Extremely low frequency magnetic field exposure modulates the diurnal rhythm of the pain threshold in mice

The aim of this study was to determine whether exposure to extremely low frequency magnetic field (ELF-MF) affects the normal diurnal rhythm of the pain threshold in mice. Pain thresholds were evaluated in mice using the hot plate test. A significant increase of pain threshold during night was observed compared to that during day. This rhythm was attenuated by both constant exposure to light (LL) and constant exposure to darkness (DD) for 5 days. Under DD exposure, the diurnal rhythm in pain threshold was restored when mice were exposed to ELF-MF (60 Hz, 1.5 mT for 12 h daily, from 08:00 to 20:00 h) for 5 days. The diurnal rhythm was not reversed under dark with reversed ELF-MF cycle (exposure to 1.5 mT from 20:00 to 08:00 h, next day) for 5 days, although pain threshold in the ELF-MF exposed period of night was slightly decreased. The diurnal rhythm of melatonin analgesic effect related to pain threshold was also observed under DD by the exposure of ELF-MF for 5 days, but not for 5 nights. The present results suggest that ELF-MF may participate in the diurnal rhythm of pain threshold by acting on the system that is associated with environmental light-dark cycle.     

Regulation of asexual reproduction in the planarian Dugesia tigrina

We studied asexual reproduction of planarians under the natural and artificial photoperiodic conditions. It was shown that light inhibits the fission of planarians, while darkness stimulates it. The diurnal dynamics of the fission of planarians demonstrated a circadian rhythm. This rhythm is stable, which is expressed when the conditions are experimentally changed: constant darkness, unnatural rhythm of light-darkness succession). However, this stability is affected at the time zone change. The planarians are adapted to new conditions and begin to fission at once in correspondence with the new diurnal regime.     

Circadian mating activity and effect of pheromone pre-exposure on pheromone response rhythms in the moth Spodoptera littoralis

Mating in moths is generally mediated by female-produced sex pheromones. Mating activity, female pheromone production/release and male pheromone responsiveness all show diurnal variations in many species. We found that the response of the male Egyptian cotton leafworm, Spodoptera littoralis, to sex pheromone gland extracts showed a diel rhythm in olfactometer tests, and the variation was persistent for at least 1 day in constant darkness. High male response to sex pheromone was correlated in time with high mating and locomotor activity. Male S. littoralis, maintained in constant darkness and exposed to pheromone gland extracts on a daily basis, showed an induced temporal variation in response after several days, in contrast to unexposed males. This suggests that in the absence of other external zeitgebers, exposure to sex pheromone may function to synchronise circadian behavioural rhythms in male moths. The daily rhythm in mating activity in S. littoralis is also shown to be persistent for at least 2 days in constant darkness. Pairs mated significantly less when either the male or female had been raised in a light:dark cycle 10 h out of phase, indicating that the proposed circadian rhythm in mating activity is composed of rhythmic mating preference/ability in both sexes.     

Regulation of Asexual Reproduction of the Planarians Dugesia tigrina

We studied asexual reproduction of planarians under the natural and artificial photoperiodic conditions. It was shown that light inhibits the fission of planarians, while darkness stimulates it. The diurnal dynamics of the fission of planarians demonstrated a circadian rhythm. This rhythm is stable, which is expressed when the conditions are experimentally changed: constant darkness, unnatural rhythm of light-darkness succession). However, this stability is affected at the time zone change. The planarians are adapted to new conditions and begin to fission at once in correspondence with the new diurnal regime.     

Responsiveness to light of the circadian clock controlling eclosion in the blowfly, Lucilia cuprina

ABSTRACT. Eclosion in Lucilia cuprina (Wiedemann) occurs near dawn. The rhythm of eclosion persists in both darkness and constant light of high intensity (490μW cm^-2) with a period close to 24h. The sensitivity to light of the circadian clock controlling eclosion varies greatly according to the stage of the life cycle. During larval life the free running rhythm in darkness can be phase shifted by light pulses of 100μW cm^-2 intensity, with the transition from a Type 1 phase response curve to a Type 0, occurring with pulses of between 1 and 8h. Extending the last light period of LD to 24 h followed by constant darkness resets the phase of the rhythm by 12h, a transition from constant light to constant darkness initiates rhythmicity in flies made arrhythmic by being reared from eggs collected from adults maintained in constant light. After pupariation, the rhythm is relatively insensitive to light. Rhythmicity is sometimes induced by a transition from constant light to constant darkness, but the phase of the rhythm is not shifted by extending the last light period of LD before entering constant darkness. Repeated LD cycles applied after pupariation initiate and entrain the rhythm.     

Photoperiod influences endogenous rhythm of ambient temperature selection by the honeybee Apis mellifera

The aim of this study was to evaluate whether the day–night cycle phase is a critical factor modulating diurnal rhythm of isolated honeybee's thermal preference or other factors are involved. The insects were exposed to standard (LD 12:12) and reversed (DL 12:12) photoperiods as well as to constant light and constant darkness conditions. Thermal preference and motor activity of honeybees were recorded for 3–5 days in a thermal gradient system. Under the standard (control) photoperiod conditions mean values of temperature selected by honeybees changed rhythmically within the period of about 24 h. Honeybees, exposed to the modified light–darkness cycle distinctly modified their rhythm of thermal preference. Under the reversed photoperiod conditions period of selected ambient temperature was much longer than before, until a complete reversal of the circadian oscillation was established at the end of the experiment. Experiments performed under constant light and constant darkness yielded undisturbed 24 h rhythms of both ambient temperature selection and locomotor activity. Under these conditions only a slight, nonsignificant flattening of the temperature selection curves was noticed. Both lack of substantial changes in the amplitude and occurring phase shifts of the rhythm, recorded in our experiments suggest its endogenous character. Our results prove that diurnal rhythm of ambient temperature selection by bee workers may be entrained by light–dark cycles. This implies a critical role of photoperiod in the modulation of nychthemeral oscillations of thermal preference in honeybees.     

Effects of changes in environmental lighting upon levels of hydroxyindole O-methyltransferase activity in the developing-chick gland.

The level of hydroxyindole O-methyltransferase (HIOMT) activity in the pienal gland of developing chicks raised under constant illumination rose more rapidly and to higher values than in the gland of birds maintained in constant darkness. Rates of net increase in activity, and levels of activity attained, for birds raised under a diurnal cycle of illumination were intermediate between those maintained in constant light or darkness. Under each of the lighting conditions, the course of increase in enzymic activity was markedly affected by variations in an unidentified factor, the source of which appeared to be the hatching eggs. Birds transferred from constant light to the dark showed either an arrest of increase in enzyme activity or a loss of activity until the levels equalled that observed for chicks of the same age raised in constant darkness. Chicks transferred from constant darkness to constant illumination showed marked increases in levels of enzyme activity at rates comparable with the maximal values observed with birds maintained under constant illumination, regardless of age and without delay. No diurnal cycle in level of HIOMT activity was observed in the pineals of 15-day birds.     

The effects of periodic alteration of the temperature on the rhythmic melatonin release of explanted chicken pineals

The melatonin rhythm of cultured chicken pineal cells can be synchronized by cyclic environmental effects. Unlike the effects of light on the melatonin secretion, those of the temperature changes are much less known. Similarly, only a few data are available on the interactions between environmental illumination and periodic temperature changes and on the sensitivity of the pineal gland to temperature changes in different ages of animals. We monitored the effects of temperature on chicken pineals for several days in vitro, in a perifusion system under different illumination patterns. The effects of temperature on pineals from chicken of different age were also compared. The phase of the melatonin rhythm was controlled by periodic elevations of temperature under both constant darkness and continuous illumination. These results show that rhythmic changes of temperature prevent desynchronization induced by constant light. Following elevation of the temperature, the melatonin rhythm of pineals of young chickens (less, than 14 weeks old) was altered for 16 - 18 hours. Similar changes in melatonin rhythm were not found in older animals. It is concluded that the sensitivity for temperature changes of the pineal cells is varying with age.     

Circadian rhythm of tryptophan hydroxylase activity in chicken retina

1. Retinal tryptophan hydroxylase activity in chickens (1-4 weeks old and embryos) was estimated by determination of levels of 5-hydroxytryptophan (5HTP) in retinas at defined intervals after inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD1015). 2. The relationship of tryptophan hydroxylase activity to photoperiod was explored. In chickens maintained on a 12-hr light: 12-hr dark cycle, a diurnal cycle in tryptophan hydroxylase activity was observed. Activity during middark phase was 4.4 times that seen in midlight phase. Cyclic changes in tryptophan hydroxylase activity persisted in constant darkness with a period of approximately 1 day, indicating regulation of the enzyme by a circadian oscillator. The phase of the tryptophan hydroxylase rhythm was found to be determined by the phase of the light/dark cycle. The relationship of the tryptophan hydroxylase rhythm to the light/dark cycle mirrors previously described rhythms of melatonin synthesis and serotonin N-acetyltransferase (NAT) activity in the retina. 3. Light exposure for 1 hr during dark phase suppressed NAT activity by 82%, while tryptophan hydroxylase activity was suppressed by only 30%. 4. Based on the differential responses of retinal NAT activity and tryptophan hydroxylase activity to acute light exposure during dark phase, it was predicted that exposure to light during dark phase would divert serotonin in the retina from melatonin biosynthesis to oxidation by MAO. In support of this, levels of 5-hydroxyindole acetic acid (5HIAA) in retina were found to be elevated approximately two-fold in chickens exposed to 30 min of light during dark phase. In pargyline-treated chickens, 2 hr of light exposure during dark phase was found to increase retinal serotonin levels by 64% over pargyline-treated controls. 5. Cyclic changes in tryptophan hydroxylase activity and NAT activity persisted for 2-3 days in constant light. Tryptophan hydroxylase activity at mid-night gradually decreased on successive days in constant light; on the first day of constant light, tryptophan hydroxylase activity at mid-night was 70% of activity seen during middark phase of the normal light/dark cycle and decreased further on subsequent days. In contrast, on each of 3 days of constant light, NAT activity at mid-night was approximately 15% of normal middark phase activity. 6. Cycloheximide completely inhibited the nocturnal increase in tryptophan hydroxylase activity when given immediately before light offset. The nocturnal increase in NAT activity was inhibited in a similar fashion. 7. Like the development of the NAT rhythm, cyclic changes of tryptophan hydroxylase activity in the retinas of chickens began on or immediately before the day of hatching. hatching.(ABSTRACT TRUNCATED AT 400 WORDS)     

Diurnal nitrate uptake in young tomato (Lycopersicon esculentum Mill.) plants: test of a feedback-based model

A simple model is proposed to describe diurnal net nitrate uptake rate patterns observed experimentally on young plants grown under constant non-limiting nutrition. It rests on two hypotheses: net uptake rate is under negative feedback control by internal plant nitrate content, and nitrogen metabolism occurs only during the light period. The model parameters were determined from the results of three independent experiments performed under non-disturbing conditions in a growth room at constant air and solution temperatures. Net hourly nitrate uptake rate was measured through a diurnal cycle and after an extended 28 h period of darkness. It increased continuously during the light period and decreased during the dark period. Under prolonged darkness, net uptake declined to an asymptotic positive uptake rate of about 10^-5 mol h^-1 g^-1 total plant dry weight. The measured hourly nitrate uptake rate values were consistent with independent determinations of long-term nitrate and total N accumulations in the plant. Realistic simulations of experimental data are achieved with the proposed model. Furthermore, the maintenance of a positive net uptake rate, measured in non-growing plants subjected to prolonged darkness, is explained in the model by the continuous increase of plant water content. The importance of the diurnal variations of plant water content for nitrate uptake rate is emphasized and gives consistency to the homeostasis hypothesis of the model. The diurnal changes in nitrate uptake predicted by the model are strongly dependent on the assumption made for diurnal changes in nitrate assimilation. While the purely photosynthetic assumption is convenient, a more realistic metabolism sub-model is needed.     

Chick pineal serotonin acetyltransferase: a diurnal cycle maintained in vitro and its regulation by light.

We have reproduced in vitro the diurnal cycles in levels of serotonin acetyltransferase activity found in the chick pineal gland in vivo. The more closely the lighting conditions of culture matched those under which the birds were raised, the closer was the similarity between cycles in levels of enzyme activity in vitro and in vivo. Repetitive cycles in levels of acetyltransferase activity persisted in culture for at least 4 days under a diurnal cycle of illumination, and at least 2 days in continuous darkness. When glands were explanted into culture in the light phase of a cycle, short periods of further exposure to light markedly stimulated subsequent increase of acetyltransferase in the dark (after a short lag). Prolonged exposure to light in culture markedly inhibited increase of enzyme activity. Cycles in the levels of enzyme activity in glands cultured under altered light cycles were regulated primarily by changes in illumination. However, the endogenous biological 'clock' remained at least partly entrained to the original light cycle. Increase of acetyltransferase activity in vitro was markedly stimulated by theophylline plus compound Ro. 20.1724 (4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone) under all lighting conditions. Kinetics (to the time of attaining maximum levels in situ) of the increase under diurnal lighting and in constant darkness were indistinguishable from those in vivo. A high concentration of dl-propranolol markedly stimulated an increase in acetyltransferase activity in glands cultured in constant darkness but had little effect on glands under diurnal lighting or continuous illumination.     

Diurnal variations of amino acids and organic acids in xylem fluid from Lagerstroemia indica: an endogenous circadian rhythm

Diurnal variations in the concentrations of major organic compounds occurred in xylem fluid extracted from Lagerstroemia indica L. The concentration of amino acids and the N/C ratio was at a maximum and that of organic acids was at a minimum between 1230 and 2030 h. Since the concentrations of total organic nitrogen, total amino acids and most individual amino acids (but not organic acids or sugars) were also proportional to xylem tension two experiments were performed to discern whether variations in chemistry were a consequence of diurnal changes in moisture stress. In the first experiment, L. indica , exposed to variable levels of moisture stress during midday, manifested an increase in organic acids and a reduction in the N/C ratio. In the second experiment, chemical profiles of xylem fluid were collected and compared for plants exposed to a natural photoperiod, constant darkness or continuous light at noon and midnight. After 1 day amino acids increased in concentration during midday for all treatments; the variation was greatest (10-fold) for plants in constant darkness where xylem tension varied from 0.20 to 0.25 MPa. Only plants exposed to continuous light lost a diurnal rhythm after 3 days. Thus, the circadian rhythm was endogenous, terminated in continuous light and was not mediated by changes in moisture stress. Glutamine accounted for most of the diurnal variation in total amino acids, organic nitrogen and the N/C ratio in xylem fluid.     

Rhythms in magnesium ion inhibition and hysteretic properties of nitrate reductase in the CAM plant Bryophyllum fedtschenkoi

Nitrate reductase (NR, EC 1.6.6.1) was tested in crude extracts of leaves from Bryophyllum fedtschenkoi plants growing under alternating light/darkness as well as in excised leaves kept in continuous light or darkness. In most extracts NR activity was inhibited 20–80% by 5 m M Mg²⁺ A light or darkness shift (30 min darkness) during the first part of the photoperiod gave an increase in the Mg²⁺ inhibition and a decrease in NR activity. Magnesium ion inhibition of NR also showed diurnal variations. Strongest inhibition was found in extracts made during the latter part of the photoperiod and start of the dark period. Pre-incubation of crude extracts with ATP increased Mg²⁺ inhibition, indicating that phosphorylation of NR is involved in regulation of NR in Crassulacean acid metabolism (CAM) plants. In continuous light an increase in Mg²⁺ inhibition occurred after 20 h and 40 h, indicating a rhythm in the phosphorylation of NR. A delay in the production of nitrite in the assay (hysteresis) was generally seen in extracts susceptible to Mg²⁺ inhibition. The rhythms related to NR activity showed the same period length (20 h) as the rhythm in CO₂ exchange. However, in contrast to the rhythm in CO₂ exchange, NR rhythms were strongly damped in continuous light. In constant darkness the rhythms were even more damped. The results show that post-translational modification of CAM NR is influenced by light/darkness and by an endogenous rhythm.     

The effects of pinealectomy and blinding on the circadian locomotor activity rhythm in the Japanese newt,Cynops pyrrhogaster

We examined the effects of pinealectomy and blinding (bilateral ocular enucleation) on the circadian locomotor activity rhythm in the Japanese newt, Cynops pyrrhogaster. The pinealectomized newts were entrained to a light-dark cycle of 12 h light and 12 h darkness. After transfer to constant darkness they showed residual rhythmicity for at least several days which was gradually disrupted in prolonged constant darkness. Blinded newts were also entrained to a 12 h light/12 h dark cycle. In subsequent constant darkness they showed free-running rhythms of locomotor activity. However, the freerunning periods noticeably increased compared with those observed in the previous period of constant darkness before blinding. In blinded newts entrained to the light/dark cycle the activity rhythms were gradually disrupted after pinealectomy even in the presence of the light/dark cycle. These results suggest that both the pineal and the eyes are involved in the newt's circadian system, and also suggest that the pineal of the newt acts as an extraretinal photoreceptor which mediates the entrainment of the locomotor activity rhythm.Abbreviations circadian period - DD constant darkness - LD cycle, light-dark cycle - LD 12:12 light-dark cycle of 12 h light and 12 h darkness     

Regulation of Melanopsin Expression

Circadian rhythms in mammals are adjusted daily to the environmental day/night cycle by photic input via the retinohypothalamic tract (RHT). Retinal ganglion cells (RGCs) of the RHT constitute a separate light‐detecting system in the mammalian retina used for irradiance detection and for transmission to the circadian system and other non‐imaging forming processes in the brain. The RGCs of the RHT are intrinsically photosensitive due to the expression of melanopsin, an opsin‐like photopigment. This notion is based on anatomical and functional data and on studies of mice lacking melanopsin. Furthermore, heterologous expression of melanopsin in non‐neuronal mammalian cell lines was found sufficient to render these cells photosensitive. Even though solid evidence regarding the function of melanopsin exists, little is known about the regulation of melanopsin gene expression. Studies in albino Wistar rats showed that the expression of melanopsin is diurnal at both the mRNA and protein levels. The diurnal changes in melanopsin expression seem, however, to be overridden by prolonged exposure to light or darkness. Significant increase in melanopsin expression was observed from the first day in constant darkness and the expression continued to increase during prolonged exposure in constant darkness. Prolonged exposure to constant light, on the other hand, decreased melanopsin expression to an almost undetectable level after 5 days of constant light. The induction of melanopsin by darkness was even more pronounced if darkness was preceded by light suppression for 5 days. These observations show that dual mechanisms regulate melanopsin gene expression and that the intrinsic light‐responsive RGCs in the albino Wistar rat adapt their expression of melanopsin to environmental light and darkness.     

Diurnal and circadian variations in indole contents in the goose pineal gland

ABSTRACT

The diurnal and circadian profiles of pineal indoles, except melatonin, are poorly characterized in birds. Moreover, there are no data on the effect of sudden changes in the light–dark cycle on these profiles. Therefore, we investigated the diurnal (Experiment I) and circadian variation (Experiment II) of nine pineal indoles (tryptophan, 5-hydroxytryptophan, serotonin, N-acetylserotonin, melatonin, 5-hydroxyindole acetic acid, 5-methoxytryptophol, 5-methoxyindole acetic acid, 5-methoxytryptamine) in geese, as well as the changes in the profiles of these substances in geese subjected to a reversed light–dark cycle (Experiment III). For the first 12 weeks of life, all geese were kept under a diurnal cycle of 12 h of light and 12 h of darkness (12L:12D). In Experiment I (n = 48), they were kept under these conditions for another 14 days before being sacrificed at 2-h intervals for sampling of the pineal glands. In Experiment II, the geese (n = 48) were divided into three groups (12L:12D, 24L:0D, 0L:24D) for 10 days before sampling at 6-h intervals. In Experiment III, 24 geese were exposed to a reversed light–dark cycle before sampling at 14:00 and 02:00 on the first, second and third days after light–dark cycle reversal. To determine the content of the indoles in the goose pineals, HPLC with fluorescence detection was used. We found that, with the exception of tryptophan, all the investigated indoles showed statistically significant diurnal variation. When geese were kept in constant darkness, most of the indoles continued to show this variation, but when geese were kept in constant light, the indoles did not show significant variation. When the light–dark cycle was reversed (12L:12D to 12D:12L), the profiles of NAS, melatonin, 5-MTAM and 5-MTOL reflected the new cycle within 2 days. The content of serotonin in geese in 12L:12D was higher than that observed in other birds under these conditions, which suggests that this compound may play a special role in the pineal physiology of this species. In conclusion, our results show that the daily variations in the metabolism of melatonin-synthesis–related indoles in the goose pineal gland are generated endogenously and controlled by environmental light conditions, as in other birds. However, comparison of the results obtained with the goose to those obtained with other species (chicken, duck) unambiguously shows that the profiles of pineal indoles differ markedly between species, in both the quantitative proportions of the compounds and the characteristics of the diurnal changes. These findings provide strong arguments for the need for comparative studies.     

About the Existence of Circadian Activity in Cave Crayfish

Evidence of a circadian clock mechanism was found in the cave crayfish Procambarus cavernicola. Analysis of motor activity recorded in this species during 12 consecutive days in either free running (constant darkness, DD or constant light, LL) or entrainment conditions (12 h of light alternated with 12 h of darkness, 12 : 12 LD) showed a well recognized circadian rhythm. In this rhythm however, the absence of synchronization by periodical external signals was notorious. The comparison between the motor circadian rhythm in cave crayfish and epigeous crayfish Procambarus clarkii (these last studied during juvenile and adult stages), evidenced strong similitude between the motor circadian rhythm of cave crayfish and juvenile epigeous crayfish.