Neuroendocrine correlates of female-offspring interaction in maternal rats

For many years it has been debated whether maternal behavior in the rat is governed by physiological or psychological factors. In the last decade many investigators have focussed on both the non-hormonal and hormonal control of this behavior. The former has been modelled by exposing hypophysectomized rats to pups. Evidence for the latter has been obtained by exchange of blood between parturient mothers and virgins and by mimicking the changes in hormone relationship throughout pregnancy and parturation, and modelling the hormone shift of parturation at various intervals from onset of pseudopregnancy. The results support the view that the balance of estrogen and progesterone is critical for the capacity of the female to express maternal behavior, and that estrogen is the dominant steroid. Systemic injection of estrogen alone does not facilitate maternal behavior while intracranial administration of the steroid into the medial preoptic area (MPOA) does enhance it. The necessity of the intact MPOA for the normal expression of maternal behavior has been demonstrated by lesion experiments. The MPOA plays a major role in both the induction and maintenance of maternal behavior. Small lesions restricted to the dorsal MPOA eliminated only the active component of maternal behavior, i.e., retrieving and nest building. The integrity of the lateral fiber connections between the MPOA and the lateral POA is essential for the expression of the active component of maternal behavior. During the pup exposure period required to induce maternal behavior in virgin rats, the oestrous cyclicity is maintained. However, once maternal behavior is established, and the virgins receive continuous tactile stimuli from pups for an extended period of time, the oestrous cyclicity ceases and a prolonged dioestrous state (PDS) occurs. The pup-induced PDS, lasting about 2 weeks, is a true pseudopregnancy having daily nocturnal PRL surges. During postpartum diestrus, lactating females deprived of pups for several hours, show immediate prolactin elevation upon reintroduction to pups. In contrast, maternal virgins are capable of this acute PRL response only during proestrous and oestrous days of the cycle but not during the dioestrous phase of the cycle or during pup induces PDS. We propose that the mechanism governing PRL secretion in response to pup stimuli differs between postpartum mothers and maternal virgins. In lactating postpartum females, it is independent of ovarian steroids, while in maternal virgins, ovarian steroids appear to play a major role.