RESUMEN
In European newborn rabbits, once-daily nursing acts as a strong non-photic entraining cue for the pre-visual circadian system. Nevertheless, there is a lack of information regarding which of the non-photic cues are capable of modulating pup circadian system. In this study, for the first time, we determined that the mammary pheromone 2-methylbut-2-enal (2MB2) presented in the maternal milk acts as a non-photic entraining cue. We evaluated the effect of once-daily exposure to maternal olfactory cues on the temporal pattern of core body temperature, gross locomotor activity and metabolic variables (liver weight, serum glucose, triacylglycerides, free fatty acids, cholecystokinin and cholesterol levels) in newborn rabbits. Rabbit pups were separated from their mothers from postnatal day 1 (P1) to P8 and were randomly assigned to one of the following conditions: nursed by a lactating doe (NAT); exposed to a 3-min pulse of maternal milk (M-Milk), mammary pheromone (2MB2), or water (H2O). To eliminate maternal stimulation, the pups of the last three groups were artificially fed once every 24-h. On P8, the rabbits were sacrificed at different times of the day. In temperature and activity, the NAT, M-Milk and 2MB2 groups exhibited clear diurnal rhythmicity with a conspicuous anticipatory rise hours prior to nursing. In contrast, the H2O group exhibited atypical rhythmicity in both parameters, lacking the anticipatory component. At the metabolic level, all of the groups exhibited a diurnal pattern with similar phases in liver weight and metabolites examined. The results obtained in this study suggest that during pre-visual stages of development, the circadian system of newborn rabbits is sensitive to the maternal olfactory cues contained in milk, indicating that these cues function as non-photic entraining signals mainly for the central oscillators regulating the expression of temperature and behavior, whereas in metabolic diurnal rhythmicity, these cues lack an effect, indicating that peripheral oscillators respond to milk administration.