Circadian rhythm in inducibility of rat pineal N-acetyltransferase after brief light pulses at night: control by a morning oscillator

Summary

• 1. The organization of the pacemaker driving the rhythm of N-acetyltransferase inducibility by darkness at night in the rat pineal gland was studied in male Wistar rats by observing changes of the rhythm caused by brief light pulses.

• 2. When rats maintained for a second day in constant darkness were exposed to a 1-min light pulse before or at 0000 hours, N-acetyltransferase activity, following the initial drop, began to rise anew to the usual high night level. After a pulse at or past 0030 hours, N-acetyltransferase activity did not increase throughout the rest of the night. The boundary between re-inducibility and non-inducibility of N-acetyltransferase by darkness after a 1-min light pulse was thus around 0015 hours.

• 3. When rats were exposed to a 1-min light pulse the first night and then released into constant darkness, the boundaries between N-acetyltransferase re-inducibility and non-inducibility by darkness after a pulse presented in the second night were phase-shifted relative to the boundary in unpulsed rats. Pulses given before midnight the first night phase-delayed the boundary the second night; pulses given after midnight, phase-advanced the boundary. The advances were more pronounced than the delays.

• 4. The phase-response curve (PRC) showing shifts of the boundary the next night after a 1-min light pulse was the same as the PRC reported for shifts of the morning N-acetyltransferase decline, which are thought to represent phase-shifts of the morning oscillator controlling the decline. It differed markedly from the PRC reported for shifts in the evening N-acetyltransferase rise, which is thought to represent phase-shifts of the evening oscillator controlling the rise.

• 5. As the PRC showing shifts of the boundary between N-acetyltransferase re-inducibility and non-inducibility after light pulses and the PRC showing shifts of the morning N-acetyltransferase decline are the same, they may be both manifestations of phase-shifts of the morning oscillator. The boundary may indicate a certain phase of the morning oscillator, in which the oscillator begins to be sensitive to light and can be phase-advanced to a phase when N-acetyltransferase activity is low. N-acetyltransferase inducibility at night might thus be solely a function of the phase of the morning oscillator at the time of light pulse presentation.