Figure 1. Schematic representation of the changing hormonal milieu and food intake during the rat four-day oestrous cycle, followed by pregnancy and lactation (see text for sources of data).
Levels are expressed as a percentage of maximum levels. During the oestrous cycle surges of oestradiol, progesterone and prolactin occur on pro-oestrus (P). Food intake on oestrous (E) displays the oestrogen-induced decrease in appetite, while food intake increases on metoestrus (M) and dioestrus (D), coincident with increased progesterone levels at this time. After mating, prolactin surges are initiated, which provide luteotrophic support for corpus luteum production of progesterone to be maintained throughout pregnancy. The placenta forms around day 7–8 of pregnancy and secretes placental lactogen at relatively constant levels, resulting in inhibition of endogenous prolactin secretion via negative feedback. Oestradiol levels gradually rise during pregnancy, and just prior to parturition the changing ratio between increasing oestradiol and decreasing progesterone causes the late-parturition surge of prolactin. Prolactin levels are then constantly elevated during lactation due to the suckling stimulus. Leptin levels show a circadian fluctuation that persists into pregnancy, when overall levels rise to be significantly elevated by day 7 of pregnancy and remain elevated until the day prior to parturition. Even in the face of elevated leptin, appetite and food intake are increased during pregnancy (significantly by day 4), creating a state of positive energy balance. Non-pregnant and early pregnant rats are responsive to the satiety actions of leptin. However by day 14 of pregnancy, rats are leptin resistant. Coincident with the timing of pregnancy-induced leptin resistance is the changing pattern of lactogen hormone secretion, from phasic prolactin surges to chronic placental lactogen secretion. During lactation the rat is in a state of negative energy balance, as food intake is increased by up to 300% to meet the high metabolic demands of the suckling young.