24-HOUR RHYTHMS OF SERUM PROLACTIN, GROWTH-HORMONE AND LUTEINIZING-HORMONE LEVELS, AND OF MEDIAL BASAL HYPOTHALAMIC CORTICOTROPIN-RELEASINGHORMONE LEVELS AND DOPAMINE AND SEROTONIN METABOLISM IN RATS NEONATALLY ADMINISTERED MELATONIN

To obtain information on long-term circadian consequences of administe ring 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 6 0-day old male rats injected with 100 mu g of melatonin on the 5th day of life. Controls receiving vehicle alone showed serum PRL concentrat ion (when 60 days of age) attaining its maximum at the end of the ligh t 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 exten t in vehicle- and melatonin-treated rats. Neonatal melatonin injection did not affect serum GH concentration when the rats were adult. In th e medial basal hypothalamus (MBH), the dihydroxyphenyl acetic acid (DO PAC)/DA ratio attained a maximum at midnight, its amplitude being sign ificantly higher in melatonin- than in vehicle-treated rats. Neonatall y 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-hy droxyindoleacetic acid (5 HIAA)/5-HT ratio in MBH showed significant d iurnal 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 maxima correlated with minima in 5-HT content of MBH. Neonatal melatonin injection brought about a significant increase in t he CRH content of MBH, as well as distortion of its diurnal rhythmicit y, a maximum being found at noon in controls and at 1800 hr in melaton in-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.