Summary
Migraine is a chronic condition of recurrent, throbbing headache generally felt on one side of the head but can also switch from one side to another. Migraines usually begin in early childhood adolescence or young adult life. The headache is characteristically accompanied by nausea, vomiting or loss of appetite. Activity, bright light or loud noises may make the headache worse, so migraineurs often seek out cool, dark, quiet rooms. Most migraine attacks are lasting from four to 12 hours, although shorter or much longer headaches can occur. It interferes with the physical ability to function, sometimes requiring bed rest. Although there are several kinds of migraines, the most common are classic migraine - a migraine with aura - and common migraine, which has no aura. Migraine attack consists of four phases, namely prodrome phase, aura phase, headache phase, and postdrome phase. Despite all the suggested hypotheses of migraine etiology, it remains unknown. Migraine headache is believed to be the result of abnormal activity in the brain that leads to dilation of the blood vessels on the surface of the brain as well as the tissues that surround the brain. The dilation of the blood vessels is believed to be associated with inflammation mechanism. This cerebral/ cranial vasodilation is referred to as “neurogenic inflammation”. Unfortunately, more is known about the factors involved in the pathophysiology of migraine headache pain after CNS dysfunction, and not before. In general, the mechanism of migraine attack is believed to consist of four steps after CNS dysfunction: 1) A local vasodilatation of the intracranial extracerebral blood vessels / meningeal blood vessel promoted by specific triggers. 2) Stimulation of pain pathways of the surrounding trigeminal sensory nervous through sensory nerve discharge causing pain impulses to be transmitted to caudal brain stem nuclei 3) Increased pain response by neuroinflammation process and release of the vasoactive neuropeptides (CGRP, NK1, and SP). 4) Transportation of pain signals to higher centers where headache pain is recognized due to trigeminal nerves activation. This in respect, enforces scientists to test variant sorts of anti- migraines to inhibit the migraine attack in one of the four mentioned steps. Treatment of migraine could be prophylactic or abortive. Prophylactic anti- migraines are usually used to reduce the frequencies of headache attacks. They are mostly non- selective drugs and their action mechanism in migraine is still unknown, which leads to undesired side effects. Abortive anti- migraines are used to prevent or reduce headache attack in migraine. Based on the pathophysiology of migraine, scientists believed that the neurotransmitter serotonin (5-HT) is involved in the initiation of the pain in migraine. These evidence includes the reduction of urinary serotonin and elevation of its major metabolite 5-HIAA during a migraine attack, the sudden and rapid drop of platelet serotonin in addition to the relieve effect of 5-HT1 agonists. The triptan class of drugs, including sumatriptan, zolmitriptan, eletriptan, naratriptan, rizatriptan, almotriptan, and frovatriptan constricts the blood vessels by affecting on 5-HT1B/1D receptors. All triptans are 5-HT1B/1D agonists, they prevent or reduce migraine headache and are very effective in relieving migraine. However, they do not prevent or reduce the number of attacks of migraine.
In the last ten years, new studies were done to understand the etiology of migraine. The new discovery of mutations in the calcium channel gene CACNA1A in migraineurs with familial hemiplegic migraine (FHM) gives link to suggest that migraine (with and without aura) is caused by ion channel abnormalities. This suggestion is recently supported by identifying a mutation in the potassium channel encoding gene KCNN3. This gene plays a critical role in determining the firing outline of neurons and acts to regulate intracellular calcium channels. This potassium channel gene KCNN3 my thus be of pathophysiological importance in migraine (with and without aura) and in the near future migraine treatments.
Chapter 1: Targets for drugs action
1.1 Receptors
G- protein- coupled receptors (GPCRs)
1.2 Ion channels
Ligand- gated ion channels
Voltage- gated ion channels
1.3 Enzymes
1.4 Carrier molecules
A drug is a chemical that affects physiological function in a certain way. Since the 1970s, scientists discovered that different types of drugs seem to interact in some specificity with different types of target proteins in the mammalian cells in a way that might modulate their functions. In general, drugs act on four target proteins in the mammalian cells (figure 1), namely: 16
- Receptors
- Ion channels
- Enzymes
- Carrier molecules (Transporters)
In this section, we shall focus only on the receptors and ion channels as main targets for drug actions related to migraine.
1.1 Receptors
A receptor is a protein molecule that can span the cellular membrane region to the extracellular environment. This protein contains an area that faces the outside of the cell in which a binding site exists that receives incoming messengers.12 These receptors binding sites are usually the analogous to the active site of an enzyme. Considering drugs as chemical messengers and neurotransmitters that could fit the binding site and “switch on” the receptor molecule, it will deliver the message and lead to a biological effect. Subsequently, other components will be involved in the transmission of the message, namely the ion channels and membrane- bound enzymes.12 The drug- receptor interaction that brings about the conformational change is a combination of lipophilic attraction, ionic bonding (acid/base) and hydrogen bonding. Thus, the effectiveness (efficacy) of a drug to 'switch on' the biochemical steps is dependent upon these lipophilic and polar interactions; the 'fit' (affinity) of the compound into the receptor pocket is dependant upon its three dimensional shape. Usually, the pharmacological effects of a drug are based on its chemical structure. Generally, a compound with good affinity and good efficacy will be a full agonist and give the maximum effect at a very low dose. A compound with good affinity and no efficacy will be an antagonist, i.e. it will elicit its desired pharmacological effect by blocking endogenous mediators. On the other hand, some drugs are partial agonists and their overall biological effects are a combination of their affinity and their efficacy.121,122,16
Depending on the structure and the transductions mechanism, there are four receptor types in human bodies:17
- Ion channel receptors (Ionotropic receptors)
- The G- protein- coupled receptors (GPCRs)
- Kinas- linked receptors
- Nuclear receptors.
Only the first two receptors are involved in the migraine attack, which we shall discuss extensively in the next sections.
G- Protein coupled receptors (GPCRs)
These metabotropic receptors consist of 7-TM (transmembrane), and have regions exposed both to the outside and the inside of the cell. In general, this 7-TM, consists of two binding domains and one G- protein coupling domain (figure 2).The protein chain winds back and forth through the cell membrane seven times. Each of the seven transmembrane sections is hydrophobic and helical in shape. The N-terminal chain is to the exterior of the cell and is variable in the length depending on the receptor. Transmembrane domains are connected by extracellular loops (EI-III) and intracellular loops (CI-III). 16,17,19
For many years, it is well known that these receptors couple to intracellular effectors system and have an influence on the biological activities in our bodies. 12,16 Each GPCR is involved in a different process depending on the coupled chemical messenger (neurotransmitters/ hormones). Some chemical messengers are simple in structure, such as monoamines (dopamine, histamine, serotonin, acetylcholine, noradrenaline), others are more complicated, such as nucleotides, lipids, neuropeptides, peptide hormones, protein hormones, glycoprotein hormones, glutamate, etc.12 Until now, there are many receptors discovered in the mammalians; such as NMDA receptors, AMPA receptors, ACh receptors, GABA receptors, 5-HT receptors, and Purinergic receptors. Some of these receptors were drug targets before being discovered, others became more interesting after the discovery of their molecular pharmacological importance. Interestingly, the rate of message transmission via GPCRs is varying. This process is very rapid when it involves the synaptic transmission (milliseconds), while it has mediated effects when it involves hormones of steroid or thyroid which needs hours or days.17 Most of the G- protein coupled receptors (GPCRs) have at least two conformational states, the inactive- (R) and the active- state (R*), which exist in equilibrium (figure 3). To promote the activity of these receptors agonists are used, while inverse agonist might reduce this activity shifting the equilibrium towards (R). An antagonist has no effect on the equilibrium, it reduces both by competition with other binding ligands. Lately, scientists discovered that some of the GPCRs are signalling even in the absence of any ligand, the so- called constitutively active GPCRs.122
It is important here to elucidate briefly the regulation of enzyme coupling and the signal transduction pathways in GPCRs. Each G-protein consists of a complex of three subunits, namely β- γ, and a. The signal transduction stages depend on the type of G-proteins e.g. Gi- inhibitory, or Gs- stimulatory, depending on the coupling to the a- subunit. Different a- subunits have different targets and different effects. To date, there are four identified a- subunits, namely as, ai, aq, and ao. [i] When a ligand binds to Gs receptor, the receptor will bind internally to β- γ complex of the Gs protein leaving αs to bind to the target enzyme and activate it. Conversely, when a ligand binds to an inhibitory receptor it leads to inactivation of the target enzyme (figure 4).16,17,19
The main targets for G- proteins are enzymes and ion channels such us adenylate cyclase (AC), phospholipase C (PC), cGMP phosphodiesterase, and K+ and Ca+2 ion channels. These targets enable GPCRs to modulate the cellular functions by producing the “second messengers” that has cAMP, IP3, and DAG inside the cell. cAMP regulates the energy metabolism by activation of protein kinase A (PKA), while IP3 regulates the ion channels by releasing of Ca+2, and DAG activates protein kinase C (PKC) in the phosphatidylinositol (PI) cycle (figure 5).16,17
In this thesis we shall not consider all these pathways in details. Instead, we shall focus on one pathway, namely the activation of adenylate cyclase due to its importance in migraine treatments. When PKA is activated by cAMP, it will catalyse the phosphorylation of proteins and enzymes at their serine and threonine residues. This will change the shape of the enzyme leading to the exposure or closure of the active site and hence will activate the enzyme to perform a specific function, e.g. nuclear phosphorylation targets include a group of transcription factors that modulate the expression of cAMP- responsive genes. These factors form a family of both activators and repressors that bind as homo- and heterodimers to cAMP- responsive elements (CREs). They belong to the basic leucine zipper (bZip) class of transcription factors, and their function is tightly regulated by phosphorylation. Constitutively expressed factors such as CREB (CRE- binding protein) are phosphorylated by PKA and thereby converted into transcriptional activators.135 In fat cells PKA activates enzymes and catalyse the breakdown of fat. Alternatively, it might have a deactivating effect, which increase lipolysis, decrease the glycogen synthesis, and eventually increasing the break down of glycogen.17
1.2 Ion channels
Definitely, many cellular functions require the passage of ions and other hydrophilic molecules across the plasma membrane. Ion channels affect the function of the neurotransmitters, cardiac conductions, and muscle contractions through the regulation of ion exchange cross the cell.19,21 Recently, experience with DNA sequence linked many diseases to defects in ion channels “channelopathies”. Plasma membrane ion channels are often expressed at relatively low concentrations in specific cells and tissues, which make them easy accessible targets for drug design.23 Basically, there are two groups of ion channels, non- gated and gated channels26. Non- gated or leakage channels are open even in the unstimulated state. In contrast, gated ion channels are activated / inactivated in response to specific stimuli. There are two types of gated channels:19,21,26,56
- Ligand- gated ion channels
- Voltage- gated ion channels
Each type has a different mechanism and activation/ inactivation function regarding its ion selectivity.21 However, they all share some structural similarity. They all tend to be tube- like macromolecules, which are made of a number of protein subunits that are designed to form water- filled pores pass through the plasma membrane, and are able to switch between open and closed states.19,21 Usually, the ligand- binding domain is extracellular (within the channel), or intracellular.21 The selectivity of these ion channels is strongly dependent on the size of the pore and the nature of its lining.19
Ligand- gated ion channels
These receptors are able to regulate the ion channels functions, and are also known as ion channels receptors and ionotropic receptors.17 They are part of five- protein ion channel structure including a ligand- binding site. They form the extracellular domain of the proteins
There are three known kinds of ionotropic receptors according to the form of their transmembrane; 2- TM, 3- TM, and 4- TM receptors constitute of four to five subunits.17 One of the major differences between these three ionotropic receptors is the location of the N- and C- terminal chain (figure 6). 17 There are two kinds of ion channels; cationic ion channels and anionic ion channels. Some of the cationic channels are non- selective, while others select only one of the following cations Na+, K+, or Ca2+. Anionic channels are primarily permeable to Cl-.17,21 The cationic channels are usually controlled by nACh (4- TM) receptors, the 5HT3 (4- TM) receptors, L- glutamate (3- TM) receptors, and ATP (2- TM) receptors, while the anionic ion channels are controlled by the GABAA and glycine receptors.11,17 When the cationic ion channels are opened, depolarization of the cell takes place and nerve stimulation occur. In contrast, when the anionic channels get opened, they will have an inhibitory effect on the cells.17,16 Interestingly, the selectivity of the ion channels also depends on the amino acids of these ion channels. When a mutation in these amino acids occurs, the selectivity of the ion channels might be changed from cationic to anionic and vice versa, resulting in dysregulation of cellular function.17