Circadian Locomotor Rhythms Research Articles

A diffusible coupling signal from the transplanted suprachiasmatic nucleus controlling circadian locomotor rhythms.

The mammalian suprachiasmatic nuclei (SCN) transmit signals to the rest of the brain, organizing circadian rhythms throughout the body. Transplants of the SCN restore circadian activity rhythms to animals whose own SCN have been ablated. The nature of the coupling signal from the grafted SCN to the host brain is not known, although it has been presumed that functional recovery requires re-establishment of appropriate synaptic connections. We have isolated SCN tissue from hamsters within a semipermeable polymeric capsule before transplantation, thereby preventing neural outgrowth but allowing diffusion of humoral signals. Here we show that the transplanted SCN, like neural pacemakers of Drosophila and silkmoths, can sustain circadian activity rhythms by means of a diffusible signal.

Restoration of circadian rhythmicity by transplants of SCN "micropunches".

Although it is widely accepted that the suprachiasmatic nuclei (SCN) of the hypothalamus serve as biological pacemakers regulating circadian rhythmicity, a number of studies suggest that some circadian rhythms may be controlled by extra-SCN structures. Transplantation of fetal anterior hypothalamic tissue containing the SCN restores circadian locomotor rhythms in SCN-lesioned hosts. Such transplants, however, contain substantial extra-SCN hypothalamic tissue. In the present study, the authors examined the recovery of circadian locomotor rhythms in animals implanted with small grafts harvested by taking "micropunches" from vibratome-sectioned brain slices. Micropunches were taken from three areas of the hypothalamus known to receive retinal input: the SCN, the subparaventricular zone, and the supraoptic nucleus. The results indicate that transplants restricted to the SCN region are necessary and sufficient for restoration of circadian locomotor activity rhythms and that micropunches of tissues from other sources are ineffective.

Seasonal Variations of Pineal Involvement in the Circadian Organization of the Ruin Lizard Podarcis Sicula

To establish whether the effects of pinealectomy on circadian locomotor rhythmicity vary with season, we examined, in constant temperature and darkness, the locomotor behaviour of ruin lizards Podarcis sicula collected and subjected to pinealectomy at different times of the year. Changes in the freerunning period in response to pinealectomy were found to be significantly greater in summer than in winter, spring and autumn. Circadian activity time changed significantly in response to pinealectomy only in spring and summer. Furthermore, while pinealectomy was effective in altering the locomotor rhythms of all individual lizards tested in summer, the same surgery was found to leave locomotor rhythmicity of many lizards tested in autumn and winter completely undisturbed. These results demonstrate for the first time in a non-mammalian vertebrate that the pineal gland is centrally involved in determining circadian organization in some seasons and is only marginally involved in others.

The circadian system of c-fos deficient mice.

We examined the role of c-fos in the synchronization of circadian rhythms to environmental light cycles using a line of gene-targeted mice carrying a null mutation at this locus. Circadian locomotor rhythms in mutants had similar periods as wild-type controls but took significantly longer than controls to entrain to 12:12 light-dark cycles. Light-induced phase shifts of rhythms in constant dark were attenuated in mutants although the circadian timing of phase delays and advances was not changed. A functional retinohypothalamic projection was indicated from behavioral results and light-induced jun-B expression in the SCN. The results indicate that while c-fos activation is not an absolute requirement for rhythm generation nor photic responses, it is required for normal entrainment of the mammalian biological clock.

Circadian locomotor rhythms in aged hamsters following suprachiasmatic transplant.

Circadian activity rhythms that have been eliminated by lesions of the suprachiasmatic nucleus (SCN) can be restored by fetal SCN grafts. Partial lesions of the host allow simultaneous expression of both donor and host rhythms. Because partial SCN ablation produces characteristic changes in activity rhythms that are similar to those that occur with age, including shortened period, reduced amplitude, and fragmentation, we investigated the extent to which fetal SCN grafts may be expressed by an animal whose activity rhythm exhibits these age-dependent changes. The results indicate that expression of a transplanted clock is possible in an unlesioned aged host. Grafts of fetal SCN into young hosts and cortical tissue grafts into intact aged hosts have no effect. In those aged animals that received SCN grafts, three patterns of expression emerged in the subsequent locomotor activity record: complete dominance of locomotor rhythmicity by the donor; relative coordination between donor and host rhythms; and spontaneous switching between host and donor phenotypes. The results suggest that the expression of rhythmicity by the grafted SCN may depend on the relative amplitude or strength of signals produced by the host and donor SCN.

Circadian rhythms in mice can be regulated by photoreceptors with cone-like characteristics

In this report we have characterized the photopigments mediating circadian phase shifts in retinal degenerate (rd) mice. In aged rd rd mice, which lack detectable opsin, high performance liquid chromatography (HPLC) was used to quantify the photopigment chromophore 11- cis-retinaldehyde. This chromophore was photoisomerized in whole eyes, suggesting the presence of a functional opsin-based photopigment system. We also analyzed the spectral sensitivity of phase shifting circadian locomotor rhythms. Our data implicate a photopigment that is consistent with the involvement of the middle wavelength-sensitive cone photoreceptors (M-cones; Λ max = 511 nm) found in the mouse retina. In addition, discrete near-ultraviolet (UV-A) pulses were capable of eliciting large phase shifts in circadian locomotor activity rhythms. This result is consistent with the involvement of the short wavelength-sensitive cone photoreceptors (UV-cones; Λ max = 359 nm) in photoentrainment. Collectively, these data suggest that both cone classes of the mouse may mediate the photic regulation of circadian rhythms. If this is the case, circadian sensitivity can be maintained by very few degenerate cones. Alternatively, an unknown class of ocular photoreceptor may fulfill this function.

Waterborne lead affects circadian variations of brain neurotransmitters in fathead minnows

Lead is a potent neurotoxin affecting brain levels of a number of vertebrate neurotransmitters. Reports on these effects are, however, not consistent either among or within species. For example, with lead-intoxicated rats there are reports of decreased acetylcholine (ACh) release and decreased ACh brain levels as well as reports of increased levels or no change in levels. Also, with rats there are reports of increased levels, decreased levels, or no change in brain catecholamines, with lead producing similar changes in both norephinephrine (NE) and dopamine (DA) in some cases and differences in response between the two in others. Although most early reports dealt with whole brain levels, reports on neurotransmitter levels in specific brain regions can be equally conflicting. Similar sorts of discrepancies exist among studies with fishes. Much of the variation among studies on lead effects on neurotransmitters is, no doubt, due to differences among the studies in variables such as: species, age, dosage and duration, route of administration. However, lead can apparently affect circadian locomotor rhythms of both rats and fishes. Therefore, another possible cause for the variation among studies is that there is an interaction among dosage, sampling time and endogenous rhythms. A lead-produced phase shift ormore » disruption in endogenous neurotransmitter rhythms could in turn elicit a host of varying results and interpretations depending on the circadian time of sampling. We elected to examine this possibility in the fathead minnow, Pimephales promelas, a freshwater species widely used for toxicity studies. 15 refs., 3 figs.« less

Diurnal variations in vasoactive intestinal polypeptide-like immunoreactivity in the suprachiasmatic nucleus of congenitally anophthalmic mice.

The present study has combined recording of circadian locomotor rhythms with light microscopic immunocytochemistry for vasoactive intestinal polypeptide (VIP) in the suprachiasmatic nucleus (SCN) of congenitally anophthalmic mice. These mice, which never develop retinae or optic nerves and do not perceive light, are thus in constant darkness. Our data show a circadian rhythm in expression of VIP in the SCN of anophthalmic mice--expression is maximal during late subjective night/early subjective day and minimal in late subjective day/early subjective night. These observations support the hypothesis that expression of VIP is related to regulation of circadian rhythms by the SCN.

Neural pathways involved in mutual interactions between optic lobe circadian pacemakers in the cricket Gryllus bimaculatus

The circadian locomotor rhythm of the cricketGryllus bimaculatus is primarily generated by a pair of optic lobe circadian pacemakers. The two pacemakers mutually interact to keep a stable temporal structure in the locomotor activity. The interaction has two principal effects on the activity rhythm, i.e., phase-dependent modulation of the freerunning period and phase-dependent suppression of activity driven by the partner pacemaker. Both effects were mediated by neural pathways, since they were immediately abolished after the optic stalk connecting the optic medulla to the lobula was unilaterally severed. The neural pathways were examined by recording locomotor activity, under a 13 h light to 13 h dark cycle, after the optic nerves were unilaterally severed and the contralateral optic stalk was partially destroyed near the lobula. When the dorsal half of the optic stalk was severed, locomotor rhythm mostly split into two components: one was readily entrained to the given light-dark cycle and the other freeran with a marked fluctuation in freerunning period, where the period of the freerunning component was lengthened or shortened when the onset of the entrained component occurred during its subjective night or day, respectively. The phase-dependent modulation of activity was also observed in both components. However, severance of the ventral half of the optic stalk resulted in appearance only of the freerunning component; neither the phase-dependent modulation of its freerunning period nor the change in activity level was observed. These results suggest that neurons driving the mutual interaction and the overt activity rhythm run in the ventral half of the proximal optic stalk that includes axons of large medulla neurons projecting to the cerebral lobe and the contralateral medulla.

Circadian organization in the ruin lizard Podarcis sicula: the role of the suprachiasmatic nuclei of the hypothalamus

The effects of electrolytic lesions to the suprachiasmatic nuclei of the hypothalamus (SCN) on circadian rhythms of locomotor activity were examined in ruin lizards Podarcis sicula maintained in constant darkness and constant temperature (29°C). All lizards (N=15) in which the lesion damaged 80% or more of the SCN became behaviorally arrhythmic. On the contrary, locomotor rhythms persisted in all cases (N=11) when the SCN remained intact and lesions were confined to neighbouring regions of the preoptic area. Taken together with previous work which demonstrates that the pineal and the retinae are not essential for the persistence of circadian locomotor rhythmicity in Podarcis sicula and with recent evidence showing the homology between the SCN of lizards and those of mammals the present results strongly support the view that the SCN of Podarcis sicula contain the primary pacemaker(s) for locomotor rhythms.

Gene expression in suprachiasmatic nucleus and circadian rhythms

The suprachiasmatic nuclei (SCN) contain a circadian system consisting of circadian oscillator (clock) that is normally synchronized by the light/dark cycle (input) and drives circadian rhythms (output) that are intrinsic to the SCN. Gene expression of immediate-early genes, such as c- fos and jun-B, in the ventrolateral SCN is associated with circadian synchronization by light pulses and subjected to circadian control. Vasopressin and somatostatin gene expression show distinct circadian rhythms intrinsic to the dorsomedial SCN with higher peptide levels occurring during the day. In addition, embryonic SCN grafted into the brain of an SCN-lesioned arrhythmic host define the period of the restored circadian locomotor rhythm. Taken together, these and other findings support the notion that the expression of genes underlying circadian synchronization, oscillation and output takes place within individual SCN neurons. However, no information regarding the nature and number of those neurons as well as the molecular mechanisms of the single cell-circadian oscillator and output is currently available. Therefore, we propose a simple two-neuron model as a framework for critically discussing the molecular genetic strategies to analyze the circadian system in SCN.

Host resets phase of grafted suprachiasmatic nucleus: a 2-DG study of time course of entrainment

The object of the present experiment was to examine whether in an intact animal implanted with a hypothalamic graft, the phase of the host and grafted suprachiasmatic nucleus (SCN) would become synchronized. To this end, we first established the time at which daily fluctuations in local cerebral glucose utilization were maximal in the SCN in our population of adult hamsters. Next, we verified that rhythms of ( 14C)2-deoxyglucose uptake could be measured on the day after birth in pups that were to provide donor tissue. Host and donor animals were housed in opposite light:dark cycles. We then transplanted fetal SCN tissue into the third ventricle of intact hamsters, placed the grafted animals in constant darkness with access to running wheels and examined the phase of metabolic activity in host and donor SCN. For several days after grafting, there was no circadian fluctuation in the metabolic activity of either the host SCN or of the grafted SCN. During this time, the circadian locomotor rhythms were not disrupted, suggesting that pacemaker activity was not interrupted. By day 14 after transplantation, metabolic activity in the host SCN was elevated during subjective day and host and donor SCN were in synchrony, invariably with the phase of the host animal. We conclude that a signal from the host SCN resets the grafted SCN and not vice versa and that pacemaker cells communicate with each other rather than exerting independent effects on target sites.

Visual and circadian responses to light in aged retinally degenerate mice

The progression of photoreceptor degeneration in retinally degenerate ( rd) mice commences early in postnatal development resulting in the complete loss of rods by 60–70 days of age followed by the more protracted loss of cones. We have previously shown that rd mice 80 days of age are capable of phase shifting their circadian locomotor rhythms in response to brief pulses of light and these animals show the same sensitivity as wild-type ( + / + ) controls. If surviving cones mediate these circadian responses, then one would expect the sensitivity of the circadian system in rd mice to decline with age and parallel the loss of cones. We demonstrate that aging rd mice (80–767 days of age) remain capable of photically regulating circadian locomotor rhythms in a manner indistinguishable from + / + mice. Circadian responses to light do not parallel cone cell degeneration in rd mice. In contrast to the circadian responses to light, old (> 210 days of age) rd mice show no visually-evoked behavioral or electroretinogram (ERG) responses.

Circadian wheel running activity rhythms in two strains of domestic rabbit

Results from experiments on two strains of domestic rabbit show that despite their hopping gait, circadian locomotor rhythms can be successfully measured using running wheels specially designed for large laboratory mammals. Three female rabbits of an English outbred strain were monitored in a 13:11 light:dark (LD) cycle and in constant light (LL), while 12 Dutch-Himalayan rabbits were monitored in a 14:10 LD cycle and constant dark (DD). Much stronger rhythms were observed in the latter strain, which under LD conditions showed individuals with patterns that varied from nocturnal ( n = 5), diurnal ( n = 4), crepuscular ( n = 1), and predominantly arrhythmic ( n = 2). In DD, 10 rabbits showed strong free-running rhythms, five with tau (τ) less than 24 h and five longer. The predominantly arrhythmic activity pattern in the remaining two made estimation of τ difficult. Differences in circadian patterns both under LD and DD were unrelated to sex. Results show that locomotor rhythms of the rabbit can be measured using the running wheel, the traditional apparatus of chronobiology. However, the experiments suggest also that the large-bodied breed of rabbits that tend towards obesity, such as the English outbred strain, are relatively inactive and that smaller, more active breeds such as the Dutch-Himalayan may be more suitable for studying circadian rhythmicity if the running wheel is employed.

Electrolytic lesions to the optic chiasm affect circadian locomotor rhythms in lizards.

Electrolytic lesions to 85-95% of both optic nerves at the level of the optic chiasm (OC-X) induce a significant shortening of the free-running period (tau) of locomotor rhythms in Podarcis sicula held in constant temperature (29 degrees C) and constant darkness. Together with previous data in P. sicula, showing that retinalectomy (RET-X) in constant darkness also mostly induces a shortening of tau, the present results demonstrate that the retinae play a central role in the control of behavioural circadian rhythmicity independently of light perception. The fact that OC-X and RET-X affect locomotor rhythms in the same way (mostly by shortening tau), strongly supports the contention that the influence of the retinae on the circadian system is neurally mediated.

Light and temperature entrainment of a locomotor rhythm in honeybees

Abstract. The circadian locomotor (walking) rhythms of forager honeybees (Apis mellifera ligustica L.) were entrained to eight different 24 h light‐dark cycles. The phases of activity onset, peak activity, and offset were correlated with the lights‐off transition, suggesting lights‐off as the primary zeitgeber for the rhythm. Further support for this hypothesis was provided by LD 1:23 experiments, in which entrainment occurred when the light pulse was situated at the end, but not at the beginning, of the subjective photophase. Steady‐state entrainment of the locomotor rhythm was achieved with square‐wave temperature cycles of 10oC amplitude under constant dark: most of the activity occurred within the early thermophase. Smaller amplitude temperature cycles yielded relative coordination of the rhythm. Interactions of temperature and light‐dark cycles resulted in entrainment patterns different from those elicited in response to either cycle alone or those formed by a simple combination of the two separate responses. Furthermore, temperature cycles having amplitudes insufficient for entrainment of the rhythm nevertheless modified the pattern of entrainment to light ‐ dark cycles, suggesting a synergism of light and temperature effects on the underlying circadian clock system.

Circadian phase-shifts induced by chlordiazepoxide without increased locomotor activity

Phase advances of circadian locomotor rhythms in response to the benzodiazepine triazolam (TRZ) administered at circadian time 6 appear to be mediated by locomotor activity. The possibility that increased activity also mediates shifts induced by the benzodiazepine chlordiazepoxide (CDZ) was investigated. Hamsters injected with CDZ 7 h before dark onset exhibited phase advances despite being confined to their nest ☐es for 3 h after receiving the drug. These shifts did not differ significantly from advances seen in hamsters which were allowed access to the rest of the cage after injections. In other experiments, the behavioral effects of both CDZ and TRZ were examined during the 5 h after injection. There was significantly more motor activity observed after injection of TRZ than CDZ. These results suggest that phase-shifting effects of some benzodiazepines can occur without inducing activity.

Ocular regression conceals adaptive progression of the visual system in a blind subterranean mammal.

The mole rat, Spalax ehrenberghi, is an extreme example of natural visual degeneration in mammals: visual pathways are regressed and incomplete, and the absence of visual cortical potentials or an overt behavioural response to light have led to the conclusion that Spalax is completely blind. But structural and molecular investigations of the atrophied, subcutaneous eye suggest a functional role for the retina in light perception, and entrainment of circadian locomotor and thermoregulatory rhythms by ambient light demonstrates a capacity for photoperiodic detection. We report here that severe regression of thalamic and tectal structures involved in form and motion perception is coupled to a selective hypertrophy of structures subserving photoperiodic functions. As an alternative to the prevalent view that ocular regression results from negative or nonselective evolutionary processes, the differential reduction and expansion of visual structures in Spalax can be explained as an adaptive response to the underground environment.

Behavioral inhibition of circadian responses to light.

Circadian locomotor rhythms in rodents may be synchronized by either photic or nonphotic events that produce phase shifts of the rhythm. Little is known, however, about how these two types of stimuli interact to produce entrainment. The well-characterized circadian photic response of the golden hamster was examined in situations where a short light pulse and locomotor activity, a nonphotic event, occurred simultaneously. Light-induced phase advances were attenuated when animals were active during light exposure. The results show that circadian responses to light depend upon the environmental situation in which the light is given, and call into question the implicit assumption in circadian rhythm research that phase shifting and entrainment to light-dark cycles depend simply on photic activation of well-known retinofugal pathways. Moreover, since light therapy is becoming an important component in the treatment of circadian-based disorders in humans, the results emphasize the need for evaluation of the behavioral aspects of light therapy protocols.

Locomotor rhythms induced by methylphenidate in suprachiasmatic nuclei-lesioned rats

A robust locomotor activity rhythm with a circadian period appeared by chronic treatment with methylphenidate or methamphetamine in the suprachiasmatic nuclei (SCN)-lesioned rats whose circadian rhythms had been abolished. The appearance of the rhythmicity was accompanied by an increase in the activity level. However, significant circadian periods were detected only in rats whose locomotor activity increased more than 200% of the pretreatment levels. An increase in the activity level is necessary to generate and/or express the circadian locomotor rhythm in SCN-lesioned rats and there seems to be a threshold.