Effects of ghrelin on nocturnal melatonin secretion in sheep: An in vitro and in vivo approach

• Published in: Journal of animal science 2017-09, Vol.95 (9), p.4101-4112
• Main Authors: Kirsz, K, Szczesna, M, Molik, E, Zieba, D A
• Format: Article
• Language: English
• Subjects: Acyltransferases - genetics; Acyltransferases - metabolism; Animals; Aralkylamine N-acetyltransferase; Chlorophenylpiperazine; Circadian Rhythm - drug effects; Explants; Female; Ghrelin; Ghrelin - administration & dosage; Growth hormones; Hydroxylase; In vivo methods and tests; Melatonin; Melatonin - secretion; Photoperiod; Pineal gland; Pineal Gland - drug effects; Pineal Gland - metabolism; Pineal Gland - secretion; Piperazines - administration & dosage; Receptors; Reproduction - drug effects; Seasons; Secretion; Serotonin; Serotonin Receptor Agonists - administration & dosage; Serotonin receptors; Serotonin S2 receptors; Sheep; Sheep - physiology; Tryptophan; Tryptophan Hydroxylase - genetics; Tryptophan Hydroxylase - metabolism
• ISSN: 1525-3163; 0021-8812; 1525-3163
• DOI: 10.2527/jas2017.1737

Recent studies confirmed that pineal melatonin (MEL) secretion is regulated by ghrelin (GHRL) in seasonally reproductive sheep. The first in vivo experiment investigated whether the effect of GHRL on nocturnal secretion of MEL in sheep is mediated by type 2 serotonin receptors. Sheep ( = 16) were intravenously injected with GHRL (2.5 μg/kg of BW) and meta-Chlorophenylpiperazine (mCPP; a mixed agonist of 5-HT2B/5-HT2C receptors; 1 mg/kg BW), either combined or individually, during the short-day (SDS) and long-day (LDS) seasons. Blood samples were collected at 15-min intervals for 4 h. The second in vitro experiment examined the effect of GHRL (10 ng/mL) on MEL secretion by pineal gland (PG) explants incubated for 5 h. The expression levels and/or concentrations of tryptophan 5-hydroxylase 1 (TPH1), aralkylamine N-acetyltransferase (AA-NAT), and the phosphorylated form of AA-NAT (p31T-AA-NAT) were determined at selected time points during the SDS and LDS seasons. The experiments demonstrated that GHRL reduced MEL secretion ( < 0.01) during the SDS season. Administration of mCPP or a combination of GHRL + mCPP stimulated MEL secretion ( < 0.01) regardless of the season. Furthermore, GHRL regulated nightly MEL secretion in a TPH1-independent manner. However, during the SDS season, GHRL reduced p31T-AA-NAT expression and the AA-NAT concentration ( < 0.01) and inhibited MEL secretion ( < 0.001), whereas during the LDS season, GHRL had no effect on MEL secretion or on the expression of the examined enzymes. These findings indicate that GHRL directly and indirectly affects PG activity in sheep and that the photoperiod modulates the effects of GHRL.