The effects of a single long photoperiod on induction and dissipation of reproductive photorefractoriness in European starlings

In many birds, long photoperiods stimulate gonadal maturation but also cause photorefractoriness, leading to gonadal regression. While much is known about the neuroendocrinology of photostimulation, little is known about photorefractoriness, partly due to lack of an experimental model. This study aimed to develop a model to test whether a single long photoperiod (LP) initiates the mechanism leading to photorefractoriness. It made use of the fact that in castrated European starlings, luteinizing hormone (LH) is low in photorefractory birds and high in photosensitive birds. In the first experiment, groups of castrated photorefractory birds were transferred from a long to a short photoperiod and then exposed to one LP every 5, 10, 20, 30, 40, or 60 days. In birds exposed to one LP every 5 days, LH stayed low. In birds exposed to one LP every 10 days, each LP caused a pulse in LH, but mean LH remained low. One LP every 20 days increased LH 6 days later, followed by a decrease at 11 days and then a further increase, so that overall, LH increased slowly. In birds exposed to one LP every 30, 40, or 60 days, LH increased at the same rate as in short photoperiod controls. Each LP did not cause a significant increase in LH, but did cause a decrease. A second experiment examined the changes in LH following a LP in more detail. Castrated starlings had been exposed to one LP every 14 days for 16 weeks. On the day of the final LP, LH values were midway between photorefractory and photosensitive values. The LP caused an increase in LH from the second day to a peak after 7 days. Thereafter, LH declined to initial values after 14 days, followed, in the absence of further LP, by a second increase to photosensitive values. These results suggest that the mechanisms causing photostimulation and photorefractoriness are both initiated during the first long photoperiod. That gonadal maturation precedes regression must reflect the relative rates at which the two processes reach completion, (C) 2001 Academic Press.