The effect of photoperiod on adrenocortical stress response in mountain chickadees (Poecile gambeli)

Birds respond to environmental changes by modulating their levels of plasma corticosterone, a stress hormone produced by the adrenal glands. Baseline levels of corticosterone and the magnitude of adrenocortical response to acute stress are known to vary seasonally. Photoperiod is one of several potential factors which could affect the seasonal modulation of corticosterone secretion. In this study, we examined the effects of photoperiod on baseline and acute-stress-induced levels of corticosterone in adult mountain chickadees (Poecile gambeli). We exposed 20 mountain chickadees to short days (8L:16D h) for 60 days, after which half of the chickadees were switched to long days (14L:10D h). We collected blood samples from both groups 33 days after the switching. Our results showed that photoperiod had no significant effect either on baseline levels of corticosterone or on the maximum levels reached during the adrenocortical response to acute stress. However, photoperiod had a significant effect on the way that chickadees responded to acute stress: birds maintained on long days reached significantly higher levels of corticosterone between 5 and 20 min after capture than birds maintained on short days, which reached their maximum levels of corticosterone between 20 and 50 min. Females reached significantly higher levels of corticosterone than males in response to acute stress. Our results suggest that factors other than photoperiod are responsible for the observed seasonal changes in baseline levels of corticosterone, whereas photoperiod is directly involved in regulation of adrenocortical stress response.     

Effects of nutritional restrictions during post-hatching development on adrenocortical function in western scrub-jays (Aphelocoma californica)

Altricial birds grow rapidly during post-hatching period and are developmentally sensitive to variations in food supply. Limited food results in elevated corticosterone levels in chicks of semi-precocial birds but it is not clear whether altricial songbirds show similar adrenocortical stress response to nutritional restrictions during early development. It is also unknown how nutritional stress during early development affects the adrenocortical function later in life in altricial birds which show tremendous variation in the magnitude of adrenocortical stress response. Using western scrub-jays (Aphelocoma californica), we experimentally demonstrated that moderate food restrictions (65% of ad libitum) during post-hatching development caused significant elevation of baseline corticosterone levels in nest-bound chicks. Compared to controls, 1-year-old scrub-jays that experienced nutritional deficits during post-hatching development also showed a marginally significant trend to have stronger adrenocortical stress response and significantly greater degree of fluctuating asymmetry in bone and feather measurements. Thus, our results demonstrated that developing altricial birds show adrenocortical response to nutritional deficits, which might produce long-term changes in responsiveness of the adrenal system. Our study suggests that baseline corticosterone levels are a good indicator of physiological conditions of developing birds and that individual variance in adrenocortical stress response commonly observed in many species might, at least in part, be explained by environmental conditions during early development. Considering that nutritional restrictions during early development are linked to many permanent changes including impaired cognitive abilities, corticosterone levels in developing young might be a reliable predictor of their future fitness.     

Assessment of the role of the glucocorticoid system in aging processes and in the action of food restriction

The Glucocorticoid Cascade Hypothesis of Aging and the hypothesis that food restriction retards the aging processes by preventing the development with age of hyperadrenocorticism were investigated. A longitudinal life span study of the daily concentration pattern of plasma corticosterone was conducted in male Fischer 344 rats fed ad libitum or restricted to 60% of the mean food intake of ad libitum fed rats. In another group of ad libitum fed and food-restricted rats, the influence of age on the response of plasma corticosterone levels to restraint stress was measured as was the time course of the return of plasma corticosterone to basal levels following the stress. The findings do not support the hypothesis that food restriction retards the aging processes by preventing the development of hyperadrenocorticism with advancing age. They also indicate that the Glucocorticoid Cascade Hypothesis does not describe a major aspect of the aging processes. Rather, the results suggest the possibility that a lifetime of daily periods of mild hyperadrenocorticism may, if anything, retard the aging processes.     

The effects of acute and life-long food restriction on basal and stress-induced serum corticosterone levels in young and aged rats

The effects of short term (1-month) and life-long 60% ad libitum food restriction on the adrenocortical response to restraint stress were compared in young and aged Fischer 344 rats. In rats restricted for 1 month (study 1), the adrenocortical response differed as a function of age. In 8-month-old animals, the initial steep rise in corticosterone in response to stress was of similar magnitude in ad libitum and restricted animals. In 23-month-old animals the corticosterone response was severely blunted in restricted animals. In life-long restricted animals (study 2), the corticosterone response to restraint stress was tested at 8, 16, and 24 months of age. The general pattern of response to stress in these animals was similar to that in study 1. The 16- and 24-month-old animals showed the same blunted response to stress found in the 23-month-old animals restricted for only 1 month, suggesting that the severe restriction per se and not life-long food restriction blunted the response to stress in aged animals. The similarity between the response to stress in study 1 and study 2 was evident even though animals were tested in one case before feeding when corticosterone levels were high, and in the other 4-5 h after feeding when corticosterone levels were lower. In study 3 it was found that in food-restricted young rats, the mean corticosterone level over a 24-h period was significantly elevated above that in ad libitum fed young rats. In aged rats, however, except before daily feeding, corticosterone levels of food-restricted rats remained significantly below those of ad libitum fed animals, whose levels were, in turn, significantly elevated compared to those of ad libitum fed young rats. These findings suggest that in aged animals severe food restriction reduces basal corticosterone levels, adrenal responsiveness to stress, and adrenal size and has the potential to protect against the consequences of high corticosterone levels in aging.     

Baseline and acute levels of corticosterone in Florida Scrub-Jays (Aphelocoma coerulescens): effects of food supplementation, suburban habitat, and year

Recent work has shown that Florida Scrub-Jays (Aphelocoma coerulescens) living in suburban habitats, as well as birds that reside in a natural habitat (hereafter referred to as wildland birds) provided supplemental food during the pre-breeding period, have lower baseline corticosterone (CORT) levels than nonsupplemented wildland birds, suggesting that predictable and abundant foods can lower stress levels. Here, we investigate whether the acute CORT response to capture stress reflects our earlier findings. During the pre-breeding seasons in 2004 and 2005 we captured over 200 Florida Scrub-Jays and collected four blood samples from each bird over a 30 min time period. We compared hormonal and morphometric data from birds in four treatment groups: wildland birds that were provided with supplemental food during the pre-breeding period, wildland birds that were provided with supplemental food during the winter and pre-breeding period, nonsupplemented wildland birds, and suburban birds that effectively have access to ad libitum human-source food year-round. Suburban birds had lower baseline levels, and a more acute initial CORT response (i.e., rate of increase) than wildland birds, but we did not observe any differences among the treatment groups of wildland birds. Baseline CORT concentrations were unusually low across treatments in 2005, a year that was characterized by increased adult body mass, superior body condition, and a high fledgling success rate, all of which suggest favorable environmental conditions. Thus, abundant or more reliable food sources in 2005 may have overwhelmed any potential benefits of supplemental feeding and explain the absence of any strong experimental effects on CORT for that year.     

Changes in patterns of corticosterone secretion concurrent with migratory fattening in a neotropical migratory bird.

Several studies on free-living birds have shown a change in corticosterone secretion (elevated baseline levels and a reduced corticosterone response to stress) during migration. It was not known, however, if this change was concurrent with the development of migratory condition or if it was an independent response to unknown environmental stressors experienced by the birds prior to capture. In this study, a Neotropical annual migrant, the yellow-rumped warbler (Dendroica coronata), held under controlled laboratory conditions, was used to test the Migration Modulation Hypothesis (MMH): during the migratory period migrants exhibit (1) elevated baseline corticosterone to facilitate migratory fattening and (2) a reduced corticosterone stress response, a means by which skeletal muscle needed for migration can be protected against catabolism by high levels of corticosterone. Fifteen hatching-year warblers were maintained on insect larvae and water ad libitum for 43 weeks, experiencing two transitions from a short- to long-day photoperiod to bring them into spring migratory condition. Corticosterone profiles comprising three blood samples from each individual (baseline at the time of initial disturbance and 30 and 60 min later), body mass, fat reserves, molt, and state of cloacal protuberance (males only) were measured at key intervals throughout the study. Over the entire study, mean baseline corticosterone levels were positively correlated with mean body mass, which increased predictably in response to long days. Individual baseline corticosterone was not correlated with individual body mass at any time. During periods when the birds were lean and held on short days, the corticosterone stress profiles were characterized by low initial hormone concentration followed by a significant increase in corticosterone with handling time. In response to long days, the warblers showed a significant increase in body mass and fat reserves concurrent with corticosterone stress profiles characterized by significantly elevated baseline levels and no further increase in corticosterone with handling time. These results support both components of the MMH illustrating changes in corticosterone secretion concurrent with migratory fattening but the exact nature of this change is unknown.     

Corticosterone levels during post-natal development in captive American kestrels (Falco sparverius)

We investigated post-natal development of the adrenocortical stress-response system in captive American kestrels (Falco sparverius) by measurements of baseline and stress-induced levels of corticosterone at ages 10, 16, 22, and 28 days post-hatching. Baseline levels of corticosterone increased significantly during post-natal development and although chicks aged 10- and 16-days old exhibited comparable baseline corticosterone levels, those of 22-day-old chicks were significantly higher and those of 28-day-old chicks close to fledging were higher than all younger groups. Chicks in this study exhibited low levels of stress-induced corticosterone early in development and did not exhibit adult-type stress-induced levels of corticosterone until 22 days of age post-hatching. Finally, although baseline and stress-induced levels of 28-day-old birds were significantly higher than one-year-old adults, there was no relationship between baseline corticosterone concentrations and time to nest departure. The fact that baseline levels of corticosterone are low during early development and then increase during later development may be an adaptation to the negative effects of chronically elevated corticosterone levels and as previously noted in other studies may minimize these negative effects on rapid growth and development in young birds, potentially maximizing normal growth. The ability of even young kestrel chicks to elevate corticosterone levels in response to stress suggests that they may be able to physiologically cope with food shortages associated with unpredictable food resources which wild kestrels often face.     

Enhancement by food restriction of liver protein synthesis in the aging Fischer 344 rat

Age-related changes in liver protein synthesis of ad libitum fed and food-restricted SPF Fischer 344 rats have been measured. In ad libitum fed animals, valine incorporation increased from 3 to 6 mo of age, decreased to the 3-mo level at 12 mo, remained relatively constant through 18 mo, and then declined further at 24 mo of age. By 24 mo valine incorporation was 30% of the 6-mo value. Food restriction had no effect at 3 mo of age (6 wk of restriction) but at 6 mo the rate of valine incorporation was 35% greater than the control, an increment that was maintained throughout the life of the animal. Therefore, although food restriction does not prevent the age-related decline in protein synthesis, it does maintain the rate of liver valine incorporation at levels greater than those observed in the liver of the ad libitum fed animal.     

Effects of isolation and food restriction begun at 50 days on the development of age-associated renal disease in the male Wistar rat

The development of proteinuria with increasing age was studied in three groups of male Wistar rats: ad libitum fed and isolated, ad libitum fed and group housed 6 to 8 rats per cage, and food restricted (one-third of the isolated ad libitum food intake) and isolated. Studies were begun at age 50 days and continued throughout life. Ad libitum fed rats when isolated ate more food, grew faster, had larger maximum body weights and developed proteinuria at a faster rate than those that were group housed. There was a small increase in the severity of glomerular pathology in old age. However, systolic blood pressure was not affected significantly by isolation, nor was life duration. Food restriction of isolated rats inhibited body growth, prevented the development of proteinuria, reduced the incidence of glomerular and tubular pathology in old age and prolonged life. Electron microscopic examination of the kidneys of old food-restricted rats revealed a much lower incidence of foot process retraction and spreading on the basement membrane of the glomerulus than in ad libitum fed rats. Cardiac enlargement was also prevented by long-term food restriction.     

Food restriction enhances melatonin effects on the pituitary-gonadal axis in female rats

The effects of melatonin (25 micrograms s.c. daily in the late afternoon for 10 days) on the ovarian morphometry and the gonadotropin secretion were investigated in food-restricted rats and rats fed ad libitum. In food-restricted rats melatonin produced the significant decrease of the surface area of the ovary and of zona granulosa. Moreover, melatonin treatment of food-restricted rats resulted in significant diminishment of the relative areas of Graafian follicles and zona granulosa estimated in relation to the ovary cross-sectional surface area. On the other hand, melatonin did not produce significant alterations of the morphometric indices of the ovary in rats fed ad libitum. Melatonin administration was found to inhibit the effects of gonadoliberin on gonadotropin release in food-restricted but not in normally-fed rats. These findings suggest that food restriction sensitizes the pituitary-ovarian axis to antigonadotropic melatonin action.     

Circulating corticosterone levels in breeding blue tits Parus caeruleus differ between island and mainland populations and between habitats

Little is known about whether adaptations to an insular life also involve adaptations in basal corticosterone levels or in the adrenocortical stress response, thus being part of a genetically based island syndrome. However, differences in corticosterone between island and mainland may also be a direct phenotypic response to differences in environmental conditions or may depend on individual characteristics of the animal such as body condition or parental investment. In this paper, we investigated whether insular (Island of Corsica) and mainland (nearby Southern France) blue tits Parus caeruleus populations differed in baseline and handling-stress induced corticosterone levels during the breeding season as a response to biological changes of insular biota. We also examined whether corticosterone levels of both mainland and insular blue tits differed between birds living in two different habitats (summergreen and evergreen oak woods) that differ in food availability and whether individual characteristics affected corticosterone levels. We found (a) differences in baseline corticosterone plasma levels between Corsica and the mainland, independent of regional differences in fat scores, (b) a regional difference in the relationship between corticosterone levels and brood size, (c) a difference in the rapidity of onset of the stress response to handling between habitats, independent of region, and (d) a negative relationship between body fat stores and baseline corticosterone levels independent of region. Reduced baseline corticosterone levels on Corsica may be a component of the insular syndrome, allowing birds to be less aggressive and to enhance parental investment despite higher breeding densities. We suggest that baseline corticosterone levels are only elevated if food availability affects directly the parents. However, when conditions deteriorate unexpectedly (as mimicked by handling stress), food allocation between parents and offspring needs to be re-adjusted in favor of the parents, possibly by increased circulating corticosterone levels. The switch to self-maintenance seems to be modified by the amount of body energy stores.     

Daily rhythms in adrenal responsiveness to adrenocorticotropin are determined primarily by the time of feeding in the rat.

These experiments were done to determine: 1) whether feeding-related shifts in daily corticosterone rhythms are dependent upon changes in ACTH rhythms, 2) whether restricted feeding of rats results in abnormally high ACTH and corticosterone levels (i.e. stress), and 3) whether changes in either insulin or glucose levels might be the concomitants of feeding that change adrenal responsiveness to ACTH. Young male rats (80--90 g) on a 12-h light, 12-h dark cycle were allowed access to one of three diets for 2 h/day beginning either at lights off or lights on. The diets contained 3%, 4.5%, or 11% fat. A group of rats had ad libitum access to the food containing 4.5% fat. On day 20 of this regimen, rats were killed at 2- to 4-h intervals during the next 24 h, and plasma ACTH, corticosterone, insulin, and glucose were measured. Adrenal weight and corticosterone content were also determined. In none of these experiments was an ACTH rhythm demonstrable by analysis of variance. Neither ACTH levels nor adrenal and plasma corticosterone levels were higher in animals fed 2 h/day than in rats eating ad libitum. Peak corticosterone levels occurred just before feeding, and the restricted feeding paradigm appeard to sharpen the daily rhythms. However, there was also an effect of the light-dark cycle on corticosterone measures. Dietary fat content was directly related to increases in body weight and mean insulin levels and infersely related to adrenal responsiveness to ACTH. The data show that: 1) the time of feeding determines the timing of the corticosterone rhythm in the absence of a rhythm in ACTH, 2) restricted feeding is not a stress, and 3) neither insulin nor glucose has a substantial influence on adrenal responsiveness to ACTH.     

Changes in plasma corticosterone and adrenocortical response to stress during the breeding cycle in high altitude flycatchers

Plasma corticosterone levels were monitored in a breeding population of Dusky Flycatchers (Empidonax oberholseri) at Tioga Pass, in the eastern Sierra Nevada. Seasonal changes in baseline plasma corticosterone levels were largely related to changes in reproductive status. Levels in both sexes were highest during the period preceding the female's initiation of a clutch. Females, alone, incubated, but males provided food regularly for their incubating mates. Excepting transient peaks in female corticosterone levels that immediately preceded ovipositions, steep declines in baseline levels of corticosterone in both sexes corresponded to the onset of parental attentiveness. Serial measurements of plasma corticosterone levels over a 1-h period of restraint, indicated that these declines originate at the level of the hypothalamo-pituitary-adrenal response system. Increases in stress-induced plasma corticosterone were significantly greater during the period preceding clutch initiation (prenesting) than during the nesting period (incubation and nestling periods). Despite the changes in corticosterone between prenesting and nesting periods, variability in corticosterone levels during both stages was negatively correlated with body condition (body mass and fat, corrected for size), and level of parental investment. Highest levels of plasma corticosterone were observed in lighter, leaner flycatchers, and during the hours when self-foraging activities were highest; lowest levels were seen in heavier, fatter birds, and those foraging for chicks. This relationship between corticosterone and parental care extended to a small number of accessory males that were acting as helpers at the nest. These results suggest that variation in the length of the reproductive cycle and degree of parental investment may help to explain the level of adaptive modulation of the adrenal stress response in species that breed in unpredictable environments.     

Lifelong dietary modulation of calcitonin levels in rats

Studies were carried out on specific pathogen-free rats to evaluate the effects of aging and dietary manipulation on serum and thyroid calcitonin (CT) levels. Male Fischer 344 rats were randomized at 6 weeks of age to six dietary groups and subsequently maintained on the following dietary regimens. Group 1 rats were fed ad libitum throughout life; group 2 rats were fed 60% of the ad libitum food uptake, but received the same amounts of calcium, phosphorus, and vitamin D; group 3 rats were fed as the group 2 animals until 6 months of age and from then on were fed ad libitum; group 4 rats were fed ad libitum until 6 months of age and then switched to 60% food restriction; group 5 rats were fed ad libitum on food isocaloric with that of group 1 rats, but containing only 60% of the protein. Group 6 rats were killed at 6 weeks of age to serve as baseline controls. Ten rats were killed in each of the remaining five groups 15 h postprandial at 6-month intervals. The following observations were made. Serum CT increased with age similarly in the ad libitum fed group 1 and 5 rats. Food restriction markedly inhibited the increase in serum CT, and the effect was more profound in animals whose food intake was restricted after 6 months of age (group 4) than in animals on lifelong food restriction (group 2). In rats switched from food restriction to ad libitum feeding (group 3) at 6 months of age, serum CT increased with age to levels identical with those of lifelong ad libitum fed group 1 animals. Thyroid CT showed a similar pattern of age-dependent and dietary modulated changes. In contrast, aging and dietary modulation had no appreciable effect on serum calcium levels, except at 27 months of age when the serum calcium level of group 1 animals increased dramatically from the level for 24-month-old animals. There was a weak positive correlation between serum calcium and serum CT (r = 0.627; P = 0.02) and a highly significant positive correlation between serum CT and thyroid CT (r = 0.917; P = 0.001). These findings indicate that elective and therapeutic restriction of food intake might also attenulate CT levels in humans, with potentially adverse implications for skeletal homeostasis.     

Prolonged food restriction and mild exercise in Shetland ponies: effects on weight gain, thyroid hormone concentrations and muscle Na(+),K(+)-ATPase

We determined the effects of food supply and low-intensity training on growth, serum thyroid hormone levels and the Na(+),K(+)-pump concentration in equine skeletal muscle. Twenty-two Shetland ponies were subjected to two different feeding regimes for 2(1/2) years (11 ponies per group): food restriction (body condition score kept at 2) or ad libitum fed (body condition score kept at 8). Five ponies in each group underwent low-intensity training. Gluteus medius muscle and serum samples were obtained in April 1998. Subsequently, all ponies were fed ad libitum and the training programme was stopped. Muscle biopsies and serum samples were collected again in November 1998. Food restriction was associated with a 30-50% reduction of body weight gain. While the total thyroxine (T(4)) level was increased, the free T(4) remained at the control level. The serum total tri-iodothyronine (T(3)) and free T(3) were reduced by 30% and 49% respectively. After 6 months of refeeding there were no differences in any of the hormone levels between the ad libitum fed and the food-restricted groups. Food restriction produced a minor, but not significant, decrease in the Na(+),K(+)-pump concentration in the gluteus medius muscle of the Shetland ponies. Low-intensity training reduced weight gain of the ad libitum fed group by 25%, but had no detectable effect on the concentration of the Na(+), K(+)-pumps. We conclude that prolonged food restriction in Shetland ponies results in a weight gain reduction of 30-50%, and is associated with similar decreases in serum total and free T(3). The reduction in serum T(3) only slightly influenced the Na(+), K(+)-ATPase concentration in skeletal muscle, indicating that muscle tissue of different species may respond differently to changes in circulating thyroid hormones.     

Inhibitory effect of hypophysectomy and food restriction on glomerular basement membrane thickening, proteinuria and renal enlargement in aging male Wistar rats

Age-related thickening of the glomerular basement membrane (GBM) was studied in three groups of male Wistar rats: (a) ad libitum fed, (b) hypophysectomized and (c) food-restricted eating the same amount of food as hypophysectomized rats, but about 45% of the ad libitum fed group. Studies were begun at 50 days (2 months) and continued throughout life. Multiple regression was used to statistically assess the effects of age and treatments. In ad libitum fed male rats GBM thickness increased from 114 nm at 50 days (2 months) to 632 nm at 1,000 days (33 months). GBM thickness at 1,000 days was 296 nm in hypophysectomized rats and 392 nm in food restricted rats. Hypophysectomy had a significantly greater inhibitory action on GBM thickening than food restriction, in rats eating the same quantity of food per day. However, a major part of the effect of hypophysectomy may be due to the permanent fall in food intake (from 16.3 to 7.9 g/day) resulting from the operation. Accompanying the age-related thickening of the GBM in ad libitum fed rats were proteinuria and renal enlargement, both of which were inhibited by hypophysectomy and food restriction.     

Plasma concentrations of glucose, insulin, and percent glycosylated hemoglobin are unaltered by food restriction in rhesus and squirrel monkeys.

Plasma concentrations of glucose and percentage of glycosylated hemoglobin have been reported to be reduced in food-restricted rats when compared with ad libitum-fed controls (Masoro et al., 1989). A similar experiment in primates, in which we also measured plasma insulin, is reported in this article. Rhesus and squirrel monkeys were fed either ad libitum or 30% less than weight-matched controls for up to 36 months. No significant age or diet effects on plasma concentration of glucose, insulin, or percentage glycosylated hemoglobin were observed, except that glucose concentration decreased with age in ad libitum-fed rhesus monkeys. Moreover, no correlations between glucose levels and the percentage of glycosylated hemoglobin could be established within any group of animals. These results suggest possible differences in glucose-related metabolism in ad libitum and food-restricted primates as compared to observations made in rats.     

Reversed hatching order, body condition and corticosterone levels in chicks of southern rockhopper penguins (Eudyptes chrysocome chrysocome)

In altricial and semi-altricial species, asynchronous hatching gives the first chicks to hatch an initial advantage over other siblings and often leads to the elimination of the smallest chicks. Both baseline corticosterone and acute stress-induced corticosterone levels have been shown to be higher in food deprived chicks than in chicks fed ad libitum. However, first-hatched chicks have also been shown to exhibit higher corticosterone levels than last-hatched chicks, suggesting an influence of the initial differences between eggs on corticosterone levels. We subjected single-chicks of southern rockhopper penguins Eudyptes chrysocome chrysocome to a standardised capture-stress protocol. In this species having very dimorphic two-egg clutches, we examined whether corticosterone levels were different between the two chick categories and tested for the effect of body condition controlled by the chick category. Neither body sizes, nor corticosterone levels differed between A- and B-chicks at 18 days. In contrast to baseline corticosterone levels, acute stress-induced levels of corticosterone were negatively correlated to body condition: chicks with a good body condition had lower acute stress-induced levels of corticosterone than chicks with a poor condition, whatever the chick category. Our results do not support the idea that initial differences in egg characteristics could drive the difference in corticosterone levels between siblings. On the contrary, they show that the A-egg of rockhopper penguins has, when reared alone, the same intrinsic potential to develop into a fledged chick as the B-egg. Later differences in body condition appear to lead to variation in the acute stress-induced levels of corticosterone.