Melanocortins Mimic the Effects of Leptin to Restore Reproductive Function in Lean Hypogonadotropic Ewes

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Abstract
Background/Aims: Leptin restores gonadotropic function in lean hypogonadotropic animals by an unknown mechanism. We aimed to test the hypothesis that restoration of gonadotropic function is a result of an upregulation of central acetylated melanocortin production. Methods and Results: Lean ovariectomised (OVX) ewes received intracerebroventricular (i.c.v.) infusions of leptin (or vehicle) for 3 days, which upregulated proopiomelanocortin (POMC) mRNA and restored pulsatile luteinizing hormone (LH) secretion. A melanocortin agonist (MTII), but not naloxone treatment, reinstated pulsatile LH secretion in lean OVX ewes. We treated (i.c.v.) lean OVX ewes with leptin (or vehicle) and measured peptide levels and post-translational modification in the arcuate nucleus (ARC). Levels of β-endorphin (β-END) were lower in lean animals, with no effect of leptin treatment. Desacetyl-α-MSH was the predominant form of α-melanocyte-stimulating hormone (α-MSH) in the ARC and levels were similar in all groups. In another group of lean and normal-weight OVX ewes, we measured the different forms of α-MSH in ARC, hypothalamus (ARC-removed) and the preoptic area (POA). Acetylated α-MSH levels were lower in lean animals in the terminal beds of the hypothalamus and POA but not the ARC. Conclusions: Leptin corrects the hypogonadotropic state in the lean condition by upregulation of POMC gene expression, and may increase transport and acetylation of melanocortins to target cells in the brain. Melanocortin treatment restores LH secretion in lean animals.

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