The PAG, via connections with the RVM (Fields and Heinricher, 1985
), contributes to the control of pain transmission in the spinal cord dorsal horn (Fields et al., 1983
). Endocannabinoids are involved in this process, through activation of the CB1
receptor, and descending modulation of nociceptive neuronal firing at the spinal level (Meng et al., 1998
; Palazzo et al., 2001
; Finn et al., 2003
; Meng and Johansen, 2004
; Maione et al., 2006
). In this study the hypothesis that endocannabinoid mechanisms in the vlPAG can also contribute to the descending modulation of dural trigeminovascular nociceptive traffic was tested in anesthetized rats. The potent and highly specific CB1
receptor agonist, ACPA, and the less specific CB agonist, WIN55,212, locally applied to the vlPAG, attenuated the dural-evoked Aδ-fiber neuronal activation in the TCC, at a similar level of response (~20%) found previously with naratriptan (Bartsch et al., 2004
). Further, the effects of WIN55,212 were reversed by a specific CB1
receptor antagonist applied directly in the vlPAG. There was no effect of CB1
receptor activation on either innocuous or noxious ophthalmic (V1) cutaneous receptor field activation or V1 corneal activation. ACPA and WIN55,212 also caused a significant inhibition of basal trigeminal neuronal tone over 45 min. These studies show for the first time that specific CB1
receptor activation in the vlPAG attenuates dural-evoked nociceptive Aδ-fiber neuronal firing and basal trigeminal tone in the TCC. The data imply that the endocannabinoid system may contribute to the descending modulation of trigeminovascular nociceptive traffic through the brainstem, which is hypothesized to play a role in migraine pathophysiology (Akerman et al., 2011
Previous studies indicate that the PAG-RVM pathway provides descending control of only noxious cutaneous pinch-evoked C-fiber responses at the spinal level (Waters and Lumb, 1997
), with innocuous inputs and spinal tone unaffected. The PAG's descending control of trigeminovascular responses, similar to responses at the spinal level, have no effect on cutaneous innocuous inputs. CB1
receptor activation in the vlPAG did not affect innocuous V1 cutaneous receptive field and V1 corneal brush responses in the TCC, and previous studies with naratriptan similarly show no effects on innocuous V1 corneal brush (Bartsch et al., 2004
). However, there are differences in the way the PAG-RVM pathway provides descending modulatory control of noxious inputs and basal neuronal tone. In this study dural-evoked Aδ-fiber TCC neuronal responses and basal spontaneous trigeminal tone were significantly inhibited by specific CB1
receptor activation, but there was no effect on the noxious V1 cutaneous receptive field. Sample size of C-fiber-responsive neurons was not sufficient for statistical testing. However, in previous studies both noxious Aδ-fiber and C-fiber dural-evoked, and basal spontaneous trigeminal neuronal responses were inhibited by electrical (Knight and Goadsby, 2001
) or chemical manipulation in the vlPAG (Knight et al., 2002
; Bartsch et al., 2004
). Noxious heat applied to the V1 cutaneous receptive field was unaffected by vlPAG naratriptan (Bartsch et al., 2004
). The PAG provides inhibitory control of dural nociceptive Aδ-fiber and C-fiber trigeminovascular neurons, but has no effect on noxious V1 cutaneous inputs of either Aδ-fiber or C-fiber latency, whereas only C-fiber noxious cutaneous responses at the spinal level are modulated by the PAG.
Interestingly, anandamide, an endogenous endocannabinoid, which acts at CB1
receptors, as well as TRPV1 ion channels, had no effect on dural nociceptive trigeminovascular activation. However, previous studies indicate that the effects of anandamide acting via CB1
receptors or TRPV1 ion channels, in the midbrain PAG, are limited by enzymatic degradation by FAAH (Kawahara et al., 2011
). Blockade of FAAH activity unmasks the inhibition and excitation of presynaptic glutamatergic transmission mediated via the CB1
receptors and TRPV1, respectively. We believe that rapid enzymatic degradation of anandamide in the vlPAG is likely to explain its lack of effect here.
These studies demonstrate that trigeminal neurons do not always respond in the same way as spinal neurons with respect to somatosensory modulation. In this case the vlPAG's descending inhibitory control of somatosensory nociceptive inputs at the spinal and trigeminal levels seems to differ. These differences are further highlighted pharmacologically by the response to systemic application of 5-HT1B/1D
receptor agonists. Noxious, mechanical trigeminal neuronal responses are inhibited by naratriptan while noxious, mechanical spinal dorsal horn neuronal responses are unaffected (Cumberbatch et al., 1998
). Despite these differences an interaction between endocannabinoid and serotonergic receptor systems in providing descending modulation of nociceptive inputs may also be shared by both neuronal populations. In the present study, in the vlPAG, the CB1
receptor-mediated trigeminovascular responses are modulated by the serotoninergic system, specifically via the 5-HT1B/1D
, triptan receptor. Previous studies in the brainstem dorsal raphe have shown that changes in firing of serotonergic neurons, particularly in the chronic constriction injury model of neuropathic pain, are modulated by CB1
receptor activation (Palazzo et al., 2006
; Haj-Dahmane and Shen, 2009
). These data taken together highlight that within the brainstem, endocannabinoid and serotonergic neurons can modulate the effects of either system in the way both spinal and trigeminal nociceptive inputs are processed.
It is not known how these separate transmitter systems are able to modulate the effects of either system; however, it is possible they modulate descending projections to spinal and trigeminal neurons in a similar way. Triptans in the PAG are thought to act by inhibiting GABAergic and glutamatergic transmission, probably by preventing their release from nerve terminals (Jeong et al., 2008
). Likewise, endocannabinoids are described as “synaptic circuit breakers” (Katona and Freund, 2008
), acting as retrograde neurotransmitters in the PAG and RVM, inhibiting GABAergic and glutamatergic transmission by preventing the release of transmitters from nerve terminals, via activation of CB1
receptors (Vaughan et al., 1999
). This synaptic mechanism may also explain the inhibitory effects on basal trigeminal tone that both naratriptan and CB1
receptor activation have in the PAG, by blocking the known tonic release of GABA and glutamate from nerve terminals, where they provide descending modulatory control of trigeminovascular neurons. Furthermore, the specific CB1
receptor antagonist, SR141716, alone causes an increase in spontaneous firing, indicating the inhibitory effects of likely endocannabinoid-mediated tone involved in controlling trigeminovascular neuronal transmission.
Drugs that inhibit trigeminovascular dural nociception have been shown to be predictive of therapeutic efficacy in migraine. These new data, alongside our previous work using systemic administration of endocannabinoids (Akerman et al., 2004
), provide evidence that specific CB1
receptor activation may be therapeutic in migraine. Furthermore, some part of this action may be through descending modulation from the vlPAG to trigeminal neurons. The use of endocannabinoids and molecules that activate the CB1
receptor as therapeutics are known to have the potential for overuse, and this may therefore serve as a limitation in their development for pain indications. However, the inhibitory actions of 5-HT1B/1D
receptor antagonists on the CB1
responses in the vlPAG provide interesting clinical implications into the pharmacology of the therapeutic action of triptans in migraine. The acute anti-migraine triptan action may, in part, be acting via the modulation of endocannabinoidergic neurons, potentially in the brainstem, and descending control of trigeminovascular nociceptive transmission may therefore occur via an interaction between endocannabinoid and serotonergic receptor systems. Further evidence is necessary to dissect the exact mechanism of the endocannabinoid–triptan interaction before definitive conclusions can be made.
Brainstem modulation of trigeminovascular nociceptive transmission is thought to be involved in the pathophysiology of migraine (Akerman et al., 2011
), and these data provide support for the argument that endocannabinoids, through brainstem mechanisms, may contribute to the modulation of trigeminovascular nociceptive transmission. Activation of CB1
receptors in the vlPAG was able to modulate and inhibit trigeminovascular nociceptive processing of inputs generated from the dura mater, which are hypothesized to be involved in the mechanism of pain in migraine. It is known already that there is dysfunction in the regulation of endocannabinoids (Sarchielli et al., 2007
; Cupini et al., 2008
; Rossi et al., 2008
) in other primary headache patients, contributing to lower levels systemically. Furthermore, using nitroglycerin, a migraine trigger, in animals, increases activity of endocannabinoid enzymes that break down endogenous endocannabinoids, in the midbrain, where the PAG is located (Greco et al., 2010
). These data point to an involvement of endocannabinoids in headache disorders, and this may be through changes in the midbrain vlPAG, contributing to altered trigeminovascular nociceptive processing, believed to be involved in migraine mechanisms.
In summary, the data show that endocannabinoid mechanisms are involved in the descending modulatory control of trigeminovascular nociceptive transmission from the brainstem, a mechanism hypothesized to contribute to the pathophysiology of migraine. Additionally, these effects may imply that endocannabinoids could be therapeutic in migraine, and as the CB1 receptor agonist responses are reversed by a triptan receptor antagonist, when injected directly into the vlPAG, this interaction may suggest they are already involved in the mechanism of action of triptans. Clinically, the data may offer the promise of an interesting avenue for therapeutic development, although with agonist approaches there is a theoretical potential for cannabinoid agonist overuse. This may limit the development of targeted therapeutics despite a potential efficacy in the clinic.