Modifying cannabis to treat chronic pain
New research shows how a cannabinoid precursor reduces hypersensitivity in a chronic neuropathic pain rat model, but only in male animals. Zhu et al. (2020). British Journal of Pharmacology. https://doi.org/10.1111/bph.14997
Chronic pain, defined as any pain that lasts for 3 months or more, currently affects more people worldwide than cancer, heart disease and diabetes combined, and is the leading cause of disability in the US. These conditions include neuropathies, inflammatory conditions and joint pain (1).
Despite being an illegal drug in the majority of countries worldwide, cannabis and its chemical derivatives are becoming well known for their pain-killing abilities, particularly for common treatment-resistant conditions that cause chronic pain. For example, ∆9-tetrahydrocannabinol (THC) and cannabidiol (CBD), both derivatives of the cannabis plant, have been trialed for the management of pain in fibromyalgia (2), as well as other forms of neuropathic pain (3). The treatment of chronic pain is, in fact, the most common reason for the use of medical cannabis, making up 64.5% of users in 2016 (4). One of the key benefits of cannabinoids compared to traditional opioid painkillers is that they do not cause addiction, which is becoming a huge public health issue in many countries. Moreover, these derivatives do not produce psychomimetic effects such as hallucinations. Hence, the widespread use of medical cannabis and its derivatives is one of the major driving forces behind the campaign to legalise cannabis in the UK.
One such cannabinoid is the upstream precursor to cannabidiol, cannabidiolic acid (CBDA). In 2018, a study was published showing that CBDA has an anti-hyperalgesia effect in an inflammatory pain model in rats. However, CBDA is a difficult drug to study as it is rapidly decarboxylated to form CBD once it has entered the body. Moreover, CBD is highly hydrophilic, meaning after consumption is remains in the water in the body, rather than being absorbed into the bloodstream and into cells, and is subsequently excreted.
To counter these issues, a simple chemical modification can be used to stabilise CBDA. This involves adding a methyl ester group to CBDA to make CBDA-ME, making it easier to absorb, as well as having significant anti-emetic (meaning anti-nausea) properties (5). This makes CBDA-ME much more desirable for long-term treatment regimes.
And this is exactly what was studied in a recent paper accepted into the British Journal of Pharmacology, which aimed to investigate the efficacy of CBDA in a common rat model of chronic neuropathic pain (NEP). To produce this model, researchers placed a cuff around the sciatic nerve in the leg of each rat, putting pressure on the nerve and resulting in chronic hypersensitivity.
In order to assess the sensitivity of these rats to mechanical stimulation, the Von Frey test of paw withdrawal was used both before the operation and for 8 weeks following the surgery to assess the development of tactile hypersensitivity.
By gently touching finer and finer filaments to the pads of the paws of the rats, the researchers could assess the smallest filament that they were able to feel, based on how the rats retracted their paws following stimulation. This could be used to establish the threshold detection of animals — that is, the minimum stimulation needed for the rats to be able to detect it. A more sensitive rat would be able to detect a smaller filament diameter, meaning it had a lower paw withdrawal threshold. Given chronic nerve damage is associated with hypersensitivity, it was hypothesised that the mice with the nerve cuffs would have lower withdrawal thresholds, and that treatment with CBDA-ME may reduce this hypersensitivity.
What they found was extremely interesting. As hypothesised, all the rats that had the sciatic nerve cuff placed showed a significant decrease in withdrawal threshold, indicating increased paw sensitivity, consistent with the emergence of NEP. Beginning in the second week after the surgery for a further 6 weeks, CBDA-ME treatment in males resulted in a continuous increase in withdrawal threshold, suggesting a pain-relieving property of the drug. This response was concentration-dependent, meaning the higher the dose of CBDA-ME, the greater the improvement. There was no effect of vehicle treatment at any point in the experiment, ruling out a habituation effect with repeated experience of the test.
However, the most intriguing result in this paper was not the anti-hyperalgesic effect of chronic CBDA-ME treatment in the male rats, but the lack of any treatment effect in the females. Despite using the same paradigm of CBDA-ME treatment and testing in the female rats, there was no change in paw withdrawal threshold at any point in the experiment. Moreover, two higher doses were also tested in the females, but still CBDA-ME had no effect. Astonishingly, this means that the females did not respond at all to CBDA-ME treatment at concentrations where male rats were seeing significant reductions in their hypersensitivity.
After conducting the behavioural experiments, the researchers also investigated the electrophysiological effects of CBDA-ME treatment. To do this, they recorded from peripheral pain fibres, finding that in males treatment with CBDA-ME increased their threshold for activation, meaning more stimulation is needed for them to fire action potentials (how neurons conduct information). This makes the pain fibres less sensitive to external stimuli, correlating with the decreased hypersensitivity seen in the rats given CBDA-ME. In stark contrast, they saw no such increase in activation threshold in the female rats, potentially explaining why these mice displayed no improvement to hypersensitivity.
One of the key selling points of this paper is the large amounts of data the researchers generate from each rat — as well as information on chronic pain development, they take multiple behavioural measures and go on to do electrophysiological experiments as well. A critical consideration of animal research is to reduce the number of animals being used in experiments, and this paper is an excellent example of producing a lot of data with a minimal number of animals. However, these animals could be used further, for example post-mortem analysis of the nervous tissue, assessment of the inflammatory response to the placement of cuff (how the neurons respond to the injury), as well as changes to transmission on a cellular level.
However, we must not overlook what is surely the take-home message of this paper — that female rats respond differently, or in this case not at all, to a compound that produced a robust effect in males. Biological sciences have historically almost exclusively studied male animals, in part due to added complications of female animals during the estrous cycle, as well as a general underestimation of sex differences in drug responses. This paper demonstrates why the study of female animals is also critically important in understanding the mechanisms and efficacies of drugs.
Overall, this paper suggests that treatment with CBDA-ME could limit or delay the development of hypersensitivity, at least in the model of chronic pain explored here. Given the prevalence of hypersensitivity in many chronic pain conditions, this research could have interesting ramifications for how we approach the treatment of chronic pain and have a more holistic impact on the use of medical cannabis in healthcare. More importantly, however, the paper highlights the incredible sex-dependency of the anti-hyperalgesia effects of CBDA-ME and demonstrates the importance of using both male and female animals in biological experiments.
References
1. SPINE-health. Glossary: https://www.spine-health.com/glossary/chronic-pain
2. van de Donk et al. (2019). Pain, 160(4): 860–69.
3. Campbell G et al. (2018). The Lancet Public Health, 3(7): e341–50.
4. Boehnke KF et al. (2019). Health Affairs, 38:2.
5. Pertwee RG et al. (2018) British Journal of Pharmacology, 175(1): 100–112.
6. Ferrier J, Marchand F, Balayssac D (2016). Bio-protocol, 6(18): e1933.
