Effects of gender and menstrual cycle phase on food-maintained responding under a progressive-ratio schedule in cynomolgus monkeys

Clinical and preclinical data suggest that fluctuations in ovarian steroid hormones across the menstrual/estrous cycle influence spontaneous feeding behavior in females. The effects of gender, menstrual cycle phase, and ovarian hormone fluctuations on food-maintained responding under a progressive-ratio schedule were investigated in four female and three male cynomolgus monkeys. Females were studied across 21 menstrual cycles, and ovulatory cycles were defined by analysis of ovarian steroid hormone levels. Data were analyzed for the early and mid-follicular phase and the mid- and late luteal phase of the menstrual cycle. Progressive-ratio break points for food were significantly higher in males than in females (p < 0.01). However, progressive-ratio break points did not vary consistently as a function of menstrual cycle phase during ovulatory cycles. There were no systematic patterns of progressive-ratio break points in anovulatory menstrual cycles. Only one female monkey reached significantly higher break points during the mid- and late luteal phases in comparison to the mid-follicular phase of the menstrual cycle (p < 0.05). There was also a significant positive correlation between progressive-ratio break points and progesterone levels and a significant negative correlation with estradiol in that monkey. Although fluctuations in ovarian steroid hormones may influence food consumption under some conditions, consistent patterns of food-maintained responding were not detected during ovulatory menstrual cycles in cynomolgus monkeys.