Effects of light on circadian pacemaker development

The effects of raising cockroaches, Leucophaea maderae, in non-24-h light cycles on the response of the circadian system to light was examined. 1. Phase response curves (PRC) were measured for 6-h light pulses for animals raised in LD 11:11 (T22), LD 12:12 (T24), and LD 13:13 (T26). The delay portion of the PRC was found to be significantly reduced in T22 animals (compared to T24 animals) while the advance portion of the PRC was reduced in T26 animals. Compared to T26 animals, phase shifts were more positive at every phase for animals raised in T22. 2. When transferred from constant darkness (DD) to constant light (LL) the freerunning period lengthened significantly less for T22 animals than T24 animals, and in some cases tau in LL was actually shorter than tau in DD in T22 animals. Animals raised in LL were inactive when exposed to LL as adults, and unlike T24 animals, were consistently reset to the beginning of the subjective night (near CT 12) when transferred to DD. 3. Roaches raised in T22 would entrain to LD 6:18, but a few animals exhibited periods of relative coordination indicating that the 24-h light cycle was near the limits of entrainment. These results indicate that the circadian system's responsiveness to light, as well as its freerunning period (Barrett and Page 1989), is dependent on the lighting conditions to which the animals are exposed during development.