Juan José Chiesa

Effects of photoperiod on rat motor activity rhythm at the lower limit of entrainment.

The experiment described here studied the rat motor activity pattern as a function of the photoperiod of circadian light-dark cycles in the limits of entrainment (22-and 23-h periods). In most cases, the overt rhythm showed 2 circadian components: 1 that followed the external LD cycle and a 2nd rhythm that was free run. The expression of these components was directly dependent on the photoperiod, and there was a gradual transition in the manifestation of 1 or the other. The component with a period equal to that of the external cycle was more manifested under long photoperiods, while the other 1 was more expressed during short photoperiods. Also, the period of the free-running component was longer under T22 than T23. For each period, the free-running component was longer under a longer photoperiod. At first sight, the presence of these 2 components in most of the rats might appear to be due to the fact that in the limits of entrainment, some rats do not entrain and thus show a free-running rhythm plus masking. However, the gradation observed in the different patterns of the overt motor activity rhythm, especially those patterns related to the different balance between the 2 components and the length of the period of the free-running component under LD as a function of the photoperiod, suggests that the circadian system can be functionally dissociated.

Effects of transient and continuous wheel running activity on the upper and lower limits of entrainment to light-dark cycles in female hamsters.

The entrainment limits to light‐dark cycles can be modified by the experimental conditions under which they are tested. Among the factors that may influence entrainment is the amount of wheel running exerted by the animal. In the present work, the effects of transitory and continuous wheel running on entrainment to light‐dark cycles were tested using a range of T cycles at the entrainment limits. Four groups of female hamsters were submitted to 1 h stepwise changes in T cycles. Two groups were exposed to T cycles of which the period was shortened at the lower limit from T22 to T18, and the other two groups were exposed to cycles that lengthened at the upper limit from T27 to T32. One of the groups at the lower limit and one at the upper limit had continuous access to a running wheel, while the others had the wheel locked, except at certain T when a lack of period control by T cycle appeared. The study demonstrates that access to running wheel widens the limits of entrainment to LD cycles. Specifically, the following observations were made: the effects of wheel running for entrainment were more evident in the groups with continuous access to wheel, as they did entrain to T19 and T32; continuous access to a wheel produced aftereffects only after T19, but not under T32; and when animals without a wheel showed relative coordination, unlocking the wheel favored entrainment in all the animals at T31, but in only 1 out 6 at T19. All of these indicate a different effect of the wheel running on the upper and lower limits of entrainment.

Effect of melatonin and diazepam on the dissociated circadian rhythm in rats.

Abstract:u2002 The main structures involved in the circadian system in mammals are the suprachiasmatic nuclei (SCN) of the hypothalamus. The SCN contain multiple autonomous single‐cell circadian oscillators that are coupled among themselves, generating a single rhythm. However, under determined circumstances, the oscillators may uncouple and generate several rhythmic patterns. Rats exposed to an artificially established 22‐h light–dark cycle (T22) express two stable circadian rhythms in their motor activity that reflect the separate activities of two groups of oscillators in the morphologically well‐defined ventrolateral and dorsomedial SCN subdivisions. In the experiments described in this paper, we studied the effect of melatonin and diazepam (DZP) administration in drinking water on the dissociated components of rat motor activity exposed to T22, to deduce the possible mechanism of these drugs on the circadian system. In order to suppress the endogenous circadian rhythm of melatonin, in some of the rats the pineal gland or the superior cervical ganglia were removed. The results show that melatonin or DZP treatment increased the manifestation of the light‐dependent component to the detriment of the manifestation of the non‐light‐dependent component and that melatonin, but not DZP, shortens the period of the non‐light‐dependent component. These findings suggest that both DZP and melatonin favor entrainment to external light, and that melatonin could also act on the SCN, producing changes in the period of the circadian cycle.

Motor activity rhythms of forced desynchronized rats subjected to restricted feeding.

Although light is the strongest zeitgeber for the circadian pacemaker, other stimuli can also produce entrainment. In the rat, periodic restricted feeding (RF) is a weak stimulus that may act as a zeitgeber. We tested the effect of RF on the motor activity rhythms of rats subjected to forced dissociation. In this situation two components, supposed to be related with the ventrolateral and dorsomedial areas of the suprachiasmatic nucleus, are detected in their motor activity. One component is entrained to the external light-dark cycle (Light Dependent Component, LDC) and thus has the same period, while the other has a period longer than 24 h (Non-Light Dependent Component, NLDC). This experiment examined whether RF can act on one or both of these two rhythms. Rats were maintained under the light-dark cycles of 22 h (T22) or 23 h (T23) for 44 days with food available for four hours per day. Afterwards the rats received food ad libitum, to test the effect of the previous RF condition. Results show that RF modifies the manifestation of the two initial rhythms, being this effect stronger under T23 than under T22. However RF does not affect the NLDC period. The results reveal that the animal can manifest simultaneously several rhythmic patterns.

Cardiac Activity of Nephrops Norvegicus (Decapoda: Nephropidae): The Relationship between Circadian and Ultradian Rhythms

Abstract The relationship between rhythms of circadian periodicity (i.e., from 20 to 28 h) and ultradian periodicity (i.e., less than 20 h) was studied in the burrowing decapod Nephrops norvegicus at the level of its cardiac activity. Animals were kept over a month under constant darkness (DD) interrupted by a few days of light-darkness regime (LD) at the beginning of the experiment. Time series (beats per 10 min) were subdivided into stages of similar numbers of days. A general mean waveform was computed per stage by averaging 24 h segments of different time series per corresponding 10 min intervals. Marked fluctuations were observed at the beginning of tests in DD, being disrupted during animals exposure to LD. Fluctuations progressively recovered over following stages of DD. The activity part of a rhythm (α) was computed in mean waveforms of different time series per each stage. Resulting values were averaged at corresponding stages. A significant increment of mean α was observed from DD to LD, decreasing over the following prolonged DD exposure. Periodogram analysis was used to assess periodicities of time series at each stage. Fourier analysis was undertaken to assess the transformation of cardiac rhythms over consecutive stages not only in terms of periodicity but also as amplitude. Both analyses showed the presence of different circadian and ultradian (i.e., 12 h and 18 h) rhythms varying in their amplitude at different stages of testing. A preponderance of time series with ultradian periodicities took place in the first stage of DD. Under LD, the number of time series showing 18 h periodicity increased, but their amplitude of fluctuation was lower compared to the previous stage. In contrast, the circadian periodicity present in the first stage of DD disappeared in LD, to be restored over prolonged DD exposure. Present results suggest that a disruption of the circadian rhythm in cardiac activity generated ultradian periods when controlling oscillators became uncoupled. Results are discussed in the context of the ecology of the species, and a model based on the phase decoupling of circadian oscillators is presented to account for the generation of ultradian 12 h and 18 h periodicities.

History-Dependent Changes in Entrainment of the Activity Rhythm in the Syrian Hamster (Mesocricetus auratus)

The authors have studied the activity rhythm of Syrian hamsters exposed to square LD cycles with a 22-h period (T22) with the aim of testing the effects of the previous history on the rhythmic pattern. To do so, sequential changes of different lighting environments were established, followed by the same LD condition. Also, the protocol included T22 cycles with varying lighting contrasts to test the extent to which a computational model predicts experimental outcomes. At the beginning of the experiment, exposure to T22 with 300 lux and dim red light occurring respectively at photophase and scotophase (LD300/dim red) mainly generated relative coordination. Subsequent transfer to cycles with ∼0.1-lux dim light during the scotophase (LD300/0.1) promoted entrainment to T22. However, a further reduction in light intensity to 10 lux during the photophase (LD10/0.1) generated weak and unstable T22 rhythms. When, after that, animals were transferred again to the initial LD300/dim red cycles, the amplitude of the rhythm still remained very low, and the phases were very unstable. Exposure to constant darkness partially restored the activity rhythm, and when, afterwards, the animals were submitted again to LD300/dim red cycles, a robust T22 rhythm appeared. The results demonstrate history-dependent changes in the hamster circadian system because the locomotor activity pattern under the same T22 cycle can show relative coordination or unstable or robust entrainment depending on the prior lighting condition. This suggests that the circadian system responds to environmental stimuli depending on its previous history. Moreover, computer simulations allow the authors to predict entrainment under LD300/0.1 cycles and indicate that most of the patterns observed in the animals due to the light in the scotophase can be explained by different degrees of coupling among the oscillators of the circadian system.

Evidences on the transient disruption of Sabella spallanzanii (Polychaeta, Sabellidae) fan activity rhythm in laboratory constant darkness

Circadian and ultradian rhythms were analyzed in Sabella spallanzanii tested in laboratory constant darkness conditions. A video camera took images per 30 s during a week. The number of times animals opened the fan per 30 min was counted by an observer. Resulting time series were subdivided into 24‐h segments in order to assess the change of the activity rhythm over consecutive days. Periodogram and waveform analyses measured the periodicity and the pattern of fluctuation, at each cycle. Results indicate that animals express a major circadian peak in the first day of test, and this progressively spits into subcomponents from days 2 to 4. At days 5 and 6 the circadian peak is reconstituted. The activity phase of a rhythm (α) and its amplitude of oscillation were also computed per 24‐h segments. The values of different time series were averaged at corresponding 24‐h stages. A significant increment of mean α was observed from days 1 to 4, while conversely the amplitude decreased. From days 5 to day 6, an opposite trend in both variables was observed. These results are descriptive but can be interpreted in the context of models accounting for the generation of ultradian rhythms when controlling oscillators became transiently uncoupled.

Method for studying behavioural activity patterns during long-term recordings using a force-plate actometer

The motor activity (MA) patterns of rodents are commonly detected in the laboratory using infrared photo-beams or running wheels. In chronobiological studies, the MA rhythm is considered as a behavioural output of the circadian pacemaker. This paper describes a method to obtain long-term records of MA in rodents, with a 1mm spatial resolution and a 1s temporal resolution. The device comprised a square platform laid on top of three force transducers, allowed the calculation of the coordinates of the centre of force exerted by a freely moving rodent, and continuously monitored its displacements. A specific computer program processed the trajectories, providing an exhaustive analysis of motion. To test this method, motor behavioural activity was studied in rats exposed to conditions that favoured rhythmicity: light-dark cycles of both 24 h (LD) and 22 h (T22), and constant darkness (DD). In addition, arrhythmicity patterns were studied under constant light (LL) conditions, and in animals with permanent lesions of the hypothalamic suprachiasmatic nucleus (SCNx). A single description of the general MA distribution at 24 h was obtained using infrared photo-beams. By contrast, under LD conditions, a 24-h rhythm with ultradian components was seen in the total distance travelled, whereas that detected in the number of slow motions showed less ultradian components. In addition, a regional preference in the nesting place was detected under both LD and DD conditions. In one SCNx animal and another exposed to LL conditions, both showing arrhythmicity in photo-beam-detected MA, a 24 h rhythm was observed in regional preference; other LL animal presented a 24 h rhythm in the number of slow motions. This method has capabilities lacking in commonly used techniques. The potential uses of this approach, principally in cronobiological studies, are discussed.