Circadian clocks: showtime for the adrenal cortex

Differential effects of photophase irradiance on metabolic and urinary stress hormone concentrations in blind and sighted rodents

The effects of different photophase irradiance levels on the daily rhythms of energy expenditure (DEE, calculated from oxygen consumption, VO(2)) and urinary metabolites of stress hormones in sighted (Microtus socialis) and blind (Spalax ehrenbergi) rodents were compared. Five groups of each species were exposed to different irradiance levels (73, 147, 293, 366, and 498 microW/cm(2)) under short photoperiod (8L:16D) condition with constant ambient temperature 25 +/- 2 degrees C for 21 days before assessments. As light intensity increased from 73 microW/cm(2), both species reduced DEE, especially among M. socialis. Cosinor analysis revealed significant ultradian rhythms in VO(2) of M. socialis with period length being inversely related to irradiance level. Conversely, in S. ehrenbergi, robust 24 h VO(2) rhythms were detected at all irradiances. In M. socialis, significant 24 h rhythms in urinary output of adrenaline were detected only at 293 microW/cm(2), whereas for cortisol, unambiguous rhythms were detected at 73 and 147 microW/cm(2). Distinct adrenaline daily rhythms of S. ehrenbergi were observed at 73 and 293 microW/cm(2), whereas this species exhibited significant rhythms in cortisol at 147 and 293 microW/cm(2). Changes in photophase irradiance levels affected stress hormone concentrations in a dose-dependent manner. There were significant negative and positive correlations of M. socialis and S. ehrenbergi stress hormones, respectively, with increasing irradiance. Our results indicate photophase light intensity is another environmental factor that can significantly affect entrainment of mammalian daily rhythms. Both low and high irradiance conditions can trigger stress responses, depending on the species' natural habitat.

Oscillators entrained by food and the emergence of anticipatory timing behaviors

Circadian rhythms are adjusted to the external environment by the light-dark cycle via the suprachiasmatic nucleus, and to the internal environment of the body by multiple cues that derive from feeding/fasting. These cues determine the timing of sleep/wake cycles and all the activities associated with these states. We suggest that numerous sources of temporal information, including hormonal cues such as corticoids, insulin, and ghrelin, as well as conditioned learned responses determined by the temporal relationships between photic and feeding/fasting signals, can determine the timing of regularly recurring circadian responses. We further propose that these temporal signals can act additively to modulate the pattern of daily activity. Based on such reasoning, we describe the rationale and methodology for separating the influences of these diverse sources of temporal information. The evidence indicates that there are individual differences in sensitivity to internal and external signals that vary over circadian time, time since the previous meal, time until the next meal, or with duration of food deprivation. All of these cues are integrated in sites and circuits modulating physiology and behavior. Individuals detect changes in internal and external signals, interpret those changes as "hunger," and adjust their physiological responses and activity levels accordingly.

Cryptochrome 2 expression level is critical for adrenocorticotropin stimulation of cortisol production in the capuchin monkey adrenal

Timely production of glucocorticoid hormones in response to ACTH is essential for survival by coordinating energy intake and expenditure and acting as homeostatic regulators against stress. Adrenal cortisol response to ACTH is clock time dependent, suggesting that an intrinsic circadian oscillator in the adrenal cortex contributes to modulate the response to ACTH. Circadian clock gene expression has been reported in the adrenal cortex of several species. However, there are no reports accounting for potential involvement of adrenal clock proteins on cortisol response to ACTH. Here we explored whether the clock protein cryptochrome 2 (CRY2) knockdown modifies the adrenal response to ACTH in a primate. Adrenal gland explants from adult capuchin monkey (n = 5) were preincubated for 6 h with transfection vehicle (control) or with two different Cry2 antisense and sense probes followed by 48 h incubation in medium alone (no ACTH) or with 100 nm ACTH. Under control and sense conditions, ACTH increased cortisol production, whereas CRY2 suppression inhibited ACTH-stimulated cortisol production. Expression of the steroidogenic enzymes steroidogenic acute regulatory protein and 3beta-hydroxysteroid dehydrogenase at 48 h of incubation was increased by ACTH in control explants and suppressed by Cry2 knockdown. Additionally, we found that Cry2 knockdown decreased the expression of the clock gene brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein (Bmal1) at the mRNA and protein levels. Altogether these results strongly support that the clock protein CRY2 is involved in the mechanism by which ACTH increases the expression of steroidogenic acute regulatory protein and 3beta-hydroxysteroid dehydrogenase. Thus, adequate expression levels of components of the adrenal circadian clock are required for an appropriate cortisol response to ACTH.