Twenty-four hour rhythms of serum prolactin, growth hormone and luteinizing hormone levels, and of medial basal hypothalamic corticotropin-releasing hormone levels and dopamine and serotonin metabolism in rats neonatally administered melatonin.

To obtain information on long-term circadian consequences of administering melatonin neonatally to rats, we assessed the 24-hour rhythms of 1) serum prolactin (PRL), luteinizing hormone (LH), and growth hormone (GH), and 2) medial basal hypothalamic dopamine (DA) and serotonin (5-HT) metabolism and corticotropin-releasing hormone (CRH) content in 60-day old male rats injected with 100 micrograms of melatonin on the 5th day of life. Controls receiving vehicle alone showed serum PRL concentration (when 60 days of age) attaining its maximum at the end of the light period (i.e., at 2000 hr), while in melatonin-injected rats high PRL values were found between 1200 and 2000 hr. Twenty-four hour changes in serum LH levels exhibited a maximum at noon, and to a similar extent in vehicle- and melatonin-treated rats. Neonatal melatonin injection did not affect serum GH concentration when the rats were adult. In the medial basal hypothalamus (MBH), the dihydroxyphenyl acetic acid (DOPAC)/ DA ratio attained a maximum at midnight, its amplitude being significantly higher in melatonin- than in vehicle-treated rats. Neonatally melatonin-injected rats also exhibited a second maximum in DOPAC/DA ratio at noon, coinciding with a minimum in DA levels of MBH. The 5-hydroxyindoleacetic acid (5-HIAA)/5-HT ratio in MBH showed significant diurnal variations in vehicle-injected controls with maxima at 1200 and 0400 hr, while in melatonin-treated rats a single maximum occurred at 2400 hr. These maximum correlated with minima in 5-HT content of MBH. Neonatal melatonin injection brought about a significant increase in the CRH content of MBH, as well as distortion of its diurnal rhythmicity, a maximum being found at noon in controls and at 1800 hr in melatonin-treated rats. The results indicate that exposure to melatonin early in life affects subsequent diurnal rhythmicity of PRL release, and of DA and 5-HT turnover and CRH content in the MBH of rats.