Interesting to note that he is saying that the primary mode of action is NOT as a vasoconstrictor but on its effect on the central nervous system. Doesn't change our appreciation of this class of meds but suggests we need to change how we think about the nature of CH.
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Handb Exp Pharmacol. 2007;(177):129-43.   


Serotonin receptor ligands: treatments of acute migraine and cluster headache.


Goadsby PJ.

Institute of Neurology, Queen Square, London WC1N 3BG, UK. peterg@ion.ucl.ac.uk

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.

PMID: 17087122 [PubMed]
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J Clin Neurosci. 2010 Mar 11.

What has functional neuroimaging done for primary headache ... and for the clinical neurologist?
Sprenger T, Goadsby PJ.

UCSF Headache Centre, Department of Neurology, University of California, 1701 Divisadero St, Suite 480, San Francisco, CA 94115, USA.

Our understanding of mechanisms involved in primary headache syndromes has been substantially advanced using functional neuroimaging.

THE DATA HAVE HELPED ESTABLISH THE NOW-PREVAILING VIEW OF PRIMARY HEADACHE SYNDROMES, SUCH AS MIGRAINE AND CLUSTER HEADACHE, AS BRAIN DISORDERS WITH NEUROVASCULAR MANIFESTATIONS, NOT AS DISORDERS OF BLOOD VESSELS.

PMID: 20227279 [PubMed]
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A bit complex and, while it says we understand where the source of Clusters appear to be, there is still no clear understanding of WHY this changes in our brain occur.

Bob G: My understanding is that dilation as the source of pain was the accepted explanation for some years only to be displaced when the role of the hypothalamus came to be seen as the central causal site in Cluster.

This does not deny that blood vessels dilate but that this is secondary to the primary activity of nerves as the source of pain.
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Headache:lessons learned from functional imaging
British Medical Bulletin 2003; 65: 223-234

Arne May
Department of Neurology, University of Regensburg, Regensburg, Germany

Using PET in a larger patient series, significant activations ascribable to the acute cluster headache were observed in the ipsilateral hypothalamic grey matter when compared to the headache-free state44. This highly significant activation was not seen in cluster headache patients out of the bout when compared to the patients experiencing an acute cluster headache attack45. In contrast to migraine25, no brainstem activation was found during the acute attack compared to the resting state. This is remarkable, as migraine and cluster headache are often discussed as related disorders and identical specific compounds, such as ergotamine and sumatriptan, are currently used in the acute treatment of both types of headache46. These data suggest that while primary headaches such as migraine and cluster headache may share a common pain pathway, the trigeminovascular innervation, the underlying pathogenesis differs significantly as might be inferred from the different patterns of presentation and responses to preventative agents46.
Just as it is striking that no brainstem activation occurs in contrast to acute migraine, no hypothalamic activation was seen in experimental pain induced by capsaicin injection into the forehead47. This is important because injection of the forehead would activate first (ophthalmic) division afferents which are the trigeminal division predominantly responsible for pain activation in cluster headache. Thus two other types of first division of trigeminal nerve pain, while sharing neuro-anatomical pathways with cluster headache, do not give rise to


VASCULAR HEADACHE: ARE BLOOD VESSELS INVOLVED?

Taking these observations on acute cluster headache together with what has been observed in experimental head-pain and migraine, the data establish that migraine and CLUSTER HEADACHE, FAR FROM BEING PRIMARILY VASCULAR DISORDERS, ARE CONDITIONS WHOSE GENESIS IS TO BE FOUND IN THE CENTRAL NERVOUS SYSTEM IN PACEMAKER OR CIRCADIAN REGIONS SPECIFIC TO THE SYNDROME. If further studies confirm these findings, a better understanding will be gained of where and how acute and preventative therapy can be targeted.
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(And more complex yet....)

Ital J Neurol Sci. 1996 Apr;17(2):179.

Curr Pain Headache Rep. 2009 Apr;13(2):155-9.
The electrophysiology of cluster headache.
Holle D, Obermann M, Katsarava Z.
SourceDepartment of Neurology, University of Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany. dagny.holle@uk-essen.de

Abstract
Cluster headache (CH) is a neurovascular headache disease characterized by recurrent, strictly unilateral, severe pain attacks. Despite its typical clinical features, including circadian rhythm of the attacks and ipsilateral autonomic dysfunction, the underlying pathophysiology of CH is still unclear. Electrophysiological data point to central disinhibition of the trigeminal nociceptive system as one of the key mechanisms of CH pain. Therefore, altered habituation pattern and changes within trigeminal-facial neuronal circuits due to central sensitization seem to be involved. One biochemical correlate is probably represented in dysfunctions of serotonergic raphe nuclei-hypothalamic pathways. STRUCTURAL AND FUNCTIONAL IMAGING DATA SHOW AN ALTERATION OF HYPOTHALAMIC STRUCTURES IN CH PATIENTS, SUPPORTING THE HYPOTHESIS THAT THE HYPOTHALAMUS, ACCORDING TO ITS FUNCTION AS A CIRCADIAN PACEMAKER, PLAYS A PIVOTAL ROLE IN CH PATHOLOGY. Cortical and brainstem reflexes are reviewed to illuminate the pathophysiology of CH.

PMID:19272282[PubMed

Ok, well it's tough to jump in with people responding who actually know what they are talking about, so my response comes from a place of total medical ignorance.

I always felt that it had to do with blood flow and the reason for the varying shadow symptoms and pain locations was the result of where in the head it was occuring.