DEVELOPMENT OF 24-HOUR RHYTHMS IN SERUM PROLACTIN AND LUTEINIZING-HORMONE LEVELS IN RATS NEONATALLY ADMINISTERED MELATONIN

To obtain information on the chronobiological consequences of neonatal melatonin administration to rats, we examined the 24h variations of s erum prolactin and luteinizing hormone (LH) levels in 21-, 25-, and 60 -day-old male rats injected with 100 mu g of melatonin or vehicle on t he fifth day of life and kept under light from 08:00 to 20:00 daily. I n 21-day-old rats, circulating prolactin varied diurnally in melatonin -treated rats only, showing a maximum at 16:00 and a secondary peak at 08:00. In 25-day-old rats, both vehicle-and melatonin-injected rats e xhibited significant time-of-day-related changes in serum prolactin, w ith 77% of the variation in melatonin-injected rats explained by a sin gle 24h component. Acrophase for prolactin rhythm of melatonin-treated rats occurred at 19:51. At 60 days of age, rats receiving vehicle or melatonin showed similar, significant 24h variations in serum prolacti n, with acrophases at 17:31-18:15. Serum LH levels attained a maximum at midnight in 21-day-old rats treated with vehicle and at noon in tho se injected with melatonin. After a cosinor analysis, 71% of the varia tion in melatonin-injected rats was explained by a single 24h componen t, with the acrophase at 11:08. In 25-day-old rats, a complex pattern of serum LH concentration was found, with two maxima in vehicle-inject ed controls, at noon and mid-night, and a maximum at noon together wit h a plateau from 20:00 to 04:00 in melatonin-treated rats. In 60-day-o ld rats, the maximum of serum LH was found at noon, with a similar ext ent showing in vehicle-and melatonin-treated animals. The acrophases w ere at 10:20 (vehicle) and 10:57 (melatonin) and did not differ signif icantly from that found in 21-day-old melatonin-treated rats. The resu lts suggest that exposure to melatonin early in life may accelerate ma turation of 24h prolactin and LH profiles toward an adult form.