How we keep track of the seasons

Researchers have discovered a key mechanism underlying how animals keep track of the seasons.

The RIKEN Brain Science Institute study shows how 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.

Seasonal time keeping is important for animals as well as people, and recent studies indicate that it is accomplished by the same part of the brain that governs our daily circadian rhythms.

This brain area, called the suprachiasmatic nucleus (SCN), cyclically expresses certain "clock" genes during a 24-hour period, but not all of the neurons march to the same beat. Two regions in the SCN are slightly out of phase, and as day length increases, so does the phase gap between them.

The research team found that the neurotransmitter GABA plays an important role in this process. In most cases, GABA inhibits the activity of neurons. However, some SCN neurons are actually excited by GABA.

Lead author Jihwan Myung explained that GABA becomes excitatory when chloride levels inside neurons are high. We suspected that changes in GABA function across the SCN could represent the repulsive force that pushes these two clusters of neurons out of phase.

Myung added that just like in other animals, our bodies keep track of the seasons and sudden changes in seasonal day length can cause severe mood disorder in some individuals. Understanding how to adjust our internal seasonal clock could lead to effective ways of helping people whose internal clocks have been disrupted.

The study appears in Proceedings of the National Academy of Sciences.