Endocannabinoids in the brainstem modulate dural trigeminovascular nociceptive traffic via CB1 and "triptan" receptors: implications in migraine

Abstract

Activation and sensitization of trigeminovascular nociceptive pathways is believed to contribute to the neural substrate of the severe and throbbing nature of pain in migraine. Endocannabinoids, as well as being physiologically analgesic, are known to inhibit dural trigeminovascular nociceptive responses. They are also involved in the descending modulation of cutaneous-evoked C-fiber spinal nociceptive responses from the brainstem. The purpose of this study was to determine whether endocannabinoids are involved in the descending modulation of dural and/or cutaneous facial trigeminovascular nociceptive responses, from the brainstem ventrolateral periaqueductal gray (vlPAG). CB1 receptor activation in the vlPAG attenuated dural-evoked Aδ-fiber neurons (maximally by 19%) and basal spontaneous activity (maximally by 33%) in the rat trigeminocervical complex, but there was no effect on cutaneous facial receptive field responses. This inhibitory vlPAG-mediated modulation was inhibited by specific CB1 receptor antagonism, given via the vlPAG, and with a 5-HT1B/1D receptor antagonist, given either locally in the vlPAG or systemically. These findings demonstrate for the first time that brainstem endocannabinoids provide descending modulation of both basal trigeminovascular neuronal tone and Aδ-fiber dural-nociceptive responses, which differs from the way the brainstem modulates spinal nociceptive transmission. Furthermore, our data demonstrate a novel interaction between serotonergic and endocannabinoid systems in the processing of somatosensory nociceptive information, suggesting that some of the therapeutic action of triptans may be via endocannabinoid containing neurons in the vlPAG.

Figure 1.

Overview of the experimental setup and neuronal characteristics. A, The basic experimental setup with dural stimulation and recording in the TCC, and modulating the descending contribution of the vlPAG by direct microinjection of compounds. B, The location of all microinjection sites within the vlPAG according to the template from Paxinos and Watson (1998) over an example Pontamine Sky Blue injection site in the vlPAG. All neurons studied were WDR with cutaneous receptive field in at least the first (ophthalmic) division of the trigeminal nerve, but also with corneal and sometimes cutaneous receptive field from the second (mandibular) region of the trigeminal nerve (C). The green shading represents an example of receptive characterization in an animal. D, The location of recording sites in the TCC of nociceptive neurons receiving convergent input from the dura mater and facial receptive field, predominantly in laminae I–II and V. These locations were reconstructed from lesions (●) or from microdrive readings (○) and an original lesion site is included in E. F, An original tracing from a typical unit responding to dural stimulation with latencies in the Aδ-fiber and C-fiber range. 5GN, trigeminal ganglion; Aq, aqueduct; DLPAG, dorsolateral PAG; DMPAG, dorsomedial; V1, first trigeminal (ophthalmic) division; V2, second trigeminal (mandibular) division; V3, third trigeminal (maxillary) division.

Figure 2.

Summary of changes in dural-evoked neuronal firing in the TCC in response to microinjection of the GABA_A receptor antagonist bicuculline methiodide into the vlPAG. Dural-evoked Aδ-fiber activity in the TCC is stable and not significant across 30 min of observations with aqueous solution (▵) microinjection in the vlPAG. After microinjection of bicuculline methiodide (●) into the vlPAG there is a significant inhibition of evoked firing in the TCC of (A) Aδ-fiber neurons and (B) basal spontaneous trigeminal tone. C, Original tracing from a dural-evoked Aδ-fiber neuronal response before and after bicuculline that is significantly inhibited. Data are presented as mean ± SEM; *p < 0.05 significance when compared with an average of the three baselines, using Student's paired t test.

Figure 3.

Summary of changes in dural-evoked neuronal firing in the TCC in response to microinjection of a cannabinoid receptor agonist, WIN55,212, into the vlPAG. A, Dural-evoked Aδ-fiber neuronal activity in the TCC was stable after microinjection of vehicle (DMSO) into the vlPAG across 45 min (▿). After microinjection of WIN55,212 (●) into the vlPAG there was significant inhibition of evoked firing in the TCC of neurons with Aδ-fiber latency. These responses were significantly reversed by the specific CB₁ receptor antagonist, SR141716 (▵). This was also the case with spontaneous neuronal firing in the TCC (B). WIN55,212 significantly inhibited responses and SR141716 was able to reverse this effect. C, Example of poststimulus histogram (cumulative over 20 dural stimulations) identifying baseline Aδ-fiber responses that are inhibited by WIN55,212 application; a response that is reversed by coapplication with SR141716. In each group poststimulus histograms are taken at the 10 min time point after drug intervention. D, Activation of the facial receptive field with cutaneous V1 pinch and V1 brush or V1 corneal brush was not significant across the treatment groups of baseline: WIN55,212 and WIN55,212 and SR141716. Data are presented as mean ± SEM; *p < 0.05 significance when compared with an average of the three baselines using Student's paired t test.

Figure 4.

Summary of changes in dural-evoked neuronal firing in the TCC in response to microinjection of a specific CB₁ receptor agonist (ACPA) and reversal with a 5-HT_1B/1D receptor antagonist (GR127935), into the vlPAG. Dural-evoked Aδ-fiber neuronal activity in the TCC was stable after microinjection of vehicle (Tocrisolve) into the vlPAG across 45 min (▵). A, After microinjection of ACPA in the vlPAG there was significant inhibition of evoked firing in the TCC of neurons with Aδ-fiber latency (●). Basal spontaneous trigeminal tone was also significantly reduced (B). C, Responses to ACPA were significantly reversed with prior treatment with either intravenous administration (▿) or microinjection (▵) of GR127925. D, Activation of the facial receptive field with cutaneous V1 pinch and V1 brush or V1 corneal brush was not significant across the treatment groups of baseline, ACPA and ACPA/GR127935 (intravenous; iv) or ACPA/GR137935 (micropipette; mp), respectively. E, Example of poststimulus histogram (cumulative over 20 dural stimulations) identifying baseline Aδ-fiber responses that are inhibited by ACPA application, a response that is reversed by co-microinjection with GR137935. In each group poststimulus histograms are taken at the 15 min time point after drug intervention. Data are presented as mean ± SEM; *p < 0.05 significance when compared with an average of the three baselines or a single baseline for the receptive field, using Student's paired t test.