Differential maturation of the molecular clockwork in the olfactory bulb and suprachiasmatic nucleus of the rabbit.

Recent studies suggest that the main olfactory bulb (OB) represents a functional circadian pacemaker. In many altricial mammals, during pre-visual stages of development the olfactory system plays a vital role in their survival. One remarkable example is the European rabbit; the newborns are normally raised in a dark nursery burrow, and the lactating female briefly visits her young approximately once every 24 h. Under these conditions, newborn rabbits depend on the circadian system to anticipate the arrival of the lactating doe as well as on pheromonal cues on the mother's ventrum to locate nipples and suckle efficiently. To investigate the development of the rabbit's circadian system, we characterized the 24-h pattern of expression of clock genes in the OB and suprachiasmatic nucleus (SCN) of pre-visual week-old rabbits and compared this with the pattern of expression in visual juvenile rabbits several weeks after weaning. We report for the first time that Per1, Cry1, and Bmal1 are expressed in the OB of newborn and juvenile rabbits. In addition, the diurnal pattern of clock gene expression develops earlier in the OB than in the SCN of newborn rabbits. Given the early maturation of the molecular clockwork and the biological relevance of this structure during development, it is possible that the OB plays an important role in temporal regulation during pre-visual life in rabbits.