Autoradiographic localization of 5-HT1E and 5-HT1F binding sites in rat brain: effect of serotonergic lesioning

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.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

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.

Distribution of 5-ht(1E) receptors in the mammalian brain and cerebral vasculature: an immunohistochemical and pharmacological study

BACKGROUND AND PURPOSE

The 5-ht(1E) receptor is highly expressed in the human brain and its structure is conserved in humans, suggesting an important physiological role for 5-ht(1E) receptors. However, neither the function nor the distribution of this receptor has been characterized in the mammalian brain.

EXPERIMENTAL APPROACH

Rats and mice lack the 5-ht(1E) receptor gene; thus, we used guinea pig brain tissue and immunofluorescent staining techniques to provide the first specific localization of 5-ht(1E) receptors in the mammalian brain.

KEY RESULTS

High levels of 5-ht(1E) receptors are detected in olfactory bulb glomeruli as well as the molecular layer of the dentate gyrus (DG). In DG membranes, BRL54443, a 5-ht(1E) /5-HT(1F) agonist, selectively stimulated 5-ht(1E) receptors and potently inhibited forskolin-dependent cAMP production (IC₅₀ = 14 nM). The staining pattern of 5-ht(1E) receptors in brain tissue suggests that this receptor is expressed predominantly in neurons rather than in glia. Additionally, 5-ht(1E) receptors were detected in the adventitial layer of cerebral arteries but not in the microvasculature, venous tissue or other brain arteries.

CONCLUSIONS AND IMPLICATIONS

These observations should help to predict clinical effects of 5-ht(1E) -selective drugs. For example, the stimulation of 5-ht(1E) receptors and subsequent inhibition of adenylate cyclase activity in the DG suggests that 5-ht(1E) receptors may mediate regulation of hippocampal activity by 5-HT, making it a possible drug target for the treatment of neuropsychiatric disorders characterized by memory deficits (such as Alzheimer's disease) or as a target for the treatment of temporal lobe epilepsy.

Guinea pig hippocampal 5-HT(1E) receptors: a tool for selective drug development

Recent studies have indicated that the serotonin [5-hydroxytryptamine (5-HT)] 1E receptor, originally discovered in human brain tissue, is not expressed in rat or mouse brain. Thus, there have been few reports on 5-HT(1E) receptor drug development. However, expression of 5-HT(1E) receptor mRNA has been shown in guinea pig brain. To establish this species as an animal model for 5-HT(1E) drug development, we identified brain regions that exhibit 5-carboxyamidotryptamine, ritanserin, and LY344864 - insensitive [(3)H]5-HT binding (characteristic of the 5-HT(1E) receptor). In hippocampal homogenates, where 5-HT(1E) receptor density was sufficiently high for radioligand binding analysis, 100 nM 5-carboxyamidotryptamine, 30 nM ritanserin, and 100 nM LY344864 were used to mask [(3)H]5-HT binding at non-5-HT(1E) receptors. The K(d) of [(3)H]5-HT was 5.7 +/- 0.7 nM and is indistinguishable from the cloned receptor K(d) of 6.5 +/- 0.6 nM. The affinities of 16 drugs for the cloned and hippocampal-expressed guinea pig 5-HT(1E) receptors are essentially identical (R(2) = 0.97). These findings indicate that using these conditions autoradiographical distribution and signal transduction studies of the 5-HT(1E) receptor in guinea pig brain are feasible. Using the guinea pig as an animal model should provide important insights into possible functions of this receptor and the therapeutic potential of selective human 5-HT(1E) drugs.

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.

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.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.

New targets in the acute treatment of headache

PURPOSE OF REVIEW

The aim of this article is to review recently identified targets for the acute treatment of primary headache disorders.

RECENT FINDINGS

Calcitonin gene-related peptide (CGRP) receptor blockade has been shown to be an effective acute anti-migraine strategy and is a non-vasoconstrictor in terms of the mechanism of action. It is likely that direct blockade of CGRP release by inhibition of trigeminal nerves would be similarly effective in both migraine and cluster headache. Options for acute treatment based on preclinical work and initial clinical studies include: serotonin 5HT1F and 5HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Proof of principle studies with octreotide, a somatostatin receptor agonist, demonstrated it to be better than placebo in the acute treatment of cluster headache but not in the acute management of migraine.

SUMMARY

The prospect of a non-vasoconstrictor acute migraine therapy offers a real opportunity to patients, and perhaps more importantly, provides a therapeutic rationale to plant migraine and cluster headache firmly in the brain as neurological problems.

[3H]LY334370, a novel radioligand for the 5-HT1F receptor. II. Autoradiographic localization in rat, guinea pig, monkey and human brain

LY334370 is a high affinity, selective agonist at the 5-HT(1F) receptor. On this basis, the tritiated compound was examined for its utility in autoradiography to localize the 5-HT(1F) receptor in rat and guinea pig brain regions. Specific 5-HT(1F) receptor binding in rat brain was found in layers 4-5 of all cortical regions examined, as well as olfactory bulb and tubercle, nucleus accumbens, caudate putamen, parafascicular nucleus of the thalamus, medial mammillary nucleus, the CA3 region of the hippocampus, subiculum, and several amygdaloid nuclei. In guinea pig brain, the [(3)H]LY334370 binding sites were found at highest density in claustrum, but also in a layer of the cortex, caudate putamen, nucleus accumbens, thalamus, and medial mammillary nucleus. Some species differences in the distribution of the 5-HT(1F) receptor were noted. Side by side comparison of rat brain autoradiography with [(3)H]LY334370 and [(3)H]sumatriptan showed labeling in the same brain regions. Preliminary binding studies in rhesus monkey and human brain sections showed [(3)H]LY334370 binding in cortical layers 4-5, subiculum (in the monkey), and the granule cell layer of the cerebellum. These findings suggest a discrete localization of the 5-HT(1F) receptor in the rat, guinea pig, monkey and human brain, and confirms the utility of [(3)H]LY334370 as a potential tool to explore further the localization and possible functions of the 5-HT(1F) receptor.

Post-triptan era for the treatment of acute migraine

There now is one realized and several attractive targets for the treatment of acute attacks of migraine that will follow and augment the use of serotonin 5-HT1B/1D receptor agonists, the triptans. Calcitonin gene-related peptide (CGRP) receptor blockade recently has been shown to be an effective acute antimigraine strategy; therefore, blockade of CGRP release by inhibition of trigeminal nerves would seem a logical approach. A number of targets are reviewed in this article including serotonin 5-HT1F and 5-HT1D receptors, adenosine A1 receptors, nociceptin, vanilloid TRPV1 receptors, and anandamide CB1 receptors. Development of one or more such compound offers the exciting prospect of new non-vasoconstrictor treatments for migraine and cluster headache.

Molecular cloning and pharmacological characterization of the guinea pig 5-HT1E receptor

The human 5-HT(1E) receptor gene was cloned more than a decade ago. Little is known about its function, and there have been no reports of its existence in the genome of small laboratory animals. In this study, attempts to clone the 5-HT(1E) gene from the rat and mouse were unsuccessful. In fact, a search of the mouse genome database revealed that the 5-HT(1E) receptor gene is missing from the mouse genome. However, the 5-HT(1E) gene was cloned from guinea pig genomic DNA and was characterized. The guinea pig 5-HT(1E) receptor gene encodes a protein of 365 amino acids. It shares 88% (nucleic acid) and 95% (amino acid) homology with the human receptor. The guinea pig 5-HT(1E) receptor showed similar pharmacology to the human 5-HT(1E) receptor in radioligand binding assays. Serotonin (5-hydroxytryptamine, 5-HT) dose-dependently stimulated [35S]GTPgammaS binding to the guinea pig 5-HT(1E) receptor with an EC(50) of 13.6+/-1.92 nM, similar to that of the human 5-HT(1E) receptor (13.7+/-1.78 nM). Activation of the guinea pig 5-HT(1E) receptor was also achieved by ergonovine, alpha-methyl-5-HT, 1-naphthylpiperazine, methysergide, tryptamine, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Methiothepin exhibited antagonist activity. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis showed that 5-HT(1E) mRNA was present in the guinea pig brain with the greatest abundance in the hippocampus, followed by the olfactory bulb. Lower levels were detected in the cortex, thalamus, pons, hypothalamus, midbrain, striatum, and cerebellum. Our current study marks the first identification of the 5-HT(1E) receptor gene in a commonly used laboratory animal species. This finding should allow the elucidation of the receptor's role(s) in the complex coordination of central serotonergic effects.

Effects of acute or chronic administration of anti-migraine drugs sumatriptan and zolmitriptan on serotonin synthesis in the rat brain

Triptans are 5-HT1 receptor agonists used as anti-migraine drugs. They act primarily on meningeal blood vessels and on trigeminovascular afferents, but they may also exert central effects. We studied the regional effects of acute and chronic treatment with sumatriptan or zolmitriptan on the rate of serotonin (5-HT) synthesis in the rat brain, using the alpha-14C-methyl-L-tryptophan quantitative autoradiographic method. Sumatriptan at low (300 microg/kg, s.c.) and high (1 mg/kg) doses, as well as zolmitriptan (100 microg/kg), acutely decreased (15-40%, P < 0.05-0.001) 5-HT synthetic rate in many brain regions, including the dorsal raphe nucleus. Chronically, sumatriptan (21 days, approximately 300 microg/kg per day via osmotic minipumps) induced significant increases in the 5-HT synthesis rate in many projection areas but had no effect in the dorsal raphe nucleus. The acute effects on 5-HT synthesis rate would be compatible with activation of 5-HT1 autoreceptors that inhibit serotonin release. In contrast, the increased 5-HT synthesis rate observed after chronic sumatriptan might possibly result from a down-regulation/desensitization of 5-HT1 receptors and/or unmasking of excitatory triptan-sensitive 5-HT receptors. Overall, the present findings indicate that not only zolmitriptan but also sumatriptan affect brain serotonergic neurotransmission.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Multiple 5-HT(1) autoreceptor subtypes govern serotonin release in dorsal and median raphé nuclei

The present study investigated the possibility of multiple 5-HT(1) autoreceptor subtypes in the rostral raphé nuclei. Slices (350 microm) of rat dorsal or median raphé nucleus (DRN/MRN) were taken from male Wistar rats and superfused with artificial cerebrospinal fluid at 32 degrees C. Fast cyclic voltammetry at carbon fibre microelectrodes was used to monitor serotonin (5-HT) release following local electrical stimulation. In both DRN and MRN, 5-HT release on short trains was reduced by the selective 5-HT(1A) agonist 8-OH-DPAT (1 microM), an effect blocked by the selective 5-HT(1A) antagonist WAY 100635 (0.1 microM) but not by SB 216641 (0.05 and 0.2 microM) or BRL 15572 (0.5 microM), selective antagonists at the 5-HT(1B) and 5-HT(1D) receptors respectively. The selective 5-HT(1B) agonist CP 93129 (0.3 microM) also reduced 5-HT release in both nuclei. Its effect was blocked by SB 216641 but not by WAY 100635 or BRL 15572. The 5-HT(1D/1B) agonist sumatriptan (0.5 microM) decreased 5-HT release in both DRN and MRN. In DRN, the effect of sumatriptan was blocked by BRL 15572 but not by WAY 100635 or SB 216641. In MRN, the effect of sumatriptan was not blocked by any of the above antagonists. BRL 15572 increased 5-HT release on long stimulations in DRN and MRN while WAY 100635 had no effect. SB 216641 increased 5-HT release in MRN but not DRN. WAY 100635 potentiated the effect of SB 216641 in DRN but not MRN. The data suggest that 5-HT release in DRN is controlled by 5-HT(1A), 5-HT(1B) and 5-HT(1D) autoreceptors. 5-HT release in MRN is controlled by 5-HT(1A) and 5-HT(1B) autoreceptors and another, as yet unidentified mechanism.

The pharmacology of headache

Headache is a common problem which besets most of us at some time or the other. The pharmacology of headache is complex in an overall sense but can be understood in terms of the anatomy and physiology of the pain-producing structures. Migraine can be used as a template to understand the activation of nociceptive systems in the head and thus their neurotransmitter mediation and modulation. In recent years, the role of serotonin (5-HT) in headache pharmacology has been unravelled in the context of both understanding its role in the nociceptive systems related to headache and by exploiting its 5-HT1 receptor subtypes in headache therapeutics. The pharmacology of the head pain systems, as they are known and as they might evolve, are explored in the context of both, the anatomy and physiology of trigeminovascular nociception and in the context of clinical questions, such as those of efficacy, headache recurrence and adverse events.

Serotonin 5-HT(1B/1D) agonist-stimulated [(35)S]GTPgammaS binding in rat and guinea pig striatal membranes

Serotonin (5-hydroxytryptamine; 5-HT) receptor ligands were used to assess agonist-stimulated [(35)S]GTPgammaS binding in rat and guinea pig striatal membranes. The assay contained 45-60 microgram protein, 300 microM GDP and 0.1 nM [(35)S]GTPgammaS, incubated at 37 degrees C for 20 min. The non-selective agonists 5-HT, 5-CT (5-carboxyamidotryptamine), and L-694,247, and the selective 5-HT(1B) receptor agonist CP 93,129 produced concentration-dependent increases in [(35)S]GTPgammaS binding in rat striatum, whereas the selective 5-HT(1A) receptor agonist R(+)-8-OH-DPAT [R(+)-8-hydroxy-2-(di-n-propylamino)tetralin] was inactive. Cyanopindolol, a 5-HT(1A/1B) receptor antagonist, completely blocked the effect of 5-HT. Methiothepin, yohimbine and cyanopindolol also blocked 5-CT-stimulated [(35)S]GTPgammaS binding with the following rank order of potency: cyanopindolol >/= methiothepin >>> yohimbine, consistent with rat 5-HT(1B) receptor pharmacology. Neither cyanopindolol nor methiothepin altered basal [(35)S]GTPgammaS binding by themselves while yohimbine had weak partial agonist activity. Furthermore, cyanopindolol shifted the 5-CT concentration-response curve rightward, increasing the EC(50) and decreasing the maximal response, but did not affect L-694, 247-stimulated [(35)S]GTPgammaS binding. The ability of cyanopindolol or spiperone (a 5-HT(1A/1D) receptor antagonist) to alter CP 93,129-stimulated [(35)S]GTPgammaS binding was determined. Cyanopindolol produced a rightward shift in the CP 93,129 concentration-response curve, while spiperone had no affect. Finally, in guinea pig striatum and hippocampus, L-694,247 produced a concentration dependent increase in [(35)S]GTPgammaS binding. In conclusion, these studies indicate that 5-HT(1B) receptor function can be assessed using agonist-stimulated [(35)S]GTPgammaS binding in rat striatal membranes using CP 93,129, 5-HT or 5-CT, but not L-694, 247.