Abstract
Pregnancy and lactation, trigger many metabolic adaptations, including increased food intake, to support the energy demands of a growing fetus and for milk production after birth. The agouti related peptide (AgRP)/ neuropeptide Y (NPY) neurons within the arcuate nucleus are important for regulation of food intake. AgRP/NPY neurons play a pivotal role in driving food intake by integrating signals of energy need from within the body with external cues. Ghrelin is an orexigenic hormone secreted from the stomach, which directly activates AgRP/NPY neurons leading to rapid behavioural changes. Ghrelin activation of AgRP/NPY neurons triggers food seeking behaviours and food consumption. I aimed to determine the role of ghrelin in driving elevated maternal food intake. My hypothesis was sensitivity to ghrelin would increase during pregnancy and lactation leading to elevated maternal food intake.
To determine whether increased sensitivity to ghrelin contributes to maternal hyperphagia, female mice were injected (I.P) with either ghrelin or vehicle (saline) at four physiological timepoints: prior to pregnancy (virgin), pregnancy day 8 and 15, and lactation day 10 and acute food intake was measured. Ghrelin injection induced food intake in virgin and lactating females. Surprisingly, on pregnancy day 15 ghrelin did not induce an increase in food intake (RM mixed model; virgin vs P15 interaction reproductive state x treatment p = 0.0444; unpaired t-test p = 0.3333), suggesting a pregnancy-induced insensitivity to ghrelin occurs.
Activation of AgRP neurons by ghrelin in pregnancy and lactation was determined using immunostaining for c-Fos, a neuronal marker of activation. Virgin, pregnancy day 15 and lactation day 10, AgRP neuron reporter (AgRP-Cre x td tomato) mice were treated (I.P) with ghrelin or vehicle and food removed. After 2 hours mice were perfused, and brains processed for immunohistochemistry. Ghrelin administration increased the number of c-Fos positive cells in AgRP neurons in virgin, pregnant and lactating mice (two-way ANOVA; effect of treatment p < 0.0001). An increase in c-Fos immunoreactive cells compared to saline controls suggests ghrelin administration activates AgRP neurons across the reproductive states.
Lastly, the AgRP calcium dependent neuron response to ghrelin, chow and palatable food was recorded using the in vivo calcium imaging technique fibre photometry. Ghrelin administration increased AgRP neuron activity in virgin, pregnant and lactating mice. Surprisingly, on pregnancy day 15 there was no attenuation of the AgRP neuron response to ghrelin and the response was not different to virgins (RM one way ANOVA p = 0.1248). At the virgin and pregnancy timepoints, chow presentation after ghrelin administration decreases AgRP neuron activity. Interestingly, this decrease in AgRP activity in response to chow is not seen on day 10 of lactation compared to the virgin state (Paired t-test p = 0.0129).
In conclusion these results suggest a pregnancy-induced insensitivity to the food intake effects of ghrelin occurs. Furthermore, the ghrelin insensitivity is not mediated by the AgRP neurons as they are still responsive to ghrelin during pregnancy. During lactation ghrelin sensitivity is not different from the virgin state however, in lactation AgRP neuron activity is not rapidly suppressed by consuming chow. This lack of AgRP neuron response to chow may contribute to the elevated maternal drive for food and increase meal size.