Scientists at the RIKEN Brain Science Institute in Japan have conducted the study to find out how animals understand the changing of the seasons. They have discovered a key mechanism of how animals keep track of the seasons. Turns out, circadian clock machinery in the brain encodes seasonal changes in daylight duration through GABA activity along with changes in the amount of chloride located inside certain neurons.
Knowing time of the seasons is very important for animals living in the wild as well as humans. Recent researches showed keeping track of seasons is accomplished by the same part of the brain that governs our daily circadian rhythms. This part of the brain, called the suprachiasmatic nucleus, regularly expresses certain “clock” genes during a 24-hour period. However, not all of the neurons work according to the same rhythm. Two smaller regions in the suprachiasmatic nucleus are slightly out of this rhythm and as day length increases, so does the phase gap between them. Scientists wanted to understand the actual mechanism, so, as usual, mouse models were used.
At first researchers measured expression levels of the clock gene Bmal1 in explanted dorsal and ventral suprachiasmatic nucleus of mice that had been living in long-day or short-day light cycles. Results were not surprising – scientists found that cyclical Bmal1 levels in dorsal and ventral regions of the mice in the short-day group were synchronized and out of sync of the long-day group. Analysis showed that coupling between the two regions is not a two-way street, which causes the dorsal region to become out of phase when daylight increases.
The researchers found that the neurotransmitter called GABA is very important in this process. GABA inhibits the activity of neurons in most cases. However, some neurons in the suprachiasmatic nucleus actually get excited by GABA. Jihwan Myung, lead author of the study, said that “GABA becomes excitatory when chloride levels inside neurons are high” and scientists thought that “changes in GABA function across the suprachiasmatic nucleus could represent the repulsive force that pushes these two clusters of neurons out of phase”.
To test this, scientists blocked GABA activity, and noticed that the large phase gap seen in the long-day group disappeared and the cycles of Bmal1 levels came to resemble those of the short-day group. This means that GABA has a special effect on the dorsal suprachiasmatic nucleus. Scientists also measured expression of two other genes—Nkcc1 and Kcc2—that are responsible for importing and exporting chloride. Researchers found that in the long day group the expression ratio of the two genes changed so that much more chloride was imported. And that is what made GABA have excitatory effect. This also meant that blocking chloride import abolishes the phase gap seen in the long-day group.
Even though research is interesting in itself, it also provides some important knowledge. Not only animals, but also humans keep track of the seasons. If the internal clock of our body gets disrupted, it can lead to severe mood disorder. This new knowledge will allow scientists to create new effective ways to adjust the internal clock that nowadays can be disrupted by many factors related to contemporary lifestyle.