Ovine pineal indoles: effects of L-tryptophan or L-5-hydroxytryptophan administration

L-5-Hydroxytryptophan (L-5-HTP) (20 or 200 mg/kg i.p.) but not L-tryptophan (500 mg/kg i.p.) loading substantially increases serum melatonin in sheep. In the present study we examined the effects of these compounds on pineal serotonin and six serotonin metabolites. L-Tryptophan failed to increase 5-hydroxytryptamine (5-HT; serotonin) or any of its metabolites despite a five-fold increase in pineal tryptophan. In contrast, L-5-HTP loading produced a marked increase in pineal 5-HT and its metabolites, including N-acetylserotonin (NAS) and melatonin, indicating that an increased synthesis of melatonin is responsible for the increased serum melatonin concentration after loading with this precursor. No change in pineal indoleamine N-acetyltransferase (NAT) activity was seen. These results are consistent with the suggestion that, during daytime in the sheep, 5-HT availability may limit the production of melatonin.