Rats anticipate a scheduled daily meat by entrainment of a circadian pacema
ker separate from the light-entrainable circadian pacemaker located in the
suprachiasmatic nuclei (SCN). The site and molecular mechanisms of the food
-entrainable pacemaker are unknown. The intrinsic period (tau) of the SCN p
acemaker is significantly lengthened by deuteriation. Sensitivity of food-e
ntrained circadian rhythms to D2O (25% in drinking water) was evaluated in
intact and SCN-ablated rats entrained to daily feeding schedules. In intact
rats fed ad-libitum, D2O lengthened tau sufficiently to drive activity rhy
thms out of entrainment to the light-dark cycle. By contrast, food-entraine
d rhythms were surprisingly resistant to modulation by D2O. The mean daily
onset time of food anticipatory activity in rats with complete SCN-ablation
s was not affected by up to 28 days of D2O intake. Transient delays and dis
ruption of anticipatory activity were evident in intact and one partial SCN
-ablated rat during D2O treatment, but these are interpretable as effects o
f coupling and/or masking interactions between a D2O-sensitive light-entrai
nable pacemaker, and a D2O-resistant food-entrained pacemaker. Differential
sensitivity to D2O suggests diversity in the molecular mechanisms of food-
and light-entrainable circadian pacemakers in mammals. D2O may have utilit
y as a screening., test to identify putative food-entrainable pacemakers fr
om among those central and peripheral tissues that can express circadian os
cillations of clock genes independent of the SCN. (C) 2001 Elsevier Science
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