Resetting of the mammalian circadian clock through lowering of the amplitude: rat pineal N-acetyltransferase rhythm as a model.

During resetting of the mammalian circadian clock, not only phase of the clock is shifted, but amplitude of overt rhythms driven by the clock may be temporarily reduced or even abolished. The present paper is aimed to elucidate the mechanism of amplitude reduction of the overt circadian rhythm in the rat pineal N-acetyltransferase (NAT). The rhythm has two phase markers, namely the time of the evening NAT rise and that of the morning decline. When the phase relationship between both markers is compressed drastically, the NAT rise may occur just close to or at the time of the decline and consequently the NAT rhythm with a full amplitude cannot be expressed. Such a compression may occur in two ways: either animals are subjected to a considerable advance in the light onset which phase advances the morning NAT decline and at the same time phase delays the evening NAT rise, or they are subjected to a considerable delay in the light offset, which primarily phase delays more the NAT rise than the decline. While in the former case the phase markers move in opposite directions, in the latter case they move in the same direction, but to a different extent. The data suggest a complex structure of the underlying clock.