Rats Get Depression In Spring Not Winter
April 26, 2013

Rats Get The ‘Winter Blues’ In Spring

Alan McStravick for redOrbit.com - Your Universe Online

Here in the latter days of April, much of the nation and the world is struggling to once and for all escape the cold grasp of the long arm of winter. For many, their mental health can be affected by the shorter hours of daylight. Whether you refer to the condition as the ℠winter blues´ or the ℠black dogs,´ or even the more formal title of ℠seasonal affective disorder´ (SAD), going for an extended period of time with a noticeable lack of natural light can cause feelings of ennui, lethargy, irritability and anxiety.

Biologists at the University of California San Diego (UCSD) claim they have found that rats actually experience the same symptoms as humans, but at a different time of the year. During the late spring and early summer months, when daylight is present for longer periods, rats exhibit an increased level of anxiety and depression. They claim this could possibly occur as a result of a discovery they made showing how the rat´s brain cells adopt a new chemical code when drastic changes occur in the day and night cycles. Effectively, a switch is flipped in the rat brain that allows an entirely different neurotransmitter to stimulate the same part of the brain.

Their findings, published in tomorrow´s issue of the journal Science, also demonstrate that the adult mammalian brain is more adaptive and malleable than neurobiologists had once thought.

The rat brain is a favorite study model due to the fact it shares so many similarities with the human brain. For this reason, the team believes their recent findings will provide much needed insight into changes that occur in the human brain as a result of altered light reception. Furthermore, they see potential applications of their findings as being important in future treatment of brain disorders such as Parkinson´s.

The study involved exposing separate groups of rats to differing amounts of light and darkness. The first group was exposed daily to 19 hours of darkness and 5 hours of light over the course of a week. Conversely, the second group was exposed to 19 hours of light and 5 hours of darkness. The team found the first group of rats had a greater number of nerve cells producing dopamine than the second group. This higher dopamine production resulted in rats that seemingly were less stressed and anxious when measured using standardized behavioral testing. Group 2 had more neurons synthesizing the neurotransmitter somatostatin, which resulted in a more stressed and anxious subject group.

Two examples of the behavioral testing involved the rats and their willingness to explore the open end of an elevated maze. This exploratory instinct has been shown to represent a diminished level of anxiety. These rats were more likely to take a swim, which researchers note shows that the animals were less stressed.

According to Nicholas Spitzer, a professor of biology at UCSD and director of the Kavli Institute for Brain and Mind, “We´re diurnal and rats are nocturnal. So for a rat, it´s the longer days that produce stress, while for us it´s the longer nights that create stress.”

This evolution in brain chemistry between rats and humans does make sense, however. Rats tend to forage for food during the night time hours while humans long ago opted for the daylight hours for their hunting-gathering mode of life.

Also contributing to the study was Davide Dulcis, a research assistant in Spitzer´s laboratory and first author of the study. Dulcis stated, “Light is what wakes us up and if we feel depressed we go for a walk outside.” He continued, “When it´s spring, I feel more motivation to do the things I like to do because the days are longer. But for the rat, it´s just the opposite. Because rats are nocturnal, they´re less stressed at night, which is good because that´s when they can spend more time foraging or eating.”

Learning of this difference between two otherwise very similar mammalian brains is just the beginning. Scientists now want to learn how the human brain changed as a result of evolution some millions of years ago and how that evolution was able to accommodate the behavioral changes that occurred as a result of switching over from a nocturnal to a diurnal existence.

“We think that somewhere in the brain there´s been a change,” commented Spitzer. “Sometime in the evolution from rat to human there´s been an evolutionary adjustment of circuitry to allow switching of neurotransmitters in the opposite direction in response to the same exposure to a balance of light and dark.”

The results of Spitzer and Dulcis´ study seems to be supported by another study out of San Diego State University published earlier this month. The previous study, led by John Ayers, culled over Google search data that referenced mental health issues and found these searches occurred 14 percent more often in US winters and 11 percent more often in the Australian winter.

To date, the researchers are not entirely certain how the dopamine to somastatin switch occurs in the individual neurotransmitter. Additionally, they have not yet determined the exact ratio of dark and light that causes this switch to occur. “Is it 50-50? Or 80 percent light versus dark and 20 percent stress? We don´t know,” said Spitzer. “If we just stressed the animal and didn´t change their photoperiod, would that lead to changes in transmitter identity? We don´t know, but those are all doable experiments.”

Despite the questions that still remain, the team sees a viable human application for their findings. They believe it will soon be possible to use the neurotransmitter switch to aid in the delivery of dopamine to parts of the brain that no longer receive dopamine in Parkinson´s patients.

“We could switch to a parallel pathway to put dopamine where it´s needed with fewer side effects than pharmacological agents,” Dulcis concluded.