Reentrainment of motor activity and spontaneous neuronal activity in the suprachiasmatic nucleus of Djungarian hamsters.

Neurons in the suprachiasmatic nucleus (SCN) of the hypothalamus exhibit a daily rhythm in spontaneous electrical activity. Essentially two methods have been employed to record this circadian rhythm: (1) an in vitro brain slice technique and (2) in vivo multiunit recordings. Reentrainment of a circadian output to a shifted light:dark cycle commonly takes several cycles (depending on the amount of shift) until completed. Such a resetting kinetic has also been shown to be valid for SCN electrical activity if recorded in vivo. In an in vitro slice preparation, however, pharmacologically induced resetting is much faster and lacks transients; that is, a shift is completed within one cycle. This study was designed to probe for the presence of transients in the neuronal activity of the SCN in a brain slice preparation. The authors exposed Djungarian hamsters to an 8-h advanced or delayed light:dark cycle and monitored wheel-running activity during reentrainment. Additional groups of identically treated hamsters were used to record the pattern of spontaneous neuronal activity within the SCN using the brain slice preparation. Neuronal activity exhibited the usual rhythm with high firing rates during the projected day and low firing rates during the projected night. However, following 1 day of exposure to the 8-h advanced light:dark cycle, this rhythm disappeared in 6 of 7 slices. Rhythmicity was still absent following 3 days of exposure to the advanced light:dark cycle (n = 4). By contrast, 3 of 7 slices prepared from hamsters exposed to a delayed light:dark cycle for 3 days exhibited a daily rhythm in electrical activity. Although pharmacological agents reset the in vitro SCN neuronal activity almost instantaneously and in in vivo studies a stable phase relationship to a shifted light:dark cycle occurs gradually over several cycles, the authors did not detect either of these patterns. Such differences in resetting kinetics (e.g., rapid resetting, gradual reentrainment, temporary lack of measurable rhythmicity) may be due to (a) application of a resetting stimulus in vivo versus in vitro, (b) duration of the resetting stimulus, (c) the nature of the resetting stimulus, or (d) the recording technique employed.

Photoperiodic time measurement in Djungarian hamsters evaluated from T-cycle studies.

We investigated the photoperiodic response to T-cycles (0.5 h of light at intervals ranging from 23.0 h to 25.3 h) of two phenotypes of Djungarian hamsters that either exhibit or lack physiological short-day adjustments under a photoperiod of 9 h light:15 h darkness. Illumination of the same circadian time caused a similar photoperiodic response in both phenotypes. Thus hamsters found to be insensitive under a full short-day photoperiod can exhibit short-day adjustments after exposure to certain T-cycles. Given these results we conclude that the absence of photoperiodic adjustments normally found in short-day-insensitive hamsters results from their atypical entrainment under a full short-day photoperiod. We further suggest that the photoperiodic phenomena seen in Djungarian hamsters cannot be adequately explained by an external coincidence model of photoperiodic time measurement. As a more suitable model, we propose one form of internal coincidence where duration of daily motor activity reflects the phasing of multiple, endogenous oscillators. This conclusion is supported by the close relationship between duration of activity and the photoperiodic response as well as by the observation that a light pulse modulates duration of activity in a phase-dependent manner.

Absence of a daily neuronal rhythm in the suprachiasmatic nuclei of acircadian Djungarian hamsters.

The suprachiasmatic nuclei (SCN) in mammals generate and maintain a variety of daily rhythms. In nocturnally active rodents, SCN electrical activity is high during the subjective day (that time of locomotor inactivity) and low during the subjective night. The present experiment examines the relationship between SCN neuronal activity and the expression of overt circadian behavior in the Djungarian hamster (Phodopus sungorus). Wheel-running locomotor activity was measured in 2 groups of hamsters housed in constant dark (DD). Twelve hamsters, selected from ongoing experiments in our laboratory, showed no overt circadian rhythm in locomotor activity. A second group of 9 control animals exhibited rhythmic wheel-running activity in DD. In contrast to control animals, in vitro SCN electrical activity in brain slices prepared from acircadian hamsters showed no circadian variation. These data indicate that in acircadian animals SCN electrical activity properly reflects behavioral patterns.

Relationship between phase resetting and the free-running period in Djungarian hamsters.

A data set of 293 phase shifts was analyzed in order to determine the relationship between phase resetting and the free-running period (tau) in Djungarian hamsters. Phase shifts in response to a 15-min light pulse were assigned to one of two groups (tau short, less than 24 hr; tau long, greater than 24 hr), and two phase response curves (PRCs) were constructed. The two PRCs differed predominantly in the advance region, which extended so far into the subjective day of PRClong that a dead zone was lacking. The functional significance of PRC differences was assessed by computer simulations of entrainment to varying skeleton photoperiods and entrainment to a 12-hr skeleton photoperiod with varying tau's. Results from these simulations confirmed the theoretical predictions by Pittendrigh and Daan: Stability of entrainment under varying photoperiods depended on the ratio of the PRC slopes at the phases illuminated by light (SE/SM). This ratio was always larger than 1 for PRClong. It approached 0 for PRCshort as soon as the evening light illuminated the dead zone; this occurred for entrainment to very short photoperiods. Stability of entrainment to lights-off was in general better for PRClong than for PRCshort, especially if PRClong was used in combination with tau long. This suggests that it can be advantageous for stability of entrainment to lights-off to express a tau greater than 24 hr in combination with a PRC lacking a dead zone. Stability of entrainment under varying tau's was not much different for PRClong or PRCshort. However, stability of entrainment deteriorated for PRClong in combination with short tau's, whereas it deteriorated for PRCshort in combination with long tau's.

Circadian characteristics of Djungarian hamsters: effects of photoperiodic pretreatment and artificial selection.

Bidirectional artificial selection for (High Line) and against (Low Line) photoresponsiveness altered the percent of photoresponsive hamsters within lines and affected circadian function of hamsters identical in photoresponsiveness. For example, free-running period was shorter in responsive relative to nonresponsive hamsters. Between-line differences for responders and nonresponders were also found: hamsters from the High Line had a shorter free-running period relative to Low Line hamsters. However, phase angle of entrainment to long and short days was not affected. In general, expression of circadian rhythmicity was extraordinarily inflexible in photononresponsive hamsters from both lines: 1) phase angle of entrainment to lights on was similar under short and long day; 2) activity duration was similar under long and short days, although some decompression occurred in constant dark; 3) aftereffects on the free-running period were absent; and 4) amplitude of the phase-response curve was small (+/- 1 h) and present only at circadian times 10-24. We propose that selection for or against photoresponsiveness may have affected the interaction of component oscillators underlying circadian rhythmicity.

Expression of circadian rhythmicity in Djungarian hamsters under constant light: effects of light intensity and the circadian system's state.

Djungarian hamsters (Phodopus sungorus), were exposed to constant light with increasing intensities (20, 60, 350 lux), and wheel running activity was recorded. With increasing light intensity the percentage of hamsters showing a split in their daily activity pattern increased and the free running period was lengthened for both the unsplit and the split state. The fact that the free running period of both states depended on the light intensity together with the observation that the highest incidence of a circadian activity occurred under 350 lux, provoked the idea that the emergence of splitting or a circadian rhythmicity is a direct consequence of the light induced lengthening of the free running period. However, analysis of the data failed to support the idea that emergence of a split or a circadian activity is a threshold phenomenon with respect to the free running period. Due to differences in circadian function some Djungarian hamsters do not exhibit photoinduction following short day exposure. In these individuals splitting also occurred but required exposure to a higher light intensity than in photo-responsive hamsters. This observation is in accordance with the idea that the two phenotypes differ in the interaction of the two component oscillators underlying circadian rhythmicity.

Daily melatonin injections affect the expression of circadian rhythmicity in Djungarian hamsters kept under a long-day photoperiod.

Djungarian hamsters (Phodopus sungorus) kept under a long-day photoperiod (16 h light:8 h dark) were injected with melatonin each day. Hamsters which responded physiologically to this treatment (gonadal regression, molt, body weight loss) phase-advanced onset and extended duration of activity. Hamsters which were physiologically insensitive to melatonin injections did not exhibit such changes in activity pattern and often failed to entrain to the light:dark cycle. Hamsters given saline injections did not alter activity or exhibit gonadal regression, weight loss and molt to the winter pelt. Melatonin-sensitive hamsters compressed duration of activity when they became physiologically refractory to the melatonin treatment (weeks 27-29). At the same time, melatonin-insensitive hamsters became entrained to the light:dark cycle. Thus, daily melatonin injections induce short-day-like adjustments in activity under a long-day photoperiod. These changes in activity are correlated with melatonin-induced gonadal regression, weight loss and molt.

Characterization of circadian function in Djungarian hamsters insensitive to short day photoperiod.

Djungarian hamsters (Phodopus sungorus sungorus) depend mainly on day length to cue seasonal adjustments. However, not all individuals respond to short day conditions. A previous study from this laboratory proposed that nonresponsiveness to short day conditions rests with a defect in the circadian organization of these hamsters. In this study we found pronounced differences between responsive and nonresponsive hamsters in the expression of circadian rhythmicity under constant darkness and under constant illumination. While responsive hamsters showed a free-running activity pattern with a period of 23.86 +/- 0.04 h and responded to brief light pulses with the expected phase delays and phase advances, nonresponsive hamsters exhibited a period of 24.04 +/- 0.05 h and responded to light pulses with phase advances. Furthermore, 9 out of 15 responsive hamsters showed a clear split in the activity pattern within 8 weeks under constant light (80-100 lux), while only 1 of the 7 nonresponsive hamsters exhibited a split activity pattern. As a result of these differences in circadian function, nonresponsive Djungarian hamsters are incapable of proper photoperiod time measurement and photoperiod-induced seasonality.

Photoperiod, temperature and melatonin effects on thermoregulatory behavior in Djungarian hamsters.

Nesting and burrowing activity were measured in hamsters acclimated to either long or short day photoperiod in thermoneutrality and at 10 degrees C. Hamsters build larger nests under short day photoperiod or at 10 degrees C as compared to long day photoperiod or thermoneutrality. Both environmental cues contributed about 50% to a total increase in nest size from 1.8 g cotton/day to 7.7 g cotton/day (long day thermoneutral versus short day at 10 degrees C). Burrowing activity was suppressed by both cold or short day exposure. Daily melatonin injections, effective in inducing physiological short day adjustment under a long day photoperiod, also increased nesting scores. Hamsters which did not respond to short day conditions or to melatonin treatment physiologically lacked behavioral adjustments as well. Collectively, these results demonstrate analogies in the environmental control of physiological thermoregulatory adjustment and nesting behavior. Burrowing activity seems to be more related to reproductive needs than to thermoregulatory requirements in this hamster.

Differential effects of short day pretreatment on melatonin-induced adjustments in Djungarian hamsters.

Djungarian hamsters which did not respond physiologically to short day conditions were injected daily with melatonin. Hamsters responded to this treatment with typical body weight alterations and molt. Therefore, we concluded that the lack of short day adjustments is not based on insensitivity to melatonin in this species. Pretreatment with short days affected the timing of melatonin-induced body weight loss and molt. Hamsters became refractory to melatonin injections earlier for both traits if pretreated with short days. Low body weight level was maintained for a shorter period of time, whereas duration of molt was not affected. These results might indicate differences in the control of melatonin-induced body weight adjustments and molt.

Organ blood flow and brown adipose tissue oxygen consumption during noradrenaline-induced nonshivering thermogenesis in the Djungarian hamster.

During NA-induced NST blood flow through BAT increased from 0.18 ml min-1 to 3.21 ml min-1 in 23 degrees C acclimated (equals thermoneutrality) and from 0.61 ml min-1 to 9.67 ml min-1 in outdoors (-2 to 12 degrees C Ta) acclimated Djungarian hamsters. In 23 degrees C acclimated hamsters this increase was accomplished by a diversion of blood flow from visceral organs without a change in cardiac output (19.7 versus 20.5 ml min-1 before and after NA). In outdoors acclimated hamsters we also observed a redistribution of blood flow from the viscera to BAT. In addition, cardiac output increased from 24.3 to 38.8 ml min-1. Metabolic rate of BAT in situ was determined from organ blood flow and the (A-V)O2 of blood across the interscapular BAT. BAT of outdoor acclimated hamsters showed a significantly higher metabolism in comparison to 23 degrees C acclimated hamsters (81.1 versus 30.4 mlO2h-1). Furthermore, this calculation revealed that 28% (23 degrees C acclimated hamsters) and 61% (outdoors acclimated hamsters) of total NST were located in BAT of Phodopus sungorus.

Evidence for differences in the circadian organization of hamsters exposed to short day photoperiod.

Djungarian hamsters, Phodopus sungorus, depend mainly on day length to cue seasonal adjustments in reproduction and thermoregulation. These photoperiod-induced changes are mediated by changes in the daily release of pineal melatonin. However, some hamsters fail to respond to chronic short day exposure, and these individuals lack typical short day rhythms for both daily activity and pineal melatonin content. These results indicate that nonresponding hamsters lack the circadian organization responsible for proper coding of day length. Although the nature of the disruption in circadian organization is yet not known, these results clearly demonstrate the central importance of circadian rhythms in regulating photoperiod-induced adjustments in reproduction and thermoregulation.

Seasonal changes of heart weight and erythrocytes in the Djungarian hamster, Phodopus sungorus.

Djungarian dwarf hamsters (Phodopus sungorus) show an annual cycle in weight-specific metabolic rate with a high level during winter. These seasonal changes in oxygen demand are met by hematological adjustments, primarily based on an increased number of erythrocytes, but a decreased erythrocyte volume during winter. Subsequently, the diffusion area for blood gas exchange is increased during this time of high metabolic capabilities. Blood oxygen capacity (hemoglobin, 2,3-diphosphoglycerate (2,3-DPG) does not change with the season. However, seasonal changes in heart weight suggest changes in cardiac output, causing an increased blood flow per unit tissue weight during winter. This increase in circulatory efficiency, as well as changes in erythrocyte surface, are primarily controlled by photoperiod, since it occurred in hamsters living indoors at thermoneutrality but subjected to seasonal changes in photoperiod to the same extent as in hamsters living outdoors.