A new study shows that the Siberian hamster, whose reproductive organs only become capable in spring and summer, temporarily retains information in its nervous system on the amount of daylight it experiences, and uses this information to make sure the reproductive organs become active at the right time of the year.
Researchers working to understand human memory through studies of the simpler brains of animals have a new experimental model to investigate in the hamster, according to a new paper in the Proceedings of the National Academy of the Sciences.
"There was some resistance to calling this phenomena 'memory,' because it doesn't fit into the established categories," lead author Brian Prendergast, now a postdoctoral researcher at the Johns Hopkins University, says. He notes that scientists had previously theorized that the hamsters were simply responding to their immediate circumstances. "What we've shown, though, is that their response to a stimulus is modified by previous circumstances. That implies a history-dependent response, and that's one definition of memory."
The big challenge for a hamster memory system that notes the length of days, Prendergast explains, is that the amount of daylight changes throughout the year in a constant, wavelike pattern. The longest day occurs roughly every June 21 and the shortest day roughly every December 21. In the periods between, the days get longer or shorter.
To anticipate the oncoming winter, the hamster uses as its cue longer daylengths followed by intermediate daylengths.
Hamster brains being what they are, though--small and relatively unsophisticated-- Prendergast and others wondered how the hamsters made sure the memory they used as a cue was in fact the most recent one.
Working at the University of California-Berkeley with support from the National Institutes of Health, Prendergast exposed groups of male hamsters to changing periods of daylight. To assess what time of year the hamster thought it was in, he measured their testicles, which grow dramatically in size with the arrival of spring and shrink back down in the fall.
He started by exposing groups to different durations of 15-hour days; some groups saw a week, others saw two weeks, and so on up to 3 months of long days. All were shifted afterwards into intermediate-length days. Any animal that remembered the summer-like days would have a reduction in testicle size.
"Animals that saw one week had no response," he says. "Two weeks or more, and they interpreted an intermediate-length day as 'short.' That says they acquired the long- day memory. But 3 months was no more effective than 2 weeks in acquiring the long-day photoperiodic memory. Unlike strong or weak memories, there doesn't seem to be any sort of gradations; the animal either gets the memory or it doesn't."
Prendergast also "taught" a group of hamsters the long-day regimen and then surgically removed their pineal glands. Found at the base of the brain, pineal glands secrete a hormone called melatonin at night. During the day, this secretion shuts off.
"When we take out the pineal gland, the animal is no longer able to communicate seasonal information into its brain," he explains. "Whatever they've learned, they can't update thereafter."
Prendergast simulated the effect of an intermediate daylength by giving the hamsters infusions of melatonin. The hamsters continued to respond normally to the melatonin signals, which represented an intermediate daylength, until about 13 weeks after the removal of the pineal gland. After 13 weeks, the reproductive system would not shift into its inactive phase--a sign that the long-day exposures were apparently forgotten.
"That makes perfect sense in terms of what goes on in the wild," Prendergast says, noting that 13 weeks is a fourth of a year or about one season. "Any day length information that the hamster learned more than 13 weeks ago would lead it to make the wrong 'decision' about an intermediate day."
Prendergast, who now works in the labs of Hopkins neuroscientist Randy Nelson, plans to try to follow the encoding signal further into the hamster brain, possibly to the hippocampus, an area linked to other types of memory.
Additional authors on the project were Irving Zucker of the University of California-Berkeley and Michael Gorman of the University of California-San Diego.
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