Effects of pinealectomy on circadian locomotor activity rhythms in european starlings,Sturnus vulgaris

Summary

Removal of the pineal organ from starlings had drastic effects on their freerunning circadian activity rhythms; in birds kept in continuous darkness the period (τ) shortened and the activity time (α) lengthened. Concomitantly bothτ andα became relatively unstable and the separation between activity and rest time was obscured. In a few birds activity became continuous and apparently arrhythmic. The activity rhythms of the birds were entrainable to a 12∶12 h light-dark cycle.

These results differ from those obtained previously by Menaker and his co-workers on the house sparrow, in which pinealectomy always resulted in arrhythmia. Nevertheless, it is suggested that the differences between these two species are only quantitative in nature rather than an expression of a qualitative difference in the organization of the circadian system. The results are consistent with the following modification of the model developed for the house sparrow: The avian pineal is the seat of a self-sustaining circadian pacemaker that acts on a population of secondary self-sustaining oscillators. These secondary oscillators, located outside the pineal, are only weakly coupled to each other but strongly dependent on the pineal driver. Their net circadian period is shorter than that of the pineal driver but, like the pineal driver, they can be synchronized by light. — It is proposed that this model fits both the starling and the sparrow data and clarifies some previously unexplained results.