No abstract

Melanophores were studied in tadpoles of the South African clawed toad, Xenopus laevis, during the first week after hatching (stages 46-49) at 25 degrees C. The tadpoles had melanophores with dispersed melanosomes in the light and punctate melanophores in the dark in LD 12:12. The melanophores remained punctate in constant dark and the melanosomes remained dispersed in constant light. Lights-out (in the light-time of LD 12:12) caused the melanophores to become punctate, which occurred more quickly than the dispersion of melanosomes, which commenced when the lights were turned on (in the dark-time of LD 12:12). Melanophores with dispersed melanosomes in tadpoles (in constant light) became punctate in response to a series of melatonin concentrations (0.2-5 ng/ml) in their bathing water irrespective of the time of day melatonin was administered. An image-analysis technique for assessing melanophore responses was tested.

Circadian responses to photoperiod were studied in house sparrows (Passer domesticus) by subjecting them to 4-h light pulses and measuring the subsequent phases of their circadian rhythms. The direction and magnitude of phase shifts in response to 4-h light pulses following pretreatment with light-dark cycles (LD) 16:8 or LD 8:16 varied with time of day; advances (3.4 h) occurred when pulses were imposed in the late subjective night on both groups of birds; delays (-2.1 h) occurred when the pulses were imposed in the early subjective night on the LD 8:16 birds. The time profiles for responses to light pulses that scanned 24 h (phase-response curves) were modified by long and short photoperiod. Short photoperiod 1) increased amplitude (1.7 h), 2) increased time from the prior lights-out to the peak of advances (6 h), and 3) decreased the mean phase shift (0.9 h).

No abstract

N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is thought to be responsible for melatonin circadian rhythms. The enzyme has circadian properties--its rhythm persists in constant conditions, and it is precisely controlled by light and dark. Experiments are reported in which 4-h light or dark pulses were imposed on chicks (Gallus domesticus) over a 24-h period. Pineal NAT profiles were measured during and subsequent to the pulses. The phase of the NAT cycle following pulses was plotted to obtain phase-response curves. Light pulses produced a maximum phase shift (advance of 5 h) 8 h after the expected time of lights-out; dark pulses produced a maximum phase shift (advance of 4 h) 3 h after the expected time of lights-out. Maximum phase delays (-2 h) occurred 1-2 h after the expected lights-out for light pulses and 8 h after expected lights-on for dark pulses.