Behavioral evidence of thermal hyperalgesia and mechanical allodynia induced by intradermal cinnamaldehyde in rats

TRPA1 agonists cinnamaldehyde (CA) and mustard oil (allyl isothiocyanate = AITC) induce heat hyperalgesia and mechanical allodynia in human skin, and sensitize responses of spinal and trigeminal dorsal horn neurons to noxious skin heating in rats. TRPA1 is also implicated in cold nociception. We presently used behavioral methods to investigate if CA affects sensitivity to thermal and mechanical stimuli in rats. Unilateral intraplantar injection of CA (5–20%) induced a significant, concentration-dependent reduction in latency for ipsilateral paw withdrawal from a noxious heat stimulus, peaking (61.7% of pre-injection baseline) by 30 min with partial recovery at 120 min. The highest dose of CA also significantly reduced the contralateral paw withdrawal latency. CA significantly reduced mechanical withdrawal thresholds of the injected paw that peaked sooner (3 min) and was more profound (44.4% of baseline), with no effect contralaterally. Bilateral intraplantar injections of CA resulted in a significant cold hyperalgesia (cold plate test) and a weak enhancement of innocuous cold avoidance (thermal preference test). The data are consistent with roles for TRPA1 in thermal (hot and cold) hyperalgesia and mechanical allodynia.     

Involvement of peripherally released substance P and calcitonin gene-related peptide in mediating mechanical hyperalgesia in a traumatic neuropathy model of the rat

We hypothesized that neuropeptides released from the peripheral terminals of primary afferents play an important role in mechanical hyperalgesia after peripheral nerve injury. Nerve injury was performed on rats with lumbar 5 spinal nerve lesion (L5 SNL), which was preceded by L5 dorsal rhizotomy (L5 DR) to avoid the potential central effects induced by L5 SNL through the L5 dorsal root. L5 DR produced a short-lasting (<6 days) decrease in paw withdrawal threshold (PWT) while the following L5 SNL produced a persistent (>42 days) PWT decrease. When intraplantar injection to the affected hind paw was given immediately before L5 SNL, antagonists for both neurokinin 1 (NK1) and calcitonin gene-related peptide 1 (CGRP1) receptors delayed the onset of the PWT decrease for 2–4 days. However, when the same injection was given after L5 SNL, CGRP1, but not NK1, receptor antagonist reversed the decreased PWT for 105 min. It is suggested that peripherally released neuropeptides contribute to the generation of neuropathic pain, with substance P and CGRP contributing to its induction phase, but only CGRP to its maintenance phase.     

Involvement of transient receptor potential vanilloid 1 in ectopic pain following inferior alveolar nerve transection in rats

Chronic pain often develops in the orofacial region after inferior alveolar nerve (IAN) injury. In animal models IAN injury often causes severe neuropathic pain-like behavior in the IAN-innervated region as well as the adjacent region that includes the whisker pad skin. However, the basis for the spreading of pain to adjacent facial areas after IAN injury is still unknown. In this study we determined if the transient receptor potential vanilloid 1 (TRPV1) was associated with altered nocifensive behavior evoked by stimulation of the whisker pad skin following IAN transection. Grooming behavior after capsaicin injection into the whisker pad region was significantly increased after IAN transection and the increase in the behavior was reversed by systemic administration of a TRPV1 antagonist. The number of phosphorylated extracellular signal-regulated kinase immunoreactive (IR) neurons in trigeminal spinal subnucleus caudalis and upper cervical spinal cord following capsaicin injection into the whisker pad region was significantly greater in IAN-transected rats than sham-operated rats. The number of TRPV1-IR trigeminal ganglion (TG) neurons innervating the whisker pad skin was also increased significantly after IAN transection. The present findings suggest that an increase in TRPV1 expression in TG neurons innervating the whisker pad skin after IAN transection may underlie the spreading of pain to the adjacent whisker pad skin.     

Expression of TrkA receptor for neurotrophins in trigeminal neurons innervating the rat gingivomucosal tissue

The purpose of this study was to characterize and evaluate the expression of TrkA receptor in trigeminal ganglion (TG) neurons that innervate the rat gingivomucosal tissue. A retrograde nerve tracer Fluorogold (FG) was injected into the gingiva (group 1) or applied into the gingival sulcus (group 2) of the first right maxillary molar to identify the neurons in TG that innervate the gingivomucosa. After 10 days TG were dissected and FG fluorescence in neurons was observed under UV light microscope. To draw a comparison, approximately 1000 neurons per ganglion from the entire TG (group 3) and approximately 350 neurons per ganglion from the maxillary region in TG (group 4), were analyzed. Expression of TrkA receptor in TG neurons was investigated by immunohistochemistry. About 70% of neurons in groups 1 and 2 contained TrkA receptor, which was statistically significantly more than in groups 3 (41%) and 4 (38%). FG-labeled TrkA-immunopositive neurons were predominantly small or medium-sized (less than 1200microm(2)). However, the neurons innervating the rat gingivomucosa were on average larger than the neurons in the entire TG or in the maxillary region. In conclusion, the majority of neurons in TG that innervate the rat gingivomucosa are small or medium-sized, contain TrkA receptor and are most probably nociceptive.     

The effect of high cervical spinal cord stimulation on the expression of SP, NK-1 and TRPV1 mRNAs during cardiac ischemia in rat

Spinal cord stimulation (SCS) is used to reduce angina that accompanies cardiac ischemia, but little is known about the molecular mechanisms mediating this effect. We studied the expression of SP, neurokinin-1 (NK-1) receptor, and transient receptor potential vanilloid type 1 (TRPV1) mRNA in the rat spinal cord at thoracic 4 (T4), cervical 2 (C2) and caudal brain stem by RT-PCR during intermittent occlusion of the left anterior descending coronary artery (CoAO), during sustained SCS by itself at the C2 spinal segment, and during sustained SCS plus intermittent CoAO. Only SP mRNA was increased significantly in T4 and brainstem during CoAO, while SCS decreased the mRNA levels of SP, NK-1 and TRPV1 significantly in T4 and the brainstem. SCS attenuated the increase of SP and TRPV1 mRNA levels at T4 level induced by intermittent CoAO when the stimulation was applied prior to the initiation of the cardiac ischemia. These results support the role for SP as a putative neurotransmitter for the myocardial ischemia-sensitive afferent neuron signal to the spinal level. They suggest that modification of the ischemic cardiac nociceptive afferent signal by SCS involves a change in SP and TRPV1 expression.     

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.     

Inhibitory effects of calcitonin gene-related peptide on long-term potentiation induced in hippocampal slices of rats

It is well known that hippocampus plays important roles in learning and memory. Both calcitonin gene-related peptide (CGRP) and CGRP receptors are found in hippocampus. In the present study we explored the influence of CGRP on long term potentiation (LTP) in hippocampal Schaffer collateral-CA1. Our results demonstrated that CGRP inhibited the LTP induced by high frequency stimulation (HFS) in hippocampal CA1 neurons in rat brain slices. The inhibitory effect was blocked by CGRP receptor-1 antagonist CGRP 8-37. The results indicate that both CGRP and CGRP receptor 1 are involved in the modulation process of LTP in hippocampus of rats.     

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 γ 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.     

Blockade effects of BIBN4096BS on CGRP-induced inhibition on whole-cell K currents in spinal dorsal horn neuron of rats

Calcitonin gene-related peptide (CGRP) plays an important role in the transmission and modulation of nociceptive information in the spinal cord. BIBN4096BS, a nonpeptide CGRP receptor antagonist, has been shown to be efficiency in clinical migraine treatment. The present study was performed to investigate the effects of BIBN4096BS on the CGRP-induced inhibition to whole-cell K⁺ currents in spinal wide dynamic range (WDR) neuron of rats. Application of BIBN4096BS inhibited the neuronal activity of WDR neurons in lumbar dorsal horn of the spinal cord in rats tested by extracellular recording method. Furthermore, CGRP induced inhibition on whole-cell K⁺ currents in cultured dorsal horn neurons of rats tested by whole-cell patch-clamp recording, and the effect was significantly blocked by BIBN4096BS. The results indicate that BIBN4096BS may produce antinociceptive effects at the spinal level in rats.     

Altered thermal sensitivity in neurons injured by infraorbital nerve lesion

Nerve lesions are common injuries. While peripheral sensitivity is lost, the partially regenerating nerve undergoes a complex transformation, occasionally leading to persistent pain syndromes. Changes of thermal perception following nerve injury have received little attention. This study investigates the sensitivity of trigeminal neurons after infraorbital nerve lesion in guinea-pigs. Cultured trigeminal neurons innervating the area of denervation were identified by retrograde transport of DiI deposited at the site of the lesion. The standardized protocol consisted of cold and heat stimulation starting from body temperature as well as application of menthol and capsaicin, while activation was quantified by Fura-2-based calcium microfluorimetry. Compared to neurons from control animals, DiI-positive neurons were similar in the percentage and extend of the responses to menthol and capsaicin. However, DiI-positive neurons were less responsive to cold stimulation and had a lower cold threshold when compared to DiI-negative or control neurons. At the same time, DiI-positive neurons were more responsive to heat stimulation and had a lower heat threshold compared to control neurons. In summary, the percentage of trigeminal neurons responsive to thermal or chemical stimulation did not change after axotomy. However, thermal sensitivity of these neurons was altered.     

The synaptic connectivity that underlies the noxious transmission and modulation within the superficial dorsal horn of the spinal cord

Noxious stimuli can usually cause the aversive sensations, pain and itch. The initial integration of such noxious information occurs in the superficial dorsal horn of the spinal cord (SDH), which is very important for understanding pain sensation and developing effective analgesic strategies. The circuits formed by pools of neurons and terminals within SDH are accepted as the platform for such complicated integrations and are highly plastic under conditions of inflammatory or neuropathic pain. Recent literature offers a complicated, yet versatile view of SDH intrinsic circuits with both inhibitory and excitatory components. However, our knowledge about the adaptative regulation of SDH local circuits is still far from sufficient due to the incomplete understanding of their organization as they are intermingled with primary afferent fibers (PAFs), poorly understood or identified SDH neurons, somehow contradictory data for descending control systems. A more positive view emphasizes abundant modern data on SDH neuron morphology and physiology riding on the back of significant technological advancements used in neuroscience. Reviewing the current literature on this topic thus produced an integrated understanding of SDH neurons and the SDH local circuits involved in noxious transmission and modulation.     

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.     

Repairing injured peripheral nerves: Bridging the gap

Peripheral nerve injuries that induce gaps larger than 1-2 cm require bridging strategies for repair. Autologous nerve grafts are still the gold standard for such interventions, although alternative treatments, as well as treatments to improve the therapeutic efficacy of autologous nerve grafting are generating increasing interest. Investigations are still mostly experimental, although some clinical studies have been undertaken. In this review, we aim to describe the developments in bridging technology which aim to replace the autograft. A multi-disciplinary approach is of utmost importance to develop and optimise treatments of the most challenging peripheral nerve injuries.     

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: A double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.