Why are tricyclic antidepressants used to prevent migraines?

[papillon]

I don't understand the role of serotonin in the vasodilation of scalp blood vessels that occurs with a migraine. For example, imitrex is an agonist for the 1B & 1D serotonin receptors to stop an ongoing migraine. And now I've seen that one of the medications used as a prophylactic for migraines is tricyclic antidepressants, which block norepinephrine and serotonin reuptake. Can anyone explain this mechanism?
Well, I thought that the major location of action of antidepressants was at neurons, not blood vesses.

[andrewbentley2001]

you've basically said it yourself-- the increase of serotonin helps to stabilize the dilation/constriction of the blood vessels in the head. What exactly is it that you're having trouble with? I'm not sure anyone understands it 100%, but your question seems to say all the basics of it.