Measurement of circadian rhythms of ocular melatonin in the pigeon by in vivo microdialysis

OCULAR melatonin rhythms were measured in pigeons (Columba livia) by in vivo microdialysis. The birds were placed in light-dark cycles with 12 h of light and 12 h of darkness (LD12:12) or continuous dim light (LLdim) after LD12:12. Under LD12:12, melatonin levels were low during the light and high during the dark. The rhythms persisted under LLdim with lower levels during the subjective day and higher levels during the subjective night. Two hours of light exposure in the middle of the night acutely suppressed melatonin to the daytime level. These results indicate that microdialysis is useful for studies investigating the mechanism regulating ocular melatonin rhythms.