Melatonin is induced by light information through the retina and leads to growth factor activation. Thus, we investigated the effects of melatonin by controlling the photoperiod of growing young rats. Male Sprague‐gDawley rats (n=6; 4 weeks old) were divided into two experimental groups: the L/D group (normal photoperiod; light/dark: 12/12 h; lights on at 9:00 a.m.) and the L/L group (light/light: 24 h). Rat body weight and food consumption were measured daily for 8 weeks. After 8 weeks, the rats were anesthetized with a mixture of ketamine (50 mg/kg) and xylazine (10 mg/kg) and sacrificed. Tissue was then collected for RNA isolation (from brain, heart, liver, kidney, adrenal gland, testis, tibia, hind limb muscles). Also, serum was isolated from blood using a centrifugal separation. The L/L group had significantly lower body weight than the L/D group from 4 to 6 weeks (p<0.05). The L/D group had increased tissue mass, compared with the L/L group, but the difference was not statistically significant. The L/D group had a significantly higher melatonin concentration than the L/L group between the hours of midnight and 2:00 a.m (p<0.01). These results indicate that photoperiod length may affect the secretion of melatonin from the pineal gland. Also, the reduction of nocturnal melatonin secretion may retard the development of growing young rats. In future studies, we plan to compare exogenous melatonin administration with endogenous melatonin concentration induced by photoperiod control. Moreover, we will confirm whether the effects seen in pathological animal models can be reversed by controlling the photoperiod.

ABSTRACT   INTRODUCTION   MATERIALS AND METHODS   RESULTS   DISCUSSION   REFERENCES

• Seunghoon Lee(1Department of Rehabilitation Science in Interdisciplinary PhD Program, Graduate School of Inje University, Gimhae 621-749, Korea)
• Han-Ki Lee(Deaprtment of Physical Therapy, Masan College, Masan 630-729, Korea)
• Jinhee Shin(Department of Physical Therapy)
• Yunkyung Hong(Department of Rehabilitation Science in Interdisciplinary PhD Program, Graduate School of Inje University, Gimhae 621-749, Korea)
• Sang-Kil Lee(Cardiovascular & Metabolic Disease Center, College of Biomedical Science & Engineering, Inje University, Gimhae 621-749, Korea)
• Sangun Lee(Department of Physical Therapy, Aomori University of Health and Welfare, Aomori 030-8505, Japan)
• Takao Suzuki(Department of Physical Therapy, Aomori University of Health and Welfare, Aomori 030-8505, Japan)
• Taeyoung Kang(College of Veterinary Medicine, Jeju National University, Jeju 690-756, Korea)
• Yonggeun Hong(Department of Rehabilitation Science in Interdisciplinary PhD Program, Graduate School of Inje University, Gimhae 621-749, Korea/Department of Physical Therapy/Cardiovascular & Metabolic Disease Center, College of Biomedical Science & Engineering, Inje University, Gimhae 621-749, Korea)