The meaning of weaning: influence of the weaning period on behavioral development in mice

Abstract

Maternal care during the first week postpartum has long-term consequences for offspring development in rodents. However, mother-infant interactions continue well beyond this period, with several physiological and behavioral changes occurring between days 18 and 28 PN. In the present study, we investigate the long-term effects on offspring behavior of being weaned at day 21 PN versus day 28 PN. We found that male and female offspring engage in higher initial levels of social interaction if weaned at day 28 PN, as well as sexually dimorphic changes in exploratory behavior. Females who were themselves weaned earlier also appeared to wean their own pups earlier. Sex-specific effects of weaning age were found on levels of oxytocin and vasopressin V1a receptor density in the hypothalamus, central nucleus of the amygdala and nucleus accumbens. These results indicate that altering weaning age in mice may be a useful model for investigating the development of sexual dimorphism in neurobiology and behavior.

Fig. 1

Frequency of postpartum maternal behavior (± SEM) in B6 mice from days 14 PN to 28 PN. a Frequency of nursing decreases across this period with increases in the frequency of nonnursing contact. b Maternal licking/grooming (LG) and nest building occur at a low frequency across this period with a slight increase in LG at day 28 PN. Self-grooming increases in frequency across the postpartum period. c Frequency of eating and drinking increase and then decrease as pups switch to eating solid food.

Fig. 1

Frequency of postpartum maternal behavior (± SEM) in B6 mice from days 14 PN to 28 PN. a Frequency of nursing decreases across this period with increases in the frequency of nonnursing contact. b Maternal licking/grooming (LG) and nest building occur at a low frequency across this period with a slight increase in LG at day 28 PN. Self-grooming increases in frequency across the postpartum period. c Frequency of eating and drinking increase and then decrease as pups switch to eating solid food.

Fig. 1

Frequency of postpartum maternal behavior (± SEM) in B6 mice from days 14 PN to 28 PN. a Frequency of nursing decreases across this period with increases in the frequency of nonnursing contact. b Maternal licking/grooming (LG) and nest building occur at a low frequency across this period with a slight increase in LG at day 28 PN. Self-grooming increases in frequency across the postpartum period. c Frequency of eating and drinking increase and then decrease as pups switch to eating solid food.

Fig. 2

Duration of social interaction in seconds (± SEM) amongst R21- and R28- weaned male and female offspring. Duration of social interaction during the first minute of testing was higher amongst R28- weaned offspring (a), whereas total duration of social interaction during the 10- min test did not differ as a function of weaning age (b). ∗ p < 0.05.

Fig. 3

Open-field behavior of R21- and R28-weaned offspring. a Time spent immobile (± SEM) was found to be decreased in R21-weaned females compared to R21-weaned males with no sex differences in R28-weaned offspring. b Time spent in the inner area of the field (± SEM) was not altered as a function of weaning age. c Total squares crossed during testing (± SEM) indicated sex differences in R21-weaned offspring, with females more active than males. Consistent with immobility findings, there were no sex differences in activity amongst R28-weaned offspring. ∗ p < 0.05.

Fig. 4

Maternal behavior of R21- and R28-weaned offspring. a Frequency of nursing and non-nursing contact (± SEM) during days 14–28 PN, which diverge more significantly in R21 females after day 21 PN. b Frequency of pup mounting (± SEM) from days 18–28 PN. R21-weaned females were observed to engage in significantly higher levels of this behavior at days 22 PN and 23 PN. ∗ p < 0.05.

Fig. 4

Maternal behavior of R21- and R28-weaned offspring. a Frequency of nursing and non-nursing contact (± SEM) during days 14–28 PN, which diverge more significantly in R21 females after day 21 PN. b Frequency of pup mounting (± SEM) from days 18–28 PN. R21-weaned females were observed to engage in significantly higher levels of this behavior at days 22 PN and 23 PN. ∗ p < 0.05.

Fig. 5

Receptor autoradiography measurement of OT and V1a receptor density (fmol/mg ± SEM) in the in central nucleus of the amygdala (CA), bed nucleus of the stria terminalis (BNST), lateral septum (LS), medial preoptic area (MPOA), nucleus accumbens core (NAc), nucleus accumbens shell (NAs) and ventral medial hypothalamus (VMH). Comparison of OT receptor density in (a) R21- and R28-weaned males, and (b) R21- and R28-weaned females. V1a receptors were also compared in R21- and R28-weaned males (c) and females (d). ∗ p < 0.05, ∗∗ p < 0.001.