Calcitonin gene-related peptide 8-37 inhibits the evoked discharge frequency of wide dynamic range neurons in dorsal horn of the spinal cord in rats

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-expressing sensory neurons and spinal microglial reactivity contribute to pain states in collagen-induced arthritis

OBJECTIVE

To evaluate the contribution of sensory neurons in ankle joints and adjacent tissue to the development of pain in collagen-induced arthritis (CIA), and to determine the relationship between pain and the appearance of clinical signs.

METHODS

Mechanical and heat hypersensitivity and hind paw swelling were assessed in Lewis rats before and until 18 days following collagen immunization. We examined the effect of intrathecal administration of a calcitonin gene-related peptide (CGRP) antagonist (CGRP(8-37) ) from day 11 to day 18 postimmunization on CIA-induced hypersensitivity. During CIA development, CGRP and p-ERK immunoreactivity was quantified in lumbar dorsal root ganglia in which sensory neurons innervating the ankle joint were identified by retrograde labeling with Fluoro-Gold. Microgliosis in the lumbar dorsal horn was assessed by immunohistochemistry, and release of CGRP evoked by activity of primary afferent fibers was measured using a preparation of isolated dorsal horn with dorsal roots attached.

RESULTS

CIA was associated with mechanical hypersensitivity that was evident before hind paw swelling and that was exacerbated with the development of swelling. Heat hyperalgesia developed along with swelling. Concomitant with the development of mechanical hypersensitivity, joint innervating neurons exhibited enhanced CGRP expression and an activated phenotype (increased p-ERK expression), and significant microgliosis became evident in the dorsal horn; these peripheral and central changes were augmented further with disease progression. CGRP release evoked by dorsal root stimulation was higher in the dorsal horn on day 18 in rats with CIA compared to control rats. Prolonged intrathecal administration of CGRP(8-37) attenuated established mechanical hypersensitivity and reduced spinal microgliosis.

CONCLUSION

Sensory neuron-derived CGRP sustains mechanical hypersensitivity and spinal microglial reactivity in CIA, suggesting that central mechanisms play critical roles in chronic inflammatory pain. Blockade of these central events may provide pain relief in rheumatoid arthritis patients.

Antinociception induced by chronic glucocorticoid treatment is correlated to local modulation of spinal neurotransmitter content

Background

While acute effects of stress on pain are well described, those produced by chronic stress are still a matter of dispute. Previously we demonstrated that chronic unpredictable stress results in antinociception in the tail-flick test, an effect that is mediated by increased levels of corticosteroids. In the present study, we evaluated nociception in rats after chronic treatment with corticosterone (CORT) and dexamethasone (DEX) in order to discriminate the role of each type of corticosteroid receptors in antinociception.

Results

Both experimental groups exhibited a pronounced antinociceptive effect after three weeks of treatment when compared to controls (CONT); however, at four weeks the pain threshold in CORT-treated animals returned to basal levels whereas in DEX-treated rats antinociception was maintained. In order to assess if these differences are associated with altered expression of neuropeptides involved in nociceptive transmission we evaluated the density of substance P (SP), calcitonin gene-related peptide (CGRP), somatostatin (SS) and _B2-γ-aminobutiric acid receptors (GABA_B2) expression in the spinal dorsal horn using light density measurements and stereological techniques. After three weeks of treatment the expression of CGRP in the superficial dorsal horn was significantly decreased in both CORT and DEX groups, while GABA_B2 was significantly increased; the levels of SP for both experimental groups remained unchanged at this point. At 4 weeks, CGRP and SP are reduced in DEX-treated animals and GABA_B2 unchanged, but all changes were restored to CONT levels in CORT-treated animals. The expression of SS remained unaltered throughout the experimental period.

Conclusion

These data indicate that corticosteroids modulate nociception since chronic corticosteroid treatment alters the expression of neuropeptides involved in nociceptive transmission at the spinal cord level. As previously observed in some supraspinal areas, the exclusive GR activation resulted in more profound and sustained behavioural and neurochemical changes, than the one observed with a mixed ligand of corticosteroid receptors. These results might be of relevance for the pharmacological management of certain types of chronic pain, in which corticosteroids are used as adjuvant analgesics.

Material basis for inhibition of Dragon's Blood on evoked discharges of wide dynamic range neurons in spinal dorsal horn of rats

In vivo experiments were designed to verify the analgesic effect of Dragon's Blood and the material basis for this effect. Extracellular microelectrode recordings were used to observe the effects of Dragon's Blood and various combinations of the three components (cochinchinenin A, cochinchinenin B, and loureirin B) extracted from Dragon's Blood on the discharge activities of wide dynamic range (WDR) neurons in spinal dorsal horn (SDH) of intact male Wistar rats evoked by electric stimulation at sciatic nerve. When the Hill's coefficients describing the dose-response relations of drugs were different, based on the concept of dose equivalence, the equations of additivity surfaces which can be applied to assess the interaction between three drugs were derived. Adopting the equations and Tallarida's isobole equations used to assess the interaction between two drugs with dissimilar dose-response relations, the effects produced by various combinations of the three components in modulating the evoked discharge activities of WDR neurons were evaluated. Results showed that Dragon's Blood and its three components could inhibit the evoked discharge frequencies of WDR neurons in a concentration-dependent way. The Hill's coefficients describing dose-response relations of three components were different. Only the combined effect of cochinchinenin A, cochinchinenin B and loureirin B was similar to that of Dragons Blood. Furthermore, the combined effect was synergistic. This investigation demonstrated that through the synergistic interaction of the three components Dragon's Blood could interfere with the transmission and processing of pain signals in spinal dorsal horn. All these further proved that the combination of cochinchinenin A, cochinchinenin B, and loureirin B was the material basis for the analgesic effect of Dragon's Blood.

Spinal CGRP1 receptors contribute to supraspinally organized pain behavior and pain-related sensitization of amygdala neurons

CGRP receptor activation has been implicated in peripheral and central sensitization. The role of spinal CGRP receptors in supraspinal pain processing and higher integrated pain behavior is not known. Here we studied the effect of spinal inhibition of CGRP1 receptors on supraspinally organized vocalizations and activity of amygdala neurons. Our previous studies showed that pain-related audible and ultrasonic vocalizations are modulated by the central nucleus of the amygdala (CeA). Vocalizations in the audible and ultrasonic range and hindlimb withdrawal thresholds were measured in awake adult rats before and 5-6h after induction of arthritis by intra-articular injections of kaolin and carrageenan into one knee. Extracellular single-unit recordings were made from neurons in the latero-capsular division of the CeA (CeLC) in anesthetized rats before and after arthritis induction. CGRP1 receptor antagonists were applied to the lumbar spinal cord intrathecally (5 microl/min) 6h postinduction of arthritis. Spinal administration of peptide (CGRP8-37, 1 microM) and non-peptide (BIBN4096BS, 1 microM) CGRP1 receptor antagonists significantly inhibited the increased responses of CeLC neurons to mechanical stimulation of the arthritic knee but had no effect under normal conditions. In arthritic rats, the antagonists also inhibited the audible and ultrasonic components of vocalizations evoked by noxious stimuli and increased the threshold of hindlimb withdrawal reflexes. The antagonists had no effect on vocalizations and spinal reflexes in normal rats. These data suggest that spinal CGRP1 receptors are not only important for spinal pain mechanisms but also contribute significantly to the transmission of nociceptive information to the amygdala and to higher integrated behavior.

Pain-related synaptic plasticity in spinal dorsal horn neurons: role of CGRP

Background

The synaptic and cellular mechanisms of pain-related central sensitization in the spinal cord are not fully understood yet. Calcitonin gene-related peptide (CGRP) has been identified as an important molecule in spinal nociceptive processing and ensuing behavioral responses, but its contribution to synaptic plasticity, cellular mechanisms and site of action in the spinal cord remain to be determined. Here we address the role of CGRP in synaptic plasticity in the spinal dorsal horn in a model of arthritic pain.

Results

Whole-cell current- and voltage-clamp recordings were made from substantia gelatinosa (SG) neurons in spinal cord slices from control rats and arthritic rats (> 6 h postinjection of kaolin/carrageenan into the knee). Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents in the dorsal root near the dorsal root entry zone. Neurons in slices from arthritic rats showed increased synaptic transmission and excitability compared to controls. A selective CGRP1 receptor antagonist (CGRP8-37) reversed synaptic plasticity in neurons from arthritic rats but had no significant effect on normal transmission. CGRP facilitated synaptic transmission in the arthritis pain model more strongly than under normal conditions where both facilitatory and inhibitory effects were observed. CGRP also increased neuronal excitability. Miniature EPSC analysis suggested a post- rather than pre-synaptic mechanism of CGRP action.

Conclusion

This study is the first to show synaptic plasticity in the spinal dorsal horn in a model of arthritic pain that involves a postsynaptic action of CGRP on SG neurons.

Nociceptive transmitter release in the dorsal spinal cord by capsaicin-sensitive fibers after noxious gastric stimulation

Little is known about transmitters that encode noxious gastric stimuli in the spinal cord. The release of glutamate, substance P, and CGRP from the spinal cord was therefore investigated in response to acid injury of the gastric mucosa. Dorsal halves of the caudal thoracic spinal cord (T7-T13) were removed 6 h after oral application of 0.5 M HCl or saline, transferred to a superfusion chamber, and the basal and capsaicin-stimulated (3.3 microM) transmitter release was determined. After acid injury, basal glutamate release increased 134% as compared to saline-treated animals. Capsaicin-stimulated release of CGRP and SP was 48% and 58% lower in acid- than in saline-treated animals, indicating that capsaicin-sensitive fibers in the dorsal spinal cord were already partially depleted by acid treatment. Capsaicin denervation reduced basal glutamate release by 33% after acid injury as compared to non-denervated acid-treated animals. Gastric origin and capsaicin sensitivity of glutamatergic, CGRP- and SP-containing primary afferents in thoracic dorsal root ganglia were then determined by retrograde tracing with True Blue and immunohistochemical labeling with the vanilloid receptor TRPV1. About 65% of True Blue-labeled cells were glutamatergic and more than 73% of this population expressed the TRPV1 receptor. Nearly all True Blue/CGRP (85%)- and True Blue/SP-positive cells (97%) coexpressed TRPV1. We conclude that noxious gastric stimulation with acid induces release of glutamate, SP, and CGRP from capsaicin-sensitive sensory afferents in the dorsal horn of the spinal cord where they may play an important role in gastric nociception and hyperalgesia.

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.

Disruption of communication between peripheral and central trigeminovascular neurons mediates the antimigraine action of 5HT 1B/1D receptor agonists

Triptans are 5HT(1B/1D) receptor agonists commonly prescribed for migraine headache. Although originally designed to constrict dilated intracranial blood vessels, the mechanism and site of action by which triptans abort the migraine pain remain unknown. We showed recently that sensitization of peripheral and central trigeminovascular neurons plays an important role in the pathophysiology of migraine pain. Here we examined whether the drug sumatriptan can prevent and/or suppress peripheral and central sensitization by using single-unit recording in our animal model of intracranial pain. We found that sumatriptan effectively prevented the induction of sensitization (i.e., increased spontaneous firing; increased neuronal sensitivity to intracranial mechanical stimuli) in central trigeminovascular neurons (recorded in the dorsal horn), but not in peripheral trigeminovascular neurons (recorded in the trigeminal ganglion). After sensitization was established in both types of neuron, sumatriptan effectively normalized intracranial mechanical sensitivity of central neurons, but failed to reverse such hypersensitivity in peripheral neurons. In both the peripheral and central neurons, the drug failed to attenuate the increased spontaneous activity established during sensitization. These results suggest that neither peripheral nor central trigeminovascular neurons are directly inhibited by sumatriptan. Rather, triptan action appears to be exerted through presynaptic 5HT(1B/1D) receptors in the dorsal horn to block synaptic transmission between axon terminals of the peripheral trigeminovascular neurons and cell bodies of their central counterparts. We therefore suggest that the analgesic action of triptan can be attained specifically in the absence, but not in the presence, of central sensitization.

Involvement of calcitonin gene-related peptide and its receptor in anti-nociception in the periaqueductal grey of rats

The present study investigated the role of calcitonin gene-related peptide (CGRP) in the modulation of nociception in periaqueductal grey (PAG) of rats. Hindpaw withdrawal latencies (HWLs) were tested by hot-plate and Randall Selitto tests. The HWLs to thermal and mechanical stimulation increased significantly after intra-PAG administration of 0.26 or 0.13 nmol of CGRP, but not 0.026 nmol of CGRP. The anti-nociceptive effects induced by CGRP were significantly blocked by intra-PAG administration of 0.026 or 0.26 nmol of the CGRP1 receptor antagonist CGRP8-37. Furthermore, administration of CGRP into the decussation of superior cerebellar peduncle, out of PAG, did not elicit anti-nociceptive effects during 60 min after the injection. The results demonstrated that CGRP plays an important role in anti-nociception in PAG of rats, and CGRP1 receptor is involved in the CGRP-induced anti-nociception.

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.

Crossed and uncrossed projections to the cat sacrocaudal spinal cord: III. Axons expressing calcitonin gene-related peptide immunoreactivity

We have investigated the projection patterns of peptidergic small-diameter primary afferent fibers to the cat sacrocaudal spinal cord, a region associated with midline structures of the lower urogenital system and of the tail. Calcitonin gene-related peptide (CGRP)-immunoreactive (CGRP-IR) primary afferent fibers were observed within the superficial laminae, rostrally as the typical inverted U-shaped band that capped the separate dorsal horns (S1 to rostral S2) and caudally as a broad band that spanned the entire mediolateral extent of the fused dorsal horns (caudal S2 and caudal). Within the dorsal gray commissure, labeling was seen as a periodic vertical, midline band. CGRP-IR labeling was prevalent in an extensive mediolateral distribution at the base of the dorsal horn, originating from both lateral and medial collateral bundles that extend from the superficial dorsal horn. Some bundles, in part traveling within the dorsal commissure, conspicuously crossed the midline. In addition to the robust projection to the superficial dorsal horn, there was a more extensive distribution of CGRP-IR fibers within the deeper portions of the cat sacrocaudal dorsal horn than has been reported for other regions of the cat spinal cord. Presumably, these deep projections convey visceral information to projection or segmental neurons at the neck of the dorsal horn and in the region of the central canal. This deep distribution overlaps the reported projections of the pelvic and pudendal nerves. In addition, the contralateral projections of CGRP-IR fibers may form an anatomical substrate of the bilateral receptive fields for selective dorsal horn neurons. The density and variety of CGRP-IR projection patterns is a reflection of the functional attributes of the innervated structures.

The effect of galanin on wide-dynamic range neuron activity in the spinal dorsal horn of rats

The present study investigated the effect of galanin on wide-dynamic range (WDR) neuron activity in the dorsal horn of the spinal cord of rats. The evoked discharge of WDR neurons was elicited by transdermic electrical stimulation applied on the ipsilateral hindpaw of rats. Galanin was administered directly on the spinal dorsal surface of L3-L5. The evoked discharge frequency of the WDR neurons decreased significantly after the administration of galanin and the effect lasted for more than 30 min. Furthermore, the inhibitory effect of galanin on the evoked discharge frequency of WDR neurons was blocked by following administration of the galanin antagonist galantide, indicating that the inhibitory effect of galanin on the activity of WDR neurons was induced by activating galanin receptors in the dorsal horn of the spinal cord. The results suggest that galanin has an inhibitory role in the transmission of presumed nociceptive information in the dorsal horn of the spinal cord in rats.

Effects of galanin on wide-dynamic range neuron activity in the spinal dorsal horn of rats with sciatic nerve ligation

Galanin is a 29-amino acid peptide with a suggested role in nociception. The effect of galanin on wide-dynamic range neuron discharge frequency in rats with nerve ligation, used as a model of neurogenic pain, was investigated by extracellular recording methods. Seven to 14 days after sciatic nerve ligation, 0.1, 0.5 or 1 nmol of galanin was administered directly on the dorsal surface of the L3-L5 spinal cord of rats with sciatic nerve ligation. It was found that galanin inhibited the activity of wide-dynamic range neurons dose-dependently, an effect was more pronounced in sciatic nerve ligated rats than intact rats. Furthermore, when 1 nmol of galantide, the galanin antagonist, was administered on the dorsal surface of the L3-L5 spinal cord, the wide-dynamic range neuron discharge frequency increased significantly. The results suggest that galanin plays an important role in the modulation of presumed nociception in mononeuropathy.