Differential distribution of [3H]sumatriptan binding sites (5-HT1B, 5-HT1D and 5-HT1F receptors) in human brain: focus on brainstem and spinal cord

Evidence-based review and frontiers of migraine therapy

BACKGROUND

Cyclic vomiting syndrome (CVS) is identified as one of the "episodic syndromes that may be associated with migraine," along with benign paroxysmal torticollis, benign paroxysmal vertigo, and abdominal migraine. It has been proposed that CVS and migraine may share pathophysiologic mechanisms of hypothalamic activation and altered dopaminergic signaling, and impaired sensorimotor intrinsic connectivity. The past decade has brought groundbreaking advances in the treatment of migraine and other headache disorders. While many of these therapies have yet to be studied in episodic syndromes associated with migraine including CVS and abdominal migraine, the potential shared pathophysiology among these conditions suggests that use of migraine-specific treatments may have a beneficial role even in those for whom headache is not the primary symptom.

PURPOSE

This manuscript highlights newer therapies in migraine. Calcitonin gene-related peptide (CGRP) and its relation to migraine pathophysiology and the therapies that target the CGRP pathway, as well as a 5HT1F receptor agonist and neuromodulation devices used to treat migraine are briefly discussed as they may potentially prove to be useful in the future treatment of CVS.

The 5-HT1F Receptor Agonist Lasmiditan improves Cognition and Ameliorates Associated Cortico-Hippocampal Pathology in Aging Parkinsonian Mice

While the etiopathology of Parkinson's disease (PD) is complex, mitochondrial dysfunction is established to have a central role. Thus, mitochondria have emerged as targets of therapeutic interventions aiming to slow or modify PD progression. We have previously identified serotonergic 5-HT1F receptors as novel mediators of mitochondrial biogenesis (MB) - the process of producing new mitochondria. Given this, here, we assessed the therapeutic potential of the FDA-approved 5-HT1F receptor agonist, lasmiditan, in a chronic progressive PD model (Thy1-aSyn 'line 61' mice). It was observed that systemic lasmiditan exhibited robust brain penetration and reversed cognitive deficits in young (4-5.5 months old) Thy1-aSyn mice (1mg/kg, every other day). Anxiety-like behavior was also improved while motor function remained unaffected. These behavioral changes were associated with enhanced MB and mitochondrial function, paired with reduced alpha-synuclein aggregation particularly in cortico-hippocampal regions. Furthermore, in older (10-11.5 months old) mice, although the effects were milder, daily lasmiditan administration increased MB and bettered cognitive abilities. In essence, these findings indicate that repurposing lasmiditan could be a potent strategy to address PD-related cognitive decline.

Regional distribution of unbound eletriptan and sumatriptan in the CNS and PNS in rats: implications for a potential central action

BACKGROUND

Triptans are potent 5-HT1B/1D/1F receptor agonists used in migraine therapy, thought to act through peripheral mechanisms. It remains unclear whether triptans cross the blood-brain barrier (BBB) sufficiently to stimulate central 5-HT1B/1D/1F receptors. This study investigates the disposition of eletriptan and sumatriptan in central nervous system (CNS) and peripheral nervous system (PNS) regions and predicts regional 5-HT1B/1D/1F receptor occupancies at clinically relevant concentrations.

METHODS

Using the Combinatory Mapping Approach (CMA) for regions of interest (ROI), we assessed the unbound tissue-to-plasma concentration ratio (Kp, uu, ROI) in rats at steady state across CNS (hypothalamus, brain stem, cerebellum, frontal cortex, parietal cortex, striatum, hippocampus, whole brain, and spinal cord) and PNS (trigeminal ganglion and sciatic nerve) regions. We used Kp, uu, ROI values to estimate unbound target-site concentrations and 5-HT1B/1D/1F receptor occupancies in humans.

RESULTS

We observed heterogenous triptan transport across CNS and PNS regions with the highest extent of unbound drug transport across the blood-nerve barrier in the trigeminal ganglion (Kp, uu, TG: eletriptan: 0.519, and sumatriptan: 0.923). Both drugs displayed restricted entry across the BBB (Kp, uu, whole brain: eletriptan: 0.058, and sumatriptan: 0.045) combined with high inter-regional variability. We estimated near-complete receptor occupancy in the trigeminal ganglion, while lower occupancies were observed in the whole brain, irrespective of the drug or receptor subtype. For instance, eletriptan was predicted to achieve 84% 5-HT1B receptor occupancy in the trigeminal ganglion and 37% in the whole brain at clinically relevant concentrations.

CONCLUSIONS

This study suggests that despite low BBB transport, both eletriptan and sumatriptan achieve unbound concentrations sufficient to stimulate 5-HT1B, 5-HT1D, and 5-HT1F receptors not only in the trigeminal ganglion, but also in the CNS. Further research is needed to determine whether central mechanisms contribute to triptan's antimigraine effect and/or side effects.

The 5-HT1F receptor as the target of ditans in migraine - from bench to bedside

Migraine is a leading cause of disability in more than one billion people worldwide, yet it remains universally underappreciated, even by individuals with the condition. Among other shortcomings, current treatments (often repurposed agents) have limited efficacy and potential adverse effects, leading to low treatment adherence. After the introduction of agents that target the calcitonin gene-related peptide pathway, another new drug class, the ditans - a group of selective serotonin 5-HT_1F receptor agonists - has just reached the international market. Here, we review preclinical studies from the late 1990s and more recent clinical research that contributed to the development of the ditans and led to their approval for acute migraine treatment by the US Food and Drug Administration and the European Medicines Agency.

Profile of Lasmiditan in the Acute Treatment of Migraine in Adults: Design, Development, and Place in Therapy

Migraine is a common neurological disorder that is present in a large proportion of the global population. It is estimated to occur in around 20.7% of women and 10.7% of men in the United States. The pathophysiology of migraine is a major focus of research, and medications have been developed to interrupt the processes that generate headache and other bothersome symptoms of migraine attacks. The triptan class of medications acts as a direct agonist at the 5-HT1B/D receptor but its use is limited by contraindications for those with coronary or cerebrovascular disease. Lasmiditan is a first-in-class agonist at the 5-HT1F serotonin receptor that does not appear to generate vasoconstriction. This article reviews the design, development, and place in therapy for lasmiditan. A narrative review of the literature using the Ovid MEDLINE database was performed. The rationale behind the development of lasmiditan and pre-clinical, proof-of-concept, Phase II, pivotal, Phase III trials and post-hoc data is covered. Additionally, the efficacy and safety of lasmiditan when compared to other acute treatments in migraine is described, including lasmiditan's side effect profile and status as a Schedule V substance. Further, head-to-head studies of lasmiditan compared with other acute treatments are required.

Changes in 5-HT1F receptor expression in rats with spasticity following spinal cord injury

Spasticity is a common complication in patients with spinal cord injury (SCI) and adversely affects patients' quality of life. Little is known about the distribution of the serotonin 1F receptor (5-HT1FR) in the spinal cord, especially in relation to the spasticity caused by SCI. Adult male Wistar rats were divided into a sham-operation group and spinalized group. SCI-induced spasticity was caused by spinal transection at the second sacral segment. The spinal cord below the transection was obtained at the end of the experiment. The expression and distribution of 5-HT1FR in the spinal cord were analyzed. The results showed that the expression of 5-HT1FR (mRNA and protein) exhibited the same downward trend after spinal transection and reached the lowest expression level at 2 and 5 days, respectively. The expression of 5-HT1FR (mRNA and protein) thereafter gradually approached the levels in the sham-operation group after 60 days. Immunostaining suggested that 5-HT1FR showed particularly strong expression in the ventral horn (VH) region. The time course of 5-HT1FR mRNA downregulation is positively correlated with the development of tail spasticity after sacral spinal cord transection. There may be a connection between 5-HT1FR and the occurrence of spasticity, but elucidation of the specific mechanism needs further experimental verification.

The protective effect of chemical and natural compounds against vincristine-induced peripheral neuropathy (VIPN)

Vincristine, an alkaloid extracted from Catharanthus rosea, is a class of chemotherapy drugs that act by altering the function of the microtubules and by inhibiting mitosis. Despite its widespread application, a major adverse effect of vincristine that limits treatment duration is the occurrence of peripheral neuropathy (PN). PN presents with several symptoms including numbness, painful sensation, tingling, and muscle weakness. Vincristine-induced PN involves impaired calcium homeostasis, an increase of reactive oxygen species (ROS), and the upregulation of tumor necrosis factor-alpha (TNF-α), and interleukin 1 beta (IL-1β) expression. Several potential approaches to attenuate the vincristine-induced PN including the concomitant administration of chemicals with vincristine have been reported. These chemicals have a variety of pharmaceutical properties including anti-inflammation, antioxidant, and inhibition of calcium channels and calcineurin signaling pathways and increased expression of nerve growth factor (NGF). This review summarized several of these compounds and the mechanisms of action that could lead to effective options in improving vincristine-induced peripheral neuropathy (VIPN).

Serotonin 1B receptor density mapping of the human brainstem using positron emission tomography and autoradiography

The serotonin 1B (5-HT_1B) receptor has lately received considerable interest in relation to psychiatric and neurological diseases, partly due to findings based on quantification using Positron Emission Tomography (PET). Although the brainstem is an important structure in this regard, PET radioligand binding quantification in brainstem areas often shows poor reliability. This study aims to improve PET quantification of 5-HT_1B receptor binding in the brainstem.Volumes of interest (VOIs) were selected based on a 3D [³H]AZ10419369 Autoradiography brainstem model, which visualized 5-HT_1B receptor distribution in high resolution. Two previously developed VOI delineation methods were tested and compared to a conventional manual method. For a method based on template data, a [¹¹C]AZ10419369 PET template was created by averaging parametric binding potential (BP_ND) images of 52 healthy subjects. VOIs were generated based on a predefined volume and BP_ND thresholding and subsequently applied to test-retest [¹¹C]AZ10419369 parametric BP_ND images of 8 healthy subjects. For a method based on individual subject data, VOIs were generated directly on each individual parametric image.Both methods showed improved reliability compared to a conventional manual VOI. The VOIs created with [¹¹C]AZ10419369 template data can be automatically applied to future PET studies measuring 5-HT_1B receptor binding in the brainstem.

The selective 5-HT1F receptor agonist lasmiditan inhibits trigeminal nociceptive processing: Implications for migraine and cluster headache

BACKGROUND AND PURPOSE

Lasmiditan is a novel selective 5-HT1F receptor agonist, recently approved for acute treatment of migraine. 5-HT1F receptors are widely expressed in the CNS and trigeminovascular system. Here, we have explored the therapeutic effects of 5-HT1F receptor activation in preclinical models of migraine and cluster headache.

EXPERIMENTAL APPROACH

Electrical stimulation of the dura mater or the superior salivatory nucleus in anaesthetised rats evoked trigeminovascular or trigeminal-autonomic reflex activation at the level of the trigeminocervical complex. Additionally, cranial autonomic manifestations in response to trigeminal-autonomic reflex activation were measured, via anterior choroidal blood flow alterations. These responses were then challenged with lasmiditan. We explored the tissue distribution of mRNA for 5-HT1F receptors in human post-mortem tissue and of several 5-HT1 receptor subtypes in specific tissue beds.

KEY RESULTS

Lasmiditan dose-dependently reduced trigeminovascular activation in a preclinical model of migraine. Lasmiditan also reduced superior salivatory nucleus-evoked activation of the trigeminal-autonomic reflex, but had no effect on cranial autonomic activation. mRNA profiling in human tissue showed expression of the 5-HT1F receptor in several structures relevant for migraine and cluster headache.

CONCLUSION AND IMPLICATIONS

Our data suggest that lasmiditan acts, at least in part, as an anti-migraine agent by reducing trigeminovascular activation. Furthermore, our results highlight a clear action for lasmiditan in a preclinical model of cluster headache. Given the proven translational efficacy of this model, our data support the potential utility of lasmiditan as a therapeutic option for the acute treatment of cluster headache attacks.

LINKED ARTICLES

This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

FDA-approved 5-HT1F receptor agonist lasmiditan induces mitochondrial biogenesis and enhances locomotor and blood-spinal cord barrier recovery after spinal cord injury

Vascular and mitochondrial dysfunction are well-established consequences of spinal cord injury (SCI). Evidence suggests mitigating these dysfunctions may be an effective approach in treating SCI. The goal of this study was to elucidate if mitochondrial biogenesis (MB) induction with a new, selective and FDA-approved 5-hydroxytryptamine receptor 1F (5-HT_1F) receptor agonist, lasmiditan, can stimulate locomotor recovery and restoration of the blood-spinal cord barrier (BSCB) after SCI. Female C57BL/6 J mice were subjected to moderate SCI using a force-controlled impactor-induced contusion model followed by daily administration of lasmiditan (0.1 mg/kg, i.p.) beginning 1 h after injury. In the naïve spinal cord, electron microscopy revealed increased mitochondrial density and mitochondrial area, as well as enhanced mitochondrial DNA content. FCCP-uncoupled oxygen consumption rate (OCR), a functional marker of MB, was also increased in the naïve spinal cord following lasmiditan treatment. We observed disrupted mitochondrial DNA content, PGC-1α levels and FCCP-OCR in the injury site 3d after SCI. Lasmiditan treatment attenuated, and in some cases restored these deficits. Lasmiditan treatment also resulted in increased locomotor capability as early as 7d post-SCI, with treated mice reaching a Basso-Mouse Scale score of 3.3 by 21d, while vehicle-treated mice exhibited a score of 2.0. Integrity of the BSCB was assessed using Evans Blue dye extravasation. While SCI increased dye extravasation at 3d and 7d, dye accumulation in the spinal cord of lasmiditan-treated mice was attenuated 7d post-SCI, suggesting accelerated BSCB recovery. Finally, lasmiditan treatment resulted in decreased lesion volume and spared myelinated tissue 7d post-SCI. Collectively, these data reveal that 5-HT_1F receptor agonist-induced MB using the FDA-approved drug lasmiditan may be an effective therapeutic strategy for the treatment of SCI.

A new era for migraine: The role of calcitonin gene-related peptide in the trigeminovascular system

There is a huge improvement in our understanding of migraine pathophysiology in the past decades. The activation of the trigeminovascular system has been proved to play a key role in migraine. Calcitonin gene-related peptide (CGRP) and CGRP receptors are widely distributed in the trigeminovascular system. The CGRP is expressed on the C-fibers, and the CGRP receptors are distributed on the A-δ fibers of the trigeminal ganglion and nerves. Further studies found elevated serum CGRP level during migraine attacks, and infusion of CGRP can trigger migraine-like attacks, provide more direct evidence of the link between CGRP and migraine attack. Based on these findings, several treatment options have been designed for migraine treatment, including CGRP receptor antagonists (gepants) and monoclonal antibodies targeting CGRP or CGRP receptors. The clinical trials show both gepants and monoclonal antibodies are effective for migraine treatment. In this section, we describe the roles of the trigeminovascular system in migraine, the discovery of CGRP, and the CGRP signaling pathway.

Lasmiditan mechanism of action - review of a selective 5-HT1F agonist

Migraine is a leading cause of disability worldwide, but it is still underdiagnosed and undertreated. Research on the pathophysiology of this neurological disease led to the discovery that calcitonin gene-related peptide (CGRP) is a key neuropeptide involved in pain signaling during a migraine attack. CGRP-mediated neuronal sensitization and glutamate-based second- and third-order neuronal signaling may be an important component involved in migraine pain. The activation of several serotonergic receptor subtypes can block the release of CGRP, other neuropeptides, and neurotransmitters, and can relieve the symptoms of migraine. Triptans were the first therapeutics developed for the treatment of migraine, working through serotonin 5-HT1B/1D receptors. The discovery that the serotonin 1F (5-HT1F) receptor was expressed in the human trigeminal ganglion suggested that this receptor subtype may have a role in the treatment of migraine. The 5-HT1F receptor is found on terminals and cell bodies of trigeminal ganglion neurons and can modulate the release of CGRP from these nerves. Unlike 5-HT1B receptors, the activation of 5-HT1F receptors does not cause vasoconstriction.The potency of different serotonergic agonists towards 5-HT1F was correlated in an animal model of migraine (dural plasma protein extravasation model) leading to the development of lasmiditan. Lasmiditan is a newly approved acute treatment for migraine in the United States and is a lipophilic, highly selective 5-HT1F agonist that can cross the blood-brain barrier and act at peripheral nervous system (PNS) and central nervous system (CNS) sites.Lasmiditan activation of CNS-located 5-HT1F receptors (e.g., in the trigeminal nucleus caudalis) could potentially block the release of CGRP and the neurotransmitter glutamate, thus preventing and possibly reversing the development of central sensitization. Activation of 5-HT1F receptors in the thalamus can block secondary central sensitization of this region, which is associated with progression of migraine and extracephalic cutaneous allodynia. The 5-HT1F receptors are also elements of descending pain modulation, presenting another site where lasmiditan may alleviate migraine. There is emerging evidence that mitochondrial dysfunction might be implicated in the pathophysiology of migraine, and that 5-HT1F receptors can promote mitochondrial biogenesis. While the exact mechanism is unknown, evidence suggests that lasmiditan can alleviate migraine through 5-HT1F agonist activity that leads to inhibition of neuropeptide and neurotransmitter release and inhibition of PNS trigeminovascular and CNS pain signaling pathways.

Mitochondrial biogenesis as a therapeutic target for traumatic and neurodegenerative CNS diseases

Central nervous system (CNS) diseases, both traumatic and neurodegenerative, are characterized by impaired mitochondrial bioenergetics and often disturbed mitochondrial dynamics. The dysregulation observed in these pathologies leads to defective respiratory chain function and reduced ATP production, thereby promoting neuronal death. As such, attenuation of mitochondrial dysfunction through induction of mitochondrial biogenesis (MB) is a promising, though still underexplored, therapeutic strategy. MB is a multifaceted process involving the integration of highly regulated transcriptional events, lipid membrane and protein synthesis/assembly and replication of mtDNA. Several nuclear transcription factors promote the expression of genes involved in oxidative phosphorylation, mitochondrial import and export systems, antioxidant defense and mitochondrial gene transcription. Of these, the nuclear-encoded peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) is the most commonly studied and is widely accepted as the 'master regulator' of MB. Several recent preclinical studies document that reestablishment of mitochondrial homeostasis through increased MB results in inhibited injury progression and increased functional recovery. This perspective will briefly review the role of mitochondrial dysfunction in the propagation of CNS diseases, while also describing current research strategies that mediate mitochondrial dysfunction and compounds that induce MB for the treatment of acute and chronic neuropathologies.

5-hydroxytryptamine 1F Receptor Agonist Induces Mitochondrial Biogenesis and Promotes Recovery from Spinal Cord Injury

Spinal cord injury (SCI) is characterized by vascular disruption leading to ischemia, decreased oxygen delivery, and loss of mitochondrial homeostasis. This mitochondrial dysfunction results in loss of cellular functions, calcium overload, and oxidative stress. Pharmacological induction of mitochondrial biogenesis (MB) may be an effective approach to treat SCI. LY344864, a 5-hydroxytryptamine 1F (5-HT1F) receptor agonist, is a potent inducer of MB in multiple organ systems. To assess the efficacy of LY344864-induced MB on recovery post-SCI, female mice were subjected to moderate force-controlled impactor-induced contusion SCI followed by daily LY344864 administration for 21 days. Decreased mitochondrial DNA and protein content was present in the injury site 3 days post-SCI. LY344864 treatment beginning 1 h after injury attenuated these decreases, indicating MB. Additionally, injured mice treated with LY344864 displayed decreased Evan's Blue dye accumulation in the spinal cord compared with vehicle-treated mice 7 days after injury, suggesting restoration of vascular integrity. LY344864 also increased locomotor capability, with treated mice reaching a Basso-Mouse Scale score of 3.4 by 21 days, whereas vehicle-treated mice exhibited a score of 1.9. Importantly, knockout of the 5-HT1F receptor blocked LY344864-induced recovery. Remarkably, a similar degree of locomotor restoration was observed when treatment initiation was delayed until 8 h after injury. Furthermore, cross-sectional analysis of the spinal cord 21 days after injury revealed decreased lesion volume with delayed LY344864 treatment initiation, emphasizing the potential clinical applicability of this therapeutic approach. These data provide evidence that induction of MB via 5-HT1F receptor agonism may be a promising strategy for the treatment of SCI. SIGNIFICANCE STATEMENT: Treatment with LY344864 induces mitochondrial biogenesis in both the naive and injured mouse spinal cord. In addition, treatment with LY344864 beginning after impactor-induced contusion spinal cord injury improves mitochondrial homeostasis, blood-spinal cord barrier integrity, and locomotor function within 7 days. Importantly, similar locomotor results are observed whether treatment is initiated at 1 h after injury or 8 h after injury. These data indicate the potential for pharmacological induction of mitochondrial biogenesis through a 5-hydroxytryptamine 1F agonist as a novel therapeutic approach for spinal cord injury.

The protective effects of sumatriptan on vincristine - induced peripheral neuropathy in a rat model

Clinical use of vincristine (VCR), an effective chemotherapeutic agent, has been limited due to its peripheral neuropathy toxicity. Sumatriptan, which is an anti-migraine agent is a specific agonist for 5-hydroxytryptamine 1B, 1D (5HT1B, 1D) receptors. Several studies have shown that sumatriptan exerts anti-inflammatory and immunomodulatory properties. This study aimed to investigate the effects of sumatriptan on VCR-induced peripheral neuropathy in a rat model. Male Wistar rats were intraperitoneally injected with VCR and normal saline four times per week for 2 weeks. In the treatment group, sumatriptan (1 mg/kg) was administered intraperitoneally 30 min prior to VCR injection every day. Mortality rate, weight variations and histopathological changes were monitored. Hot plate, tail flick and motor nerve conduction velocity (MNCV) tests were used to evaluate sensory and motor neuropathy. Levels of tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β) and caspase-3 in the dorsal ganglion root were assessed by quantitative reverse transcription-PCR (qRT-PCR). Moreover, the protein levels of p65 nuclear factor kappa B (NF- B) and phospho-p65 NF- B were examined by Western blot analysis. Co-administration of sumatriptan with VCR significantly reversed alterations in the hot plate, tail flick threshold and sciatic MNCV induced by VCR and also prevented mixed sensory-motor neuropathy, as indicated by better general conditions, behavioral and electrophysiological results. In addition, sumatriptan improved the body weight loss caused by VCR. The mRNA levels of TNF-α, IL-1β and caspase-3 were significantly diminished in the treatment group. These findings were confirmed by histopathological analysis. In conclusion, this study demonstrated that sumatriptan significantly attenuated VCR-induced neuropathy and could be considered as a neuroprotective agent to prevent the VCR-induced neuropathy.

Is selective 5-HT1F receptor agonism an entity apart from that of the triptans in antimigraine therapy?

Migraine is a neurovascular disorder that involves activation of the trigeminovascular system and cranial vasodilation mediated by release of calcitonin gene-related peptide (CGRP). The gold standard for acute migraine treatment are the triptans, 5-HT1B/1D/(1F) receptor agonists. Their actions are thought to be mediated through activation of: (i) 5-HT1B receptors in cranial blood vessels with subsequent cranial vasoconstriction; (ii) prejunctional 5-HT1D receptors on trigeminal fibers that inhibit trigeminal CGRP release; and (iii) 5-HT1B/1D/1F receptors in central nervous system involved in (anti)nociceptive modulation. Unfortunately, coronary arteries also express 5-HT1B receptors whose activation would produce coronary vasoconstriction; hence, triptans are contraindicated in patients with cardiovascular disease. In addition, since migraineurs have an increased cardiovascular risk, it is important to develop antimigraine drugs devoid of vascular (side) effects. Ditans, here defined as selective 5-HT1F receptor agonists, were developed on the basis that most of the triptans activate trigeminal 5-HT1F receptors, which may explain part of the triptans' antimigraine action. Amongst the ditans, lasmiditan: (i) fails to constrict human coronary arteries; and (ii) is effective for the acute treatment of migraine in preliminary Phase III clinical trials. Admittedly, the exact site of action is still unknown, but lasmiditan possess a high lipophilicity, which suggests a direct action on the central descending antinociceptive pathways. Furthermore, since 5-HT1F receptors are located on trigeminal fibers, they could modulate CGRP release. This review will be particularly focussed on the similarities and differences between the triptans and the ditans, their proposed sites of action, side effects and their cardiovascular risk profile.

Targeted 5-HT1F Therapies for Migraine

Migraine is a common neurological disease characterised by the presence of attacks of unilateral, severe head pain accompanied by other symptoms. Although it has been classified as the sixth most disabling disorder, the available therapeutic options to treat this condition have not progressed accordingly. The advance in the development of 5-HT1 receptor agonists for migraine, including 5-HT1B/D and 5-HT1F receptor agonists, has meant a major step forward towards the progression of a better treatment for migraine. Triptans have a limited efficacy, and their effect on vasoconstriction makes them unsafe for patients with cardiovascular and/or cerebrovascular diseases. Therefore, novel effective antimigraine treatments without cardiovascular effects are required, such as selective 5-HT1F receptor agonists (ditans). Lasmiditan has much higher affinity for the 5-HT1F receptor than for the vasoconstrictor 5-HT1B receptor. This has been confirmed in preclinical studies performed to date, where lasmiditan showed no effect on vasoconstriction, and in clinical trials, where healthy individuals and patients did not report cardiac events due to treatment with lasmiditan, although it should be confirmed in larger cohorts. Lasmiditan crosses the blood-brain barrier and may act both centrally and peripherally on 5-HT1F receptors expressed on trigeminal neurons. It is a well-tolerated compound that does not induce major adverse events. Although ongoing phase III clinical trials are needed to confirm its efficacy and safety, lasmiditan might offer an alternative to treat acute migraine with no associated cardiovascular risk. This review will focus on the characterisation of 5-HT1 receptor agonists and their effects as migraine therapies.

5-HT1F receptor-mediated mitochondrial biogenesis for the treatment of Parkinson's disease

BACKGROUND AND PURPOSE

Parkinson's disease is characterized by progressive decline in motor function due to degeneration of nigrostriatal dopaminergic neurons, as well as other deficits including cognitive impairment and behavioural abnormalities. Mitochondrial dysfunction, leading to loss of ATP-dependent cellular functions, calcium overload, excitotoxicity and oxidative stress, is implicated in the pathophysiology of Parkinson's disease. Using the 5-HT1F receptor agonist LY344864, a known inducer of mitochondrial biogenesis (MB), we investigated the therapeutic efficacy of stimulating MB on dopaminergic neuron loss in a mouse model of Parkinson's disease.

EXPERIMENTAL APPROACH

Male C57BL/6 mice underwent bilateral intrastriatal 6-hydroxydopamine or saline injections and daily treatment with 2 mg·kg-1 LY344864 or vehicle for 14 days beginning 7 days post-lesion. Tyrosine hydroxylase immunoreactivity (TH-ir) and MB were assessed in the brains of all groups following treatment, and locomotor activity was evaluated prior to lesioning, 7 days post-lesion and after treatment.

KEY RESULTS

Increased mitochondrial DNA content and nuclear- and mitochondrial-encoded mRNA and protein expression was observed in specific brain regions of LY344864-treated naïve and lesioned mice, indicating augmented MB. LY344864 attenuated TH-ir loss in the striatum and substantia nigra compared to vehicle-treated lesioned animals. LY344864 treatment also increased locomotor activity in 6-hydroxydopamine lesioned mice, while vehicle treatment had no effect.

CONCLUSIONS AND IMPLICATIONS

These data revealed that LY344864-induced MB attenuates dopaminergic neuron loss and improves behavioural endpoints in this model. We suggest that stimulating MB may be beneficial for the treatment of Parkinson's disease and that the 5-HT1F receptor may be an effective therapeutic target.

The prevalence, course and clinical correlates of migraine in Parkinson's disease: A multicentre case-controlled study

Background Previous studies have reported a lower migraine prevalence in Parkinson's disease (PD) patients and improvements in migraine headaches after PD onset, but the clinical association of migraines with PD is unclear. Methods We analysed headache and migraine prevalence and clinical correlates in 436 PD patients (mean age, 69.3 ± 7.8 years) and 401 age- and sex-matched controls (mean age, 69.2 ± 8.6 years) in a case-controlled, multicentre study. Migraines were diagnosed by a questionnaire developed according to the International Classification of Headache Disorders, second edition. We evaluated changes in headache intensity, frequency and severity over several years around the onset of PD among PD patients with headaches or migraines, and over the past several years among control subjects with headaches or migraines. Results PD patients had lower lifetime (9.6% vs. 18.0%) and 1-year (6.7% vs. 11.0%) migraine prevalences than controls. However, lifetime (38.5% vs. 38.9%) and 1-year (26.1% vs. 26.2%) headache prevalence did not differ between PD patients and controls. After adjusting for gender, timing of the evaluation of headache changes, and recall period, PD patients with headaches or migraines exhibited a pronounced reduction in the intensity, frequency and overall severity of their headaches and migraines after the onset of PD compared with controls with headaches or migraines. PD patients with migraines exhibited a higher rate of depression and higher Pittsburgh Sleep Quality Index and PD sleep scale-2 scores than those without headaches. Conclusion While overall headache and migraine severity reduced after PD onset, the presence of migraines was associated with sleep disturbances and depression in PD patients.

Serotonin receptor targeted therapy for migraine treatment: an overview of drugs in phase I and II clinical development

Research has focused on serotonin (5-HT) 5-HT1D and 5-HT1F receptors to develop drugs acting through non-vasoconstrictive mechanisms for treating acute migraine and those targeting 5-HT2B and 5-HT7 receptors for preventing migraine. Areas covered: This paper reviews antimigraine drugs targeting 5-HT receptors in one phase I trial (sumatriptan iontophoretic transdermal system, TDS) and five phase II clinical trials (PNU-142633, LY334370, lasmiditan, NOX-188). Expert opinion: Data from our overview on investigational drugs in phase I and II clinical trials using the 5-HT1B/1D receptor agonist (sumatriptan TDS), 5-HT1D receptor agonist (PNU-142633), 5-HT1F receptor agonists (LY334370, lasmiditan) and a combined 5-HT1B/1D receptor agonist with nNOS inhibition (NOX-188) provided encouraging data for sumatriptan TDS and lasmiditan, disappointing results for PNU-142633, and promising findings for NOX-188. The 5-HT1F receptor agonist lasmiditan, a drug acting through non-vasoconstrictive mechanisms, represents a promising safe, effective and tolerated acute migraine therapy also for patients at cardiovascular risk. Upcoming phase III trials should clarify the optimal lasmiditan dose and eventual clinical advantages over triptans. The negative results for the PNU-142633 trial prompt further studies using specific compounds more precisely targeting 5-HT1D receptors. Antagonism at 5-HT2B and 5-TH7 receptors, a promising strategy to prevent migraine, is still limited to experimental migraine models.

Marked sexual dimorphism in 5-HT1 receptors mediating pronociceptive effects of sumatriptan

Amongst the side effects of triptans, a substantial percentage of patients experience injection site pain and tenderness, the underlying mechanism of which is unknown. We found that the dose range from 10fg to 1000ng (intradermal) of sumatriptan induced a complex dose-dependent mechanical hyperalgesia in male rats, with distinct peaks, at 1pg and 10ng, but no hyperalgesia at 1ng. In contrast, in females, there was 1 broad peak. The highest dose (1000ng) did not produce hyperalgesia in either sex. We evaluated the receptors mediating sumatriptan hyperalgesia (1pg, 1 and 10ng). In males, the injection of an antagonist for the serotonin (5-HT) receptor subtype 1B (5-HT_1B), but not 5-HT_1D, markedly inhibited sumatriptan (1pg)-induced hyperalgesia, at 10ng a 5-HT_1D receptor antagonist completely eliminated hyperalgesia. In contrast, in females, the 5-HT_1D, but not 5-HT_1B, receptor antagonist completely blocked sumatriptan (1pg and 10ng) hyperalgesia and both 5-HT_1B and 5-HT_1D receptor antagonists attenuated hyperalgesia (1ng) in females, which is GPR30 estrogen receptor dependent. While selective 5-HT_1D or 5-HT_1B, agonists produce robust hyperalgesia in female and male rats, respectively, when co-injected the hyperalgesia induced in both sexes was attenuated. Mechanical hyperalgesia induced by sumatriptan (1pg and 10ng) is dependent on the G-protein α_i subunit and protein kinase A (PKA), in IB4-positive and negative nociceptors. Understanding the mechanisms responsible for the complex dose dependence for triptan hyperalgesia may provide useful information for the design of anti-migraine drugs with improved therapeutic profiles.

Spinal 5-HT1A, not the 5-HT1B or 5-HT3 receptors, mediates descending serotonergic inhibition for late-phase mechanical allodynia of carrageenan-induced peripheral inflammation

Previous electrophysiological studies demonstrated a limited role of 5-hydroxytryptamine 3 receptor (5-HT3R), but facilitatory role of 5-HT1AR and 5-HT1BR in spinal nociceptive processing of carrageenan-induced inflammatory pain. The release of spinal 5-HT was shown to peak in early-phase and return to baseline in late-phase of carrageenan inflammation. We examined the role of the descending serotonergic projections involving 5-HT1AR, 5-HT1BR, and 5-HT3R in mechanical allodynia of early- (first 4h) and late-phase (24h after) carrageenan-induced inflammation. Intrathecal administration of 5-HT produced a significant anti-allodynic effect in late-phase, but not in early-phase. Similarly, intrathecal 5-HT1AR agonist (8-OH-DPAT) attenuated the intensity of late-phase allodynia in a dose dependent fashion which was antagonized by 5-HT1AR antagonist (WAY-100635), but produced no effect on the early-phase allodynia. However, other agonists or antagonists of 5-HT1BR (CP-93129, SB-224289) and 5-HT3R (m-CPBG, ondansetron) did not produce any anti- or pro-allodynic effect in both early- and late- phase allodynia. These results suggest that spinal 5-HT1A, but not 5-HT1B or 5-HT3 receptors mediate descending serotonergic inhibition on nociceptive processing of late-phase mechanical allodynia in carrageenan-induced inflammation.

Orofacial pain management: current perspectives

Some of the most prevalent and debilitating pain conditions arise from the structures innervated by the trigeminal system (head, face, masticatory musculature, temporomandibular joint and associated structures). Orofacial pain (OFP) can arise from different regions and etiologies. Temporomandibular disorders (TMD) are the most prevalent orofacial pain conditions for which patients seek treatment. Temporomandibular disorders include a number of clinical problems that involve the masticatory musculature, the temporomandibular joint (TMJ) or both. Trigeminal neuropathic pain conditions can arise from injury secondary to dental procedures, infection, neoplasias, or disease or dysfunction of the peripheral and/or central nervous system. Neurovascular disorders, such as primary headaches, can present as chronic orofacial pain, such as in the case of facial migraine, where the pain is localized in the second and third division of the trigeminal nerve. Together, these disorders of the trigeminal system impact the quality of life of the sufferer dramatically. A multidisciplinary pain management approach should be considered for the optimal treatment of orofacial pain disorders including both non-pharmacological and pharmacological modalities.

Activation of 5-hydroxytryptamine1B/1D/1F receptors as a mechanism of action of antimigraine drugs

INTRODUCTION

The introduction of the triptans (5-hydroxytryptamine (5-HT)1B/1D receptor agonists) was a great improvement in the acute treatment of migraine. However, shortcomings of the triptans have prompted research on novel serotonergic targets for the treatment of migraine.

AREAS COVERED

In this review the different types of antimigraine drugs acting at 5-HT receptors, their discovery and development are discussed. The first specific antimigraine drugs were the ergot alkaloids, consisting of ergotamine, dihydroergotamine and methysergide, which are agonists at 5-HT receptors, but can also bind α-adrenoceptors and dopamine receptors. In the 1990s, the triptans became available on the market. They are 5-HT1B/1D receptor agonists, showing fewer side effects due to their receptor specificity. In the last years, compounds that bind specifically to 5-HT1D, 5-HT1F and 5-HT7 receptors have been explored for their antimigraine potential. Furthermore, the serotonergic system seems to act in tight connection with the glutamatergic as well as the CGRP-ergic systems, which may open novel therapeutic avenues.

EXPERT OPINION

Although the triptans are very effective in treating migraine attacks, their shortcomings have stimulated the search for novel drugs. Currently, the focus is on 5-HT1F receptor agonists, which seem devoid of vascular side effects. Moreover, novel compounds that affect multiple transmitter and/or neuropeptide systems that are involved in migraine could be of therapeutic relevance.

Serotonergic modulation of neuronal activity in rat midbrain periaqueductal gray

Serotonin (5-HT) modulates pain and anxiety from within the midbrain periaqueductal gray (PAG). In the present study, the effects of 5-HT- and 5-HT1/2 subtype-selective ligands on rat PAG neurons were examined using whole cell patch-clamp recordings in brain slices. In voltage clamp, 5-HT produced outward and inward currents in distinct subpopulations of neurons that varied throughout different subregions of the PAG. The 5-HT1A agonist R(+)-8-OH-DPAT (1 μM) produced outward currents in subpopulations of PAG neurons. By contrast, sumatriptan (1 μM) and other 5-HT1B, -D, and -F subtype agonists had little or no postsynaptic activity. The 5-HT2A/C agonists DOI (3 μM) and TCB-2 (1 μM) produced inward currents in subpopulations of PAG neurons, and DOI enhanced evoked inhibitory postsynaptic currents via a presynaptic mechanism. In current clamp, both R(+)-8-OH-DPAT and sumatriptan produced an excitatory increase in evoked mixed postsynaptic potentials (PSPs). In addition, R(+)-8-OH-DPAT, but not sumatriptan, directly hyperpolarized PAG neurons. By contrast, the 5-HT2 agonist DOI depolarized subpopulations of neurons and produced an inhibitory decrease in evoked mixed PSPs. These findings indicate that 5-HT1A and 5-HT1B/D ligands have partly overlapping inhibitory effects on membrane excitability and synaptic transmission within the PAG, which are functionally opposed by 5-HT2A/C actions in specific PAG subregions.

A PET study with [11C]AZ10419369 to determine brain 5-HT1B receptor occupancy of zolmitriptan in healthy male volunteers

Aim

To investigate the occupancy at brain 5-hydroxytryptamine (5-HT) 1B receptors in human subjects after administration of the antimigraine drug zolmitriptan.

Methods

Positron emission tomography (PET) studies were undertaken using the radioligand [11C]AZ10419369 in eight control subjects at baseline and after administration of zolmitriptan orodispersible tablets. The subjects were examined after two consecutive administrations of 10 mg zolmitriptan, approximately 1 week apart. Two of the subjects were subsequently examined after administration of 5 mg zolmitriptan. One week after the last administration of zolmitriptan five of the subjects underwent additional PET measurements without drug pretreatment.

Results

After administration of 10 mg zolmitriptan, mean receptor occupancy was 4–5%. No consistent changes in 5-HT1B receptor binding were observed for subjects who received 5 mg zolmitriptan. There was a statistically significant negative relationship between binding potential ( BPND) and plasma concentration of zolmitriptan and the active metabolite 183C91, respectively. All of the five subjects who were examined 1 week after dosing with zolmitriptan showed higher BPND post drug administration compared with baseline.

Conclusion

This is the first demonstration of CNS 5-HT1B receptor occupancy of a triptan. The findings are consistent with the low receptor occupancy previously reported in PET studies with agonists at other G protein coupled receptors.

Autoradiographic Mapping of 5-HT(1B/1D) Binding Sites in the Rhesus Monkey Brain Using [carbonyl-C]zolmitriptan

Zolmitriptan is a serotonin 5-HT(1B/1D) receptor agonist that is an effective and well-tolerated drug for migraine treatment. In a human positron emission tomography study, [(11)C]zolmitriptan crossed the blood-brain barrier but no clear pattern of regional uptake was discernable. The objective of this study was to map the binding of [(11)C]zolmitriptan in Rhesus monkey brain using whole hemisphere in vitro autoradiography with [(11)C]zolmitriptan as a radioligand. In saturation studies, [(11)C]zolmitriptan showed specific (90%) binding to a population of high-affinity binding sites (Kd 0.95-5.06 nM). There was regional distribution of binding sites with the highest density in the ventral pallidum, followed by the external globus pallidus, substantia nigra, visual cortex, and nucleus accumbens. In competitive binding studies with 5-HT(1) receptor antagonists, [(11)C]zolmitriptan binding was blocked by selective 5-HT(1B) and 5-HT(1D) ligands in all target areas. There was no appreciable change in binding with the addition of a 5-HT(1A) receptor antagonist.

Serotonin and molecular neuroimaging in humans using PET

The serotonergic system is one of the most important modulatory neurotransmitter systems in the human brain. It plays a central role in major physiological processes and is implicated in a number of psychiatric disorders. Along with the dopaminergic system, it is also one of the phylogenetically oldest human neurotransmitter systems and one of the most diverse, with 14 different receptors identified up to this day, many of whose function remains to be understood. The system's functioning is even more diverse than the number of its receptors, since each is implicated in a number of different processes. This review aims at illustrating the distribution and summarizing the main functions of the serotonin (5-hydroxytryptamin, 5-HT) receptors as well as the serotonin transporter (SERT, 5-HTT), the vesicular monoamine transporter 2, monoamine oxidase type A and 5-HT synthesis in the human brain. Recent advances in in vivo quantification of these different receptors and enzymes that are part of the serotonergic system using positron emission tomography are described.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Does sumatriptan cross the blood-brain barrier in animals and man?

Sumatriptan, a relatively hydrophilic triptan, based on several animal studies has been regarded to be unable to cross the blood–brain barrier (BBB). In more recent animal studies there are strong indications that sumatriptan to some extent can cross the BBB. The CNS adverse events of sumatriptan in migraine patients and normal volunteers also indicate a more general effect of sumatriptan on CNS indicating that the drug can cross the BBB in man. It has been discussed whether a defect in the BBB during migraine attacks could be responsible for a possible central effect of sumatriptan in migraine. This review suggests that there is no need for a breakdown in the BBB to occur in order to explain a possible central CNS effect of sumatriptan.

Future drugs for migraine

Migraine is a complex, neurovascular disorder in which genetic and environmental factors interact. At present, frontline therapies in the acute treatment of migraine include the use of non-steroidal anti-inflammatory drugs and triptans. Evidence indicates that calcitonin gene-related peptide (CGRP) plays a fundamental role in the mechanism of migraine. CGRP is a strong vasodilatatory neuropeptide that is released from activated trigeminal sensory nerves. The development of CGRP antagonists has also been driven by the fact that triptans are vasoconstrictive and cannot be safely used in patients with cardiovascular risk factors. Olcegepant (BIBN4096) is the first CGRP antagonist for the treatment of migraine that has been tested in clinical trials, but because of its poor oral bioavailability, only the intravenous formulation has been tested. The first oral non-peptide CGRP antagonist, telcagepant, has been shown recently to be highly effective in the treatment of migraine attacks. This development can be considered as the most important pharmacological breakthrough for migraine treatment since the introduction of sumatriptan in the early 1990s. These results are also of importance, since they support an interesting pathophysiological hypothesis of migraine. The pipeline of future compounds for the treatment of acute migraine headaches include TPRV1 antagonists, prostaglandin E receptor 4 (EP(4)) receptor antagonists, serotonin 5HT1(F) receptor agonists and nitric oxide synthase inhibitors. The immediate future of a preventative treatment for migraine headaches is well represented by botulinum toxin type-A, glutamate NMDA receptor antagonists, gap-junction blocker tonabersat and an angiotensin type 1 blocker candesartan.

Sumatriptan inhibits synaptic transmission in the rat midbrain periaqueductal grey

Background

There is evidence to suggest that the midbrain periaqueductal grey (PAG) has a role in migraine and the actions of the anti-migraine drug sumatriptan. In the present study we examined the serotonergic modulation of GABAergic and glutamatergic synaptic transmission in rat midbrain PAG slices in vitro.

Results

Serotonin (5-hydroxytriptamine, 5-HT, IC₅₀ = 142 nM) and the selective serotonin reuptake inhibitor fluoxetine (30 μM) produced a reduction in the amplitude of GABA_A-mediated evoked inhibitory postsynaptic currents (IPSCs) in all PAG neurons which was associated with an increase in the paired-pulse ratio of evoked IPSCs. Real time PCR revealed that 5-HT1A, 5-HT1B, 5-HT1D and 5-HT1F receptor mRNA was present in the PAG. The 5-HT1A, 5-HT1B and 5-HT1D receptor agonists 8-OH-DPAT (3 μM), CP93129 (3 μM) and L694247 (3 μM), but not the 5-HT1F receptor agonist LY344864 (1 – 3 μM) inhibited evoked IPSCs. The 5-HT (1 μM) induced inhibition of evoked IPSCs was abolished by the 5-HT1B antagonist NAS181 (10 μM), but not by the 5-HT1A and 5-HT1D antagonists WAY100135 (3 μM) and BRL15572 (10 μM). Sumatriptan also inhibited evoked IPSCs with an IC₅₀ of 261 nM, and reduced the rate, but not the amplitude of spontaneous miniature IPSCs. The sumatriptan (1 μM) induced inhibition of evoked IPSCs was abolished by NAS181 (10 μM) and BRL15572 (10 μM), together, but not separately. 5-HT (10 μM) and sumatriptan (3 μM) also reduced the amplitude of non-NMDA mediated evoked excitatory postsynaptic currents (EPSCs) in all PAG neurons tested.

Conclusion

These results indicate that sumatriptan inhibits GABAergic and glutamatergic synaptic transmission within the PAG via a 5-HT1B/D receptor mediated reduction in the probability of neurotransmitter release from nerve terminals. These actions overlap those of other analgesics, such as opioids, and provide a mechanism by which centrally acting 5-HT1B and 5-HT1D ligands might lead to novel anti-migraine pharmacotherapies.

Reversal of inflammatory and noninflammatory visceral pain by central or peripheral actions of sumatriptan

BACKGROUND & AIMS

Sumatriptan is used specifically to relieve headache pain. The possible efficacy of sumatriptan was investigated in 2 models of visceral pain.

METHODS

Pancreatic inflammation was induced by intravenous injection of dibutyltin dichloride. Noninflammatory irritable bowel syndrome was induced by intracolonic instillation of sodium butyrate. The effects of systemic sumatriptan on referred hypersensitivity were tested in both models. Effects of sumatriptan within the rostral ventromedial medulla (RVM), a site of descending modulation of visceral pain, was determined by (1) testing the effects of RVM administration of 5HT1(B/D) antagonists on systemic sumatriptan action and (2) determining whether RVM application of sumatriptan reproduced the actions of systemic drug administration.

RESULTS

Systemic sumatriptan elicited a dose- and time-related blockade of referred hypersensitivity in both models that was blocked by systemic administration of either 5HT1(B) or 5HT1(D) antagonists. Sumatriptan administered into the RVM similarly produced dose- and time-related blockade of referred hypersensitivity in both visceral pain models. This was blocked by local microinjection of the 5HT1(B) antagonist but not the 5HT1(D) antagonist. Microinjection of 5HT1(B) or 5HT1(D) antagonists into the RVM did not block the effects of systemic sumatriptan.

CONCLUSIONS

Our findings suggest that sumatriptan suppresses either inflammatory or noninflammatory visceral pain, most likely through peripheral 5HT1(B)/(D) receptors. Actions at 5HT1(B) receptors within the RVM offer an additional potential site of action for the modulation of visceral pain by triptans. These studies offer new insights into the development of strategies that may improve therapy of visceral pain conditions using already available medications.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

Emerging therapies for migraine

Migraine is a common disabling brain disorder that--considering its clinical and economic impact--is understudied and in need of additional management options. Currently, treatments are classified as preventive or acute-attack therapies, although it is expected that this distinction will become blurred over time. The gap-junction blocker tonabersat, an inducible nitric oxide synthase (NOS) inhibitor and botulinum toxin A are all being investigated in clinical trials as preventive therapies. Device-based approaches using neurostimulation of the occipital nerve have provided promising results, whereas the first study of patent foramen ovale closure for migraine prevention produced disappointing results. Calcitonin gene-related peptide receptor antagonists, vanilloid TRPV1 receptor antagonists and NOS inhibitors are all being investigated in clinical trials for acute migraine. There is much cause for optimism in this area of neurology and considerable benefit awaits our patients.

New Migraine and Pain Research

An understanding of the pathophysiology and pharmacology of migraine has been driven by astute clinical observations, elegant experimental medicine studies and importantly by the introduction of highly effective selective anti-migraine agents such as the Triptan 5-HT(1B/1D) agonists. New investigational migraine therapies such CGRP antagonists target key components of the trigeminal sensory neuroinflammatory response and show promise for the future. Cutting edge molecular profiling studies looking at gene expression during chronic pain are now being used to reveal the cell biology of pain and new potential therapeutic targets. Translational neuroimaging research can link the laboratory and the clinic and is now being used to help understand the neural systems biology of migraine. Research into migraine has generated sophisticated hypotheses that encompass primary CNS dysfunction, trigeminovascular activation, pain perception and activation of associated neural circuits involved in affective functions providing a rich framework within which to design and test future migraine treatment strategies.

Activation of 5-HT1B and 5-HT1D receptors in the rat nucleus tractus solitarius: opposing action on neurones that receive an excitatory vagal C-fibre afferent input

BACKGROUND AND PURPOSE

Central 5-HT-containing pathways are known to be important in cardiovascular regulation and a crucial area for this regulation is the nucleus tractus solitarius (NTS), which contains many of the known 5-HT receptor subtypes. In this study the role of 5-HT(1B) and 5-HT(1D) receptors, targets for the antimigraine drugs known collectively as triptans, was examined in the NTS. EXPERIMENT APPROACH: Extracellular recordings were made, in anaesthetized rats, from 109 NTS neurones that were excited by electrical stimulation of the vagus and drugs were applied ionophoretically to these neurones.

KEY RESULTS

The 5-HT(1B/1D) receptor agonist sumatriptan applied to 64 neurones produced a 64% reduction in the firing rate of 54 of these neurones. Ketanserin, a 5-HT(1D/2A) receptor antagonist, alone had little effect, but co-applied with sumatriptan significantly attenuated this inhibition, whilst co-application of the 5-HT(1B) receptor antagonist GR55562 resulted in potentiation of this inhibition. Sumatriptan also caused a 25% reduction in vagal afferent evoked activity as well as that caused by stimulation of cardiopulmonary afferents. In another 41 neurones the 5-HT(1B) receptor agonist CP-93 129 produced a doubling of the background firing rate in 31 of these neurones and a significant increase in both vagal afferent evoked activity and that evoked by cardiopulmonary afferent activation.

CONCLUSIONS AND IMPLICATIONS

Activation of 5-HT(1B) and 5-HT(1D) receptors have opposing actions on NTS neurones of excitation and inhibition, respectively. As both receptors are negatively coupled to adenylate cyclase this would indicate that they have different anatomical locations within NTS.

Serotonin receptor ligands: treatments of acute migraine and cluster headache

Fuelled by the development of the serotonin 5-HT(1B/1D) receptor agonists, the triptans, the last 15 years has seen an explosion of interest in the treatment of acute migraine and cluster headache. Sumatriptan was the first of these agonists, and it launched a wave of therapeutic advances. These medicines are effective and safe. Triptans were developed as cranial vasoconstrictors to mimic the desirable effects of serotonin, while avoiding its side-effects. It has subsequently been shown that the triptans' major action is neuronal, with both peripheral and central trigeminal inhibitory effects, as well as actions in the thalamus and at central modulatory sites, such as the periaqueductal grey matter. Further refinements may be possible as the 5-HT(1D) and 5-HT(1F) receptor agonists are explored. Serotonin receptor pharmacology has contributed much to the better management of patients with primary headache disorders.

New targets in acute migraine treatment

Migraine is a common and highly disabling neurological problem, whose acute treatment was revolutionized by the triptans, serotonin 5-HT1B/1D receptor agonists. Some patients do not respond to triptans, while others are not suitable for them largely because of contraindications based on vascular disease. The exploration of nonvasoconstrictor treatments for acute migraine offers the prospect of dramatic improvements in patient care, as well as important insights into the mechanisms of migraine. Possibilities for such developments include, calcitonin gene-related peptide receptor antagonists, serotonin 5-HT1F and 5-HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Taken together, the future for migraine and affected patients is bright and promising.