Endocrine control in birds

Introduction

The endocrine system consists of a number of organs and major glands located in different areas of the body which play an important role in the proper functioning of the animal. Reproduction in poultry is controlled by a neuroendocrine axis comprised of the hypothalamus, the anterior pituitary gland, and the gonads hypothalamo-pituitary-gonadal (HPG) axis.

Hypothalamus

The hypothalamus is a major part of the brain and is located at the base and approximately in the skull. As far as its endocrine functions in the bird is concerned, they include the production of the releasing factors that act as a control on the anterior pituitary gland, and oxytocin that plays a part in the release of the yolk. The quantity of the releasing factors and oxytocin released is influenced by day length. The longer the day is to 18 hours, the greater the amount of these compounds released and the greater the effect on the target gland or function. The hypothalamus serves as an integration centre and coordinates the activation and inhibition of the axis by releasing neuropeptides in the portal vascular system.

Pituitary gland or hypophysis

The pituitary gland is often called the master gland because many of the compounds it produces target other similar glands to trigger them to produce their compounds that, in turn, influence the functioning of a particular system or organ. Thus, it can be said it is a controlling gland. It consists of two parts:

• Anterior pituitary
• Posterior pituitary

The anterior pituitary gland is stimulated by special releasing factors from the hypothalamus of the brain to produce and release a number of hormones. These include:

Sex hormones– stimulates the sex glands:

• Luteinising Hormone (LH)
• Follicle Stimulating Hormone (FSH)

The quantity of these hormones produced by the pituitary gland will influence the level of activity of the target organ or response. The more that is produced, the greater will be the response. Oxytocin plays a part in the release of the yolk into the oviduct and the actual laying of the egg or oviposition.

In turn, upon binding to their cognate-specific receptors on pituitary gonadotropes, these peptides control the synthesis and release of gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH) into the systemic circulation. Both LH and FSH act at the level of the gonads to initiate sexual maturation, by stimulating gametogenesis and the synthesis of sex steroid hormones. Like most endocrine axes, the HPG is primarily under feedback control.

At the level of the hypothalamus, stimulatory and inhibitory neuropeptides correspond to gonadotropin releasing hormones (GnRHs) and gonadotropin inhibitory hormone (GnIH), respectively. In chickens, two GnRHs have been characterized [chicken cGnRH-I (cGnRH-1) and cGnRH-II] (Miyamoto et al.,1982), and although both GnRHs have the ability to stimulate the production of gonadadotropins, it is well-established that cGnRH-I is the hypophysiotropic peptide.

This is supported by the fact that cGnRH-I perikarya are mainly located in the septal/preoptic hypothalamus projecting to the median eminence (ME), while cGnRH-II perikarya are mainly located around the occulomotor complex extending to the mesencephalon (Van Gils et al.,1993), and immunization against cGnRH-I (not cGnRH-II) induces gonadal reduction in laying hens. On the other hand, GnIH, a peptide from the RF-amide [carboxy-terminal arginine (R) with an amidated phenylalanine (F) motif] family first isolated from quail brain (Tsutsui et al., 2000), was shown to inhibit LH synthesis and release (Ciccone et al.,2004; Ubuka et al.,2006).

Control and integration of stimulatory and inhibitory inputs

As seasonal breeders, chickens have the ability to utilize external environmental cues to initiate and terminate reproduction. Under temperate latitudes, photoperiod is the predominant indicator with increase in day length resulting in gonadal recruitment. This has been extensively utilized by the poultry industry to manage and maximize reproduction under controlled environments. Although it has been well-established that an increase in photoperiod beyond 12 h can stimulate the synthesis and release of GnRH by the hypothalamus (Dunn and Sharp,1990), the exact mechanisms involved in the transduction of light energy into a neuroendocrine signal is still not fully understood.

However, evidence point to direct detection by deep brain photoreceptors located within the hypothalamus (Saldanha et al., 2001). The possible location and mode of action of such photoreceptors is reviewed in this issue by Kuenzel as part of this symposium, and may involve the action of TSH and thyroid hormones as previously reported (Yoshimura et al., 2003; Nakane and Yoshimura, 2014). On the other hand, experimental evidence shows that the synthesis and release of GnIH is primarily under the influence of melatonin, produced by both the pineal gland and the retina of the eye.

Melatonin receptor Mel1c is present on GnIH neurons in quail, and removal of endogenous melatonin sources (pinealectomy and enuclation) reduces levels of GnIH mRNA and peptide within the hypothalamus. Furthermore, this effect was reversed when exogenous melatonin was administered. More recently, it has also been shown that melatonin can stimulate the release of GnIH by hypothalamic explants (Chowdhury et al.,2010), and in vivo melatonin and GnIH follow similar diurnal patterns (Chowdhury et al., 2010; 2013). As a matter of fact, the effect of melatonin may not be limited to birds and may be conserved amongst multiple seasonal breeders including mammals (Tsutsui et al.,2013). Thus, under short days, melatonin produced by the pineal gland and retina of the eye is at its highest, stimulating the release of GnIH and maintaining an inhibition on the HPG axis.

As day length increases, reduced melatonin production lifts GnIH inhibition, while this increase in exposure to light directly stimulates hypothalamic photoreceptors, leading to the activation of the stimulatory pathway via GnRH. Under this model, the hypothalamus modulates pituitary gonadotropes’ function by changing the ratio of inhibitory (GnIH) versus stimulatory (GnRH-I) neuropeptides released into the portal vascular system.

At the level of the pituitary gland, we have also shown that the sensitivity to hypothalamic neuropeptides switches around the time of photo stimulation. In both males and females, levels of cGnRHR-III mRNA are lowest in immature birds, highest post photo stimualtion and progressively decrease towards the end of an active reproductive period (Shimizu and Bedecarrats,2006). Conversely in the same pituitary samples, levels of cGnIHR mRNA are the highest in immature birds and lowest post photo stimulation (Shimizu and Bédécarrats, 2010). This suggests that the activation/inhibition of the HPG is not only under the control of the ratio of hypothalamic neuropeptides, but also possibly regulated at the levels of the anterior pituitary gland by changing the ratio of receptors.