Pineal gland N-acetyltransferase and hydroxyindole-O-methyltransferase activity in the rainbow trout (Salmo gairdneri): seasonal variation linked to photoperiod.

The activities of rainbow trout pineal gland N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were determined at various times during a 24 h period in mid-winter and mid-summer. The results indicate a small diurnal fluctuation in HIOMT and a marked diurnal fluctuation in NAT activity with peak enzyme activities occurring during the dark phase. NAT can be assumed to be the rate limiting enzyme for the production of melatonin and, as NAT activity increases were associated with the dark phase, it can also be assumed that melatonin production would be greater in winter than in summer.