Deep-brain photoreceptors (DBPs) involved in the photoperiodic gonadal response in an avian species, Gallus gallus

Role of deep brain photoreceptors in regulation of daily and seasonal responses in birds

Birds exhibit an extraordinary morphological, physiological, and behavioral diversity that allows them to adapt to the diverse environments of our planet. To achieve this, they utilize different sensory structures. One of these structures is located in the deep brain and contains neurons with photopigments (Deep Brain Photoreceptors, DBP) that detect daily and seasonal changes in ambient light (photoperiod), allowing the individual to adjust and synchronize physiological processes with the environment. This DBPs detects and transmits light information to the hypothalamic-pituitary-gonadal axis, regulating the gonadal recrudescence/regression cycle and possibly daily responses in birds. This work reviews and discusses the state of the art about the presence and functionality of DBPs in a phylogenetic context, with a particular focus on annual reproductive responses and their little-known relationship with daily responses. Exceptions to the seasonal reproductive regulation mechanism, as observed in opportunistic bird species such as the eared dove, where food availability appears to drive the activity of the gonadal oscillator are also discussed. Finally, the possible neural pathways through which DBPs transmit photoperiodic information to the circadian system in birds are proposed.

Transcriptome and metabolome reveal the mechanism of neuroendocrine regulation in ovarian development of broiler breeders

The long-term selection for meat has led to the poor egg production efficiency in broiler. In this study, we analyzed the transcriptional levels of hypothalamus and ovary during the pre-laying (PP) and laying periods (LP) of broiler breeders. By combining these with the levels of reproductive hormones and ovarian metabolism, to reveal the neuroendocrine control mechanism of ovarian development. Results showed that during LP, the number of LYFs, SYFs and WFs, the thickness of the granular cell layer, and the serum LH, FSH, P4 and E2 levels were significantly increased (P < 0.05). A total of 1188 and 2481 differentially expressed genes (DEGs) were detected in hypothalamus and ovary, respectively. 1972 significantly differentially metabolites (DMs) were detected in ovary. In hypothalamus, the expression of neuroendocrine regulatory genes such as TRH, AVT, VIP, and NYB in the Neuroactive ligand-receptor interaction pathway regulated the LH and FSH secretion via the HPG axis. In ovary, the promotion of GCs proliferation may occur through the glycerophospholipid metabolism pathway, which increased the thickness of the GCs layer. This helped to receive gonadotropin signals and increased P4 and E2 secretion. Meanwhile, the decreased expression levels of ovarian development inhibitory factors in the TGF-beta signaling pathway, including BMP2, BMP4, BMP15 and AMHR2, and the increased expression levels of MMPs, including MMP9, MMP11 and MMP13, may regulate the synthesis of metabolites associated with steroid hormone secretion and ovarian development, such as E2, E2-3S, 7α-OH-DHEA, CHO and AD. These genes and metabolites may play an important role in HPG axis in regulating ovarian development.

Immunization with OPN5 increased seasonal degradation of reproductive activity in Magang ganders

To investigate the regulatory mechanism mediated by hypothalamic OPN5 on seasonal changes in the reproductive activities of domestic geese, 60 Magang ganders in their breeding period were selected for the experiment and evenly divided into an immunization group(OPN5-IM) and a control group. On days 0, 15 and 30, ganders in the immunized group were immunized with OPN5-KLH protein vaccine, and ganders in the control were immunized with the same amount of blank emulsified vaccine. Additionally, 120 female geese were provided to stimulate the reproductive activities of male geese. The results showed that the arrangement of spermatogenic cells was disturbed, the number of sperm decreased, and the testicular weight, seminiferous tubule area, length diameter, spermatogenic epithelium thickness decreased significantly with the natural day length prolonged. Moreover, the concentration of testosterone and LH decreased significantly while PRL increased. The prolonged photoperiod significantly affected the gene expression of GnRH-I, VIP, FSHβ, FSHR, LHβ, PRL, and PRLR in ganders. Specifically, the gene expression of GnRH-I, FSHβ, and LHβ in the hypothalamus and pituitary decreased, while the gene expression of VIP, PRL, and PRLR increased. Following OPN5 immunization, the anti-OPN5 antibody titer of ganders in the OPN5-IM group was notably higher than in the control group. The testicular degeneration was severe in OPN5-IM group compared with the control, as evidenced by a significant reduction in seminiferous tubule area, length diameter, and thickness of spermatogenic epithelium in the immunized group on day 60. Additionally, the concentrations of testosterone and LH were lower in the OPN5-IM group than in the control group, whereas PRL was higher. Moreover, OPN5 immunization significantly affected the expression of GnRH-I, PRL, and PRLR. OPN5 mRNA and protein expression were higher in the immunized group, whereas TRH, DIO2, and TSHR mRNA expressions were lower. However, DIO3 mRNA and protein were up-regulated in the immunized group. In conclusion, our results indicated that the reproductive performance of Magang geese degraded from the breeding to the non-breeding period as daylight was extended. Immunization against OPN5 increased OPN5 expression and down-regulated the TSH-DIO2/DIO3 pathway, further to affect the HPG axis and accelerate the degradation of reproductive activity. Therefore, OPN5 may play an important mediating role in light-regulating seasonal reproductive degradation in Magang geese.

Photostimulation decreases fearfulness, but improves growth performance and egg quality of breeder Pekin ducks

Lighting is a critical environmental factor that influences production performance and welfare of poultry, however Pekin ducks can typically be housed under 24 h (24 h) of light. 460 hatchlings were randomly allocated to 4 rooms with two pens in each room. The rooms were allocated to 24 h light or PS. PS was achieved by gradually increasing photoperiod by half an hour per week from days 112 to 238 and held at 18 h light for the rest of life. Prior to the onset of lay, pens were organized with 30 hens and 7 drakes/pen (4 pens/treatment). We measured bodyweight at weeks 0, 1, 2, 5, 10, 19, and 29, feed intake at weeks 1 to 7 and feed conversion ratio (FCR) was calculated for weeks 1, 2, and 5. Eggs laid were recorded daily from weeks 20 to 31. Novel object test (NOT) was done at weeks 1, 5, 10, 19, and 29, transect welfare scoring at week 30, egg quality assessment at weeks 28 and 29, and fertility was determined at weeks 28 and 29. Statistical analyses were done using 2-way ANOVA, T-test, or Friedman Test with a Tukey-Kramer test as post-hoc. A p ≤ 0.05 was considered significant. Drakes bodyweight was higher in the PS compared to 24h treatment at weeks 5 (p < 0.01) and 10 (p < 0.001). No difference was observed in FCR, fertility, or transect data. NOT showed lower fear response (p < 0.05) at weeks 5 and 10 in the PS compared to 24 h treatment. The 24 h treatment increased the percent eggs laid at weeks 23 (p < 0.05), 24 (p < 0.01) and 25 (p < 0.05) compared to the PS. However, there was a higher egg weight (p < 0.001), yolk weight (p < 0.05), Haugh unit (p < 0.05), and shell weight (p = 0.05) in the PS compared to 24 h treatment. Our findings support that PS may help decrease fearfulness, improve growth performance and egg quality of breeder ducks.

Effect of active immunization with OPN5 on follicular development and egg production in quail under different photoperiods

OPN5 is one of the main deep brain photoreceptors (DBPs), converting photoperiodic information into neuroendocrine signals to regulate reproduction in birds. This study investigated the mechanism of OPN5-mediated photoperiodic regulation of reproduction by active immunization against OPN5. 96 female quail were divided into OPN5-immunized and control group under the same photoperiod: 16 L:8 D (d 1 to d 35), 8 L:16 D (d 36 to d 70) and 12 L:12 D (d 71 to d 126). OPN5-immunized group was conducted with OPN5 protein vaccination and control group was given a blank vaccine. Samples were collected on d 1, d 30, d 60, and d 126. Results showed switching photoperiod to 8 L:16 D decreased the laying rate, GSI%, numbers of YFs and WFs, serum levels of PRL, P4 and E2, and pituitary PRL and TSHβ protein expressions in both groups (P < 0.05). Whereas the OPN5-immunized group exhibited higher laying rates than the control group (P < 0.05). The control group showed reduced GnRHR and TSHβ gene expressions in the pituitary and increased GnIH and DIO3 transcript and/or protein abundance in the hypothalamus. (P < 0.05). The OPN5-immunized group had lower DIO3 expression at both mRNA and protein levels. (P < 0.05). Switching photoperiod from 8 L:16 D to 12 L:12 D increased the laying rates, GSI%, numbers of YFs and WFs, serum levels of PRL, and PRL protein expression in both groups (P < 0.05), and the responses were more pronounced in OPN5-immunized group (P < 0.05). In contrast to the control group, quail with OPN5-immunization had higher OPN5 and DIO2 transcript and/or protein levels but lower DIO3 expressions in the hypothalamus along the transition photoperiods (P < 0.05). The results revealed that OPN5 responds to photoperiod transition, and its activation mediates related signaling to up-regulate TSH-DIO2/DIO3 pathway and VIP-PRL secretion to prime quail reproductive functions.

Effect of immunization against OPN5 on the reproductive performance in Shan Partridge ducks under different photoperiods

Photoperiod is an important environmental factor that influences seasonal reproduction behavior in birds. Birds translate photoperiodic information into neuroendocrine signals through deep brain photoreceptors (DBPs). OPN5 has been considered candidate DBPs involved in regulating seasonal reproduction in birds. We found that OPN5 could mediate light to regulate the follicle development in ducks. In this study, we further verified the effect of OPN5 on follicular development in Shan Partridge ducks by immunizing against the extracellular domain (ECD) of OPN5. We investigated the specific regulatory mechanism of photoperiod mediated by OPN5 on the reproductive activity of ducks. The trial randomly divided 120 Shan Partridge ducks into 3 groups with different treatments: the immunization of OPN5 group was done at d0, d15, d30, and d40 with 1 mL of vaccine containing OPN5 protein (thus containing 1, 1, 0.5, and 0.5 mg of OPN5-KLH protein), and the control group (CS and CL groups) was injected at the same time with the same dose of OPN5-uncontained blank vaccine. The group of CS (900 lux), OPN5 (600 lux), and CL (600 lux) lasted for 40 d in 12 L:12 D photoperiods, respectively. Then, the groups of CS, OPN5, and CL subsequently received 12 L:12 D, 12 L:12 D, and 17 L:7 D light treatments for 33 d, respectively. The ducks were caged in 3 constant rooms with the same feeding conditions for each group, free water, and limited feeding (150 g per duck each day). Duck serum and tissue samples were collected at d 40, d 62, and d 73 (n = 12). It was found that before prolonged light, the group of immunization (group OPN5) and the group of strong light intensity (group CS) were higher than the group of CL in egg production. Subsequent to prolonged light, the group CL in egg production rose about the same as the group immunization, while the strong light group (group CS) was lower. Group OPN5 increased the ovarian index of ducks, and both the immunization of group OPN5 and group CL (extended light) increased the thickness of the granular layer and promoted the secretion of E2, P4, LH, and PRL hormones. Compared with group CS, group CL and OPN5 increased the mRNA level and protein expression of OPN5 in the hypothalamus on d 62 and d 73 (P < 0.05). The gene or protein expression patterns of GnRH, TRH, TSHβ, DIO2, THRβ, VIP, and PRL were positively correlated with OPN5, whereas the gene expression patterns of GnIH and DIO3 were negatively correlated with OPN5. The results showed that immunization against OPN5 could activate the corresponding transmembrane receptors to promote the expression of OPN5, up-regulate the expression of TSHβ and DIO2, and then regulate the HPG axis-related genes to facilitate the follicular development of Shan Partridge ducks. In addition, in this experiment, prolonging the photoperiod or enhancing the light intensity could also enhance follicle development, but the effect was not as significant as immunizing against OPN5. Our results will offer beneficial data and more supportive shreds of evidence in favor of elucidating the role of OPN5 in relation to photoperiods and reproduction.

Three different seasonally expressed opsins are present in the brain of the Eared Dove, an opportunist breeder

Birds living at high latitudes perceive the photoperiod through deep-brain photoreceptors (DBP) located in deep-brain neurons. During long photoperiods the information transmitted by these photoreceptors increases the activity of the hypothalamic-pituitary-gonadal (HPG) axis, leading to gonadal development. The presence of photopigments such as VA-Opsin, Opn4, Opn5 and Opn2 in brain areas implicated in reproductive behaviors has been firmly established in several avian species with seasonal breeding, whereas their existence in opportunistic breeding birds remains unconfirmed. The Eared Dove is an urban and peri-urban dove that breeds throughout the year. Males of this species do not exhibit the typical gonadal regression/recrudescence cycle, thus posing the question of what occurs upstream of the HPG axis. We addressed this issue by first studying the presence of diverse opsins located in DBP in the brains of Eared Dove males and whether these photopigments changed their expression throughout the year. We carried out an immunohistochemistry analysis on three different opsins: Opn2 (rhodopsin), Opn3 and Opn5. Our results demonstrate the discrete neuroanatomical distribution of these opsins in the brain of Eared Dove males and strongly indicate different seasonal expressions. In the anterior region of the hypothalamus, Opn2-positive cells were detected throughout the year. By contrast, Opn5 was found to be strongly and seasonally expressed during winter in the anterior and the hypothalamic region. Opn3 was also found to be significantly and seasonally expressed during winter in the hypothalamic region. We thus demonstrate for the first time that males of the Eared Dove, have three different deep-brain opsin-expressing photoreceptors with differential location/distribution in the anterior and hypothalamic region and differential seasonality. The persistence of Opn2 and the strong seasonal expression of nonvisual photopigments Opn3 and Opn5 in two areas of the avian brain, which are associated with reproduction, could be the primary distinction between seasonal and opportunistic breeders.

Endocrine and molecular regulation mechanisms of follicular development and egg-laying in quails under different photoperiods

Photoperiod is a key environmental factor in regulating bird reproduction and induces neuroendocrine changes through the hypothalamic-pituitary-gonadal (HPG) axis. OPN5, as a deep-brain photoreceptor, transmits light signals to regulate follicular development through TSH-DIO2/DIO3. However, the mechanism among OPN5, TSH-DIO2/DIO3, and VIP/PRL in the HPG axis underlying the photoperiodic regulation of bird reproduction is unclear. In this study, 72 laying quails with 8-week-old were randomly divided into the long-day (LD) group [16 light (L): 8 dark (D)] and the short-day (SD) group (8 L:16 D), and then samples were collected on d 1, d 11, d 22, and d 36 of the experiment. The results showed that compared with the LD group, the SD group significantly inhibited follicular development (P < 0.05), decreased the P4, E2, LH, and PRL in serum (P < 0.05), downregulated the expression of GnRHR, VIP, PRL, OPN5, DIO2, and LHβ (P < 0.05), reduced the expression of GnRH and TSHβ (P > 0.05), and promoted DIO3, GnIH gene expression (P < 0.01). The short photoperiod downregulates OPN5, TSHβ, and DIO2 and upregulates DIO3 expression to regulate the GnRH/GnIH system. The downregulation of GnRHR and upregulation of GnIH resulted in a decrease in LH secretion, which withdrew the gonadotropic effects on ovarian follicles development. Slow down of follicular development and egg laying may also arise from lack of PRL potentiation to small follicle development under short days.

Adaptation to the shallow sea floor environment of a species of marine worms, Oligobrachia mashikoi, generally inhabiting deep-sea water

Beard worms from the family Siboglinidae, are peculiar animals and are known for their symbiotic relationships with sulfur bacteria. Most Siboglinids inhabit the deep-sea floor, thus making difficult to make any observations in situ. One species, Oligobrachia mashikoi, occurs in the shallow depths (24.5 m) of the Sea of Japan. Taking advantage of its shallow-water habitat, the first ecological survey of O. mashikoi was performed over a course of 7 years, which revealed that its tentacle-expanding behavior was dependent on the temperature and illuminance of the sea water. Furthermore, there were significantly more O. mashikoi with expanding tentacles during the nighttime than during the daytime, and the prevention of light eliminated these differences in the number of expending tentacles. These results confirmed that the tentacle-expanding behavior is controlled by environmental light signals. Consistent with this, we identified a gene encoding a photoreceptor molecule, neuropsin, in O. mashikoi, and the expression thereof is dependent on the time of day. We assume that the described behavioral response of O. mashikoi to light signals represent an adaptation to a shallow-water environment within the predominantly deep-sea taxon.

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.

Seasonality in tropical birds

The survival of offspring depends on environmental conditions. Many organisms have evolved with seasonality, characterized as initiation-termination-reinitiation of several physiological processes (i.e., body fattening, molt, plumage coloration, reproduction, etc). It is an adaptation for the survival of many species. Predominantly seasonal breeders use photoperiod as the most reliable environmental cue to adapt to seasonal changes but supplementary factors like temperature and food are synergistically involved in seasonal processes. Studies from diverse vertebrate systems have contributed to understanding the mechanism involved in seasonal reproduction at the molecular and endocrine levels. Long-day induced thyrotropin (thyroid-stimulating hormone) released from the pars tuberalis of the pituitary gland triggers local thyroid hormone activation within the mediobasal hypothalamus. This locally produced thyroid hormone, T3, regulates seasonal gonadotropin-releasing hormone secretion. Most of the bird species studied are seasonal in reproduction and linked behavior and, therefore, need to adjust reproductive decisions to environmental fluctuations. Reproductive strategies of the temperate zone breeders are well-documented, but less is known about tropical birds' reproduction and factors stimulating the annual breeding strategies. Here, we address seasonality in tropical birds with reference to seasonal reproduction and the various environmental factors influencing seasonal breeding.

OPN5 Regulating Mechanism of Follicle Development Through the TSH-DIO2/DIO3 Pathway in Mountain Ducks Under Different Photoperiods

Abstract: Photoperiod is an important environmental factor that influence seasonal reproduction behavior in bird. Birds translates photoperiodic information into neuroendocrine signals through deep brain photoreceptors (DBPs). OPN5 has been considered as candidate DBPs involving in regulation of seasonal reproduction in birds. However, little is known about the effect of OPN5 in non-seasonal breeding birds. Thus, we pondered on whether OPN5 regulating follicular development through TSH-DIO2/DIO3 system responds to different photoperiods in non-seasonal laying ducks. As an ideal non-seasonal breeding bird, a total of 120 mountain ducks were randomly divided into three groups and treated respectively to a different photoperiod: group S (8 L:16D), group C (17 L:7D), and group L (24 L:0D). The ducks were caged in a fully enclosed shelter with the same feeding conditions for each group, free water and limited feeding (150 g per duck each day). Samples were collected from each group at d 0, d 5, d 8, d 20, and d 35 (n = 8). The ducks in 24 h photoperiod had the highest laying rate and the lowest feed-to-egg ratio, while the ducks in 8 h photoperiod had the lowest laying rate and the highest feed-to-egg ratio. Long-day photoperiod for 24 h significantly increased the ovarian index and GnRH, LH, E2, and P4 levels in serum; short-day photoperiod for 8 h increased testosterone levels in serum. Compared with 8 h photoperiod, long-day photoperiod significantly or highly significantly increased the mRNA level and protein expression of OPN5 in the hypothalamus of long-day photoperiod on d 35 (p &lt; 0.05). The gene or protein expression patterns of GnRH, TRH, TSHβ, DIO2, THRβ, VIP, and PRL were positively correlated with OPN5, whereas the gene expression patterns of GnIH and DI O 3 were negatively correlated with OPN5. The results revealed that OPN5 mediated the effect of light on follicular development through the TSH-DIO2/DIO3 pathway, the expression of OPN5 increased with light duration and improved the efficiency of the HPG axis to promote follicular development in mountain ducks.

Effect of pre-hatch incubator lights on the ontogeny of CNS opsins and photoreceptors in the Pekin duck

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Incubated eggs with and without light had no effect on post-hatch production.

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Light does not influence the ontogeny of retinal rod and cone photoreceptors.

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Brain OPN4 mRNA is increased the later stages of embryonic development.

The Pekin duck is a valuable agricultural commodity globally and in the United States. Pekin ducks are seasonal breeders; they are sensitive to light and thus, research on the neuroendocrine and behavioral responses are needed to maximize production and to improve their welfare. There is compelling evidence that specific wavelengths of light may adversely alter the growth and welfare of meat (grow out) ducks. However, despite a birds’ dependence upon light, in commercial poultry hatcheries, incubators almost exclusively hold eggs in the dark. Therefore, our objective was to determine the effects of lighting on the expression of retina photoreceptors (RPs) and deep brain photoreceptors (DBPs) during duck embryological development. Two groups of ducks were raised with and without light over 21 d from egg laying, embryonic day 0. Brain and retinal tissues were collected at embryonic days 3, 7, 11, 16, and 21 of a 24 d incubation period. qRT-PCR was performed on RPs (OPN1LW, OPN2SW, OPN1SW, MAFA, RHO, and RBP3) and the DBP OPN4M from retinal and brain samples, respectively. We find that the presence and absence of light during pre-hatch incubation, had no influence on the expression of any retinal photoreceptor. However, a late embryological increase in DBP OPN4M expression was observed. Taken together, the impact of light during pre-hatch incubation does not impact the overall post-hatch production. However, future directions should explore how OPN4M pre-hatch activation impacts Pekin duck post-hatch development and growth.

Non-visual Opsins and Novel Photo-Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spectrum Region

In recent decades, a number of novel non-visual opsin photopigments belonging to the family of G protein- coupled receptors, likely involved in a number of non-image-forming processes, have been identified and characterized in cells of the inner retina of vertebrates. It is now known that the vertebrate retina is composed of visual photoreceptor cones and rods responsible for diurnal/color and nocturnal/black and white vision, and cells like the intrinsically photosensitive retinal ganglion cells (ipRGCs) and photosensitive horizontal cells in the inner retina, both detecting blue light and expressing the photopigment melanopsin (Opn4). Remarkably, these non-visual photopigments can continue to operate even in the absence of vision under retinal degeneration. Moreover, inner retinal neurons and Müller glial cells have been shown to express other photopigments such as the photoisomerase retinal G protein-coupled receptor (RGR), encephalopsin (Opn3), and neuropsin (Opn5), all able to detect blue/violet light and implicated in chromophore recycling, retinal clock synchronization, neuron-to-glia communication, and other activities. The discovery of these new photopigments in the inner retina of vertebrates is strong evidence of novel light-regulated activities. This review focuses on the features, localization, photocascade, and putative functions of these novel non-visual opsins in an attempt to shed light on their role in the inner retina of vertebrates and in the physiology of the whole organism.

Central Nervous System Associated With Light Perception and Physiological Responses of Birds

Environmental light that animal receives (i.e., photoperiod and light intensity) has recently been shown that it affects avian central nervous system for the physiological responses to the environment by up or downregulation of dopamine and serotonin activities, and this, in turn, affects the reproductive function and stress-related behavior of birds. In this study, the author speculated on the intriguing possibility that one of the proposed avian deep-brain photoreceptors (DBPs), i.e., melanopsin (Opn4), may play roles in the dual sensory-neurosecretory cells in the hypothalamus, midbrain, and brain stem for the behavior and physiological responses of birds by light. Specifically, the author has shown that the direct light perception of premammillary nucleus dopamine-melatonin (PMM DA-Mel) neurons is associated with the reproductive activation in birds. Although further research is required to establish the functional role of Opn4 in the ventral tegmental area (VTA), dorsal raphe nucleus, and caudal raphe nucleus in the light perception and physiological responses of birds, it is an exciting prospect because the previous results in birds support this hypothesis that Opn4 in the midbrain DA and serotonin neurons may play significant roles on the light-induced welfare of birds.

Ontogeny of OPN4, OPN5, GnRH and GnIH mRNA Expression in the Posthatch Male and Female Pekin Duck (Anas platyrhynchos domesticus) Suggests OPN4 May Have Additional Functions beyond Reproduction

The hypothalamic-pituitary-gonadal axis (HPG) is known to be regulated by daylength through the deep brain photoreceptor (DBP) system. The post-hatch ontogeny is not known for any of the DBPs. We set out to determine the ontogeny of OPN4 and OPN5 gene expression relative to GnRH and GnIH using qRT-PCR. Brains and serum were collected from five drakes and five hens on the day of hatching (Day 0) and again at 2, 4, 6, 10, 14, 19, 25 and 31 weeks of age and analyzed by qRT-PCR. Hen and drake serum was assayed for circulating levels of estradiol and testosterone, respectively. Data were analyzed between sexes over time using a repeated measures two-way ANOVA. Interestingly, the results show that on the day of hatching (Day 0), ducks showed adult-like levels of relative OPN4, but not OPN5, gene expression. During week 10, DBP levels increased, achieving highest relative expression levels at week 19 that maintained through week 31, typically peak fertility in ducks. GnRH mRNA levels increased following the DBP expression at the onset of puberty, and gonadal steroids increased after GnRH at week 14 while estradiol preceded testosterone. GnIH mRNA levels did not appreciably change during the time course of this experiment. These observations suggest that OPN4 may be active during the peri-hatch period and may have physiological roles beyond puberty and fertility.

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).

Effects of light intensity and dual light intensity choice on plasma corticosterone, central serotonergic and dopaminergic activities in birds, Gallus gallus

Light intensity plays an important role in the regulation of growth, behavior, reproduction, and welfare of avian species. Light intensity preference behavior has been suggested to be involved in welfare of birds. This study aims to investigate the effects of different light intensity and dual light intensity choice (DLIC) lighting program on plasma corticosterone (CORT), and tryptophan hydroxylase 2 (TPH2, the rate-limiting enzyme of serotonin biosynthesis) and tyrosine hydroxylase (TH, the rate-limiting enzyme of dopamine biosynthesis) gene expression in the brainstem of male chickens. Day old broilers were housed in two commercial houses, and placed in 24 pens. All the treatment groups were provided with 23 h light (L) /1 h dark (D) and 30 lx (lx) light intensity during the first week and then 18L:6D (10 lx) from day 7 to 14. Blood and brain were sampled at 14 days of age (10 lx) before the onset of light treatments. On day 15, four treatments (2, 10, 20, and 100 lx), and DLIC treatment (2/20 lx) were initiated. Samples were collected on days 15, 16, 17, 30 and 41. TPH2 expression in the dorsal raphe nucleus (DRN) and caudal raphe nucleus (CRN) of brainstem, and TPH2 and TH expression in ventral tegmental areas (VTN) of the midbrain were determined by qPCR. Results showed that bright light and DLIC lighting program temporarily attenuated plasma CORT, suggesting the short-term stress attenuating effect of bright light and DLIC lighting program. Differential TPH2 expression in the DRN and CRN observed in the DLIC birds indicate a significant effect of DLIC lighting program on the serotonergic activity in the avian brainstem. At the 41 days of age, the significant downregulation of TPH2 and TH expression occurred in the VTA of DLIC treated birds compared to the other group of birds. Taken together, temporal and spatial regulation of TPH2 and TH expression by DLIC lighting program indicate that compensatory regulation of serotonergic and dopaminergic activities might be involved in the light intensity preference behavior of birds, suggesting a possible beneficial effect of the DLIC lighting program on broiler welfare.

Intensity dependent disruptive effects of light at night on activation of the HPG axis of tree sparrows (Passer montanus)

Artificial light at night (ALAN) has become increasingly recognized as a disruptor of the reproductive endocrine process and behavior of wild birds. However, there is no evidence that ALAN directly disrupt the hypothalamus-pituitary-gonadal (HPG) axis, and no information on the effects of different ALAN intensities on birds. We experimentally tested whether ALAN affects reproductive endocrine activation in the HPG axis of birds, and whether this effect is related to the intensity of ALAN, in wild tree sparrows (Passer montanus). Forty-eight adult female birds were randomly assigned to four groups. They were first exposed to a short light photoperiod (8 h light and 16 h dark per day) for 20 days, then exposed to a long light photoperiod (16 h light and 8 h dark per day) to initiate the reproductive endocrine process. During these two kinds of photoperiod treatments, the four groups of birds were exposed to 0, 85, 150, and 300 lux light in the dark phase (night) respectively. The expression of the reproductive endocrine activation related TSH-β, Dio2 and GnRH-I gene was significantly higher in birds exposed to 85 lux light at night, and significantly lower in birds exposed to 150 and 300 lux, relative to the 0 lux control. The birds exposed to 85 lux had higher peak values of plasma LH and estradiol concentration and reached the peak earlier than birds exposed to 0, 150, or 300 lux did. The lower gene expression of birds exposed to 150 and 300 lux reduced their peak LH and estradiol values, but did not delay the timing of these peaks compared to the control group. These results reveal that low intensity ALAN accelerates the activation of the reproductive endocrine process in the HPG axis, whereas high intensity ALAN retards it.

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.

Expression of deep brain photoreceptors in the Pekin drake: a possible role in the maintenance of testicular function

Several putative deep brain photoreceptors (DBPs) have been identified, such as melanopsin, opsin 5, and vertebrate ancient opsin. The aim of this study was to elucidate the role of DBPs in gonadal regulation in the Pekin drake. As previously reported, we observed opsin-like immunoreactivity (-ir) in the lateral septum (LS), melanopsin-ir in the premammillary nucleus (PMM), and opsin 5-ir in the periventricular organ. To determine the sensitivity of the DBPs to specific wavelengths of light, drakes were given an acute exposure to red, blue, or white light. Blue light stimulated an increase (P < 0.01) in the immediate early gene fra-2-ir co-expression in melanopsin-ir neurons in the PMM, and red light increased (P < 0.05) fra-2-ir co-expression in opsin-ir neurons, suggesting these neurons are blue- and red-receptive, respectively. To further investigate this photoperiodic response, we exposed drakes to chronic red, long-day white, short-day white, or blue light. Blue light elicited gonadal regression, as testes weight (P < 0.001) and plasma luteinizing hormone (LH) levels (P < 0.001) were lower compared to drakes housed under long-day white light. Photo-regressed drakes experienced complete gonadal recrudescence when housed under long-day red and blue light. qRT-PCR analyses showed that gonadally regressed drakes showed reduced levels (P < 0.01) of gonadotropin releasing hormone (GnRH) mRNA but not photoreceptor or GnIH mRNAs compared to gonadally functional drakes. Our data suggest DBP in the LS may be rhodosin and multiple DBPs are required to fully maintain gonadal function in Pekin drakes.

Seasonal Reproduction in Vertebrates: Melatonin Synthesis, Binding, and Functionality Using Tinbergen's Four Questions

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen's Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

16S rRNA gene sequencing reveals effects of photoperiod on cecal microbiota of broiler roosters

Photoperiod is an important factor in stimulating broiler performance in commercial poultry practice. However, the mechanism by which photoperiod affects the performance of broiler chickens has not been adequately explored. The current study evaluated the effects of three different photoperiod regimes (short day (LD) = 8 h light, control (CTR) = 12.5 h light, and long day (SD) = 16 h light) on the cecal microbiota of broiler roosters by sequencing bacterial 16S rRNA genes. At the phylum level, the dominant bacteria were Firmicutes (CTR: 68%, SD: 69%, LD: 67%) and Bacteroidetes (CTR: 25%, SD: 26%, and LD: 28%). There was a greater abundance of Proteobacteria (p < 0.01) and Cyanobacteria (p < 0.05) in chickens in the LD group than in those in the CTR group. A significantly greater abundance of Actinobacteria was observed in CTR chickens than in SD and LD chickens (p < 0.01). The abundance of Deferribacteres was significantly higher in LD chickens than in SD chickens (p < 0.01). Fusobacteria and Proteobacteria were more abundant in SD chickens than in CTR and LD chickens. The predicted functional properties indicate that cellular processes may be influenced by photoperiod. Conversely, carbohydrate metabolism was enhanced in CTR chickens as compared to that in SD and LD chickens. The current results indicate that different photoperiod regimes may influence the abundance of specific bacterial populations and then contribute to differences in the functional properties of gut microbiota of broiler roosters.

Mapping the brain of the chicken (Gallus gallus), with emphasis on the septal-hypothalamic region

There has been remarkable progress in discoveries made in the avian brain, particularly over the past two decades. This review first highlights some of the discoveries made in the forebrain and credits the Avian Brain Nomenclature Forum, responsible for changing many of the terms found in the cerebrum and for stimulating collaborative research thereafter. The Forum facilitated communication among comparative neurobiologists by eliminating confusing and inaccurate names. The result over the past 15yearshas been a standardized use of avian forebrain terms. Nonetheless, additional changes are needed. The goal of the paper is to encourage a continuing effort to unify the nomenclature throughout the entire avian brain. To emphasize the need for consensus for a single name for each neural structure, I have selected specific structures in the septum and hypothalamus that our laboratory has been investigating, to demonstrate a lack of uniformity in names applied to conservative brain regions compared to the forebrain. The specific areas reviewed include the distributions of gonadotropin-releasing hormone neurons and their terminal fields in circumventricular organs, deep-brain photoreceptors, gonadotropin inhibitory neurons and a complex structure and function of the nucleus of the hippocampal commissure.

Effects of thyroid hormone manipulation on pre-nuptial molt, luteinizing hormone and testicular growth in male white-crowned sparrows (Zonotrichia leuchophrys gambelii)

Most seasonal species rely on the annual change in day length as the primary cue to appropriately time major spring events such as pre-nuptial molt and breeding. Thyroid hormones are thought to be involved in the regulation of both of these spring life history stages. Here we investigated the effects of chemical inhibition of thyroid hormone production using methimazole, subsequently coupled with either triiodothyronine (T3) or thyroxine (T4) replacement, on the photostimulation of pre-nuptial molt and breeding in Gambel's white-crowned sparrows (Zonotrichia leuchophrys gambelii). Suppression of thyroid hormones completely prevented pre-nuptial molt, while both T3 and T4 treatment restored normal patterns of molt in thyroid hormone-suppressed birds. Testicular recrudescence was blocked by methimazole, and restored by T4 but not T3, in contrast to previous findings demonstrating central action of T3 in the photostimulation of breeding. Methimazole and replacement treatments elevated plasma luteinizing hormone levels compared to controls. These data are partially consistent with existing theories on the role of thyroid hormones in the photostimulation of breeding, while highlighting the possibility of additional feedback pathways. Thus we suggest that regulation of the hypothalamic pituitary gonad axis that controls breeding may be more complex than previously considered.

Peropsin modulates transit of vitamin A from retina to retinal pigment epithelium

Peropsin is a non-visual opsin in both vertebrate and invertebrate species. In mammals, peropsin is present in the apical microvilli of retinal pigment epithelial (RPE) cells. These structures interdigitate with the outer segments of rod and cone photoreceptor cells. RPE cells play critical roles in the maintenance of photoreceptors, including the recycling of visual chromophore for the opsin visual pigments. Here, we sought to identify the function of peropsin in the mouse eye. To this end, we generated mice with a null mutation in the peropsin gene (Rrh). These mice exhibited normal retinal histology, normal morphology of outer segments and RPE cells, and no evidence of photoreceptor degeneration. Biochemically, Rrh^-/- mice had ∼2-fold higher vitamin A (all-trans-retinol (all-trans-ROL)) in the neural retina following a photobleach and 5-fold lower retinyl esters in the RPE. This phenotype was similar to those reported in mice that lack interphotoreceptor retinoid-binding protein (IRBP) or cellular retinol-binding protein, suggesting that peropsin plays a role in the movement of all-trans-ROL from photoreceptors to the RPE. We compared the phenotypes in mice lacking both peropsin and IRBP with those of mice lacking peropsin or IRBP alone and found that the retinoid phenotype was similarly severe in each of these knock-out mice. We conclude that peropsin controls all-trans-ROL movement from the retina to the RPE or may regulate all-trans-ROL storage within the RPE. We propose that peropsin affects light-dependent regulation of all-trans-ROL uptake from photoreceptors into RPE cells through an as yet undefined mechanism.

The distribution of melanopsin (OPN4) protein in the human brain

Until now, melanopsin (OPN4) - a specialized photopigment being responsive especially to blue light wavelengths - has not been found in the human brain at protein level outside the retina. More specifically, OPN4 has only been found in about 2% of retinal ganglion cells (i.e. in intrinsically photosensitive retinal ganglion cells), and in a subtype of retinal cone-cells. Given that Allen Institute for Brain Science has described a wide distribution of OPN4 mRNA in two human brains, we aimed to investigate whether OPN4 is present in the human brain also at protein level. Western blotting and immunohistochemistry, as well as immunoelectron microscopy, were used to analyse the existence and distribution of OPN4 protein in 18 investigated areas of the human brain in samples obtained in forensic autopsies from 10 male subjects (54 ± 3.5 years). OPN4 protein expression was found in all subjects, and, furthermore, in 5 out of 10 subjects in all investigated brain areas localized in membranous compartments and cytoplasmic vesicles of neurons. To our opinion, the wide distribution of OPN4 in central areas of the human brain evokes a question whether ambient light has important straight targets in the human brain outside the retinohypothalamic tract (RHT). Further studies are, however, needed to investigate the putative physiological phototransductive actions of inborn OPN4 protein outside the RHT in the human brain.

Sixteen kiwi (Apteryx spp) transcriptomes provide a wealth of genetic markers and insight into sex chromosome evolution in birds

Background

Kiwi represent the most basal extant avian lineage (paleognaths) and exhibit biological attributes that are unusual or extreme among living birds, such as large egg size, strong olfaction, nocturnality, flightlessness and long lifespan. Despite intense interest in their evolution and their threatened status, genomic resources for kiwi were virtually non-existent until the recent publication of a single genome. Here we present the most comprehensive kiwi transcriptomes to date, obtained via Illumina sequencing of whole blood and de novo assembly of mRNA sequences of eight individuals from each of the two rarest kiwi species, little spotted kiwi (LSK; Apteryx owenii) and rowi (A. rowi).

Results

Sequences obtained were orthologous with a wide diversity of functional genes despite the sequencing of a single tissue type. Individual and composite assemblies contain more than 7900 unique protein coding transcripts in each of LSK and rowi that show strong homology with chicken (Gallus gallus), including those associated with growth, development, disease resistance, reproduction and behavior. The assemblies also contain 66,909 SNPs that distinguish between LSK and rowi, 12,384 SNPs among LSK (associated with 3088 genes), and 29,313 SNPs among rowi (associated with 4953 genes). We found 3084 transcripts differentially expressed between LSK and rowi and 150 transcripts differentially expressed between the sexes. Of the latter, 83 could be mapped to chicken chromosomes with 95% syntenic with chromosome Z.

Conclusions

Our study has simultaneously sequenced multiple species, sexes, and individual kiwi at thousands of genes, and thus represents a significant leap forward in genomic resources available for kiwi. The expression pattern we observed among chromosome Z related genes in kiwi is similar to that observed in ostriches and emu, suggesting a common and ancestral pattern of sex chromosome homomorphy, recombination, and gene dosage among living paleognaths. The transcriptome assemblies described here will provide a rich resource for polymorphic marker development and studies of adaptation of these highly unusual and endangered birds.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2714-2) contains supplementary material, which is available to authorized users.