Further evidence that circadian rhythm of blinded rat pups is entrained by the nursing dam.

To determine the importance of the nursing mother in setting the phase of the endogenous rhythm in blinded rat pups, free-running rhythm of locomotor activity in the pups was observed during 4-5 wk after weaning. Blinded pups, optically enucleated on day 1, were raised by either an intact natural mother or an intact foster mother with a rhythm reversed to that of the natural mother. The locomotor activity was continuously recorded for 4-5 wk, starting between 3 and 5 postnatal wk. At 5 wk of age the offset time of activity phase was found during the periods corresponding to the middle half of the light phase in the nursing period, regardless of whether they were raised by a natural or a foster mother. The free-running periods, in the blinded pups tested, were fairly close, with the majority ranging between 24.3 and 24.5 h. These findings indicate that the endogenous rhythm of the blinded pups is entrained by the nursing mother during the nursing period.