Effect of Photoperiod

The finding that an early night light stimulus phase delays primarily the evening marker of the SCN and pineal rhythms and a late night light stimulus phase advances primarily the morning marker of both rhythms suggests that on long days light perturbing into the late evening and early morning hours may compress the waveform of the SCN and pineal rhythms; on short days, the waveform may decompress. This actually happens. The interval between the evening rise in c-fos photoinduction and the morning decline under unmasked conditions in darkness as well as that between the evening NAT rise and the morning decline were by about 5 h longer under a short, LD 8:16 photoperiod, than under a long, LD 16:8 photoperiod (Figure 4A,B) (8,9,12,36). In both rhythms, the interval under the short photoperiod was extended assymetricaly, into the morning hours. This indicates a more important role of the morning than of the evening light in entrainment of the rat circadian pacemaking system (9,1.5). Similarly, in Syrian and European hamsters, the interval between the evening rise and the morning decline in the SCN c-fos photoinduction is also longer on short than on long days (41). Importantly, the photoperiod affected the waveform of the SCN rhythm in

Figure 4. Effect of a photoperiod and of a light stimulus encompassing midnight on the SCN rhythm in c-Fos photoinduction and on the pineal N-acetyltransferase rhythm, respectively. A: Rats were maintained in LD 16:8 (circles) or LD 8:16 (squares) and the SCN rhythm in photic induction of c-fos mRNA (in situ hybridization) was followed in darkness. Full bars indicate original dark periods. B: rats were maintained in natural daylight and the pineal N-acetyltransferase rhythm was followed on June 20 (circles) or on December 19 (squares). Full bars indicate dark periods. C: Rats maintained in LD 12:12 were exposed to a light stimulus from 23 h till 08 h (squares) or left untreated (circles), then released into darkness and the next day the rhythm in the light-induced c-Fos immunoreactivity was followed. Full bar indicates original dark period and the dark period during the night when rats were exposed to a long light stimulus, respectively. D: Rats maintained in LD 12:12 were exposed to bringing forward the light onset to 23h (squares) or left untreated (circles); the next day, they experienced darkness already since 14 h and the pineal N-acetyltrans-ferase rhythm was followed. Full bar indicates original dark period and the dark period during the night when the light onset was brought forward to before midnight, respectively. Data from Sumova et al., 1995 (36) (A), Illnerova and VangCek, 1980 (12) (B), Sumova and Illnerova, 1998 (33) (C) and Illnerova and Vangiek, 1987 (17) (D).

Figure 4. Effect of a photoperiod and of a light stimulus encompassing midnight on the SCN rhythm in c-Fos photoinduction and on the pineal N-acetyltransferase rhythm, respectively. A: Rats were maintained in LD 16:8 (circles) or LD 8:16 (squares) and the SCN rhythm in photic induction of c-fos mRNA (in situ hybridization) was followed in darkness. Full bars indicate original dark periods. B: rats were maintained in natural daylight and the pineal N-acetyltransferase rhythm was followed on June 20 (circles) or on December 19 (squares). Full bars indicate dark periods. C: Rats maintained in LD 12:12 were exposed to a light stimulus from 23 h till 08 h (squares) or left untreated (circles), then released into darkness and the next day the rhythm in the light-induced c-Fos immunoreactivity was followed. Full bar indicates original dark period and the dark period during the night when rats were exposed to a long light stimulus, respectively. D: Rats maintained in LD 12:12 were exposed to bringing forward the light onset to 23h (squares) or left untreated (circles); the next day, they experienced darkness already since 14 h and the pineal N-acetyltrans-ferase rhythm was followed. Full bar indicates original dark period and the dark period during the night when the light onset was brought forward to before midnight, respectively. Data from Sumova et al., 1995 (36) (A), Illnerova and VangCek, 1980 (12) (B), Sumova and Illnerova, 1998 (33) (C) and Illnerova and Vangiek, 1987 (17) (D).

the light-induced c-Fos immunoreactivity directly, and not via the pineal melatonin (32).

A long light stimulus encompassing the middle of the night compressed the waveform of the SCN rhythm in c-Fos photoinduction (Figure 4C) and of the pineal NAT rhythm (Figure 4D) in a manner similar to the effect of a long photoperiod, i.e., by phase delaying the evening marker of both rhythms and phase advancing the morning one (9,17,33). When rats were maintained under an extremely long, LD 18:6 photope-riod, even a 5-min or a shorter pulse had such an effect, i.e., it phase-delayed the evening marker, phase-advanced the morning one and further compressed the SCN and the pineal rhythm waveform (15,33).

When rats were transferred from a long, LD 16:8, photoperiod to a short, LD 8:16, photoperiod, the waveform of the SCN rhythm in c-Fos photoinduction (Figure 5A), as well as that of the pineal NAT rhythm (Figure 5B) extended just gradually and it took two weeks before the full extension was achieved (11,34). However, when rats were transferred from a short to a long photoperiod, compression of the interval enabling high SCN c-Fos photoinduction occurred within three days (Figure 5C) (34). It appears that the memory on long but not on short days is stored in the SCN itself.

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