Critical period for the entrainment of the circadian rhythm in blinded pups by dams.

The critical period for the entrainment of circadian rhythm in blinded pups by a foster mother with a rhythm inverted to that of the natural mother was determined by a convenient method for the analysis of the circadian rhythm of drinking behavior. The pups, optically enucleated on day 1, were transferred to the foster mother at various ages, measured in days from birth, and raised until weaning. When the pups were transferred before 4 days of age, the drinking rhythm of the adopted pups was in phase with that of the blinded natural pups born of and raised by the foster mother. On the other hand, when the mother was exchanged after 10 days of age, the rhythm of the adopted pups was out of phase with that of the natural pups of the foster mother. Pups exchanged on 5 or 7 days of age showed inconsistent phase of rhythm, either being entrained by the foster or the natural mothers, or affected by both mothers. These results indicate that blinded pups must be adopted by the foster mothers earlier than day 4 for the foster mothers to fully entrain the circadian rhythm of blinded pups.