Circadian variations in plasma LH and FSH in juvenile and adult male mice

Experiments were performed to ascertain circadian fluctuations in plasma levels of LH and FSH in juvenile and adult male mice. Animals under natural lighting (11 h day/13 h night) were killed at 1-hour intervals over a 24-hour period. There were large variations in plasma LH concentrations between animals sacrificed within each killing period. Baseline LH levels (values lower than 60 ng/ml) showed a significant 24-hour periodicity in adult males. FSH concentrations exhibited significant diurnal variations in juvenile and adult males. There was significant influence of age on the temporal pattern and 24-hour mean plasma hormone levels.     

Effect of neonatal androgenization on the testosterone feedback sensitivity in adult rats in two lighting conditions

Neonatally androgenized and intact adult male Wistar rats received daily, during 1 week, either testosterone propionate or sesame oil injections in periodic or constant light. Serum and pituitary gonadotropins and hypothalamic LHRH were measured. In periodic light, neonatal androgenization did not change the serum concentration or pituitary contents of gonadotropins, or the hypothalamic content of LHRH. Testosterone injections decreased serum concentration and pituitary content of gonadotropin of intact rats but failed to decrease the pituitary gonadotropin content of neonatally androgenized rats. In constant light, serum FSH was decreased in neonatally androgenized rats. Testosterone injections decreased both serum LH and FSH concentrations of intact rats but only serum LH of androgenized rats. Pituitary gonadotropin and hypothalamic LHRH contents remained unchanged. We conclude that neonatal androgenization renders the male rat hypothalamo-pituitary axis more resistant to changes of testosterone concentration in adulthood. Constant light did not sensitize the neonatally androgenized rats to testosterone, but on the contrary, testosterone injections were less effective in constant than in periodical light.     

Developmental patterns of FSH and LH in female rats deprived of light before puberty.

Plasma FSH and LH levels were examined in female rats reared in the dark at different ages from birth until sexual maturation to investigate whether, and to what extent, external factors such as light, influence gonadotropin levels during development. Control animals were raised in diurnal lighting consisting of 12 hours of light and 12 hours of dark. Light deprivation did not eliminate the characteristic peak of gonadotropins seen in early postnatal development but significantly increased levels of FSH and slightly decreased levels of LH (except for a transient rise at day 12). Constant darkness tended to lower whole body, ovarian and pituitary weights but to increase pineal weight. Whereas the time of eye-opening was the same in control and light-deprived animals, puberty (as judged by vaginal opening and first ovulation) was delayed in animals raised in the dark. The data suggest that environmental light has a mediating action on patterns of gonadotropin release, particularly on FSH, during prepuberal development.     

Circadian rhythm of blood glucose and tissue glycogen in fed and fasted rats

Blood glucose and tissue glucogen circadian rhythms were determined in male Wistar rats adapted 3 weeks to an artificial lighting regimen of 12 hours' light and 12 hours' darkness. Over a period of 24 hours we examined at 3-hour intervals the blood glucose concentration and the glycogen content of the liver, heart, skeletal muscle (quadriceps femoris) and white (epididymal) and brown (interscapular) adipose tissue of fed rats and rats fasted for 24 hours. The experiments were carried out in the autumn and the results were evaluated statistically by an analysis of variance and the cosinor test. The blood glucose level and the glycogen concentration in all the given tissues, in both fed and starved rats, displayed rhythmic oscillations with a 24- or 12-hour period in the course of the day, with the exception of glycogen in the white adipose tissue of fed rats, in which cosinor analysis failed to demonstrate any rhythm. One day's fasting did not affect the character of circadian rhythm.     

The relation of hepatic in vitro inactivation of corticosteroids to the circadian rhythm of plasma corticosterone.

Plasma corticosterone levels and the in vitro capacity of the liver to reduce the delta(4)3-ketone group of corticosterone were ascertained at 4 hr intervals in male rats maintained on a normal lighting schedule (12L:12D). The rates of delta(4)3-ketone reduction, as well as wet liver weight, were highest during the early portion (08.00 hr) of the light period when liver protein and plasma corticosterone concentrations were low. Shortly (2000 hr) after the beginning of the dark period plasma corticosterone reached peak levels, while hepatic inactivation of corticosterone was markedly depressed. This inverse relationship suggests that the rhythmicity in the capacity of the liver to inactivate corticosterone may contribute to the circadian periodicity of plasma corticosterone.     

Entrainment of circadian rhythms of cholesterol‐LDL,corticosterone and motor activity by feeding schedules in rats

Abstract

The daily variations of locomotor activity, plasma and adrenal corticosterone levels and cholesterol‐LDL were studied in male Wistar rats with food ad libitum and feeding restricted to the first 4 hours of the light phase in LD 12:12..

Under LD 12:12 (light on from 9:00 to 21:00h) rats with food ad libitum were eating and moving during the dark period and the locomotor activity clearly showed a biphasic pattern with three harmonic components. Plasma and adrenal corticosterone levels increased during the light period and reached a maximum value just before the dark period whereas the acrophase of cholesterol‐LDL is found at the beginning of the light phase.

The acrophases of activity, plasma and adrenal corticosterone levels in the restricted feeding schedule rats occurred in the first three hours of lighting and the cholesterol‐LDL acrophase at the beginning of the dark phase.

These results confirm a previous report that the shift of feeding to the light phase seems to cause a concomitant phase‐shift in all the variables measured.     

Influence of light regimens on circadian changes in the blood glucose and tissue glycogen concentration in the rat

Adult male Wistar rats allowed food and drinking water ad libitum were kept 2- 5 weeks under standard conditions, but with a different artificial light regimens. The standard regimen was 12:12 h light and dark (LD) and the other variants were 12:12 h dark and light (DL), continuous darkness (DD) and continuous light (LL). The blood glucose level and the liver, skeletal muscle, heart and brown and white adipose tissue glycogen concentration were tested during the day at 3-hour intervals. The experiments were carried out during the winter and were evaluated by an analysis of variance and the cosinor test. Circadian variation of the blood glucose level and the liver and both the adipose tissue glycogen concentrations was only slightly affected by changes in the light regimen. Conversely, the oscillations of the skeletal muscle and heart glycogen concentration reacted markedly to the variants of the light regimen, their reaction being manifested in the localization of the acrophases in different parts of the day, especially when comparing the DD and LD regimens.     

The effect of testosterone on serum gonadotropins of castrated rats kept under different lighting conditions.

Testosterone feedback sensitivity was measured as the ability of testosterone propionate to decrease serum LH and FSH of long-term castrated (4 wk) rats under four different lighting conditions: periodic light (12L:12D), constant light (LL), constant darkness (DD), and dim night illumination (1 lx) with a 12L:12D photoperiod. Rats were exposed to the different lighting conditions for 1 wk, during which they received daily testosterone propionate (125 micrograms or 250 micrograms s.c.). At the end of the experiment the rats were decapitated at 1100 h, and serum gonadotropin levels were measured by RIA. Serum LH of the rats kept under LL was reduced to the level of the intact rats with the smaller testosterone dose (125 micrograms/day). Under all other lighting conditions only the large dose (250 micrograms/day) was able to restore the serum LH concentration to the level of the intact rats. Serum FSH was restored only partially, and the effect was the same with both doses and similar under all lighting conditions. We conclude that the increase in testosterone negative feedback sensitivity was not caused by the lack of periodicity of illumination alone, but that sufficient intensity of lighting throughout the 24 h was needed as well.     

Comparison of the effects of CRF and stress on levels of pituitary cyclic AMP and plasma ACTH in vivo

We have previously reported that acute stress increases levels of rat pituitary cyclic AMP in vivo. The present study was conducted to test the hypothesis that stress-induced increases in pituitary cyclic AMP in vivo were mediated via CRF. We compared the effects of various stressors with the effects of CRF or epinephrine administration on pituitary cyclic AMP and plasma ACTH responses in vivo. Stressors, epinephrine or CRF increased levels of pituitary cyclic AMP. Pituitary cyclic AMP response to either immobilization or CRF was much greater at light onset than at lights off in rats maintained on at 12 hr light: 12 hr dark lighting regimen. In rats with pituitary stalk transections, footshock did not increase levels of pituitary cyclic AMP, suggesting that some factor of central origin was required for this stress response. Exogenous CRF administration did increase levels of pituitary cyclic AMP in stalk-transected rats, while epinephrine increased levels in sham-operated but not in stalk-transected rats. Antisera to CRF markedly decreased pituitary cyclic AMP response to exogenous CRF administered 6 min following antisera and partially attenuated pituitary cyclic AMP response to forced running. Taken as a whole these data support a major role for CRF in the pituitary cyclic AMP response to stress.     

Environmental lighting and muricidal behaviour in the male Wistar rat.

Effects of different conditions of environmental lighting on the appearance of the muricidal behaviour in male Wistar rats have been studied. The animals were kept under different conditions of environmental lighting: 1) natural day light alternated with the dark of the night; 2) sodium, continuous light emitted by a sodium steam lamp; 3) neon, continuous light emitted by fluorescent neon tubes. The continuous sodium steam light increased the percentage of animals becoming muricide when compared to animals bred in a natural environment with a normal succession of day-night lighting. On the contrary, this percentage decreased if the rats of the same group are exposed to continuous light emitted by fluorescent neon tubes. As the exposure of rats to an environment under continuous light causes a reduction of the cerebral content of serotonin, the muricidal behaviour provoked in naturally non-muricide rats by this type of lighting could be related to this depletion.     

Maternal influences on the ontogeny of basal levels of plasms corticosterone in the rat.

To study the effect of maternal factors on circadian periodicity in plasma corticosterone in weanling rats, base levels of this steroid were assessed at 4-hr intervals over a 24-hr period in both blind and normal offspring of blind or normal dams. Blind litters reared by blind mothers do not show a daily periodicity in plasma corticosterone levels. However, blind litters reared by normal mothers do display significant daily fluctuations in the plasma levels of corticoids. All normal weanlings exhibit a significant daily periodicity in plasma corticosterone. Peak levels of corticosterone in normal weanlings, which were separated from their dams at Day 21 or reared by blind dams, were present at the onset of the dark period (2130). However, in normal weanlings which were not separated from their mothers, maximal corticosterone occurred several hours after lights off. These results demonstrate that the pattern of daily periodicity in plasma corticosterone levels in the weanling rat can be influenced both by maternal factors and by the light cycle.     

Changes in the gonadotropic function of the rat hypophysis during development of experimental alcoholism

Luteinizing hormone (LH) and prolactin blood plasma levels and LH level in the pituitary of alcoholic male rats were studied Alcoholic rats (heavy drinkers) have revealed hyperprolactinemia and inadequate LH secretions. The possible role of gonadotropins in the development of hypogonadism in alcoholic rats is discussed.     

Day–Night Differences in Membrane‐Bound Pyroglutamyl‐2‐Naphthylamide‐Hydrolyzing Activity in Rat Hypothalamus,Pituitary, and Retina

Membrane‐bound pyroglutamyl‐2‐naphthylamide‐hydrolyzing enzyme activity was analyzed fluorometrically in the anterior hypothalamus, pituitary, and retina of adult male rats to investigate day–night differences. Six groups (n=6 per group) were assessed—three during the light span and three during the dark span—under a standard 12 h–12 h light–dark cycle (light on from 07:00 to 19:00 h) and controlled temperature environment, with food and water available ad libitum. In the hypothalamus, enzyme activity levels were higher for time points of the dark than the light period. In contrast, the pituitary and retina exhibited the highest levels at the time points of the light period. The pituitary and retina also exhibited significant differences between the clock‐hour means of the light period. Day–night differences in membrane‐bound pyroglutamyl‐2‐naphthylamide‐hydrolyzing activity may reflect differences in its susceptible endogenous substrates.     

Stressful colours: corticosterone concentrations in a free-living songbird vary with the spectral composition of experimental illumination

Organisms have evolved under natural daily light/dark cycles for millions of years. These cycles have been disturbed as night-time darkness is increasingly replaced by artificial illumination. Investigating the physiological consequences of free-living organisms in artificially lit environments is crucial to determine whether nocturnal lighting disrupts circadian rhythms, changes behaviour, reduces fitness and ultimately affects population numbers. We make use of a unique, large-scale network of replicated field sites which were experimentally illuminated at night using lampposts emanating either red, green, white or no light to test effect on stress hormone concentrations (corticosterone) in a songbird, the great tit (Parus major). Adults nesting in white-light transects had higher corticosterone concentrations than in the other treatments. We also found a significant interaction between distance to the closest lamppost and treatment type: individuals in red light had higher corticosterone levels when they nested closer to the lamppost than individuals nesting farther away, a decline not observed in the green or dark treatment. Individuals with high corticosterone levels had fewer fledglings, irrespective of treatment. These results show that artificial light can induce changes in individual hormonal phenotype. As these effects vary considerably with light spectrum, it opens the possibility to mitigate these effects by selecting street lighting of specific spectra.     

Day-night differences in membrane-bound pyroglutamyl-2-naphthylamide-hydrolyzing activity in rat hypothalamus, pituitary, and retina

Membrane-bound pyroglutamyl-2-naphthylamide-hydrolyzing enzyme activity was analyzed fluorometrically in the anterior hypothalamus, pituitary, and retina of adult male rats to investigate day-night differences. Six groups (n=6 per group) were assessed—three during the light span and three during the dark span—under a standard 12 h-12 h light-dark cycle (light on from 07:00 to 19:00 h) and controlled temperature environment, with food and water available ad libitum. In the hypothalamus, enzyme activity levels were higher for time points of the dark than the light period. In contrast, the pituitary and retina exhibited the highest levels at the time points of the light period. The pituitary and retina also exhibited significant differences between the clock-hour means of the light period. Day-night differences in membrane-bound pyroglutamyl-2-naphthylamide-hydrolyzing activity may reflect differences in its susceptible endogenous substrates.     

Effect of aging on 24-hour pattern of stress hormones and leptin in rats

This work analyzes the 24-hour changes of hypothalamic-pituitary-adrenal (HPA) axis activity and leptin release in aged rats. Three- and 22-month-old male Wistar rats were killed at 6 time intervals during a 24-hour cycle (n=8-10 rats/group). Aging augmented plasma ACTH while it decreased plasma and adrenal gland corticosterone levels. Plasma and adrenal corticosterone levels attained high levels during all the scotophase, concomitantly with the maxima in ACTH levels, whereas in aged rats only a brief plasma corticosterone peak at the early scotophase and no time of day variations of adrenal corticosterone were observed. Aging augmented circulating leptin, with a significant interaction "agextime" in the factorial ANOVA, i.e. only in young rats time of day changes were significant, with the lowest values of leptin at the middle of the light period and higher values at night. When plasma leptin was expressed on body weight basis, the age-related differences became not significant but the daily pattern of plasma leptin found in young rats persisted. Plasma and adrenal corticosterone levels correlated significantly with plasma ACTH only in young rats. Likewise, plasma leptin correlated with plasma corticosterone only in young rats. These changes can be attributed to a disrupting effect of aging on the homeostatic mechanisms modulating HPA activity and leptin release.     

Postcastration rise in plasma gonadotropins is blocked by a luteinizing hormone-releasing hormone antagonist

The dependence of the acute increases in plasma gonadotropins following castration on luteinizing hormone-releasing hormone (LHRH) was assessed with the use of a potent LHRH antagonist [ALHRH; (Nac-L-Ala1,p-Cl-D-Phe2,D-Trp3,6) LHRH]. Blood samples were collected from male and female rats at the time of castration and 2, 4, 8, 12, 24 and 48 h following and plasma gonadotropin levels were determined. Immediately following castration (diestrus I for females) animals received one of the following treatments: females-vehicle, 100 micrograms ALHRH, 50 micrograms estrogen benzoate (EB), or 100 micrograms ALHRH + 50 micrograms EB; males-vehicle, 100 micrograms ALHRH, 500 micrograms testosterone propionate (TP), or 100 micrograms ALHRH + 500 micrograms TP. ALHRH blocked the selective increase in plasma follicle-stimulating hormone (FSH) observed in female rats as well as the parallel increases in both gonadotropins seen in male rats following castration. Administration of EB or ALHRH + EB to females significantly suppressed both gonadotropins compared with control levels. However, EB alone did not completely block the rise in plasma FSH in females. In males, all three treatments significantly suppressed the increases in both gonadotropins when compared with control levels. These data demonstrate that hypothalamic LHRH plays an essential role in the acute elevations of plasma gonadotropins following castration in rats. In addition, these data suggest that the selective rise of FSH in females is dependent on LHRH stimulation of pituitary gonadotropes.     

Daily and annual variations of free fatty acid, glycerol and leptin plasma concentrations in goats (Capra hircus) under different photoperiods

The aim of the study was to differentiate the impact of lighting conditions and feeding times on the regulation of lipid metabolism of goats under different photoperiods throughout the year. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60 degrees N (the longest light period 18 h, the shortest 6 h). Ambient temperature and feeding regime were kept constant. Blood samples were collected six times a year at 2-h intervals for 2 days, first in light/dark (LD) conditions and then after 3 days in constant darkness (DD). Significant daily variations were detected in the concentrations of plasma free fatty acids (FFA) and glycerol throughout the year. The nocturnal decrease and morning rise of FFA levels were related to the photoperiod, while the trough levels of glycerol were associated with the concentrate meal times. In DD conditions, FFA and glycerol rhythms were unstable. A significant seasonal variation was detected in the overall FFA and glycerol levels suggesting decreased lipogenesis in winter, increased lipolysis in spring and high lipogenesis in summer and fall. There was no significant daily rhythm in serum leptin levels, nor did the profiles in LD and DD conditions differ. The leptin level was slightly lower in early fall than in the other seasons, paralleling a small decrease of body mass in the goats after the grazing season. The daily or annual variations of FFA and glycerol levels were not clearly related to leptin concentrations. The results suggest that lipid metabolism of goats is regulated by light even in constant temperature and feeding conditions; however, no significant contribution of leptin levels could be shown.     

Floral Inhibition of Biloxi Soybean During a 72-hour Cycle

The inhibitory effect of light interruptions given during the photophobe phases of a 72-hour cycle was studied with Biloxi soybean [Glycine max (L.) Merr.]. The basic 72-hour cycle consisted of 8 hours of light followed by 64 hours of darkness and was repeated 7 times. Supplementary white light treatments given at the twenty-fourth and/or forty-eighth hour of the cycle (photophil phases) promoted the flowering levels of the controls and kept light treatments given at the most inhibitory points from inhibiting flowering completely. Such supplementary light treatments did not affect the time of maximum sensitivity to light interruptions. When 30-minute light breaks were used, maximum inhibition occurred at the 16-, 43-, and 63-hour points. The duration of the light breaks affected the time of maximum inhibition when given during the second photophobe phase. The time of maximum inhibition occurred earlier with 4-hour light breaks than with either 3-minute or 2-hour light interruptions.

Three-minute red light interruptions produced essentially the same effect as 3-minute white light interruptions. Such treatments inhibited flowering completely in the first photophobe phase, inhibited flowering to only a small degree in the second photophobe phase, and inhibited flowering to an intermediate degree in the third photophobe phase. Far-red light interruptions strongly inhibited flowering in the first photophobe phase, especially when given early in the dark period. Three minutes of supplementary white light given at the twenty-fourth or forty-eighth hour of the cycle partially overcame the inhibitory effect of far-red light. Four hours of supplementary white light at these times completely overcame the far-red inhibition.

     

Entrainment of rhythmic melatonin secretion in man to a 12-hour phase shift in the light/dark cycle

Daily rhythms in melatonin secretion were monitored in four healthy adult males by measuring the melatonin contents of sequential 4-hour urine specimens and of plasma samples collected at 12-hour intervals, or, in one subject, continuously for 24 hours. All subjects exhibited similar diurnal rhythms, with peak urinary melatonin excretion rates and blood melatonin levels occurring during the daily period of darkness and sleep. When the daily light/dark regimen was phase-shifted by 180°, the plasma and urinary melatonin rhythms required 5–7 days (depending on the subject) to re-entrain to the new schedule. Simultaneous measurements of plasma melatonin levels and melatonin excretion rates indicate that urinary melatonin reflects, with remarkable fidelity, circulating melatonin levels.