Conclusions from the studies with PACAP infusion and knockouts
Human provocation studies of migraine using systemic administration of different molecules have generated important insights into the possible mechanisms in primary headache diseases. CGRP, VIP, and PACAP-38 are all strong vasodilators, sharing the activation of adenylate cyclase. The first direct evidence that highlighted the potential significance of PACAP in the pathophysiology of migraine came from observations made in a human study [
60] where intravenous administration of PACAP-38 caused headache and vasodilation in healthy subjects as well as in migraineurs, and lead to delayed-type migraine-like attacks exclusively in migraine patients without aura (65–75%), a feature remarkably similar to that previously reported after nitroglycerin infusion [
61,
62]. Because VIP had no such effect, it was suggested that this might be secondary to the activation of the PAC
1 receptor, which is now under study in a clinical trial. For comparison, CGRP induces delayed migraine-like attacks in 65% of migraine patients, but not in normal subjects [
63]. Today numerous CGRP active agents are on the verge of going to the market as effective therapy both for acute treatment and as prophylaxis.
In support of the above, a magnetic resonance imaging (MRI) angiographic study revealed that PACAP-38-induced headache was associated with significant dilation of the extracranial part of the middle meningeal arteries (MMAs), but not of the middle cerebral arteries (MCAs). This effect preceded the onset of migraine-like headache and was sensitive to the administration of a triptan class of drug [
64]. A subsequent MRI angiographic human study demonstrated a remarkable and sustained vasodilation of extracranial but not intracranial arteries (including the MCAs) following PACAP-38 administration [
65]. In addition to these, intravenous PACAP-38-induced migraine-like attacks have been found associated with alterations in brain network connectivity, by means of a resting-state functional MRI study [
66]. The role of VIP in PACAP-induced vasodilation is unclear, as data regarding its levels after PACAP administration are contradictory [
65,
67], and its vasodilation is less long-lasting than that of PACAP [
65]. The migraine response to PACAP-38 has not been found to be influenced by the presence of the risk allele MEF2D [
67,
68].
Experimental studies support the relevance of PACAP in sensory functions as well as in pain disorders, including primary headaches. Neurograms obtained following stimulation of the ipsilateral peroneal or sciatic nerve suggested that intrathecal PACAP may act as a neurotransmitter or modulator in sensory C-fibers, possibly with a contribution of A-fibers [
69]. Interestingly, PACAP-27 was more potent than PACAP-38 or VIP at this function. On the other hand, the direct intrathecal administration of PACAP elicited a dose-dependent decrease in the tail-flick latency, whereas higher doses resulted in biting and scratching; these behaviors were interpreted as pain-like syndrome and supported a role of PACAP as a sensory neurotransmitter involved in nociception [
70]. Intrathecal administration of PACAP(6–38), a PAC
1 receptor antagonist reduced mechanical and thermal hyperalgesia, suggesting that these receptors are important for PACAP-induced pain [
71]. The role of PAC
1 receptor in pain sensation has also been supported by studies using PAC
1 knockout mice, as these animals showed reduced chronic responses to chemical, mechanical, and thermal stimuli [
72,
73]. Similar results were obtained with PACAP-knockout mice, presenting with diminished light-aversive behavior (i.e., photophobia), decreased c-fos expression in the TNC and TG, and reduced meningeal blood flow following nitroglycerin administration compared to wild-type controls [
74].
PACAP has also been implicated in higher order processing of pain in brain regions such as thalamus and amygdala [
75,
76], relevant in the central sensitization and emotional load of pain. Intracerebroventricularly applied PAC
1
receptor inhibitor decreased the delayed activation and sensitization of second-order nociceptive neurons within the trigeminocervical complex following dural stimulation [
77]. This supports a potential role of PACAP in the development of central sensitization of pain in migraine. There are experimental results suggesting the role of PACAP in the development of peripheral sensitization as well in certain models of pain [
78]. The data on the possible role of PACAP and its receptors in migraine pathogenesis by means of modulating neurogenic inflammation [
73,
79‐
81] and mast cell degranulation [
65,
82‐
86] are, however, conflicting.
PACAP as a biomarker in primary headaches
A support for a sensory role of PACAP in nociception came from experiments with capsaicin administration in vivo in rats [
26], as it resulted in elevated cerebrospinal fluid concentrations of PACAP-27 by 177%, PACAP-38 by 93%, and CGRP by 692% [
87]. Studies on animal models of trigeminal activation confirm the potential role of PACAP in the pathogenesis of migraine in particular, as immunoreactivity of both PACAP-27 and PACAP-38 were found elevated in the TNC of rats following electrical stimulation (at the TG) or chemical stimulation (by nitroglycerin) of the trigeminovascular system [
88]. Furthermore, PACAP-38 levels were found elevated in the blood plasma of rats after electrical stimulation of the TG [
88]. Likewise, elevated levels of PACAP were measured in the external jugular vein samples of cats following electrical stimulation of the superior sagittal sinus [
89]. In fact, stimulation of the trigeminal system directly showed the release of not only PACAP but also VIP and CGRP into the external jugular venous blood [
89,
90].
Human clinical studies provide concordant data, as of note, elevated levels of plasma PACAP-38 were revealed in the ictal period of migraineurs (i.e., during a spontaneous migraine attack) compared to the interictal phase [
89,
91]. Interestingly, supportive data were reported in episodic cluster headache patients as well [
92]. The interictal plasma PACAP concentrations of migraineurs were, however, significantly lower compared to the levels measured in healthy controls or in patients with tension-type headache [
89,
91,
93].