Effects of neonatal rearing by different types of foster mother on the distribution of corticotropin-releasing factor neurons in the central amygdaloid nucleus in rats

The mother–child relationship of newborns plays an essential role in the development of the central nervous system, and an inadequate relationship, such as mother–child separation, can cause deficits of mental function in adulthood. However, insufficient research has examined the effects of foster mothers. We assigned some neonatal rats to one of two foster mothers: one that was lactating and feeding her first litter (FL group) and one that had one previous experience of childbirth and feeding but no current litter (FE group). Other pups were raised by their own mother (OM group) or subjected to maternal separation (MS group). Pups were placed with the foster mother (FL and FE groups) or separated from their mother (MS group) for 3 h/day on postnatal days 1–20. At age 6 weeks, each group was divided into two subgroups, one with 30 min of acute restraint stress loading (FL-R, FE-R, OM-R, and MS-R) and one without it (FL, FE, OM, and MS). Then, we compared the density of corticotropin-releasing factor-immunoreactive (CRF-ir) neurons in the central amygdaloid nucleus (CeA). The density of CRF-ir neurons in the CeA was significantly lower in the FL-R and MS-R subgroups than in the FL and MS subgroups, respectively. The results suggest that differences in care received during the neonatal period affect maturation of CRF neurons in the CeA and may have negative effects on the synthesis and release of CRF.