The effect of food restriction on the regulation of gonadotropin-releasing hormone in male house finches (Haemorhous mexicanus)

Adjustments to energy provisioning and oxidative balance in response to temperature in a wild passerine

Climate change and urbanization are associated with elevated ambient temperature (Ta). This increase may negatively impact organisms by creating conditions that are outside their resilience limits, but the physiological mechanisms that limit phenotypic plasticity in response to Ta variation remain poorly understood. We investigated these mechanisms in captive House Finches, Haemorhous mexicanus, a common native resident of rural and urban environments. We exposed finches to temperatures either slightly below the species' lower critical temperature (constant 20 °C; COOL group) or close to its upper critical temperature (daily min. 27 °C, daily max. 35 °C; WARM group) for two weeks. Birds in the COOL group ate more than birds in the WARM group, which is consistent with the prediction that cool Ta exposure increased the metabolic rate. However, finches of the two groups did not differ with regard to their body masses, fat reserves, or blood concentrations of ketone bodies, uric acid, and erythrocytic peroxidized lipids. Thus, exposure to the two experimental treatments did not result in major metabolic differences between groups. Acute stress caused by handling and restraint for 30 min decreased plasma uric acid, which may have been associated with its utilization as a free radical scavenger and so may have decreased stress-associated oxidative damage. Acute stress also increased plasma ketone bodies, suggesting increased lipid oxidation. These stress-related metabolic changes did not differ in the COOL and WARM groups, indicating within the range of Ta to which birds were exposed that temperature did not affect the birds' physiological sensitivity to acute stress.

Adiponectin inhibits GnRH secretion via activating AMPK and PI3K signaling pathways in chicken hypothalamic neuron cells

It has been reported that adiponectin (AdipoQ), an adipokine secreted by white adipose tissue, plays an important role in the control of animal reproduction in addition to its function in energy homeostasis by binding to its receptors AdipoR1/2. However, the molecular mechanisms of AdipoQ in the regulation of animal reproduction remain elusive. In this study, we investigated the effects of AdipoQ on hypothalamic reproductive hormone (GnRH) secretion and reproduction-related receptor gene (estrogen receptor [ER] and progesterone receptor [PR]) expression in hypothalamic neuronal cells (HNCs) of chickens by using real-time fluorescent quantitative PCR (RT-qPCR), enzyme-linked immunosorbent assay (ELISA), Western blot (WB) and cell counting kit-8 (CCK-8) assays and found that overexpression of AdipoQ could increase the expression levels of AdipoR1/2 and reproduction-related receptor genes (P < 0.05) while decreasing the expression level of GnRH. In contrast, interference with AdipoQ mRNA showed the opposite results in HNCs. Furthermore, we demonstrated that AdipoQ exerts its functions through the AMPK and PI3K signaling pathways. Finally, our in vitro experiments found that AdipoRon (a synthetic substitute for AdipoQ) treatment and AdipoR1/2 RNAi interference co-treatment resulted in no effect on GnRH secretion, suggesting that the inhibition of GnRH secretion by AdipoQ is mediated by the AdipoR1/2 signaling axis. In summary, we uncovered, for the first time, the molecular mechanism of AdipoQ in the regulation of reproductive hormone secretion in hypothalamic neurons in chickens.

Past and future: Urbanization and the avian endocrine system

Urban environments are evolutionarily novel and differ from natural environments in many respects including food and/or water availability, predation, noise, light, air quality, pathogens, biodiversity, and temperature. The success of organisms in urban environments requires physiological plasticity and adjustments that have been described extensively, including in birds residing in geographically and climatically diverse regions. These studies have revealed a few relatively consistent differences between urban and non-urban conspecifics. For example, seasonally breeding urban birds often develop their reproductive system earlier than non-urban birds, perhaps in response to more abundant trophic resources. In most instances, however, analyses of existing data indicate no general pattern distinguishing urban and non-urban birds. It is, for instance, often hypothesized that urban environments are stressful, yet the activity of the hypothalamus-pituitary-adrenal axis does not differ consistently between urban and non-urban birds. A similar conclusion is reached by comparing blood indices of metabolism. The origin of these disparities remains poorly understood, partly because many studies are correlative rather than aiming at establishing causality, which effectively limits our ability to formulate specific hypotheses regarding the impacts of urbanization on wildlife. We suggest that future research will benefit from prioritizing mechanistic approaches to identify environmental factors that shape the phenotypic responses of organisms to urbanization and the neuroendocrine and metabolic bases of these responses. Further, it will be critical to elucidate whether factors affect these responses (a) cumulatively or synergistically; and (b) differentially as a function of age, sex, reproductive status, season, and mobility within the urban environment. Research to date has used various taxa that differ greatly not only phylogenetically, but also with regard to ecological requirements, social systems, propensity to consume anthropogenic food, and behavioral responses to human presence. Researchers may instead benefit from standardizing approaches to examine a small number of representative models with wide geographic distribution and that occupy diverse urban ecosystems.

Effects of timed food availability on reproduction and metabolism in zebra finches: Molecular insights into homeostatic adaptation to food-restriction in diurnal vertebrates

Food availability affects metabolism and reproduction in higher vertebrates including birds. This study tested the idea of adaptive homeostasis to time-restricted feeding (TRF) in diurnal zebra finches by using multiple (behavioral, physiological and molecular) assays. Adult birds were subjected for 1 week or 3 weeks to food restriction for 4 h in the evening (hour 8-12) of the 12 h light-on period, with controls on ad lib feeding. Birds on TRF showed enhanced exploratory behavior and plasma triglycerides levels, but did not show differences from ad lib birds in the overall food intake, body mass, and plasma corticosterone and thyroxine levels. As compared to ad lib feeding, testis size and circulation testosterone were reduced after first but not after third week of TRF. The concomitant change in the mRNA expression of metabolic and reproductive genes was also found after week 1 of TRF. Particularly, TRF birds showed increased expression of genes coding for gonadotropin releasing hormone (GnRH) in hypothalamus, and for receptors of androgen (AR) and estrogen (ER-alpha) in both hypothalamus and testes. However, genes coding for the deiodinases (Dio2, Dio3) and gonadotropin inhibiting hormone (GnIH) showed no difference between feeding conditions in both hypothalamus and testes. Further, increased Sirt1, Fgf10 and Ppar-alpha, and decreased Egr1 expression in the liver suggested TRF-effects on the overall metabolism. Importantly, TRF-effects on gene expressions by week 1 seemed alleviated to a considerable extent by week 3. These results on TRF-induced reproductive and metabolic effects suggest homeostatic adaptation to food-restriction in diurnal vertebrates.

Disruption of energy homeostasis by food restriction or high ambient temperature exposure affects gonadal function in male house finches (Haemorhous mexicanus)

Reproductive success requires that individuals acquire sufficient energy resources. Restricting food availability or increasing energy expenditure (e.g., thermoregulation) inhibits reproductive development in multiple avian species, but the nature of the energy-related signal mediating this effect is unclear. To investigate this question, we examined reproductive and metabolic physiology in male house finches that either underwent moderate food restriction (FR) or were exposed to high temperature (HT), in which birds were held at a high, but not locally atypical, ambient temperature cycle (37.8 °C day, 29.4 °C night) compared to a control group (CT; 29.4 °C day, 21.1 °C night). We hypothesized that FR and HT inhibit reproductive development by lowering available metabolic fuel, in particular plasma glucose (GLU) and free fatty acids (FFA). Following FR for 4 weeks, finches lost body mass, had marginally higher plasma FFA, and experienced a 90% reduction in testis mass compared to CT birds. Four weeks of HT exposure resulted in reduced voluntary food consumption and muscle mass, as well as an 80% reduction in testis mass relative to CT birds. Both FR and HT birds expressed less testicular 17β-hydroxysteroid dehydrogenase (17β-HSD) mRNA than controls but the expression of other testicular genes measured was unaffected by either treatment. Neither treatment significantly influenced plasma GLU. This study is among the first to demonstrate a negative effect of HT on reproductive development in a wild bird. Further studies are needed to clarify the role of metabolic mediators and their involvement under various conditions of energy availability and demand.