25 years ago, pharmacologist M. E. West of the University of the West Indies in Kingston, Jamaica, noted that local fisherman who smoke cannabis or drink rum made with the leaves and stems of the plant had “an uncanny ability to see in the dark,” which enabled them to navigate their boats through coral reefs. “It was impossible to believe that anyone could navigate a boat without compass and without light in such treacherous surroundings,” he wrote after accompanying the crew of a fishing boat one dark night, “[but] I was then convinced that the man who had taken the rum extract of cannabis had far better night vision than I had, and that a subjective effect was not responsible.”

Some of these crew members told West that Moroccan fishermen and mountain dwellers experience a similar improvement after smoking hashish, and in 2002, another research team travelled to the Rif mountains in Morocco to investigate further. They gave a synthetic cannabinoid to one volunteer, and hashish to three more, then used a newly developed piece of kit to measure the sensitivity of their night vision before and after. Confirming West’s earlier report, they found that cannabis improved night vision in all three of their test subjects.

Now, another study provides hard evidence for the claim, revealing a cellular mechanism by which cannabis might improve night vision. The findings, published recently in the open access journal eLife, could eventually be applied to the treatment patients with degenerative eye diseases such as retinitis pigmentosa.

West had suggested that cannabis might improve vision by acting on the eye muscles to dilate the pupils, so that more light falls on the retina, but other experiments ruled this out by showing that marijuana constricts the pupils. It’s also possible that the drug can influence activity in the visual cortex at the back of the brain, but the CB1 receptor protein, which binds the psychoactive ingredient of cannabis, is found at far higher levels in the eye than in the visual cortex, suggesting that any effects the drug has on vision are likely due to its actions on retinal cells.

Lois Miraucourt of the Montreal Neurological Institute and his colleagues looked not to stoned fishermen, but to tadpoles of the African clawed toad, Xenopus laevis, which are transparent and, therefore, amenable to all sorts of experiments that cannot be performed in humans or other lab animals.

In one set of experiments, they applied a synthetic cannabinoid to eye tissue preparations from the tadpoles, and used microelectrodes to measure how retinal ganglion cells, whose fibres form the optic nerve, respond to light. The researchers found that this made the cells more sensitive, increasing the rate at which they fired to both bright and dim light stimuli. Closer investigation revealed that this occurred due to inhibition of a protein called NKCC1, via its actions on the CB1 receptor.

NKCC1 is a co-transporter protein that normally shuttles sodium, potassium, and chloride ions in and out of cells, and their concentrations determine the electrical properties of nerve cells. Overall, these experiments show that cannabinoids reduce the concentration of chloride ions inside the retinal ganglion cells, making them more excitable and more sensitive to light.

Miraucourt and his colleagues then carried out another set of experiments to determine if the cellular responses they observed could contribute to vision. Tadpoles have a natural tendency to avoid dark moving dots, and the researchers exploited this by putting some tadpoles into a Petri dish, showing them dark dots under various lighting conditions, while using specially designed video-tracking software to track the movements of the tadpoles and the dots, and to measure the tadpoles’ avoidance responses.

Under normal lighting conditions, they observed no differences between tadpoles treated with a synthetic cannabinoid and untreated ones. In the dark, however, tadpoles given the cannabinoid avoided significantly more dots than untreated ones, which only responded to the dots as if by chance. Thus, the researchers conclude that the enhanced cellular responses observed in their first set of experiments improved the tadpoles’ sensitivity to contrast under low-light conditions.

Whether the findings are applicable to humans remains to be seen but, if so, they could pave the way to treatments for diseases such as retinitis pigmentosa and glaucoma, which cause blindness by killing off cells in the retina. Cannabinoids are known to have a neuroprotective effect on retinal cells, so treatments based on the drug may, in theory, not only improve vision for patients with deteriorating eyesight, but also slow down the progression of such diseases.

References

Miraucourt, L. S., et al. (2016). Endocannabinoid signaling enhances visual responses through modulation of intracellular chloride levels in retinal ganglion cells. eLife, 5: e15932. DOI: 10.7554/eLife.15932 [Full text]

Russo, E. B., et al. (2004). Cannabis improves night vision: a case study of dark adaptometry and scotopic sensitivity in kif smokers of the Rif mountains of northern Morocco. J. Ethnopharmacol., 93: 99–104 [PDF]