Pineal, retinal, serum melatonin, and N-acetylserotonin rhythms in chicks on reversing light regimen and eye covering

After reversing a 12:12-h light-dark regimen of environmental lighting, pineal and retinal melatonin levels of white leghorn chicks recorded at the first mid-darkness were greatly enhanced and reached maximum levels after three more days; those recorded during the light periods indicate gradual decline but were far from the nadir of the original light period, even at the end of the experiment. The first mid-darkness serum melatonin levels recorded after photoperiod reversal were not much different from their original mid-light values. However, on continuing with the reversed regimen, the next mid-darkness levels were sharply increased, and maximum levels were reached after a further 2 days. Under the same experimental conditions light had much more drastic effects, and 6 h on the reversed regimen were sufficient to bring down completely the high value of the original mid-darkness period to the level of the starting mid-light nadir. Retinal N-acetylserotonin (NAS) measured simultaneously had a pattern similar to that of melatonin, but the pineal NAS rhythm did invert completely, albeit gradually. Eye covering did not prevent inversion of pineal and serum melatonin rhythms, which were identical in eye-covered and sighted control chicks on the reversed regimen of light. However, retinal melatonin values of the light periods were significantly less depressed in eye-covered than in sighted control chicks. Moreover, eye covering completely prevented the retinal NAS depression under light but did not affect pineal NAS. During darkness retinal and pineal NAS elevation was sluggish in the eye-covered chicks.