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

Abstract

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.

Daily Melatonin Injections Affect the Expression of Circadian Rhythmicity in Djungarian Hamsters Kept under a Long-Day Photoperiod

“…Daily melatonin injections given in the afternoon extend the duration of activity, resulting in an activity pattern which is similar to that seen under a short-day photoperiod [25,26], Thus, melatonin's effectiveness at eliciting short-day ad justments under a long-day photoperiod is not restricted to physiological traits, such as gonadal regression or thermo regulatory enhancement [17,18], In fact, the melatonin-in duced alteration of activity is correlated with the ability of this hormone to induce physiological short-day adjustments since only hamsters which exhibited short-day characteris tics in activity underwent short-day-like adjustments in re production and thermoregulation. Further, the timing of spontaneous refractoriness to melatonin is the same for ac tivity and the physiological traits.…”

“…Short-day-insensitive hamsters exhibit a different phase re sponse curve under constant darkness than responsive hamsters. Additionally, short-day-insensitive hamsters have a longer free-running period in constant darkness, and rarely exhibit a split in their activity rhythm under constant light [26], However, we have not examined whether ham sters physiologically insensitive to a short-day photoperiod are the same individuals which are insensitive to melatonin injections. The possibility that insensitivity to both melato nin and short days might have a common circadian basis is currently under investigation.…”

“…6). In order to maintain the positive phase relationship between onset of activity and lights off, melatonin injections must override the phase-delaying effect of the light expe rienced in the early subjective night [26] by a strong phaseadvancing action. Whether melatonin's phase-advancing action depends on the timing of the injection, as demon strated for other physiological adjustments [20], has yet to be tested.…”

Temperature dependence of gonadal regression in Syrian hamsters exposed to short day lengths

“…In contrast to deer mice, nonphotoresponsiveness in Siberian hamsters appears to be due to a change in the generation of the melatonin signal rather than to tissue insensitivity to melatonin. Nonresponder Siberian hamsters do not maintain extended ␣-values when transferred to SDs (18,32), but they respond appropriately to long melatonin signals produced endogenously in constant darkness or exogenously by daily melatonin infusions (12,16,25,32). Nonphotoresponsiveness in a primarily photoperiodic species allows for more opportunistic reproductive responses to environmental conditions (5).…”

Photoperiodic Responses Differ among Inbred Strains of Golden Hamsters (Mesocricetus Auratus)1

“…Within-species differences in reproductive responses to day length have been described in a variety of small mammals, including deer mice [12], white-footed mice [13], voles [14], and Djungarian hamsters [15]. Although many of these intraspecific differences in response to photoperiod have a demonstrable genetic basis [16][17][18], this is the first study to directly examine genetic influences on critical photoperiod in a mammalian species.…”