Lesions of the thalamic intergeniculate leaflet alter hamster circadian rhythms

We have investigated the effects of destruction of the geniculo-hypothalamic tract (GHT) on the circadian system of golden hamsters. In the first experiment, intact hamsters were housed in constant darkness, and phase shifts in running-wheel activity rhythms were assessed following 15-min light pulses administered at circadian time (CT) 12 (defined as the beginning of activity), CT 14, CT 18, and CT 20. Responses to light pulses at the same CTs were then reassessed after GHT lesions. Hamsters with complete lesions showed decreases in phase advances caused by light pulses at CT 18 and CT 20. Phase delays elicited by light at CT 12 and CT 14 were not altered. In a second study, intact and GHT-ablated hamsters housed in constant light received 6-hr dark pulses at various CTs. Hamsters with complete GHT ablation showed smaller advances than controls to dark pulses centered on CT 8-10. After 110 days in constant light, 7 of 10 intact hamsters showed splitting of their activity rhythms into two components, while only 1 of the 8 similarly treated ablated hamsters displayed dissociated activity components. Ablated hamsters had significantly shorter free-running periods during the first 35 days of exposure to constant light than did the intact hamsters. These results demonstrate that destruction of the GHT in the hamster alters phase shifting in response to periods of light or dark, and they indicate a role for the GHT in mediating several photic effects on the circadian system.     

The intergeniculate leaflet,but not the visual midbrain,mediates hamster circadian rhythm response to constant light

Several studies have demonstrated a variety of effects of intergeniculate leaflet (IGL) lesions on circadian rhythm regulation. Recent studies have suggested the possibility that certain rhythm functions attributed to the IGL are actually controlled by retinorecipient midbrain nuclei or other brain areas connected to the IGL. The present investigations evaluated whether midbrain lesions previously shown to block the phasic actions of benzodiazepine would also reduce or eliminate the period-lengthening effect of constant light (LL), a function that has been attributed to the IGL. Experiment 1 established that the circadian period of controls lengthened by about 0.57 h when the animals were transferred from constant dark (DD) to LL, but the magnitude of change was attenuated by about 50% in animals with IGL lesions caused by the neurotoxin N-methyl-D-aspartate (NMDA). In experiment 2, controls were compared to groups receiving either NMDA lesions of the pretectum or tectum or knife cuts designed to sever connections between the IGL and more medial retinorecipient nuclei. As in experiment 1, there were no differences between groups with respect to circadian period in DD. However, unlike experiment 1, all groups lengthened period equally in LL (overall mean increase = 0.57 h). Thus, the effect of LL on circadian period appears to be a joint result of photic information arriving at the circadian clock directly from the retinohypothalamic tract and indirectly through the IGL via the geniculohypothalamic tract, without involvement of visual midbrain. The results may have implications for the anatomical basis of Aschoff's rule.     

The thalamic intergeniculate leaflet mediates locomotor activity-induced reversal of phenotype in photoperiod nonresponsive Siberian hamsters

The role of the intergeniculate leaflet of the thalamus (IGL) in photoperiod responsiveness was examined in a laboratory-selected line of photoperiod nonresponsive (NR) Siberian hamsters. NR hamsters fail to exhibit typical winter-type responses (i.e., gonadal regression and development of winter-type pelage) when exposed to short day lengths (e.g., 10 h of light/day). Earlier studies revealed that NR hamsters will exhibit winter-type responses when exposed to short photoperiod if they are given free access to a running wheel. The present study tested the hypothesis that this locomotor activity-induced reversal of phenotype is dependent on the IGL. Male NR hamsters underwent destruction of the IGL prior to being housed in short day lengths in cages equipped with running wheels. Activity rhythms were monitored for 8 weeks, after which time pelage response and paired testes weights were obtained. In contrast to sham-operated NR animals given access to running wheels, IGL-ablated animals showed no increase in the duration of nocturnal running wheel activity and became active later in the night than sham-lesioned animals. Lesioned animals also failed to exhibit the typical short photoperiod-induced gonadal regression and pelage molt. The results implicate the IGL in the mechanism by which running wheel activity can influence photoperiodic responses.     

Running activity mediates the phase-advancing effects of dark pulses on hamster circadian rhythms

Summary Pulses of darkness can phase-shift the circadian activity rhythms of hamsters,Mesocricetus auratus, kept in constant light. Dark pulses under these conditions alter photic input to the circadian system, but they also commonly trigger wheel-running activity. This paper investigates the contribution of running activity to the phase-shifting effects of dark pulses. A first experiment showed that running activity by itself can phaseshift rhythms in constant light. Hamsters were induced to run by being confined to a novel wheel for 3–5 h. When this was done at circadian times (CT) 0, 6, and 9, the mean steady-state phase-shifts were 0.6 h, 3.5 h, and 2.3 h, respectively. The latter two values are at least as large as those previously obtained with dark pulses of similar durations and circadian phases. A second experiment showed that restricting the activity of hamsters during 3-h dark pulses at CT 9 reduces the amplitude of the phase-shifts. Unrestrained animals phase-advanced by 1.1 h, but this shift was halved in animals whose wheel was locked, and completely abolished in animals confined to nest boxes during the dark pulse. Activity restriction in itself (without dark pulses) had only minimal phase-delaying effects on free-running rhythms when given between ca. CT 10 and CT 13. These results support the idea that, in hamsters at least, dark pulses affect the circadian system mostly by altering behavioural states rather than by altering photic input to the internal clock.Abbreviations CT circadian time - DD constant darkness - LD light-dark - LL constant light - PRC phase response curve - period of rhythm     

Effects of light on human circadian rhythms.

Blind subjects with defective retinal processing provide a good model to study the effects of light (or absence of light) on the human circadian system. The circadian rhythms (melatonin, cortisol, timing of sleep/wake) of individuals with different degrees of light perception (n = 67) have been studied. Blind subjects with some degree of light perception (LP) mainly have normally entrained circadian rhythms, whereas subjects with no conscious light perception (NPL) are more likely to exhibit disturbed circadian rhythms. All subjects who were bilaterally enucleated showed free running melatonin and cortisol rhythms. Studies assessing the light-induced suppression of melatonin show the response to be intensity and wavelength dependent. In contrast to ocular light exposure, extraocular light failed to suppress night-time melatonin. Thus, ocular light appears to be the predominant time cue and major determinant of circadian rhythm type. Optimisation of the light for entrainment (intensity, duration, wavelength, time of administration) requires further study.     

Light exposure mediates circadian rhythms of rhizosphere microbial communities

Microbial community circadian rhythms have a broad influence on host health and even though light-induced environmental fluctuations could regulate microbial communities, the contribution of light to the circadian rhythms of rhizosphere microbial communities has received little attention. To address this gap, we monitored diel changes in the microbial communities in rice (Oryza sativa L.) rhizosphere soil under light–dark and constant dark regimes, identifying microbes with circadian rhythms caused by light exposure and microbial circadian clocks, respectively. While rhizosphere microbial communities displayed circadian rhythms under light–dark and constant dark regimes, taxa possessing circadian rhythms under the two conditions were dissimilar. Light exposure concealed microbial circadian clocks as a regulatory driver, leading to fewer ecological niches in light versus dark communities. These findings disentangle regulation mechanisms for circadian rhythms in the rice rhizosphere microbial communities and highlight the role of light-induced regulation of rhizosphere microbial communities.Subject terms: Microbial ecology, Community ecology     

Phase-shifting effects of dusklike and dawnlike light pulses on the circadian activity rhythms of Syrian hamsters

This study tested whether light pulses with a dusklike offset or a dawnlike onset caused phase shifts of different sizes in the circadian wheel-running activity of Syrian hamsters, Mesocricetus auratus. Six experiments were conducted, each with 30 hamsters; the hamsters received first one type of pulse and then the other type a few weeks later, allowing a paired comparison. The six experiments represented the combination of two maximum light intensities (150 and 250 lux) and three zeitgeber times (ZTs) at which the pulses were given (ZT13.5, ZT14.5, and ZT20). Pulses were 30 minutes long, a relatively short duration to minimize circadian time effects. Aschoff's type II method of measuring phase shifts was used. In none of the six experiments did a two-tailed paired t test detect a significant difference in the size of phase shifts caused by dusklike versus dawnlike pulses. A three-way analysis of variance (ANOVA) on the combined data from all six experiments (with pulse type, pulse intensity, and ZT as factors) also failed to detect a significant effect of pulse type. Statistical power was calculated and found to be reasonably good. These negative results are in line with those of a previous study in which a different methodology was used. (Chronobiology International, 18(3), 413-421, 2001)     

Melatonin facilitates synchronization of sparrow circadian rhythms to light

We recorded circadian locomotor activity rhythms of house sparrows (Passer domesticus) exposed to low-amplitude light-dark cycles (2∶1 lux) with periods of 22.5 or 24.5 h. Under these conditions the circadian rhythms of the majority of the birds were not synchronized by the light cycle but either free-ran or showed relative coordination. However, when melatonin was administered continuously via subcutaneous silastic implants the rhythms became synchronized. It is proposed that melatonin facilitates synchronization either by weakening the circadian oscillatory system thereby increasing its range of entrainment, or by enhancing circadian sensitivity to the light Zeitgeber. In general, the results suggest that melatonin, besides its well-known phasic effects on the circadian system also has important tonic effects modifying the ease with which circadian systems can be entrained.     

The pineal gland influences rat circadian activity rhythms in constant light.

Mammalian circadian organization is believed to derive primarily from circadian oscillators within the hypothalamic suprachiasmatic nuclei (SCN). The SCN drives circadian rhythms of a wide array of functions (e.g., locomotion, body temperature, and several endocrine processes, including the circadian secretion of the pineal hormone melatonin). In contrast to the situation in several species of reptiles and birds, there is an extensive literature reporting little or no effect of pinealectomy on mammalian circadian rhythms. However, recent research has indicated that the SCN and circadian systems of several mammalian species are highly sensitive to exogenous melatonin, raising the possibility that endogenous pineal hormone may provide feedback in the control of overt circadian rhythms. To determine the role of the pineal gland in rat circadian rhythms, the effects of pinealectomy on locomotor rhythms in constant light (LL) and constant darkness (DD) were studied. The results indicated that the circadian rhythms of pinealectomized rats but not sham-operated controls dissociated into multiple ultradian components in LL and recoupled into circadian patterns only after 12-21 days in DD. The data suggest that pineal feedback may modulate sensitivity to light and/or provide coupling among multiple circadian oscillators within the SCN.     

Proportional effect of light on entrained circadian rhythms of birds and mammals

Summary Circadian rhythms of locomotor activity of two species of finches,Fringilla coelebs andCarduelis chloris, and four rodent species,Mesocricetus auratus, Glis glis, Eutamias sibiricus andFunambulus sp., were studied under the influence of 1212 h light-dark (LD) cycles with different levels of illumination during L and D. The LD intensity-ratio (Zeitgeber amplitude) was approximately constant at different mean levels of light intensity per cycle (Zeitgeber level). Theproportional effect of light, measured as the influence of mean light intensity per cycle (geometric mean) on the phase-angle differences between activity onset and light-on (in day-active species) or light-off (in night-active species) showed large interspecific variability. The variations—in extent and direction—were congruous with the effects of light on the free-running rhythms in the same species suggesting a functional relationship between effects of light on free-running and entrained circadian rhythms.     

Acute light exposure suppresses circadian rhythms in clock gene expression

Light can induce arrhythmia in circadian systems by several weeks of constant light or by a brief light stimulus given at the transition point of the phase response curve. In the present study, a novel light treatment consisting of phase advance and phase delay photic stimuli given on 2 successive nights was used to induce circadian arrhythmia in the Siberian hamster ( Phodopus sungorus). We therefore investigated whether loss of rhythms in behavior was due to arrhythmia within the suprachiasmatic nucleus (SCN). SCN tissue samples were obtained at 6 time points across 24 h in constant darkness from entrained and arrhythmic hamsters, and per1, per2 , bmal1, and cry1 mRNA were measured by quantitative RT-PCR. The light treatment eliminated circadian expression of clock genes within the SCN, and the overall expression of these genes was reduced by 18% to 40% of entrained values. Arrhythmia in per1, per2, and bmal1 was due to reductions in the amplitudes of their oscillations. We suggest that these data are compatible with an amplitude suppression model in which light induces singularity in the molecular circadian pacemaker.     

Environmental effects on circadian rhythms in photosynthesis and stomatal opening

Persistent circadian rhythms in photosynthesis and stomatal opening occurred in bean (Phaseolus vulgaris L.) plants transferred from a natural photoperiod to a variety of constant conditions. Photosynthesis, measured as carbon assimilation, and stomatal opening, as conductance to water vapor, oscillated with a freerunning period close to 24 h under constant moderate light, as well as under light-limiting and CO₂-limiting conditions. The rhythms damped under constant conditions conducive to high photosynthetic rates, as did rates of carbon assimilation and stomatal conductance, and this damping correlated with the accumulation of carbohydrate. No rhythm in respiration occurred in plants transferred to constant darkness, and the rhythm in stomatal opening damped rapidly in constant darkness. Damping of rhythms also occurred in leaflets exposed to constant light and CO₂-free air, demonstrating that active photosynthesis and not simply light was necessary for sustained expression of these rhythms. This is CIWDPB Publication No. 1142 This research was supported by National Science Foundation grant BSR 8717422 (C.B.F.) and a U.S. Department of Agriculture training grant to Stanford University (T.L.H.).     

Masking of circadian activity rhythms in canaries by light and dark

Canaries (Serinus canaria) were kept singly in cages placed in an artificially illuminated, soundproof cabinet. Perch-hopping activity was recorded by means of a computer system. In three series of experiments, the activity rhythms of the birds were entrained to 24 hr by light-dark (LD) cycles with 4, 12, or 20 hr of light (L), respectively. The intensity of illumination was 10 lux in L and 0.25 lux in darkness (D). Under LD 4:20 and 12:12, the intensity of D was increased daily at the same zeitgeber time to 1 lux for 1 hr (L pulse) during about 8 consecutive days. This sequence was followed by 8 days without L pulses before giving another series of L pulses at a different zeitgeber time. Under LD 20:4, the intensity of L was decreased to 1 lux for 1 hr (D pulse). The activity of all birds was more or less increased by the L pulses (positive masking) and decreased by the D pulses (negative masking). The level of masking activity during the L and D pulses depended on the circadian phase at which the pulses were administered. Positive masking by L pulses was minimal about 5 hr after the beginning of D, and increased steadily thereafter. Negative masking by D pulses was maximal at the beginning and the end of L, and minimal during the middle.