Photic entrainment: Mechanisms in the SCN and rhythm expression to the pineal.

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Light is the strongest zeitgeber for the mammalian circadian pacemaker which locates in the suprachiasmatic nucleus (SCN) and receives the photic stimuli directly through retinohypothalamic tract. In this experiment, the mechanism of the light entrainment was studied by measuring rat pineal melatonin, which shows a marked circadian rhythm and the synthesis which is directly suppressed by the light.1. Light suppression of melatonin synthesis: Monochromatic light pulse of 520nm (green) or 660nm (red), with the same intensity, was given at the midnight. The pineal serotonin N-acetyl transferase activity decreased faster and stronger than pineal or plasma melatonin. Decrease in the plasma melatonin was the smallest.2. Discrepancy in the light effects on the pineal melatonin and pacemaker: Pineal melatonin was suppressed by light more strongly at the late dark phase than at midnight, and by green light than by red light. Light suppression depends on the circadian phase and on the wavelength. However, the phase shift of the free-running locomotor rhythm was larger by green than by red light given at either phase. Therefore, the threshold for light suppression is lower than that for phase-shift. The retinohypothalamic tract is composed of functionally different fibers.3. Pineal in vivo microdialysis: Using in vivo microdialysis technique, extracelluar melatonin was measured continuously for 4 consecutive days in the same rat. Extracellular pineal melatonin was decreased rapidly by 3 min light pulse and the shift of the melatonin rhythm already completed on the next day of the pulse.4. Does acetylcholine transmitters the photic information ?: Microinjection of cholinergic agonist, carbachol, into the SCN neither suppressed the synthesis nor entrained the rhythm of extracellular pineal melatonin. Acetylcholine is unlikely to transmit the photic stimuli to the circadian clock and to the pineal gland.