Effect of shortened photoperiods on pineal serotonin n‐acetyltransferase activity and rhythmicity

Abstract

Adult male albino rats were acclimated to shortened light/dark (L/D) photoperiods (L/D:1/4/23 3/4 or L/D.2/22) concurrently with rats maintained in a L/D: 14/10 photoperiodic cycle. The activity and rhythmicity of pineal serotonin N‐acetyl‐transferase (NAT) was examined in the rats maintained in the shortened photoperiod at 4‐hr intervals for 24 hrs and compared to pineal NAT activity in rats maintained in a L/D:14/10 photoperiod. The results indicate that NAT activity exhibits a refractory period to darkness. During the refractory period the pineal NAT activity is approximately coincident with the period of minimal NAT activity exhibited by rats during the light period in a L/D:14/10 photoperiod. At the end of the refractory period there exists a dramatic rise in NAT activity reaching a peak at 04.00 hr which is 110–130 fold greater than the level during either the light phase or the refractory period. This zenith of NAT activity in rats maintained in shortened photoperiods is approximately 2 fold greater than the NAT activity peak exhibited by rats in the L/D:14/10 photoperiod which exhibits a 50–60 fold increase greater than daytime levels. Since NAT is believed to be the rate limiting enzyme in the formation of melatonin in the pineal gland, the findings imply an increase in the amount of melatonin synthesized in pineals of rats maintained in a short photoperiod. Suppression of reproduction in animals maintained in short photoperiods may be related to the increased melatonin formed as a result of increased pineal NAT activity.