Restricted access to natural mother shifted endogenous rhythm of rat pups

For further investigation of the role of rat dams in setting the phase of pups' endogenous circadian rhythm, periodic maternal deprivation experiments were performed. Access of intact original mothers to blinded pups was restricted to light phase (L-Suckling pups) or dark phase (D-suckling pups) under 12-12 h light-dark alternation throughout the nursing period. Free-running patterns of drinking and corticosterone rhythms were determined for 3 consecutive weeks after weaning. In each determination of the rhythm, water consumption and corticosterone levels were measured every 4 hr. Both rhythms were clearly observed in both L- and D-suckling pups. Acrophase of water intake rhythm at 5 weeks of age in L-suckling and D-suckling groups were 0758 h and 1954 h while those of corticosterone rhythm were 0143 h and 0948 h respectively. Reversed phase relationship of rhythms between L- and D-suckling groups were observed throughout the period examined. These results indicate that suckling can act as a Zeitgeber for the endogenous rhythm of pups.