Daily variation in the concentration of 5‐methoxytryptophol and melatonin in the duck pineal gland and plasma

Abstract

The duck pineal gland rhythmically produces two 5-methoxyindole compounds, i.e. 5-methoxytryptophol and melatonin. 5-Methoxytryptophol levels are low at night and high during the day, while melatonin concentrations are high at night and low during the day. The melatonin rhythm reflects oscillations in the activity of serotonin N-acetyltransferase (AA-NAT; a penultimate and key regulatory enzyme in the melatonin biosynthetic pathway). The activity of hydroxyindole-O-methyltransferase (HIOMT; an enzyme involved in the synthesis of both 5-methoxytryptophol and melatonin) does not exhibit any significant rhythmic changes throughout the 24-hr period. Plasma levels of melatonin exhibited daily changes that were parallel to fluctuations in pineal melatonin content. Although plasma concentrations of 5-methoxytryptophol were low in ducks, they showed daily variations. The mean 5-methoxytryptophol concentration between zeitgeber time 9 (ZT9) and ZT15 was 2.4-times higher than the mean value for samples collected between ZT18 and ZT3. These findings indicate that in the duck the pineal production of 5-methoxytryptophol and melatonin may be inversely correlated.

Diurnal Profiles of Melatonin Synthesis-Related Indoles, Catecholamines and Their Metabolites in the Duck Pineal Organ

“…Of note, ASMT activity did not show diurnal fluctuation in the pineal organs of the majority of the investigated species [2,61], including the domestic duck [33]. For many years, it has been believed than the enzymatic capacity of ASMT is adequate to convert a large amount of NAS, which is produced at night, into MLT.…”

“…Analyzing the literature data, notably, Zawilska et al . [33] reported an ca. 3-fold increase in the activity of AA-NAT and an ca .…”

“…Therefore, the diurnal rhythms of 5-MTOL, with the highest levels during the photophase, were reported in the pineal organs and in the blood plasma of several species [25,32,34,35,44,45,46,62,63], including the domestic duck [33], whereas daily changes in the 5-MIAA level were described in the pineal organs of only a few species (rainbow trout [25], pigeon [35], Japanese quail [64], rat [65], and golden hamster [66]). Studies on Japanese quail demonstrated the presence and diurnal fluctuations of 5-MIAA in blood plasma and identified the pineal organ as a source of this indole in the circulation [64].…”

“…Our knowledge concerning the indole contents in the avian pineal organ is almost exclusively limited to MLT and 5-MTOL, which have been measured in several species by radioimmunoassays [32,33,34]. The data concerning other pineal indoles are only fragmentary [35,36,37].…”

Diurnal Rhythm of Plasma Melatonin Concentration in the Domestic Turkey and Its Regulation by Light and Endogenous Oscillators

“…In the face of a growing number of studies proving the significant role of the pineal gland and its chief hormone in poultry farming, there are very few studies focusing on the profiles of plasma MLT concentration in domestic birds [18,[22][23][24][25][26][27][28]. In most cases, the published data involve very young birds aged 2-4 weeks.…”