Long‐term daily melatonin infusion induces a large increase in N‐acetyltransferase activity, hydroxyindole‐O‐methyltransferase activity, and melatonin content in the Harderian gland and eye of pinealectomized male Siberian hamsters (Phodopus sungorus)

Abstract

The effects of long-term daily melatonin infusions on the melatonin synthetic pathway in the Harderian glands and eyes of male Siberian hamsters were studied. Hamsters were pinealectomized (PX) and infused daily for 8 hr with either melatonin (6 microg/hr) or vehicle for 7 days in short photoperiod (SP, 10L:14D), followed by 14 wk in either SP (SP group) or in constant darkness (DD group). After the infusion period (15 wk), the infusion was stopped and animals were transferred into SP for 3 wk. The hamsters were then killed at midday or midnight. Exogenous melatonin infusion caused an increase in the Harderian gland weight, which was still evident 3 wk after the end of the treatment. In addition, exogenous melatonin increased endogenous melatonin concentrations (4-fold) and hydroxyindole-O-methyltransferase (HIOMT) activity (2-fold). N-acetyltransferase (NAT) activity, however, was not increased, and no day/night difference in melatonin content and HIOMT activity was observed in the Harderian glands. In the eye, melatonin infusions significantly increased day and night-time melatonin levels (up to 3-fold) and both NAT and HIOMT activities (up to 3.5-fold). This effect of melatonin treatment was observed in both SP and DD groups. These observations demonstrate that exogenously-infused melatonin at relatively high doses activates the synthesis of endogenous melatonin in the Harderian gland and eye of the Siberian hamster. Circulating levels of melatonin were also markedly increased, indicating that in these conditions melatonin may be released from extra-pineal sites.

Entrainment of Circadian Activity Rhythms in Rats to Melatonin Administered at T Cycles Different from 24 Hours

“…The MEL profile showed a relatively high basal level. Recently, it was reported that chronic MEL infusion enhanced the ability of the Harderian gland to produce MEL, thus leading to an increased basal level of the hormone in the Djungarian hamster [20]. Such a mechanism might explain the elevated basal level in our study, because the MEL-producing Harderian gland also exists in the rat [21].…”

Long-term melatonin administration enhances the antioxidant potential of human plasma maintained after discontinuation of the treatment

“…Our results are in accordance with other authors who have demonstrated that oral administration of melatonin to patients with type 2 diabetes or primary essential hypertension correlates with the increased antioxidant capacity of plasma (KedzioraKornatowska et al 2008(KedzioraKornatowska et al , 2009). Similarly, infusion or oral administration of melatonin precursor L-tryptophan caused a dose-dependent increase of endogenous melatonin level in plasma (Djeridane et al 2000;Jaworek et al 2007). In the present study, because the volunteers were banned from intake of other antioxidants, the increase in FRAP value could be related to enhancing the synthesis of melatonin after oral supplementation of the hormone.…”

Melatonin Synthesis in the Rat Harderian Gland: Age- and Time-related Effects

“…This organ is presumed to serve as an extraretinal phototransducer involved in the retinal± pineal axis (in Payne, 1994) and is intimately associated with eyes and opens by a duct onto the surface of the nictitating membrane (Sakai, 1981;Djeridane, 1992Djeridane, , 1994Djeridane, , 1996. In rodents, the Harderian gland synthesizes lipids (Seyama et al, 1992), photosensitive porphyrin pigments (Spike, Payne and Moore, 1992), and indoles (MenendezPelaez and Buzzell, 1992;Menendez-Pelaez et al, 1993;Djeridane et al, 1999a,b), mainly melatonin (Djeridane et al, 1998(Djeridane et al, , 2000. The organs also exhibit activity of serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), enzymes necessary for melatonin production (Pe Âvet et al, 1980;Balemans et al, 1983;Menendez-Pelaez and Buzzell, 1992;Menendez-Pelaez et al, 1993).…”