General Discussion

A striking similarity exists between resetting of the SCN rhythm in c-Fos photoinduction and resetting of the pineal NAT rhythm. Following photic stimuli in the early, middle and late night, respectively, both the SCN and the pineal rhythms phase shift in a similar way during the first transient cycle. Importantly, the evening markers of both rhythms do not necessarily phase shift in parallel with the morning ones, suggesting a complex nature of the underlying pacemaking system (9,13). Following melatonin administration in late day, the evening rise in the SCN c-Fos photoinduction as well as the pineal NAT rise are phase advanced instantaneously, by about the same amount.

A striking similarity exists also in the response of the SCN rhythm in c-Fos photoinduction and in the response of the pineal NAT rhythm to the photoperiod. Both rhythms are photoperiod-dependent: under a long photoperiod, the interval enabling high c-Fos photoinduction as well as the interval of elevated NAT activity are short and under a short photoperiod they are long. Under LD 16:8, LD 12:12 and LD 8:16, respectively, the morning decline in the light-induced c-Fos occurs at about the same time as that in the NAT activity, whereas the evening NAT rise occurs by 1 to 2 h later than the rise in c-Fos photoinduction. The 1 to 2h delay in the NAT rise may be explained by the time interval necessary for the NAT mRNA and protein formation (20): following administration of isoproterenol, a beta adrenergic agonist, it takes 1 to 2h before the NAT activity markedly increases (Figure 8) (14). The gradual evening rise in the number of the light-induced c-Fos immunopositive cells suggests that more and more SCN neurons start to be in a light-responsive phase, and towards the middle of the night all cells capable of c-Fos photoinduction may respond. Similarly, strength of the signal coming from the SCN into the pineal may gradually increase; under an extremely long, LD 20:4 photoperiod, NAT activity rises as rapidly as after isoproterenol administration (Figure 8) (14).

After transition of rats from a long to a short photoperiod, decompression of the waveform of the SCN rhythm in c-Fos photoinduction as well as of the pineal NAT rhythm waveform proceeds just gradually and is roughly completed within two weeks whereas compression of the SCN rhythm after a change from a short to a long photoperiod is roughly achieved already within three days. Memory on long days stored in

Time after onset of darkness (h)

Figure 8. The evening rise in the pineal N-acetyltransferase activity under various photoperiods and following isoprotenerol administration. Rats were maintained in LD 20:4 (squares), LD 18:6 (up-triangles), LD 16:8 (down-triangles), LD 12:12 (rhombs) and LD 8:16 (hexagons), respectively, and killed at various times after the onset of darkness. Isoproterenol (0.5mg/kg) was administered to rats maintained in LD 12:12 at the time of the usual dark onset; the rats were left in light and killed at various times after isoproterenol administration (circles). Data from Illnerova and Vani ôek, 1983 (14).

Time after onset of darkness (h)

Figure 8. The evening rise in the pineal N-acetyltransferase activity under various photoperiods and following isoprotenerol administration. Rats were maintained in LD 20:4 (squares), LD 18:6 (up-triangles), LD 16:8 (down-triangles), LD 12:12 (rhombs) and LD 8:16 (hexagons), respectively, and killed at various times after the onset of darkness. Isoproterenol (0.5mg/kg) was administered to rats maintained in LD 12:12 at the time of the usual dark onset; the rats were left in light and killed at various times after isoproterenol administration (circles). Data from Illnerova and Vani ôek, 1983 (14).

the SCN pacemaking system may explain "the carry-over phenomenon" when infrequent long day treatment induces long day responses (2).

And finally, a striking similarity exists also between the SCN rhythm in c-Fos photoinduction and the pineal NAT rhythm during ontogenesis. In newborn rats, the interval enabling high c-Fos photoinduction in the SCN as well as the interval of elevated NAT activity in the pineal are markedly extended as compared with those in adult animals. In 10-day-old rats, though the intervals are already reduced, the SCN as well as the pineal rhythm are not yet fully photoperiod-dependent.

The aforementioned data indicate that changes in the pineal NAT rhythm reflect mostly changes of the intrinsic rhythmicity in the VL-SCN. Though we do not have yet enough data which would allow us to compare in detail the rhythm in the spotaneous c-Fos immunoreactivity, which appears mostly in the DM-SCN, with the pineal NAT rhythm, preliminary results suggest that even that rhythm shows similarity with the pineal rhythmicity. First, the rhythm in the spontaneous c-Fos induction in the DM-SCN is photoperiod-dependent similarly as the NAT rhythm. The interval of low c-Fos immunoreactivity which indicates low neuronal activity and at the same time the subjective night, is shorter on long than on short days, similarly as the interval of elevated NAT activity. And second, in newborn rats, the period of a low spontaneous c-Fos immunoreactivity is considerably longer than in adult animals, similarly as the interval of elevated NAT activity is longer in newborn than in adult rats.

The Insomnia Battle

The Insomnia Battle

Who Else Wants To Sleep From Lights Out 'Til Sunrise Without Staring At The Ceiling For Hours Leaving You Feeling Fresh And Ready To Face A New Day You know you should be asleep. You've dedicated the last three hours in the dark to trying to get some sleep. But you're wide awake.

Get My Free Ebook


Post a comment