Photoperiodic information acquired and stored in vivo is retained in vitroby a circadian oscillator, the avian pineal gland

Endogenous circadian rhythms have been described in a wide range of organis ms from prokaryotes to man. Although basic circadian mechanisms at the mole cular level are genetically fixed, certain properties of circadian rhythms at the organismic level can be; modified by environmental conditions and su bsequently retained for some time, even in organisms shielded from 24-hr en vironmental variations. To investigate the capacity of animals to acquire a nd store photoperiodic information, we examined activity and melatonin rhyt hms in house sparrows during synchronization to two different photoperiods and during subsequent prolonged darkness. Under constant environmental cond itions, intact animals continued to have long feeding activity times when p reviously exposed to long days and short feeding activity times when previo usly exposed to short days. Correspondingly, significantly different durati ons of elevated melatonin in the plasma directly reflected the differences in night length during synchronization as well as during prolonged darkness . Additionally, we found a significant difference in the amplitude of the n octurnal melatonin signal, which also was conserved in prolonged darkness. To investigate whether the photoperiodic experience of an intact animal can be "memorized" by an isolated component of its circadian pacemaking system , we have investigated in vitro melatonin release during continuous darknes s from explanted pineal glands of house sparrows after in vivo synchronizat ion to two distinct photoperiods. Differences in the durations of elevated melatonin occurred during the first two cycles in culture and a difference in melatonin: amplitude was detectable during the first night in culture. O ur data indicate that photoperiodic patterns imposed on sparrows during in vivo synchronization can be maintained as an internal representation of tim e within the isolated pineal gland. Hence, the pineal gland, as one of the most significant components of the songbird circadian pacemaker, not only h as the capacity to autonomously produce circadian rhythms of melatonin rele ase but also is capable of storing biologically meaningful information expe rienced during previous cycles.