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 serum prolactin and luteinizing hormone (LH) levels in 21-, 25-, and 60-day-old male rats injected with 100 microg of melatonin or vehicle on the fifth day of life and kept under light from 08:00 to 20:00 daily. In 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 exhibited significant time-of-day-related changes in serum prolactin, with 77% of the variation in melatonin-injected rats explained by a single 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 prolactin, 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 those injected with melatonin. After a cosinor analysis, 71% of the variation in melatonin-injected rats was explained by a single 24h component, 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-injected controls, at noon and midnight, and a maximum at noon together with a plateau from 20:00 to 04:00 in melatonin-treated rats. In 60-day-old rats, the maximum of serum LH was found at noon, with a similar extent showing in vehicle- and melatonin-treated animals. The acrophases were at 10:20 (vehicle) and 10:57 (melatonin) and did not differ significantly from that found in 21-day-old melatonin-treated rats. The results suggest that exposure to melatonin early in life may accelerate maturation of 24h prolactin and LH profiles toward an adult form.