Photoperiodic control of melanoma growth in hamsters: influence of pinealectomy and melatonin.

Pinealectomy (PX) increased MM1 (melanotic melanoma no. 1) hamster melanoma growth in animals held under a 14-h light, 10-h dark (14:10) photoperiod without altering tumor latency. Hamsters maintained under a 6-h light, 18-h dark (6:18) photoperiod exhibited gonadal collapse, a longer tumor latency, and slower tumor growth rate than animals held under 14:10. PX produced a further increase in tumor latency and a decrease in growth in these animals. In contrast, acute morning injection of low doses (50 micrograms/day) of melatonin or delivery by Silastic capsule (35 micrograms/day) implanted at the time of tumor cell inoculation increased MM1 melanoma growth in hamsters held under 14:10 photocycle, without affecting testicular or adrenal function. Treatment of hamsters 11 weeks before tumor cell inoculation with 14 micrograms/day melatonin via Silastic capsule produced a decrease in serum PRL but no change in tumor growth or testicular or adrenal weights in animals held under 14:10. Treatment of hamsters with 17.7 micrograms/day melatonin (Silastic capsule) 11 weeks before tumor cell inoculation increased testes and adrenal weights as well as serum PRL and androgen levels, but significantly decreased tumor growth in hamsters held under a short daily photoperiod. These results suggest that the photoperiod under which hamsters are maintained dictates the growth rate of MM1 tumors and the effect of PX on tumor behavior. When photoperiod significantly alters gonadal and adrenal function, the quantity, time, and duration of melatonin presentation are all important variables in the effect of melatonin on tumor growth.