Abstract
Circadian rhythms of mammals are generated endogenously, the master oscillator system residing in the suprachiasmatic nuclei (SCN). Previous experiments have indicated that the rabbit has a feeding entrainable circadian oscillator (FEO) which is supposed to be of greatest importance during the early infancy of the rabbit. Here we report the course of telemetrically monitored core body temperature of rabbit pups and of their does. Temperature increased from 37.6 ± 0.3 °C on day 2 to 39.5 ± 0.1 °C on day 28 of life. The pups showed a 24 h temperature rhythm even during their first days of life. Temperature increased 2½-3 h prior to nursing for 0.4-0.8 °C and rose for an additional 0.4-0.6 °C immediately after milk ingestion. The anticipatory, but not the postprandial component persisted when nursing was skipped twice. The persistence of a rhythm in the absence of any entraining agent is crucial for its endogenous generation. In the doe, the core body temperature gradually decreased during the last 2/3 of pregnancy. During parturition it steeply rose for 1.5-1.7 °C and attained a plateau of 39.7 ± 0.2 °C during lactation. The circadian rhythm persisted during the whole course of pregnancy and lactation. Thus, in the rabbit an endogenous, feeding entrainable circadian oscillator appears to operate from the first days of life. It is of functional significance in that it alerts the pup in time so that it is able to utilize the singular short presence of the doe for maximal milk intake.