Steroid Dependent Effects of Oxytocin on Spontaneous Motor Activity in Female Rats

In the present study, dose relationships for effects of oxytocin (OXY) on spontaneous motor activity in female rats were investigated. Ovariectomized (OVX) and cycling female Sprague-Dawley rats were given OXY 10–1000 μg/kg s.c. or saline, 10 min before registration of motor activity in an open-field arena. In the OVX rats, 100 μg/kg of OXY increased the activity in the center of the arena, whereas 1000 μg/kg decreased locomotor activity (LA). In the cycling rats, OXY 100–1000 μg/kg decreased LA during diestrus, while 1000 μg/kg also decreased LA during metestrus. The latter dose also reduced the exploratory behavior during estrus. In a second experiment, OVX rats were pretreated with estradiol benzoate (EB) and progesterone (P). When P levels were predominant, OXY 10–1000 μg/kg decreased LA. Oxytocin 10–100 μg/kg given after pretreatment with EB increased the activity in the center of the arena, whereas 1000 μg/kg given in the presence of both EB and P increased peripheral activity (PA). These results show that the effects of OXY on motor activity in female rats are modified by female sex steroid hormones.