Calcitonin gene-related peptide enhances substance P-induced behaviors via metabolic inhibition: in vivo evidence for a new mechanism of neuromodulation

The Effect and Possible Mechanism of Intradiscal Injection of Simvastatin in the Treatment of Discogenic Pain in Rats

To study the effect of intradiscal injection of simvastatin on discogenic pain in rats and its possible mechanism, 30 adult female rats were used in this experiment. Twenty rats were randomly divided into sham operation group (Control group), intervertebral disk degeneration group (DDD group), intervertebral disk degeneration + hydrogel group (DDD + GEL group), and intervertebral disk degeneration + simvastatin group (DDD + SIM group). The mechanical pain threshold and cold sensation in rats were measured. The contents of NF-kappa B1, RelA, GAP43, SP, CGRP, TRPM 8, IL-1β, and TNF-α in the intervertebral disk (IVD), the corresponding contents of dorsal root ganglion (DRG) and plantar skin GAP43 and TRPM 8 were quantitatively detected by PCR. The corresponding IVDs were stained to detect their degeneration. There was no significant difference in the mechanical pain threshold between the groups at each time point. From the first day to the 8th week after surgery, the cold-sensing response of the DDD group was significantly higher than that of the Control group (P < 0.05). At 7 and 8 weeks postoperatively, the cold-sensing response of the DDD + SIM group was significantly lower than that of the DDD + GEL group (P < 0.05). The levels of NF-κB1, RelA, GAP43, SP, CGRP, TRPM8, IL-1β, and TNF-α in the IVD of DDD + SIM group were significantly lower than those in DDD group (P < 0.05). The content of GAP43 and TRPM8 in rat plantar skin decreased significantly and TRPM8 in DRG decreased significantly (P < 0.05).

Interaction of calcitonin gene related peptide (CGRP) and substance P (SP) in human skin

Calcitonin gene related peptide (CGRP) and substance P (SP) are neuropeptides that are simultaneously released from nociceptive C-fibers. CGRP is a potent vasodilator, inducing a long-lasting increase in superficial skin blood flow, whereas SP induces only a brief vasodilation but a significant plasma extravasation. CGRP and SP may play important roles in the pathophysiology of various pain states but little is known about their interaction. Different concentrations of SP (ranging from 10^-5M to 10^-9M) were applied to the volar forearm of 24 healthy subjects via dermal microdialysis. SP was applied either alone or in combination with CGRP10^-9M and CGRP 10^-6M. As expected, SP induced a transient increase in skin blood flow that decayed shortly after application. This transient blood flow peak was blunted with co-application of CGRP 10^-9M and inhibited with co-application of CGRP10^-6M. SP alone induced plasma protein extravasation (PPE). However, when CGRP10^-6M was added, the PPE significantly increased. Our results demonstrate a complex interaction of the neuropeptides CGRP and SP. CGRP10^-6M prevented SP-induced early vasodilation but augmented SP-induced PPE. These interactions might explain why vascular symptoms in chronic pain can differ strikingly between individuals.

Spinal astrocytic activation contributes to both induction and maintenance of pituitary adenylate cyclase-activating polypeptide type 1 receptor-induced long-lasting mechanical allodynia in mice

Background

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are present in the spinal dorsal horn and dorsal root ganglia, suggesting an important role of PACAP–PACAP receptors signaling system in the modulation of spinal nociceptive transmission. We have previously reported that a single intrathecal injection of PACAP or a PACAP specific (PAC1) receptor selective agonist, maxadilan, in mice induced dose-dependent aversive behaviors, which lasted more than 30 min, and suggested that the maintenance of the nociceptive behaviors was associated with the spinal astrocytic activation.

Results

We found that a single intrathecal administration of PACAP or maxadilan also produced long-lasting hind paw mechanical allodynia, which persisted at least 84 days without affecting thermal nociceptive threshold. In contrast, intrathecal application of vasoactive intestinal polypeptide did not change mechanical threshold, and substance P, calcitonin gene-related peptide, or N-methyl-D-aspartate induced only transient mechanical allodynia, which disappeared within 21 days. Western blot and immunohistochemical analyses with an astrocytic marker, glial fibrillary acidic protein, revealed that the spinal PAC1 receptor stimulation caused sustained astrocytic activation, which also lasted more than 84 days. Intrathecal co-administration of L-α-aminoadipate, an astroglial toxin, with PACAP or maxadilan almost completely prevented the induction of the mechanical allodynia. Furthermore, intrathecal treatment of L-α-aminoadipate at 84 days after the PAC1 stimulation transiently reversed the mechanical allodynia accompanied by the reduction of glial fibrillary acidic protein expression level.

Conclusion

Our data suggest that spinal astrocytic activation triggered by the PAC1 receptor stimulation contributes to both induction and maintenance of the long-term mechanical allodynia.

Calcitonin Gene-Related Peptide Modulates Heat Nociception in the Human Brain - An fMRI Study in Healthy Volunteers

Background

Intravenous infusion of calcitonin-gene-related-peptide (CGRP) provokes headache and migraine in humans. Mechanisms underlying CGRP-induced headache are not fully clarified and it is unknown to what extent CGRP modulates nociceptive processing in the brain. To elucidate this we recorded blood-oxygenation-level-dependent (BOLD) signals in the brain by functional MRI after infusion of CGRP in a double-blind placebo-controlled crossover study of 27 healthy volunteers. BOLD-signals were recorded in response to noxious heat stimuli in the V1-area of the trigeminal nerve. In addition, we measured BOLD-signals after injection of sumatriptan (5-HT_1B/1D antagonist).

Results

Brain activation to noxious heat stimuli following CGRP infusion compared to baseline resulted in increased BOLD-signal in insula and brainstem, and decreased BOLD-signal in the caudate nuclei, thalamus and cingulate cortex. Sumatriptan injection reversed these changes.

Conclusion

The changes in BOLD-signals in the brain after CGRP infusion suggests that systemic CGRP modulates nociceptive transmission in the trigeminal pain pathways in response to noxious heat stimuli.

Calcitonin gene-related peptide and migraine with aura: A systematic review

Background

Calcitonin gene-related peptide (CGRP) is a key molecule in migraine pathophysiology. Most studies have focused on CGRP in relation to migraine without aura (MO). About one-third of migraine patients have attacks with aura (MA), and this is a systematic review of the current literature on CGRP and MA.

Methods

We performed a systematic literature search on MEDLINE for reports of CGRP and MA, covering basic science, animal and human studies as well as randomized clinical trials.

Results

The literature search identified 594 citations, of which 38 contained relevant, original data. Plasma levels of CGRP in MA patients are comparable to MO, but CGRP levels varied among studies. A number of animal studies, including knock-ins of familial hemiplegic migraine (FHM) genes, have examined the relationship between CGRP and cortical spreading depression. In patients, CGRP does not trigger migraine in FHM, but is a robust trigger of migraine-like headache both in MA and MO patients. The treatment effect of CGRP antagonists are well proven in the treatment of migraine, but no studies have studied the effect specifically in MA patients.

Conclusion

This systematic review indicates that the role of CGRP in MA is less studied than in MO. Further studies of the importance of CGRP for auras and migraine are needed.

Effects of acute adult and early-in-life bladder inflammation on bladder neuropeptides in adult female rats

Background

The purpose of the present study was to determine how acute adult and/or prior early-in life (EIL; P14-P16) exposure to bladder inflammation affects bladder content of calcitonin gene related peptide (CGRP) and substance P (SP). Estrous cycle influences were also studied in the adult-treatment conditions.

Methods

In Experiment 1, intravesical zymosan or isoflurane anesthesia alone was administered to adult female rats. Bladders and serum were collected 24 hours later during each phase of the estrous cycle. In Experiment 2, zymosan or anesthesia alone was administered EIL and as adults, with bladder tissue collection 24 h later.

Results

In general, Experiment 1 showed that bladder content of both CGRP and SP was increased by inflammation. This effect was significant when data were collapsed across all phases of the estrous cycle, but was only significant during proestrus when individual comparisons were made during each phase of estrous. Also, adult bladder inflammation significantly reduced estradiol levels. In Experiment 2, bladder content of CGRP and SP was significantly increased in rats receiving EIL and/or adult inflammation. Bladder weights were also significantly increased by inflammation.

Conclusions

These data indicate that bladder CGRP and SP are maximally increased during the proestrus phase of the estrous cycle in inflamed adult female rats. EIL exposure to bladder inflammation alone can also produce an increase in CGRP and SP lasting into adulthood. Therefore, EIL experience with bladder inflammation may predispose an organism to experience a painful bladder disorder as an adult by increasing primary afferent content of CGRP and/or SP.

Calcitonin gene-related peptide receptor antagonists for migraine

IMPORTANCE OF THE FIELD

Migraine is a highly prevalent disabling condition, and the current treatment options are not satisfactory. The role of calcitonin gene-related peptide (CGRP) in migraine pathophysiology is well established. CGRP receptor antagonists address this new target and have the potential to improve therapy for both responders and non-responders to previous options.

AREAS COVERED IN THIS REVIEW

This review describes CGRP, its receptors and their role in the pathophysiology of migraine. CGRP receptor antagonists are a recent development; all reported antagonists are reported in chronological order. The experimental evidence, as well as all clinical trials since the first proof-of-concept study in 2004, is discussed.

WHAT THE READER WILL GAIN

An overview of the CGRP system and why it provides an attractive drug target for headache. The main focus is on the currently presented CGRP receptor antagonists and clinical evidence for this new therapeutic option.

TAKE HOME MESSAGE

CGRP receptor antagonists will provide an additional and valuable therapeutic option for the treatment of headaches.

Peptidomics of prolyl endopeptidase in the central nervous system

Prolyl endopeptidase (Prep) is a member of the prolyl peptidase family and is of interest because of its unique biochemistry and connections to cognitive function. Using an unbiased mass spectrometry (MS)-based peptidomics platform, we identified Prep-regulated peptides in the central nervous system (CNS) of mice by measuring changes in the peptidome as a function of Prep activity. This approach was validated by the identification of known Prep substrates, such as the neuropeptide substance P and thymosin-beta4, the precursor to the bioactive peptide Ac-SDKP. In addition to these known substrates, we also discovered that Prep regulates many additional peptides, including additional bioactive peptides and proline rich peptides (PRPs). Biochemical experiments confirmed that some of these Prep-regulated peptides are indeed substrates of the enzyme. Moreover, these experiments also supported the known preference of Prep for shorter peptides while revealing a previously unknown cleavage site specificity of Prep when processing certain multi-proline-containing peptides, including PRPs. The discovery of Prep-regulated peptides implicates Prep in new biological pathways and provides insights into the biochemistry of this enzyme.

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.

Intraarticular induction of interleukin-1beta expression in the adult mouse, with resultant temporomandibular joint pathologic changes, dysfunction, and pain

OBJECTIVE

To examine the effects of intraarticular induction of interleukin-1beta (IL-1beta) expression in adult mice.

METHODS

We used somatic mosaic analysis in a novel transgenic mouse with an inducible IL-1beta transcription unit. Transgene activation was induced by Cre recombinase in the temporomandibular joints (TMJs) of adult transgenic mice (conditional knockin model). The effects of intraarticular IL-1beta induction were subsequently evaluated at the cellular, histopathologic, and behavioral levels.

RESULTS

We developed transgenic mice capable of germline transmission of a dormant transcription unit consisting of the mature form of human IL-1beta as well as the reporter gene beta-galactosidase driven by the rat procollagen 1A1 promoter. Transgene activation by a feline immunodeficiency virus Cre vector resulted in histopathologic changes, including articular surface fibrillations, cartilage remodeling, and chondrocyte cloning. We also demonstrated up-regulation of genes implicated in arthritis (cyclooxygenase 2, IL-6, matrix metalloproteinase 9). There was a lack of inflammatory cells in these joints. Behavioral changes, including increased orofacial grooming and decreased resistance to mouth opening, were used as measures of nociception and joint dysfunction, respectively. The significant increase in expression of the pain-related neurotransmitter calcitonin gene-related peptide (CGRP) in the sensory ganglia as well as the auxiliary protein CGRP receptor component protein of the calcitonin-like receptor in the brainstem further substantiated the induction of pain.

CONCLUSION

Induction of IL-1beta expression in the TMJs of adult mice led to pathologic development, dysfunction, and related pain in the joints. The somatic mosaic model presented herein may prove useful in the preclinical evaluation of existing and new treatments for the management of joint pathologic changes and pain, such as in osteoarthritis.

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.

Role of calcitonin gene-related peptide in the sensitization of dorsal horn neurons to mechanical stimulation after intradermal injection of capsaicin

This study was designed to assess the role of calcitonin gene-related peptide (CGRP) and its receptor in the sensitization of dorsal horn neurons induced by intradermal injection of capsaicin in rats. Extracellular recordings were made from wide dynamic range (WDR) dorsal horn neurons with receptive fields on the hindpaw in the lumbar enlargement of anesthetized rats. The background activity and responses to brushing, pressing, and pinching the skin were assessed. A postsuperfusion or a presuperfusion of CGRP(8-37) paradigm was followed. When tested 30 min after capsaicin injection, there was an increase in background activity and responses to brush, press, and pinch applied to the receptive field. Superfusion of CGRP(8-37) into the spinal cord at 45 min after capsaicin injection significantly reversed the increased background activity and responses to brush, press, and pinch applied to the receptive field. On the other hand, spinal superfusion of CGRP(8-37) prior to capsaicin injection prevented the increased background activity and responses to brush, press, and pinch of WDR neurons that occurred following capsaicin injection in control experiments. A sensitization of spinal dorsal horn neurons could also be induced by superfusion of the spinal cord with CGRP. The effect could be blocked by CGRP(8-37) dose-dependently. Collectively, these results suggest that CGRP and its receptors are involved in the spinal cord central sensitization induced by intradermal injection of capsaicin.

Calcitonin gene-related peptide regulates expression of neurokinin1 receptors by rat spinal neurons

Although neurokinin 1 (NK1) receptors contribute to hyperalgesia, and their expression is increased in the spinal cord during peripheral inflammation, little is known regarding the signaling molecules and the second messenger pathways that they activate in regulating the expression of the NK1 receptor gene. Because the promoter region of the NK1 receptor contains a cAMP response element (CRE), we tested the hypothesis that calcitonin gene-related peptide (CGRP) regulates the expression of NK1 receptors via a pathway involving activation of the transcription factor cAMP response element binding protein (CREB). Experiments were conducted on primary cultures of neonatal rat spinal neurons. Treatment of cultures with CGRP for 8-24 hr increased (125)I-substance P binding on spinal neurons; the increase in binding was preceded by an elevation in NK1 receptor mRNA. The CGRP-induced change in (125)I-substance P binding was concentration-dependent and was inhibited by the antagonist CGRP(8-37). CGRP increased phosphorylated CREB immunoreactivity and CRE-dependent transcription in neurons, indicating the involvement of the transcription factor CREB. Evidence that CGRP increased cAMP levels in spinal neurons and that the protein kinase A inhibitor H89 attenuated CGRP-induced CRE-dependent transcription suggests that the intracellular pathway stimulated by CGRP leads to activation of protein kinase A. Collectively these data define a role for CGRP as a signaling molecule that induces expression of NK1 receptors in spinal neurons. The data provide evidence that a neuropeptide receptor controls gene expression in the CNS and add another dimension to understanding the cotransmission of substance P and CGRP by primary afferent neurons.

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.

Capsaicin-evoked release of immunoreactive calcitonin gene-related peptide from the spinal cord is mediated by nitric oxide but not by cyclic GMP

Recent data support a role for nitric oxide (NO) in pain processing at the level of the spinal cord, possibly via regulation of neuropeptide release. The goal of this study was to determine whether capsaicin, which selectively activates primary afferent neurons and evokes neuropeptide release, acts in an NO-dependent manner. Our results indicate that capsaicin (1 microM)-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP) is significantly reduced in the presence of the NO synthase inhibitor, L-NAME (10-400 nM; F(3,45)=68.38; P<0.001) and, the selective nNOS inhibitor, 3-bromo-7-nitroindazole (170-680 nM; F(5,48)=56.2; P<0. 01). D-NAME (200 nM) had no effect on capsaicin-evoked iCGRP release. Hemoglobin (an extracellular scavenger of NO; 3 mg/ml) significantly reduced the effect of capsaicin on the release of iCGRP (F(1,8)=9.12; P<0.05). The NOS substrate, L-arginine, effectively reversed the inhibitory effect of 3-bromo-7-nitroindazole on capsaicin-evoked iCGRP release. To determine whether the NO-mediated release was NMDA-driven, we superfused spinal cord slices with competitive and non-competitive NMDA antagonists in the presence and absence of capsaicin. MK-801 (0. 1-10 microM; F(4,33)=8.49; P<0.0001) and AP-5 (0.01-10 microM; F(4, 38)=3.34; P<0.05) reduced capsaicin-evoked iCGRP release. CNQX, an AMPA/kainate antagonist (10 nM-10 microM), significantly decreased capsaicin-evoked release of iCGRP (F(6,42)=8.76; P<0.01) in a dose-dependent fashion. Additionally, our results demonstrate that while capsaicin-evoked release is significantly reduced in the presence of LY-83583 (10 microM; F(2,18)=3.46; P<0.01; a cyclic GMP lowering agent), there is no effect of ODQ (a potent and selective inhibitor of guanylate cyclase). Moreover, the application of a cell permeable analog of cyclic GMP (8-bromo-cGMP; 0.01-1000 microM) is without effect on both basal and evoked iCGRP release. Finally, we observed no colocalization of immunoreactive neuronal NOS (nNOS) with CGRP in the dorsal horn. In summary, these data indicate that capsaicin evokes the release of iCGRP, in part, via the production of NO which enters the extracellular space prior to having an effect. Moreover, iCGRP and nNOS are produced in distinct populations of neurons within the dorsal horn. We conclude that capsaicin-evoked release involves the activation of the NMDA receptor but is also modified by the activation of AMPA or kainate receptors. Finally, these data suggest that while capsaicin-evoked iCGRP release is modified by NO, this release does not require the activation of guanylate cyclase and subsequent production of cyclic GMP.

Trigeminal innervation of the mammalian pineal gland

There is evidence that the trigeminal (Gasserian) ganglia innervate the mammalian pineal gland and serve in its regulation in addition to the sympathetic and cholinergic as well as further influences. By means of immunohistochemical methods, previous studies demonstrated fibers containing calcitonin gene-related peptide (CGRP-LI) or substance P (SP) in the superficial pineal of various mammalian species. In addition, SP and the related tachykinin, neurokinin A, were detected by radioimmunoassay and HPLC, respectively, in the rat and human pineal gland. In the present study, retrograde neuronal tracing upon injection of a tracer substance into the superficial pineal gland of rats was used in combination with immunohistochemistry to show that trigeminal ganglionic neurons innervate the gland, and that a considerable amount of these neurons are also CGRP- or SP-immunoreactive. These results reveal that afferent neuropeptidergic entering the superficial pineal gland originate from the trigeminal ganglia. The present paper reviews the evidence for a CGRP- and SP-ergic innervation of the mammalian pineal gland and discusses the possible role of these neuropeptides with regard to pineal function.

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

Although the distribution of calcitonin gene-related peptide has been extensively studied in the spinal cord, little is known about the precise subcellular localization of receptors for calcitonin gene-related peptide. The present study was undertaken to localize calcitonin gene-related peptide receptors in both the dorsal and ventral horns of the rat spinal cord. Immunocytochemical localization with specific monoclonal antibodies was performed at the light and electron microscopic levels. Calcitonin gene-related peptide receptor was expressed in neuronal but not glial elements. Discrete postsynaptic localization of receptor for the calcitonin gene-related peptide was evident in the cells and dendrites of the superficial dorsal horn. Some of the terminal endings apposing the stained synapses formed the central terminals of glomerular complexes. The endings were scallop shaped (Type I), typical of primary afferent terminations. Other dorsal horn structures with postsynaptic labeling were contacted by dome-shaped or elongated axonal endings. Presynaptic localization on some dorsal horn terminations may serve an autoreceptor function. Motoneurons, on the other hand, were contacted by axonal terminals with presynaptic calcitonin gene-related peptide receptors. These data suggest that (i) dorsal horn neurons are capable of direct primary afferent, calcitonin gene-related peptide receptor-mediated interactions and (ii) neuronal terminals contacting motor horn cells can be influenced through presynaptic paracrine-like calcitonin gene-related peptide receptor-mediated interactions. Thus, calcitonin gene-related peptide can have multiple modulatory effects on spinal cord neurons through site-specific receptors.

Neuropeptide-mediated facilitation and inhibition of sensory inputs and spinal cord reflexes in the lamprey

The effects of neuromodulators present in the dorsal horn [tachykinins, neuropeptide Y (NPY), bombesin, and GABAB agonists] were studied on reflex responses evoked by cutaneous stimulation in the lamprey. Reflex responses were elicited in an isolated spinal cord preparation by electrical stimulation of the attached tail fin. To be able to separate modulator-induced effects at the sensory level from that at the motor or premotor level, the spinal cord was separated into three pools with Vaseline barriers. The caudal pool contained the tail fin. Neuromodulators were added to this pool to modulate sensory inputs evoked by tail fin stimulation. The middle pool contained high divalent cation or low calcium Ringer to block polysynaptic transmission and thus limit the input to the rostral pool to that from ascending axons that project through the middle pool. Ascending inputs and reflex responses were monitored by making intracellular recordings from motor neurons and extracellular recordings from ventral roots in the rostral pool. The tachykinin neuropeptide substance P, which has previously been shown to potentiate sensory input at the cellular and synaptic levels, facilitated tail fin-evoked synaptic inputs to neurons in the rostral pool and concentration dependently facilitated rostral ventral root activity. Substance P also facilitated the modulatory effects of tail fin stimulation on ongoing locomotor activity in the rostral pool. In contrast, NPY and the GABAB receptor agonist baclofen, both of which have presynaptic inhibitory effects on sensory afferents, reduced the strength of ascending inputs and rostral ventral root responses. We also examined the effects of the neuropeptide bombesin, which is present in sensory axons, at the cellular, synaptic, and reflex levels. As with substance P, bombesin increased tail fin stimulation-evoked inputs and ventral root responses in the rostral pool. These effects were associated with the increased excitability of slowly adapting mechanosensory neurons and the potentiation of glutamatergic synaptic inputs to spinobulbar neurons. These results show the possible behavioral relevance of neuropeptide-mediated modulation of sensory inputs at the cellular and synaptic levels. Given that the types and locations of neuropeptides in the dorsal spinal cord of the lamprey show strong homologies to that of higher vertebrates, these results are presumably relevant to other vertebrate systems.

Testes exhibit elevated expression of calcitonin gene-related peptide receptor component protein

Calcitonin gene-related peptide (CGRP) receptor component protein (RCP) is a novel protein that modulates CGRP responsiveness in a variety of cell types. Using probes based on the isolation of CGRP-RCP complementary DNA (cDNA) from a guinea pig organ of Corti cDNA library, we cloned human (h) and mouse (m) CGRP-RCP cDNAs, both of which encode 148-residue proteins that at the amino acid levels are approximately 88% identical to each other and to the 146-residue guinea pig CGRP-RCP. Northern blot analysis confirmed the presence of CGRP-RCP messenger RNA in all of the human and mouse tissues tested. In these human tissues, hCGRP-RCP messenger RNA (major band at approximately 3.1 kb, minor band at approximately 7.5 kb) was most prevalent in the testis. In the mouse, the highest abundance of CGRP-RCP RNA was clearly in the testis (major band at approximately 1.6 kb, minor band at approximately 1.1 kb). Based on this tissue distribution of RNA, we sought to identify the cells in the murine testis that contained CGRP-RCP protein. Numerous antisera generated against hCGRP-RCP, including one to recombinant hCGRP-RCP, exhibited strong immunoreactivity localized to the head region of spermatozoa. No CGRP-RCP immunoreactivity was observed in other cells at less mature stages of sperm maturation, in Sertoli or interstitial (Leydig) cells, or in human spermatozoa. Murine epididymal (mature) spermatozoa exhibited CGRP-RCP immunoreactivity identical to that of testicular spermatozoa. Spermatozoa that underwent an experimentally induced acrosome reaction (acrosomal discharge) lost their CGRP-RCP immunoreactivity. Therefore, it appears that CGRP-RCP is associated with the acrosome, suggesting that it may play an important role in reproduction.

CGRP activates renal pelvic substance P receptors by retarding substance P metabolism

Substance P and calcitonin gene-related peptide (CGRP) are colocalized in renal pelvic sensory nerves. Increasing renal pelvic pressure results in an increase in afferent renal nerve activity that is blocked by a substance P receptor antagonist but not by a CGRP receptor antagonist. CGRP potentiates the effects of substance P by preventing the metabolism of substance P. Therefore, we examined whether CGRP enhanced the afferent renal nerve activity responses to substance P and increased renal pelvic pressure, a stimulus known to increase substance P release. Combined administration of substance P and CGRP into the renal pelvis resulted in an increase in afferent renal nerve activity (1392+/-217%. s; area under the curve of afferent renal nerve activity versus time) that was greater (P<0.01) than that produced by substance P (620+/-156%. s) or CGRP (297+/-96%. s) alone. Likewise, CGRP enhanced the afferent renal nerve activity response to increased renal pelvic pressure. During renal pelvic administration of the neutral endopeptidase inhibitor thiorphan, the afferent renal nerve activity response to substance P plus CGRP was similar to that produced by either neuropeptide alone. Because these studies suggested that CGRP potentiated the afferent renal nerve activity responses to substance P, we examined whether the afferent renal nerve activity response to CGRP was blocked by a substance P receptor antagonist, RP67580. RP67580 blocked the afferent renal nerve activity response to CGRP by 85+/-12% (P<0.02). We conclude that CGRP activates renal pelvic sensory nerves by retarding the metabolism of substance P, thereby increasing the amount of substance P available for stimulation of substance P receptors.

Neuropathic pain in the cancer patient

Cancer presents itself in numerous ways, adding to the complexity of any pain syndrome with which it is associated. Neuropathic pain, unlike many other pain syndromes, is difficult to treat even in the absence of cancer. The combination results in a heterogeneous group of patients with a complex set of symptoms. This makes the assessment of pain, classification of syndromes, and clinical study a challenge. If the disease is nonprogressive, general principles of care are essentially the same as in those without cancer. In patients with progressive disease and more refractory painful conditions, spinal anesthetic and neurosurgical therapies must often be considered. Under such circumstances, caregivers are forced to carefully balance uncertain benefits and risks, often without the luxury of time. More careful observation and controlled trials in these patients help facilitate this challenging process.

Action of substance P and interaction of calcitonin gene-related peptide and substance P on the cat antral circular muscle

The actions of substance P (SP) and calcitonin gene-related peptide (CGRP) and their interaction were examined in vitro in the feline antrum and colon. Circular muscle contraction was seen in the antrum to both peptides, but only to SP in the proximal colon. Antral contraction was enhanced when both peptides were given together. This interaction was inhibited by tetrodotoxin or atropine. SP acted at the antrum via a smooth muscle neurokinin receptor which is not a (NK)-1 receptor. SP binding was displaced by neurokinin A but not by the NK-1 receptor antagonist, CP-96345. The colonic response was inhibited by CP-96345. Immunohistochemistry revealed SP-like immunoreactivity (SP-LI) in fibers in the antral myenteric plexus and circular muscle, while CGRP-like immunoreactivity (CGRP-LI) was seen in the myenteric plexus only, without co-localization. These studies supported the hypothesis that SP acted via the NK-2 receptor at the feline circular muscle in the antrum to induce contraction and at the NK-1 receptor in the proximal colon. CGRP enhanced the effect of SP via a cholinergic pathway.

Increased concentrations of calcitonin gene-related peptide-like immunoreactivity in rat brain and peripheral tissue after ischaemia: correlation to flap survival

The effects of experimentally induced ischaemia after free-flap surgery on concentrations of neuropeptide Y (NPY), neurokinin A (NKA), substance P (SP) and calcitonin gene-related peptide (CGRP)-like immunoreactivity (-LI) were studied in flap tissue and in different regions of the rat brain (striatum, hippocampus, pituitary, hypothalamus, frontal and occipital cortex). Ten days after the operation, CGRP-LI and NKA-LI were decreased in the ischaemic tissue but increased in the surrounding tissue. In the brain, CGRP-LI was increased in five of six regions analysed, with the exception of the striatum. SP-LI and NKA-LI were increased in the pituitary and hippocampus, but decreased in other brain regions. Changes of CGRP-LI in the brain correlated positively with the CGRP-LI concentrations in the surrounding flap tissue and the CGRP-LI concentrations in the ischaemic flap tissue with the extent of flap survival. The results of the present study suggest that higher concentrations of CGRP-LI are related to tissue survival and that endogenous CGRP has a regulatory effect in ischaemia.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Isolation and identification of CGRP C-terminal fragments in the rat spinal cord

In this study, we have used a liquid chromatography micropurification system in combination with fast atom bombardment mass spectrometry (FAB-MS) and N-terminal sequencing to characterize 3 calcitonin gene-related peptide (CGRP) immunoreactivities present in the rat spinal cord. Full-length CGRP contributed to approximately 68% of the total immunoreactive material, while approximately 23% consisted of 2 C-terminal fragments, CGRP(18-37) and CGRP(19-37). Synthetic C-terminal fragments of CGRP, e.g. CGRP(19-37), have been shown to antagonize CGRP effects in vitro. We show that such fragments exist in relatively substantial amounts in the rat spinal cord.

Intrathecal CGRP8-37-induced bilateral increase in hindpaw withdrawal latency in rats with unilateral inflammation

1. Recent work in our laboratory has demonstrated that intrathecal administration of a selective antagonist of calcitonin gene-related peptide (CGRP), CGRP8-37, increased the hindpaw withdrawal latency (HWL) to thermal stimulation and hindpaw withdrawal threshold (HWT) to pressure in normal rats, and that these effects were more pronounced than in rats with mononeuropathy. 2. The present study was performed to investigate the effects of intrathecal administration of CGRP8-37 on the HWL and HWT in rats with unilateral hindpaw inflammation induced by subcutaneous injection of carrageenin. The effect of naloxone was also studied. 3. Subcutaneous injection of 0.1 ml of carrageenin into the plantar region of the left hindpaw induced a significant increase in the volume of the ipsilateral hindpaw (P < 0.001), and significant bilateral decreases of the HWL to thermal stimulation (ipsilateral: P < 0.001; contralateral: P < 0.01) and HWT to pressure (ipsilateral: P < 0.001; contralateral: P < 0.01). 4. Intrathecal administration of 10 nmol of CGRP8-37, but not of 1 or 5 nmol, induced a significant bilateral increase in the HWL and HWT in rats with experimentally induced inflammation (thermal test: P < 0.001; mechanical test: P < 0.001). 5. The effect of intrathecal administration of 10 nmol CGRP8-37 on HWL and HWT was significantly more pronounced in intact rats than in rats with experimentally induced inflammation (ipsilateral: P < 0.001; contralateral: P < 0.001). 6. The effect of CGRP8-37 on withdrawal responses in the inflamed paw was partly reversed by intrathecal injection of naloxone at a dose of 88 nmol in the thermal (ipsilateral: P < 0.01; contralateral: P = 0.14) and mechanical tests (ipsilateral: P < 0.05; contralateral: P = 0.60). 7. A significant bilateral increase in the concentration of CGRP-like immunoreactivity in the perfusate of both hindpaws was demonstrated 24 h after unilateral injection of carrageenin (ipsilateral: P < 0.001; contralateral: P < 0.05). There was also an increase in the amount of CGRP-like immunoreactivity in the cerebrospinal fluid (P < 0.001), but not in plasma (P = 0.75). 8. The present study demonstrates that acute experimentally-induced unilateral hindpaw inflammation, induces bilateral increases in the amount of CGRP-like immunoreactivity in hindpaw perfusates. Intrathecal administration of CGRP8-37 increased the HWL to thermal stimulation and HWT to pressure bilaterally. 9. The results indicate that CGRP plays a role in the transmission of presumed nociceptive information in the spinal cord of rats with experimentally induced inflammation. Furthermore, our findings suggest that opioids can modulate CGRP-related effects in the spinal cord.

Opioid antagonists naloxone, beta-funaltrexamine and naltrindole, but not nor-binaltorphimine, reverse the increased hindpaw withdrawal latency in rats induced by intrathecal administration of the calcitonin gene-related peptide antagonist CGRP8-37

We recently demonstrated that intrathecal administration of calcitonin gene-related peptide 8-37 (CGRP8-37), a selective antagonist of calcitonin gene-related peptide receptors, dose-dependently increased the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in intact rats, indicating a role for CGRP and its receptors in the transmission of presumed nociceptive information in the spinal cord. The present study was performed to explore the interaction between CGRP and opioids in the spinal cord of rats. The effects of naloxone, a non-selective opioid receptor antagonist, and three different selective opioid receptor antagonists on the increased latency to withdrawal response induced by intrathecal injection of CGRP8-37 were explored. Intrathecal administration of 10 nmol of CGRP8-37 induced a significant bilateral increase in hindpaw withdrawal latency to both thermal and mechanical stimulation. The effect was partly reversed by intrathecal injection of 4 or 8 micrograms of naloxone, 10 nmol of either the mu opioid receptor antagonist beta-funaltrexamine or the delta opioid receptor antagonist naltrindole, but not by 10 nmol of the kappa opioid receptor antagonist nor-binaltorphimine. These results indicate that mu and delta, but not kappa, opioid receptors are involved in the modulation of post-synaptic effects and/or release of CGRP and other neurotransmitters.

Increased fos expression in preoptic calcitonin gene-related peptide (CGRP) neurones following mating but not the luteinizing hormone surge in female rats

The functional relationship between sexually dimorphic neural populations and sex differences in reproductive functioning is unclear. The present study has investigated the function of the sexually dimorphic, estrogen-receptive, calcitonin gene-related peptide (CGRP) neurones in the female preoptic area by examining patterns of Fos immunoreactivity within these cells in relation to the luteinizing hormone surge and lordosis behaviour. In the first experiment, ovariectomized rats were treated with estradiol alone or estradiol plus progesterone to induce the luteinizing hormone surge. The percentage of CGRP neurones with Fos-positive nuclei was not different in estradiol alone (18 +/- 4%) and estradiol/progesterone-treated (24 +/- 3%) rats although the number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 2-fold (P < 0.01) in estrogen/progesterone-treated rats and 40 +/- 5% of luteinizing hormone-releasing hormone neurones were found to express Fos in this group. In the second experiment, ovariectomized rats were treated with estradiol and progesterone and either, mated with a single male or placed in an empty cage, for 30 min. The number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 4-fold in mated rats (P < 0.01) and the percentage of CGRP neurones with Fos-positive nuclei increased from 24 +/- 3% to 38 +/- 2% (P < 0.01) in mated animals. No differences were detected in the number of luteinizing hormone-releasing hormone neurones with Fos-positive nuclei in mated and non-mated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Controlled superfusion of the rat spinal cord for studying non-synaptic transmission: an autoradiographic analysis

Recently, evidence has been raised that long-term changes in the central nervous system are mediated by extrasynaptic spread of neuropeptides ('volume transmission'). To study the effects of volume transmission in the spinal cord we developed the technique of controlled superfusion of the rat cord dorsum. This paper presents quantitative data about the spread, local spinal tissue concentration and redistribution of (2-[125I]iodohistidyl)neurokinin applied for 15, 30 or 60 min to the spinal cord dorsum in concentrations of 0.05 or 50 microM (10 microliters). Analysis of autoradiograms of sagittal and transverse spinal cord sections was done by computer-assisted densitometry. Under all experimental conditions, the spread of radiolabel into the superfused spinal cord segments reached Rexed's laminae V and VI; maximal spread (1.6 +/- 0.3 mm) was measured after superfusion for 30 min. The amount of radiolabel decreased in ventral direction as a function of distance. Highest tissue concentrations of neurokinin A (NKA) were obtained within the superficial spinal cord up to a depth of 0.5 mm and ranged from 700 to 2000 pmol/g following superfusions for 15 or 30 min with 50 microM NKA. Thus, these tissue concentrations were 25-70 times lower than the concentration of NKA in the superfusate. Since pool content was not exchanged, the radioactivity within the spinal cord was lower after superfusion periods of 60 min than after 15 or 30 min. Detection of radiolabel in blood and urine suggests that capillary clearance is relevant and limits the accumulation of the peptide within the spinal cord tissue and the spread into deeper laminae. The controlled superfusion of the rat cord dorsum is a useful method to mimick the spinal release of endogenous neuropeptides such as NKA during intense noxious stimulation, and it can be employed for versatile investigations of the effects of neuroactive molecules on the processing of sensory information in the intact spinal network.

Intraspinal release of immunoreactive calcitonin gene-related peptide during development of inflammation in the joint in vivo--a study with antibody microprobes in cat and rat

This study addressed the intraspinal release of immunoreactive calcitonin gene-related peptide in vivo during mechanical stimulation of the normal joint and during the development of an acute experimental inflammation in the knee joint in the anaesthetized cat (spinalized) and rat (not spinalized). Release was assessed using microprobes coated with antibody to calcitonin gene-related peptide; inhibition of binding of [125I]calcitonin gene-related peptide to these probes following insertion into the spinal cord is equated with intraspinal release of the endogenous (unlabelled) peptide. Probes inserted prior to inflammation showed marked basal release of immunoreactive calcitonin gene-related peptide in the dorsal horn with a maximum in the superficial dorsal horn in the absence of intentional stimulation. The pattern of binding of [125I]calcitonin gene-related peptide was not or only minimally changed by innocuous mechanical stimuli (flexion of and innocuous pressure to the knee in the cat and innocuous pressure to the knee of the rat) but was significantly altered by electrical stimulation of the tibial nerve in the cat (sufficient to excite unmyelinated afferent fibres), indicating release of the peptide by the latter stimulus. During the first hours of the development of an experimental inflammation in the knee joint induced by intra-articular injections of kaolin and carrageenan, the pattern of binding of [125I]calcitonin gene-related peptide changed. In the cat, the level of immunoreactive calcitonin gene-related peptide showed a persistent increase in the gray matter and up to the surface of the cord and release was slightly increased by innocuous stimuli. In the rat, increased levels of immunoreactive calcitonin gene-related peptide were mainly seen in the superficial and deep dorsal horn during innocuous pressure (this stimulus did not evoke release of the peptide prior to inflammation) and noxious pressure applied to the injected knee, whereas increased basal levels were only observed at later stages. These data show that the development of an acute experimental inflammation in the joint is associated with an enhancement of the intraspinal release of immunoreactive calcitonin gene-related peptide. Since the changes in the release were noted at an early stage, within the first hours, they could contribute to the generation of inflammation-evoked changes of the responsiveness of spinal cord neurons and hence to the mechanisms inducing inflammatory pain.

The calcitonin gene-related peptide antagonist CGRP8-37 increases the latency to withdrawal responses in rats

The present study explored the effects of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 on the latency to hindpaw withdrawal responses induced by both thermal and mechanical stimulation in rats. (1) Intrathecal injection of 10 nmol of CGRP had no effects on the latency to hindpaw withdrawal; intrathecal injection of 5 nmol of substance P (SP) decreased the latency to both withdrawal responses. (2) Intrathecal administration of 5 nmol or 10 nmol of CGRP8-37, but not 1 nmol, induced a significant increase in hindpaw withdrawal latency. (3) Intrathecal administration of CGRP8-37 not only reversed the SP-induced decrease in latency to both withdrawal responses but also mediated a significant increase in response latency compared to basal levels. The demonstrated results suggest that intrathecal administration of CGRP8-37 has a possible antinociceptive effect, and CGRP receptors in the spinal cord may be involved.

Regulation of the second-messenger systems in the rat spinal cord during prolonged peripheral inflammation

Unilateral intraplantar injection of Freund's complete adjuvant (FCA) into 1 hind paw of rats was used as a model of peripheral inflammation and persistent pain in order to examine time course effects of a continuous barrage of nociceptive input on the second-messenger transducing systems in the spinal cord. cAMP, cGMP and inositol 1,4,5-trisphosphate (insP3) were extracted from the lumbosacral cord at days 1, 7, 14, 21 and 42 following FCA injection and quantified by either radioreceptor-assay (RRA) or radioimmunoassay (RIA). The lumbosacral contents of cAMP and cGMP when quantified in whole lumbosacral cord segment were not significantly changed by FCA treatment at all time points. InsP3 accumulation was significantly increased on days 14, 21 and 42 following FCA injection relative to sham-treated time-matched controls. However, cGMP and insP3 contents were significantly increased in the left longitudinal half of the lumbar enlargement ipsilateral to the injected paw on day 21 following FCA treatment, but not in the sham-treated time-matched controls. With [3H]insP3 as a ligand, Scatchard (Rosenthal) analyses of the concentration-dependent saturation curves showed that the densities (Bmax) of insP3 receptors (insP3R) were significantly increased throughout the time course of adjuvant-induced peripheral inflammation. The binding affinities (KD) for insP3R were significantly decreased on days 7, 14 and 21 following FCA injection corresponding to the times of most stable and peak inflammation. InsP3R from the cerebelli of the same rats as used in the lumbosacral insP3R characterization was used as a positive control in this study and did not show any change in both Bmax and KD as a result of FCA treatment, thus demonstrating that the changes in lumbosacral insP3R characteristics might be specific to the nociceptive sensory pathway such as the spinal cord. Thus it appears that sustained afferent nociceptive input induced by FCA injection increased the accumulation of cGMP, insP3 and insP3R density in the spinal cord through increased neuronal activities of functional receptors coupled to major classes of chemical mediators of nociception including neuropeptides and excitatory aminoacids. Changes in insP3 accumulation in the lumbosacral cord following FCA injection were significantly correlated with changes in insP3R density. Changes in the ratios of lumbosacral insP3 contents and insP3R density were also significantly correlated with changes in body weight and hind paw size induced by FCA injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Messenger plasticity in primary sensory neurons following axotomy and its functional implications

Following peripheral axotomy, long-lasting changes in the expression of neuropeptides and their receptors in primary sensory neurons are observed. These changes involve the downregulation of the excitatory peptides substance P and calcitonin gene-related peptide and the upregulation of the inhibitory peptides neuropeptide tyrosine and galanin, resulting in a reduction of transmission in the dorsal horn. The changes observed are thought to represent adaptive responses to limit the consequences of peripheral nerve damage to the organism as a whole and to promote survival and recovery of the individual neuron.

Behavioral and immunohistochemical changes in an experimental arthritis model in rats

An experimental arthritis induced by injection of kaolin and carrageenan into the knee joint resulted in a temporal relationship between glutamate dorsal horn content and paw withdrawal latency (PWL) which was positively correlated. Limping, guarding, increased response to heat stimuli (hyperalgesia) and altered staining patterns for glutamate (GLU), substance P (SP), and calcitonin gene-related peptide (CGRP) were monitored in the awake behaving arthritic rat over a 1 week time course. A decrease in PWL occurred on the side ipsilateral to the inflamed knee as early as 4 h after the induction of arthritis indicating the animals are hyperalgesic. The PWL remained decreased through the first 24 h. Computer-assisted quantification of the density of immunohistochemical staining indicated the content of GLU, SP and CGRP was altered differentially throughout the time course of the arthritis. The changes observed for all three substances occurred across the entire superficial dorsal horn. There was an initial depletion of SP followed by an increase in both SP and CGRP content which was maintained through 1 week. The GLU content was increased during the hyperalgesic period. The GLU changes followed the same time course and were positively correlated with the changes in PWL. In a small group of animals injected with kaolin and carrageenan, hyperalgesia did not develop. In this group of animals, no change in dorsal horn GLU or SP content occurred. Rather, there was an increase in CGRP content in the middle portion of the superficial dorsal horn which is the termination site of knee joint afferents. These data indicate that the development of heat hyperalgesia is dependent on GLU and possibly SP. Since inflammation of the knee joint does not involve the foot pad, the heat hyperalgesia observed during the first 24 h following induction of arthritis represents a central neuronal sensitization.

Spinal cord amino acid release and content in an arthritis model: the effects of pretreatment with non-NMDA, NMDA, and NK1 receptor antagonists

An experimental arthritis, induced by injection of the knee joint with kaolin and carrageenan, results in guarding of and decreased weight bearing on the limb. At the time of injection, a transient increased release of all amino acids examined is measurable in samples collected by microdialysis. A second and prolonged increase of aspartate (ASP), glutamate (GLU), and glutamine (GLN) concentrations follows after 3 h. The increased release at time of injection is blocked by microdialysis application of a non-N-methyl-D-aspartate (non-NMDA) or an NMDA receptor antagonist, and the release of ASP, GLU, and GLN in the late phase is blocked by pretreatment with a non-NMDA (CNQX), an NMDA (AP7) or a neurokinin 1 (NK1; CP-96,345) antagonist. Dorsal horn immunoreactive staining of GLU, substance P (SP), and calcitonin gene-related peptide (CGRP) is reflective of the events occurring in the late phase of amino acid release since GLU release is positively correlated with GLU staining density. Increased immunoreactivity for GLU, SP, and CGRP at 8 hr in the arthritic animals is differentially altered by pretreatment of the spinal cord dorsal horn with non-NMDA, NMDA, or NK1 receptor antagonists. The differential staining pattern for GLU, SP, and CGRP, the differential release of ASP and GLU, and the differential activation of the EAA and NK1 receptors implies that ASP, GLU, SP, and CGRP are each involved in the processing of sensory information and that their roles in the central sensitization occurring with the inflammatory process, are unique.

Ultrastructural studies on peptides in the dorsal horn of the spinal cord--I. Co-existence of galanin with other peptides in primary afferents in normal rats

The aim of the present study was to investigate galanin-like immunoreactivity in primary afferent terminals and its relationship to other neuropeptides in laminae I and II of the fourth and fifth lumbar segments of normal rat spinal cord using immunofluorescence and pre- and post-embedding electron-microscopic immunocytochemistry. Triple-immunofluorescence staining showed that galanin-like immunoreactivity co-localized with substance P- and calcitonin gene-related peptide-like immunoreactivities in many nerve fibres and terminals in laminae I and II of the dorsal horn. At the ultrastructural level, using pre-embedding immunocytochemistry, galanin-like immunoreactivity was found in type I glomeruli with an electron-dense central terminal containing many densely packed synaptic vesicles and several large dense-core vesicles. Both the cytoplasm and the core of the large vesicles were immunoreactive. In type II glomeruli with an electron-lucent central terminal and loosely packed synaptic vesicles the large dense-core vesicles and the cytoplasm were only weakly galanin-positive. Post-embedding immunocytochemistry revealed that galanin-like immunoreactivity co-existed with substance P- and calcitonin gene-related peptide-like immunoreactivities in many terminals and in individual large dense-core vesicles in lamina II. These terminals were considered to represent primary afferents, since there is evidence that calcitonin gene-related peptide in the dorsal horn only occurs in nerve endings originating in dorsal root ganglia. Evidence was also unexpectedly obtained for the occurrence of several other peptides in calcitonin gene-related peptide-positive terminals, i.e. in presumably primary afferents. Thus galanin-like immunoreactivity sometimes also co-localized with cholecystokinin- and neuropeptide tyrosine-like immunoreactivities in calcitonin gene-related peptide-immunoreactive terminals and in some large dense-core vesicles in such terminals. A small number of calcitonin gene-related peptide immunoreactive, presumably primary afferent terminals contained enkephalin-, neurotensin- (and galanin-)like immunoreactivities. These results indicated that galanin can be co-stored with several other neuropeptides in large dense-core vesicles in primary afferent terminals and may presumably be released together with them in the superficial layer of the dorsal horn. Since various combinations of peptides, presumably at varying concentrations, occur in the large dense-core vesicles in a given nerve ending, it is likely that the individual large dense-core vesicles produced in a neuron are heterogenous with regard to peptide content and thus to the message that they transmit upon release.

Central hyperexcitability triggered by noxious inputs

Repetitive activity in unmyelinated sensory afferent neurones, arising from electrical stimuli, tissue injury or nerve damage, can induce long-lasting sensitization in dorsal horn neurones. This process can be blocked by antagonists of the NMDA receptor. In the past year it has emerged that sensory neuropeptides and nitric oxide are also essential mediators of this phenomenon.