Photoperiod modulates melanoma growth in C57BL/6 mice

Seasonal variations can be found in almost any parameter of an organism's biochemistry, physiology, endocrinology, and behaviour. This phenomenon, generally called photoperiodism, results from one of the major functions of the circadian system, i.e. the translation of environmental information into rhythmic intraorganismic signals, which then regulate or influence physiology and pathology. We induced melanoma in three groups of syngeneic C57BL/6 mice synchronised to different photoperiods (8, 12, or 18 h of light within 24-h days) by subcutaneous injections of HFH18 melanoma cell suspensions. All animals from all three photoperiodic groups developed exponentially growing tumors. The average tumor volume on day 31 post injection was significantly smaller in animals exposed to light/dark conditions (LD) 8:16h as compared with animals held in LD 18:6h and intermediate in animals from the equinox group. These results indicate that C57BL/6 mice react to photoperiod, which can exert a significant effect on tumor growth.