Characterization of opioid receptors that modulate nociceptive neurotransmission in the trigeminocervical complex

Pathophysiology of Migraine

OBJECTIVE

This article provides an overview of the current understanding of migraine pathophysiology through insights gained from the extended symptom spectrum of migraine, neuroanatomy, migraine neurochemistry, and therapeutics.

LATEST DEVELOPMENTS

Recent advances in human migraine research, including human experimental migraine models and functional neuroimaging, have provided novel insights into migraine attack initiation, neurochemistry, neuroanatomy, and therapeutic substrates. It has become clear that migraine is a neural disorder, in which a wide range of brain areas and neurochemical systems are implicated, producing a heterogeneous clinical phenotype. Many of these neural pathways are monoaminergic and peptidergic, such as those involving calcitonin gene-related peptide and pituitary adenylate cyclase-activating polypeptide. We are currently witnessing an exciting era in which specific drugs targeting these pathways have shown promise in treating migraine, including some studies suggesting efficacy before headache has even started.

ESSENTIAL POINTS

Migraine is a brain disorder involving both headache and altered sensory, limbic, and homeostatic processing. A complex interplay between neurotransmitter systems, physiologic systems, and pain processing likely occurs. Targeting various therapeutic substrates within these networks provides an exciting avenue for future migraine therapeutics.

KCl-induced repetitive cortical spreading depression inhibiting trigeminal neuronal firing is mediated by 5-HT1B/1D and opioid receptors

Background

We aimed to examine the effects of repetitive cortical spreading depression on the responses of nociceptive trigeminal neurons with dural afferents and characterize the role of 5-HT_1B/1D and opioid receptors.

Methods

Trigeminocervical complex neurons (n = 53) responsive to nociceptive activation of the dura mater were studied in rats using electrophysiological techniques.

Results

A sub-population (n = 32) showed an average inhibition of dural-evoked responses of 65 ± 14% from baseline with cortical spreading depression. This response was reversed by the selective 5-HT_1B/1D receptor antagonist, GR127935 (3 mg/kg; n = 6, iv), and a non-selective opioid receptor antagonist, naloxone (1.5 mg/kg; n = 6, iv), five minutes after injection. To determine the role of the nucleus raphe magnus in the trigeminocervical complex inhibitory effect, microinjection of lidocaine (2%, n = 6) or muscimol (100 mM, n = 5) into the nucleus raphe magnus was performed. There was no effect on cortical spreading depression-induced inhibition of neuronal firing in trigeminocervical complex by either.

Conclusion

The data demonstrate that repetitive cortical spreading depression inhibits a subpopulation of dural nociceptive trigeminocervical neurons, an effect mediated by serotonin and opioid receptors. This inhibition does not involve modulation of nucleus raphe magnus neurons.

Role of Omics in Migraine Research and Management: A Narrative Review

Migraine is a neurological disorder defined by episodic attacks of chronic pain associated with nausea, photophobia, and phonophobia. It is known to be a complex disease with several environmental and genetic factors contributing to its susceptibility. Risk factors for migraine include head or neck injury (Arnold, Cephalalgia 38(1):1-211, 2018). Stress and high temperature are known to trigger migraine, while sleep disorders and anxiety are considered to be the comorbid conditions with migraine. Studies have reported various biomarkers, including genetic variants, proteins, and metabolites implicated in migraine's pathophysiology. Using the "omics" approach, which deals with genetics, transcriptomics, proteomics, and metabolomics, more specific biomarkers for various migraine can be identified. On account of its multifactorial nature, migraine is an ideal study model focusing on integrated omics approaches, including genomics, transcriptomics, proteomics, and metabolomics. The current review has been compiled with an aim to focus on the genomic alterations especially involved in the regulation of glutamatergic neurotransmission, cortical excitability, ion channels, solute carrier proteins, or receptors; their expression in migraine patients and also specific proteins and metabolites, including some inflammatory biomarkers that might represent the migraine phenotype at the molecular level. The systems biology approach holds the promise to understand the pathophysiology of the disease at length and also to identify the specific therapeutic targets for novel interventions.

Tear Opiorphin Levels in Ocular Pain Caused by Corneal Foreign Body

PURPOSE

Opiorphin is an endogenous inhibitor of enkephalin-degrading enzymes. It has a strong analgesic effect in chemical and mechanical pain models. We aimed to evaluate the tear opiorphin levels in ocular pain caused by corneal foreign bodies and demonstrate whether there is any correlation with pain levels obtained from the Visual Analog Scale (VAS) score and tear opiorphin level.

METHODS

Thirty-two healthy individuals and 34 individuals diagnosed with corneal foreign bodies were included in this study. Tear opiorphin levels were measured by the ELISA method using a commercially available kit. The difference in tear opiorphin levels between the patient and control groups were evaluated using the Mann-Whitney U test. The correlation between VAS scores and tear opiorphin levels was evaluated using the Spearman rank correlation coefficient.

RESULTS

The median values of tear opiorphin levels of the patient and control groups were 134 pg/mL (86.86-296.25) and 109.80 pg/mL (66.15-191.49), respectively. The Mann-Whitney U test showed a statistically significant difference in tear opiorphin levels between patient and control groups (P < 0.05). No ocular pain was reported in the control group. The median VAS score of the patient group was 6 points (1-9). No correlation was found between VAS scores and tear opiorphin levels in the patient group.

CONCLUSIONS

The cornea is the most densely innervated tissue, and the highest opiorphin concentrations have been observed in tear. It is, therefore, expected that the stimulation or damage to the nerve endings in cornea would cause an increase in opiorphin secretion as a pain relief mechanism.

Characterization of opioidergic mechanisms related to the anti-migraine effect of vagus nerve stimulation

Vagus nerve stimulation (VNS) is a promising neuromodulation approach used in the treatment of migraine, whose therapeutic mechanism is largely unknown. Previous studies suggest that VNS's anti-nociceptive effects may, in part, involve engaging opioidergic mechanisms. We used a validated preclinical model of head pain, with good translational outcomes in migraine, acute intracranial-dural stimulation, which has responded to invasive VNS. We tested the effects of μ (MOR), δ (DOR) and κ (KOR) opioid receptor agonists in this model, and subsequently the effects of opioid receptor antagonists against VNS-mediated neuronal inhibition. MOR, DOR, and KOR agonists all inhibited dural-evoked trigeminocervical neuronal responses. Both DOR and KOR agonists also inhibited ongoing spontaneous firing of dural responsive neurons. Both DOR and KOR agonists were more efficacious than the MOR agonist in this model. We confirm the inhibitory effect of invasive VNS and demonstrate that this effect was prevented by a broad-spectrum opioid receptor antagonist, and by a highly selective DOR antagonist. Our data confirm the role of MOR in dural-trigeminovascular neurotransmission and additionally provide evidence of a role of both DOR and KOR in dural-nociceptive transmission of trigeminocervical neurons. Further, the results here provide evidence of engagement of opioidergic mechanisms in the therapeutic action of VNS in headache, specifically the DOR. These studies provide further support for the important role of the DOR in headache mechanisms, and as a potential therapeutic target. The data begin to dissect the mode of action of the analgesic effects of VNS in the treatment of primary headache disorders.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.

Different processing of meningeal and cutaneous pain information in the spinal trigeminal nucleus caudalis

Introduction

Within superficial trigeminal nucleus caudalis (Sp5C) (laminae I/II), meningeal primary afferents project exclusively to lamina I, whereas nociceptive cutaneous ones distribute in both lamina I and outer lamina II. Whether such a relative absence of meningeal inputs to lamina II represents a fundamental difference from cutaneous pathways in the central processing of sensory information is still unknown.

Methods

We recorded extracellular field potentials in the superficial Sp5C of anesthetised rats evoked by electrically stimulating the dura mater, to selectively assess the synaptic transmission between meningeal primary afferents and second-order Sp5C neurons, the first synapse in trigeminovascular pathways. We tested the effect of systemic morphine and local glycinergic and GABAAergic disinhibition.

Results

Meningeal stimulation evokes two negative field potentials in superficial Sp5C. The conduction velocities of the activated primary afferents are within the Aδ- and C-fibre ranges. Systemic morphine specifically suppresses meningeal C-fibre-evoked field potentials, and this effect is reversed by systemic naloxone. Segmental glycinergic or GABAAergic disinhibition strongly potentiates meningeal C-fibre-evoked field potentials but not Aδ-fibre ones. Interestingly, the same segmental disinhibition conversely potentiates cutaneous Aδ-fibre-evoked field potentials and suppresses C-fibre ones.

Conclusion

These findings reveal that the different anatomical organization of meningeal and cutaneous inputs into superficial Sp5C is associated with a different central processing of meningeal and cutaneous pain information within Sp5C. Moreover, they suggest that the potentiation upon local disinhibition of the first synapse in trigeminovascular pathways may contribute to the generation of headache pain.

Inhibitory effect of high-frequency greater occipital nerve electrical stimulation on trigeminovascular nociceptive processing in rats

Electrical stimulation of the greater occipital nerve (GON) has recently shown promise as an effective non-pharmacological prophylactic therapy for drug-resistant chronic primary headaches, but the neurobiological mechanisms underlying its anticephalgic action are not elucidated. Considering that the spinal trigeminal nucleus (STN) is a key segmental structure playing a prominent role in pathophysiology of headaches, in the present study we evaluated the effects of GON electrical stimulation on ongoing and evoked firing of the dura-sensitive STN neurons. The experiments were carried out on urethane/chloralose-anesthetized, paralyzed and artificially ventilated male Wistar rats. Extracellular recordings were made from 11 neurons within the caudal part of the STN that received convergent input from the ipsilateral facial cutaneous receptive fields, dura mater and GON. In each experiment, five various combinations of the GON stimulation frequency (50, 75, 100 Hz) and intensity (1, 3, 6 V) were tested successively in 10 min interval. At all parameter sets, preconditioning GON stimulation (250 ms train of pulses applied before each recording) produced suppression of both the ongoing activity of the STN neurons and their responses to electrical stimulation of the dura mater. The inhibitory effect depended mostly on the GON stimulation intensity, being maximally pronounced when a stimulus of 6 V was applied. Thus, the GON stimulation-induced inhibition of trigeminovascular nociceptive processing at the level of STN has been demonstrated for the first time. The data obtained can contribute to a deeper understanding of neurophysiological mechanisms underlying the therapeutic efficacy of GON stimulation in primary headaches.

Topiramate Inhibits Trigeminovascular Neurons in the Cat

To facilitate understanding the action of antimigraine preventives the effect of topiramate on trigeminocervical activation in the cat was examined. Animals ( n = 7) were anaesthetized and physiologically monitored. The superior sagittal sinus (SSS) was stimulated to produce a model of trigeminovascular nociceptive activation. Cumulative dose-response curves were constructed for the effect of topiramate at doses of 3, 5, 10, 30 and 50 mg/kg on SSS-evoked firing of trigeminocervical neurons. Topiramate reduced SSS evoked firing in a dose-dependent fashion. The maximum effect was seen over 30 min for the cohort taken together. At 3 mg/kg firing was reduced by 36 ± 13% (mean ± SEM) after 15 min. At 5 and 50 mg/kg firing was reduced by 59 ± 6% and 65 ± 14%, respectively, after 30 min. Inhibition of the trigeminocervical complex directly, or neurons that modulate sensory input, are plausible mechanisms for the action of preventives in migraine.

Nociception-specific blink reflex: pharmacology in healthy volunteers

BACKGROUND

The physiology and pharmacology of activation or perception of activation of pain-coding trigeminovascular afferents in humans is fundamental to understanding the biology of headache and developing new treatments.

METHODS

The blink reflex was elicited using a concentric electrode and recorded in four separate sessions, at baseline and two minutes after administration of ramped doses of diazepam (final dose 0.07 mg/kg), fentanyl (final dose 1.11 μg/kg), ketamine (final dose 0.084 mg/kg) and 0.9 % saline solution. The AUC (area under the curve, μV*ms) and the latency (ms) of the ipsi- and contralateral R2 component of the blink reflex were calculated by PC-based offline analysis. Immediately after each block of blink reflex recordings certain psychometric parameters were assessed.

RESULTS

There was an effect due to DRUG on the ipsilateral (F 3,60 = 7.3, P < 0.001) AUC as well as on the contralateral (F 3,60 = 6.02, P < 0.001) AUC across the study. A significant decrement in comparison to placebo was observed only for diazepam, affecting the ipsilateral AUC. The scores of alertness, calmness, contentedness, reaction time and precision were not affected by the DRUG across the sessions.

CONCLUSION

Previous studies suggest central, rather than peripheral changes in nociceptive trigeminal transmission in migraine. This study demonstrates a robust effect of benzodiazepine receptor modulation of the nociception specific blink reflex (nBR) without any μ-opiate or glutamate NMDA receptor component. The nociception specific blink reflex offers a reproducible, quantifiable method of assessment of trigeminal nociceptive system in humans that can be used to dissect pharmacology relevant to primary headache disorders.

Intravenous dextromethorphan/quinidine inhibits activity of dura-sensitive spinal trigeminal neurons in rats

BACKGROUND

Migraine is a chronic neurological disorder characterized by episodes of throbbing headaches. Practically all medications currently used in migraine prophylaxis have a number of substantial disadvantages and use limitations. Therefore, the further search for principally new prophylactic antimigraine agents remains an important task. The objective of our study was to evaluate the effects of a fixed combination of dextromethorphan hydrobromide and quinidine sulphate (DM/Q) on activity of the spinal trigeminal neurons in an electrophysiological model of trigemino-durovascular nociception.

METHODS

The study was performed in 15 male Wistar rats, which were anaesthetized with urethane/α-chloralose and paralysed using pipecuronium bromide. The effects of cumulative intravenous infusions of DM/Q (three steps performed 30 min apart, 15/7.5 mg/kg of DM/Q in 0.5 mL of isotonic saline per step) on ongoing and dural electrical stimulation-induced neuronal activities were tested in a group of eight rats over 90 min. Other seven animals received cumulative infusion of equal volumes of saline and served as control.

RESULTS

Cumulative administration of DM/Q produced steady suppression of both the ongoing activity of the spinal trigeminal neurons and their responses to electrical stimulation of the dura mater.

CONCLUSIONS

It is evident that the observed DM/Q-induced suppression of trigeminal neuron excitability can lead to a reduction in nociceptive transmission from meninges to higher centres of the brain. Since the same mechanism is believed to underlie the pharmacodynamics of many well-known antimigraine drugs, results of the present study enable us to anticipate the potential efficacy of DM/Q in migraine.

Animal models of headache: from bedside to bench and back to bedside

In recent years bench-based studies have greatly enhanced our understanding of headache pathophysiology, while facilitating the development of new headache medicines. At present, established animal models of headache utilize activation of pain-producing cranial structures, which for a complex syndrome, such as migraine, leaves many dimensions of the syndrome unstudied. The focus on modeling the central nociceptive mechanisms and the complexity of sensory phenomena that accompany migraine may offer new approaches for the development of new therapeutics. Given the complexity of the primary headaches, multiple approaches and techniques need to be employed. As an example, recently a model for trigeminal autonomic cephalalgias has been tested successfully, while by contrast, a satisfactory model of tension-type headache has been elusive. Moreover, although useful in many regards, migraine models are yet to provide a more complete picture of the disorder.

Large conductance calcium-activated potassium channels (BKCa) modulate trigeminovascular nociceptive transmission

Migraine is a common, disabling, neurological problem whose acute management would benefit from the development of purely neurally acting therapies. The trigeminocervical complex is pivotal in nociceptive signaling in migraine, and is an accepted target for putative antimigraine agents. Whole-cell patch-clamp or extracellular recordings were made of trigeminal neurons identified in rat brainstem slices. Bath application of the large conductance calcium-activated potassium (BKCa) channel opener NS1619 caused a dramatic decrease of cell firing that could be reversed by the co-application of iberiotoxin. NS1619 hyperpolarized the resting membrane potential and reduced the frequency of spontaneous action potentials in these neurons. These data suggest the presence of BKCa channels in the trigeminocervical complex. In vivo in cat L-glutamate-evoked firing was facilitated in nociceptive neurons, also responding to stimulation of the superior sagittal sinus, in the trigeminal nucleus caudalis by the BKCa peptide antagonists, iberiotoxin and slotoxin. Of units tested, 70% responded to microiontophoretic application of the blockers, identifying a subpopulation of trigeminal neurons expressing toxin-sensitive BKCa channels. NS1619 inhibited 74% of cells tested, and this was reversed by slotoxin, suggesting that the action of NS1619 in these cells was mediated through BKCa channels. These data are consistent with the presence of BKCa channels in the trigeminal nucleus caudalis that are potential targets for the development of antimigraine treatments, and may also offer insights into receptor mechanisms involved in sensitization and thus allodynia, in migraine.

Buprenorphine inhibits bradykinin-induced release of calcitonin gene-related peptide from rat trigeminal neurons via both mu-opioid and nociceptin/orphanin peptide receptors

In this study we used the dual opioid and nociceptin/orphanin peptide (NOP) agonist buprenorphine to investigate the relative contributions of opioid and NOP systems in regulating bradykinin-stimulated calcitonin-gene related peptide (CGRP) release from primary cultures of neonatal rat trigeminal neurons. We found that: bradykinin stimulates CGRP secretion either by a direct effect or after applying so-called "bradykinin-priming" protocol. In both cases, buprenorphine was able to inhibit bradykinin-stimulated CGRP secretion; however, inhibition was mediated by NOP receptors when buprenorphine was added to the incubation medium along with bradykinin, whereas it appeared to be mediated by mu-opioid receptors in bradykinin priming experiments. Bradykinin treatments also caused an increase in neuronal prostaglandin production; prostanoids appeared to be involved in the stimulatory effects of bradykinin as well as in buprenorphine inhibition, through apparently unrelated mechanisms.

Animal models of migraine: looking at the component parts of a complex disorder

Animal models of human disease have been extremely helpful both in advancing the understanding of brain disorders and in developing new therapeutic approaches. Models for studying headache mechanisms, particularly those directed at migraine, have been developed and exploited efficiently in the last decade, leading to better understanding of the potential mechanisms of the disorder and of the action for antimigraine treatments. Model systems employed have focused on the pain-producing cranial structures, the large vessels and dura mater, in order to provide reproducible physiological measures that could be subject to pharmacological exploration. A wide range of methods using both in vivo and in vitro approaches are now employed; these range from manipulation of the mouse genome in order to produce animals with human disease-producing mutations, through sensitive immunohistochemical methods to vascular, neurovascular and electrophysiological studies. No one model system in experimental animals can explain all the features of migraine; however, the systems available have begun to offer ways to dissect migraine's component parts to allow a better understanding of the problem and the development of new treatment strategies.

Topiramate inhibits trigeminovascular activation: an intravital microscopy study

Activation, or the altered perception of activation, of trigeminal nerves that innervate the cranial vasculature is considered to be a pivotal component of the pathophysiology of acute migraine. Calcitonin gene-related peptide (CGRP) levels are increased during migraine and after trigeminal nerve stimulation in the cat. Both CGRP and nitric oxide (NO) infusion causes headache and delayed migraine in migraineurs. Neurogenic stimulation of a cranial window, CGRP and NO injection all cause meningeal artery dilation in the rat when viewed using intravital microscopy. Topiramate is an antiepileptic drug with established efficacy as a migraine preventive, and has recently been shown to inhibit neurons of the trigeminocervical complex after superior sagittal sinus stimulation. In this study, we used intravital microscopy with neurogenic dural vasodilation, and CGRP- and NO-induced dilation to examine whether intravenous topiramate has effects on the trigeminovascular system. Topiramate was able to attentuate neurogenic dural vasodilation maximally after 15 min by 52% at 30 mg kg(-1) (t(5) = 6.78, n = 6); there was no significant inhibition at 10 mg kg(-1). There was also significant attenuation of the NO-induced dilation maximally after 15 min, at both 10 and 30 mg kg(-1) by 21% (t(6) = 6.09, n = 7) and 41% (t(6) = 5.3, n = 7), respectively. CGRP-induced dilation was not inhibited at either dose of topiramate. The study demonstrates that topiramate is likely to inhibit neurogenic dural vasodilation by inhibiting the release of CGRP from prejunctional trigeminal neurons, thus attenuating the dural vasodilation. Topiramate is not able to act postsynaptically at the blood vessels themselves as the CGRP-induced dilation was not attenuated. The data are consistent with an effect of topiramate on trigeminovascular activation which may form part of its preventive antimigraine mechanisms of action.

Attenuation of pain-related behavior in a rat model of trigeminal neuropathic pain by viral-driven enkephalin overproduction in trigeminal ganglion neurons

Trigeminal neuropathic pain represents a real challenge to therapy because commonly used drugs are devoid of real beneficial effect or patients frequently become intolerant or refractory to some of these compounds. In a rat model of trigeminal neuropathic pain, which shares numerous similarities with human trigeminal neuralgia and trigeminal neuropathic pain, we used a genomic herpes simplex virus-derived vector (HSVLatEnk) to examine the possible effect of a local overproduction of proenkephalin A (PA) targeted to the trigeminal primary sensory neurons. Unilateral peripheral inoculation of recombinant vectors on the vibrissal pad territory resulted in an about ninefold increase in proenkephalin A mRNA levels in trigeminal ganglion ipsilateral to the infected side. Transgene-derived met-enkephalin accumulated in numerous nerve cell bodies of trigeminal ganglion and was transported through the sensory nerve fibers located in the infraorbital nerve. Bilateral mechanical hyperresponsiveness, which developed 2 weeks after chronic constrictive injury of the left infraorbital nerve, was significantly attenuated in animals overproducing PA in the trigeminal ganglion ipsilateral to the lesioned infraorbital nerve. This antiallodynic effect was reversed by both the opioid receptor antagonist naloxone and the peripherally acting antagonist naloxone methiodide. Our data demonstrate that the local overproduction of PA-derived peptides in trigeminal ganglion sensory neurons evoked a potent antiallodynic effect through the stimulation of mainly peripherally located opioid receptors and suggest that targeted delivery of endogenous opioids may be of interest for the treatment of some severe forms of neuropathic pain.

Convergence of cervical and trigeminal sensory afferents

Cranial nociceptive perception shows a distinct topographic distribution, with the trigeminal nerve receiving sensory information from the anterior portions of the head, the greater occipital nerve, and branches of the upper cervical roots in the posterior regions. However, this distribution is not respected during headache attacks, even if the etiology of the headache is specific for only one nerve. Nociceptive information from the trigeminal and cervical territories activates the neurons in the trigeminal nucleus caudalis that extend to the C2 spinal segment and lateral cervical nucleus in the dorsolateral cervical area. These neurons are classified as multimodal because they receive sensory information from more than one afferent type. Clinically, trigeminal activation produces symptoms in the trigeminal and cervical territory and cervical activation produces symptoms in the cervical and trigeminal territory. The overlap between the trigeminal nerve and cervical is known as a convergence mechanism. For some time, convergence mechanisms were thought to be secondary to clinical observations. However, animal studies and clinical evidence have expanded our knowledge of convergence mechanisms. In this paper, the role of convergence mechanisms in nociceptive physiology, physiopathology of the headaches, clinical diagnosis, and therapeutic conduct are reviewed.