Components of the melatonin message in the response to photoperiod of the tammar wallaby (Macropus eugenii).

Female tammars experiencing long-day photoperiod (LD 15:9) are in a reproductive state termed seasonal quiescence. After a change to LD 12:12, a sequence of endocrine events, dependent on a melatonin message being interpreted during the next 3 days, leads to the disappearance of a morning pulse of prolactin on day 4, and to reactivation of the quiescent corpus luteum by day 8. Elements of the message were investigated in three experiments. In experiment 1, tammars on LD 15:9 were injected with melatonin 2.5 hr before lights out on 0-5 successive days. All those receiving four or five daily injections, and six of ten receiving two or three injections, reactivated. In experiment 2, tammars on LD 15:9 were injected with melatonin or oil on 5 consecutive days to achieve a lengthening of the period of elevated melatonin from 9.0 hr to 9.5 hr, 10 hr, 10.5 hr, and 11.5 hr respectively. One of five tammars experiencing 10 hr and seven of ten experiencing the longer periods reactivated. In experiment 3, tammars on LD18:6 received melatonin 6 hr, 6 + 3 hr or 3 hr before lights out for ten successive days. Between injections the melatonin concentrations returned to basal levels. The treatments thus provided a skeletal increase in the profile of melatonin from 6 to 9 hr or 6 to 12 hr. All those treated at 6 + 3 hr reactivated and three of five treated at 6 hr only reactivated. We conclude that the tammar stores information about the change in the maximum duration of elevated melatonin experienced each night and, after three successive periods in which the duration is greater than 10.5 hr, it reactivates. Since reactivation cannot occur until completion of the third dark phase, the processing center must be active during the fourth day, prior to the abolition of the prolactin peak.