Effects of Photoperiod or Exogenous Melatonin Administration on the Activity of N‐Acetyltransferase and Hydroxyindole‐O‐Methyltransferase and the Melatonin Content of the Harderian Gland of Two Strains of Female Syrian Hamsters

The activities of N‐acetyltransferase (NAT) and hydroxyindole‐O‐methyltransferase (HIOMT) and the melatonin concentration of the Harderian glands of two strains of Syrian hamster females (outbred and inbred LSH/SsLak) exposed to two different photoperiods (14:10 h and 8:16 h) were studied. The Harderian glands of the inbred hamsters showed greater NAT activity than those of the outbred animals. On the other hand, the glands of the outbred hamsters exhibited higher HIOMT activity and melatonin content than those of the inbred LSH/SsLak. Short photoperiod exposure, which produced gonadal regression in the inbred but not in the outbred hamsters, decreased the NAT activity in the inbred animals to the levels of the outbred. HIOMT activity was not affected by the lighting conditions. After the exposure to short days, the melatonin content of the inbred hamster Harderian glands increased to that in the outbred animals. Daily melatonin injections, which caused gonadal regression in the LSH/SsLak but not in the outbred hamsters, did not simulate the effect of the short photoperiod on the Harderian gland NAT activity and melatonin content of the inbred hamsters.