Establishment and persistence of photoperiodic memory in hamsters

Long summer days unequivocally stimulate, and short winter days inhibit rep roduction in Siberian hamsters. By contrast, intermediate-duration day leng ths (12.5-14 h long) either accelerate reproductive development or initiate regression of the reproductive apparatus. Which of these outcomes transpir es depends on an animal's photoperiodic history, suggesting that hamsters m ust encode a representation of prior photoperiods. The duration of nocturna l melatonin secretion is the endocrine representation of day length, but no thing is known about how long it takes to establish photoperiodic histories or how long they endure. Hamsters exposed for 2 or more weeks to long summ er day lengths acquired a long-day photoperiodic history that determined su bsequent reproductive responses to intermediate-duration day lengths and me latonin signals. The memory for long-day lengths persisted in pinealectomiz ed hamsters for 6.5 weeks, faded significantly after 13 weeks, and was func tionally absent after 20 weeks. These findings indicate that hamsters are i nfluenced only by relatively recent day lengths and melatonin signals and i gnore earlier ones that might cause them to misinterpret the salience of cu rrent day lengths.