Uncoupling protein 1 (UCP1) effects the production of heat from brown adipose tissue (BAT) in non-shivering thermogenesis. Prolactin (PRL) receptors are widely distributed and, in ovine BAT, their expression peaks with the appearance of UCP1 (Symonds et al. 1998). Subsequently, in the newborn period, there are concomitant decreases in both PRL receptor and UCP1 abundance (Budge et al. 2001). In addition, the administration of PRL to lambs during the first week of neonatal life improves thermoregulation (Pearce et al. 2001). This study aimed to examine the hypothesis that maternal PRL administration can promote UCP1 development in the fetal rat pup. Female rats were randomly allocated to PRL or control (C) groups. Recombinant PRL (25 mg kg^-1 day^-1), or an equivalent volume of saline, were infused throughout pregnancy from 12 h after mating (Yang et al. 2001). Each dam was randomised to sampling before or after birth. For fetal BAT sampling, dams were humanely killed by overdose of metafane at 19.5 days gestation (term = 22 days; n = 4, both groups) and pups removed from the uterus. Neonatal rat pups delivered spontaneously and, at 18 h after birth (n = 7, both groups), newborns were weighed and humanely killed before sampling of interscapular BAT. All animal procedures were carried out in accordance with US legislation. The abundance of UCP1 in BAT was determined by immunoblotting and expressed relative to a reference sample from a control fetus (mean ± S.E.M.). In addition, histological sections of BAT were prepared for lipid and UCP1 analysis.

Maternal PRL infusion resulted in a greater abundance of UCP1 in fetal rats at both 19.5 days gestation (C: 97.2 ± 8.4 % ref.; PRL: 525.6 ± 74.4 % ref.; P ▓le│ 0.04, ANOVA) and 18 h after birth (BAT from individual neonates pooled for analysis – C: 955.5 % ref.; PRL: 2201.1 % ref.). Neonatal BAT possessed fewer lipid droplets but more UCP1, as determined by histology. There were no significant effects of PRL on the duration of pregnancy, offspring survival or pup weight. In conclusion, as rats are altricial, and the potential thermogenic activity of BAT develops over the first few days of postnatal life, these changes in BAT recruitment prior to birth further implicate PRL in fetal and neonatal BAT maturation.