Pup age and aggressive behavior in lactating rats

Abstract

High levels of aggressive behaviors against intruders in the nest area are displayed by female rats during the first 10 days after delivery, declining thereafter to very low levels, even though lactation continues. Cross-fostering experiments were undertaken to test the hypothesis that pup age may affect aggression in lactating rats. The behavior of females on the 8th day after delivery when raising fostered 8-day-old pups was compared to that of females on the 8th postpartum day raising older pups (18 days old) for the last 5 days, and females on the 18th day after delivery raising fostered 18-day-old pups were compared to females in the same postpartum period nursing younger pups (8 days of age at the time of the maternal aggression test) for 5 days. Pup retrieval activity and plasma prolactin level were also analyzed. Females on the 8th postpartum day nursing 18-day-old pups were less aggressive than females in the same postpartum period, but with 8-day-old pups. Likewise, females on the 18th postpartum day nursing younger pups were more aggressive and presented higher levels of prolactin than females nursing older pups. Thus, pup development can alter the natural decline of maternal aggressive behavior.

Freezing suppression by oxytocin in central amygdala allows alternate defensive behaviours and mother-pup interactions

“…Evidence that rat dams suppress their own defense responses when threatened in the presence of pups is scarce (Pinel et al, 1990; Sukikara et al, 2010). However, it is well described that the social environment modulates defensive behavior, such as freezing in adult rats (Kikusui et al, 2006; Kiyokawa et al, 2004 Knapska et al, 2010) and that rat dams will attack an adult male that poses a danger to the pups but not themselves, whereas virgin females do not (Bosch, 2013; Giovenardi et al, 2000; Knapska et al, 2010; Mayer and Rosenblatt, 1987). …”

The changing role of the medial preoptic area in the regulation of maternal behavior across the postpartum period: Facilitation followed by inhibition

“…These changes result in distinct patterns of maternal behavior and care at different postpartum time points [25, 28, 29, 57, 79, 81, 86, 87, 104]. This exquisite capacity for plasticity in the behavioral repertoire of postpartum females must necessarily involve structures in the neural network that mediate maternal responsiveness, which are sensitive to the incoming changing information from the developing pups.…”

Early undernourishment interferes with the maternal aggressive response triggered by an intruder entering the homing cage

“…Our findings suggest that perinatal undernourishment disturbs the mechanism that underlies the MAR, because for U females, the frequency of entries by the intruder male into the nest area (Zone 1) was less than those for the C group. Another possible explanation of the increased MAR of U dams is a disruption in nitric oxide (NO) synthesis or activity in neuronal structures implicated in the maternal behaviour, because it has been demonstrated that during high levels of MAR, there are increases in NO synthesis that continue until postpartum day 8 (Giovenardi et al 2000). In mice with targeted disruption of the neuronal NO synthase gene (nNOS 2 /2) there was a significant decrease in both the percentage of animals displaying aggression, and in the average number of attacks against a male intruder, but these mice displayed normal retrieving behaviour.…”

“…Through the first 10 days after delivery, female rats vigorously attack intruders in the nest area to protect the newborn pups. In order to assess the MAR in rodents, an adult intruder, male or female, is introduced into the home cage of a resident female, and the frequency of sniffing, (the female approaches the intruder and sniffs its body and/or genitals), of lateral attacks (the female moves laterally towards the intruder) usually in association with piloerection and biting the back of the intruder, of frontal attacks (the female with the body orientated frontally lunges toward the intruder followed by biting of the intruder's head and shoulders and of biting (the females bites any part of the intruder body) (Giovenardi et al 2000). Later, the increased independence of the pups in terms of feeding and environmental exploration may decrease the mother's protective behaviours.…”

“…The typical MAR may be triggered by numerous environmental stimuli, including progesterone, estradiol, and prolactin, or by cues arising from the pups that increase the neuronal excitability underlying the directed aggression (Wise 1974;Svare and Gandelman 1976;Gandelman and Simon 1980;Svare et al 1982;Giovenardi et al 2000). Here, we used the early MAR as a model to evaluate the noxious long-term effects of early food restriction and sensory deprivation.…”