Serotonin promotes patience in parts of brain, reveal scientists

Okinawa: Serotonin, the hormone which determines the level of patience, has been found by a study done on mice to also dictate whether one will crave instant gratification or will calmly anticipate the reward. Patience is often called a virtue because having the ability to suppress the need for instant gratification is difficult to attain.

Recently, a study conducted by scientists on mice revealed to them the action of serotonin in individually promoting patience in the specific areas of the brain. The researchers in Japan found that the mice became more patient in waiting for food when the hormone was artificially triggered in them during the lab experiment.

The `Neural Computation Unit at the Okinawa of Science and Technology Graduate University (OIST)` conducted this study by authors, Dr. Katsuhiko Miyazaki and Dr. Kayoko Miyazaki, and the results were published in the journal, Science Advances.

Dr Katsuhiko Miyasaki, who is one of the authors in the journal, said "Serotonin is one of the most famous neuromodulators of behavior, helping to regulate mood, sleep-wake cycles and appetite," and Dr Miyasaki further stated that, "Our research shows that release of this chemical messenger also plays a crucial role in promoting patience, increasing the time that mice are willing to wait for a food reward."

This recent work draws very heavily from the previous research, in which a powerful technique called optogenetics was used by the unit. They used the light to stimulate specific neurons inside of the brain, in order to establish a causal link between patience and serotonin.

The genetically engineered mice which were bred by the scientists had neurons that released serotonin and expressed a light-sensitive protein. All this meant that by shining light at precise times, the researchers could stimulate these neurons to release serotonin, using an optical fiber implanted in the brain.

In the research, it was found that the waiting time of the mice for the food increased when these neurons were stimulated in them. The maximum effort was observed when the timing of the reward was uncertain but the probability of receiving it was high.

"In other words, for the serotonin to promote patience, the mice had to be confident that a reward would come but uncertain about when it would arrive", stated Dr. Miyazaki. 

The main focus of the previous study was on an area of the brain named `dorsal raphe nucleus`, which is also called the central hub of neurons that produce serotonin in the brain. 

In their most recent study, scientists particularly studied the other areas of the brain which contribute to regulating patience due to these neurons from the `dorsal raphe nucleus` reaching out in these other areas of the forebrain.

There are three main areas in the brain which when damaged have shown to have increased impulsive behavior - the nucleus accumbens, which is a deep brain structure, and two frontal lobe parts called the `medial prefrontal cortex` and `orbitofrontal cortex`.

Dr. Miyazaki further explains that "Impulse behaviors are intrinsically linked to patience -- the more impulsive an individual is, the less patient -- so these brain areas were prime candidates".

During the study, optical fibers were implanted by the scientists into one of either, the medial prefrontal cortex, the orbitofrontal cortex, or the nucleus accubens, along with the `dorsal raphe nucleus`.The mice were trained by the researchers to do a waiting task, in which they had to put their nose inside a hole, called "nose poke", and then a food pellet was delivered to them. In 75 per cent of the trials, the mice were rewarded by the scientists. In some trials when the mice started the nose poke, the reward timing varied while during others it was fixed at six or ten seconds.

During the omission trials, which were the remaining of the 25 per cent trials, no food rewards were provided to the mice. The motive was to measure how long the mice will continue to perform the nose pokes during the omission trials. This was done to check how patient the mice would remain when the serotonin releasing hormones were not being stimulated. The researchers found that there was no increase in the wait time when the serotonin-releasing neural fibres that reach into the nucleus accumbens were stimulated. This suggested that no role is played by serotonin in this area of the brain in regulating patience.

Interestingly, when the serotonin release was stimulated in the medial prefrontal cortex and the orbitofrontal cortex, it was found that with only a few crucial differences, the mice waited longer while holding the nose poke. The release of serotonin in the orbitofrontal cortex promoted patience with the same effectiveness as serotonin in the dorsal raphe nucleus: both when reward timing was uncertain and also when it was fixed, with stronger effects in the former.

The scientists on the other hand saw that when the timing of the reward varied, patience in the medial prefrontal cortex increased, with there being no effect when the timing was fixed.

Dr. Miyazaki concluded that "The differences seen in how each area of the brain responded to serotonin suggests that each brain area contributes to the overall waiting behavior of the mice in separate ways".