The Pineal NAT Rhythm

When rats were exposed to a light stimulus in the early night and then released into darkness, the next day both the SCN rhythm in c-Fos photoinduction and the pineal

Scn Rhythm

Figure 1. Example of the light-evoked induction of the immunoreactive c-Fos protein in the suprachiasmatic nucleus. A rat was exposed to a 30-min light pulse at 21 h in the night, then returned to darkness and killed 30 min later. Note that the photic c-Fos induction occurred predominantly in the VL-SCN. Adapted from Sumova et al., 1998 (35).

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Figure 2. The SCN rhythm in the light-induced c-Fos immunoreactivity (A) and the pineal rhythm in N-acetyltransferase (B). Rats maintained in LD 12:12 were released into darkness and the next day the SCN and the pineal rhythms were followed. Full bars indicate original dark periods. Data from Sumova and 1ll-nerova, 1998 (33) and from Illnerova and Van ek, 1987 (16), respectively.

Scn Rhythm

Figure 3. Phase delays of the SCN rhythm in the light-induced c-Fos immunoreactivity (A) and of the pineal N-acetyltransferase rhythm (B) following a photic stimulus in the early night and phase-advances of the SCN (C) and of the pineal rhythm (D) following a photic stimulus in the late night. Rats maintained in LD 12:12 were exposed to a light-stimulus (squares) or left untreated (circles), then they were released into darkness and the next day the SCN and pineal rhythms were followed. Rats were exposed to a 1-h light pulse at 23 h (A), to a 1-min pulse at 22h (B), to a 1-h light pulse at 02h (C) and to a 1-min pulse at 03h (D), respectively. Data from Sumova and Illnerova, 1998 (33) and from Illnerova and Vangdek, 1987 (16), respectively.

Figure 3. Phase delays of the SCN rhythm in the light-induced c-Fos immunoreactivity (A) and of the pineal N-acetyltransferase rhythm (B) following a photic stimulus in the early night and phase-advances of the SCN (C) and of the pineal rhythm (D) following a photic stimulus in the late night. Rats maintained in LD 12:12 were exposed to a light-stimulus (squares) or left untreated (circles), then they were released into darkness and the next day the SCN and pineal rhythms were followed. Rats were exposed to a 1-h light pulse at 23 h (A), to a 1-min pulse at 22h (B), to a 1-h light pulse at 02h (C) and to a 1-min pulse at 03h (D), respectively. Data from Sumova and Illnerova, 1998 (33) and from Illnerova and Vangdek, 1987 (16), respectively.

NAT rhythms were phase-delayed as compared with the rhythm profiles in control, unexposed animals; in both rhythms, the evening marker was delayed to a larger extent than the morning one (Figure 3A,B) (9,16,17,33). However, when such delays of both rhythms were studied under an extremely long photoperiod, LD 18:6, the morning markers were phase delayed more than the evening ones, due apparently to the state of the underlying pacemaking system (7,33,37). When rats were exposed to a light stimulus in the late night and then released to darkness, the next day, during the first tran sient cycle, only the morning decline in the SCN c-Fos photoinduction and in the pineal NAT, respectively, was phase advanced, as compared with the decline in control rats, but not the evening rise (Figure 3C,D) (16,17,19,33). Apparently, the evening and the morning SCN and pineal markers do not necessarily phase shift in parallel. The finding suggests a complex nature of the underlying SCN pacemaker where groups of neurons might be first reset together and via coupling might entrain other groups (25,42). The evening NAT rise started to be phase advanced only within four days following a late night light stimulus, and even then to a still lesser extent than the morning decline (16).

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