Circannual reproductive rhythm in the European hamster (Cricetus cricetus): demonstration of the existence of an annual phase of sensitivity to short photoperiod.

In the European hamster (Cricetus cricetus) short photoperiod (SP) is responsible for the transition between the breeding and the resting season and data obtained previously suggest that a circannual "clock" drives the annual rhythm of reproduction. This hypothesis implies the existence of a SP-sensitive phase of the circannual system that occurs independently of the photoperiodic regime perceived by the animals after their arousal from hibernation at the end of March. In control animals kept outside, testicular atrophy occurs in August. When the animals were transferred from outdoors to controlled SP conditions (LD 10:14 and ambient temperature Ta = 18+/-2 degrees C), immediately (Group II) or 2, 4, 6 wk after capture (Groups IV, V, VI, respectively), sexual arrest occurs at the same time between mid-June and mid-July. In the other groups, transfer from outdoors to SP either after 6, 8, 10, 12 or 14 wk (Groups VI, VII, IX, X, XI, respectively) after capture, is followed directly within 4 wk by the gonadal atrophy. When SP was applied from the beginning of August (Group XII) gonadal atrophy was observed after only 2 wk. In this last group, however, the rapid involution is the consequence of the already initiated decline in sexual activity induced by the short daylengths from July. When comparing the effect of SP in two different ambient temperatures (Ta: 18+/-2 degrees C vs 7+/-2 degrees C), immediately (Groups II vs III), 8 (Groups VII vs VIII) or 16 (Groups XII vs XIII) wk after capture, it appears that low temperature does not affect the physiological process described above. In the European hamster, after the gonadal regrowth at the end of hibernation, the animals do not need to experience increasing long days to react against SP. Gonadal inhibition is induced when, following our hypothesis, SP coincides with an endogenous period of sensitivity that extends from mid-May to at least July-August. The present findings complement and extend earlier evidence to support the existence of an endogenous circannual control of seasonal reproduction in the European hamster.

Environmental control of the seasonal variations in the daily pattern of melatonin synthesis in the European hamster, Cricetus cricetus.

Nocturnal patterns of pineal melatonin concentrations were measured at hourly intervals in the European hamster, Cricetus cricetus, maintained under different natural or experimental environmental conditions. There were pronounced variations in the night peak of pineal melatonin both in the duration and the amplitude of the melatonin peak and in the onset and decline of melatonin synthesis. The duration of the melatonin peak increased proportionally with increased dark period. The amplitude increased abruptly from LD 16/8 to LD 15/9 and remained constant in all other photoperiods. The onset of synthesis started 6:00 hours after the onset of darkness in LD 16/8, 15/9, and 14/10, while it started 4:00 hours after dark onset in shorter photoperiods (LD 12/12 and 10/14). This result is opposite to that observed in the rat. The decline of synthesis was delayed as darkness increased and was directly related to lights on in long photoperiods, while it was endogenous in short photoperiods. Temperature, under a long photoperiod, also seems to be implicated in the regulation of the amplitude of the melatonin peak.

[Is the animal in hibernal sleep awake?]. / L'animal en sommeil hibernal est-il éveillé?

It has been recognized for a long time that hibernation and slow wave sleep are homologous processes for energy conservation. Numerous EEG studies have demonstrated that during entrance into hibernation rapid eye movement sleep (REM) disappeared under cerebral temperature below 25 degrees C and that in deep hibernation, animals were preferentially in NREM sleep. Hibernation was thought to be an extension of NREM sleep. Nevertheless, other observations suggest that hibernation is not an homogeneous state. For example, in deep hibernation the activity of single thalamic units occurs with periods of activation and decline. High unit activity is associated with high electromyographic (EMG) activity, whereas low unit activity is associated with low EMG activity. To test the hypothesis that NREM sleep would have a restorative function, the EEG SWA activity (EEG delta power) was recorded during an arousal from hibernation and the following euthermic bout. Contrary to expectations, EEG SWA was maximal after an arousal and declined during the euthermic period. These findings suggest that a bout of hibernation is not NREM sleep, but would be the equivalent of a sleep debt.

[Is the length of gestation a multiple of a fundamental cycle?]. / La durée de gestation est-elle un multiple d'un cycle fondamental?

Pregnancy duration was accurately determined using early ultrasonography in 21 patients prior to spontaneous abortion and in two successive pregnancies in each of 57 women prior to spontaneous delivery. Pregnancy durations before abortion or delivery approximated multiples of 14 days (14.4 and 13.3 days, respectively). In addition, in a given woman, the duration of two successive pregnancies carried to term was often identical; in a smaller number of cases, a difference of about 12 days (calculated value, 11.8 days) or, rarely, 24 days, was found. These values may correspond to the same functional period. These observations suggest that the duration of pregnancy (mean, 266 days) may be a multiple of the duration of the luteal phase of the menstrual cycle (12 to 14 days). Interindividual variations in luteal phase duration (standard deviation, 0.5 to 1 day) may explain inter-individual variations in pregnancy duration (standard deviation of about 15 days). The mean multiple may be 20. Thus, pregnancy duration may be determined both by fetal maturation and by a maternal time-counting mechanism. This hypothesis is discussed in the light of current data.

The photoperiodic response in Syrian hamster depends upon a melatonin-driven circadian rhythm of sensitivity to melatonin.

The pineal gland, via the daily pattern of melatonin (MEL) secretion, is directly involved in the conduction of photoperiodic information. The duration of MEL secretion is proportional to the duration of the dark period and, whatever the photoperiod is, MEL synthesis occurs 3 or 4 h after the dark onset in Syrian hamsters. In order to determine the relative importance of the duration or the coincidence hypothesis, a daily infusion protocol was used in sexually active pinealectomized hamsters. Long duration of MEL infusion (10 h) completely inhibit testes whereas short duration infusion (5 h) had no effect. When the animals were infused twice within 2 h 30 min separated by 3 h, they presented a complete gonadal atrophy, similar to the one observed with the 10 h infusion. Measurement of plasma MEL during the infusion and separation periods revealed that MEL reached physiological nighttime values during the infusion period and fell to daytime values 1 h after the end of an infusion period. Thus, the results could not be due to a time additive action of the two MEL pulses. An intermediate response was observed when the 2 signals were applied across the light/dark transition. Gonadal regression did not occur when the 2 periods of infusion were separated by 5 h 30 min. The efficiency of this type of infusion was not dependent on the ambiant photoperiod since similar results were obtained in long and short photoperiods. The infusion was also as effective during the day as well as during the night. These results suggest that there is a rhythm of sensitivity to MEL, based on the coincidence hypotheses, that are important for transmission of photoperiodic information. This rhythm of sensitivity to MEL seems to be entrained by MEL itself, since the efficiency of the two pulses of MEL is not dependent of time of application and/or of photoperiod.

Are the annual reproductive and body weight rhythms in the male European hamster (Cricetus cricetus) dependent upon a photoperiodically entrained circannual clock?

Most of the data obtained so far on the European hamster (Cricetus cricetus) suggest direct photoperiodically driven seasonal changes in sexual activity and body weight. The results of the present long-term study support the hypothesis that these annual changes are the expression of photoperiodically driven endogenous circannual rhythms. When subjected following capture (April-May) to constant conditions of long photoperiod (LP) and constant temperature a large number of the European hamsters present, in September-December, complete gonadal atrophy associated with a decrease in body weight. A sexual reactivation as well as an increase in body weight are observed in the same animals between January and April. Of the six animals that survived long enough, two only presented partial gonadal atrophy during the second year. These observations clearly demonstrate that the decline in sexual activity in subjective autumn does not require a decrease in photoperiod, at least in the first year. Theoretically, the observed rhythms, if circannual in nature, would be generated by a self-sustained annual oscillator (circannual clock) able to function in the absence of a photoperiodic input. Pinealectomy makes animals unable to detect or measure photoperiodic information. Of the six European hamsters tested (pinealectomized in June and then kept continuously under LP), five showed clear annual rhythms in body weight and reproductive capacities for two consecutive years. Clearly endogenous annual rhythms were expressed in these conditions. To be entrained to a 1-year period, such a circannual clock should, however, be able to react to either LP and/or to short-photoperiod (SP), at least at certain periods of the annual cycle. In animals exposed to LP in August or October, after gonadal atrophy had been established by exposure to natural SP, gonadal regrowth started in December or January, about 2 to 3 months earlier than in animals kept outside or in experimental SP. With the same experimental conditions, exactly the same results were obtained in pinealectomized animals; thus stimulatory effect of LP or LP-induced phase advance of the circannual clock can be excluded. The absence of the SP information would then induce such reaction. In animals kept under constant LP and temperature following capture, however, pinealectomy in January--when all animals are sexually active--induces gonadal atrophy within--weeks. This clearly demonstrates that LP is stimulatory at this time of the subjective year.

Search for rhythmicity during hibernation in the European hamster.

Temporal patterns of hibernation were studied by continuous monitoring of body temperature by radiotelemetry over 6 months in European hamsters, Cricetus cricetus, at constant temperature and photoperiod. Entrances into hibernation occurred mostly at the end of the night (0000-0800 hours), while arousals were randomly distributed between day and night. This is at variance with a control of bout duration by a clock with a period of 24 h. Consequently, the timing of entrances implies a phase-resetting of the circadian clock on each arousal. Persistence of circadian rhythmicity with a period different from 24 h during deep hibernation was investigated examining whether the durations of torpor bouts were integer multiples of a constant period. A non-parametric version of the classical contingency test of periodicity was developed for this purpose. Periods ranging from 21 to 29 h were tested. Nine animals out of ten showed at least one significant period in this range (P < 0.01), either below 24 h (21.8 +/- 0.5 h, n = 4) or above (27.3 +/- 0.5 h, n = 7). However, we have found a theoretical model of bout durations for which the contingency test of periodicity sometimes gives false significant results. This indicates that the power of the test is weak. With this reservation our results suggest that a circadian oscillator controls the duration of a bout of hibernation, which would occur after an integer, but variable and possibly temperature-dependent number of cycles.

Role of cardiac innervation in atrial natriuretic peptide secretion in transplanted heart recipients.

To investigate whether cardiac innervation modulates atrial natriuretic peptide (ANP) secretion, we performed acute volume expansion on eight normal and eight matched (age, weight, and total blood volume) transplanted denervated heart patients (Htx), while monitoring fluid-regulating hormone, systemic blood pressure, and echocardiographic atrial area changes. At rest, plasma ANP and guanosine 3',5'-cyclic monophosphate (cGMP) were lower in control subjects than in Htx (45 +/- 16 vs. 103 +/- 35 pg/l and 0.9 +/- 0.3 vs. 3.5 +/- 1.4 pM, respectively; P < 0.001). Plasma active renin, aldosterone, and catecholamines did not differ significantly in the two populations, whereas arginine vasopressin and cortisol were higher in controls (P < 0.01 and P < 0.005). Although volume expansion (+15%) and atrial stretch were similar in the two groups, plasma ANP and cGMP increased significantly only in the Htx group (103 +/- 35 to 189 +/- 69 pg/l and 3.5 +/- 1.4 to 5.8 +/- 1.4 pM, respectively; P < 0.001). The decrease observed for the other hormones was not significant except for arginine vasopressin and cortisol (P < 0.05 and P < 0.001) in the control group. These results support the hypothesis of an inhibitory role of cardiac innervation in biologically active ANP secretion in humans, at rest and after acute volume expansion.

Viscosity of water in hibernating and nonhibernating mammals estimated by proton NMR relaxation times.

Longitudinal (T1) and transverse (T2) nuclear magnetic resonance relaxation times were measured in vitro at 37, 30, 25, 15, and 5 degrees C on serum, brain, liver, kidney, and heart samples from a hibernator, the European hamster, active in summer (SA), active in winter, or in the hibernating state in winter; from a less efficient hibernator, the golden hamster; and from a homeotherm, the rat. T1 and T2 relaxation times varied between species and in the European hamster between the active and hibernating subjects. Despite the major relaxation time differences between the organs, NMR relaxation time measurements showed a general trend to an increase in the viscosity of water for the European hamster in the active state. Although these modifications were not directly related to the process of hibernation itself, the relaxation times observed in the hibernating animals were closer to those seen in the rat. This evidenced that changes of physical properties of water reflect a better adaptation to low temperatures of the hamster, as compared to the nonhibernator, given that the low water viscosity of SA hamster allows the decrease of the viscosity with temperature during the hibernating state. These in vitro studies permit the study the viscosity which is an important physicochemical parameter involved in NMR longitudinal relaxation time of water proton. More detailed studies of other physiological parameters must be undertaken by further in vivo measurements.

Seasonal variations in the daily rhythm of pineal gland and/or circulating melatonin and 5-methoxytryptophol concentrations in the European hamster, Cricetus cricetus.

Day-night variations in pineal and/or circulating melatonin and 5-methoxytryptophol (5-ML) concentrations were measured monthly throughout the year in female European hamsters, Cricetus cricetus, maintained under natural conditions. Pronounced seasonal variations in the day-night rhythm of both melatonin and 5-ML were observed. As previously reported for melatonin, the daily rhythm of both methoxyindoles disappeared in spring and early summer, while a clear day-night rhythm occurred in autumn, winter, and early spring. The amplitude of the day-night variations appeared to be maximum from October until January. An inverse relationship existed between the rhythms of melatonin and 5-ML.

Endogenous, photoperiodic and hormonal control of the body weight rhythm in the female European hamster, Cricetus cricetus.

1. Body weight and hibernation rhythms were followed on normal and castrated female European hamsters raised in different conditions of photoperiod and ambient temperature. 2. In the normal females, the photoperiod was more effective than the ambient temperature regarding the control of the body weight rhythm. 3. In the castrated females, testosterone was more effective than oestradiol in suppressing both body weight and hibernation rhythms. 4. In short photoperiod conditions, the existence of endogenous rhythmicity depends upon prior photoperiodic exposure of the animals.

Effects of different doses and durations of melatonin infusions on plasma melatonin concentrations in pinealectomized Syrian hamsters: consequences at the level of sexual activity.

The effect of different doses and durations of melatonin infusions on plasma melatonin concentrations has been studied in pinealectomized Syrian hamsters maintained under short photoperiod at either 7 degrees C or 18 degrees C. The effects of the infusions on plasma melatonin concentrations and on gonadal activity were compared. The results show that the minimal effective quantity of infused melatonin that induced gonadal atrophy was 40 ng/h at 7 degrees C and 20 ng/h at 18 degrees C. An infusion of 8 hr duration per day is necessary to inhibit sexual activity, while an infusion of 6 hr duration was ineffective. This finding suggests that the critical duration of melatonin infusion is between 6 and 8 hr. Despite the various doses of melatonin infused, plasma melatonin concentrations measured in the middle of the infusion period did not differ significantly from concentrations measured in intact animals. This finding suggests that the metabolism of infused melatonin increases as the dose of melatonin increases. Moreover, the different physiological effects observed after the various melatonin infusions cannot be explained by variations in plasma melatonin concentrations.

Long-term lithium treatment does not suppress hibernation in European hamsters.

European hamsters were fed LiCl-supplemented food for a long time before, during and after the hibernating season. Long-term administration of LiCl does not suppress hibernation, which occurs normally during the first part of the experiment. Moreover lithium-treated hibernating hamsters tolerate very high plasma lithium levels. This tolerance is not explained. The results are discussed in relation with the recent theories on the similarities between depression, seasonal affective disorder and hibernation.

Melatonin, serotonin, 5-hydroxyindole-3-acetic acid and N-acetyltransferase in the pineal of the European hamster (Cricetus cricetus) kept under natural environmental conditions: lack of a day/night rhythm in melatonin formation in spring and early summer.

In female European hamsters killed in spring and early summer, pineal melatonin content exhibited no day/night rhythm. Absolute levels measured were relatively low, being on the order of daytime levels detected in other hamster species. An absence of day/night changes in the activity of N-acetyltransferase was also observed. However, a marked rhythm in pineal serotonin (5-HT) was found, an abrupt large increase being observed at the beginning of the light period. The day/night rhythm of pineal 5-HIAA content is similar to that of 5-HT. This absence of rhythm in pineal melatonin formation might mean that in the European hamster it is not melatonin but another substance that is of importance in photoperiodism. An absence of melatonin rhythm, however, could also be simply a peculiar pattern of melatonin production observed at a given period of the year. In this case, melatonin would be able to transduce photoperiodic information in the European hamster, as in other photoperiodic species.

Photoperiodic regulation of body mass, food intake, hibernation, and reproduction in intact and castrated male European hamsters, Cricetus cricetus.

A group of sexually active male European hamsters were raised either in short-photoperiod conditions (SP; LD 8:16) or in long-photoperiod conditions (LP; LD 16:8) from their capture at the end of the hibernation period. Another group of hamsters was castrated in April and gonadectomized animals were maintained in SP and cold (7 degrees C) or in a succession of SP and LP plus cold. Another group, castrated in May or in September and raised in LP conditions, was transferred in September to SP conditions and cold. 1. Normal hamsters raised in continuous SP or LP apparently did not show signs of rhythmic behavior, except possibly in gonadal activity. 2. Body weight increased continuously, plasma testosterone levels oscillated between 1.5 and 2.5 ng/ml, and animals raised in SP and in cold did not enter hibernation. 3. Similar results were also found in castrated animals kept in SP conditions and cold. 4. The sequence LP-SP induced a decrease in food intake and body weight and a decrease in plasma testosterone levels and triggered entry into hibernation in both intact and castrated animals. 5. After 6 months continuously in SP and with exposure to cold spontaneous recrudescence in food intake and body weight occurred and hibernation ended in both intact and castrated animals. 6. In normal animals a spontaneous increase in plasma testosterone levels was observed. 7. In both normal and gonadectomized animals the phase of refractoriness could be broken by exposure to LP conditions. 8. The critical photoperiod lies between 15 and 15.5 h. These results demonstrate that the European hamster is a photoperiodic species.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation of intestinal enzymes to seasonal and dietary changes in a hibernator: the European hamster (Cricetus cricetus).

Effects of diet, hibernation and seasonal variations on hydrolase activities were determined in mucosa and purified brush border membranes of the small intestine of European hamsters. Wild hamsters captured in April and fed for several weeks with an equilibrated laboratory chow (20% protein, 50% carbohydrates) exhibited a rise in disaccharidase activities (sucrase, isomaltase, lactase) but no changes in aminopeptidase N activity. During deep hibernation, in contrast to sucrase and isomaltase activities which showed only minor changes, lactase activity was significantly enhanced along the jejunoileum, and aminopeptidase N activity was maximum in the ileum. After a short period (48 h) of wakefulness and feeding following 10 days of starvation during the hibernation period, the activities of the disaccharidases and of aminopeptidase N returned to values measured in active animals. In contrast to the nutritional state, which has an important impact on the activities of intestinal enzymes, season has little effect on the intestine of the active animal under a controlled environment. The pattern of enzyme activities which occurs along the small intestine in the hibernating animal may be a prerequisite for optimum digestion during the short phases of waking during the hibernation period of the European hamster.