The nervous system is the major communication network in the body and its normal functioning is highly dependent on many different complex metabolic processes as well as maintenance of its structural integrity. Accordingly, any disruption in the nervous system’s normal metabolic processes or structure, or both, can lead to various neurological conditions such as depression, anxiety and cognitive impairment. Specifically, alteration in behavior is caused by a functional deficit of neurotransmitters (brain chemicals) including norepinephrine, serotonin and dopamine at certain sites in the brain.[1] Interestingly, studies on mice show that using natural remedies such as garlic can enhance the production of these brain chemicals, thereby improving behavior.
Garlic as a Medicinal Food Ingredient
Garlic, also known as Allium sativum, is one of the most widely grown vegetable crops in Asia and has long been used as a medicinal food ingredient in many different countries. In pharmacological research, an overwhelming body of evidence supports the health benefits of Allicin, the key ingredient in raw garlic.[2] Although Allicin is the most important compound that is generally claimed to be responsible for the many health benefits of garlic, studies show that other sulfur compounds such as diallyl disulphide (DDS), S-allylcysteine (SAC) and diallyl trisulfide (DTS) also play many important roles in the treatment of certain medical conditions. However, not everyone can freely consume fresh raw garlic because of its unpleasant taste and strong smell. Therefore, pharmaceutical companies developed many different formulations of garlic preparations such as aged garlic extract, garlic oil, and dehydrated garlic powder in order to enjoy its therapeutic benefits.
The Antidepressant Effect of Garlic
The role of garlic in mice behavior is well established. Garlic may help modify depressive behavior in mice because of its crucial role in preventing functional deficit of neurotransmitters at certain sites in the brain. In a study by Dhingra et al., they found that oral garlic extract supplementation in young Swiss albino mice produced significant antidepressant-like activity.[3]
The researchers observed that the garlic extract exerted its antidepressant-like activity by inhibiting Monoamine oxidase A (MAO-A) and Monoamine oxidase B (MAO-B) levels. MAO-A and MAO-B are involved in the breakdown of the neurotransmitters dopamine, serotonin, epinephrine, and norepinephrine. Serotonin transmits signals within the brain that help regulate mood and emotion, whereas epinephrine, norepinephrine and dopamine transmit signals to produce smooth physical movements. As a result, the mice fed with oral garlic extract displayed improvement in depression-like activity as evidenced by a significant decrease in immobility time in a dose-dependent manner in both tail suspension test (TST) and forced swim test (FST). In addition to the above findings, the researchers also observed that the efficacy of the garlic extract was comparable to antidepressant medications such as fluoxetine and imipramine.
The Effect of Garlic on Cognition
Animal models of cognitive impairment and severe behavioral abnormalities (irritability, impaired communication, inability to process sensory information, and restlessness) have high levels of oxidative stress which are thought to cause programmed cell death (apoptosis) in the brain. Interestingly, a study by Jeong et al. found that ethyl acetate from garlic may help improve mice behavior and protect against cognitive impairment caused by oxidative stress through its antioxidant activities.[4] The researchers exposed PC12 cells (brain cells of mice) to amyloid beta (sticky, abnormal protein plaques) for two hours in order to obtain a 144% increase in oxidative stress levels. Pretreatment of the PC12 cell samples by ethyl acetate fraction from aged garlic extract significantly prevented the increase in oxidative stress, suggesting that it has potent antioxidant activities. To assess the neuroprotective effects of ethyl acetate, the researchers fed a group of mice with freeze-dried ethyl acetate fraction from aged garlic extract for 3 weeks and injected them with amyloid beta to induce memory impairment. The researchers observed that the mice performed better in Y maze test, a behavioral test that measures the willingness of rodents to explore new environments. The mice was able to perform successive entries into the three arms of the maze, suggesting that ethyl acetate fraction from aged garlic extract attenuated both memory and cognitive impairment induced by amyloid beta.
The Anti-Anxiety Effect of Garlic
Deficiency in gamma amino butyric acid (GABA) has been reported to play a crucial role in the development of psychiatric disorders, including anxiety and major depression.[5] A study by Gilhotra et al. found that garlic exerts its antianxiety-like activity by increasing the levels of GABA.[6] Male Swiss albino mice were used in the study and were fed with ethanolic extract of garlic. Stress was produced in mice by immobilizing them for 6 hours. In order to assess anxiety in mice, elevated plus maze was used. This test is based on the natural dislike of mice for open and elevated areas. The results of the study showed that the ethanolic extract of garlic produced significant antianxiety-like activity and increased the levels of GABA in the brain of mice.
Commonly used as a medicinal food ingredient, garlic is critical for a healthy mood and improving cognitive function in mice. By increasing certain chemicals in the brain and protecting against oxidative stress in mouse models, consumption of garlic can potentially treat depression, anxiety, and cognitive impairment in humans. With these beneficial effects, garlic can be considered as a therapeutic option in patients with altered mood and psychiatric disorders.
References
- Gold PW, Goodwin FK, Chrousus GP. Clinical and biochemical manifestations of depression: Relation to the neurobiology of stress: Part 1. N Engl J Med. 1988;319:348–53.
- Banerjee SK, Mukherjee PK, Maulik SK. Garlic as an antioxidant: the good, the bad and the ugly. Phytother res. 2003;13:97–106. doi: 10.1002/ptr.1281.
- Dhingra D, Kumar V. Evidences for the involvement of monoaminergic and GABAergic systems in antidepressant-like activity of garlic extract in mice. Indian Journal of Pharmacology. 2008;40(4):175-179. doi:10.4103/0253-7613.43165.
- Jeong JH, Jeong HR, Jo YN, Kim HJ, Shin JH, Heo HJ. Ameliorating effects of aged garlic extracts against Aβ-induced neurotoxicity and cognitive impairment. BMC Complementary and Alternative Medicine. 2013;13:268. doi:10.1186/1472-6882-13-268.
- Cryan JF, Slattery DA. GABAB receptors and depression. Current status. Advances in pharmacology (San Diego, Calif.). 2010; 58:427-51.
- Gilhotra, N., Dhingra, D. (2016, April). GABAergic and nitriergic influence in antianxiety-like Activity of Garlic in Mice. Journal of Applied Pharmaceutical Science. Available from https://www.researchgate.net/publication/303368687_GABAergic_and_nitriergic_influence_in_antianxiety-like_Activity_of_Garlic_in_Mice.