Genetic analyses of photoresponsiveness in the Djungarian hamster,Phodopus sungorus

Summary Endotherms living at temperate and arctic latitudes must adjust their physiology and behavior in order to survive seasonal change. The Djungarian hamster uses photoperiod to cue annual cycles of reproduction and thermoregulation, and its responses to short photoperiod include loss of body weight and change in pelage color. Some individuals do not exhibit these responses when exposed to short days.In this study individual variation in photoresponsiveness is quantified, and four lines of evidence for a genetic component to that variation are provided. First, two separate breeding stocks differed in both the percent of animals responding to a short-day lighting regimen (SD) and in the degree and timing of their response. Second, analysis of variance within and between families of full sibs for a photoresponsive index, PI (body weightloss +2 (molt index –1)) following 12 weeks in SD demonstrated a significant family resemblance (intraclass correlation of 0.36±0.03). Third, heritability estimates from regression of offspring scores on parent scores for body weight loss, molt index and PI after 12 weeks in SD were 0.340.13, 0.36±0.10 and 0.37±0.12, respectively, indicating a strong additive genetic component for the three characters. Finally, a significant response occurred after one generation of artificial selection for and against photoresponsiveness.Abbreviations PI photoresponsive index (body weight loss+2 (molt index –1)) - SD short day light regimen     

Seasonal variability of sialo-glycoconjugates in the brain of the Djungarian hamster (Phodopus sungorus)

The influence of season, photoperiod and ambient temperature on the content of proteins, sialo-glycoproteins and gangliosides and on the composition of gangliosides of three different brain regions (cortex, cerebellum and basalbrain) of the Djungarian dwarf hamster (Phodopus sungorus) had been investigated. Concomittantly changes in body wt and fur colouration were recorded. Dwarf hamsters living under natural photoperiod and ambient temperature conditions ("outside") showed a distinct annual cycle in body wt (summer: about 45 g; winter: about 25 g) and fur colouration (summer: dark grey; winter: whitish). Among the three brain regions the mean concentration of proteins ranged between 120 and 155 mg protein/g wet wt. The sialo-glycoprotein content varied between 260 and 410 micrograms NeuAc/g wet wt, and that of gangliosides between 800 and 1650 micrograms NeuAc/g wet wt. Seasonal fluctuations were not found. The composition of brain gangliosides remained uninfluenced throughout the year in the cerebellum, whereas seasonal variations were observed in cortex and basalbrain. Consequently the concentration ratio of the two major mammalian ganglioside fractions GD1a vs GT1b remained almost stable in cerebellum (0.3). In contrast to this the seasonal values of cortex and basalbrain changed from 0.6 and 0.8 in winter to 0.7 and 1.1 in summer. This indicated a higher polarity of the gangliosides in these brain regions during cold adaptation. The results are discussed with regard to modulatory functions of neuronal gangliosides for the process of synaptic transmission during seasonal adaptation.     

Ketamine-xylazine anaesthesia in the Djungarian hamster (Phodopus sungorus)

The combination of ketamine-xylazine was assessed as a surgical anaesthetic in Djungarian hamsters acclimatized to both long (16 h light: 8 h dark) and short (8 h light: 16 h dark) photoperiods. It was concluded that 50 mg/kg of ketamine with 10 mg/kg of xylazine or 100 mg/kg of ketamine with 5-10 mg/kg of xylazine when given together by intraperitoneal injection was a satisfactory general anaesthetic. Two hundred mg/kg of ketamine with 10 mg/kg xylazine caused death in 13 of 24 animals. There were no clinically significant effects on depth of anaesthesia due to photoperiod.     

Chromosome replication patterns in the Djungarian hamster (Phodopus sungorus)

Patterns of early and late replication in the individual chromosomes of the Djungarian hamster (Phodopus sungorus) have been studied using the techniques of Giemsa staining suppression when bromodeoxyuridine is incorporated into the DNA. — Late replicating autosome regions correspond to G-band regions, early replication regions are less clearly demarcated but correspond to R-band regions plus some G-band zones. In part this reduction in sharpness of early replication bands may be due to the fact that nearly all metaphase G-bands contain R-band material since they are compounded from blocks of sub-G bands. — The long arm of the X chromosomes in the female differ in the start time of synthesis but are rarely separable at the close of S. There are no differences between the short arms. In the male, Y starts very late but finishes about the same time as the X which behaves like the early replicating X of the female.Visiting worker from Department of Biological Sciences, Sambulpur University, Burla 768017, India     

Photoperiod-dependent changes in exocytotic activity in the hypophyseal pars tuberalis of the Djungarian hamster,Phodopus sungorus

The pars tuberalis of the hypophysis of the Djungarian hamster, Phodopus sungorus, was investigated with regard to secretory activity by applying the tannic acid-Ringer perfusion technique. Two groups were maintained under long photoperiods (16 h light: 8 h dark) or short photoperiods (8 h light: 16 h dark), respectively. Perfusion with tannic acid showed that specific pars tuberalis cells release some of their secretory granules as indicated by typical exocytotic figures. The percentage of cells displaying exocytotic activity was significantly higher in the pars tuberalis of hamsters kept under long photoperiods. The number of exocytotic figures per single cell was not increased. These results provide further evidence for a secretory activity of the pars tuberalis and support the hypothesis of its involvement as a mediator between photoperiodic stimuli and the endocrine system.     

Fenestrated capillaries in the ventral sebaceous gland of the Djungarian hamster, Phodopus sungorus

The ventral sebaceous gland of the Djungarian hamster is a macroscopically visible organ situated in the midventral area of the abdominal wall. It consists of densely packed acini arranged in lobules with common excretory ducts. The rich vascular network of the gland is characterized by fenestrated capillaries. Fenestrated endothelium has not yet been reported as a characteristic and regular finding within sebaceous glands. Results are discussed with regard to proliferation rate of sebocytes and the demand of fluid and nutrient supply.     

Maximum energy assimilation rates in the Djungarian hamster (Phodopus sungorus)

Summary Physiological limits to energy budgets were estimated in Djungarian hamsters (Phodopus sungorus) using food balance and respirometric methods. The summer acclimatized, reproductively inactive hamsters could balance their energy budget at-2° C, assimilating 91.1 kJ·animal^-1· day^-1 after gradual cold acclimation, whereas non-acclimated hamsters showed negative energy balance assimilating only 54.4 kJ·animal^-1·day^-1. At the same ambient temperature, multiparous females (although neither pregnant nor lactating at the time) maintained positive energy balance assimilating 81.6 kJ·animal^-1·day^-1. Hamsters are capable of rapid adjustments of their maximum assimilation rates to meet their current energy demands, but only up to the value of about 3.5xBMR. It is concluded, that the actual energy budgets of small mammals keep, all the time, fairly near the upper physiological limit, with body reserves ready to buffer short-term oscillations.     

Modulation of leptin sensitivity by short photoperiod acclimation in the Djungarian hamster, Phodopus sungorus

During seasonal acclimation, Djungarian hamsters spontaneously exhibit a reduction in food intake, body mass and body fat stores, which is externally cued by shortening of day length in autumn and controlled by a sliding set-point. We investigated the function of the leptin adipostatic feedback system in the photoperiodic control of seasonal acclimation. In response to mouse recombinant leptin injections for 10 days, long day photoperiod (LD) and short day photoperiod (SD)-acclimated hamsters decreased food intake and body mass. The reduction of body mass was due to the depletion of body fat, as revealed by carcass composition analysis. In SD hamsters, leptin caused a larger reduction of body fat mass than observed under LD conditions, whereas the anorectic effect was similar in both photoperiods. The serum leptin concentration was 9.3 ± 1.2 ng/ml in LD-acclimated hamsters and decreased significantly to 4.2 ± 0.8 ng/ml and 2.1 ± 0.6 ng/ml in hamsters exposed to SD for 66 days and 116 days, respectively (P < 0.001). A strong positive correlation between total body fat mass and serum leptin concentration was found (r _S=0.935, P < 0.0001, n=70). Despite the anorectic action of exogenous leptin, higher endogenous leptin levels in LD hamsters were paralleled by higher food intake in LD hamsters as compared to SD hamsters. This paradoxical finding further supports the increased leptin sensitivity in SD hamsters as judged from leptin treatment experiments. We tested the functional significance of leptin for the controlled down-regulation of food intake and body mass induced by short photoperiod. Food restriction for 10 days during the transition phase decreased body mass below the desired sliding set-point, which was recovered in control hamsters following ad libitum refeeding. Treatment with mouse recombinant leptin during ad libitum refeeding inhibited the recovery of body mass and blunted the increase of food intake observed in controls, indicating that the sliding set-point utilizes leptin as a signal for the adjustment of the appropriate body mass level. Accepted: 15 October 1999     

Cardiac dynamics during daily torpor in the Djungarian hamster (Phodopus sungorus)

Djungarian or Siberian hamsters (Phodopus sungorus) acclimated to short photoperiod display episodes of spontaneous daily torpor with metabolic rate depressed by approximately 70% and body temperature (T(b)) reduced by approximately 20 degrees C. To study the cardiovascular adjustment to daily torpor in Phodopus, electrocardiogram (ECG) and T(b) were continuously recorded by telemetry during entrance into torpor, in deep torpor, and during arousal from torpor. Minimum T(b) during torpor bouts was approximately 21 degrees C, and heart rate, approximately 349 beats/min at euthermy, displayed marked sinus bradyarrhythmia at approximately 70 beats/min. Arousal was typically completed within approximately 40 min, followed by a sustained post-torpor inactivity tachycardia ( approximately 540 beats/min). The absence of episodes of conduction block, tachyarrhythmia, or other forms of ectopy throughout the torpor cycle demonstrates a remarkable resistance to arrhythmogenesis. The ECG morphology lacks a distinct isoelectric interval following the QRS complex, and the ST segment resembles the ECG pattern in mice, with a prominent fast transient outward K(+) current (I(to,f)) determining the early phase of ventricular repolarization. During low-temperature torpor, the amplitudes of the QRS complex substantially increased, suggesting that in the euthermic state the terminal portion of ventricular depolarization is fused with the beginning of repolarization, low T(b) acting to decorrelate the superposition between depolarization and repolarization by delaying the repolarization onset. Atrioventricular and ventricular conduction times were prolonged as function of T(b). In contrast, the QT vs. T(b) relationship showed marked hysteresis indicating the operation of nonlinear control mechanisms whereby the rapid QT shortening during arousal results from additional mechanisms (probably sympathetic stimulation) other than temperature alone.     

Nucleolus-like bodies in the pineal gland of the Djungarian hamster (Phodopus sungorus)

Summary Light- and electron-microscopic observations on the pineal gland of Phodopus sungorus revealed intracytoplasmic inclusions resembling nucleolus-like bodies similar to those found in other regions of the central nervous system. Bernhard's EDTA method was used to confirm that these inclusions were nucleolus-like bodies. These structures were rarely found in pinealocytes of sexually active longday animals, whereas large numbers of them were observed in pinealocytes of sexually quiescent short-day animals. Nucleolus-like bodies may therefore be involved in pineal secretion.     

Cold prevents the light induced inactivation of pineal N-acetyltransferase in the Djungarian hamster,Phodopus sungorus

Summary In the Djungarian hamster seasonal acclimatization is primarily controlled by photoperiod, but exposure to low ambient temperature amplifies the intensity and duration of short day-induced winter adaptations. The aim of this study was to test, whether the pineal gland is involved in integrating both environmental cues. Exposure of hamsters to cold (0 °C) reduces the sensitivity of the pineal gland to light at night and prevents inactivation of N-acetyltransferase (NAT). The parallel time course of NAT activity and plasma norepinephrine content suggests that circulating catecholamines may stimulate melatonin synthesis under cold load.Abbreviations NAT N-acetyltransferase - NE norepinephrine - T _a ambient temperature     

A high incidence of abdominal pregnancy in the Djungarian hamster (Phodopus sungorus).

Eight animals with abdominal fetuses were found in a relatively small colony of Djungarian hamsters. All the fetuses were macerated. Some had remains of an amniotic sac with an umbilical attachment to a placenta adherent to the peritoneum, but others were without placental remains. Gross and histological examinations failed to reveal any ruptures of the uterus or oviducts. It was not possible to decide whether the abdominal pregnancies were primary or secondary.     

The role of ozone/oxygen in clindamycin-associated enterocolitis in the Djungarian hamster (Phodopus sungorus sungorus)

Rectal and oral ozone-O2 gas insufflations prevented a fatal clindamycin-induced enterocolitis in Djungarian dwarf hamsters. In spite of no gross lesions in the colon of these species, the term 'colitis' is justified because of intracellular degenerative changes of colon epithelial cells. Dwarf hamsters are omnivores and are, therefore, more relevant to man than other animal species as an animal model for studying colitis.     

Ventilation and metabolism of the Djungarian hamster (Phodopus sungorus) and the albino mouse

Metabolic rates (O2 consumption and CO2 production) were similar in the Djungarian hamster and the white mice. The Djungarian hamster had a decreased tidal volume, an increased frequency of breathing (due to a lengthened expiratory time) and a decreased minute ventilation compared to that of the mouse. Although the relative ventilatory increase in response to hypercapnia or hypoxia was similar in the hamster and the mouse, the breathing patterns during exposures differed.     

Pineal and pituitary involvement in the photoperiodic regulation of body weight, coat color and testicular size of the Djungarian hamster, Phodopus sungorus

Hypophysectomy of 40-day-old male hamsters kept under long photoperiods resulted in a change in coat color from the brownish summer coat to the whitish winter coat, within 5 wk of the surgery, cessation of growth, and, most frequently, an additional loss in body weight. The testes of the hypophysectomized animals were involuted within 14 days after surgery. Continuous substitution of prolactin by implantation of two anterior pituitaries under the kidney capsule was able to reverse partly or totally the inhibitory effects of hypophysectomy on coat color and body weight, although there was no influence on testicular volume. The same effects could be achieved by daily s.c. injections of prolactin (100 micrograms ovine-PRL/day) into hypophysectomized hamsters, whereas the injection of adrenocorticopropic hormone (5 IU porcine-ACTH/day) or melanocyte-stimulating hormone (35 micrograms synthetic alpha-MSH/day) was ineffective. Additional pinealectomy influenced neither the inhibitory effects of hypophysectomy on coat pigmentation and body weight nor the stimulatory effects of prolactin substitution in hypophysectomized animals. Thus, the study provides evidence that the effects of the pineal gland on annual changes in body weight and coat color are mediated via the pituitary.     

Circadian rhythms of pineal N-acetyltransferase activity in the Djungarian hamster,Phodopus sungorus,in response to seasonal changes of natural photoperiod

Summary The aim of this study was to describe the regular annual pattern of the daily melatonin synthesis in Djungarian hamsters,Phodopus sungorus sungorus. The hamsters were maintained from birth in natural photoperiodic conditions and in bimonthly intervals the day/night rhythms of pineal N-acetyltransferase (NAT) were measured. Analysis of the circadian profiles of NAT activity showed that the duration of elevated melatonin synthesis closely reflects the duration of the scotophase throughout the seasons. Thus the duration of elevated melatonin seems to represent a direct humoral signal transmitting the photoperiodic message. The duration of the nightly melatonin pulse appears to be influenced mainly by the time of dawn rather than by the time of dusk. Additional information about the time of year might be encoded in the total amount of melatonin synthesized per day, whereas the amplitude of the nightly melatonin peak seems to be of minor importance.Abbreviation NAT N-acetyltransferase Dedicated to Dr. Klaus Hoffmann on the occasion of his 60th birthday     

Seasonal changes in circulating levels of thyroid hormones are not dependent on the age in Djungarian hamsters Phodopus sungorus

1. Plasma levels of total and free thyroxine (T4; FT4) and triiodothyronine (T3; FT3) were measured by radioimmunoassay in adult and senescent Djungarian hamsters at different times of the year. 2. Seasonal changes of both hormones were found in adult and senescent hamsters. 3. However, except for total thyroxine, the patterns were different in both groups of hamsters. 4. These results suggest, that in Djungarian hamsters age-related changes of thyroid function do not affect the secretory activity of the thyroid gland rather than the phase of the seasonal cycle.     

Evidence for genetic variation in the occurrence of the photoresponse of the Djungarian hamster, Phodopus sungorus.

The Djungarian hamster generally responds to a short-day photoperiod with a complex syndrome of physiological and behavioral changes; however, not all hamsters are photoresponsive. The phenotypic difference is, in part, genetically determined. Parent-offspring regression on a number of continuous and discontinuous measures indicated significant heritability for photoresponsiveness. Four generations of replicated bidirectional selection on a photoresponse index (PI) resulted in significant shifts in the percentage of responsive hamsters, although the average PI of responsive individuals was not significantly changed. Eight estimates of heritability ranged from 0.20 to 0.52. We hypothesize that the circadian system is responsible for the occurrence of the photoresponse, but that the extent of photoresponse is controlled by a separate functional system.