The effects of Δ9 THC on cutaneous sensitivity and its relation to personality

Cannabinoid effects on responses to quantitative sensory testing among individuals with and without clinical pain: a systematic review

There has been an explosion of interest in the utility of cannabinoids as potential analgesics. This systematic review critically synthesizes the evidence for cannabinoid analgesic effects on quantitative sensory testing outcomes in both healthy adults and patients with chronic noncancer pain. Our systematic review protocol is preregistered on PROSPERO (CRD42018117367). An electronic search was made in PsycINFO, Cochrane, Google Scholar, Embase, and Pubmed of all literature published until August 2018. Of the 1217 studies found from the search, a total 39 placebo-controlled studies that met the eligibility criteria were synthesized for this study. Because of substantial heterogeneity of study designs, populations, cannabinoid compounds, and quantitative sensory testing outcomes, meta-analysis was not conducted. More consistent evidence of cannabinoid analgesia was observed for inhaled cannabis than synthetic cannabinoids. Analgesic effects were most commonly observed in tests of cold pain sensitivity, and hyperalgesic effects were most commonly observed in tests of electrical stimulation. Patterns of findings from studies with healthy subjects did not substantively differ from those with chronic noncancer pain. However, these observations are qualified by the high degree of inconsistency across studies and methodological heterogeneity. We offer recommendations for future studies to improve study rigor and reproducibility.

Association of Cannabinoid Administration With Experimental Pain in Healthy Adults: A Systematic Review and Meta-analysis

Importance

Cannabinoid drugs are widely used as analgesics, but experimental pain studies have produced mixed findings. The analgesic properties of cannabinoids remain unclear.

Objective

To conduct a systematic review and meta-analysis of the association between cannabinoid drug administration and experimental pain outcomes in studies of healthy adults.

Design, Setting, and Participants

A systematic search of PubMed, EMBASE, MEDLINE, PsycINFO, and CINAHL was conducted from the inception of each database to September 30, 2017. Studies were eligible for inclusion if they met criteria, including healthy participants and an experimentally controlled administration of any cannabinoid preparation in a quantified dose. Studies that used participants with chronic pain were excluded. Data extracted included study characteristics, cannabinoid types and doses, sex composition, and outcomes. Study quality was assessed using a validity measure previously established in published reviews. Random-effects meta-analyses were used to pool data and generate summary estimates.

Main Outcomes and Measures

Experimental pain threshold, pain tolerance, pain intensity, pain unpleasantness, and mechanical hyperalgesia.

Results

Eighteen placebo-controlled studies (with 442 participants) were identified. Of the 442 participants, 233 (52.7%) were male and 209 (47.3%) were female. For sample ages, 13 (72%) of the 18 studies reported a mean sample age (26.65 years), 4 (22%) reported a range, and 1 (6%) reported a median value. The search yielded sufficient data to analyze 18 pain threshold comparisons, 22 pain intensity comparisons, 9 pain unpleasantness comparisons, 13 pain tolerance comparisons, and 9 mechanical hyperalgesia comparisons. Cannabinoid administration was associated with small increases in pain threshold (Hedges g = 0.186; 95% CI, 0.054-0.318; P = .006), small to medium increases in pain tolerance (Hedges g = 0.225; 95% CI, 0.015-0.436; P = .04), and a small to medium reduction in the unpleasantness of ongoing experimental pain (Hedges g = 0.288; 95% CI, 0.104-0.472; P = .002). Cannabinoid administration was not reliably associated with a decrease in experimental pain intensity (Hedges g = 0.017; 95% CI, -0.120 to 0.154; P = .81) or mechanical hyperalgesia (Hedges g = 0.093; 95% CI, -0.059 to 0.244; P = .23). The mean quality rating across studies was good.

Conclusions and Relevance

Cannabinoid drugs may prevent the onset of pain by producing small increases in pain thresholds but may not reduce the intensity of experimental pain already being experienced; instead, cannabinoids may make experimental pain feel less unpleasant and more tolerable, suggesting an influence on affective processes. Cannabis-induced improvements in pain-related negative affect may underlie the widely held belief that cannabis relieves pain.

Amygdala activity contributes to the dissociative effect of cannabis on pain perception

Cannabis is reported to be remarkably effective for the relief of otherwise intractable pain. However, the bases for pain relief afforded by this psychotropic agent are debatable. Nonetheless, the frontal-limbic distribution of cannabinoid receptors in the brain suggests that cannabis may target preferentially the affective qualities of pain. This central mechanism of action may be relevant to cannabinoid analgesia in humans, but has yet to be demonstrated. Here, we employed functional magnetic resonance imaging to investigate the effects of delta-9-tetrahydrocannabinol (THC), a naturally occurring cannabinoid, on brain activity related to cutaneous ongoing pain and hyperalgesia that were temporarily induced by capsaicin in healthy volunteers. On average, THC reduced the reported unpleasantness, but not the intensity of ongoing pain and hyperalgesia: the specific analgesic effect on hyperalgesia was substantiated by diminished activity in the anterior mid cingulate cortex. In individuals, the drug-induced reduction in the unpleasantness of hyperalgesia was positively correlated with right amygdala activity. THC also reduced functional connectivity between the amygdala and primary sensorimotor areas during the ongoing-pain state. Critically, the reduction in sensory-limbic functional connectivity was positively correlated with the difference in drug effects on the unpleasantness and the intensity of ongoing pain. Peripheral mechanisms alone cannot account for the dissociative effects of THC on the pain that was observed. Instead, the data reveal that amygdala activity contributes to interindividual response to cannabinoid analgesia, and suggest that dissociative effects of THC in the brain are relevant to pain relief in humans.

An analgesia circuit activated by cannabinoids

Although many anecdotal reports indicate that marijuana and its active constituent, delta-9-tetrahydrocannabinol (delta-9-THC), may reduce pain sensation, studies of humans have produced inconsistent results. In animal studies, the apparent pain-suppressing effects of delta-9-THC and other cannabinoid drugs are confounded by motor deficits. Here we show that a brainstem circuit that contributes to the pain-suppressing effects of morphine is also required for the analgesic effects of cannabinoids. Inactivation of the rostral ventromedial medulla (RVM) prevents the analgesia but not the motor deficits produced by systemically administered cannabinoids. Furthermore, cannabinoids produce analgesia by modulating RVM neuronal activity in a manner similar to, but pharmacologically dissociable from, that of morphine. We also show that endogenous cannabinoids tonically regulate pain thresholds in part through the modulation of RVM neuronal activity. These results show that analgesia produced by cannabinoids and opioids involves similar brainstem circuitry and that cannabinoids are indeed centrally acting analgesics with a new mechanism of action.