Receptor for calcitonin gene-related peptide: localization in the dorsal and ventral spinal cord

Tyrosine Kinase Inhibitors Reduce NMDA NR1 Subunit Expression, Nuclear Translocation, and Behavioral Pain Measures in Experimental Arthritis

In the lumbar spinal cord dorsal horn, release of afferent nerve glutamate activates the neurons that relay information about injury pain. Here, we examined the effects of protein tyrosine kinase (PTK) inhibition on NMDA receptor NR1 subunit protein expression and subcellular localization in an acute experimental arthritis model. PTK inhibitors genistein and lavendustin A reduced cellular histological translocation of NMDA NR1 in the spinal cord occurring after the inflammatory insult and the nociceptive behavioral responses to heat. The PTK inhibitors were administered into lumbar spinal cord by microdialysis, and secondary heat hyperalgesia was determined using the Hargreaves test. NMDA NR1 cellular protein expression and nuclear translocation were determined by immunocytochemical localization with light and electron microscopy, as well as with Western blot analysis utilizing both C- and N-terminal antibodies. Genistein and lavendustin A (but not inactive lavendustin B or diadzein) effectively reduced (i) pain related behavior, (ii) NMDA NR1 subunit expression increases in spinal cord, and (iii) the shift of NR1 from a cell membrane to a nuclear localization. Genistein pre-treatment reduced these events that occur in vivo within 4 h after inflammatory insult to the knee joint with kaolin and carrageenan (k/c). Cycloheximide reduced glutamate activated upregulation of NR1 content confirming synthesis of new protein in response to the inflammatory insult. In addition to this in vivo data, genistein or staurosporin inhibited upregulation of NMDA NR1 protein and nuclear translocation in vitro after treatment of human neuroblastoma clonal cell cultures (SH-SY5Y) with glutamate or NMDA (4 h). These studies provide evidence that inflammatory activation of peripheral nerves initiates increase in NMDA NR1 in the spinal cord coincident with development of pain related behaviors through glutamate non-receptor, PTK dependent cascades.

Cross-talk signaling in the trigeminal ganglion: role of neuropeptides and other mediators

The trigeminal ganglion with its three trigeminal nerve tracts consists mainly of clusters of sensory neurons with their peripheral and central processes. Most neurons are surrounded by satellite glial cells and the axons are wrapped by myelinating and non-myelinating Schwann cells. Trigeminal neurons express various neuropeptides, most notably, calcitonin gene-related peptide (CGRP), substance P, and pituitary adenylate cyclase-activating polypeptide (PACAP). Two types of CGRP receptors are expressed in neurons and satellite glia. A variety of other signal molecules like ATP, nitric oxide, cytokines, and neurotrophic factors are released from trigeminal ganglion neurons and signal to neighboring neurons or satellite glial cells, which can signal back to neurons with same or other mediators. This potential cross-talk of signals involves intracellular mechanisms, including gene expression, that can modulate mediators of sensory information, such as neuropeptides, receptors, and neurotrophic factors. From the ganglia cell bodies, which are outside the blood–brain barrier, the mediators are further distributed to peripheral sites and/or to the spinal trigeminal nucleus in the brainstem, where they can affect neural transmission. A major question is how the sensory neurons in the trigeminal ganglion differ from those in the dorsal root ganglion. Despite their functional overlap, there are distinct differences in their ontogeny, gene expression, signaling pathways, and responses to anti-migraine drugs. Consequently, drugs that modulate cross-talk in the trigeminal ganglion can modulate both peripheral and central sensitization, which may potentially be distinct from sensitization mediated in the dorsal root ganglion.

Immunohistochemical localization of the calcitonin gene-related peptide binding site in the primate trigeminovascular system using functional antagonist antibodies

Calcitonin gene-related peptide (CGRP) is a potent vasodilator and a neuromodulator implicated in the pathophysiology of migraine. It binds to the extracellular domains of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein (RAMP) 1 that together form the CGRP receptor. Antagonist antibodies against CGRP and its binding site at the receptor are clinically effective in preventing migraine attacks. The blood-brain barrier penetration of these antagonist antibodies is limited, suggesting that a potential peripheral site of action is sufficient to prevent migraine attacks. To further understand the sites of CGRP-mediated signaling in migraine, we used immunohistochemical staining with recently developed antagonist antibodies specifically recognizing a fusion protein of the extracellular domains of RAMP1 and CLR that comprise the CGRP binding pocket at the CGRP receptor in monkey and man. We confirmed binding of the antagonist antibodies to human vascular smooth muscle cells (VSMCs) of dural meningeal arteries and neurons in the trigeminal ganglion, both of which are likely sites of action for therapeutic antibodies in migraine patients. We further used one of these antibodies for detailed mapping on cynomolgus monkey tissue and found antagonist antibody binding sites at multiple levels in the trigeminovascular system: in the dura mater VSMCs, in neurons and satellite glial cells in the trigeminal ganglion, and in neurons in the spinal trigeminal nucleus caudalis. These data reinforce and clarify our understanding of CGRP receptor localization in a pattern consistent with a role for CGRP receptors in trigeminal sensitization and migraine pathology.

The CGRP receptor antagonist BIBN4096BS peripherally alleviates inflammatory pain in rats

Calcitonin gene-related peptide (CGRP) is known to play a major role in the pathogenesis of pain syndromes, in particular migraine pain. Here we focus on its implication in a rat pain model of inflammation, induced by injection of complete Freund adjuvant (CFA). The nonpeptide CGRP receptor antagonist BIBN4096BS reduces migraine pain and trigeminal neuronal activity. Here we demonstrate that the compound reduces inflammatory pain and spinal neuronal activity. Behavioural experiments reveal a reversal of the CFA-induced mechanical hypersensitivity and monoiodoacetate (MIA)-induced weight-bearing deficit in rats after systemic drug administration. To further investigate the mechanism of action of the CGRP antagonist in inflammatory pain, in vivo electrophysiological studies were performed in CFA-injected rats. Recordings from wide dynamic range neurons in deep dorsal horn layers of the lumbar spinal cord confirmed a reduction of neuronal activity after systemic drug application. The same amount of reduction occurred after topical administration onto the paw, with resulting systemic plasma concentrations in the low nanomolar range. However, spinal administration of BIBN4096BS did not modify the neuronal activity in the CFA model. Peripheral blockade of CGRP receptors by BIBN4096BS significantly alleviates inflammatory pain.

Calcitonin receptor-like receptor and receptor activity modifying protein 1 in the rat dorsal horn: localization in glutamatergic presynaptic terminals containing opioids and adrenergic alpha2C receptors

Calcitonin gene-related peptide (CGRP) is abundant in the central terminals of primary afferents. However, the function of CGRP receptors in the spinal cord remains unclear. CGRP receptors are heterodimers of calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1). We studied the localization of CRLR and RAMP1 in the rat dorsal horn using well-characterized antibodies against them, which labeled numerous puncta in laminae I-II. In addition, RAMP1 was found in cell bodies, forming patches at the cell surface. The CRLR- and RAMP1-immunoreactive puncta were further characterized using double and triple labeling. Colocalization was quantified in confocal stacks using Imaris software. CRLR did not colocalize with primary afferent markers, indicating that these puncta were not primary afferent terminals. CRLR- and RAMP1-immunoreactive puncta contained synaptophysin and vesicular glutamate transporter-2 (VGLUT2), showing that they were glutamatergic presynaptic terminals. Electron microscopic immunohistochemistry confirmed that CRLR immunoreactivity was present in axonal boutons that were not in synaptic glomeruli. Using tyramide signal amplification for double labeling with the CRLR and RAMP1 antibodies, we found some clear instances of colocalization of CRLR with RAMP1 in puncta, but their overall colocalization was low. In particular, CRLR was absent from RAMP1-containing cells. Many of the puncta stained for CRLR and RAMP1 were labeled by anti-opioid and anti-enkephalin antibodies. CRLR and, to a lesser extent, RAMP1 also colocalized with adrenergic alpha(2C) receptors. Triple label studies demonstrated three-way colocalization of CRLR-VGLUT2-synaptophysin, CRLR-VGLUT2-opioids, and CRLR-opioids-alpha(2C) receptors. In conclusion, CRLR is located in glutamatergic presynaptic terminals in the dorsal horn that contain alpha(2C) adrenergic receptors and opioids. Some of these terminals contain RAMP1, which may form CGRP receptors with CRLR, but in others CRLR may form other receptors, possibly by dimerizing with RAMP2 or RAMP3. These findings suggest that CGRP or adrenomedullin receptors modulate opioid release in the dorsal horn.

In vitro effects of adrenomedullin and calcitonin gene related peptide on the release of serotonin and amino acids from rat dorsal spinal cord

Adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) are structurally related, interact with each others receptors and show overlapping biological activities. Immunoreactivity (IR) and mRNAs along with binding sites for both CGRP and adrenomedullin have been shown in the rat spinal cord. CGRP mediates the transmission of nociceptive information at the spinal cord level and both peptides has shown to induces c-fos expression and accumulation of cAMP in spinal cells. In this study, HPLC methods were used to investigate the effects of AM and CGRP on the basal and K+-evoked release of serotonin, glutamate (Glu), aspartate (Asp), glycine (Gly) and gamma amino butyric acid (GABA) from the slices prepared from the rat spinal cord. Neither CGRP (10(-7) and 10(-6) M) nor AM (10(-7) and 10(-6) M) had significant effects on the basal release of serotonin and the amino acids tested in this study. However, CGRP produced statistically significant increases in the K+-evoked release of Asp and Glu, whereas AM failed to do so. Neither AM nor CGRP (10(-7) and 10(-6) M) showed any significant effects on the K+-evoked release of serotonin, GABA and Gly. Present data suggest that the stimulatory effects of CGRP on the release of Asp and Glu were exerted by distinct types of CGRP receptors.

The colocalization of CGRP receptor and AMPA receptor in the spinal dorsal horn neuron of rat: A morphological and electrophysiological study

Both the calcitonin gene-related peptide (CGRP) receptor and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor are involved in the transmission of sensory information from primary afferent to the spinal cord. The present study found that there was a colocalization of CGRP receptor and AMPA receptor in a single spinal dorsal horn neuron in rat determined by double immunofluorescence labeling image methods. Furthermore, our results showed that the evoked discharge frequency of the wide dynamic range (WDR) neuron, one type of the dorsal horn neurons, increased significantly after micro-iontophoretic delivery of CGRP or AMPA alone tested by extracellular recording, indicating a functional colocalization of CGRP receptor and AMPA receptor in a single spinal dorsal horn neuron. The results of the present study found a morphological and functional colocalization of the CGRP receptor and AMPA receptor in a single dorsal horn neuron that involved in the transmission and modulation of sensory information from primary afferent to the spinal cord in rats.

Neuropeptide and kinin antagonists

Neuropeptides and kinins are important messengers in the nervous system and--on the basis of their anatomical localisation and the effects produced when the substances themselves are administered, to animals or to human subjects-a significant number of them have been suggested to have a role in pain and inflammation. Experiments in gene deletion (knock-out or null mutant) mice and parallel experiments with pharmacological receptor antagonists in a variety of species have strengthened the evidence that a number of peptides, notably substance P and calcitonin gene-related peptide (CGRP), and the kinins have a pathophysiological role in nociception. Clinical studies with non-peptide pharmacological antagonists are now in progress to determine if blocking the action of these peptides might have utility in the treatment of pain.

Calcitonin gene-related peptide enhances release of native brain-derived neurotrophic factor from trigeminal ganglion neurons

Activity-dependent plasticity in nociceptive pathways has been implicated in pathomechanisms of chronic pain syndromes. Calcitonin gene-related peptide (CGRP), which is expressed by trigeminal nociceptors, has recently been identified as a key player in the mechanism of migraine headaches. Here we show that CGRP is coexpressed with brain-derived neurotrophic factor (BDNF) in a large subset of adult rat trigeminal ganglion neurons in vivo. Using ELISA in situ, we show that CGRP (1-1000 nM) potently enhances BDNF release from cultured trigeminal neurons. The effect of CGRP is dose-dependent and abolished by pretreatment with CGRP receptor antagonist, CGRP(8-37). Intriguingly, CGRP-mediated BDNF release, unlike BDNF release evoked by physiological patterns of electrical stimulation, is independent of extracellular calcium. Depletion of intracellular calcium stores with thapsigargin blocks the CGRP-mediated BDNF release. Using transmission electron microscopy, our study also shows that BDNF-immunoreactivity is present in dense core vesicles of unmyelinated axons and axon terminals in the subnucleus caudalis of the spinal trigeminal nucleus, the primary central target of trigeminal nociceptors. Together, these results reveal a previously unknown role for CGRP in regulating BDNF availability, and point to BDNF as a candidate mediator of trigeminal nociceptive plasticity.

Localization of calcitonin receptor-like receptor and receptor activity modifying protein 1 in enteric neurons, dorsal root ganglia, and the spinal cord of the rat

Calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) comprise a receptor for calcitonin gene related peptide (CGRP) and intermedin. Although CGRP is widely expressed in the nervous system, less is known about the localization of CLR and RAMP1. To localize these proteins, we raised antibodies to CLR and RAMP1. Antibodies specifically interacted with CLR and RAMP1 in HEK cells coexpressing rat CLR and RAMP1, determined by Western blotting and immunofluorescence. Fluorescent CGRP specifically bound to the surface of these cells and CGRP, CLR, and RAMP1 internalized into the same endosomes. CLR was prominently localized in nerve fibers of the myenteric and submucosal plexuses, muscularis externa and lamina propria of the gastrointestinal tract, and in the dorsal horn of the spinal cord of rats. CLR was detected at low levels in the soma of enteric, dorsal root ganglia (DRG), and spinal neurons. RAMP1 was also localized to enteric and DRG neurons and the dorsal horn. CLR and RAMP1 were detected in perivascular nerves and arterial smooth muscle. Nerve fibers containing CGRP and intermedin were closely associated with CLR fibers in the gastrointestinal tract and dorsal horn, and CGRP and CLR colocalized in DRG neurons. Thus, CLR and RAMP1 may mediate the effects of CGRP and intermedin in the nervous system. However, mRNA encoding RAMP2 and RAMP3 was also detected in the gastrointestinal tract, DRG, and dorsal horn, suggesting that CLR may associate with other RAMPs in these tissues to form a receptor for additional peptides such as adrenomedullin.

Calcitonin gene-related peptide enhances TTX-resistant sodium currents in cultured dorsal root ganglion neurons from adult rats

The neuropeptide calcitonin gene-related peptide (CGRP) binds to a subpopulation of dorsal root ganglion (DRG) neurons, elevates intracellular calcium, and causes inward currents in about 30% of lumbar DRG neurons. Using whole-cell patch clamp recordings, we found in the present study that application of CGRP to isolated and cultured DRG neurons from the adult rat enhances voltage-gated TTX-resistant (TTX-R) Na(+) inward currents in about 30% of small- to medium-sized DRG neurons. During CGRP, peak densities of Na(+) currents increased significantly. CGRP shifted the membrane conductance of the CGRP-responsive cells towards hyperpolarization without changing the slope of the peak conductance curve. The effect of CGRP was blocked by coadministration of CGRP8-37, an antagonist at the CGRP receptor. The effect of CGRP was also blocked after bath application of PKA14-22, a membrane-permeant blocker of protein kinase A, and PKC19-31, a PKC inhibitor, in the recording pipette. These data show pronounced facilitatory effects of CGRP on TTX-R Na(+) currents in DRG neurons which are mediated through CGRP receptors and intracellular pathways involving protein kinases A and C. Thus, in addition to prostaglandins, CGRP is another mediator that affects TTX-R Na(+) currents which are thought to occur mainly in nociceptive DRG neurons.

Variable sensitivity to noxious heat is mediated by differential expression of the CGRP gene

Heat sensitivity shows considerable functional variability in humans and laboratory animals, and is fundamental to inflammatory and possibly neuropathic pain. In the mouse, at least, much of this variability is genetic because inbred strains differ robustly in their behavioral sensitivity to noxious heat. These strain differences are shown here to reflect differential responsiveness of primary afferent thermal nociceptors to heat stimuli. We further present convergent behavioral and electrophysiological evidence that the variable responses to noxious heat are due to strain-dependence of CGRP expression and sensitivity. Strain differences in behavioral response to noxious heat could be abolished by peripheral injection of CGRP, blockade of cutaneous and spinal CGRP receptors, or long-term inactivation of CGRP with a CGRP-binding Spiegelmer. Linkage mapping supports the contention that the genetic variant determining variable heat pain sensitivity across mouse strains affects the expression of the Calca gene that codes for CGRPalpha.

The spinal basis of opioid tolerance and physical dependence: Involvement of calcitonin gene-related peptide, substance P, and arachidonic acid-derived metabolites

Chronic opioid use in the management of pain is limited by development of analgesic tolerance and physical dependence. The mechanisms underlying tolerance-dependence are not entirely clear, however, recent evidence suggests that spinal adaptations leading to increased activity of sensory neuropeptides (calcitonin gene-related peptide (CGRP), substance P) and their downstream signaling messengers derived from metabolism of arachidonic acid: prostaglandins (PG), lipoxygenase (LOX) metabolites, and endocannabinoids, plays an important role in this phenomenon. In this communication we review the evidence implicating these factors in the induction and expression of opioid tolerance and physical dependence at the spinal level.

Calcitonin gene-related peptide receptor activation produces PKA- and PKC-dependent mechanical hyperalgesia and central sensitization

Calcitonin gene-related peptide (CGRP), acting through CGRP receptors, produces behavioral signs of mechanical hyperalgesia in rats and sensitization of wide dynamic range (WDR) neurons in the spinal cord dorsal horn. Although involvement of CGRP receptors in central sensitization has been confirmed, the second-messenger systems activated by CGRP receptor stimulation and involved in pain transmission are not clear. This study tested whether the hyperalgesia and sensitizing effects of CGRP receptor activation on WDR neurons are mediated by protein kinase A or C (PKA or PKC) signaling. Intrathecal injection of CGRP in rats produced mechanical hyperalgesia, as shown by paw withdrawal threshold tests. CGRP-induced hyperalgesia was attenuated significantly by the CGRP1 receptor antagonist, CGRP8-37. The effect was also attenuated significantly by a PKA inhibitor (H89) or a PKC inhibitor (chelerythrine chloride). Electrophysiological experiments demonstrated that superfusion of the spinal cord with CGRP-induced sensitization of spinal dorsal horn neurons. The CGRP effect could be blocked by CGRP8-37. Either a PKA or PKC inhibitor (H89 or chelerythrine) also attenuated this effect of CGRP. These results are consistent with the hypothesis that CGRP produces hyperalgesia by a direct action on CGRP1 receptors in the spinal cord dorsal horn and suggest that the effects of CGRP are mediated by both PKA and PKC second-messenger pathways.

Expression of spinal cord Fos protein in response to intrathecal adrenomedullin and CGRP in conscious rats

Adrenomedullin (AM) immunoreactivity and mRNA, in addition to a large number of specific AM-binding sites, exist in the rat spinal cord. However, no phenotype has been reported for AM in the spinal cord. Here, expression of c-fos in response to intrathecal (i.t.) administration of AM, proadrenomedullin N-terminal 20 peptide (PAMP) and calcitonin gene-related peptide (CGRP) was examined in the thoracic, lumbar and sacral regions of spinal cord in conscious rats. Two hours after i.t. administration of either CGRP (2.5 and 10 microg) or AM (10 microg), the number of c-Fos immunoreactive nuclei was increased in all the spinal regions examined in this study, with the highest increase observed in the superficial dorsal horn. Few cells with c-fos immunoreactivity were found in the spinal cord of rats 2 h after i.t. injection of either saline or PAMP. Effects of AM (10 microg) and CGRP (2.5 microg) on c-fos expression were blocked when rats were pretreated with 40 microg of intrathecal CGRP8-37 (CGRP1 receptor antagonist). Fos-like immunoreactivity induced by i.t. CGRP and/or AM were also significantly abolished by i.t. administration of the nitric oxide (NO) inhibitor, l-NAME, indicating that endogenous NO is a necessary intermediary in CGRP and AM induced c-fos expression in the rat spinal cord. In conclusion, AM induces c-fos expression in rat spinal cord when administered intrathecally, with the pattern being similar to those produced by i.t. CGRP. Effects of the two peptides are sensitive to CGRP8-37 and l-NAME.

Involvement of opioid receptors in the CGRP8-37-induced inhibition of the activity of wide-dynamic-range neurons in the spinal dorsal horn of rats

The present study was performed to explore the involvement of opioid receptors in the calcitonin gene-related peptide 8-37 (CGRP8-37, an antagonist of CGRP receptor)-induced inhibition of the activity of wide-dynamic-range (WDR) neurons in the spinal dorsal horn of rats. Extracellular recording was performed with a multibarrelled glass micropipette, and the chemicals were delivered by micro-iontophoresis. The discharge frequency of WDR neurons was evoked by subcutaneous electrical stimulation applied to the ipsilateral hindpaw. Iontophoretic application of CGRP8-37 by an ejection current of 160 nA induced significant inhibition of the discharge frequency of WDR neurons. The inhibitory effect of CGRP8-37 on the activity of WDR neurons was attenuated by later iontophoretic application of the opioid antagonist naloxone. Furthermore, the effect of CGRP8-37 was attenuated by either iontophoretic application of the kappa-receptor antagonist nor-binaltorphimine (nor-BNI) or the mu-receptor antagonist beta-funaltrexamine (beta-FNA) but not by the delta-receptor antagonist naltrindole. The results indicate that kappa- and mu-opioid receptors on the membrane of WDR neurons are involved in the modulation of CGRP8-37-induced antinociception in dorsal horn of the spinal cord in rats.

Spinal administration of lipoxygenase inhibitors suppresses behavioural and neurochemical manifestations of naloxone-precipitated opioid withdrawal

1. This study investigated the role of spinal lipoxygenase (LOX) products in the induction and expression of opioid physical dependence using behavioural assessment of withdrawal and immunostaining for CGRP and Fos protein expression in the spinal cord. 2. Administration of escalating doses (5-50 mg kg-1; i.p.) of morphine for 5 days markedly elevated CGRP-like immunoreactivity in the dorsal horn of the rat spinal cord. Naloxone (2 mg kg-1; i.p.) challenge precipitated a robust withdrawal syndrome that depleted CGRP-like immunoreactivity and increased the number of Fos-like immunoreactive neurons in the dorsal horn. 3. Intrathecal administration of NDGA (10, 20 microg), a nonselective LOX inhibitor, AA-861 (1.5, 3 microg), a 5-LOX selective inhibitor, or baicalein (1.4, 2.8 microg), a 12-LOX selective inhibitor, concurrently with systemic morphine for 5 days or as a single injection immediately preceding naloxone challenge, blocked the depletion of CGRP-like immunoreactivity, prevented increase in the number of Fos-like immunoreactive neurons in the dorsal horn, and significantly attenuated the morphine withdrawal syndrome. 4. The results of this study suggest that activity of LOX products, at the spinal level, contributes to the expression of opioid physical dependence, and that this activity may be expressed through increased sensory neuropeptide release.

Inhibition of neurokinin-1-substance P receptor and prostanoid activity prevents and reverses the development of morphine tolerance in vivo and the morphine-induced increase in CGRP expression in cultured dorsal root ganglion neurons

Chronic treatment with opioid drugs such as morphine leads to the development of tolerance, which manifests as a loss of drug potency. The mechanisms underlying this phenomenon are poorly understood, but recent evidence suggests that increased activity of nociceptive sensory transmitters [calcitonin gene-related peptide (CGRP) and substance P] and other signalling messengers (prostaglandins) contribute to its development. Chronic intrathecal morphine administration to rats for 7 days produced analgesic tolerance. Co-administration of SR140333, a selective substance P receptor (neurokinin-1) antagonist, or nimesulide, a cyclooxygenase-2-selective inhibitor, augmented the acute effects of morphine, prevented morphine tolerance and reversed established tolerance. In cultured adult dorsal root ganglion neurons, exposure to morphine for 5 days increased the number of neurons expressing CGRP immunoreactivity. Co-exposure with the peptide CGRP receptor antagonist CGRP8-37, SR140333 or nimesulide prevented the morphine-induced increase in the expression of CGRP immunoreactivity. Additionally, BIBN4096BS, a nonpeptide CGRP receptor antagonist, stereoselectively produced similar effects. In summary, this investigation demonstrates that activity of CGRP and substance P contributes to both the induction and expression of opioid analgesic tolerance. Additionally, it highlights the involvement of prostaglandins generated by spinal cyclooxygenase-2 activity in the genesis of opioid tolerance. The neuropeptide and prostanoid activity contributing to tolerance is expressed at the level of the primary afferents terminating in the spinal cord. The combination of opioids with agents that block this activity may represent a useful strategy for the prevention as well as the reversal of clinical opioid tolerance.

The role of calcitonin gene-related peptide (CGRP) in the generation and maintenance of mechanical allodynia and hyperalgesia in rats after intradermal injection of capsaicin

This study was designed to assess the role of calcitonin gene-related peptide (CGRP) and its receptor in the generation and maintenance of secondary mechanical allodynia and hyperalgesia induced by intradermal injection of capsaicin in rats. Paw withdrawal responses (PWRs) to von Frey hairs with different bending forces applied on the rat paw were tested in this study. CGRP(8-37), a specific antagonist of CGRP 1 receptors, was delivered through a microdialysis fiber inserted across the dorsal horn. Post- and pretreatment paradigms were followed. When CGRP(8-37) was administered 1h after capsaicin injection, the mechanical allodynia and hyperalgesia were partially reversed in a dose-dependent manner. On the other hand, when rats were treated with CGRP(8-37) prior to capsaicin injection, the PWRs to von Frey applications were significantly reduced as compared to control animals. Collectively, these results suggest that CGRP receptors present in the dorsal horn are involved in the generation and maintenance of nociceptive behaviors associated with cutaneous inflammation.

Evidence for the existence of a new receptor for CGRP, which is not CRLR

Receptors for calcitonin gene-related peptide (CGRP), a neuropeptide known to be the most potent vasodilator, are abundantly expressed in cerebellum. A monoclonal antibody to cerebellar CGRP receptors specifically detects a 66 kDa protein from rat cerebellum and other rat and human tissues, but not from SK-N-MC cells which express calcitonin receptor-like receptor (CRLR), a recently described component of CGRP receptors. In contrast, mRNA expression for CRLR was abundant in SK-N-MC cells, but it was undetectable in rat cerebellum. Furthermore, the antibody could not detect any immunoreactive protein in HEK 293 cells transiently transfected with CRLR and receptor activity-modifying protein 1 (RAMP(1)) indicating the possible existence of another CGRP receptor, which does not involve CRLR. Due to the absence of biochemical or structural data on the existence of a CGRP(2) receptor and the new data provided in this paper, we suggest to identify the two CGRP receptors as CGRP-A and CGRP-B.

Long-term potentiation in spinothalamic neurons

Sensitization of nociceptive dorsal horn neurons, including spinothalamic tract (STT) cells, is thought to underlie the development of secondary hyperalgesia and allodynia following tissue injury. In central sensitization, responses to stimulation of sensory receptors are enhanced without any change in the excitability of the primary afferent neurons. We hypothesize that central sensitization of STT neurons is a variety of long-term potentiation (LTP). Evidence that LTP occurs in the spinal cord is reviewed. Neurotransmitters that trigger central sensitization include excitatory amino acids and peptides. Evidence for this is that co-activation of N-methyl-D-aspartate and NK1 receptors can produce long-lasting increases in the responses of STT cells, and antagonists of these receptors prevent central sensitization. Responses to excitatory amino acids increase and those to inhibitory amino acids decrease during central sensitization, presumably accounting for the changed excitability of STT cells. We believe these changes result from the activation of signal transduction pathways, including the protein kinase C, NO/protein kinase G and protein kinase A cascades. Recent evidence shows that calcium/calmodulin dependent kinase II (CaMKII) is also upregulated early in the process of central sensitization and that several types of ionotropic glutamate receptors become phosphorylated. It is proposed that the phosphorylation of neurotransmitter receptors leads to alterations in the sensitivity of these receptors and to central sensitization. Comparable events occur during LTP in brain structures.

Calcitonin gene-related peptide in pregnancy and its emerging receptor heterogeneity

Calcitonin gene-related peptide (CGRP) is the most potent vasodilator, and there is a growing body of evidence that this peptide might have multiple other functions. During pregnancy, circulating CGRP levels in rats increase up to the time of delivery, followed by a sharp decline at term and postpartum. In addition, the sensitivity of various vascular beds to CGRP in rats appears to increase with advancing pregnancy. This increased sensitivity might be involved in regulating uteroplacental blood flow, in addition to other vascular adaptations that occur during normal pregnancy. Furthermore, the uterine relaxation response to CGRP is elevated during pregnancy and decreased at term. Sex steroid hormones, estrogens and progesterone, regulate CGRP synthesis and its effects on both myometrial and uterine vascular tissues. These changes in smooth muscle relaxation sensitivity to CGRP appear to be a consequence of changes in CGRP-receptor levels in these tissues. There appear to be two receptors for CGRP: the CGRP-A receptor, a well-characterized receptor consisting of calcitonin receptor-like receptor and receptor activity modifying protein 1, and the CGRP-B receptor. The CGRP system might play a role in the maintenance of normal pregnancy, and a defect in this system might lead to complications.

Localization of functional calcitonin gene-related peptide binding sites in a subpopulation of cultured dorsal root ganglion neurons

In this study we investigated whether cultured dorsal root ganglion (DRG) neurons from the adult rat express binding sites for calcitonin gene-related peptide (CGRP). These were identified on fixed cells by using CGRP labeled at the N-terminal site with 1.4-nm gold particles. After 1 day in culture, about 20% of small to medium-sized DRG neurons showed CGRP-gold binding. Binding of CGRP-gold was dose-dependently reduced by coadministration of CGRP. The calcium imaging technique in living cells revealed that the bath administration of CGRP evoked an increase of the intracellular calcium in up to 30% of the DRG neurons tested. Both depletion of intracellular calcium stores by thapsigargin or using a calcium-free medium blocked the CGRP-mediated increase of cytosolic calcium in most neurons. Thus intracellular and extracellular sources of calcium are relevant for the CGRP response. Using the whole-cell patch-clamp technique, about 30% of the neurons were found to exhibit an inward current and a depolarization upon administration of CGRP close to the neurons. Immunocytochemical double-labeling techniques showed that most of the CGRP-gold binding sites were expressed in unmyelinated (neurofilament 200-negative) DRG neurons. Most of the neurons with CGRP-gold binding sites also expressed the tyrosine kinase A receptor, and all of them showed CGRP-like immunoreactivity. This study shows, therefore, that a subpopulation of unmyelinated, peptidergic primary afferent neurons express CGRP binding sites that can be activated by CGRP in an excitatory direction. The binding sites may serve as autoreceptors because all of these neurons also synthesize CGRP. The activation of CGRP binding sites may sensitize primary afferent neurons and influence the release of mediators.

Binding properties of the novel, non-peptide CGRP receptor antagonist radioligand, [(3)H]BIBN4096BS

BIBN4096BS [[R-(R,(R*,S*)]-N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]carbonyl] pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2H)-quinazolinyl)-,1-Piperidinecarboxamide] is a selective calcitonin gene-related peptide (CGRP) receptor antagonist with a picomolar affinity to the CGRP receptor in human neuroblastoma SK-N-MC cells. Here, we describe the characterisation of the binding properties of the tritiated radioanalogue of BIBN4096BS in SK-N-MC cells as well as in marmoset tissue. [(3)H]BIBN4096BS showed reversible and saturable binding to SK-N-MC cells with a K(D) of 0.045 nM. In competition experiments, [3(H)]BIBN4096BS is concentration-dependently displaced from SK-N-MC cell membranes by BIBN4096BS as well as by the endogenous ligand CGRP and its analogues with the rank order of affinity BIBN4096BS>human alpha-CGRP=human beta-CGRP>[Cys(Et)(2,7)]human alpha-CGRP>adrenomedullin (high affinity site)=human alpha-CGRP-(8-37)=human beta-CGRP-(8-37)>calcitonin=amylin. In the marmoset cortex, saturable [(3)H]BIBN4096BS binding was observed with a K(D) of 0.077 nM. CGRP showed biphasic competition of [(3)H]BIBN4096BS binding, whilst BIBN4096BS monophasically displaced its radioanalogue with a K(i) of 0.099 nM. These data, using [(3)H]BIBN4096BS, confirm the high affinity of this novel antagonist for the primate CGRP receptor and demonstrate furthermore that this radioligand is a useful tool to study CGRP receptor pharmacology.

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.

Calcitonin gene-related peptide-receptor component protein expression in the uterine cervix, lumbosacral spinal cord, and dorsal root ganglia

The neuropeptide calcitonin gene-related peptide (CGRP) may play a role in neurogenic inflammation, tissue remodeling of the uterine cervix, promoting vasodilation, parturition, and processing of sensory information in the spinal cord. CGRP-immunoreactive nerves of the cervix and spinal cord have been studied but cellular identification of the CGRP receptor has received little attention. CGRP-receptor component protein (CGRP-RCP) is a small protein associated with the CGRP receptor; thus, immunostaining for the CGRP-RCP can be used to identify sites of the CGRP receptor. We determined sites of CGRP-RCP immunoreactivity relative to the presence of CGRP-ir nerve fibers in the female rat uterine cervix, spinal cord, and dorsal root ganglia. CGRP-RCP immunoreactivity was expressed in the dorsal horn of the spinal cord, venules of the uterine cervix, and perikarya of sensory neurons in dorsal root ganglia. CGRP-immunoreactive fibers were adjacent to CGRP-RCP-immunoreactive vessels in the cervix and among CGRP-RCP-immunoreactive structures in the dorsal horn of the spinal cord. This suggests CGRP-RCP is associated with structures innervated by CGRP nerves and these interactions may be changed in tissues in response to an appropriate stimulus.

Role of calcitonin gene-related peptide and its antagonist on the evoked discharge frequency of wide dynamic range neurons in the dorsal horn of the spinal cord in rats

The present study was performed to explore the effect of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the evoked discharge frequency of wide dynamic range (WDR) neurons in the dorsal horn of the spinal cord in rats. Recording was performed with a multibarrelled glass micropipette and the chemicals were delivered by iontophoresis. The discharge of WDR neurons was evoked by transdermic electrical stimulation applied on the ipsilateral hindpaw. (1) Iontophoretic application of CGRP at an ejection current of 100 nA increased the discharge frequency of WDR neurons significantly. (2) Iontophoretic application of CGRP8-37 at an ejection current of 80 or 160 nA induced significant decreases in the discharge frequency of WDR neurons, but not at 40 nA. (3) Iontophoretic application of CGRP8-37 not only antagonized the CGRP-induced increase in the evoked discharge frequency of WDR neurons but also induced a significant decrease in the evoked discharge frequency of WDR neurons compared to basal levels. The results indicate that CGRP and its receptors play a facilitary role on the transmission and/or modulation of nociceptive information in the dorsal horn of the spinal cord in rats.

Sensory nerves and neuropeptides in uterine cervical ripening

At the time of parturition (fetal delivery) the uterine cervix must "ripen," becoming soft, pliable, and dilated to accommodate the fetus' delivery. The fundamental processes underlying cervical ripening remain poorly understood. Knowledge that abundant autonomic and sensory nerves supply the uterine cervix, that transection of afferent nerves supplying the cervix blocks parturition, and that some of the changes in the cervix resemble those seen in inflammatory reactions suggests nerves may have a role in the cervical ripening changes. The present study utilized immunohistochemistry, plasma extravasation, and solution hybridization-nuclease protection assay to elucidate the complement of primary afferent nerves and some receptors in the rat cervix during pregnancy, and to determine if they may have roles in the ripening process at term. This study revealed an abundance of nerves associated with the cervical vasculature and myometrial smooth muscle containing immunoreactivity for substance P, calcitonin gene-related peptide, secretoneurin, and nitric oxide synthase throughout pregnancy. Many of these are small unmyelinated capsaicin-sensitive C-fibers. Substance P- (NK1-) and calcitonin gene-related peptide receptors were apparent on uterine cervix vasculature from pregnant, parturient, and postpartum rats. NK1 receptor mRNA was maximal at 20 days of pregnancy. Plasma extravasation of i.v. administered Evans Blue or Monastral Blue was most pronounced at parturition (shortly after NK1 mRNA is maximal); this was similar to plasma extravasation evoked by i.v. administration of substance P or capsaicin-treatment. This study revealed new data about the nervous system of the rat uterine cervix and that these nerves and their transmitters could very well be part of a neurogenic inflammatory process involved in cervical ripening.

Blockade and reversal of spinal morphine tolerance by peptide and non-peptide calcitonin gene-related peptide receptor antagonists

This study examined the effects of the peptide CGRP receptor antagonist CGRP(8-37) and the newly-developed non-peptide CGRP receptor antagonist BIBN4096BS for their potential to both inhibit the development and reverse tolerance to the antinociceptive action of morphine. Repeated administration of intrathecal morphine (15 microg), once daily, produced a progressive decline of antinociceptive effect and an increase in the ED(50) value in the tailflick and paw pressure tests. Co-administration of CGRP(8-37) (4 microg) or BIBN4096BS (0.05, 0.1 microg) with morphine (15 microg) prevented the decline of antinociceptive effect and increase in ED(50) value in the tailflick test. Intrathecal administration of the CGRP receptor antagonists did not alter the baseline responses in either tests. Acute CGRP(8-37) also did not potentiate the acute actions of spinal morphine. In animals rendered tolerant to intrathecal morphine, subsequent administration of CGRP(8-37) (4 microg) with morphine (15 microg) partially restored the antinociceptive effect and ED(50) value of acute morphine, reflecting the reversal of tolerance. Animals tolerant to intrathecal morphine expressed increased CGRP and substance P-like immunostaining in the dorsal horn of the spinal cord. The increase in CGRP, but not substance P-like immunostaining, was blocked by a co-treatment with CGRP(8-37) (4 microg). In animals already tolerant to morphine, the increase in CGRP but not substance P-like immunostaining was partially reversed by CGRP(8-37) (4 microg). These data suggest that activation of spinal CGRP receptors contributes to both the development and expression of spinal opioid tolerance. CGRP receptor antagonists may represent a useful therapeutic approach for preventing as well as reversing opioid tolerance.

Differential effects of calcitonin gene-related peptide and calcitonin gene-related peptide 8-37 upon responses to N-methyl-D-aspartate or (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate in spinal nociceptive neurons with knee joint input in the rat

Calcitonin gene-related peptide is involved in the spinal processing of nociceptive input from the knee joint and in the generation and maintenance of joint inflammation-evoked hyperexcitability of spinal cord neurons. The present study examined whether this peptide influences the excitation of nociceptive spinal cord neurons by agonists at the N-methyl-D-aspartate and the non-N-methyl-D-aspartate [(R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate] receptors, both of which are essential for the excitation and hyperexcitability of spinal cord neurons. In anaesthetized rats extracellular recordings were made from dorsal horn neurons with knee input, and compounds were administered ionophoretically close to the neurons recorded. When calcitonin gene-related peptide was administered the responses of the neurons to the application of both N-methyl-D-aspartate and AMPA were increased. The coadministration of the antagonist calcitonin gene-related peptide 8-37 had no effect on the responses to N-methyl-D-aspartate, but it prevented the enhancement of the responses to N-methyl-D-aspartate by calcitonin gene-related peptide. By contrast, the administration of calcitonin gene-related peptide 8-37 enhanced the responses of the neurons to AMPA, and it did not antagonize but rather increased the effects of calcitonin gene-related peptide on these responses. The data suggest that the facilitatory role of calcitonin gene-related peptide on the development and maintenance of inflammation-evoked hyperexcitability is caused at least in part by the modulation of the activation of the dorsal horn neurons through their N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors. The different effects of calcitonin gene-related peptide 8-37 on the respones to N-methyl-D-aspartate and AMPA suggest that different intracellular pathways may facilitate the activation of N-methyl-D-aspartate and ionotropic non-N-methyl-D-aspartate receptors.