What happens in a migraine attack is that the platelets clump together and release serotonin when it is not needed. During the first phase of a migraine attack something makes the platelets in the blood clump together – they do not actually form clots of course, but they do release large amounts of serotonin. The serotonin makes the blood vessels constrict and so reduces the blood-flow to the brain. The body has control mechanisms that counteract the effect of the serotonin, but when these come into play they cause a violent swing in the opposite direction. The blood vessels in the brain open out too much and this brings on the throbbing pain that is a feature of the second phase of the attack. Pressure on certain parts of the brain might produce the feelings of nausea.
But what makes the platelets clump together in the first place? The answer to this question may be two vasoactive amines, known as tyramine and phenylethylamine, that are found in certain foods. These substances are usually broken down by the platelets, which produce various enzymes for the purpose. Doctors suspect that some people with migraine do not produce enough of these enzymes – or produce defective versions of them. So if they eat foods that are rich in these substances they cannot break them down fast enough – and platelet clumping follows. The foods that are rich in these amines are well known as migraine triggers – principally chocolate, cheese and red wine.
However, studies of the enzymes involved in breaking down tyramine and phenylethylamine have produced rather puzzling results. Many migraine patients are defective for one of the enzymes involved, called a monoamine oxidase. But those who are sensitive to triggers such as chocolate or cheese are no more defective than other migraine patients. If tyramine and phenylethylamine are involved in migraine (and it is by no means certain that they are) then the underlying enzyme deficiencies are obviously not simple ones. For more on enzyme deficiencies in migraine.
There must be other factors that can make the platelets clump together, because it is only a minority of patients whose migraines are triggered by these foods. The hormone adrenaline is known to have similar effects on platelets, and other substances produced by the body may act in this way. Presumably the migraine-sufferer produces too much of these substances, or lacks the control mechanisms that normally keep them in check.
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