Oxytocin induction of short-latency maternal behavior in nulliparous, estrogen-primed female rats.

This study was designed to test whether intracerebroventricular (icv) administration of oxytocin can shorten the latency of estrogen-treated virgin rats to respond to foster pups with maternal care. A secondary goal was a description of the behavioral effects of icv oxytocin in rats. Forty-eight hours before pup presentation female rats (Sprague-Dawley strain, obtained from Zivic-Miller Laboratories, Inc.) with chronic lateral ventricle cannulae were ovariectomized and treated with 100 micrograms/kg body wt estradiol benzoate (EB). Immediately before pup presentation either 400 ng oxytocin or the normal saline vehicle was infused. A second control group of noncannulated animals received the same surgery, EB treatment, and behavior tests without any icv infusion. Maternal behavior was scored on a scale from 0 to 5, one point each being given for licking pups, carrying pups, grouping pups, crouching over pups, and nest building. At the end of 1 hr of pup exposure 65% of the oxytocin-infused animals received scores of 4 or 5, while only 25% of the control animals attained these scores. Maternal behavior appeared as a unit in both spontaneously maternal and oxytocin-treated animals, with fewer than 10% in either group receiving scores of 1, 2, or 3. The oxytocin group also spent significantly more time grooming. In a second experiment in which oxytocin or normal saline was administered icv to oil-treated animals, this grooming effect was shown to be estrogen independent while the induction of maternal behavior required prior treatment with estrogen. Finally, a slight, transient hyperthermic effect of oxytocin on body temperature was observed in this test paradigm. These data suggest a role for oxytocin, acting with estrogen, in the onset of maternal behavior in rats.