Lactation decreases mRNA levels of opioid peptides in the arcuate nucleus of the rat

The state of lactation results in increased food intake to compensate for the increased energy expenditure to produce nutrients supplied to the offspring. In this study, Sprague-Dawley female rats lactating for 10–16 days, and rats 7 days post-lactation were implanted with osmotic minipumps infusing either naltrexone (NTX) (70 μg/h) or saline (0.9%) over a 48 h period. mRNA levels of pro-dynorphin (proDYN), pro-opiomelanocortin (POMC) and pro-enkephalin (proENK) were measured in the arcuate nucleus (ARC) and whole pituitary of both groups. In both saline- and NTX-treated lactating subjects, food intake was higher than in post-lactating subjects (P<0.01). In post-lactating subjects, NTX decreased food intake by 27% during the infusion period (P<0.05). There were no significant differences in body weight between the treatment groups; however, naltrexone decreased body weight gain in both lactating and post-lactating subjects. In both saline and NTX-treated lactating subjects, ARC mRNA levels of proDYN, POMC and proENK were significantly decreased compared with the saline or NTX-treated post-lactating subjects (P<0.01). NTX did not significantly influence gene expression of opioid peptides in the ARC in either the lactating or the post-lactating subjects. Neither the lactation condition nor NTX administration significantly changed mRNA levels of proDYN, POMC or proENK in whole pituitary. Thus, as has been noted in energy-deprived rats, opioid peptide gene expression is decreased in the ARC of lactating rats, a period during which rats have increased energy requirements.