In female mammals, reproductive senescence is a complex process involving progressive ovarian dysfunction, associated with altered central control of the hypothalamic-pituitary-gonadal axis and desynchronization of the circadian system. The objective of this study was to investigate daily- and age-dependent changes in the regulation of Arg-Phe amide-related peptide-3 (RFRP-3), a hypothalamic peptide involved in reproduction, and correlate those changes to the luteinizing hormone (LH) secretion in female C57BL/6 J mice of different age groups (4-, 13-, and 19- months old). Plasma LH levels were similar in diestrus mice from 4 to 13 months, but were markedly increased in 19 month-old mice. We found an age-dependent decrease in the total number of RFRP-3 neurons and in the relative number of activated (c-Fos-positive) RFRP-3 neurons. In 19 month-old mice also the daily variation in RFRP-3 neuronal activation was abolished. We found no daily- or age-dependent changes in the arginine vasopressin (AVP)- and vasoactive intestinal peptide (VIP)-fiber density in the dorsomedial hypothalamus. However, we did record a daily variation in the number of RFRP-3 neurons receiving close AVP- and VIP-ergic fiber appositions in 4 and 13 month- old, but not 19 month-old mice. The present findings indicate that RFRP-3 neurons are downregulated during old age and that the daily changes in their innervation by the circadian peptides, AVP and VIP, are abolished. This age-associated reduced rhythmic activity of the inhibitory RFRP-3 system could be implicated in the elevated LH secretion observed during reproductive senescence.
Keywords: Aging hypothalamus; RFRP-3; Reproductive senescence; Vasoactive intestinal peptide; Vasopressin.
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