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.

Role of Melatonin in Temperature-Induced Activation of the Neuroendocrine Reproductive Axis in Garter Snakes.

An animal's ability to respond optimally to changing environmental conditions is paramount to successfully reproducing and thus maximizing fitness. Studies on photoperiod-induced changes in neural thyroid hormone metabolism have conclusively linked environmental cues to the neuroendocrine reproductive axis of birds and mammals. Whether this conserved mechanism also transduces changes in environmental temperature, however, has not been fully addressed.

Trans-seasonal activation of the neuroendocrine reproductive axis: Low-temperature winter dormancy modulates gonadotropin-releasing hormone neurons in garter snakes.

All animals use external cues from the environment to accurately time life-history events. How the brain decodes environmental stimuli to effect changes in physiology and behavior, however, is poorly understood, particularly with regard to supplementary environmental cues such as temperature. We asked if low-temperature dormancy alters the synthesis and/or release of gonadotropin-releasing hormone (GnRH).

Exogenous leptin promotes reproductive behavior during aphagia in red-sided garter snakes (Thamnophis sirtalis parietalis).

Despite the established dichotomy between investment in either reproduction or self-maintenance, a hormonal mechanism that influences an organism's decision to prioritize these behaviors remains elusive. The protein hormone leptin is a likely candidate because it is secreted from adipocytes in proportion to the amount of stored fat in numerous species. Although the majority of studies suggest that leptin stimulates reproduction, the actions of leptin can be context-dependent.

Energy Metrics of Red-Sided Garter Snakes () Vary with Sex but Not Life-History Stage.

Because reproduction is energetically expensive, an organism's energy stores are likely involved in mediating transitions between reproductive and self-maintenance activities. We investigated whether body condition index, adipocyte follicle size, and liver glycogen differ with the life-history transition from reproduction to migration and foraging in red-sided garter snakes (). Females primarily investing in mating behavior located at the den had a significantly higher body condition index than females migrating to summer feeding grounds.

Sexually Dimorphic Patterns of Cell Proliferation in the Brain Are Linked to Seasonal Life-History Transitions in Red-Sided Garter Snakes.

Seasonal rhythms in physiology and behavior are widespread across diverse taxonomic groups and may be mediated by seasonal changes in neurogenesis, including cell proliferation, migration, and differentiation. We examined if cell proliferation in the brain is associated with the seasonal life-history transition from spring breeding to migration and summer foraging in a free-ranging population of red-sided garter snakes () in Manitoba, Canada. We used the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) to label newly proliferated cells within the brain of adult snakes collected from the den during the mating season or from a road located along their migratory route.

Physiological correlates of reproductive decisions: Relationships among body condition, reproductive status, and the hypothalamus-pituitary-adrenal axis in a reptile.

When opportunities to feed and reproduce are limited, females are often unable to recover sufficient energy stores to reproduce in consecutive years. Body condition has been used as a proxy for recent reproductive history in such species. We previously found that glucocorticoid responses to capture stress vary with body condition in female red-sided garter snakes (Thamnophis sirtalis parietalis), a species with limited seasonal breeding opportunities.

Arginine Vasotocin and Neuropeptide Y Vary with Seasonal Life-History Transitions in Garter Snakes.

Transitions between life-history stages are often accompanied by dramatic behavioral switches that result from a shift in motivation to pursue one resource over another. While the neuroendocrine mechanisms that regulate such behavioral transitions are poorly understood, arginine vasotocin (AVT) and neuropeptide Y (NPY) are excellent candidates because they modulate reproductive and feeding behavior, respectively. We asked if seasonal changes in AVT and NPY are concomitant with the seasonal migration to and from the feeding grounds in red-sided garter snakes (Thamnophis sirtalis parietalis).

Patterns of stress responses shift during seasonal life-history transitions: An analysis comparing baseline, maximal and integrated corticosterone in female red-sided garter snakes (Thamnophis sirtalis parietalis).

Glucocorticoids often rise and fall with a variety of external and internal cues and frequently vary among life-history stages. This suggests that changing glucocorticoids may coordinate life-history transitions. To explore this hypothesis, we asked if the time-course of stress-induced glucocorticoid levels differ between two life-history transitions (i.

Biological Rhythms: Melatonin Shapes the Space-Time Continuum of Social Communication.

A new study shows that nocturnal courtship vocalization is regulated by a circadian rhythm and potentiated by melatonin at multiple timescales. These findings are in contrast to those in diurnal vocalizers and provide a striking example of melatonin's niche-specific functions.

Seasonal and sex differences in responsiveness to adrenocorticotropic hormone contribute to stress response plasticity in red-sided garter snakes (Thamnophis sirtalis parietalis).

As in many vertebrates, hormonal responses to stress vary seasonally in red-sided garter snakes (Thamnophis sirtalis parietalis). For example, males generally exhibit reduced glucocorticoid responses to a standard stressor during the spring mating season. We asked whether variation in adrenal sensitivity to adrenocorticotropic hormone (ACTH) explains why glucocorticoid responses to capture stress vary with sex, season and body condition in red-sided garter snakes.

Seasonal variation in cell proliferation and cell migration in the brain of adult red-sided garter snakes (Thamnophis sirtalis parietalis).

Plasticity in the adult central nervous system has been described in all vertebrate classes as well as in some invertebrate groups. However, the limited taxonomic diversity represented in the current neurogenesis literature limits our ability to assess the functional significance of adult neurogenesis for natural behaviors as well as the evolution of its regulatory mechanisms. In the present study, we used free-ranging red-sided garter snakes (Thamnophis sirtalis parietalis) to test the hypothesis that seasonal shifts in physiology and behavior are associated with seasonal variation in postembryonic neurogenesis.

Sex or candy? Neuroendocrine regulation of the seasonal transition from courtship to feeding behavior in male red-sided garter snakes (Thamnophis sirtalis parietalis).

This article is part of a Special Issue "Energy Balance". Seasonal modulation of glucocorticoids plays an important role in supporting critical life-history events, and probably facilitates transitions between different life-history stages. In a well-studied population of red-sided garter snakes (Thamnophis sirtalis parietalis), glucocorticoids are elevated during the mating season, but males dispersing to summer feeding grounds have significantly lower baseline glucocorticoids than courting males at the den.

Responses to capture stress and exogenous corticosterone vary with body condition in female red-sided garter snakes (Thamnophis sirtalis parietalis).

This study examined whether hormonal and behavioral responses to capture stress and exogenous corticosterone (CORT) vary with body condition in female red-sided garter snakes (Thamnophis sirtalis parietalis). Female snakes were collected during the spring mating season and treated with 4 h of capture stress. We measured plasma CORT and estradiol before, during and after capture stress treatment followed by latency to copulate, a measure of female receptivity.

Sexually dimorphic effects of melatonin on brain arginine vasotocin immunoreactivity in green treefrogs (Hyla cinerea).

Arginine vasotocin (AVT) and its mammalian homologue, arginine vasopressin (AVP), regulate a variety of social and reproductive behaviors, often with complex species-, sex- and context-dependent effects. Despite extensive evidence documenting seasonal variation in brain AVT/AVP, relatively few studies have investigated the environmental and/or hormonal factors mediating these seasonal changes. In the present study, we investigated whether the pineal hormone melatonin alters brain AVT immunoreactivity in green treefrogs (Hyla cinerea).

Pheromonal mediation of intraseasonal declines in the attractivity of female red-sided garter snakes, Thamnophis sirtalis parietalis.

During the breeding season, female red-sided garter snakes (Thamnophis sirtalis parietalis) produce and express a sexual attractiveness pheromone that elicits male courtship behavior. Composed of a homologous series of saturated and monounsaturated methyl ketones, this pheromone is expressed in female skin lipids. Recent studies have shown that the sexual attractivity of unmated female garter snakes declines as the breeding season progresses.

Chronobiology of reproduction in garter snakes: neuroendocrine mechanisms and geographic variation.

The majority of studies on reproductive neuroendocrinology in snakes have focused on one particular snake population in Manitoba, Canada, the red-sided garter snake (Thamnophis sirtalis parietalis). Although traditionally these studies have emphasized its unusual temporal dissociation between mating behavior and peak gonadal activity, current evidence suggests that reproductive regulation in this population may be more similar to the norm than previously thought. Like other ectotherms, temperature plays a critical role in activating reproductive behavior in red-sided garter snakes.

Brain nuclei in actively courting red-sided garter snakes: a paradigm of neural trimorphism.

During the breeding season, two distinct male phenotypes are exhibited by red-sided garter snakes (Thamnophis sirtalis parietalis), with courtship behavior being directed not only toward females, but also toward a sub-population of males called she-males. She-males are morphologically identical to other males except for a circulating androgen level three times that of normal males and their ability to produce a female-like pheromone. As in other vertebrates, limbic nuclei in the red-sided garter snake brain are involved in the control of sexual behaviors.

Temporally distinct effects of stress and corticosterone on diel melatonin rhythms of red-sided garter snakes (Thamnophis sirtalis).

Circadian and circannual rhythms in physiology and behavior are temporally organized via hormonal signals that reflect changing environmental cues. Interactions between endocrine signals are in turn important for integrating multiple physiological and behavioral rhythms. In the present study, we examined interactions between melatonin, the hypothalamus-pituitary-adrenal (HPA) axis, and corticosterone in a well-studied population of red-sided garter snakes (Thamnophis sirtalis parietalis).

Seasonal aromatase activity in the brain of the male red-sided garter snake.

We investigated regional and seasonal variations in neural aromatase activity (AA), the enzyme that converts androgens into estrogens, to examine a possible indirect role of testosterone (T) in mediating spring reproductive behavior of red-sided garter snakes, a species exhibiting a dissociated reproductive pattern. Neural AA in male snakes varied significantly among brain regions. Additionally, there were significant interactions between brain region and season.

Endocrine mechanisms mediating temperature-induced reproductive behavior in red-sided garter snakes (Thamnophis sirtalis parietalis).

We investigated the mechanisms by which temperature induces seasonal reproductive behavior in red-sided garter snakes (Thamnophis sirtalis parietalis). Specifically, we addressed whether elevated temperatures during winter dormancy influence (1) diel melatonin and corticosterone rhythms; (2) sex steroid hormone and corticosterone profiles; and (3) the expression of reproductive behavior following emergence. Elevated hibernation temperatures (i.

Kisspeptin-like immunoreactive neuron distribution in the green anole (Anolis carolinensis).

Kisspeptins are a recently identified class of neuropeptides belonging to the RFamide peptide family. Despite growing evidence supporting kisspeptin as a key regulator of reproduction, data addressing whether kisspeptin is a conserved reproductive signal are lacking. We investigated the distribution of kisspeptin in adult green anole lizards (Anolis carolinensis) via immunohistochemistry.

Seasonal variation in hormonal responses of timber rattlesnakes (Crotalus horridus) to reproductive and environmental stressors.

Data addressing adrenocortical modulation across taxonomic groups are limited, especially with regard to how female reproductive condition influences the sensitivity of the hypothalamus-pituitary-adrenal axis. We investigated seasonal and reproductive variation in basal and stress-induced hormone profiles in a population of free-ranging timber rattlesnakes (Crotalus horridus) in north-central Pennsylvania during spring (i.e.

Geographic variation in timekeeping systems among three populations of garter snakes (Thamnophis sirtalis) in a common garden.

Transduction of environmental cues into endocrine signals that synchronize physiology and behavior with optimal environmental conditions is central to an animal's timekeeping system. Using a common garden approach, we investigated possible geographic variation in timekeeping systems by comparing 24-h melatonin and corticosterone rhythms and reproductive behavior among three populations of garter snakes with very different life histories: red-sided garter snakes (Thamnophis sirtalis parietalis) from Manitoba, Canada; red-spotted garter snakes (Thamnophis sirtalis concinnus) from western Oregon; and eastern garter snakes (Thamnophis sirtalis sirtalis) from southern Florida. Melatonin and corticosterone cycles differed significantly among the three snake populations in a majority of the sampling periods.

Corticosterone and the transition from courtship behavior to dispersal in male red-sided garter snakes (Thamnophis sirtalis parietalis).

Seasonal modulation of baseline glucocorticoid concentrations as well as the sensitivity of the hypothalamic-pituitary-adrenal (HPA) axis plays an important role in supporting critical life-history events such as seasonal reproduction and migration. Despite numerous studies on adrenocortical modulation, little is known about the exact timing of this seasonal modulation with respect to critical life-history stages. We tested the hypothesis that seasonal modulation of the HPA axis during the spring mating season in male red-sided garter snakes (Thamnophis sirtalis parietalis) is temporally linked to the mechanisms regulating dispersal.