Green Tea Protects from the Psychological Effects of Stress in Rats
It's no secret that a cup or two of green tea can make you relaxed, but now scientists have shown that green tea can reduce the effects of psychological stress in rats.
In a paper yet to be published, rats were put under stress and given either their usual diet or a diet enriched with green tea polyphenols (link). To see how psychological stress and green tea were related, experiments measuring cognitive performance and serum levels of stress hormones were done.
Study method
The rats were divided into five groups: control group (CT), stress group (ST), and three stress groups given low, medium and high doses of green tea polyphenols (LG, MG and HG). The green tea polyphenol (GTP) content of the three diets were 0.1%, 0.5% and 1%, respectively.
Psychological stress was induced by keeping the rats restrained and inhibiting their movements six hours every day for three weeks. Their cognitive performance was then evaluated using an open-field test, a step-through test and a water maze. These tests measure both the activity level and memory ability of rats.
Green tea polyphenols and cognitive performance
In the open-field test, which measures how actively rats explore the arena, the stressed rats were much less active than the control rats. No significant improvement was seen in the rats fed the low dose of green tea polyphenols, while the rats given the medium or high dose were almost as active as the control rats.
The memory of the rats was also affected by stress during the step-through test and water maze test. These tests measure spatial memory and the ability to remember adverse stimuli. Again, only the medium and high doses of GTPs significantly reduced the harmful effects of stress on the rats' memory.
Green tea polyphenols and stress hormones
Stress activates the symphatetic nervous system, which results in a release of catecholamines. Catecholamines are "fight-or-flight" hormones that consist of epinephrine, norepinephrine and dopamine. They are involved in the modulation of cognition, awareness, attention, and emotional state, helping the body to cope with a stressful situation. According to the authors, when the stress level is too high for the body to cope with, cognitive impairments appear and the levels of these hormones begin to decline.
Plasma levels of norepinephrine (another stress hormone, also known as noradrenaline) and dopamine were remarkably reduced in the ST and LG rats. In the MG and HG groups norepinephrine and dopamine levels were lower than in the control group, but much higher than in the LG group. In other words, medium and higher levels of green tea polyphenols partially inhibited the stress-induced decrease in the levels of these hormones.
All four stress groups had higher levels of plasma cortisol than the control group. Cortisol is often referred to as the "stress hormone", since its levels increase in the presence of stress and anxiety. Cortisol also causes blood pressure to rise and immune responses to be reduced. Feeding the stressed rats green tea polyphenols lowered their cortisol levels, but the reduction was statistically significant only in the MG group.
The levels of reactive oxidative species (ROS) were increased in the brain tissue of stressed rats, but again, medium and high doses of GTPs partially inhibited this increase. Similarly, the total antioxidative capacity in brain tissue was reduced by stress but to a smaller extent in the MG and HG groups. The levels of superoxide dismutase showed a similar trend, but the differences were not statistically significant.
A different effect was seen in the levels of interleukin-6 and interleukin-2. While stress increased IL-6 and IL-2, feeding the rats GTPs did not inhibit this increase. In fact, green tea polyphenols increased IL-6 even further. This may be because increased levels of IL-6 can enhance the body's adaptability to stress. Similar effects have been reported in other studies on green tea.
Conclusion
Psychological stress negatively affected the behavior and memory of rats. This adverse effect was associated with higher levels of cortisol, reactive oxygen species, IL-2, and IL-6, and lower levels of norepinephrine, dopamine, and total antioxidative capacity.
These changes were partially inhibited by diets containing 0.5% and 1% green tea polyphenols, except for IL-6, which was further increased by GTPs. A diet containing only 0.1% GTPs did not show significant results.
For more information on green tea and cognition, see these posts:
Green Tea Protects from Bone Loss in Female Rats
Green Tea Protects from Arthritis in Rats
Does Taurine Improve Cognitive Performance and Mood?
Does Ginkgo Biloba Improve Cognitive Performance?
11 kommenttia:
great blog, i've just read it. love it
Fascinating... If I come across any stressed out rats, I'll let them know about this study! Joking. Lot's of great info in your blog.
Hehe, funny Green Tea Powder.
I absolutely love green tea, and this article just gives me another reason to love it! Sometimes I get quite stressed out throughout the day. Maybe I should increase my green tea intake.
@Anonymous & Green Tea Powder: Glad you like the blog! Thanks for visiting.
@Christina: Personally, I find that a few cups usually has a relaxing effect, while drinking lots of green tea can sometimes make me feel more energetic. It also depends on whether I'm fasting or not.
For a relaxing effect, have you tried rooibos tea? It certainly makes me feel pleasantly drowsy. Plus, no caffeine, so it's safe to drink at night.
Have you looked much at research on selenium? There is a crazy finding of that links bad mood, anxiety, confusion, and even sadness to a lack of the trace element. I've been doing a bit of bio-inorganic research involving mercury-selenium interaction with my professor. Here's a very interesting and comprehensive review: http://www.ncbi.nlm.nih.gov/pubmed/10963212
If you don't have access to the Lancet, you can find a whole lot of similar stuff free. (or email me, wink-wink, nudge-nudge... ;) *cough-for-personal-use-only-do-not-reproduce-cough*
@MedDog,
I have read some papers on selenium and it does look interesting. I'm not yet decided on whether I should supplement it or not (in addition to what I get from my multivitamin).
Most of the studies on mercury and selenium seem to indicate that selenium is good (because of selenium's ability to bind mercury), but I've also read a couple that suggest it might actually increase mercury levels in the blood and the brain. Have you found out anything interesting in your research?
Oh, and I do have access to the Lancet, but thank you for the offer!
It seems most good multivitamins have plenty anyway. Indeed co-accumulation of mercury and selenium was noted by several studies in human autopsies as well as most animal experiments. But, what is clear is the reduction of toxic effects. It seems very stable complexes are formed in erythrocytes and all over the brain and liver, and this serves to detoxify organic and inorganic mercury. However, as to complexes in the brain, it would appear that selenium helps sequester and detoxify methyl-mercury once it has already passed the blood-brain barrier. I'm not sure that there is evidence that selenium, binding mercury outside the brain, will actually draw into the ol' noggin, as such a phenomenon has not been noted with inorganic mercury experiments (selenium complexes of which do end up being stored in the liver with no apparent toxic effects).
Additionally, it seems the methylmercury in fish comes "pre-detoxified" by the high selenium levels found in most of these animals (sharks and whales have a low Se/Hg ratio however).
I would also add that most supplementation studies find that maximization of the activity of some newly-discovered plasma selenoproteins occurs around 100 micrograms/day. Anti-cancer and immunomodulating activity was noted at 200 micrograms/day. So that appears to be a good level of intake.
@MedDog,
Interesting. Do you take selenium supplements yourself?
Rats have surprising similarities to humans which is why they are good to study. This is very interesting research and I am not surprised by the results at all, tea is well known to help relax and aid in mental activities.
Like anything, if you take too much of it you will experience some negative effects. Tea does keep people awake and is a diuretic, so personally I stick to a few cups a day. I much prefer it over coffee, however most Australians have now started drinking coffee like the gloria jeans take away style ones which I don't personally think are much good for you, so it is good to hear some positive research about tea. The research I hear about tea I find easier to believe than the research I hear about coffee. After all tea is the dried leaves of a plant I don't think there is too much toxic about it.Quite the opposite.
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