Glucocorticoids in tuatara (Sphenodon punctatus): Some influential factors, and applications in conservation management

Invited review: Adrenocortical function in avian and non-avian reptiles: Insights from dispersed adrenocortical cells

Herein we review our work involving dispersed adrenocortical cells from several lizard species: the Eastern Fence Lizard (Sceloporus undulatus), Yarrow's Spiny Lizard (Sceloporus jarrovii), Striped Plateau Lizard (Sceloporus virgatus) and the Yucatán Banded Gecko (Coleonyx elegans). Early work demonstrated changes in steroidogenic function of adrenocortical cells derived from adult S. undulatus associated with seasonal interactions with sex. However, new information suggests that both sexes operate within the same steroidogenic budget over season. The observed sex effect was further explored in orchiectomized and ovariectomized lizards, some supported with exogenous testosterone. Overall, a suppressive effect of testosterone was evident, especially in cells from C. elegans. Life stage added to this complex picture of adrenal steroidogenic function. This was evident when sexually mature and immature Sceloporus lizards were subjected to a nutritional stressor, cricket restriction/deprivation. There were divergent patterns of corticosterone, aldosterone, and progesterone responses and associated sensitivities of each to corticotropin (ACTH). Finally, we provide strong evidence that there are multiple, labile subpopulations of adrenocortical cells. We conclude that the rapid (days) remodeling of adrenocortical steroidogenic function through fluctuating cell subpopulations drives the circulating corticosteroid profile of Sceloporus lizard species. Interestingly, progesterone and aldosterone may be more important with corticosterone serving as essential supportive background. In the wild, the flux in adrenocortical cell subpopulations may be adversely susceptible to climate-change related disruptions in food sources and to xenobiotic/endocrine-disrupting chemicals. We urge further studies using native lizard species as bioindicators of local pollutants and as models to examine the broader eco-exposome.

Assay temperature affects corticosteroid-binding globulin and free corticosterone estimates across species

Glucocorticoid hormones are often measured to assess how organisms physiologically respond to challenges in their environment. In plasma, glucocorticoids circulate in two forms: bound to corticosteroid-binding globulins (CBG) or unbound (free). Measuring CBG allows us to estimate the amount of free glucocorticoids present in a plasma sample. However, free glucocorticoid estimates are affected by the assay temperature used when measuring CBG, with colder temperatures maximizing specific binding but likely underestimating glucocorticoid's affinity for CBG. Here, we test how a biologically relevant incubation temperature (41 °C) changes the disassociation constant (K_d; used to estimate free glucocorticoid levels) when compared to the traditional 4 °C incubation temperature, across four commonly studied avian species. We then apply the new K_d's calculated at 41 °C to existing data sets to examine how the change in K_d affects free corticosterone estimates and data interpretation. K_d's were generally higher (lower affinity for CORT) at warmer incubation temperatures which resulted in higher levels of estimated free CORT in all four species but differed among subspecies. This increase in free CORT levels did not qualitatively change previously reported statistical relationships, but did affect variance and alpha (P) values. We suggest that future assays be run at biologically relevant temperatures for more accurate estimates of free CORT levels in vivo and to increase the chances of detecting biological patterns of free-CORT that may not be revealed with the classic methodology that tends to underestimate free CORT levels.

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.

Effects of temperature on plasma corticosterone in a native lizard

The glucocorticoid stress response is frequently used to indicate vertebrate response to the environment. Body temperature may affect glucocorticoid concentrations, particularly in ectotherms. We conducted lab manipulations and field measurements to test the effects of body temperature on plasma corticosterone (predominant glucocorticoid in reptiles) in eastern fence lizards (Sceloporus undulatus). First, we acclimated lizards to one of 4 treatments: 22 °C, 29 °C, 33 °C, or 36 °C, and measured cloacal temperatures and plasma corticosterone concentrations at baseline and after exposure to a standardized stressor (cloth bag). Both baseline and stress-induced corticosterone concentrations were lower in lizards with lower body temperatures. Second, we acclimated lizards to 22 °C or 29 °C and exposed them to a standardized (cloth bag) stressor for 3 to 41 min. Lizards acclimated to 29 °C showed a robust increase in plasma corticosterone concentrations with restraint stress, but those at 22 °C showed no such increases in corticosterone concentrations. Third, we measured lizards upon capture from the field. There was no correlation between body temperature and baseline plasma corticosterone in field-caught lizards. These results suggest body temperature can significantly affect plasma corticosterone concentrations in reptiles, which may be of particular concern for experiments conducted under laboratory conditions but may not translate to the field.

Endocrine responses to diverse stressors of capture, entanglement and stranding in leatherback turtles (Dermochelys coriacea)

Leatherback turtles (Dermochelys coriacea) are exposed to many anthropogenic stressors, yet almost no data on stress physiology exist for this species. As a first step toward understanding the physiological responses of leatherback turtles to stress, and with the particular goal of assessment of the effect of capture, we quantified corticosterone (an adrenal stress hormone) and thyroxine (a regulator of metabolic rate, often inhibited by chronic stress) in 17 healthy leatherback turtles captured at sea for scientific study, with comparisons to 15 'distressed' leatherbacks that were found entangled in fishing gear (n = 8), confined in a weir net (n = 1) or stranded on shore (n = 6). Distressed leatherbacks had significantly elevated corticosterone (mean ± SEM 10.05 ± 1.72 ng/ml, median 8.38 ng/ml) and free thyroxine (mean 0.86 ± 0.37 pg/ml, median 0.08 pg/ml) compared with healthy leatherbacks sampled immediately before release (after ∼40 min of handling; corticosterone, mean 4.97 ± 0.62 ng/ml, median 5.21 ng/ml; and free thyroxine, mean 0.05 ± 0.05 pg/ml, median 0.00 pg/ml). The elevated thyroxine in distressed turtles compared with healthy turtles might indicate an energetic burden of entanglement and stranding. Six of the healthy leatherbacks were sampled twice, at ∼25 and ∼50 min after the time of first disturbance. In all six individuals, corticosterone was higher in the later sample (earlier sample, mean 2.74 ± 0.88 ng/ml, median 2.61 ng/ml; later sample, mean 5.43 ± 1.29 ng/ml, median 5.38 ng/ml), indicating that capture and handling elicit an adrenal stress response in this species. However, the corticosterone elevation after capture appeared relatively mild compared with the corticosterone concentrations of the entangled and stranded turtles. The findings suggest that capture and handling using the protocols described (e.g. capture duration <1 h) might represent only a mild stressor, whereas entanglement and stranding might represent moderate to severe stressors.