β-endorphin influence on kisspeptin neurons of the ewes under long day (LD) or short day (SD) photoperiodic treatment
Résumé
Daylength (i.e. photoperiod) is the main synchronizer of seasonal reproduction in temperate climates albeit other factors can act as modulators. For instance, the duration of anestrus in seasonally breeding domestic mammals is dependent on the metabolic status of the animal as exemplified in horses and sheep: the lowest the body condition score, the longest the anestrus season. This suggests a tight interplay between hypothalamic neurons integrating daylength, neurons stimulating GnRH neuron activity such as Kisspeptin (KISS) neurons, and neurons involved in food intake/metabolism such as POMC neurons secreting peptides such as β-endorphin (BEND), an anorexigenic peptide also involved in reproductive function. Moreover, KISS and BEND exhibit photoperiod-driven variations of their relative levels in the ovine arcuate nucleus (ARC); KISS being more expressed during breeding season while maximal levels of BEND occur during the anestrus season.
We therefore first sought for neuroanatomical relationships between KISS and BEND neurons in the ARC of ovariectomized, estradiol replaced ewes submitted to either LD (16L:8D) to elicit an anestrus state (low LH levels) or SD (8L:16D) to reactivate the gonadotropic axis (high LH levels). A double immunocytochemistry was performed and the sections were examined with a confocal microscope. We found a significantly greater (p<0,05; Mann-Whitney U test) number of BEND fibers contacting KISS neurons in SD than in LD (44 vs. 26 terminals/neuron). There was no significant variation in the number of KISS fibers on BEND neurons between SD and LD.
We then assessed the expression of opioid receptors in the ARC of the ewe using in situ hybridization. While µ and κ opioid receptors were readily detected in numerous thalamic and hypothalamic nuclei including the ARC, opioid receptor transcript remained undetectable. No obvious variation in the distribution or relative level of μ or κ opioid receptors was observed between SD and LD.
Further experiments, using in vivo injection or hypothalamic explant culture are underway to evaluate the functional impact of BEND on KISS neuron activity in sexually active ewes.