Age affects hibernation in Syrian hamsters (Mesocricetus auratus).

In this study, we aimed to show how age affects hibernation in the Syrian hamster. Experimentally, we used 30 male animals differing in age. The old animals were 20 months of age and the adults were 8 months of age at the end of the test. The young animals were 3 weeks old at the start of testing and 5 months old at the end of the testing period. The torpor observation started October 15, 1996, and ended March 11, 1997, in the laboratory colony maintained under natural photoperiod and outdoor air. Observations were performed around noon daily. Three measures (i.e., prehibernation period [hibernation latency], proportion of hibernation spent in torpor, and proportion of animals in torpor), all of which reflect the strength of occurrence of hibernation, indicated that the older hamsters (1) started hibernation earlier, (2) spent more time in torpor, and (3) had a higher chance of being in torpor than the younger ones during the hibernation season.

Unpredictable deprivation of water increases the probability of torpor in the Syrian hamster.

The effect of unpredictable water deprivation on hibernation was investigated in the Syrian hamster under natural photoperiod and temperature. Based on our previous findings, we hypothesized that 1) deprivation of water caused testicular regression, 2) the magnitude of regression depended on unpredictability of water deprivation, 3) unpredictable deprivation of water on 50% of the days of the experimental period causes more testicular regression for animals than regular deprivation every other day, even if the total lengths were the same for the two treatments, and thus, 4) unpredictable deprivation of water induces hibernation more readily than regular deprivation or no deprivation. The results showed that the animals with the random and unpredictable deprivation a) started hibernation earlier, b) had a higher chance of being in torpor at the end of hibernation season, c) spent more time in torpor during hibernation than those animals which were deprived of water on a regular basis or the control, non-deprived animals. These findings supported our working hypothesis.

An analysis of factors that induce hibernation in Syrian hamsters.

Factors that induce hibernation of Syrian hamsters were analyzed in young adults and older adults under two photoperiodic conditions (i.e., a long photoperiod and a natural photoperiod). The experiment was performed from September 1st, 1992 to March 24th, 1993. The older hamsters entered earlier into and spent longer in hibernation than the young ones. Water deprivation caused more torpor than free access to water in the older hamsters as well as in the young ones. As the ambient temperature dropped, not only the number of animals entering hibernation but also the number of days in hibernation increased. Long photoperiod in the cold environment tended to induce torpor in the older animals but not in the young ones. An examination of testicular weight at the end of the experiment revealed that the regressed testes were well correlated with the induction and maintenance of hibernation regardless of age and photoperiod.

Stress suppresses testicular and body weight in young Syrian hamsters under short photoperiod.

Two experiments were performed to investigate the effect of stress on testicular weight and body mass in young adult male Syrian hamsters under long or short photoperiods. We hypothesized that water or food deprivation causes stress and that the amount of stress depends on unpredictable timing of deprivation. More specifically, water or food deprivation on unpredictable days was considered more stressful for animals than regular deprivation on fixed days even if the total lengths of deprivation were the same for the two treatments. Experiment 1 showed that water deprivation on unpredictable days caused more suppressive effects on testicular weight and body mass than that on fixed days under short photoperiod. Experiment 2 indicated that unpredictable food deprivation on a quarter or half of the days throughout the 12-week testing period under short photoperiod also induced more detrimental effects on testes and body growth than predictable deprivation once every 4 days or every other day. These findings clearly suggested that the stress associated with water or food caused more suppressive effects on testicular weight and body mass than the shortage of water or food per se.

Suprachiasmatic nuclei influence torpor and circadian temperature rhythms in hamsters.

Siberian hamsters were maintained in a short-day photoperiod (8 h light-day) at 15 degrees C; body temperature (Tb) and locomotor activity were telemetrically recorded at 10-min intervals over the course of 5 mo. Animals manifesting repeated torpor bouts (Tb less than 30 degrees C for several hours) were subjected to lesions of the suprachiasmatic nuclei (SCN), pinealectomy, or sham operations. In the 15 wk after surgery, none of the animals with bilateral lesions of the SCN exhibited torpor; circadian Tb and locomotor activity rhythms, as determined by cosinor and power spectral analysis, also were absent in SCN-lesioned hamsters. Pinealectomized animals and brain-lesioned hamsters with intact SCN had normal circadian temperature and activity rhythms and showed torpor for at least 4 wk postsurgically. Expression of torpor and circadian rhythms of Tb and activity are dependent on intact SCN and persist for several weeks in the absence of pineal secretory activity.

Circadian rhythms in sleep-wakefulness and wheel-running activity in a congenitally anophthalmic rat mutant.

Circadian rhythms were examined in a congenitally anophthalmic rat mutant. All bilaterally anophthalmic rats had clear circadian rhythms in sleep-wakefulness as well as in locomotor activity, but the rhythms did not entrain to the light-dark (LD) cycle. However, two unilaterally anophthalmic rats, like sighted rats, reentrained their sleep-wakefulness rhythms to the LD cycle in 4 days after a 6 hr delaying phase-shift. The hypogenesis of the suprachiasmatic nuclei (SCN) that is correlated with absence of circadian activity rhythms in anophthalmic mice was not observed in these anophthalmic rats.

[Central neural mechanism of the circadian system and its physiological psychology].

Recent advances on the study of circadian rhythm are reviewed and discussed on the following topics: its phylogeny (the existence of extraretinal photoreceptors, photoreceptors for entrainment, the location of circadian oscillators), its ontogeny and aging, its central neural mechanism (the ablation study of the suprachiasmatic nucleus (SCN), the electrophysiological study of the SCN in multiple unit activity and single neuron activity in rat hypothalamic slice preparation, the use of 2-DG as a functional marker, the transplantation of the SCN, the afferents and efferents of SCN and its peptides), multiple oscillators theory (the anticipatory activities in rats with SCN lesions in response to the restricted feeding schedule, internal desynchronization, splitting), and its psychological significance. Many lines of evidence have shown that the SCN is a putative circadian pacemaker in mammals. On the other hand, other lines of evidence suggest that there are other oscillators outside the SCN. Finally it is emphasized that the chronobiological viewpoint is necessary for the development of psychology.

Ontogenesis of circadian sleep-wakefulness rhythms and developmental changes of sleep in the altricial rat and in the precocial guinea pig.

Ontogenesis of circadian sleep-wakefulness rhythms and other developmental changes of sleep were examined in the altricial rat and in the precocial guinea pig. Circadian sleep-wakefulness rhythms began to emerge on the 4th-4.5th week after birth in rats born and raised not only under light-dark (LD) 12:12 h lighting schedule but also under a constant light condition. Only ultradian rhythms were recognized in the rat up to 2.5-3.5 weeks after birth. Unlike the rat, the guinea pig did not develop significant diurnal and circadian sleep rhythms during the course of its growth. Sleep-wakefulness rhythms of the 1-week-old infant guinea pig were very similar to those of the adult. During the development of the altricial rat the periods of slow wave sleep (SWS) and paradoxical sleep (PS) cycles, and the amounts of SWS per day increased. On the other hand, the amounts of PS per day and the length of the PS episodes decreased. For the guinea pig, however, these parameters of sleep were apparently constant throughout development.

Decreased food intake of rats kept under adiurnal feeding cycles: effect of suprachiasmatic lesions.

The circadian feeding rhythm and food intake under restricted feeding conditions, including adiurnal feeding cycles, were examined in rats with bilateral lesions of the suprachiasmatic nucleus (SCN). Although rats with SCN-lesions ate nearly as much food per day as those with control-lesions, their feeding pattern did not show circadian rhythmicity. When rats with control-lesions were fed for 5 hr once every 20 hr or for 7 hr once every 28 hr, they ate less than when they were fed for 6 hr once every 24 hr, probably due to some effect of desynchronization between the feeding cycle and an endogenous circadian oscillator. Decreased food intake under adiurnal feeding cycles was also observed in rats with SCN-lesions as in those with control-lesions. It is suggested that the circadian rhythm entrained by food is related to an endogenous time-keeping system that does not include the SCN.

Sleep-wakefulness rhythms in mice after suprachiasmatic nucleus lesions.

The experiment was performed in order to investigate whether the suprachiasmatic nucleus is a potent circadian pacemaker also in mice. Cortical EEG activity and neck muscle EMG were continuously recorded in 19 albino mice under 12 h light-12 h dark cycles. Computer analysis revealed that after bilateral suprachiasmatic nucleus (SCN) lesions, the diurnal rhythm in slow wave sleep (SWS) and paradoxical sleep (PS) stages was completely eliminated although ultradian rhythms with 2-4 h periodicity persisted. However, the daily amount of either slow wave sleep or paradoxical sleep did not show any significant change after small hypothalamic lesions with a complete destruction of SCN.

The search for circadian rhythm pacemakers in the light of lesion experiments.

Considering the huge phylogenetic tree of evolution of the animal kingdom, only very limited observations have been made on the pacemaker mechanism of the circadian rhythms of various biological phenomena. If there is a master clock controlling the circadian rhythmicity in the animal body, lesions in it should certainly produce arrhythmic activity. On the basis of very scattered data available at present, it can be presumed that in the central nervous system there is a group of cells which functions as a master clock. Its location is close enough to the primary photoreceptors to be entrained by the environmental light-dark cycle. The clock unidirectionally controls all body activities and also other secondary pacemakers, if they exist. The influence of a secondary pacemaker is seen in pathological conditions or after destruction of the master clock. Lesions in the coupling site between the clock and overt activity may result in arrhythmia without damaging clock activity. Hence, care must be taken in assessing the data recorded. Up to now, the optic lobes in several insect species and the suprachiasmatic nucleus in several rodent species are the most likely sites of a master clock, and lesions in these instantaneously induce arrhythmia. Increasing study of the retinohypothalamic projection in other mammals and birds will throw more and more light on this nucleus. A search for the circadian master clock in the body should, therefore, be extended to all species of animals and this will clarify the circadian clock mechanism, no matter whether it functions as a result of membrane activity, genetic influence or a very specific biochemical mechanism resistant to environmental changes.

Analysis of sleep-wakefulness rhythms in male rats after suprachiasmatic nucleus lesions and ocular enucleation.

To determine quantitatively characteristics of sleep-wakefulness rhythms in male albino rats, computer analysis of long term polygraphic records (24 h/day) of cortical EEG activity, neck EMG and EOG taken from 23 rats under 12:12 light-dark schedule was performed. After bilateral suprachiasmatic nucleus (SCN) lesions, the circadian rhythm in sleep-wakefulness was completely eliminated, although no attenuation or even slight enhancement of the ultradian rhythms with 2-4 h pweiodicity was observed. After enucleation of both eyes, the circadian rhythm was free-running with a phase shift in the range from --12 to +22 min/day in 6 rats. A gradual decrease of the spectral value of the circadian rhythm and inverse enhancement of the ultradian rhythms with 4--7 h periodicity (predominantly 6 h in 4 out of 6 rats) were also shown. In the spectral diagram, the appearance of paradoxical sleep (PS) paralleled slow-wave sleep (SWS), in the cases of the circadian rhythm and ultradian rhythms with 4--7 h periodicity. Behaviorally blind rats with bilateral primary optic tract (POT) lesions maintained the circadian rhythm in sleep-wakefulness entrained to the environmental light-dark cycle. Power spectral analysis showed no characteristic difference from normal rats. Based on these data, the role of the SCN as a pacemaker of endogenous circadian rhythm in sleep-wakefulness is discussed.