Changes in brain peptides associated with reproduction and energy homeostasis: Putative roles of gonadotrophin-releasing hormone-II and tyrosine hydroxylase in determining reproductive performance in response to daily food availability times in diurnal zebra finches

Neuropeptides and reproductive flexibility in songbirds: A mini review

Synchronization of physiological and behavioral activities associated with avian reproduction requires corresponding changes in the activity of the hypothalamus-pituitary-gonadal axis. This involves complex brain peptidergic pathways, which show spatial and temporal differences in their expression and distribution during the annual reproductive cycle. The well-studied pathways include gonadotropin-releasing and inhibiting hormones (GnRH, GnIH), neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), vasoactive intestinal peptide (VIP) and other peptides like arginine vasotocin (VT), oxytocin (mesotocin), and spexin. Together, these peptides form a neurochemical framework for the integration of both internal and external (environmental) cues; this results in a neuroendocrine response. Conceivably, therefore, the neurochemical framework within which brain peptides possibly interact and perform reproductive regulatory roles might show species differences. Here, we aim to review briefly the roles of these neuropeptides in reproduction in both opportunistically and seasonally breeding birds. Much of the discussion will be based on our own research on the opportunistic breeding zebra finch and the seasonally breeding redheaded bunting, Indian weaverbird, and spotted munia. The summer breeding redheaded bunting and weaverbird are typical photosensitive long-day species, but they show qualitative differences in response to stimulatory photoperiods during the post-reproductive period of their annual cycle. Buntings exhibit absolute photorefractoriness, while weaverbirds exhibit relative photorefractoriness. The autumn breeding spotted munia, on the other hand, is an atypical photosensitive species. It responds to both short and long photoperiods and presumably lacks photorefractoriness.

Integrated transcriptomic hypothalamic-pituitary-ovarian axis network analysis reveals the role of energy availability on egg production in layers

Energy is a crucial component for maintaining egg production in layers. The hypothalamic-pituitary-ovarian (HPO) axis is an energy-sensitive functional axis for follicle development, synthesis, and secretion of reproductive hormones, and plays a key role in modulating sustained ovulation in layers. To investigate the mechanism of integrated network regulation of the HPO axis under energy fluctuation, ninety Hy-line brown layers (265-day-old, 1.92 ± 0.02 kg) were randomly divided into three groups for a 17-day experiment: a control group (Con group) fed ad libitum from days 1 to 17, an energy-deprived group (ED group) that was fed ad libitum from days 1 to 12 and then underwent a fasting period from days 13 to 17 to induce a pause in laying, and a re-fed group (Rf group) that fasted for seven days (specifically, days 1 to 5, day 7, and day 9), had ad libitum access to feed on days 6 and 8, and was continuously fed from days 10 to 17. Each treatment consisted of 10 replicates with 3 birds per replicate. The study found that energy deprivation significantly decreased reproductive performance such as egg laying rate, ovarian index, number of small yellow follicles (SYF), and normal hierarchical follicles (NHIE) (P < 0.05), which recovered after refeeding, indicating the importance of energy availability for sustained ovulation in layers. In addition, estradiol (E2), estradiol to progesterone (E2/P4) ratio, and luteinizing hormone (LH) displayed changes similar to follicle number, whereas follicle-stimulating hormone (FSH) exhibited a contrasting pattern. Transcriptome analysis revealed that energy deprivation downregulated genes related to energy and appetite-regulated neurotransmitter receptors and neuropeptides in the hypothalamus. These signals combined to inhibit gonadotropin-releasing hormone (GnRH) secretion and subsequently downregulated the crucial genes responsible for synthesizing gonadotropins, gonadotropin-releasing hormone receptor (GnRHR), and glycoprotein hormones alpha chain (CGA). Consequently, this suppression of the hypothalamus and pituitary affected ovarian function through ovarian steroidogenesis and the extracellular matrix (ECM)-receptor interaction. These findings suggest that energy deprivation inhibits the function of the HPO axis, leading to impaired follicle development and reduced egg production, and that refeeding can partially restore these indicators.

Physiological effects of food availability times in higher vertebrates

Food availability is a crucial ecological determinant of population size and community structure, and controls various life-history traits in most, if not all, species. Food availability is not constant; there are daily and seasonal differences in food abundance. When coupled to appetite (urge to eat), this is expressed as the eating schedule of a species. Food availability times affect daily and seasonal physiology and behaviour of organisms both directly (by affecting metabolic homeostasis) and indirectly (by altering synchronization of endogenous rhythms). Restricted food availability times may, for example, constrain reproductive output by limiting the number or quality of offspring or the number of reproductive attempts, as has been observed for nesting frequency in birds. Consuming food at the wrong time of day reduces the reproductive ability of a seasonal breeder, and can result in quality-quantity trade-offs of offspring. The food availability pattern serves as a conditioning environment, and can shape the activity of the genome by influencing chromatin activation/silencing; however, the functional linkage of food availability times with epigenetic control of physiology is only beginning to emerge. This Review gives insights into how food availability times, affected by changes in eating schedules and/or by alterations in feeding environment or lifestyle, could have hitherto unknown consequences on the physiology and reproductive fitness of seasonally breeding vertebrates and those that reproduce year round.