FSHβ links photoperiodic signaling to seasonal reproduction in Japanese quail

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.

Time- and season-dependent changes in the steroidogenic markers in female tree sparrow (Passer montanus)

Seasonal breeders display elevated sex steroid hormone production during reproductive seasons, resulting in significant physiological and structural alterations. One such seasonal breeder adapted to the changing environment is a Tree sparrow (Passer montanus). The study aims to investigate 24-h rhythmicity and annual variations in the expression of steroidogenic gene markers of adult female tree sparrows. Two experiments were conducted; in experiment one, birds (n = 5 birds/time points) were sampled at six time points, i.e., ZT1, ZT5, ZT9, ZT13, ZT17, and ZT21 (ZT = Zeitgeber time, ZT0 = sunrise time) during the reproductive stage; subsequently, hypothalamus and ovary were harvested for gene expression analysis. In experiment two, birds (n = 5/month) were sampled at mid-day every month for a year. Feather molt, follicular diameter, body mass, and bill coloration were recorded. The hypothalamus and ovary were harvested for gene expression studies. Blood plasma cholesterol and progesterone were also measured. The study indicates a larger follicular size during May and June. Whereas, maximum molt was observed during the post-reproductive phase. Cholesterol levels were highest prior breeding phase and higher progesterone levels paralleled larger follicular size. While higher levels of GnIh (gonadotropin-inhibitory hormone) and Dio3 (type 3 deiodinase) were observed during the non-breeding phase, elevated expression of Tshβ (thyroid stimulating hormone subunit beta), Dio2 (type 2 deiodinase), and GnRh (gonadotropin-releasing hormone) was noted during the reproductive period. The study also reveals 24-h rhythmicity in selected steroidogenic markers (StAR, Nr4a1, Er, Scp2, Cyp17a1, Foxl2, Cyp11a1, Hsd11b2, Cyp11b, Cyp19a1, and Vdac1) and seasonal variations directly influence steroidogenesis, which connects with the annual reproductive cycle.

Hypotheses in light detection by vertebrate ancient opsin in the bird brain

Extra-retinal photoreception is common across fish and avian species. In birds, the hypothalamus contains non-visual photoreceptors that detect light and regulate multiple endocrine systems. To date, light-dependent control of seasonal reproduction is one of the most well-studied systems that require deep brain photoreception. However, the precise photoreceptor(s) that detect light and the neuroendocrine connection between opsin-expressing cells and the gonadotropin-releasing hormone-1 (GnRH1) system remain poorly defined. In the past couple of decades, two opsin molecules have been proposed to link light detection with seasonal reproduction in birds: neuropsin (Opn5) and vertebrate ancient opsin (VA opsin). Only VA opsin is expressed in GnRH1 cells and has an absorption spectrum that matches the action spectrum of the avian photoperiodic reproductive response. This perspective describes how the annual change in daylength, referred to as photoperiod, regulates the neuroendocrine control of seasonal reproduction. The opsin genes are then outlined, and the cellular phototransduction cascade is described, highlighting the common feature of hyperpolarization in response to light stimulation. We then discuss the latest evidence using short-hairpin RNA to temporarily knock down VA opsin and Opn5 on transcripts involved in the neuroendocrine regulation of reproduction. Based on emerging data, we outline three theoretical scenarios in which VA opsin might regulate GnRH1 synthesis and release in birds. The models proposed provide a series of testable hypotheses that can be used to improve our understanding of avian light detection by VA opsin or other opsin-expressing cells in the brain.

Molecular basis of photoinduced seasonal energy rheostasis in Japanese quail (Coturnix japonica)

Seasonal rhythms in photoperiod are a predictive cue used by many temperate-zone animals to time cycles of lipid accumulation. The neuroendocrine regulation of seasonal energy homeostasis and rheostasis are widely studied. However, the molecular pathways underlying tissue-specific adaptations remain poorly described. We conducted two experiments to examine long-term rheostatic changes in energy stability using the well-characterized photoperiodic response of the Japanese quail. In experiment 1, we exposed quails to photoperiodic transitions simulating the annual photic cycle and examined the morphology and fat deposition in liver, muscle, and adipose tissue. To identify changes in gene expression and molecular pathways during the vernal transition in lipid accumulation, we conducted transcriptomic analyses of adipose and liver tissues. Experiment 2 assessed whether the changes observed in Experiment 1 reflected constitutive levels or were due to time-of-day sampling. We identified increased expression of transcripts involved in adipocyte growth, such as Cysteine Rich Angiogenic Inducer 61 and Very Low-Density Lipoprotein Receptor, and in obesity-linked disease resistance, such as Insulin-Like Growth Factor Binding Protein 2 and Apolipoprotein D, in anticipation of body mass gain. Under long photoperiods, hepatic transcripts involved in fatty acid (FA) synthesis (FA Synthase, FA Desaturase 2) were down-regulated. Parallel upregulation of hepatic FA Translocase and Pyruvate Dehydrogenase Kinase 4 expression suggests increased FA uptake and inhibition of the pyruvate dehydrogenase complex. Our findings demonstrate tissue-specific biochemical and molecular changes that drive photoperiod-induced adipogenesis. These findings can be used to determine conserved pathways that enable animals to accumulate fat without developing metabolic diseases.

Tanycytes from a bird's eye view: gene expression profiling of the tanycytic region under different seasonal states in the Svalbard ptarmigan

In mammals and birds, tanycytes are known to regulate thyroid hormone conversion, and this process is central to the control of seasonal reproduction. In mammals, this cell type is also implicated in retinoic acid signalling, neurogenesis, and nutritional gatekeeping, all of which have been linked to hypothalamic regulation of energy metabolism. Less is known about these potential wider roles of tanycytes in birds. To address this gap, we combined LASER capture microdissection and transcriptomics to profile the tanycytic region in male Svalbard ptarmigan, a High Arctic species with photoperiod-dependent seasonal rhythms in reproductive activation and body mass. Short photoperiod (SP) adapted birds were transferred to constant light (LL) to trigger breeding and body mass loss. After five months under LL, the development of photorefractoriness led to spontaneous re-emergence of the winter phenotype, marked by the termination of breeding and gain in body mass. The transfer from SP to LL initiated gene expression changes in both thyroid hormone and retinoic acid pathways, as described in seasonal mammals. Furthermore, transcriptomic signatures of cell differentiation and migration were observed. Comparison to data from Siberian hamsters demonstrated that a photoperiod-dependent re-organisation of the hypothalamic tanycytic region is likely a conserved feature. Conversely, the spontaneous development of photorefractoriness showed a surprisingly small number of genes that reverted in expression level, despite reversal of the reproductive and metabolic phenotype. Our data suggest general conservation of tanycyte biology between photoperiodic birds and mammals and raise questions about the mechanistic origins of the photorefractory state.

Dissociating Mechanisms That Underlie Seasonal and Developmental Programs for the Neuroendocrine Control of Physiology in Birds

Long-term programmed rheostatic changes in physiology are essential for animal fitness. Hypothalamic nuclei and the pituitary gland govern key developmental and seasonal transitions in reproduction. The aim of this study was to identify the molecular substrates that are common and unique to developmental and seasonal timing. Adult and juvenile quail were collected from reproductively mature and immature states, and key molecular targets were examined in the mediobasal hypothalamus (MBH) and pituitary gland. qRT-PCR assays established deiodinase type 2 (DIO2) and type 3 (DIO3) expression in adults changed with photoperiod manipulations. However, DIO2 and DIO3 remain constitutively expressed in juveniles. Pituitary gland transcriptome analyses established that 340 transcripts were differentially expressed across seasonal photoperiod programs and 1,189 transcripts displayed age-dependent variation in expression. Prolactin (PRL) and follicle-stimulating hormone subunit beta (FSHβ) are molecular markers of seasonal programs and are significantly upregulated in long photoperiod conditions. Growth hormone expression was significantly upregulated in juvenile quail, regardless of photoperiodic condition. These findings indicate that a level of cell autonomy in the pituitary gland governs seasonal and developmental programs in physiology. Overall, this paper yields novel insights into the molecular mechanisms that govern developmental programs and adult brain plasticity.