International journal of chronobiologyJan 1, 1979

Day length controls serotonin-processing enzyme activity in the rat pineal gland

Circadian Biology Weekly Brief ↗PubMed ↗

Updated Apr 4, 2026

Abstract

Constant light exposure significantly reduces pineal serotonin N-acetyltransferase (NAT) activity in adult male albino rats.

Pineal NAT levels during nocturnal hours are depressed under constant light conditions.
Exposure to darkness with brief light interruptions leads to similarly low NAT activity and disrupted rhythmicity.
The timing of environmental light appears to play a more critical role in regulating pineal NAT activity than the total duration of darkness.
Both constant light and intermittent light exposure obliterate the natural rhythm of pineal NAT activity.

Simplified

Adult male albino rats were acclimated to constant light (light:dark-LD-24:0) or to darkness interrupted with brief periods of light at 6 h intervals (LD 1/4:5 3/4 X 4) concurrently with rats maintained in a LD 14:10 photoperiodic cycle. The activity and rhythmicity of pineal serotonin N-acetyltransferase (NAT) was examined at regular intervals for 24 hours in rats maintained in the experimental photoperiods and compared to pineal NAT activity and rhythmicity in rats maintained in the LD 14:10 photoperiod. The results indicate that constant light is capable of depressing nocturnal levels of rat pineal NAT and obliterating the pineal NAT rhythm. Likewise, rats subjected to darkness interrupted with brief periods of light at 6 h intervals experienced a similar response in pineal NAT activity to animals subjected to constant light, i.e., pineal NAT activity was persistently low and the rhythmicity was obliterated. The results are discussed relative to the hypothesis that the pineal NAT activity responds to an endogenous rhythm in photoperiodic time measurement. The evidence herein suggests that the time of occurrence of environmental light in the photoperiod is more important in determining pineal NAT activity and/or rhythmicity than is the total amount of darkness or the dark to light ratio to which animals may be subjected.

Day length controls serotonin-processing enzyme activity in the rat pineal gland

Abstract

You found one interesting study. We’ll send the next 7.

What Lands in Your Inbox Each Week:

Recent issues from the circadian biology brief

Abstract

Related papers

You found one interesting study. We’ll send the next 7.

What Lands in Your Inbox Each Week:

Recent issues from the circadian biology brief