Circulating prolactin, MPOA prolactin receptor expression and maternal aggression in lactating rats.

Maternal aggression is most intense in lactating rats from the 3rd to the 12th day postpartum. The purpose of this study was to determine if plasma prolactin (PRL) and prolactin receptor (PRL-R(L)) mRNA expression in the medial preoptic area (MPOA) of lactating rats are altered in association with maternal aggression. Lactating Sprague Dawley rats were divided into five groups and exposed for 10 min to an intruder male or to an object on postpartum day 8. Trunk blood and the brain of the dams were collected 30 or 240 min after exposure and from a non-exposed group. Lower levels of prolactin were found 30 min after the aggression test. No change was detected in the number of cells expressing PRL-R(L) mRNA by in situ hybridization histochemistry (ISHH) as a function of testing. However, the correlation between plasma PRL and PRL-R(L) mRNA expression in the mothers changed from positive in control females to negative in intruder exposed animals. These data support the concept that a maternal aggressive experience, while acutely altering PRL secretion, fails to affect PRL-R(L) mRNA expression.

Effects of oxytocin microinjected into the central amygdaloid nucleus and bed nucleus of stria terminalis on maternal aggressive behavior in rats.

The central effect of oxytocin (OT) on the aggressive behavior of lactating rats was studied. Female rats are more aggressive than nonlactating resident females, vigorously attacking conspecific intruder male or females. This behavior is considered important for pup protection against infanticide. The present work aimed to test the effects on maternal aggressive behavior of OT infused into the central amygdaloid nucleus (CeM) or bed nucleus of stria terminalis (BNST). The surgeries for bilateral cannula implantation were performed between the 2nd and 4th postpartum day. Three days after the surgery, saline or OT was infused and 5 min later a male intruder was placed in the home-cage and the behaviors were videotaped for 10 min. The frequency of the aggressive behaviors and the duration of locomotion during the aggressive behavior test were measured. The latency to retrieve the pups was also evaluated. The results showed that OT injected into CeM (10 and 20 ng/nucleus) decreased frequency of biting and frontal attack while in the BNST (10 and 20 ng/nucleus) decreased the frequency of biting. No significant change on retrieval activity was detected. OT in CeM and BNST has an inhibitory effect on the aggressive behavior of lactating female rats.