Lack of effect of ghrelin treatment on melatonin production in rat pineal and Harderian glands

The effects of ghrelin, a peptide hormone secreted from the stomach, on melatonin remain unknown. The aim of the study was to investigate possible ghrelin-melatonin interactions by studying the effect of ghrelin treatment on melatonin production in rat pineal and Harderian glands. Young (9 weeks) and old (20 months) male Wistar rats, maintained under a light:dark cycle regimen of 12:12, were assigned randomly to either a single subcutaneous (s.c.) injection of saline or ghrelin (1 μg/rat or 15 μg/rat) 1 h before sacrifice in the middle of the dark phase, or repeated s.c. saline or ghrelin injections (15 μg/rat), 3, 2 and 1 h before sacrificed in the middle of the dark phase. Neither ghrelin doses (1 μg/rat or 15 μg/rat) nor type of treatment (acute or repeated) influenced melatonin levels or the melatonin synthesizing enzymes N-acetyltransferase and hydroxyindole-O-methyltransferase activities, either in pineal gland or in Harderian glands. At the concentrations used, ghrelin does not influence melatonin production in rat pineal and Harderian glands, and therefore is not involved in the regulation of melatonin secretion, at least under our experimental conditions.