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)

The effects of long-term daily melatonin infusions on the melatonin synthet ic 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 mu g/hr) or vehicle for 7 days in short photoperiod (S P, 10L:14D), followed by 14 wk in either SP (SP group) or in constant darkn ess (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 kill ed at midday or midnight. Exogenous melatonin infusion caused an increase i n the Harderian gland weight, which was still evident 3 wk after the end of the treatment. In addition, exogenous melatonin increased endogenous melat onin concentrations (4-fold) and hydroxyindole-O-methyltransferase (HIOMT) activity (2-fold). N-acetyltransferase (NAT) activity, however, was not inc reased, and mo day/night difference in melatonin content and HIOMT activity was observed in the Harderian glands. In the eye, melatonin infusions sign ificantly 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 tr eatment was observed in both SP and DD groups. These observations demonstra te that exogenously-infused melatonin at relatively high doses activates th e synthesis of endogenous melatonin in the Harderian gland and eye of the S iberian hamster. Circulating levels of melatonin were also markedly increas ed, indicating that in these conditions melatonin may be released from extr a-pineal sites.