Biomedical subjects

John C Wingfield

Publications and source records attributed to John C Wingfield.

At least 55 records · Page 3Linked to original sources

PubMed2004

Effect of estradiol implants on reproductive behavior of female Lapland longspurs (Calcarius lapponicus).

The role of estrogens in reproductive behavior of free-living birds has been little investigated, despite the importance of estrogens in coordinating reproductive behavior and the need for information on potential effects of estrogenic contaminants on wildlife. We gave subcutaneous implants of estradiol (E2) to 15 female Lapland longspurs (Calcarius lapponicus), an arctic bird with a six-week breeding season. Controls were 9 females with empty implants and 14 unimplanted females. After implantation, E2 females had greater E2 levels and greater body mass than controls. We did not detect any increases in, or prolongation of, sexual or nest-building behavior. The incubation period was longer for E2 nests than for control nests. Nest videotapes showed that E2 females and their mates exhibited normal nestling care, and nestling growth, fledging success, and overwinter survival were also unaffected. However, whenever the males arrived at the nests, E2 females showed a significantly greater likelihood of begging from the male rather than leaving the nest. Courtship or incubation feeding have been considered atypical for this species, and it is not clear what role E2 normally plays in promoting begging behavior. Overall, elevated E2 levels did not result in any obvious costs for the female, save perhaps for slightly reduced incubation efficiency. The weak response to hormonal implants is similar to patterns seen in male Lapland longspurs, and may be related to the short breeding season. However, since little is presently known about the role of estrogens in behavior of wild female birds, it remains to be seen whether these patterns are typical of female passerines in general, or of short-season breeders. Further research on effects of estrogens (both exogenous and endogenous) on free-living female birds is needed.

Animals↗

PubMed2004

Prolactin-induced parental hyperphagia in ring doves: are glucocorticoids involved?

Hyperphagia is a prominent component of the parental behavior repertoire in male and female ring doves and is necessary in order for parents to successfully provision their growing young. Although previous studies implicate both prolactin and the endogenous glucocorticoid, corticosterone, in parental hyperphagia, the functional interactions between these two hormones in regulating changes in feeding activity have not been characterized. These studies examined the possibility that prolactin's orexigenic effects are mediated through the increased secretion of corticosterone. Twice-daily intracerebroventricular (icv) injection of prolactin increased plasma corticosterone concentration in non-breeding doves of both sexes, with males exhibiting more pronounced effects than females. To further test the importance of glucocorticoid signaling in prolactin-induced feeding responses, changes in food intake were investigated in icv prolactin-treated, non-breeding doves following icv infusion of the glucocorticoid receptor antagonist RU38486 or propylene glycol vehicle. No attenuation of prolactin-induced hyperphagia was observed in either sex following co-administration of RU38486 at a dose shown previously to block dexamethasone-induced feeding in doves. These findings suggest that elevated corticosterone titers in blood may contribute to the hyperphagia observed in response to prolactin, but corticosterone signaling through a mammalian-type glucocorticoid receptor is not essential.

Animals↗

PubMed2004

Plasma corticosterone increases during migratory restlessness in the captive white-crowned sparrow Zonotrichia leucophrys gambelli.

Plasma corticosterone increases during the period of spring migration in a variety of bird species. Long-distance migrants show elevations in corticosterone specifically in association with the stage of flight, suggesting that corticosterone may support flight-related processes, for example, locomotor activity and/or energy mobilization. The pattern of corticosterone secretion as it relates to migratory flight has hitherto not been clearly described in migrants that frequently interrupt flight to refuel, for example, the Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). The Gambel's white-crowned sparrow fuels by day and expresses peak migratory activity during the first few hours of night. To determine if plasma corticosterone increases in association with the stage of migratory flight also in this short-bout migrant, we induced captive white-crowned sparrows to enter into the migratory condition by placing photosensitive birds on long days (16L:8D) and then evaluated birds for plasma corticosterone and locomotor activity during four time points of the day. Patterns found in long-day birds were compared to those observed in short-day controls (8L:16D). Differences in energy metabolism as determined from plasma metabolites were also evaluated. We found that locomotor activity and corticosterone were significantly elevated at the onset of the dark period, but only in long-day birds. Plasma beta-hydroxybutyrate (a ketone body) was also elevated. Thus, findings suggest that plasma corticosterone and ketogenesis increase in association with migratory restlessness in a short-bout migrant. In fact, corticosterone may play a regulatory role, because it shows a trend to increase already before night-time activity.

Adaptation, Physiological↗

PubMed2004

Nonbreeding season pairing behavior and the annual cycle of testosterone in male and female downy woodpeckers, Picoides pubescens.

Studies in birds show that testosterone (T) concentrations vary over the annual cycle depending on mating system and life history traits. Socially monogamous species show pairing behavior throughout the year and low levels of male-male aggression and are underrepresented in these studies, yet the function of testosterone could be particularly important for sexual and social interactions occurring outside the breeding season. We measured fecal T concentrations over the annual cycle and the frequency of interactions between male and female downy woodpeckers (Picoides pubescens) from late fall through early spring. We validated the fecal assay by collecting blood in conjunction with a subsample of our fecal samples: fecal T correlated with circulating levels in the blood. The annual peak level of T in males was relatively low and short-lived, similar to that of other bird species with low levels of male-male aggression and high paternal care. The annual cycle of female T resembled the male pattern, and the ratio of male T to female T was close to 1.0. Likewise, the frequency of aggression among females was similar to the frequency among males. Overall, testosterone levels in both sexes were variable, even in winter. In other bird species, sexual behavior during nonbreeding periods correlates with circulating levels of T in males. Based on this observation, we tested the hypothesis that T in winter was positively related to the frequency of interaction between mated downy woodpeckers. The results showed no such relationship. We discuss this finding and further relate the annual cycle of T in both males and females to behaviors that appear to facilitate mate choice and retention of the pair bond during conspecific challenge.

Animals↗

PubMed2004

No latitudinal differences in adrenocortical stress response in wintering black-capped chickadees (Poecile atricapilla).

Birds respond to deterioration in environmental conditions by elevating their corticosterone levels, which can enhance their survival. It is less clear if animals constantly living in energetically challenging environment show similar increases in adrenocortical function. Previous work has demonstrated that under controlled conditions black-capped chickadees (Poecile atricapilla) from northern latitudes cache more food and perform better on spatial memory tasks than their southern conspecifics. As elevated levels of corticosterone have been shown previously to correlate with spatial memory performance in chickadees, this study aimed to investigate whether black-capped chickadees from northern latitudes have elevated baseline levels of corticosterone and/or a stronger adrenocortical stress response than their southern conspecifics, irrespective of their immediate environment. We found no differences between Alaskan and Colorado chickadees maintained under identical conditions for 3 months in either baseline levels of corticosterone or maximum levels of corticosterone achieved during the stress response. Baseline corticosterone levels were negatively correlated with relative body mass across both groups of birds. Our results suggest that the population differences in food catching behavior and spatial memory were not related to differences in corticosterone levels. We conclude that many reported population differences in baseline levels and in strength of adrenocortical stress response may often reflect differences in local environmental conditions rather than population-specific physiological traits.

Adrenal Cortex↗

PubMed2004

Evidence for a novel gonadotropin-releasing hormone in hypothalamic and forebrain areas in songbirds.

One or more of three distinct forms of gonadotropin-releasing hormones (GnRHs), has been found in all vertebrates studied. Birds are known to contain two types of GnRH, chicken GnRH-I and -II (cGnRH-I and -II), but to date nobody has investigated the possibility of the presence of lamprey GnRH-III in birds. Thus, our objective was to investigate the presence and distribution of immunoreactive lamprey (ir-lamprey) GnRH-III in hypothalamic and forebrain areas in house sparrows (Passer domesticus) and Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii). In addition to its distribution in the CNS, we investigated whether or not exogenous lamprey GnRH-III peptide could elicit luteinizing hormone (LH) release in vivo. This study presents two novel findings: 1) The widespread presence in the central nervous system of an immunoreactive third GnRH in birds that is clearly hypophysiotropic and has gonadotropin-releasing capabilities. Intravenous injection of 100 ng lamprey-GnRH-III peptide elicited a rapid rise in circulating luteinizing hormone as compared to controls. This third GnRH, ir-lamprey GnRH-III, likely has multiple functions, as suggested by its widespread distribution. 2) Ir-lamprey GnRH-III is present in abundance in telencephalic areas, including the hippocampal formation and the song control system. In no vertebrate has a GnRH been localized in these 'higher' control regions before. The fact that ir-lamprey-GnRH-III is present in auditory processing areas (higher vocal center, or HVc, area X, caudomedial neostriatum) and song-producing areas (HVc, robust nucleus of the neostriatum, or RA) suggests a direct way in which auditory processing and song production can affect circulating gonadotropins.

Animals↗

PubMed2004

The low-affinity glucocorticoid receptor regulates feeding and lipid breakdown in the migratory Gambel's white-crowned sparrow Zonotrichia leucophrys gambelii.

Plasma corticosterone increases during spring migration in a variety of bird species, including the Gambel's white-crowned sparrow Zonotrichia leucophrys gambelii. Corticosterone is elevated specifically in association with migratory flight, suggesting that corticosterone may promote processes such as energy mobilization and/or migratory activity. General effects of glucocorticoids support such a prediction. Because glucocorticoids exert permissive effects on food intake, corticosterone may also participate in the regulation of migratory hyperphagia. To examine the role of corticosterone during migration, we induced Gambel's white-crowned sparrows to enter the migratory condition and compared food intake and locomotor activity between controls and birds injected with RU486--an antagonist to the low-affinity glucocorticoid receptor (GR). In addition, we investigated effects of RU486 in birds that were subjected to a short-term fast. Results indicate that RU486 did not affect locomotor activity. However, consistent with its effects in mammals, RU486 suppressed food intake. Thus, hyperphagia and migratory restlessness, the two behaviors that characterize migration, may be regulated by different mechanisms. Lastly, RU486 antagonized fasting-induced lipid mobilization, as evidenced by decreased plasma free fatty acids. Thus, data on spring migrants suggest that endogenous corticosterone levels act through the GR to support hyperphagia and that the GR promotes availability of lipid fuel substrates in association with periods of energetic demand, e.g. during migratory flight.

Analysis of Variance↗

PubMed2003

Modulation of the hypothalamic-pituitary-adrenal axis of an Arctic-breeding polygynandrous songbird, the Smith's longspur, Calcarius pictus.

To successfully reproduce in the Arctic, birds must modulate their neuroendocrine and behavioural systems. These adjustments include an attenuation of the stress responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to external stimuli and a behavioural insensitivity to high corticosterone (B) levels. The HPA axis was examined in free-living territorial polygynandrous Smith's longspurs (Calcarius pictus) that migrate to breed on the Arctic tundra. Basal and stress-induced B levels were measured through the breeding season and were found to be significantly lower in females compared with males. This was not a consequence of adrenal insensitivity, because intrajugular injections of adrenocorticotrophin hormone (ACTH) enhanced B release in incubating females. In males the adrenocortical response to stress was significantly attenuated during the parental phase compared with arrival at the breeding ground. In contrast to temperate passerines, there was no significant decrease in male territorial aggressive behaviour when B was experimentally elevated, suggesting a behavioural insensitivity to glucocorticoids. This mechanism is hypothesized to increase reproductive success by preventing interruptions to parental care during transient deleterious environmental perturbations, which are often experienced in the short Arctic breeding season. Modulation of the HPA axis in this species in relation to life-history stage, lifetime reproductive success and the polygynandrous mating system is discussed.

Adrenocorticotropic Hormone↗

PubMed2003

Brain aromatase, 5 alpha-reductase, and 5 beta-reductase change seasonally in wild male song sparrows: relationship to aggressive and sexual behavior.

In many species, territoriality is expressed only during the breeding season, when plasma testosterone (T) is elevated. In contrast, in song sparrows (Melospiza melodia morphna), males are highly territorial during the breeding (spring) and nonbreeding (autumn) seasons, but not during molt (late summer). In autumn, plasma sex steroids are basal, and castration has no effect on aggression. However, inhibition of aromatase reduces nonbreeding aggression, suggesting that neural steroid metabolism may regulate aggressive behavior. In wild male song sparrows, we examined the neural distribution of aromatase mRNA and seasonal changes in the activities of aromatase, 5 alpha-, and 5 beta-reductase, enzymes that convert T to 17 beta-estradiol, 5 alpha-dihydrotestosterone (5 alpha-DHT, a potent androgen), or 5 beta-DHT (an inactive metabolite), respectively. Enzyme activities were measured in the diencephalon, ventromedial telencephalon (vmTEL, which includes avian amygdala), caudomedial neostriatum (NCM), and the hippocampus of birds captured during spring, molt, or autumn. Aromatase and 5 beta-reductase changed seasonally in a region-specific manner. Aromatase in the diencephalon was higher in spring than in molt and autumn, similar to seasonal changes in male sexual behavior. Aromatase activity in the vmTEL was high in both spring and autumn but significantly reduced at molt, similar to seasonal changes in aggression. 5 beta-Reductase was not elevated during molt, suggesting that low aggression during molt is not a result of increased inactivation of androgens. These data highlight the relevance of neural steroid metabolism to the expression of natural behaviors by free-living animals.

Aggression↗

PubMed2003

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.

Adaptation, Physiological↗

PubMed2003

Androgens and estrogens induce seasonal-like growth of song nuclei in the adult songbird brain.

In seasonally breeding songbirds, the brain regions that control song behavior undergo dramatic structural changes at the onset of each annual breeding season. As spring approaches and days get longer, gonadal testosterone (T) secretion increases and triggers the growth of several song control nuclei. T can be converted to androgenic and estrogenic metabolites by enzymes expressed in the brain. This opens the possibility that the effects of T may be mediated via the androgen receptor, the estrogen receptor, or both. To test this hypothesis, we examined the effects of two bioactive T metabolites on song nucleus growth and song behavior in adult male white-crowned sparrows. Castrated sparrows with regressed song control nuclei were implanted with silastic capsules containing either crystalline T, 5alpha-dihydrotestosterone (DHT), estradiol (E(2)), or a combination of DHT+E(2). Control animals received empty implants. Song production was highly variable within treatment groups. Only one of seven birds treated with E(2) alone was observed singing, whereas a majority of birds with T or DHT sang. After 37 days of exposure to sex steroids, we measured the volumes of the forebrain song nucleus HVc, the robust nucleus of the archistriatum (RA), and a basal ganglia homolog (area X). All three steroid treatments increased the volumes of these three song nuclei when compared to blank-implanted controls. These data demonstrate that androgen and estrogen receptor binding are sufficient to trigger seasonal song nucleus growth. These data also suggest that T's effects on seasonal song nucleus growth may depend, in part, upon enzymatic conversion of T to bioactive metabolites.

Androgens↗

PubMed2003

Effects of temperature on photoperiodically induced reproductive development, circulating plasma luteinizing hormone and thyroid hormones, body mass, fat deposition and molt in mountain white-crowned sparrows, Zonotrichia leucophrys oriantha.

The mountain white-crowned sparrow, Zonotrichia leucophrys oriantha, breeds in subalpine meadows throughout many mountainous regions of western North America. Mathematical analysis of 20 years of egg-laying dates at Tioga Pass, California (3030m elevation) indicated a highly predictable breeding season suggesting that precise environmental cues such as the annual change in day length were important for regulating reproductive function. Additionally, it appeared that there was sufficient yearly variation in the timing of breeding to suggest that other environmental cues may also be important for regulating adjustments in reproductive development and regression. Captive populations of Z. l. oriantha showed strong responses in gonadal development following transfer to longs days (15L 9D) and low temperature (5 degrees C) slowed down photoperiodically induced gonadal growth and subsequent regression, in both males and females. High temperature of 30 degrees C tended to accelerate gonadal development and regression whereas gonadal development was intermediate in a group exposed to 20 degrees C. Prior exposure to these temperature regimes while on short days (9L 15D) had no effect on body mass, fat, or plasma levels of luteinizing hormone (LH) and thyroid hormones. Curiously there was no effect of temperature on photoperiodically induced rises in LH in either sex despite marked effects on gonadal growth. Brood patch development was also enhanced in females exposed to 30 degrees C. Corticosterone levels measured in a subset of plasma samples from this experiment indicated no effect of temperature suggesting that the retarded gonadal development at 5 degrees C was not a result of thermal stress. Although there was a robust effect of photostimulation on thyroid hormone levels in blood of both sexes, temperature treatment had no effect on tri-iodothyronine (T3) concentrations. However, plasma levels of thyroxine (T4) were lower initially at 5 degrees C versus 20 and 30 degrees C treatments. This may be related to the protracted gonadal cycle at 5 degrees C versus the truncated gonadal cycle at 30 degrees C. Molt score, an indication of post-reproductive state and onset of photorefractoriness, was delayed in birds exposed to 5 degrees C. Body mass, and to a lesser extent fat score, tended to be lowest in birds exposed to 5 degrees C compared with those at 20 and 30 degrees C. These results demonstrate that ambient temperature significantly affected photoperiodically induced gonadal development and regression in these birds. The endocrine mechanisms underlying these effects require further study.

Adipose Tissue↗

PubMed2003

Short-term fasting affects locomotor activity, corticosterone, and corticosterone binding globulin in a migratory songbird.

Unpredictable events such as severe storms lead to an increase in circulating corticosterone (CORT) in breeding birds. This increase is often accompanied by elevations in foraging and irruptive behavior. We were interested in determining if acute food restriction (such as might occur during inclement weather) is a sufficient cue to elicit an increase in locomotor activity, increase CORT secretion, and/or decrease circulating levels of corticosterone binding globulin (CBG) in white-crowned sparrows (Zonotrichia leucophrys gambelii). Male Z.l. gambelii were housed individually in environmental chambers on long days (LD 20:4) to simulate breeding season daylength. Birds were fed ad libitum, and on select days, food was removed 2 h after lights on (fasted treatment), or was removed and replaced (control). We analyzed CORT and CBG levels after 1, 2, 6, 22 (lights on), and 23 h under fasted and control conditions. We also measured activity during the 23-h experiment. Activity levels were increased under fasted conditions during the daytime relative to control conditions, but activity levels did not differ between treatments during the night. Fasting as little as 1, 2, and 6 h significantly increased total CORT levels above baseline (control), although after 22 h, total CORT levels under fasted conditions matched those under control conditions. Plasma CBG decreased after the 22-h fast, and remained low after the 23-h fast. This change was sufficient to significantly elevate free CORT levels in fasted birds relative to ad libitum food conditions, despite the lack of difference in total CORT levels.

Animal Migration↗

PubMed2003

The concept of allostasis in biology and biomedicine.

Living organisms have regular patterns and routines that involve obtaining food and carrying out life history stages such as breeding, migrating, molting, and hibernating. The acquisition, utilization, and storage of energy reserves (and other resources) are critical to lifetime reproductive success. There are also responses to predictable changes, e.g., seasonal, and unpredictable challenges, i.e., storms and natural disasters. Social organization in many populations provides advantages through cooperation in providing basic necessities and beneficial social support. But there are disadvantages owing to conflict in social hierarchies and competition for resources. Here we discuss the concept of allostasis, maintaining stability through change, as a fundamental process through which organisms actively adjust to both predictable and unpredictable events. Allostatic load refers to the cumulative cost to the body of allostasis, with allostatic overload being a state in which serious pathophysiology can occur. Using the balance between energy input and expenditure as the basis for applying the concept of allostasis, we propose two types of allostatic overload. Type 1 allostatic overload occurs when energy demand exceeds supply, resulting in activation of the emergency life history stage. This serves to direct the animal away from normal life history stages into a survival mode that decreases allostatic load and regains positive energy balance. The normal life cycle can be resumed when the perturbation passes. Type 2 allostatic overload begins when there is sufficient or even excess energy consumption accompanied by social conflict and other types of social dysfunction. The latter is the case in human society and certain situations affecting animals in captivity. In all cases, secretion of glucocorticosteroids and activity of other mediators of allostasis such as the autonomic nervous system, CNS neurotransmitters, and inflammatory cytokines wax and wane with allostatic load. If allostatic load is chronically high, then pathologies develop. Type 2 allostatic overload does not trigger an escape response, and can only be counteracted through learning and changes in the social structure.

Adaptation, Physiological↗

PubMed2003

Avipox sp. in a colony of gray-crowned rosy finches (Leucosticte tephrocotis).

Members of a wild-caught colony of 16 gray-crowned rosy finches (Leucosticte tephrocotis) were presented with dermal and mucosal lesions, anorexia, emaciation, lethargy, and sudden death. Lesions included dermatitis, conjunctivitis, and glossitis. Skin scrapings from and bacterial culture of dermal lesions yielded Staphylococcus aureus and Candida albicans. Necropsy and histologic examination revealed characteristic epidermal and mucosal pox lesions, with the presence of characteristic Bollinger body intracellular inclusions. Electron microscopy (EM) provided confirmation of pox virus infection. This epornitic resulted in the death or euthanasia of 12 birds (75% morbidity and associated mortality) and was brought to conclusion through culling of affected birds. The source of infection remains unknown, although multiple modes of introduction exist. Similar epornitics may be prevented through indoor, species-specific housing, and quarantine. Vaccination and antiparasitic treatment may reduce the risk of disease spread.

Animals↗

PubMed2002

Latitudinal variation in plasma testosterone levels in birds of the genus Zonotrichia.

Birds breeding in northern latitudes generally have elevated plasma testosterone levels throughout the breeding season with a peak at the onset of the breeding season. In contrast, tropical birds tend to have extremely low plasma testosterone levels year round with only a slight increase during breeding. While these patterns have been consistent in the species investigated, closely related species have not been investigated across a range of latitudes. Birds of the genus Zonotrichia present an ideal opportunity to investigate latitudinal variation in plasma testosterone levels as breeding populations occur from northern Alaska to southern Argentina. We studied three taxa of Zonotrichia: (1) Gambel's white-crowned sparrows, Zonotrichia leucophrys gambelii, breeding at high latitude in northern Alaska, (2) Puget Sound white-crowned sparrows, Z. l. pugetensis, breeding at mid-latitude in Washington state, and (3) an equatorial population of the rufous-collared sparrow, Z. capensis, in Ecuador. To compare both baseline breeding and maximal testosterone levels, males from the three taxa were either bled immediately upon capture during the breeding season or first challenged with gonadotropin-releasing hormone (GnRH) and then bled. As a control, another group of birds experienced a saline challenge. Two-way ANCOVA, with treatment and taxa as factors and testis volume as a covariate, showed a significant effect of treatment on plasma testosterone levels. There was also a significant interaction between taxa and treatment. Contrary to expectations, the equatorial species exhibited greater plasma testosterone levels in response to the GnRH challenge than the high latitude species. There were no differences between the mid- and high-latitude species. The equatorial species had the smallest average testis size and within each taxa there were no relationships between plasma testosterone and testis size. These data suggest that latitudinal clines in plasma testosterone levels in Zonotrichia do not follow previously described patterns and that tropical birds can have levels of testosterone similar to northern latitude species.

Animals↗

PubMed2002

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.

Adrenal Cortex↗

PubMed2002

Glucocorticoids and parental hyperphagia in ring doves (Streptopelia risoria).

These studies explored the possibility that glucocorticoids promote parental care in ring doves by mediating, at least in part, the pronounced increase in food consumption that parent doves exhibit while provisioning their young. Plasma concentrations of the endogenous glucocorticoid corticosterone were found to be significantly higher in breeding females during the posthatching phase than during the incubation period. These differences were not observed in male breeding partners, but sex differences in daily activity rhythms are well documented in breeding doves, and blood sampling at different times of day would be required to adequately characterize the pattern of corticosterone in males during these breeding stages. In studies on nonbreeding doves, twice-daily intracerebroventricular (icv) injections of the synthetic glucocorticoid dexamethasone (DEX) increased food intake by 25-50% in both sexes, and further studies in males revealed that the increase was directly related to the dose of DEX administered. The highest dose of DEX given icv (1.0 microg/day) was not effective in stimulating feeding when given systemically, thereby suggesting that the hyperphagic action of DEX is exerted directly on the central nervous system. The icv infusion of the selective glucocorticoid receptor antagonist RU38486 blocked the hyperphagic effects of twice-daily icv injections of DEX in both sexes. Collectively, these data support the hypothesis that corticosterone contributes to the parental hyperphagia exhibited by breeding doves during the posthatching period. They also suggest that these orexigenic effects are mediated in part by CNS binding sites that resemble mammalian glucocorticoid receptors.

Animals↗