Abstract
Changes in appetite occur during the ovarian cycle in female mammals. Research on appetite-regulatory gastrointestinal peptides in females is limited, because reproductive changes in steroid hormones present additional experimental factors to control for. This study aimed to explore possible changes in the orexigenic (appetite-stimulating) gastrointestinal peptide hormone ghrelin during the rodent oestrous cycle. Fed and fasted plasma and stomach tissue samples were taken from female Wistar rats (32–44 weeks of age) at each stage of the oestrous cycle for total ghrelin quantification using radioimmunoassay. Sampling occurred during the dark phase when most eating takes place in rats. Statistical analysis was by paired-samples t-test, one-way ANOVA on normally distributed data, with Tukey post-hoc tests, or Kruskal-Wallis if not. GLM univariate analysis was used to assess main effects and interactions in ghrelin concentrations in the fed or fasted state and during different stages of the ovarian cycle, with age as a covariate. No consistent fed to fasted ghrelin increases were measured in matched plasma samples from the same animals, contrary to expectations. Total ghrelin concentrations did not significantly change between cycle stages with ANOVA, in either fed or fasted plasma or in stomach tissue. This was despite significantly decreased fasted stomach contents at oestrus (P = 0.028), suggesting decreased food intake. There was however a significant interaction in ghrelin plasma concentrations between fed and fasted proestrus rats and a direct effect of age with rats over 37 weeks old having lower circulating concentrations of ghrelin in both fed and fasted states. The biological implications of altered ghrelin plasma concentrations from 37 weeks of age are as yet unknown, but warrant further investigation. Exploring peripheral ghrelin regulatory factor changes with increasing age in reproductively competent females may bring to light potential effects on offspring development and nutritional metabolic programming.
Highlights
Ghrelin is a key gut peptide involved in eating behaviour that is secreted predominantly from X/A-like cells of the stomach [1]
Due to the limited previous research conducted on appetite-regulatory gastrointestinal peptides in females, because steroid hormones present additional experimental factors to control for, the present study aimed to investigate whether circulating and stomach tissue total ghrelin concentrations change in line with reported appetite changes during the oestrous cycle, in rats aged 38 ± 0.49 weeks (32–44 weeks)
Stomach contents data were analysed against the cycle stage of the preceding day, as the rats were fasted from the beginning of the cull day, analysis of remaining stomach contents provided an indication of food consumption during the previous day/cycle stage (Fig 1)
Summary
Ghrelin is a key gut peptide involved in eating behaviour that is secreted predominantly from X/A-like cells of the stomach [1]. Ghrelin exists in two forms, an acylated and de-acylated. Ghrelin effects are mediated by the acylated form of the peptide and its receptor GHSR1a, to which the des-acyl ghrelin form does not bind. Ghrelin is the only orexigenic (appetite-increasing) peptide to have been isolated that is produced outside the central nervous system, and its role in whole body energy homeostasis and feeding stimulation is a major focus of research, especially for its potential control of body mass within healthy ranges. The increasing incidence of obese reproductive age females in human populations across the globe [4] is of concern for the health of these women and their offspring, so it is essential that we understand more about the role of gut peptides in normal eating behaviour and reproductive functioning. Ghrelin concentrations are highest in the circulation prior to feeding episodes, after which they reduce and plateau [5]
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