Photoperiod effects on the expression of deep brain photoreceptors and gonadotrophin genes are sex- and age-dependent in Pekin ducks

Birds rely on a specialized set of photoreceptive neurons called deep brain photoreceptors (DBPs) for the detection of light signals which are then integrated to modulate the expression of gonadotropin hormones. Therefore, we want to investigate DBPs expression, melanopsin (OPN 4), neuropsin (OPN5), and vertebrate ancient opsin (VAOpn), and their regulation of gonadotropin releasing hormone (GnRH) and gonadotropin inhibitory hormone (GnIH) genes in regards to sex and age in Pekin ducks. 460 hatchlings were randomly allocated to 4 rooms and exposed to 24 h light (LL) or photostimulation (PS; n = 30 hens and 7 drakes/pen; 4 pens/treatment) for 19 weeks. Ducks were euthanized via an intravenous injection of sodium pentobarbital (Fatal Plus; 396 mg/mL/kg body weight) at weeks 0, 5, 10, and 19 (n = 6/sex/treatment/timepoint) for brain sample collection which was further hemisected into diencephalon. Gene expressions of OPN 4, OPN5, and VAOpn, GnRH, and GnIH were quantified using qRT-PCR. In the hens, OPN4 expression was higher (p < 0.05) in LL compared to PS at week 0, suggesting a rapid transcriptional response to continuous light but no difference in OPN5 expression. VAOpn expression was reduced (P < 0.05) in PS compared to LL at week 10 in the hens. In parallel, GnRH expression was higher (p < 0.05) under LL compared to PS at week 19, peaked at week 0 in both treatments and declined (p < 0.05) thereafter in the hens. In contrast, GnIH expression increased (p < 0.01) with age in both sexes, and was lower in the PS hens compared to LL hens at week 10. These findings demonstrate photoperiod-dependent variation in DBPs alongside GnRH and GnIH transcription which is consistent with a possible involvement in sex-specific reproductive development in ducks.