THE PHOTOPERIODIC RESPONSE IN SYRIAN-HAMSTER DEPENDS UPON A MELATONIN-DRIVEN CIRCADIAN-RHYTHM OF SENSITIVITY TO MELATONIN

The pineal gland, via the daily pattern of melatonin (MEL) secretion, is directly involved in the conduction of photoperiodic information. T he duration of MEL secretion is proportional to the duration of the da rk period and, whatever the photoperiod is, MEL synthesis occurs 3 or 4 h after the dark onset in Syrian hamsters. In order to determine the relative importance of the duration or the coincidence hypothesis, a daily infusion protocol was used in sexually active pinealectomized ha msters. Long duration of MEL infusion (10 h) completely inhibit testes whereas short duration infusion (5 h) had no effect. When the animals were infused twice within 2 h 30 min separated by 3 h, they presented a complete gonadal atrophy, similar to the one observed with the 10 h infusion. Measurement of plasma MEL during the infusion and seperatio n periods revealed that MEL reached physiological nighttime values dur ing the infusion period and fell to daytime values 1 h after the end o f an infusion period. Thus, the results could not be due to a time add itive action of the two MEL pulses. An intermediate response was obser ved when the 2 signals were applied across the light/dark transition. Gonadal regression did not occur when the 2 periods of infusion were s eparated by 5 h 30 min. The efficiency of this type of infusion was no t dependent on the ambiant photoperiod since similar results were obta ined in long and short photoperiods. The infusion was also as effectiv e during the day as well as during the night. These results suggest th at there is a rhythm of sensitivity to MEL, based on the coincidence h ypotheses, that are important for transmission of photoperiodic inform ation. This rhythm of sensitivity to MEL seems to be entrained by MEL itself, since the efficiency of the two pulses of MEL is not dependent of time of application and/or of photoperiod.