Should I Lay or Should I Grow: Photoperiodic Versus Metabolic Cues in Chickens

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).

Changes in the Control of the Hypothalamic-Pituitary Gonadal Axis Across Three Differentially Selected Strains of Laying Hens (Gallus gallus domesticus)

“…Overall, this study suggests that our initial models describing the HPG axis (Bédécarrats, 2015;Bedecarrats et al, 2016) may not be fully applicable to modern commercial layer strains. While sexual maturation has been previously identified to be dependent on photo-schedule, the "64-g window" reported in this study strongly suggests a link between body weight and the AFE, likely via a metabolic trigger (Hanlon et al, 2020). This is further reinforced by evidence of a de-synchronization in OPN5 mRNA levels between the strains, with Lohmann hens demonstrating an earlier elevation in levels during the immature pullet phase.…”

Application of Ultraviolet Light for Poultry Production: A Review of Impacts on Behavior, Physiology, and Production

“…Longer wavelengths of red light are specifically critical for stimulating the GnRH release (Reddy et al, 2012;Baxter et al, 2014) and darkness produces melatonin to stimulate the gonadotropin inhibitory hormone (GnIH) that prevents GnRH release (Tsutsui et al, 2000(Tsutsui et al, , 2010. However, as reviewed by Hanlon et al (2020), light is not the only factor triggering reproductive status within hens, other metabolic factors may also play an important role.…”

Function and Evolution of Nuclear Receptors in Environmental-Dependent Postembryonic Development

“…Steroid signaling is thereby orchestrated by a range of environmental and developmental cues, which again, are highly species specific. Vertebrates include day length and/or body size information to time puberty with season and food availability ( Leka-Emiri et al, 2017 ; Paul et al, 2018 ; Hanlon et al, 2020 ), which are the most common sources for environmentally induced variation (phenotypic plasticity).…”

“…However, sexual maturation is also associated with the reduction of growth in vertebrates. Usually, the sexual developmental switch is induced after a critical size/weight threshold is reached and the environmental conditions allow for sexual maturation ( Hyun, 2013 ; Leka-Emiri et al, 2017 ; Hanlon et al, 2020 ). This in turn causes a cease of growth in the organism and sexual maturation determines the final body size.…”