Melatonin is Not a Magic Pill

Many animals undergo seasonal physiological changes in order to ensure that their babies are born during a time of more abundant food and milder weather and to help their bodies prepare for harsh winter conditions. In order to precisely time these physiological changes with the seasons, most animals have evolved to respond to the most reliable marker for time of year, photoperiod (the number of hours of daylight in a 24-hour period).

Two different strategies have evolved to synchronize physiology with the annual cycle: In photoperiodic species, photoperiod directly changes the physiology. For example, in Syrian hamsters, a short photoperiod (as we would be exposed to in winter) causes the reproductive system to shut down and a long photoperiod (as we would see in spring and summer) is needed to reactivate it. In circannual species, the body has inherent, repetitive oscillations in physiology with cycles of about a year even if they are kept in constant conditions for years. Photoperiod can help align the physiology of a circannual species with the time of year (like resetting your clock), but is not necessary for the cycles to occur.

In mammals, the hormone melatonin, produced by the pineal gland in the brain, is thought to be essential in this process of annual body rhythms. Melatonin is produced mostly at night, which means that we produce more of it on long nights (in the winter) than on short nights (in the summer). Most research on the subject states that mammals with their pineal gland removed cannot process photoperiodic information. However, if you give these animals without pineal glands short infusions of melatonin, their bodies will enter a summer-like state; if you give long infusions of melatonin, their bodies will enter a winter-like state.

Researchers Stefanie Monecke, Dominique Sage-Ciocca and Paul Pévet from the University of Strasbourg in France and Franziska Wollnik from the University of Stuttgart in Germany tested whether melatonin is essential for annual rhythms in a circannual species, the European hamster. They performed surgeries on European hamsters in which they either removed the pineal gland or left it intact. They then put the hamsters in a photoperiod that was similar to the natural environment, but changed twice as fast (essentially speeding up a year's worth of photoperiod changes into 6 months). They regularly recorded variables known to change seasonally with this species: their body weight and temperature, activity patterns, and reproductive state. The researchers also checked their blood plasma for the presence of melatonin to be sure that the surgeries were effective.

All of the European hamsters with the fake surgeries that left the pineal glands intact showed regular seasonal fluctuations (but shortened to a 6-month period) of their body weight and temperature, activity patterns, and reproductive state, as expected. Surprisingly, most hamsters with the pineal gland removed also showed the same regular seasonal fluctuations of the same variables shortened to the same 6-month period. When the researchers checked the melatonin levels of the animals, the animals with the intact pineal glands did have melatonin, whereas those without pineal glands did not have melatonin. Clearly, melatonin is not the only thing that is communicating the photoperiod to their bodies to regulate annual cycles.

The scientists repeated the study, but this time they did some of the surgeries when animals were in a long (summer-like) photoperiod and others when animals were in a short (winter-like) photoperiod. European hamsters that had their pineal glands removed in the summer-like state were much more likely to have normal annual cycles compared to those that had their pineal glands removed in the winter-like state.

These findings may be surprising to many researchers that study annual cycles in animals, because most research claims that photoperiod influences melatonin production, which then regulates the body's annual cycle. Closer inspection of previous research shows that this indeed seems to be the case in photoperiodic species. However, many studies provide evidence that melatonin is not required to maintain body rhythms in circannual species. This evidence has often been dismissed and explained by the exposure of the animals to other environmental factors that change seasonally. We have been so convinced of the critical role of melatonin in regulating annual rhythms, that we have ignored evidence to the contrary in study after study.

This study shows that there is a fundamental difference in how photoperiodic species and circannual species regulate their bodies' annual rhythms. Furthermore, photoperiod is doing something to the bodies and brains of circannual species that does not involve melatonin, and we don't yet know what that is. Why would two different systems for regulating annual cycles evolve? The scientific mystery awaits.

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Further reading:

Monecke, S., Sage-Ciocca, D., Wollnik, F. and Pévet, P. Photoperiod can entrain circannual rhythms in pinealectomized European hamsters, Journal of Biological Rhythms, 28:4, 278-290 (2013). DOI: 10.1177/0748730413498561.

Image Credits:

European hamster by Agnieszka Szeląg at Wikimedia Commons.