R. Subbaraj

Differential Effects of Pinealectomy on Circadian Rhythms of Feeding and Perch Hopping in the European Starling

To study the effects of pinealectomy on the circadian rhythms of locomotor activity and feeding, European starlings (Sturnus vulgaris) were held in constant light (0.2 lux and 200 lux) and under constant temperature conditions. Locomotor activity was measured by means of perches with microswitches mounted underneath, and feeding with an infrared photocell system at the feeder. Pinealectomy consistently led to disturbances in perch-hopping rhythms and often to a complete loss of rhythmicity as revealed by periodogram analysis. In some birds, perch-hopping rhythms recovered following a period of initial arrhythmicity. When a perch- hopping rhythm was present, its period was usually shorter than it had been before pinealec tomy. In contrast to its effects on perch hopping, pinealectomy had no effect on the persistence of feeding rhythmicity, although its period, like that of the hopping rhythm, decreased after this operation. These results support the hypothesis derived from previous studies that the circadian organization of feeding is different from that of perch hopping. Different circadian pacemakers may be involved, but other models may possibly explain the data just as well.

RELATIONSHIP BETWEEN LIGHT INTENSITY AND PHASE RESETTING IN A MAMMALIAN CIRCADIAN SYSTEM

The light-induced phase-resetting response of the locomotor activity rhythm in the field mouse Mus booduga was studied at two phases of the circadian cycle known to respond to light stimuli of 15 min duration and 1000 lux intensity with maximum advance (at circadian time 20 [CT20]) and maximum delay phase-shifts (at CT15). The phase-shifts evoked by natural daylight stimuli of various illuminations ranging between 0.001 lux and 10,000 lux and lasting 15 min were estimated. The results clearly demonstrate that the relationship between the phase-shifts and the intensities of light stimuli is nonlinear. Furthermore, a single light stimulus of 0.001 lux, or 0.1 lux intensity for a duration of 15 min, administered at CT20, evoked unequivocal responses; phase delays were observed instead of phase advances. The critical intensities needed for light stimuli of 15 min duration to induce saturating response were calculated and were found to be about 100 lux for CT20 and about 500 lux for CT15. These results suggest that a greater intensity of light is required at the phase CT15 to induce a saturating phase shift than is required at a later phase of the circadian cycle (CT20).

Mirror imaging phase response curves obtained for the circadian rhythm of a bat with single steps of light and darkness

Abstract The circadian rhythm in the flight activity of a tropical microchiropteran bat Taphozous melanopogon responds at all phases with delay phase shifts to single light‐on steps (DD/LL transfers). The circadian rhythm responds at all phases with advance phase shifts to single light‐off steps (LL/DD transfers). Phase shifts were measured from the delays or advances of the onsets of flight activity on days following DD/LL and LL/DD transfers relative to the temporal course of the onsets of activity in controls. The magnitude of the phase shifts was a function of the phases in which the transfers were made. The On‐PRC and Off‐PRC plotted from such data are mirror‐images in their time‐course and wave‐form. The phase shifts of the circadian rhythm in either direction were accompanied by changes in period (for the duration of our recordings after die transfer). The period lengthened following a delay shift and it shortened following an advance shift. The phase shifts are abrupt and discernible in the first ...

Timely Administration of Melatonin Accelerates Reentrainment to Phase-Shifted Light-Dark Cycles in the Field Mouse Mus Booduga

The effect of melatonin on the rate of reentrainment after a 6 h phase delay and a 6 h phase advance in the light-dark (LD) cycle was assayed in the nocturnal field mouse Mus booduga. After a phase delay of 6 h in the LD cycle, a single dose of melatonin (1 mg/kg) was administered for three consecutive days at about CT4 (circadian time 4). After a phase advance of 6 h in the LD cycle, melatonin was administered for three consecutive days at about CT22. Melatonin was found to accelerate reentrainment in both cases. Melatonin-treated animals took significantly fewer cycles to reentrain compared to vehicle-treated (50% dimethylsulfoxide [DMSO]) and nontreated control animals.

Reference phases and circadian rhythms

Abstract The choice of an appropriate phase of reference (the reference phase) is important in determining (i) τ characteristics, (ii) α: p ratios and (iii) ? ϕ (phase shifts). Earlier authors have variously favoured the peaks (or troughs), onset, mid‐point or end of activity, center of gravity of the activity period or median values, as reference phases. We report on our work on the activity rhythms of microchiropteran bats in the tropics under field, seminatural and laboratory conditions. Facts and figures have been specially marshalled to show that the greater precision exhibited by bats at emergence and the lability characterizing their end of activity in the field, are also mirrored in the actograms obtained in the laboratory. We, therefore, favour the “onset” of activity as reference value. The onset of activity (i) is often very stable and may thus reflect the state of the basic oscillator better, (ii)unlike mid point it does not depend on the end of activity, (iii) and may be more directly under t...

In the field mouse Mus booduga melatonin phase response curves (PRCs) have a different time course and wave form relative to light PRC

Abstract: The phase shifting effects of the pineal hormone melatonin on the circadian locomotor activity rhythm of the field mouse Mus booduga was examined at various phases of the circadian cycle using single melatonin injections of two concentrations (10 mg/kg, high dose; and 1 mg/kg, low dose) and two phase response curves (PRCs) were constructed. A single dose of melatonin administered during the early subjective day evoked maximum phase delays, and during the late subjective night evoked phase advances in the locomotor activity rhythm. Other phases of the circadian cycle also responded to melatonin. The interval between circadian time 19 (CT19) and CT2 of the high dose melatonin PRC is marked by significant phase advances, whereas the interval between CT2 and 19 is marked by significant phase delays. A single dose of melatonin of strength 10 mg/kg was found to evoke phase shifts that were of comparable magnitude to those of the phase shifts evoked by natural daylight pulses. Control animals, treated with 50% dimethyl sulfoxide (DMSO), did not respond with phase shifts significantly greater than zero. Significant differences between the shapes of the two melatonin PRCs exist. Further melatonin PRCs appear to have a different time course and wave form relative to light‐induced PRC.

Ultraviolet-light-evoked phase shifts in the locomotor activity rhythm of the field mouse Mus booduga

The circadian rhythm of locomotor activity in the nocturnal field mouse Mus booduga has been monitored after an exposure to ultraviolet-A (UV-A; wavelength between 350 and 400 nm) radiation at various hours of the subjects cycle. The magnitude of the phase shift is used to determine the sensitivity of the circadian pacemaker underlying locomotor activity rhythm. It is observed that UV-A shifts the phase of the locomotor activity rhythm in a phase-dependent manner. A phase response curve (PRC) is constructed to depict the time course and waveform of the basic oscillators sensitivity to UV-A radiation. The UV-A PRC qualitatively resembles the PRC constructed using diffused daylight as stimulus. Furthermore, the phase response characteristics of the circadian pacemaker for UV-A suggest a periodically varying sensitivity to UV-A similar to that to daylight.

GREEN LIGHT EVOKES MAXIMUM PHASE SHIFTS IN THE LOCOMOTOR ACTIVITY RHYTHM OF THE FIELD MOUSE MUS BOODUGA

The effect of pulses of different monochromatic light wavelengths was investigated on the circadian locomotor activity rhythm of the tropical rodent Mus booduga. Blue (480 nm), green (549 nm) and red (649 nm) light were used for pulses at circadian time (CT) 14 and 20, the maximum delay and advance eliciting phases of the white light phase response curve (PRC). All three wavelengths evoked qualitatively similar responses in terms of the signs of the phase shifts. Contrary to the results obtained in rabbits and bats, M. Booduga does not exhibit differential responses to different light wavelengths at various phases. This prompts us to speculate on the existence of a single class of photoreceptors for all light wavelengths. The responses of these animals to three different energy levels (5, 25 and 50 microW cm2) of these light wavelengths were also studied and these showed quantitative differences. Green light evoked larger phase shifts in terms of both delays and advances compared with red light (P < 0.1 at both CTs). However, it failed to exhibit an energy-dependent increase in the magnitude of the phase shifts. In contrast, red light evoked an energy-dependent increase in the magnitude of the phase shifts of both CT 14 and CT 20 and blue light at CT 14 alone. It is speculated that the threshold level for perceiving green light may be lower than the energy levels used and hence saturation would have been reached at 5 microW cm-2. For red light, it is possible that this threshold is at a much higher level. This explains the failure to exhibit an energy-dependent increase by green light and the systematic increase by red light.

Seasonality in the flight activity of the tropical bat Rhinopoma hardwickei under natural photoperiod

The activity cycle of a colony of about 1500 bats Rhinopoma hardwickei was studied in 1978-1979 in Madurai, India. Variations in the times of emergence of foraging paralleled variations in sunset and return to the roosting paralleled sunrise. The releasing stimulus for departure of the first outflyer on clear evenings is the crossing of a lower threshold value of its relative decrease. Return to the roost usually occurs at a lower light intensity than the departure. In summer the nightly rate of emergence is relatively higher than in winter. The seasonal changes of phase angle differences of onset (Ψ0) and end (Ψe) of activities are roughly mirror images. The time of midpoint of foraging activity however, stays relatively constant.

Chronobiotic effect of melatonin following phase shift of light/dark cycles in the field mouse Mus booduga

The objective of this study was to assess whether melatonin accelerates the re-entrainment of locomotor activity after 6 h of advance and delay phase shifts following exposure to LD 12:12 cycle (simulating jet-lag/shift work). An experimental group of adult male field mice Mus booduga were subjected to melatonin (1 mg/kg) through i.p. and the control group were treated with 50 % DMSO. The injections were administered on three consecutive days following 6h of phase advance and delay, at the expected time of “lights off”. The results show that melatonin accelerates the re-entrainment after phase advance (29%) when compared with control mice. In the 6 h phase delay study, the experimental mice (melatonin administered) take more cycles for re-entrainment (51%) than the control. Further, the results suggest that though melatonin may be useful for the treatment of jet-lag caused by eastward flight (phase advance) it may not be useful for westward flight (phase delay) jet-lag