Our findings suggest that anaesthesia in conjunction with surgery acutely disturbed the normal circadian rhythm of melatonin by delaying the onset of nocturnal melatonin secretion.

A new radioimmunoassay (RIA) using [125I]-melatonin as tracer for determination of melatonin in biological fluids has been developed. Melatonin antisera were raised in rabbits by immunization with bovine thyroglobulin conjugate of N-acetyl-5-methoxytryptophan. Two high-affinity and specific antisera were obtained. Unlike in previous studies melatonin was radioiodinated directly. Iodo-Gen was used as the oxidant. Radioiodinated melatonin was purified by TLC for use in RIA. Melatonin was extracted from plasma, serum and urine samples (1 ml) with chloroform. Using the extraction the sensitivity of the RIA method was 18 fmol/ml of original sample. Plasma, serum and urine extracts diluted parallelly with synthetic melatonin in RIA. HPLC analysis of plasma and serum extracts showed only one immunoreactive peak co-eluting with synthetic melatonin. Majority of urine immunoreactivity co-eluted with synthetic melatonin, but 7\p=n-\23% contaminating immunoreactivity was also observed. Daytime values for rat plasma, human serum and urine melatonin were 30\p=n-\60, 20\p=n-\40 and 50\p=n-\130 fmol/ml and the respective night values were 160\p=n-\300, 180\p=n-\370 and 230\p=n-\470 fmol/ml. Thus a characteristic diurnal rhythm of melatonin was observed in all cases. The urinary excretion of immunoreactive melatonin during the day was 3\p=n-\9 and during the night 11\p=n-\28 pmol/h. Thus we have developed a specific and valid RIA method for the determination of plasma and serum melatonin. Despite the incomplete specificity of the RIA for urine determinations, a clear diurnal rhythm for urine melatonin was observed. The distinct advantage of the utilization of [125I]melatonin as tracer is that the costly and cumbersome scintillation counting can be avoided.The first quantitative determination of melatonin utilized the effect of melatonin on dermal melano-

The concentrations of melatonin in 112 preovulatory follicular fluid (FF) samples obtained from 60 women undergoing in vitro fertilization and 27 patients at laparotomy during a spontaneous cycle were measured by RIA and compared with those in peripheral serum. The circadian and seasonal variations in FF melatonin were also analyzed. The FF melatonin concentrations in stimulated (mean +/- SEM, 61.9 +/- 6.4 pmol/L) and spontaneous cycles (98.1 +/- 8.9 pmol/L) were significantly higher (P less than 0.005) than those in peripheral serum (25.4 +/- 1.2 and 38.6 +/- 1.8 pmol/L, respectively), and in the stimulated cycles there was a positive correlation between them. The FF melatonin concentration in the morning (58.9 +/- 3.8 pmol/L) was significantly higher (P less than 0.005) than that in the daytime (23.2 +/- 0.8 pmol/L), but the morning concentrations did not differ between the light and the dark seasons of the year, whereas the daytime values were higher (P less than 0.005) during the dark season (27.1 +/- 2.1 pmol/L) than during the light season (21.1 +/- 2.1 pmol/L). The FF melatonin concentration did not correlate with follicular volume, and FF and serum melatonin concentrations showed no significant correlation with the serum concentrations of estradiol, progesterone, testosterone, or PRL. There were also no differences between FF melatonin concentrations in aspirates with or without an ovum. In summary, significant circadian and circannual variations in high FF melatonin concentrations were found, which suggest that melatonin could potentially interfere with the regulation of reproduction in humans at the follicular level.

The effects of season on the activity of the pituitary-ovarian axis and the pineal gland were studied in 11 women by serum and urinary melatonin determinations and in 21 women by measurements of the serum concentrations of anterior pituitary and ovarian hormones during the dark and light seasons. A melatonin index was determined by integration of the area below the curve of serum melatonin concentrations during 24-h periods in both seasons. During the dark season, the daytime 12-h melatonin index and daytime urinary melatonin excretion were significantly higher than during the light season. In addition, the duration of the nocturnal melatonin pulse (serum melatonin levels, greater than 65 pmol/L) was lengthened during this season, whereas the mean serum estradiol concentration was significantly decreased at the time of ovulation and during the luteal phase of the cycle, indicating lowered ovarian activity. Luteal phase gonadotropin concentrations were increased during the dark season, which was also characterized by increased sex hormone-binding globulin (SHBG) and decreased free testosterone concentrations and free androgen indices (ratio of testosterone to SHBG X 700) throughout the menstrual cycle. The dark season was thus characterized by increased melatonin secretion and decreased ovarian and androgenic activities. In summary, we characterized two season-dependent hormonal phenomena. Although we did not prove any cause and effect association between melatonin and anterior pituitary-ovarian hormones, the inverse seasonal relationship in pineal gland and ovarian secretion suggests that melatonin is causally related to reproduction in humans.

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