Influences of light-dark shifting on the immune system, tumor growth and life span of rats, mice and fruit flies as well as on the counteraction of melatonin.

Animal models were designed to study the changes in immune function, oncogenicity and life span of rats, mice and fruit flies following light-dark (LD) shift manipulations. Alternating the photoperiod of LD 14:10 and DL 10:14 every 3 days in rats disrupted the circadian immune rhythm pattern, decreased the blood leukocyte concentration by 48% and lowered the percentage of lymphocytes in the blood from 71% (control) to 49.2%. In mice, the phagocytosis of neutrophils was only reduced by 7%, but the level of serum hemolysin dropped significantly in the photoperiod-shifted group as compared with animals kept under a constant photoperiod of LD 12:12 or LD 14:10. In Ehrlich-carcinoma- or sarcoma-180-injected mice, a reduction of survival duration, acceleration of tumor growth and depression of the immune system were recorded in the LD-shifted animals. In addition, the life span of fruit flies was shortened by 9.6% by photoperiodic shifting. Melatonin treatment evidently counteracted the deleterious influences of photoperiodic shifting in the above animals. It is suggested that repeated inversion of the LD cycle results in a chronobiological abnormality that, in turn, induces dysfunctions. Reentrainment by exogenous melatonin may inhibit the harmful influences of photoperiodic shifting.