar amount of these TKs from enteric nerves. Furthermore, a single anatomical substrate can express more than one TK receptor type, so that the blockade of a single receptor type may not reveal functional effects in integrated models of motility. In isolated human small intestine and colon circular muscle strips, both NK1 and NK2 receptors mediate contractile effects. Indeed, in the human small intestine, smooth muscle electrical and motor events induced by electrical field stimulation (EFS) can involve either or both NK1 and NK2 receptors or these latter receptors predominantly, depending on the experimental conditions. In contrast, in the human colonic smooth muscle, only the NK2 receptor-mediated component of the response to EFS is prominent and some evidence would suggest that this component is the main excitatory motor mechanism at this level. Furthermore, a NK2 receptor-mediated secretory component in the human colonic mucosa has been recently demonstrated. Thus, it could be speculated that the blockade of both NK1 and NK2 receptors will be necessary to antagonise motor effects induced by exogenous administration or endogenous release of TKs in the small intestine, whereas the blockade of the NK2 receptors would be sufficient to disrupt physiological motor and, possibly, secretory activity at the colonic level. Available evidence indicates that, in healthy volunteers, the infusion of NKA (25 pmol/kg/min i.v.) stimulated small intestine motility and precipitated a series of intestinal and non-intestinal adverse events. Nepadutant (8 mg i.v.), a selective NK2 receptor antagonist, antagonised small intestine motility induced by NKA and prevented associated intestinal adverse events. In another study, the same dose of nepadutant increased colo-rectal compliance during isobaric balloon distension in healthy volunteers pretreated with a glycerol enema, disclosing a NK2 receptor-mediated component in the regulation of colonic smooth muscle tone. However, the prolonged blockade of NK2 receptors by nepadutant (16 mg i.v. b.i.d. for 8 days) did not affect bowel habits, neither in term of movements nor of stool consistency. Altogether, these results indicate that, even when there is a significant redundance in the effects of TKs and in the role of their receptors, the selective blockade of tachykinin NK2 receptors can have functional consequences on human intestinal motility and perception, but this can occur without the disruption of the physiological functions.

Feed-forward inhibition: a novel cellular mechanism for the analgesic effect of substance P

Substance P (SP) is a neuropeptide well known for its contribution to pain transmission in the spinal cord, however, less is known about the possible modulatory effects of SP. A new study by Gu and colleagues, published in Molecular Pain (2005, 1:20), describes its potential role in feed-forward inhibition in lamina V of the dorsal horn of the spinal cord. This inhibition seems to function through a direct excitation of GABAergic interneurons by substance P released from primary afferent fibers and has a distinct temporal phase of action from the well-described glutamate-dependent feed-forward inhibition. It is believed that through this inhibition, substance P can balance nociceptive output from the spinal cord.

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.

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.

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.

Distinct regional distributions of NK1 and NK3 neurokinin receptor immunoreactivity in rat brainstem gustatory centers

Tachykinins and their receptors are present in gustatory centers, but little is known about tachykinin function in gustation. In this study, immunohistochemical localization of substance P and two centrally prevalent neurokinin receptors, NK1 and NK3, was carried out in the rostral nucleus of the solitary tract and the caudal parabrachial nucleus to evaluate regional receptor/ligand correspondences. All three proteins showed regional variations in labeling density that correlated with distinct sites in gustatory centers. In the rostral nucleus of the solitary tract, the relative densities of substance P and NK1 receptors varied in parallel across subnuclei, with both being moderate to dense in the dorsocentral, chemoresponsive zone. NK3 receptors had a distinct distribution in the caudal half of this zone, suggesting a unique role in processing taste input from the posterior tongue. In the caudal parabrachial nucleus, substance P and NK1 receptor immunoreactivities were dense in the pontine taste area, while NK3 receptor labeling was sparse. The external medial subnucleus had substantial NK3 receptor and substance P labeling, but little NK1 receptor immunoreactivity. These findings suggest that distinct tachykinin ligand/neurokinin receptor combinations may be important in local processing of information within brainstem gustatory centers.

Spinal and cortical spreading depression enhance spinal cord activity

Cortical spreading depression (CSD) has been suggested to underlie some neurological disorders such as migraine. Despite the intensity with which many investigators have studied SD in the brain, only a few studies have aimed to identify SD in the spinal cord. Here we described the main characteristic features of SD in the spinal cord induced by different methods including various spinal cord injury models and demonstrated that SD enhances the spinal cord activity following a transient suppressive period. These findings suggest that SD may play a role in the mechanisms of spinal neurogenic shock, spinal cord injury, and pain. Furthermore, we studied the effect of CSD on the neuronal activity of the spinal cord. CSD was induced via cortical pinprick injury or KCl injection in the somatosensory cortex. CSD did not propagate into the cervical spinal cord. However, intracellular recordings of the neurons in the dorsal horn of C2 segment, ipsilateral to the hemisphere in which CSD was evoked, showed a transient suppression of spontaneous burst discharges, followed by a significant enhancement of the neuronal activity. This indicates a link between a putative cause of the neurological symptoms and the subsequent pain of migraine.

Characterization of wide dynamic range neurons in the deep dorsal horn of the spinal cord in preprotachykinin-a null mice in vivo

We previously reported that mice with a deletion of the preprotachykinin-A (pptA) gene, from which substance P (SP) and neurokinin A (NKA) are derived, exhibit reduced behavioral responses to intense stimuli, but that behavioral hypersensitivity after injury is unaltered. To understand the contribution of SP and NKA to nociceptive transmission in the spinal cord, we recorded single-unit activity from wide dynamic range neurons in the lamina V region of the lumbar dorsal horn of urethane-anesthetized wild-type and ppt-A null mutant (-/-) mice. We found that intensity coding to thermal stimuli was largely preserved in the ppt-A -/- mice. Neither the peak stimulus-evoked firing nor the neuronal activity during the initial phase (0-4 s) of the 41-49 degrees C thermal stimuli differed between the genotypes. However, electrophysiological responses during the late phase of the stimulus (5-10 s) and poststimulus (11-25 s) were significantly reduced in ppt-A -/- mice. To activate C-fibers and to sensitize the dorsal horn neurons we applied mustard oil (MO) topically to the hindpaw. We found that neither total MO-evoked activity nor sensitization to subsequent stimuli differed between the wild-type and ppt-A -/- mice. However, the time course of the sensitization and the magnitude of the poststimulus discharges were reduced in ppt-A -/- mice. We conclude that SP and/or NKA are not required for intensity coding or sensitization of nociresponsive neurons in the spinal cord, but that these peptides prolong thermal stimulus-evoked responses. Thus whereas behavioral hypersensitivity after injury is preserved in ppt-A -/- mice, our results suggest that the magnitude and duration of these behavioral responses would be reduced in the absence of SP and/or NKA.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

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.

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.

The role of the amino-terminal domain of tachykinins in neurokinin-1 receptor signaling and desensitization

Neurokinin A (NKA) has previously been shown to be a full agonist of the neurokinin-1 receptor (NK-1R) but is only able to cause partial homologous desensitization of the receptor compared to substance P (SP). NKA and SP share the same amino acid sequence at their C-terminal active site domains but differ in structure at their N-terminal domains. These observations have led to the proposal that the N-terminal domains of tachykinin peptides affect the desensitization but not the agonist activities of the peptides. Some of the preprotachykinin proteins contain SP and the NKA-like tachykinins neuropeptide K (NPK) and neuropeptide gamma (NPgamma), which contain NKA at their C-terminals and are N-terminally extended. In this study, the abilities of NKA, NPK, and NPgamma to stimulate NK-1R second messenger (IP(3)) signaling and rapid homologous desensitization of the NK-1R were examined. In addition, a similar analysis was performed using several nonmammalian tachykinin peptides in order to obtain additional insight into the role of the tachykinin N-terminal domain in these NK-1R functions. NPK and NPgamma were found, like NKA, to be full agonists of rat NK-1R IP(3) signaling but, unlike NKA, were also able to cause full rapid homologous desensitization of the receptor. The extended N-terminal domains of NPK and NPgamma thus increase the desensitization activities of these NKA-like peptides. Of the nonmammalian tachykinins tested, all were full agonists but kassinin and eledoisin had only partial homologous desensitization activity, suggesting that the N-terminal structures of these peptides also differentially affect agonist versus desensitization activities of the NK-1R.

Urotensin-II regulates intracellular calcium in dissociated rat spinal cord neurons

Urotensin-II (U-II), a peptide with multiple vascular effects, is detected in cholinergic neurons of the rat brainstem and spinal cord. Here, the effects of U-II on [Ca2+]i was examined in dissociated rat spinal cord neurons by fura 2 microfluorimetry. The neurons investigated were choline acetyltransferase-positive and had morphological features of motoneurons. U-II induced [Ca2+]i increases in these neurons with a threshold of 10-9 m, and a maximal effect at 10-6 m with an estimated EC50 of 6.2 x 10-9 m. The [Ca2+]i increase induced by U-II was mainly caused by Ca2+ influx from extracellular space, as the response was markedly attenuated in a Ca2+-free medium. Omega-conotoxin GVIA (10-7 m), a N-type Ca2+ channel blocker, largely inhibited these increases, whereas the P/Q Ca2+ channel blocker, omega-conotoxin GVIIC (10-7 m) and the l-type Ca2+ channel blocker, verapamil (10-5 m) had minimal effects. Down-regulation of protein kinase C by 4-alpha-phorbol 12-myristate 13-acetate (10-6 m) or enzyme inhibition using the specific inhibitor bisindolylmaleimide I (10-6 m) did not inhibit the observed effects. Similarly, inhibition of protein kinase G with KT5823 (10-6 m) or Rp-8-pCPT-cGMPS (3 x 10-5 m) did not modify U-II-induced [Ca2+]i increases. In contrast, protein kinase A inhibitors KT5720 (10-6 m) and Rp-cAMPS (3 x 10-5 m) reduced the response to 25 +/- 3% and 42 +/- 8%, respectively. Present results demonstrate that U-II modulates [Ca2+]i in rat spinal cord neurons via protein kinase A cascade.

In vivo metabolism and clearance of substance P and co-expressed tachykinins in rat striatum

Neurons expressing the preprotachykinin A gene, which encodes the sequences of substance P, neurokinin A, neuropeptide gamma and neuropeptide K, exemplify peptide co-existence. Furthermore, there is also evidence that substance P fragments have biological activity. However, the relative contribution of each of these peptides to tachykinin signalling is still poorly understood. An important factor which will determine the characteristics of the signal mediated by co-localised peptides is their clearance from the extracellular space. The striatum, in which tachykinins are present and exert neuromodulatory roles, can be used as a model to investigate this aspect. Therefore, in this study we characterised in vivo in the striatum the metabolism and clearance of substance P and of the other three co-expressed peptides. After intrastriatal administration of 1 pmol, tritiated substance P disappeared too rapidly for metabolites to be detected. However, when 10 nmol substance P and 1 pmol tritiated substance P were co-injected, substance P(1-4) and substance P(1-7), which are biologically active, were detected as major metabolites. Under these conditions, the rate of decay of tritiated substance P was 0.2 nmol/min. The effects of the peptidase inhibitors thiorphan, bestatin and captopril suggested that neutral endopeptidase 24.11 and aminopeptidases were involved in primary substance P cleavages, whereas angiotensin-converting enzyme was involved in secondary cleavages. The monitoring of the decay of unlabelled substance P by high-performance liquid chromatography gave a rate of 0.16 nmol/min. Using high-performance liquid chromatography with capillary electrophoresis, the rates of decay of 10 nmol neurokinin A or neuropeptide gamma were five and seven times faster than that of substance P. In contrast, over the time course of the experiment, no significant decay of neuropeptide K was detected. These results show that substance P disappears rapidly from the extracellular space, and supports the formation in vivo of major N-terminal active substance P metabolites. Our study also highlights significant differences in the clearance of co-expressed tachykinins and suggests that certain species may disappear relatively slowly from the extracellular space, and thus may make a significant temporal and spatial contribution to signalling.

Analogs of Substance P modified at the C-terminus which are both agonist and antagonist of the NK-1 receptor depending on the second messenger pathway

The initial goal of this study was to analyze, using photolabeling, the interactions between Substance P and its tachykinin NK-1 receptor. Therefore, the photoreactive amino acid para-benzoyl-phenylalanine (pBzl)Phe was incorporated into the Substance P sequence from position 4 to 11 leading to Bapa0[(pBzl)Phex]SP analogs. Biotinyl sulfone-5-aminopentanoic acid (Bapa) was introduced in order to purify the covalent complex. These photoreactive SP analogs were first assayed for their affinity for the two binding sites associated with the NK-1 receptor, as well as for their potency in activating the phospholipase C and adenylate cyclase pathways. All analogs photoreactive from position 4 to 11 have moderate to high affinity for the two NK-1 receptor-binding sites, except for the analog modified at position 7. This affinity could be correlated to their potency to activate the phospholipase C and adenylate cyclase pathways, except for the analog photoreactive at position 11. Bapa0[(pBzl)Phe11]SP was found to be an agonist in the phospholipase C pathway and an antagonist in the adenylate cyclase pathway, other analogs modified at position 11 were therefore analyzed. Among these, Bapa0[Pro9, (pBzl)Hcy(O2)11]SP is a partial agonist, whereas Bapa0[Hcy(ethylaminodansyl)11]SP is a full agonist in the phospholipase C pathway, the two analogs being antagonist in the adenylate cyclase pathway. These results show that analogs of SP can be simultaneously agonist at one binding site and antagonist at the other binding site associated with the NK-1 receptor.

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.