The effect of pinealectomy on entrainment of the locomotor activity rhythm in sparrows maintained on various short days

Pinealectomy of house sparrows on 3L:21D (3 h light per 24 h) resulted in a significant increase in the time between the onset of perch-hopping activity and lights on (ψon) as well as the time between the offset of activity and lights of (ψoff). The daily variance in ψon and ψoff was also increased following the removal of the pineal gland. On longer light cycles (i.e., 5L:19D; 7L:17D), neither ψon or ψoff, nor the variance of ψon or ψoff was different between sham-pinealectomized and pinealectomized sparrows. Upon returning the birds to an ultrashort light cycle, 1L:23D, ψoff, as well as the variance in ψon and ψoff were again found to be significantly larger in the pinealectomized birds when compared to sham-operated controls. These results indicate that the effects of pinealectomy on the entrained rhythm of locomotor activity are most pronounced when birds are exposed to a weak entraining agent, such as an ultrashort light: dark cycle. In view of the observation that pinealectomy can alter the phase relationship between activity onset and offset, it is suggested that the pineal gland may be involved in the coupling of the oscillators that regulate activity onset and offset.