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.

Perinatal junk food exposure increases the preference for palatable diets in juvenile and adult rat offspring. Previous studies have implicated reduced sensitivity of the opioid pathway in the programming of food preferences; however it is not known when during development these changes in opioid signalling first emerge. This study aimed to determine the impact of a maternal junk food (JF) diet on mu-opioid receptor (MuR) expression and ligand binding in two key regions of the reward pathway, the nucleus accumbens (NAc) and the ventral tegmental area (VTA) in rats during the early suckling (postnatal day (PND) 1 and 7) and late suckling/early post-weaning (PND 21 and 28) periods. Female rats were fed either a JF or a control diet for two weeks prior to mating and throughout pregnancy and lactation. MuR expression in the VTA was significantly reduced in female JF offspring on PND 21 and 28 (by 32% and 57% respectively, P < 0.05), but not at earlier time points (PND 1 and 7). MuR ligand binding was also reduced (by 22%, P < 0.05) in the VTA of female JF offspring on PND 28. No effects of perinatal junk food exposure on MuR mRNA expression or binding were detected at these time points in male offspring. These findings provide evidence that the opioid signalling system is a target of developmental programming by the end of the third postnatal week in females, but not in males.