The transition to motherhood results in a number of hormonal, neurological and behavioural changes necessary to ensure offspring growth. Once motherhood is established, further neurological and behavioural changes may result in long-term memory in mothering. Recent research has shown that postpartum motherhood enhances both nurturing behaviour and oxytocin activities. The transmembrane glycoprotein, CD38, is expressed on many neuronal cells and has been shown to play a role in social behaviours through stimulating the release of oxytocin in the hypothalamus. The present study was performed to investigate the effects of reproductive experience (primi- and multiparity, dams and sires) on the degree of parental behaviour, such as retrieval. Comparisons were performed between wild-type (Cd38 +/+) and Cd38 knockout (Cd38 -/-) mice of the ICR strain. Multiparous Cd38 -/- dams retrieved pups much faster than primiparous mice, whereas there were no significant differences between primi- and multiparous Cd38 +/+ dams. Plasma oxytocin levels were significantly increased in multiparous dams of both genotypes. In addition, oxytocin levels in the hypothalamus and pituitary were lower in Cd38 -/- than in wild-type mice. ADP-ribosyl cyclase activity in the hypothalamus, but not in the pituitary, was slightly increased in Cd38 +/+ dams. In an identical test, 40% of first-time Cd38 +/+ sires showed retrieval. The time required to retrieval was shorter in second-time Cd38 +/+ sires. Both first- and second-time Cd38 -/- sires showed only 10% retrieval behaviour. These results indicate that parental behaviour is improved by reproductive experience, especially in Cd38 -/- dams, and suggest that these effects may be a result of increased oxytocin levels.
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