Influence of lighting cycles on daily rhythms in concentrations of plasma tri-iodothyronine and thyroxine in intact and pinealectomized immature broiler hens ( Gallus domesticus )

The effects of pinealectomy on the daily rhythms of concentrations of tri-iodothyronine (T3) and thyroxine (T4) were investigated in sexually immature female chickens exposed to 21-, 24- and 27-h cycles of light and darkness, or to extended periods of light or darkness for more than 24 h. In pinealectomized and control birds, rhythms in levels of plasma T3 and T4 were entrained by all lighting cycles and decreased in amplitude or disappeared in continuous light or darkness. In pinealectomized and control birds held on 21-h (11 h light:10 h darkness; 11L:10D) and 24-h (14L:10D) lighting cycles, the peak of the T4 rhythm coincided with, or lagged, the trough in the rhythm of T3 while in birds held on a 27-h (14L:13D) lighting cycle, the peak of the T4 rhythm preceded the trough in the rhythm of T3. Pinealectomy resulted in significant effects on the phases or amplitudes of rhythms of T3 or T4 in all lighting schedules except 4L:20D. However, these effects were not consistent in direction between experimental groups and were, therefore, of doubtful physiological significance. Pinealectomy increased the mean level of plasma T4 in birds exposed to continuous light or darkness or to 4L:20D. A corresponding reduction in mean levels of plasma T3 was seen in birds exposed to continuous light or darkness. It is concluded that under the lighting conditions investigated pinealectomy had no clear effect on the phases or amplitude of daily rhythms of levels of T4 or T3. However, after the effects of the feeding pattern on thyroid hormone rhythms imposed by the lighting cycle were removed by placing birds in constant lighting conditions, pinealectomy appeared to exert an inhibitory action on thyroid function.