IMMEDIATE RESPONSE TO LIGHT OF RAT PINEAL MELATONIN RHYTHM - ANALYSISBY IN-VIVO MICRODIALYSIS

Melatonin in the extracellular space of the pineal gland was measured continuously for 4 consecutive days from single, freely moving rats by means of in vivo microdialysis. A robust circadian rhythm was observe d in the pineal extracellular melatonin under both light-dark (LD) and continuous dark (DD) conditions, the patterns of which were almost id entical for 4 days within individuals but varied substantially among i ndividuals. The offset phase of melatonin rhythm was more stable than the onset phase. Light-induced phase shift of melatonin rhythm was mea sured in individual rats, which had been entrained to LD and subsequen tly released into DD. On the 1st day in DD, a 3-min light pulse of 200 lx was applied either at circadian time (CT) of 17 or 22 h (5 and 10 h after the dark onset, respectively). The light pulse rapidly suppres sed the nocturnal melatonin level. The rate as well as the level of me latonin suppression was significantly greater by the pulse at CT22 tha n at CT17. A phase shift of the melatonin rhythm was calculated on the 2nd and 3rd days in DD. Significant phase delay shift was observed af ter the pulse at CT17 and advance shift after the pulse at CT22 of sim ilar to 1 h in either case. Because the amount of phase shift was not different between the 2nd and 3rd days in DD, the phase shift of pinea l melatonin rhythm by single light pulse seems to be completed immedia tely.