Abstract
Green tea has been used as a medicine in East Asia for thousands of years. Plant-derived compounds called flavanols, which are included in green tea, may have potentials to help maintain healthy brain function. In this chapter, we review the effects of flavanols, e.g. epicatechin (EpiC), on cognitive ability in the pond snail, Lymnaea stagnalis. In this decade, the Lukowiak’s group has tested the effects of EpiC on cognition ability in Lymnaea. In a Lymnaea model system, they showed that EpiC and EpiC-containing foods have a rapid and activity-dependent effect enhancing the formation of long-term memory (LTM) following operant conditioning of aerial respiratory behavior. In the last part of this chapter, we also introduce our study for the effects of EpiC on LTM formation in another model system in Lymnaea. This study showed that EpiC increases the persistence of LTM formed by classical conditioning of feeding behavior, and suggested that EpiC alters some electrophysiological properties of a neuron in the feeding system.
Keywords
- Green tea-derived catechins
- Epicatechin
- Operant conditioning
- Classical conditioning
- Learning and memory
- Long-term memory
- Lymnaea
1. Introduction
Green tea is one of the most popular beverages in the world. It is made from
Gastropod mollusks such as
A single 30-min training session (0.5 h-TS) results in intermediate-term memory (ITM) that persists for up to 3 hours, whereas two 30-min training sessions with a 1 hour rest interval (2 h-TS) results in long-term memory (LTM) that persists for 24 hours [21]. ITM depends on the translation of existing mRNA transcripts but does not require mRNA transcription. LTM requires both the translation of mRNA and the formation of new mRNA transcripts [22]. Thus, a 2 h-TS could drive the process of mRNA transcription in addition to translation of mRNA.
To drive aerial respiration in
Individual neurons in
In the last part of this chapter, we introduce our study for the effects of EpiC on LTM formation in the feeding system in
2. Enhancing effects of epicatechin on memory formation by operant conditioning of aerial respiratory behavior
To date, the most detailed information on effects of EpiC in
When snails were given a 0.5 h-TS, which do not usually form LTM lasting 24 hours or more, in the presence of 15 mg/L EpiC, the memory persisted until 24 hours after that training. Thus, EpiC can promote LTM formation by driving the process of mRNA transcription in addition to mRNA translation. EpiC also enhances LTM formation. When snails were operantly conditioned in EpiC-containing pond water (15 mg/L) by a 2 h-TS (TS1, TS2; Figure 3A), which typically results in memory lasting only 24 hours in pond water without EpiC (control group, TS1 versus MT, n = 12, no significance; Figure 3B), they formed LTM lasting at least 72 hours (EpiC group, TS1 versus MT, n = 12,
Exposure to EpiC does not alter locomotor activity and spontaneous aerial respiratory behavior themselves in
In mice, EpiC improves retention of spatial memory by enhancing angiogenesis [8]. Oral intake of EpiC via gavage increases the level of EpiC and its metabolites in rat plasma and brain [41], and then EpiC could influence brain functions. In
Additionally, it is reported that EpiC also has a rapid and activity-depend effect on promoting LTM formation in
A specific mechanism by which EpiC enhances memory formation has not been clarified. However, there have been many studies in mammals suggesting various ways in which dietary flavonoids may exert such beneficial effects on the CNS [8]. EpiC is known as antioxidant, protecting neurons from injury caused by oxidative stress [46]. EpiC can be photo-inactivated by exposure to ultraviolet light (UV). Exposed to the sun for 6 hours, EpiC changes the molecular conformation by breaking the cyclic ether through a radical mechanism [47]. But no significant change was observed in the antioxidant activities of EpiC upon 6 h beta-UV radiation. Following photo-inactivation of EpiC, memory enhancement did not occur. Photo-inactivation of foods containing EpiC also blocked their ability to enhance LTM [7]. Thus, enhancing effect of EpiC on memory formation in
As mentioned above, LTM formation is dependent on altered gene activity and new protein synthesis [48]. It is well-known that the consolidation period following learning plays an important role for LTM formation [20, 49], in which the learning is encoded into memory. EpiC can cause an enhancement of memory formation if snails experience EpiC during training or immediately after training. However, EpiC exposure at 1 hour before training or 1 hour after training was not sufficient to cause memory enhancement [42]. In
3. Signaling pathway involved in epicatechin effect
EpiC effects on LTM formation may be due to its ability to drive an increase in intracellular kinase activity [51] in neurons such as RPeD1. It has also been shown that activation of CREB is necessary for LTM formation in
It is reported that exposure to crayfish effluent (CE), which also enhances LTM formation and significantly decreases RPeD1 excitability [60], works a serotonergic pathway that can be blocked by mianserin, a serotonin receptor antagonist [50]. As previously shown, however, mianserin does not affect the LTM formation enhancement induced by EpiC [4]. In addition, once the osphradial nerve that connects the osphradium (a sensory organ) to the CNS is severed, CE no longer enhances LTM formation [50]. Thus, the osphradial nerve must be intact in order to cause enhancement of LTM formation by exposing CE. Whereas, EpiC enhanced LTM formation after severing the osphradial nerve [4]. Thus, it appears that EpiC acts via a different mechanism and a different pathway from those caused by the perception of CE.
McComb and collaborators demonstrated the memory formation by using
Our group can perform “
These results provide the basis of future studies in
4. Effects of intaking of catechin-rich foods
Does exposure to food products containing EpiC during the training elicit similar effects seen for exposure to pure EpiC? Lukowiak et al. demonstrated interesting experiments whether foods containing substantial amounts of EpiC, such as green tea, cocoa, apple peel and black tea, can enhance memory formation in
Interestingly, black tea does not only enhance LTM formation but suppresses LTM formation in
Another component of black tea, caffeine, can inhibit cognitive function. In drosophila, caffeine reduces the performance for light aversive conditioning [66]. However, both green tea and black tea contain a same amount of caffeine. It is possible that L-theanine included in green tea in comparatively large amounts is thought to balance the effects of caffeine. The combination of these two substances may be synergistic, as one study found that people who ingested L-theanine and caffeine together had better attention than when either was used alone [67, 68]. Therefore, investigating catechin-rich foods is difficult to permit a full understanding of the specific effect of these phytochemicals.
5. Rescue effect of epicatechin on stress-impaired memory
Green tea-derived catechins do not only enhance memory formation, but also rescue impaired cognitive functions due to environmental stressors. Catechin-rich foods have been considered to improve various aspects of cognitive functions in rodents and humans, and some reports suggest that it has positive effects on mild cognitive impairment [69, 70, 71]. EpiC administration improves spatial memory in mice via an increase in cerebral angiogenesis or a direct effect on neural elements [8]. In
Most of freshwater mollusks including
6. Enhancing effect of epicatechin on memory formation by classical conditioning of feeding behavior
CTA training procedure we performed is briefly as follows. Adult snails randomly chosen were food deprived for 24 hours before being subjected to CTA training. Snails were then immersed in an appetitive solution (10 mM sucrose) for 15 s. Then, the sucrose solution was quickly replaced with distilled water, and the feeding response (i.e. number of bites) was measured in distilled water for 5 minutes (pre-test). Ten minutes after the pre-test, CTA training was performed. In CTA training, snails were immersed for 15 s in 10 mM sucrose, which were immediately immersed for 15 s in 10 mM KCl solution (i.e. the UCS). The UCS .inhibits the feeding response. After the UCS was presented, snails were immersed either in distilled water (control) or EpiC solution (15 mg/L) for 9.5 minutes. This procedure was repeated 5 times. After CTA training, snails were kept in distilled water for 24, 48 or 72 hours and then the post-test was performed, which was exactly the same as the pre-test. By comparing the number of bites in the pre-test with that in the 24 h post-test, we determined whether the snail was a ‘good’ learner or a ‘poor’ learner.
Figure 4A shows a histogram of the decreasing rate of the feeding response in the 24 h post-test (i.e. 24 hours after training). The decreasing rate was measured for 26 snails trained without EpiC and 22 snails trained with EpiC, and the data from all snails (n = 48) are combined in the histogram. As shown in Figure 4A, from their responses, the snails were roughly divided into two groups: ‘good’ and ‘poor’ learners. Good learners were defined as snails that showed at least a 40% decrease in the number of bites in the 24 h post-test compared with that in the pre-test. Thus, poor learners were defined as snails whose post-test scores decrease by less than 40%. In this data, 32 of the 48 snails (i.e. 67%) were classified as good learners.
For the good learners, we statistically analyzed on the data presented in Figure 4B (control group, n = 14; EpiC group, n = 12). In both groups, snails showed a significant decrease in the number of bites in the 24 h post-test (control group, 37.3 ± 3.5 to 8.8 ± 1.6 bites per 5 min,
We next compared the post-test scores between two groups (control group versus EpiC group) at 24, 48 and 72 hours after CTA training. A statistical analyze showed that there was a significant difference in the memory scores at 72 hours between the two groups (control group versus EpiC group,
An identified spontaneously active pair of neurons, the cerebral giant cells (CGCs), has been shown to both modulate the neuronal network underling the feeding behavior and be necessary for LTM and its retrieval following CTA training (Figure 4) [31, 32]. Therefore, a possible mechanism underlying the significant effect of EpiC on LTM persistence is an alteration in CGC activity. Our data supported this possibility [76]. Additionally, our data suggested that a GABAergic neuron may play a significant role in mediating CTA-LTM and the EpiC effect on the CGC may involve a GABAergic neuron. For example, the GABA sensitivity of a neuron (maybe the CGC itself) might be enhanced in good learners or in snails exposed to EpiC.
7. Conclusion
Studies described in this chapter have provided valuable information on a possibility of EpiC-rich foods contributing to cognition ability in
These studies suggest that EpiC has not only antioxidant properties but also targets molecules (e.g. specific receptors) directly to affect the signaling pathway. Then, the results may yield the basis of future studies to elucidate how EpiC enhances LTM formation of classical and operant conditioning in
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