HIGH REPEATABILITY OF THE AMPLITUDE AND DURATION OF THE NYCTHEMERAL RHYTHM OF THE PLASMA MELATONIN CONCENTRATION IN THE ILE-DE-FRANCE EWE

In order to determine individual variability in the amplitude and dura tion of the nycthemeral rhythm of the plasma melatonin concentration, 12 ovariectomized estradiol-treated Ile-de-France ewes were submitted to a sequence of 34 long days (LD 1, 16L:8D), 21 short days (SD, 8L:16 D), and 21 LD (LD2). Intensive blood sampling (hourly from 1 hr before dusk to 1 hr after dawn) was performed for 4 consecutive nights in ea ch of the photoperiodic situations (LD 1, SD, LD2), i.e., a total of 1 2 nights per ewe, Plasma melatonin was assayed by radioimmunoassay. In stantaneous amplitude (difference between each hourly measurement of t he plasma melatonin concentration during the nocturnal elevation and t he mean diurnal melatonin concentration), mean individual amplitude (s ame definition but using the mean melatonin concentration during the n octurnal elevation) and duration of elevation of plasma melatonin conc entrations were calculated for each ewe and each night and analyzed by variance analysis. Instantaneous amplitude varied significantly with photoperiod, with night intra-photoperiod, with ewes, and time intra-n ight. Mean individual amplitude varied significantly with photoperiod and with ewe but not with night intra-photoperiod. Dramatic difference s between individuals appeared in the amplitude and duration of melato nin rhythms, The lowest mean individual amplitude (all nights of measu rement) found was 95 pg/ml and the highest was 544 pg/ml. Overall repe atability coefficient of the mean amplitude of melatonin elevation was 0.71 (P < 0.001). Duration of melatonin elevation varied significantl y with photoperiod, with nights intra-photoperiod and with ewes. Durin g LD periods, mean duration of melatonin elevation was equivalent to d uration of the night (from 7 to 8 hr), while during SD it varied with ewes from 9 to 16 hr, Repeatability coefficient of the duration of mel atonin elevation in SD was 0.57 (P < 0.05). It is concluded that ampli tude and duration (in SD) of the nycthemeral rhythm of the plasma mela tonin concentration are highly repeatable individual characteristics i n the ewe.