In addition to the known effect of major cannabinoids, such as cannabidiol (CBD) and tetrahydrocannabinol (THC), on the endocannabinoid system and pain receptors there is growing interest in the effects of minor cannabinoids, such as cannabigerol (CBG).
Chronic pain affects a significant proportion of the global population, often having a significant impact on quality of life. However, current treatment options are largely considered to be of questionable efficacy while carrying significant risk of addiction and/or overdose. Therefore, the development of alternative therapies is a constantly significant area of research within the medical and scientific communities.
A recent study aimed to assess how several cannabis derivatives could be useful for pain relief, both alone and in combination.
Design and Methods of the Study
The researchers assessed three cannabinoids – CBD, CBG, and THC – to determine their ability to inhibit Capsaicin responses in cultured sensory neurons. Capsaicin is a compound derived from chili peppers. It is a chemical irritant for mammals, including humans, that produces a pain sensation in tissues (it is responsible for irritation and spice from chilis).
Adult rat Dorsal Root Ganglion (DRG) were harvested, and enzyme digested to obtain a neuronal suspension in BSF2 medium containing 2% fetal calf serum, and the neurotrophic factors NGF and GDNF. After 48 hours, the cultured neurons were loaded with Fura-2 AM, to determine the effects of cannabinoids on capsaicin responses using calcium imaging. Cultures were separated into two groups. Cultures in the control group were treated with a control vehicle, followed by 1 µM capsaicin. In cannabinoid-treated cultures, CBD, CBD, and THC were applied individually, or combined (ratio – 1:1:1), followed by 1 µM capsaicin.
The researchers then analysed data from six experiments with Student’s t-test and Pearson’s correlation coefficient. In each experiment, the largest calcium responses to the cannabinoid were selected for analysis.
Responses were recorded as the difference between baseline (before the addition of cannabinoid or drug) and peak after the addition. Data were recorded for the mean change in 340/380 nm λex ratio in response to added cannabinoid(s), and capsaicin, and expressed as a percentage of the control obtained from the same animal specimen.
Results of the Study
No change in baseline was observed following the application of control vehicle, while capsaicin responses were observed in phase bright neurons, within seconds of application, as a rapid and sustained increase in intracellular 340/380 ratio. In comparison, the application of cannabinoids resulted in a calcium influx after a long delay.
Effect of CBG
CBG application at 1 µM was not seen to elicit calcium influx but reduced the subsequent capsaicin response. Higher concentrations of 10 µM and 30 µM were seen to elicit dose-related increases in calcium influx in a subset of neurons; however, this was reduced at the highest concentration tested (100 µM). Capsaicin responses showed a dose-related reduction.
Effect of THC
Dose-related calcium influx was observed in a subset of neurons, following application of THC at 1, 10, 30, 100 and 150 µM. Corresponding dose-related reduction of capsaicin responses was observed in the presence of THC, with maximum reduction at 100 µM.
Effect of CBD
Application of CBD at 1, 10, 30 and 100 µM concentrations elicited dose-related calcium influx, with corresponding dose-related reduction of capsaicin responses.
Effect of combination of CBG, CBD, and THC
A combination of CBG, THC and CBD were applied in a ratio of 1:1:1. The dose-related calcium influx, with maximum influx at 90 µM. Capsaicin responses were dose-dependently diminished at 3 µM, 30 µM, and 90 µM. Interestingly, the combined inhibitory effect of combined cannabinoids at 3 µM was equivalent to the effects of CBD, CBG or THC applied individually at 10 µM.
The results from this study show that CBD, CBG, and THC demonstrate potent dose-related inhibition of capsaicin responses in DRG neurons when applied individually in an in vitro model of neuronal hypersensitivity. This effect was enhanced when cannabinoids were applied in combination, being most effective 90 µM.
The researchers conclude that the efficacy and tolerability of THC could therefore be improved when the cannabinoid is applied in combination with CBD and CBG at optimal concentrations. They recommend that further studies be carried out, in vivo, to determine the potential of this combination as a pain treatment and in particular the role of the disputed ‘entourage effect’.