Interactions between $β$-endorphin and kisspeptin neurons of the ewe arcuate nucleus are modulated by photoperiod
Opioid peptides are well-known modulators of the central control of reproduction. Among them, dynorphin coexpressed in kisspeptin (KP) neurons of the arcuate nucleus (ARC) has been thoroughly studied for its autocrine effect on KP release through $\kappa$ opioid receptors. Other studies have suggested a role for $\beta$-endorphin (BEND), a peptide cleaved from the proopiomelanocortin (POMC) precursor, on food intake and central control of reproduction. Similarly to KP, BEND content in the ARC of sheep is modulated by day length and BEND modulates food intake in a dose-dependent manner. As KP levels in the ARC vary with photoperiodic and metabolic status, a photoperiod-driven influence of BEND neurons on neighboring KP neurons is plausible. The aim of this study was therefore to investigate a possible modulatory action of BEND on KP neurons located in the ovine ARC. Using confocal microscopy, numerous KP appositions on BEND neurons were found but there was no photoperiodic variation in the number of these interactions in ovariectomized, estradiol-replaced ewes. In contrast, BEND terminals on KP neurons were twice as numerous under short days (SD), in ewes having an activated gonadotropic axis, as compared to anestrus ewes under long days (LD). Injection of 5$\mu$g BEND into the third ventricle of SD ewes induced a significant and specific increase of activated KP neurons (16% versus 9% in controls) while the percentage of overall activated (c-Fos positive) neurons, was similar between both groups. These data suggest a photoperiod-dependent influence of BEND on KP neurons of the ARC, which may influence GnRH pulsatile secretion and inform KP neurons on the metabolic status.
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