In a recent study published in Scientific Reports, researchers assessed the potential protective effect of cannabidiol (CBD)- and delta-9-tetrahydrocannabinol (THC)-based oils derived from Cannabis sativa plants against aluminum (Al)-induced neurotoxicity in the zebrafish animal model.
Study: Cannabis sativa-based oils against aluminum-induced neurotoxicity. Image Credit: Creativan/Shutterstock.com
The secondary metabolism of C. sativa generates hundreds of chemicals, some with biological activity, e.g., terpenoids and phenolic compounds. Two terpenoids, THC and CBD, are key targets for medical use as they modulate the cholinergic nervous system (CholNS), whose main excitatory neurotransmitter is acetylcholine (ACh).
While the CholNS system maintains memory, environmental perception, and movement, ACh acts on brain regions linked to learning/cognition.
Any changes in the CholNS are, thus, responsible for many neurodegenerative disorders, such as Parkinson's, Alzheimer's disease (AD), and multiple sclerosis (MS). Aluminum (Al), the third most abundant metal on the earth, toxicate the CholNS system.
It is considered a neurotoxic compound due to its ability to accumulate in biological tissues. It also interacts with the antioxidant enzymes, e.g., superoxide dismutase (SOD) and catalase (CTL), crucial for re-establishing oxidative homeostasis.
Thus, cells encounter increased oxidative stress due to Al exposure, which leads to cell death, senescence, and multiple diseases.
Al is used heavily in the cosmetic, pharma, and metallurgical industries and in the production of alloys used for making objects of domestic usage, e.g., cooking utensils, which has increased Al exposure among humans.
In high concentrations, Al could lead to intoxication and cause neurotoxicity. However, its physiological action as a neurotoxic compound is unclear.
About the study
In the present study, researchers used 144 adult zebrafish to investigate the protective effect of CBD- and THC-based oils against the neurotoxic effects of Al used at a concentration of 5.5 mg/L.
They divided test animals into six groups, with each group receiving vehicle (ultrapure olive oil), Al(−); CBD-rich oil, Al(−); THC-rich oil, Al(−); vehicle and Al(+); CBD-rich oil and Al(+); and THC-rich oil and Al( +) for 15 days, i.e., chronic exposure.
Next, they performed behavioral tests, novel tank test (NTT), and social preference test (SPT) in triplicate for three consecutive days on test animals.
In addition, they evaluated the effect of these oils on AChE activity as the marker of CholNS function and the antioxidant enzymes, superoxide dismutase (SOD), catalase (CTL), and glutathione S-transferase (GST).
SOD and CTL work sequentially, where the former reduces the superoxide anion (O2-) to hydrogen peroxide (H2O2), and the latter uses the H2O2 produced by SOD to form water and oxygen.
Further, to explore the kinetics of Al as a stressor, the team quantified the THC and CBD concentration in the aquaria water using high-performance liquid chromatography (HPLC).
In humans, excessive and persistent apprehension of day-to-day situations is a behavioral biomarker of the anxiety pattern. In fish, reduced locomotion and exploratory behavior represent an anxiety-like condition. This behavior is hazardous and originates from alterations in the central nervous system, e.g., cholinergic signaling.
Zebrafish exposed to Al showed an anxiety-like behavior, which declined their social interaction, most likely due to a decrease in AChE resulting in neurotoxicity. Exposure to Al in the presence of the CBD- and THC-rich oils negated the adverse effects of the metal, suggesting an antioxidant and neuroprotective effect of these oils.
Additionally, the zebrafish showed an increase in the concentrations of antioxidant enzymes, which suggested they developed an increased oxidative resistance to Al.
Specific strains of Cannabis sativa produce different amounts of THC and CBD; thus, CBD's presence in the THC-rich oil and vice versa is considered normal. However, intriguingly, the researchers observed a decreased concentration of both oils in the study groups treated with the oils and Al.
In the presence of Al, the concentration of both THC and CBD was reduced in the aquarium water, likely because their interaction led to the formation of conjugated compounds that neutralized the Al toxicity.
In the SPT test, Al decreased the interaction with the conspecifics. However, when applied with the THC-rich oil, test animals resumed their social interaction. While the THC-rich oil reversed the effect on cholinergic signaling and behavior or helped resume AChE activity, neither THC- nor CBD-rich oils reversed the anxiety-like pattern, likely because different behavioral paradigms were used in the study experiments.
High concentrations of THC oils caused anxiogenic effects, which, in turn, increased anxiety-like behavior. On the contrary, low concentrations of CBD helped re-establish the social behavior among test animals, likely because CBD (even at low concentrations) decreased the aggressiveness in mammals by activating the CB1 receptors, possibly linked to observed increased social behavior.
Zebrafish species gather in shoals in their natural surroundings, which is very important for protection from enemies, search for food, and reproduction. On the contrary, the anxiety-like behavior, like staying at the bottom, seemed like a defense strategy in the absence of conspecifics, a situation imposed during the NTT assessment.
Chronic exposure to Al with the THC- and CBD-rich oils increased the SOD activity that prevented cellular damage if the CTL did not increase in these groups. Conversely, in the group treated with Al, the authors noted no increase in SOD activity but enhanced CTL.
The CTL activation indicates that Al continuously generates hydrogen peroxide (H2O2), which is hazardous for cells. The non-activation of the SOD enzyme in the group treated with only Al might be linked to the metallic inhibition of the SOD.
Al increased the GST activity, further showing the neurotoxic effect of Al on cells. In the presence of THC- and CBD-rich oils, Al reduced the GST activity, reflecting the protective effect of these oils in the cells.
While Al-induced neurotoxicity caused AChE decrease in zebrafish animal models, THC- and CBD-rich oils re-established their social behavior and reversed anxiety-like patterns.
In addition, both oils re-established the SOD and CTL activity (both antioxidant enzymes) after Al exposure. Furthermore, these oils modulated GST activity to prevent cellular damage.
Future studies should explore other CNS signaling pathways implicated with anxiety-like behavior that these oils failed to re-establish.
However, since these oils have a neuroprotective effect, the researchers advocated their use in drugs combating neurological- and antioxidant impairment-related disorders after adequate clinical assessments.