Ovine Pineal Indoles: Effects of l‐Tryptophan or l‐5‐Hydroxytryptophan Administration

Abstract: 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 fivefold 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.