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

Neuroendocrinology of Reproduction and Social Behaviors in Reptiles: Advances Made in the Last Decade

Among amniotes, reptiles are ectothermic and are clearly distinguished from mammals and birds. Reptiles show great diversity not only in species numbers, but also in ecological and physiological features. Although their physiological diversity is an interesting research topic, less effort has been made compared to that for mammals and birds, in part due to lack of established experimental models and techniques. However, progress, especially in the field of neuroendocrinology, has been steadily made. With this process, basic data on selected reptilian species have been collected. This review article presents the progress made in the last decade, which includes 1) behavioral regulation by sex steroid hormones, 2) regulation of seasonal reproduction by melatonin and GnRH, and 3) regulation of social interaction by arginine vasotocin. Through these research topics, we provide insights into the physiology of reptiles and the latest findings in the field of amniote neuroendocrinology.

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. We investigated whether the hormone melatonin mediates the effects of low temperature winter dormancy on thyroid hormone metabolism within the hypothalamus of red-sided garter snakes (Thamnophis sirtalis parietalis). To address this question, we used immunohistochemistry to assess changes in thyroid stimulating hormone (TSH) in the infundibulum of the pituitary and deiodinase 3 (Dio3) and gonadotropin-releasing hormone (GnRH) in the hypothalamus. We also asked if changes in TSH, Dio3, and/or GnRH immunoreactivity are associated with changes in male courtship behavior. In contrast to our predictions, 6 weeks of low-temperature dormancy at 4°C significantly decreased TSH-labeled cell number in the infundibulum; it is possible that an initial decrease in TSH is related to the release of snakes from temperature refractoriness. Treatment of snakes with the melatonin precursor 5-hydroxytryptophan during dormancy at 4°C both reversed the temperature-induced change in TSH-immunoreactivity and disrupted the temporal pattern of male courtship behavior. These results suggest that TSH cells within the infundibulum are both modulated by temperature and sensitive to changes in melatonin. As predicted, male snakes hibernated at an elevated temperature of 12°C for 6 weeks and treated with vehicle showed no change in TSH-, Dio3-, or GnRH-immunoreactive cell number. Treatment of snakes with the melatonin receptor antagonist luzindole was not sufficient in rescuing the effects of dormancy at 12°C on TSH immunoreactivity or courtship behavior. However, luzindole-treated snakes showed a significant increase in GnRH- immunoreactive cell number, suggesting that melatonin exerts an inhibitory effect on GnRH in garter snakes. In summary, our results provide critical insights into the mechanisms that mediate the effects of temperature on reproductive physiology and behavior.

Role of melatonin in murine "restraint stress"-induced dysfunction of colonic microbiota

Intestinal diseases caused by physiological stress have become a severe public health threat worldwide. Disturbances in the gut microbiota-host relationship have been associated with irritable bowel disease (IBD), while melatonin (MT) has anti-inflammatory and antioxidant effects. The objective of this study was to investigate the mechanisms by which MT-mediated protection mitigated stress-induced intestinal microbiota dysbiosis and inflammation. We successfully established a murine restraint stress model with and without MT supplementation. Mice subjected to restraint stress had significantly elevated corticosterone (CORT) levels, decreased MT levels in their plasma, elevated colonic ROS levels and increased bacterial abundance, including Bacteroides and Tyzzerella, in their colon tract, which led to elevated expression of Toll-like receptor (TLR) 2/4, p-P65 and p-IKB. In contrast, supplementation with 20 mg/kg MT reversed the elevation of the plasma CORT levels, downregulated the colon ROS levels and inhibited the changes in the intestinal microbiota induced by restraint stress. These effects, in turn, inhibited the activities of TLR2 and TLR4, p-P65 and p-IκB, and decreased the inflammatory reaction induced by restraint stress. Our results suggested that MT may mitigate "restraint stress"-induced colonic microbiota dysbiosis and intestinal inflammation by inhibiting the activation of the NF-κB pathway.

Effect of Capture, Phenotype, and Physiological Status on Blood Glucose and Plasma Corticosterone Levels in Free-Ranging Dice Snakes

AbstractTheoretically, animals integrate intrinsic and extrinsic factors to respond appropriately to the wide range of stressors they encounter during their life span. We examined how stress response varies between sexes and among morphotypes in wild dice snakes (Natrix tessellata). We also considered reproductive and feeding status and antipredator behavior. We used two indicators of stress (glucose [GLUC] and corticosterone [CORT] levels) at eight sampling time intervals (immediately after capture, up to 17 h after) and a large sample size (N=113 snakes). Concentrations of both markers increased sharply after capture (an equivalent of predation). This acute phase occurred earlier for GLUC (30 min) compared to CORT (60 min). Then the values plateaued to very high levels without decline over time, indicating prolonged saturation of the hypothalamus-pituitary-adrenal axis. In contrast to our expectations, we found no effect of sex, morphotype, or reproductive status. Yet the CORT stress response of those individuals displaying death-feigning (DF) antipredator behavior was attenuated compared to those that did not. Low stress hormones levels may facilitate the expression of DF (high levels supporting fleeing behavior). The presence of partially digested material in the stomach was associated with higher blood GLUC during the plateau. Assaying blood GLUC requires very little blood but was as good as CORT at gauging acute stress response. The prolonged plateau suggests that captivity should be minimized during field studies.

Physiological and behavioral effects of exogenous corticosterone in a free-ranging ectotherm

In the face of global change, free-ranging organisms are expected to experience more unpredictable stressors. An understanding of how organisms with different life history strategies will respond to such changes is an integral part of biodiversity conservation. Corticosterone (CORT) levels are often used as metrics to assess the population health of wild vertebrates, despite the fact that the stress response and its effects on organismal function are highly variable. Our understanding of the stress response is primarily derived from studies on endotherms, leading to some contention on the effects of chronic stress across and within taxa. We assessed the behavioral and hormonal responses to experimentally elevated stress hormone levels in a free-ranging, arid-adapted ectotherm, the Southern Pacific rattlesnake (Crotalus helleri). Plasma CORT was significantly elevated in CORT-implanted snakes 15days after implantation. Implantation with CORT did not affect testosterone (T) levels or defensive behavior. Interestingly, we observed increased defensive behavior in snakes with more stable daily body temperatures and in snakes with higher plasma T during handling (tubing). Regardless of treatment group, those individuals with lower baseline CORT levels and higher body temperatures tended to exhibit greater increases in CORT levels following a standardized stressor. These results suggest that CORT may not mediate physiological and behavioral trait expression in arid-adapted ectotherms such as rattlesnakes.

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.e., spring and fall migration) in female red-sided garter snakes (Thamnophis sirtalis parietalis). We collected non-migratory females from a communal den and migratory females from a road along the migration route and treated them with 4h of capture stress; plasma corticosterone was measured before, during and after capture stress. During the spring, den-collected females exhibited a stress-induced peak in corticosterone at an earlier sampling time than migrating, road-collected females. Because the pattern of corticosterone responses varied with migratory state, negative feedback on and/or sensitivity of the hypothalamus-pituitaryadrenal (HPA) axis may be linked to spring migration. During the fall, capture stress elicited an increase in corticosterone in den-collected females but not in migrating, road-collected females. Baseline corticosterone was higher and both maximal and integrated corticosterone responses were lower during the fall compared to spring, indicating that stress responses are smaller when baseline corticosterone is elevated, perhaps due to a "ceiling effect". These data suggest that HPA axis regulation changes during seasonal migration, possibly via altering negative feedback, HPA axis sensitivity, or some other mechanism. This study supports the hypothesis that glucocorticoids coordinate life-history events and suggests that examining a suite of stress response characteristics is most informative for understanding the function of HPA modulation.

Good vibrations: Assessing the stability of snake venom composition after researcher-induced disturbance in the laboratory

Phenotypic plasticity contributes to intraspecific variation in traits of many animal species. Venom is an integral trait to the success and survival of many snake species, and potential plasticity in venom composition is important to account for in the context of basic research as well as in human medicine for treating the various symptoms of snakebite and producing effective anti-venoms. Researchers may unknowingly induce changes in venom variation by subjecting snakes to novel disturbances and potential stressors. We explored phenotypic plasticity in snake venom composition over time in captive Pacific rattlesnakes (Crotalus oreganus) exposed to vibration treatment, compared to an undisturbed control group. Venom composition did not change significantly in response to vibration, nor was there a detectable effect of overall time in captivity, even though snakes re-synthesized venom stores while subjected to novel disturbance in the laboratory. This result indicates that venom composition is a highly repeatable phenotype over short time spans and that the composition of venom within adult individuals may be resistant to or unaffected by researcher-induced disturbance. On the other hand, the change in venom composition, measured as movement along the first principle component of venom phenotype space, was associated with baseline corticosterone (CORT) levels in the snakes. While differential forms of researcher-induced disturbance may not affect venom composition, significant changes in baseline CORT, or chronic stress, may affect the venom phenotype, and further investigations will be necessary to assess the nature of the relationship between CORT and venom protein expression.

Seasonal biotic and abiotic factors affecting hunting strategy in free-living Saharan sand vipers, Cerastes vipera

Sit-and-wait ambushing and active hunting are two strategies used by predators to capture prey. In snakes, hunting strategy is conserved phylogenetically; most species employ only one strategy. Active hunters encounter and capture more prey but invest more energy in hunting and have higher risks of being predated. This trade-off is important to small predators. The small Cerastes vipera employs both modes of hunting, which is unlike most viperids which use only sit-and wait ambushing. This species hibernates in October and emerges in April. Energy intake should be high prior to hibernation to overcome the non-feeding hibernation period and for reproduction on their emergence. We predicted that more individuals would hunt actively towards hibernation and an abiotic factor would trigger this response. Furthermore, since more energy is required for active hunting, we predicted that snakes in good body condition would use active hunting to a greater extent than snakes in poor body condition. To test our predictions, we tracked free-living snakes year round and determined their hunting strategy, estimated their body condition index (BCI), and calculated circannual parameters of day length as environmental cues known to affect animal behaviour. Two novel findings emerged in this study, namely, hunting strategy was affected significantly by 1) the circannual change in day length and 2) by BCI. The proportion of active hunters increased from 5% in April to over 30% in October and BCI of active foragers was higher than that of sit-and-wait foragers and, therefore, our predictions were supported. The entrainment between the proportion of active hunting and the abiotic factor is indicative of an adaptive function for choosing a hunting strategy. A trend was evident among life stages. When all life stages were present (September-October), the proportion of active foragers increased with age: 0.0% among neonates, 18.2% among juveniles and 31.4% among adults. We concluded that vulnerable small neonates used sit-and-wait ambush not only as a hunting strategy but also as a hiding technique.

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

Like 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. We measured glucocorticoids at 0, 1, and 4 hours after injection with ACTH (0.1 IU/g body mass) or vehicle in males and females during the spring mating season and fall pre-hibernation period. Because elevated glucocorticoids can influence sex steroids, we also examined androgen and estradiol responses to ACTH. ACTH treatment increased glucocorticoids in both sexes and seasons. Spring-collected males had a smaller integrated glucocorticoid response to ACTH than fall-collected males. The integrated glucocorticoid response to ACTH differed with sex during the spring, with males having a smaller glucocorticoid response than females. Although integrated glucocorticoid responses to ACTH did not vary with body condition, we observed an interaction among season, sex and body condition. In males, ACTH treatment did not alter androgens in either season, but androgens decreased during the sampling period. Similar to previous studies, plasma estradiol was low or undetectable during the spring and fall and therefore any effect of ACTH treatment on estradiol could not be determined. These data provide support for a mechanism that partly explains how the HPA axis integrates information about season, sex, and body condition: namely, variation in adrenal responsiveness to ACTH.

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. We tested the hypothesis that decreased plasma glucocorticoids mediate the behavioral switch between reproduction and foraging in this species. Using a two-choice Y-maze paradigm, we demonstrate that males treated with the glucocorticoid synthesis inhibitor metyrapone (1 and 3mg implants) prefer feeding cues (worm trail) over reproductive cues (female pheromone trail) significantly earlier than control-treated snakes. The metyrapone-induced changes in appetitive feeding behavior were independent of changes in plasma androgens and body mass loss. Metyrapone-treated males continued to court females at levels similar to those of control-treated snakes, suggesting that appetitive reproductive and ingestive behaviors are not mutually exclusive during this life-history transition. Consistent with this hypothesis, metyrapone treatment did not alter the number of arginine vasotocin-immunoreactive cells in any brain region, while it significantly increased neuropeptide Y-immunoreactive cell number in both the cortex and nucleus sphericus (homologues of the mammalian hippocampus and amygdala, respectively). Our results suggest that male red-sided garter snakes have the potential to maximize reproductive opportunities by continuing to court females they encounter even as they disperse from the den in search of food. Taken together, these data have important implications for understanding the neuroecology of seasonal life-history transitions.

Facultative nocturnal behaviour in snakes: experimental examination of why and how with Ratsnakes (Elaphe obsoleta) and Racers (Coluber constrictor)

Diel activity patterns are often fixed within species such that most animals can be classified as diurnal, crepuscular, or nocturnal, and have sensory abilities that reflect when they are active. However, many snake species appear capable of switching between diurnal and nocturnal activity. Here, we evaluate the hypothesis that some species are constrained in their activity by the sensory cues used for foraging. We experimentally assessed differences between two sympatric snake species in their ability to alter diel activity patterns, to address why those snakes that switch do so (do thermal constraints force them to be active in otherwise nonpreferred conditions?), and to explore how sensory abilities to locate prey facilitate or constrain this shift. Ratsnakes (Elaphe obsoleta (Say in James, 1823)) were active when temperature was optimal, regardless of light level, suggesting their activity pattern is genuinely plastic. Consistent with our predictions, Ratsnakes successfully detected prey in low and high light using visual or chemical cues, and were most successful when cues were coupled. Racers (Coluber constrictor L., 1758) were almost exclusively diurnal, regardless of temperature, and became less active when daytime temperatures were suboptimal. The ability of Ratsnakes to shift activity may confer a foraging advantage and should buffer Ratsnakes and similarly flexible species from climate change, whereas climate change may pose a more serious threat to inflexible species such as Racers.

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. Body condition was determined as the residual from a regression of body mass on snout-vent-length. Baseline CORT did not differ between females in positive and negative body condition, but females in negative body condition showed a significantly larger increase in plasma CORT in response to capture stress. Estradiol, which is generally low during the mating season in this population, did not change in response to capture stress. Body condition, but not capture stress, influenced latency to copulate, suggesting that females are resistant to the behavioral effects of capture stress during the spring mating season. In a second experiment, only females in negative body condition increased latency to copulate in response to injection of a physiological (15 μg) dose of exogenous CORT, while all females responded to a pharmacological (60 μg) dose. These results indicate that behavioral responses to exogenous CORT vary with female body condition during the short mating season. Taken together, our data suggest that variation in body condition may be associated with differences in HPA axis sensitivity and/or glucocorticoid receptor density in the brain.

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. Diel melatonin and corticosterone rhythms appear to be important in transducing temperature cues, and it is clear that both hormones regulate courtship behavior during spring. Current evidence also suggests that sex steroid hormones are in fact central to reproductive regulation in males, although the timing of their action occurs during winter dormancy. Whether this is also true for female T. sirtalis parietalis requires further study, but it should be noted that patterns of sex steroid hormones are sexually dimorphic during winter dormancy, as are melatonin rhythms during spring emergence. While continuing to advance our understanding of reproductive regulation in this extremely well-studied population is prudent, future comparative studies are critical for understanding if and how reproductive regulatory mechanisms differ across environments, populations, and phylogenies. For example, melatonin and corticosterone responses to environmental cues vary significantly among populations of T. sirtalis in a common garden, as do male courtship behavior and androgen concentrations. These data support the hypothesis that neuroendocrine-mediated responses to environmental cues underlie phenotypic plasticity in reproductive life history traits.