Neuroendocrine regulation of appetite in the female non-human primate: Effects of menstrual cycle phase and ovarian steroids

Appetite is influenced by ovarian steroids. The aim of this thesis was to elucidate the mechanisms whereby ovarian steroids influence appetite in non-human primates. To this end, food intake was measured over the course of the menstrual cycle and in ovariectomized (OVX) monkeys treated with estrogen (E2) or E2 in combination with progesterone (P4). The effects of ovarian steroids on PYY-induced anorexia were determined in OVX monkeys treated with either E2 or E2 + P4. Also, the effect of ovarian steroids on the orexigenic actions of NPY was tested by stimulating endogenous NPY in response to an insulin-induced hypoglycemic challenge during the menstrual cycle and in OVX monkeys primed with E2 or E2 + P4.; Food intake declined during the second half of the follicular phase when E2 levels increased and P4 levels were low. Food intake reached a nadir on the day of the LH surge before rebounding during the luteal phase in association with a rise in P4 levels. OVX monkeys treated with E2 or E2 + P4 showed similar cyclic changes in food intake. Administration of PYY3-36 decreased appetite. The magnitude of response was greater in animals treated with E2 versus those treated with E2 + P4. Insulin-induced hypoglycemia increased food intake. No difference in response was observed in the follicular and luteal phases of the menstrual cycle or in OVX monkeys treated with E2 or E2 + P4.; The changes in appetite observed during the menstrual cycle are likely due to changes in ovarian steroids, since similar alterations in food consumption were achieved in ovariectomized monkeys treated with E2 and E2 + P4. PYY inhibited food intake to a greater extent in E2 treated monkeys compared to E2 + P4 treated animals. Since PYY levels did not change during the menstrual cycle, increased sensitivity rather than increased secretion may be responsible for the periovulatory decline in energy intake. Sensitivity to the orexigenic action of NPY was unchanged during the menstrual cycle since hypoglycemia-induced food intake was not influenced by the ovarian steroid milieu. These results further our understanding of the neuroendocrine mechanisms where by ovarian steroids affect the complex behaviour of food intake in primates.