They’re Alive!: Circadian Rhythms in Harvested Fruits and Veggies

fresh vegetables

Research shows that fruits and vegetables are still alive after they are harvested. (Photo credit: MelindaChan/Flickr Open/Getty Images)

Have you ever considered whether the fruits and vegetables you see on the shelves in grocery stores are still alive? Do you think they die once they’re picked from the fields or off a tree? Research indicates that some life processes still function in vegetables such as cabbage even after they’ve been harvested.

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Researchers Link Circadian Rhythms to Learning and Memory

A properly-working circadian system helps Siberian hamsters remember things, such as where to find food sources. (Credit: Eric Wong/Shutterstock)

Researchers at Stanford University, led by senior research scientist Norman Ruby, recently discovered a link between circadian rhythms and learning and memory. The subject of the researchers’ study was the Siberian hamster (also called the dwarf hamster). Their research indicated that a correctly-functioning circadian system is necessary for the hamsters’ ability to remember what they have learned. Hamsters that had their circadian systems disabled were unable to consistently recall objects in their environment as compared to hamsters with working circadian systems.

The scientists developed a non-invasive procedure to disable the hamsters’ circadian systems by manipulating the hamsters’ exposure to light. First, the hamsters were exposed to two hours of bright light late at night. Next, the hamsters’ normal cycle of light/dark was delayed by three hours. After the treatment–the single treatment is enough to destroy the hamsters’ circadian system the hamsters’ normal cycle of light and dark was resumed. To test the hamsters’ memory and learning ability, the researchers used a standard test called a “novel object recognition task.” This technique takes advantage of a hamsters’ innate interest in exploring its environment.

The Novel Object Recognition Task

In this technique, two objects are placed in the opposite corners of a box. A hamster is then placed in the box. (As shown in the box marked “A”.) The hamster will typically examine the objects in its environment, spending an equal amount of time with the two objects. After a period of five minutes, the hamster is removed from the box, and one of the objects in the box is replaced with a new one. The hamster is then placed back into the box. (As shown in the box marked “B”.) A normal hamster with an unimpaired circadian system will spend time examining both objects, but will spend twice the amount of time at the new object. In comparison, a hamster with an impaired circadian system will spend the same amount of time at both objects, as it does not remember seeing one of the objects before.

This illustration shows the two steps of the novel object recognition task.

Previous research indicates that learning retention depends on the amount of a neurochemical called GABA in the brain. GABA controls brain activity. The biological clock manages an animal’s daily cycle of sleep and alertness by inhibiting different parts of the brain through the release of GABA. The hippocampus is the part of the brain that stores memories. When the hippocampus is over-inhibited by the release of too much GABA, the hippocampus becomes overwhelmed, and memories aren’t stored correctly.

Research Implications for Human Diseases

This research has implications for several diseases that impact learning and memory. For example, those affected with Down Syndrome don’t perform well on cognitive tests due to an over-inhibited brain during development. People with Alzheimer’s disease could also benefit from this study, as memory loss is also linked with an over-inhibited brain. In addition, as people age, their circadian systems begin to degrade and break down. This breakdown could explain short-term memory loss in the elderly.

In two separate studies focused on Down Syndrome and Alzheimer’s disease, mice exhibiting the symptoms of each were given pentylenetetrazole (PTZ), a GABA antagonist. PTZ works in the brain by blocking GABA from binding to synapses, which lets them continue to fire. This continual firing of the synapses keeps the brain in an excited state. In the mice, the PTZ counteracted the inhibitory affects of GABA, and improved their ability to learn and retain memories.

Ruby and his colleagues hypothesized that giving PTZ to hamsters with impaired circadian systems would see a similar improvement. The results of their experiment confirmed the researchers’ hypothesis–after being given PTZ, the impaired hamsters showed a definite improvement in their learning and memory skills.

The results of this study were published online October 1 in an early edition of the journal Proceedings of the National Academy of Sciences. Other researchers who contributed to the paper include co-authors H. Craig Heller, Calvin Hwang, Colin Wessells, Fabian Fernandez, Pei Zhang, and Robert Sapolsky.

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