Sex and age-dependent effects of a maternal junk food diet on the mu-opioid receptor in rat offspring.

•A maternal junk food (JF) diet alters offspring mu-opioid receptor (MuR) expression.•It was unclear when these changes in MuR expression in the offspring first emerged.•No effect of a maternal JF diet on MuR was present on postnatal day 1 or 7.•Female JF offspring had reduced MuR expression on postnatal day 21 and 28.•The late suckling period maybe a time of increased susceptibility of MuR to JF diets.