Abstract
While photoperiod has been generally accepted as the primary if not the exclusive cue to stimulate reproduction in photoperiodic breeders such as the laying hen, current knowledge suggests that metabolism, and/or body composition can also play an influential role to control the hypothalamic-pituitary gonadal (HPG)-axis. This review thus intends to first describe how photoperiodic and metabolic cues can impact the HPG axis, then explore and propose potential common pathways and mechanisms through which both cues could be integrated. Photostimulation refers to a perceived increase in day-length resulting in the stimulation of the HPG. While photoreceptors are present in the retina of the eye and the pineal gland, it is the deep brain photoreceptors (DBPs) located in the hypothalamus that have been identified as the potential mediators of photostimulation, including melanopsin (OPN4), neuropsin (OPN5), and vertebrate-ancient opsin (VA-Opsin). Here, we present the current state of knowledge surrounding these DBPs, along with their individual and relative importance and, their possible downstream mechanisms of action to initiate the activation of the HPG axis. On the metabolic side, specific attention is placed on the hypothalamic integration of appetite control with the stimulatory (Gonadotropin Releasing Hormone; GnRH) and inhibitory (Gonadotropin Inhibitory Hormone; GnIH) neuropeptides involved in the control of the HPG axis. Specifically, the impact of orexigenic peptides agouti-related peptide (AgRP), and neuropeptide Y (NPY), as well as the anorexigenic peptides pro-opiomelanocortin (POMC), and cocaine-and amphetamine regulated transcript (CART) is reviewed. Furthermore, beyond hypothalamic control, several metabolic factors involved in the control of body weight and composition are also presented as possible modulators of reproduction at all three levels of the HPG axis. These include peroxisome proliferator-activated receptor gamma (PPAR-γ) for its impact in liver metabolism during the switch from growth to reproduction, adiponectin as a potential modulator of ovarian development and follicular maturation, as well as growth hormone (GH), and leptin (LEP).
Highlights
Decades of genetic selection along with significant improvements in environmental conditions and nutrition have allowed modern commercial chickens to become exceedingly efficient
OPN4, OPN5, and vertebrate ancient (VA)-Opsin, have all been proposed as deep-brain photoreceptors mediating the photoperiodic response (Foster et al, 1985, 1994; Chaurasia et al, 2005; Halford et al, 2009; Kang et al, 2010; Nakane and Yoshimura, 2010; Davies et al, 2012; Ohuchi et al, 2012), VAOpsin is the only photoreceptor that has been found to meet all four criteria outlining the activity of the opsin of interest (68,69,71,101; Table 1)
Recent evidence of the co-expression of VA-Opsin and GonadotropinReleasing Hormone I (GnRH-I) strongly suggests this deep brain photoreceptor (DBP) may be a key photoreceptor involved in the photo-induced activation of the hypothalamic-pituitary gonadal (HPG) axis (García-Fernández et al, 2015)
Summary
Decades of genetic selection along with significant improvements in environmental conditions and nutrition have allowed modern commercial chickens to become exceedingly efficient. With the rising demand for sustainable egg production, breeding companies have more recently focused on extending the laying period to achieve 500 eggs per hen at 100 weeks of age (van Sambeek, 2010; Bain et al, 2016). This will require precise co-ordination of several systems involved in the control of the reproductive axis, and metabolism and nutrient partitioning. This review aims to summarize the current knowledge on the control of sexual maturation in chickens, with a specific emphasis on the integration of photoperiodic cues while presenting evidence of possible interactions with factors involved in metabolic control
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