The major outcome of the present study is that subcutaneous administration of sumatriptan did not change the levels of circulating CGRP in the intra or extracranial circulation in healthy volunteers.
Origin and physiological role of circulating CGRP
The origin of circulating CGRP is not known in details, but in rats, the major part of circulating CGRP is released from perivascular nerve terminals [
25] and CGRP in primary afferents can be released both peripherally and centrally [
26,
27]. In rats, the 5-HT
1D receptors are found on trigeminal nociceptors [
28] and colocalized with CGRP in trigeminal ganglion neurons [
29]. CGRP might modulate nociceptive transmission [
30] and nociceptor function depends on the sensitivity to CGRP [
31].
CGRP is a potent vasodilator [
32] in both cerebral and extracerebral arteries [
33] and CGRP is broadly distributed in the nervous system [
34] including the trigeminovascular system [
35].
In the resting state, CGRP is present in plasma in both healthy volunteers [
20]. This could be due to leakage after localized release rather than to specific systemic function [
36], and CGRP does not affect the resting vascular tone [
37]. In pathological settings CGRP might, however, play a role in maintaining vascular tone [
38]. This was reported for subarachnoid haemorrhage [
39].
Triptans and CGRP
Stimulation of the trigeminal system causes releases CGRP in vitro [
26] resulting in cranial vasodilatation [
10] and sensory activation [
40].
CGRP is increased in external jugular blood during cluster headache [
12]. Migraine attacks and trigeminovascular activation have also been reported to cause a CGRP increase in internal [
13] and external jugular blood [
3], but not in peripheral blood, which could suggest a localized activity in cranial vascular afferent pathways [
3]. A larger study could not reproduce the ictal CGRP increase [
41], and increased CGRP concentration in migraineurs interictally [
42] also obscures the exact role of CGRP in the pathophysiology of primary headaches.
Triptans may lower the CGRP concentration in animals [
26,
43] and in nitric oxide induced headache [
44] and spontaneous migraine attacks in humans [
45]. The antimigraine effect of the triptans might therefore be due to inhibited release of CGRP [
3]. This effect might be mediated via the 5HT
1-receptor, because antagonists to this receptor blocks the CGRP attenuating effect of sumatriptan [
43]. The CGRP decrease was, however, present only in patients whose headache improved [
10,
44]. It is therefore possible that CGRP levels may decrease spontaneously during migraine attacks [
13].
In the present study, we report that sumatriptan does not affect CGRP levels in humans under baseline conditions. This is similar to in vitro results in cultured trigeminal neurons where sumatriptan decreased CGRP secretion from chemically stimulated sensory neurons, but not the basal secretion rate [
11]. The concentration of sumatriptan required for this in vitro inhibition was higher than the estimated sumatriptan plasma concentration in patients, and the cell culture showed a higher rate of CGRP-positive cells than seen in vivo [
11]. Another drug, topiramate, used for migraine prophylaxis, had also no effect on basal CGRP released from trigeminal neurons, but did decrease CGRP secretion from stimulated trigeminal neurons [
46].
The CGRP response to antimigraine drugs could, therefore, rely on activity in the trigeminovascular system. The kinase–phosphatase balance might control CGRP secretion and this balance might be altered during trigeminal activation [
11]. Phosphatase activity is increased by sumatriptan, and phosphatase inhibition blocks the inhibitory effect of sumatriptan on stimulated CGRP release [
11]. If sumatriptan affects the phosphatases only in activated trigeminal neurons, it could explain the apparent lack of effect in this study, with normal subjects.
In an animal study, triptans effectively prevented the induction of sensitization in central trigemino vascular neurons but not in meningeal nociceptors [
47], which could indicate that triptans act on presynaptic 5-HT
1B/1D receptors on the central terminals of meningeal nociceptors. Furthermore, it has been reported that the effects of CGRP in the meninges, including meningeal vasodilatation, are insufficient to activate or sensitize meningeal nociceptors [
48]. This points toward a central site of action for the migraine-promoting role of CGRP. Triptans might, however, bind to presynaptic 5-HT
1B/1D receptors on trigeminal sensory fibers or trigeminal ganglion cells and inhibit nerve activity and hence reduce peripheral CGRP release [
49].
Triptans might also interfere with CGRP signaling more centrally [
47,
50,
51], but sumatriptan is hydrophilic and does not easily penetrate the blood brain barrier (BBB) [
52]. After BBB disruption, however, sumatriptan alters trigeminal evoked activity in animal studies [
53]. If CGRP is mainly released from intra cerebral sources, sumatriptan will inhibit such a release only if the BBB is disrupted [
54] which might be the case during migraine attacks [
55]. In the present study, the BBB would prevent sumatriptan from reaching the central sites of CGRP release. Clinical efficacy of triptans are however, not directly linked to lipophilicity [
7] and the lacking brain penetration of sumatriptan can therefore only partly explain the lack of effect of sumatriptan in the present study. Sumatriptan had pronounced effect on blood pressure (Fig.
3), an effect comparable with earlier results [
28] indicating that the drug was present in relevant doses.
Does sumatriptan have specific intracranial effects on CGRP?
Triptans cause greater vasoconstriction in cranial arteries compared to coronary vessels [
56], and may thus be selective vasoconstrictors of cerebral arteries [
57]. Sumatriptan causes vasoconstriction of the dural vessels [
58], but acts only as a weak vasoconstrictor of the superficial temporal artery [
59]. This could reflect a different action of sumatriptan on intra and extracerebral vessels.
It has been suggested that blood from the internal jugular vein specifically reflects intracranial neurotransmitter physiology, whereas the external jugular blood can express neurotransmitter and biochemical variations in both intra and extracranial cerebral structures [
13].
We therefore explored whether sumatriptan had different effect on the CGRP concentration in blood from the internal and the external jugular system. We found excellent agreement between the levels of CGRP in the internal and the external jugular veins (Fig.
2), with a mean difference of −0.06 pmol/L between the two compartments. Under normal circumstances the internal jugular vein is considered to be the most important pathway for venous blood returning from the brain [
60]. It thus seems likely that sumatriptan has a similar lack of effect on CGRP in these two vascular beds.
Methodological considerations
CGRP released from trigeminal afferents is a marker of trigeminovascular activation and can be measured in both experimental animals and humans. The objective of our study was to examine the effect of sumatriptan on plasma levels of CGRP, not to determine whether the drug was effective in migraine treatment. We therefore conducted this experiment as a pharmacological model study in healthy volunteers. The model used in the present study might be used for the study of other important signaling molecules in both healthy volunteers and potentially in migraine patients.
Previous studies show a large variation in the CGRP levels. The baseline CGRP levels in this study are larger than jugular levels during migraine [
41] but comparable with peripheral CGRP levels during migraine [
3]. We have used the same assay as in [
41], to minimize interassay variation. CGRP plasma levels show considerable variability [
61]. The main objective of our study was to compare CGRP before and after sumatriptan rather than the absolute values, thereby minimizing variability as a potential methodological problem. We drew blood simultaneously from all four catheters thereby eliminating temporal variability as a source of bias. Even though the missing values from the external jugular vein were high (32.5%), we have data from 11 subjects, which is sufficient according to our power calculation.