Exploring avian deep-brain photoreceptors and their role in activating the neuroendocrine regulation of gonadal development

Avian migration clocks in a changing world

Avian long-distance migration requires refined programming to orchestrate the birds' movements on annual temporal and continental spatial scales. Programming is particularly important as long-distance movements typically anticipate future environmental conditions. Hence, migration has long been of particular interest in chronobiology. Captivity studies using a proxy, the shift to nocturnality during migration seasons (i.e., migratory restlessness), have revealed circannual and circadian regulation, as well as an innate sense of direction. Thanks to rapid development of tracking technology, detailed information from free-flying birds, including annual-cycle data and actograms, now allows relating this mechanistic background to behaviour in the wild. Likewise, genomic approaches begin to unravel the many physiological pathways that contribute to migration. Despite these advances, it is still unclear how migration programmes are integrated with specific environmental conditions experienced during the journey. Such knowledge is imminently important as temporal environments undergo rapid anthropogenic modification. Migratory birds as a group are not dealing well with the changes, yet some species show remarkable adjustments at behavioural and genetic levels. Integrated research programmes and interdisciplinary collaborations are needed to understand the range of responses of migratory birds to environmental change, and more broadly, the functioning of timing programmes under natural conditions.

Parental behavior and newborn attachment in birds: life history traits and endocrine responses

In birds, parental care and attachment period differ widely depending on the species (altricial or precocial), developmental strategies, and life history traits. In most bird species, parental care can be provided by both female and male individuals and includes specific stages such as nesting, laying, and hatching. During said periods, a series of neuroendocrine responses are triggered to motivate parental care and attachment. These behaviors are vital for offspring survival, development, social bonding, intergenerational learning, reproductive success, and ultimately, the overall fitness and evolution of bird populations in a variety of environments. Thus, this review aims to describe and analyze the behavioral and endocrine systems of parental care and newborn attachment in birds during each stage of the post-hatching period.

Light receptors in the avian brain and seasonal reproduction

Detection and transduction of photic cues by nonvisual photoreceptors, located in the deep brain, is a critical component of timing seasonal reproduction in birds. However, the precise identity of the photoreceptors responsible for detection of salient photic cues remains uncertain and debated. Here I review of the existing evidence for each of the three candidate photoreceptive opsins: Vertebrate Ancient Opsin, Melanopsin, and Neuropsin, including localization, action spectrum, and data from experimental manipulation of opsin expression. These findings are compared to an updated list of key criteria established in the literature as a litmus for classifying an opsin as the "breeding photoreceptor." Integrating evidence for each of the candidate photoreceptors with respect to these criteria reveals support for all three opsins in regulation of seasonal reproduction. Taken together these findings strongly suggest that transduction of seasonal photoperiodic information involves the activity of multiple photoreceptor types and populations functioning in concert. This review also highlights the need to shift attention from simply identifying "the breeding photoreceptor" to a more integrative approach aiming to parse the contribution of specific photoreceptor populations within the brain.

Effect of follicle-stimulating hormone and luteinizing hormone levels on egg-laying frequency in hens

Background and Aim

Gonadotropins, for example, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are hormones that affect the reproductive process. In hens, optimal levels of FSH and LH can stimulate follicle growth fairly rapidly and thereby increase egg production through follicle development and increased ovulation. Follicle-stimulating hormone acts in the early stages of follicular growth, whereas LH acts on pre-ovulatory follicles. Normal follicular growth is the result of the complementary action of FSH and LH. Low FSH and LH levels result in the formation of follicles but a lack of egg production in chickens. This study aimed to investigate FSH and LH hormone levels from layer chickens with different egg-laying frequencies.

Materials and Methods

Fifty blood serum samples were collected from 54-week-old ISA brown strain hens that were divided into five groups (with 10 hens per group) as follows: Hens that lay eggs (i) every day, (ii) once every 2 days, (iii) twice every 3 days, (iv) 3 times every 4 days, and (v) hens that do not lay eggs. Follicle-stimulating hormone and LH levels were measured in samples using an enzyme-linked immunosorbent assay, and the data were analyzed using multivariate analysis of variance.

Results

Follicle-stimulating hormone levels were significantly associated with the frequency of egg laying in ISA brown strain hens (p < 0.05); the highest FSH level (869.005 ± 149.194 pg/mL) was found in hens that lay eggs every day. In contrast, the highest LH level (51.386 ± 2.410 mIU/mL) was found in hens that lay eggs every 2 days.

Conclusion

High level of FSH (869.005 ± 149.194 pg/mL) was associated with a high frequency of egg laying (every day) in ISA brown strain hens, and LH level of around 30.406 pg/mL was associated with daily egg laying in these hens.

Incubation Temperature and Lighting: Effect on Embryonic Development, Post-Hatch Growth, and Adaptive Response

During incubation, the content of the egg is converted into a chick. This process is controlled by incubation conditions, which must meet the requirements of the chick embryo to obtain the best chick quality and maximum hatchability. Incubation temperature and light are the two main factors influencing embryo development and post-hatch performance. Because chicken embryos are poikilothermic, embryo metabolic development relies on the incubation temperature, which influences the use of egg nutrients and embryo development. Incubation temperature ranging between 37 and 38°C (typically 37.5–37.8°C) optimizes hatchability. However, the temperature inside the egg called “embryo temperature” is not equal to the incubator air temperature. Moreover, embryo temperature is not constant, depending on the balance between embryonic heat production and heat transfer between the eggshell and its environment. Recently, many studies have been conducted on eggshell and/or incubation temperature to meet the needs of the embryo and to understand the embryonic requirements. Numerous studies have also demonstrated that cyclic increases in incubation temperature during the critical period of incubation could induce adaptive responses and increase the thermotolerance of chickens without affecting hatchability. Although the commercial incubation procedure does not have a constant lighting component, light during incubation can modify embryo development, physiology, and post-hatch behavior indicated by lowering stress responses and fearful behavior and improving spatial abilities and cognitive functions of chicken. Light-induced changes may be attributed to hemispheric lateralization and the entrainment of circadian rhythms in the embryo before the hatching. There is also evidence that light affects embryonic melatonin rhythms associated with body temperature regulation. The authors’ preliminary findings suggest that combining light and cyclic higher eggshell temperatures during incubation increases pineal aralkylamine N-acetyltransferase, which is a rate-limiting enzyme for melatonin hormone production. Therefore, combining light and thermal manipulation during the incubation could be a new approach to improve the resistance of broilers to heat stress. This review aims to provide an overview of studies investigating temperature and light manipulations to improve embryonic development, post-hatch growth, and adaptive stress response in chickens.

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

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).

A Comparative Perspective on Extra-retinal Photoreception

Ubiquitous in non-mammalian vertebrates, extra-retinal photoreceptors (ERPs) have been linked to an array of physiological, metabolic, behavioral, and morphological changes. However, the mechanisms and functional roles of ERPs remain one of the enduring questions of modern biology. In this review article, we use a comparative framework to identify conserved roles and distributions of ERPs, highlighting knowledge gaps. We conclude that ERP research can be divided into two largely unconnected categories: (i) identification and localization of photoreceptors and (ii) linkage of non-retinal light reception to behavioral and physiological processes, particularly endocrine systems. However, the emergence of novel gene editing and silencing techniques is enabling the unification of ERP research by allowing the bridging of this divide.

Scotostimulation of reproductive neural pathways and gonadal maturation are not correlated with hypothalamic expression of deiodinases in subtropical spotted munia

Circannual rhythm regulates the annual timing of reproduction in spotted munia, with sex differences in its relationship with the external photoperiod environment. Interestingly, munia show an atypical photosensitivity and exhibit gonadal maturation when acutely exposed to an unnatural short photoperiod (eg 3 hours of light per day; ie a long scotoperiod). The proximate mechanisms regulating scotoperiod-induced hypothalamic-pituitary-gonadal (HPG) activation are unclear. Because thyroid hormone signalling plays a central role in photoperiodic induction, we hypothesised the involvement of similar mechanism, comprising alterations in hypothalamic deiodinases, under long scotoperiod-induced HPG activation. To test this, several endpoints of cellular and molecular correlates were assayed in male and female munias after 1 and 4 weeks of exposure to an 3:21 hour light/dark cycle (3L:21D), with controls on a 21:3 hour light/dark cycle (21L:3D). We measured the hypothalamic expression of mRNA and protein of light-sensitive (neuropsin, OPN5) and reproductive (vasoactive intestinal peptide [VIP], neuropeptide Y [NPY], gonadotrophin-releasing hormone [GnRH], gonadotrophin-inhibiting hormone [GnIH]) neuropeptides by quantitative polymerase chain reaction (PCR) and immunohistochemistry, respectively. In addition, we also measured mRNA expression of types 2 (DIO2) and 3 (DIO3) deiodinases that regulate triiodothyronine-mediated GnRH release and gonadal maturation in photoperiodic species. The quantitative PCR and immunohistochemistry results were consistent. Higher OPN5 levels under 21L:3D than under 3L:21D suggested its role in sensing the length of the light period. Similarly, low VIP and high NPY expression under 3L:21D than under 21L:3D were consistent with their roles as cellular correlates of photic and nonphotic environment, respectively. High GnRH-I/low GnIH levels and gonadal recrudescence under 3L:21D, and an inverse pattern under 21L:3D, confirmed the scotostimulation of HPG axis in spotted munia. However, DIO2 and DIO3 mRNA levels did not differ between 2 scotoperiods, in contrast to their reciprocal expression pattern found during long-day photostimulation. We demonstrate for the first time sex-dependent scotostimulation of reproductive neural pathways and suggest the involvement of molecules other than hypothalamic deiodinases in the regulation of gonad development cycle in 'nonphotoperiodic' seasonally breeding vertebrates.

Light at night disrupts nocturnal rest and elevates glucocorticoids at cool color temperatures

Nighttime light pollution is quickly becoming a pervasive, global concern. Since the invention and proliferation of light-emitting diodes (LED), it has become common for consumers to select from a range of color temperatures of light with varying spectra. Yet, the biological impacts of these different spectra on organisms remain unclear. We tested if nighttime illumination of LEDs, at two commercially available color temperatures (3000 and 5000 K) and at ecologically relevant illumination levels affected body condition, food intake, locomotor activity, and glucocorticoid levels in zebra finches (Taeniopygia guttata). We found that individuals exposed to 5000 K light had higher rates of nighttime activity (peaking after 1 week of treatment) compared to 3000 K light and controls (no nighttime light). Birds in the 5000 K treatment group also had increased corticosterone levels from pretreatment levels compared to 3000 K and control groups but no changes in body condition or food intake. Individuals that were active during the night did not consequently decrease daytime activity. This study adds to the growing evidence that the spectrum of artificial light at night is important, and we advocate the use of nighttime lighting with warmer color temperatures of 3000 K instead of 5000 K to decrease energetic costs for avian taxa.

Hormonally mediated effects of artificial light at night on behavior and fitness: linking endocrine mechanisms with function

Alternation between day and night is a predictable environmental fluctuation that organisms use to time their activities. Since the invention of artificial lighting, this predictability has been disrupted and continues to change in a unidirectional fashion with increasing urbanization. As hormones mediate individual responses to changing environments, endocrine systems might be one of the first systems affected, as well as being the first line of defense to ameliorate any negative health impacts. In this Review, we first highlight how light can influence endocrine function in vertebrates. We then focus on four endocrine axes that might be affected by artificial light at night (ALAN): pineal, reproductive, adrenal and thyroid. Throughout, we highlight key findings, rather than performing an exhaustive review, in order to emphasize knowledge gaps that are hindering progress on proposing impactful and concrete plans to ameliorate the negative effects of ALAN. We discuss these findings with respect to impacts on human and animal health, with a focus on the consequences of anthropogenic modification of the night-time environment for non-human organisms. Lastly, we stress the need for the integration of field and lab experiments as well as the need for long-term integrative eco-physiological studies in the rapidly expanding field of light pollution.

The Role of VIP in Social Behavior: Neural Hotspots for the Modulation of Affiliation, Aggression, and Parental Care

Although the modulation of social behaviors by most major neurochemical systems has been explored, there are still standouts, including the study of vasoactive intestinal polypeptide (VIP). VIP is a modulator of circadian, reproductive, and seasonal rhythms and is well known for its role in reproductive behavior, as it is the main vertebrate prolactin-releasing hormone. Originally isolated as a gut peptide, VIP and its cognate receptors are present in virtually every brain area that is important for social behavior, including all nodes of the core "social behavior network" (SBN). Furthermore, VIP cells show increased transcriptional activity throughout the SBN in response to social stimuli. Using a combination of comparative and mechanistic approaches in socially diverse species of estrildid finches and emberizid sparrows, we have identified neural "hotspots" in the SBN that relate to avian affiliative behavior, as well as neural "hotspots" that may represent critical nodes underlying a trade-off between aggression and parental care. Specifically, we have found that: (1) VIP fiber densities and VIP receptor binding in specific brain sites, such as the lateral septum, medial extended amygdala, arcopallium, and medial nidopallium, correlate with species and/or seasonal differences in flocking behavior, and (2) VIP cells and fibers within the anterior hypothalamus-caudocentral septal circuit relate positively to aggression and negatively to parental care while VIP elements in the mediobasal hypothalamus relate negatively to aggression and positively to parental care. Thus, while a given behavior or social context likely activates VIP circuitry throughout the SBN and beyond, key brain sites emerge as potential "hotspots" for the modulation of affiliation, aggression, and parental care.

The premammillary nucleus of the hypothalamus is not necessary for photoperiodic timekeeping in female turkeys (Meleagris gallopavo)

In birds, seasonal reproduction is regulated by day length, with long days in the spring activating the hypothalamic-pituitary-gonadal axis and reproductive behaviors. The photoreceptors mediating this process remain unknown, but recently, the premammillary nucleus (PMM) of the hypothalamus has been implicated as the site of photoperiodic signaling in turkeys. We performed electrolytic lesions of the PMM to elucidate its role in the photoactivation and maintenance of egg production in female turkeys. Our results show that ablation of the PMM does not alter the normal lay cycle. No differences were found between lesioned birds and sham controls in the latency to lay following photostimulation, nor in subsequent egg production over a period of 29 weeks. No differences in the incidence of gonadal regression were found, indicating that the PMM is not essential for the termination of breeding. We conclude that any role of the PMM in photoperiodic regulation, if it exists, is redundant with other components of the system.

Photoperiodic control of GnRH-I expression in seasonal reproduction of the Eurasian tree sparrow

Day length has been shown to be a major source of temporal information regulating seasonal reproduction in the Eurasian tree sparrow (Passer montanus).