Light receptors in the avian brain and seasonal reproduction

Seasonal food intake and energy balance: Neuronal and non-neuronal control mechanisms

Animals inhabiting temperate and high latitudes undergo drastic seasonal changes in energy storage, facilitated by changes in food intake and body mass. Those seasonal changes in the animal's biology are not mere consequences of environmental energy availability but are anticipatory responses to the energetic requirements of the upcoming season and are actively timed by tracking the annual progression in photoperiod. In this review, we discuss how photoperiod is used to control energy balance seasonally and how this is distinct from energy homeostasis. Most notably, we suggest that photoperiodic control of food intake and body mass does not originate from the arcuate nucleus, as for homeostatic appetite control, but is rather to be found in hypothalamic tanycytes. Tanycytes are specialized ependymal cells lining the third ventricle, which can sense metabolites from the cerebrospinal fluid (e.g. glucose) and can control access of circulating signals to the brain. They are also essential in conveying time-of-year information by integrating photoperiod and altering hypothalamic thyroid metabolism, a feature that is conserved in seasonal vertebrates and connects to seasonal breeding and metabolism. We also discuss how homeostatic feedback signals are handled during times of rapid energetic transitions. Studies on leptin in seasonal mammals suggest a seasonal shift in central sensitivity and blood-brain transport, which might be facilitated by tanycytes.

Neural thyroid hormone metabolism integrates seasonal changes in environmental temperature with the neuroendocrine reproductive axis

We asked if environmental temperature alters thyroid hormone metabolism within the hypothalamus, thereby providing a neuroendocrine mechanism by which temperature could be integrated with photoperiod to regulate seasonal rhythms. We used immunohistochemistry to assess the effects of low-temperature winter dormancy at 4 °C or 12 °C on thyroid-stimulating hormone (TSH) within the infundibulum of the pituitary as well as deiodinase 2 (Dio2) and 3 (Dio3) within the hypothalamus of red-sided garter snakes (Thamnophis sirtalis). Both the duration and, in males, magnitude of low-temperature dormancy altered deiodinase immunoreactivity within the hypothalamus, increasing the area of Dio2-immunoreactivity in males and females and decreasing the number of Dio3-immunoreactive cells in males after 8-16 weeks. Reciprocal changes in Dio2/3 favor the accumulation of triiodothyronine within the hypothalamus. Whether TSH mediates these effects requires further study, as significant changes in TSH-immunoreactive cell number were not observed. Temporal changes in deiodinase immunoreactivity coincided with an increase in the proportion of males exhibiting courtship behavior as well as changes in the temporal pattern of courtship behavior after emergence. Our findings mirror those of previous studies, in which males require low-temperature exposure for at least 8 weeks before significant changes in gonadotropin-releasing hormone immunoreactivity and sex steroid hormones are observed. Collectively, these data provide evidence that the neuroendocrine pathway regulating the reproductive axis via thyroid hormone metabolism is capable of transducing temperature information. Because all vertebrates can potentially use temperature as a supplementary cue, these results are broadly applicable to understanding how environment-organism interactions mediate seasonally adaptive responses.

An Expanding Role for Nonvisual Opsins in Extraocular Light Sensing Physiology

We live on a planet that is bathed in daily and seasonal sunlight cycles. In this context, terrestrial life forms have evolved mechanisms that directly harness light energy (plants) or decode light information for adaptive advantage. In animals, the main light sensors are a family of G protein-coupled receptors called opsins. Opsin function is best described for the visual sense. However, most animals also use opsins for extraocular light sensing for seasonal behavior and camouflage. While it has long been believed that mammals do not have an extraocular light sensing capacity, recent evidence suggests otherwise. Notably, encephalopsin (OPN3) and neuropsin (OPN5) are both known to mediate extraocular light sensing in mice. Examples of this mediation include photoentrainment of circadian clocks in skin (by OPN5) and acute light-dependent regulation of metabolic pathways (by OPN3 and OPN5). This review summarizes current findings in the expanding field of extraocular photoreception and their relevance for human physiology.

Endocrine effects of exposure to artificial light at night: A review and synthesis of knowledge gaps

Animals have evolved with natural patterns of light and darkness, such that light serves as an important zeitgeber, allowing adaptive synchronization of behavior and physiology to external conditions. Exposure to artificial light at night (ALAN) interferes with this process, resulting in dysregulation of endocrine systems. In this review, we evaluate the endocrine effects of ALAN exposure in birds and reptiles, identify major knowledge gaps, and highlight areas for future research. There is strong evidence for ecologically relevant levels of ALAN acting as an environmental endocrine disruptor. However, most studies focus on the pineal hormone melatonin, corticosterone release via the hypothalamus-pituitary-adrenal axis, or regulation of reproductive hormones via the hypothalamus-pituitary-gonadal axis, leaving effects on other endocrine systems largely unknown. We call for more research spanning a diversity of hormonal systems and levels of endocrine regulation (e.g. circulating hormone levels, receptor numbers, strength of negative feedback), and investigating involvement of molecular mechanisms, such as clock genes, in hormonal responses. In addition, longer-term studies are needed to elucidate potentially distinct effects arising from chronic exposure. Other important areas for future research effort include investigating intraspecific and interspecific variability in sensitivity to light exposure, further distinguishing between distinct effects of different types of light sources, and assessing impacts of ALAN exposure early in life, when endocrine systems remain sensitive to developmental programming. The effects of ALAN on endocrine systems are likely to have a plethora of downstream effects, with implications for individual fitness, population persistence, and community dynamics, especially within urban and suburban environments.