Prolactin rhythms-oscillators’ response to photoperiodic cues is age and circadian time dependent

In vivo prolactin release patterns exhibit a compound rhythm with circadian (24 h), semicircadian (12 h) and ultradian (6–8 h) periods. Changes in these rhythmic patterns were observed at different photoperiodic conditions, and in elderly. Since in vitro prolactin release was related to the photoperiodic history of the animal, we studied the effect of varying photoperioda upon the in vitro rhythmic output of prolactin release from young and old male rat pituitaries, isolated at different circadian times from animals housed at LD 12:12, 18:6 for 10 days or 6 weeks. The results indicate that, both, mean levels and rhythmic prolactin release in vitro are determined by the age of the animal, the circadian time of pituitary isolation, the photoperiodic conditions in which the animal was housed, and the duration of housing in the long day conditions. The change of the rhythmic output pattern is gradual, reflecting a process by which the oscillators respond to the external cues to fit prolactin release pattern to the environmental conditions. Each of the oscillators (e.g. circadian, semicircadian, ultradian) shows different sensitivity to the changing photoperiodic signal and is regulated at the level of phase and amplitude but not the period. In old rats the response of the oscillators to the change in photoperioda is attenuated and not sufficient to induce a change in the output of prolactin release suggesting a loss in adaptation ability.