Pain is a key symptom in patients affected by multiple sclerosis (MS) [1
] and poses a considerable burden on quality of life [3
] and disability [4
]. Its prevalence varies between 29 and 86% [5
] depending on the criteria defining the different types of MS-related pain and according to location, duration (paroxysmal, chronic) or presumed pain pathophysiology (central neuropathic or musculoskeletal) [6
]. Spasticity-related pain, together with painful tonic spasms, is believed to be of mixed neuropathic and nociceptive origin [7
] and to result from either corticospinal system disinhibition or chronic activation of nociceptive afferents [8
Because of the limited response to current therapeutic options, exogenous cannabinoids have attracted increasing interest for the treatment of peripheral [9
], central [10
], neuropathic, and cancer pain [11
]. Nabiximols (Sativex®
) have been demonstrated as effective in MS-related pain syndromes [12
] and approved in Italy since 2013 for symptomatic treatment of moderate-to-severe MS-related spasticity symptoms in adult patients refractory to other antispastic drugs [15
is an oromucosal spray formulation derived from the Cannabis sativa
plant, which contains delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) in a nearly 1:1 ratio. THC is the main active substance, acting as a partial agonist at CB1 and CB2 receptors (CB1R and CB2R). Unlike traditional neurotransmitters, endogenous cannabinoids act as retrograde synaptic messengers, as they are released postsynaptically and activate CB receptors presynaptically. They exert their action by suppressing neurotransmitter release and modulating the excitatory effects of glutamate and inhibitory function of inter-neuronal gamma-aminobutyric acid [16
The majority of these effects are mediated by CB1R, located in several pain areas such as the periaqueductal gray matter, spinal dorsal horn, and dorsal root ganglia neurons [17
]. They contribute predominantly to antihyperalgesic effects in animal models [18
]. CB2R, more diffusely distributed in peripheral tissues (immune organs and mast cells in particular) [17
], has shown major efficacy in modulating pain of inflammatory origin [19
]. Cannabinoids may also interact with many other neurotransmitters, including dopamine, acetylcholine, serotonin, and opioids [21
]. Randomized controlled trials and observational studies testing the symptomatic efficacy of oral cannabinoids for relieving pain and spasticity in MS patients have generally evaluated response to treatment through subjective outcome measures: self-administered quality-of-life questionnaires and visual or numerical scales such as the Modified Ashworth Scale (MAS) for spasticity, or a numerical rating scale (NRS) for pain or spasticity [12
The most reliable neurophysiological method to detect damage to nociceptive fibers in patients with neuropathic pain is laser-evoked potentials (LEPs) [25
]. Also quantitative sensory testing (QST), a psychophysical exam, can assess small fiber function and may be appropriate to quantify positive sensory phenomena like mechanical and thermal allodynia and hyperalgesia, which may help characterize painful neuropathic syndromes and predict or monitor treatment effects [25
Few studies to date have evaluated neurophysiological and psychophysiological data to assess pain modulation after cannabinoid intake [27
]. The aim of the present study was to examine via psychophysiological and neurophysiological testing the effects of oromucosal THC/CBD spray (Sativex®
) on the modulation of pain and thermal/pain thresholds in MS patients.
In this group of 28 MS patients, 9 dropped out of the study because of drug abuse (n = 1, smoked marijuana abuse), relapse during 1-month Sativex® therapy (n = 1), lack of compliance with neurophysiological studies (n = 4), and intolerable adverse events (dizziness) (n = 3, doses: 4 puffs for one and 6 puffs for two patients); among 19 MS patients who entered in the final analysis, 8 presented neuropathic pain, 6 nociceptive pain, and 5 mixed pain.
All patients gradually increased their dose of oromucosal spray of Sativex® until they achieved a satisfactory number of administrations per day (mean puffs/day 6.9 ± 1.9, range 4–11). Six patients reported mild side effects (dizziness in 4, drowsiness in 2, and lack of concentration in 2). A significant reduction in NRS score after drug therapy (from 6.61 to 3.55, p < 0.0001) was observed. If a 20% reduction in pain is considered clinically relevant, 14 patients (74%) responded to Sativex® therapy.
As compared to the controls, the MS patients had a significant reduction in LEP amplitude in either the dominant hand (mean N2–P2 complex amplitude (SD) 37.48 uV (13.79) for C vs. 17.41 uV (8.22) for PT0, F = 26,814, p
< 0.0001) or the feet (mean N2-P2 complex amplitude (SD) 24.95 uV (9.75) for C vs. 16.46 uV (7.63) for PT0; F = 6,502, p
= 0.006) (Figure 1
Significant differences were similarly found for N2 latency in either the dominant hand (mean N2 LEP latency (SD) 209.84 ms (14.70) for C vs. 261.18 ms (39.73) for PT0; F = 23,084, p
< 0.0001) or the feet (mean N2 LEP latency (SD) 274.72 ms (34.12) for C vs. 321.67 ms (48.53) for PT0; F = 6,408, p
= 0.002) (Figure 2
LEPs were inelicitable at the upper limbs in 11% of patients and at the lower limbs in 32%. Conversely, no significant change in LEP parameters was noted after drug administration (Figure 1
and Figure 2
Similar results were observed in the patients with neuropathic/mixed pain, with a significant decrease of amplitude and increased latency of the ‘neuropathic’ group compared to control group, in either the dominant hand or feet (dominant hand: mean N2–P2 complex amplitude (SD) 37.48 uV (13.79) for C vs. 18.08 uV (8.96) for PT0; F = 911,374, p < 0.0001; mean N2 latency (SD) 209.84 ms (14.7) for C vs. 245 ms (36.76) for PT0; F = 573,622, p < 0.0001; feet: mean N2–P2 complex amplitude (SD) 24.95 uV (9.75) for C vs. 12.85 uV (4.77) for PT0; F = 293,020, p < 0.0001; mean N2 latency (SD) 274.72 ms (34.12) for C vs. 332.5 ms (59.87); F = 752,471; p < 0.0001).
Also in patients with neuropathic or mixed pain, no change in LEP parameters was observed after Sativex® therapy.
QST cold and warm perception thresholds for both hands and feet and heat pain threshold for feet were also significantly altered in patients compared to controls [Hands: QST CDT median (5th–95th percentile) 30.67 °C (28.64–31.3) for C vs. 27.10 °C (15.77–30.9) for PT0; p
< 0.0001; QST HDT median (5th–95th percentile) 34.07 °C (32.97–36.88) for C vs. 36.95 °C (33.96–47.54) for PT0; p
< 0.0001; Feet: QST CDT median (5th–95th percentile) 29.77 °C (26.5–30.93) for C vs. 22.45 °C (4.66–29.83) for PT0; p
< 0.0001; QST HDT median (5th–95th percentile) 36.48 °C (33.95–40.61) for C vs. 42 °C (36.84–47.12) for PT0; p
< 0.0001; QST HPT median (5th–95th percentile) 44.94 °C (39.75–49.25) for C vs. 48.9 °C (43.6–51.5) for PT0; p
< 0.0001] and did not change after THC/CBD therapy (Figure 3
We observed a significant posttreatment difference between controls and patients in cold pain threshold as measured by hand stimulation (QST CPT median (5th–95th percentile) 8.67 (0–23.71) for C vs. 17.25 (0–27.8) for PT1; p
= 0016) (Figure 3
Comparison of the number of abnormal test results for patients before and after therapy showed a significant reduction in abnormal cold perception thresholds in feet (from 39.5 to 28.9%, p
= 0.048) and a trend towards a reduction in abnormal CDT also in hands (from 35.5 to 25%, p
= 0.06, not significant) (Figure 4
). No differences in the other QST tests or LEP parameters were seen. No correlation between nabiximol dosage and expanded disability status scale (EDSS) change was recorded.
Consistent with previous studies [31
], our results show that Sativex®
therapy is effective for relieving pain, as seen in the significant reduction in the NRS scores. Furthermore, a significant reduction in amplitude and increased latency, probably caused by conduction block and demyelination damage of the spinothalamic pathway, was observed in the MS patients as compared to controls. This observation is shared by other studies [27
]. Conversely, we observed no change in LEP parameters after drug therapy. Like other studies [27
], this lack of change may be linked to several different factors, including severe impairment of nociceptive pathways, high disease burden, and coexistence of nociceptive and neuropathic pain in MS patients.
Other studies have demonstrated a significant decrease in the N2–P2 complex amplitude after tramadol injection in healthy volunteers [34
] and a decrease in amplitude and an increase in latency after oral carbamazepine in patients with trigeminal neuralgia [35
]. We argue that severe damage to the nociceptive system related to MS may prevent significant changes in LEP; otherwise, cannabinoids may exert modulation on nociceptive pathways different from that of opioids or anticonvulsants, as shown by the reported finding of a decrease in N2 latency in a MS pain-free group after nabimixols therapy [27
], suggesting recovery of conduction along pain pathways.
Concerning QST, our results demonstrated a significant reduction in CDT and an increase in HDT in patients compared to controls. Intriguingly, there was a significant increase in the hand CPT of patients after Sativex®
, and a significant percentage reduction in abnormal CDT after therapy, although the difference did not reach statistical significance between patients before and after treatment. Only one randomized, double-blind, placebo-controlled, crossover trial [36
] compared QST thresholds before and after 3 weeks of dronabinol treatment in MS patients affected by central neuropathic pain, and obtained a significant reduction in pain without any changes in thermal and thermal pain thresholds. The discrepancy with our data may be due to different reasons, such as the effect of cannabidiol, the shorter treatment period in their trial, and the different selection criteria to enroll MS patients with central neuropathic pain.
Thermal sensation is known to be mediated by transient receptor potential (TRP) ion channels implicated in many physiological processes, including temperature sensation, pain, regulation of neurotransmitter release, and immune function. It is also established that THC and CBD may act as agonists of TRP channels, which is why they are also called ionotropic cannabinoid receptors to differentiate them from the classic CB1 and CB2 metabotropic receptors [37
]. The sub-class TRPA1 are cold-sensitive cation channels that function as sensors of painful cold [38
]. They are located predominantly in the nociceptive neurons of the peripheral nervous system (PNS) and may act as mediators of mechanical hyperalgesia and cold hypersensitivity [39
]; their transcription is increased during inflammation and its suppression reduces hypersensitivity to cold temperature (cold allodynia) in rat models of inflammation and nerve injury [41
]. Although the contribution of TRPA1 to thermosensation is controversial [38
], TRPA1 has been implicated also in cold sensation in vitro and in vivo [42
]. For these reasons, TRPA1 are important for the detection of noxious stimuli and pain transduction [44
The mechanism of action of cannabinoids on TRP are multiple and complex, as some cannabinoids may indirectly suppress TRPV1 and TRPA effects on pain and inflammation by acting on CB1, but directly activate the TRP channel at higher concentrations [18
]. Furthermore, non-psychotropic cannabinoids (such as CBD) can activate and desensitize TRPV1 and TRPA1 [47
TRPM8, another member of the TRP superfamily, is also involved in thermosensation and has a role in the detection and transmission of cold stimuli [38
] and cold hypersensitivity [49
]. De Petrocellis and co-workers [50
] demonstrated that certain phyocannabinoids (THC and CBD in particular) can efficaciously antagonize the effect of TRPM8 agonists.
In our study, the increase in cold pain threshold in hands and the normalization of cold detection threshold in MS patients after therapy could reflect a cannabinoid-mediated modulation on either TRPA1 or TRPM8 channels, probably via activation (and consequent desensitization) of TRPA1 and inhibition of TRPM8, although the mechanisms of action of the single channels need to be elucidated [40
]. If cooling increases the open probability of the TRPA1 channel [40
], the fact that the patients felt noxious cold at higher temperatures could mean activation (and consequent desensitization) of this channel and, at the same time, an antagonistic effect on TRPM8 [48
]. The lack of the same finding at the feet could be interpreted as a failure of Sativex®
to modulate the more severely damaged nociceptive pathways, as suggested by the high number of abnormal thermal detections and sensory impairment evidenced on clinical examination of the feet. Furthermore, although the cold pain threshold was increased (from 9.55 °C to 17.25 °C) in the patients after Sativex®
therapy, its value remained within the normal range (<23.71 °C) and they did not complain of thermal allodynia before or after drug intake.
We acknowledge several limitations of our study: first, the small sample size and high EDSS; second, the lack of a placebo group, although Sativex®
efficacy for the relief of both nociceptive and neuropathic pain has been demonstrated in some controlled studies [10
]; third, the short observation period, though the long-term efficacy of nabimixols has only recently been proved [43
]. Further studies with larger patient samples and lower EDSS are needed to better clarify how Sativex®
relives pain symptoms in MS.
In summary, our study further corroborates evidence for the effectiveness of Sativex®
in reducing pain in MS patients. Furthermore, our findings suggest a possible direct effect of cannabinoids on TRP channels by modulating or desensitizing cold-channel functions, in particular TRPA1 and TRPM8. This underscores the importance of TRP channels as a therapeutic target by acting through peripheral desensitization and inhibition of sensory neurons [53
]. Several TRP subtypes may not only function as an integral part of the endocannabinoid system but also represent promising molecular targets for pain alleviation, especially when upregulated/sensitized in pathological or inflammatory conditions [37